摘要:

Thermodynamic Properties of Thin Films of Some Dipolar Liquids Adjacent to Fused Silica Surfaces BY K. H. ADLFINGER AND G. PESCHEL Institut fur Physikalische Chemie der Universitat Wurzburg 87 Wurzburg Markus- strai3e 9-1 1 Germany Received 31st March 1970 The disjoining pressure of some dipolar organic liquids (PhN02 PhCN PhCH,CN PhCF3 MeCN) forming thin layers between two fused silica surfaces shows maximum values in temperature ranges which refer to thermodynamic transitions of higher order. Here the molecules in the bulk liquid seem to gain an additional rotational degree of freedom about one axis whereas the molecules in the oriented surface zone still show a rotational restriction about this axis. Thermodynamic quantities for the liquid boundary layers are introduced and discussed.Since the experiments of Deryaguin l* evidence has been accumulated that a solid surface can alter the structure of an adjacent liquid layer 3-5 up to a thickness of about cm. This phenomenon is assumed to be caused by the solid surface inducing a molecular long-range orientation in the vicinal liquid. This orientation implies a decrease of the thermodynamic potential of the surface zone compared with that of the bulk liquid; it gives rise to the disjoining pressure. Thus if two solid plates immersed in a liquid showing long-range orientation approach to distances smaller than about cm under the action of an external force an oppositely directed dis- joining force arises holding the plates apart by a distance which is dependent on the external force. Knowing the geometry of the plates the disjoining pressure can be evaluated.Hitherto only few details are known about the mechanism of the long- range orientation which also alters the visc0sity,~9 ' the thermal conduction,8 and the dielectric constant of liquid surface The most frequently investigated liquid in this respect is water where the dis- joining pressure is of the order of magnitude of lo5 dyn/cm2.10* l1 The most inter- esting effect however is represented by the temperature dependence of the disjoining pressure. The present authors found significant maxima of the disjoining pressure of water at temperatures at which structural transitions of higher order occur.12* l3 These temperatures at about 14 32 45 and 61°C are repeatedly quoted and are discussed by Dro~t-Hansen.'~ By a special experimental device 7 * lo* l1 we succeeded also in determining the viscosity of aqueous boundary layers which show likewise maxima at the characteristic temperatures.Our interpretation agrees with that of Deryaguin,l viz. that small molecular aggregates determine the structure of aqueous boundary layers. The four maxima suggest the existence of four different molecular aggregates being stabitized in different temperature ranges. Applying the theory of viscous flow of Eyring l6 and determin- ing approximate values for the activation energy of viscous flow it can be assumed that the aqueous boundary layers above 14°C consist of small aggregates mainly built up of three or four water molecules. Above 32°C the aggregates are larger and above 89 90 THIN FILMS OF DIPOLAR LIQUIDS 45°C the size of the aggregates seems to be comparable with that of the molecu1ar entities existing above 14°C.Results for the temperature range about 61°C are not yet available because of experimental difficulties. All these effects get smaller with increasing plate distance and vanish at about 1.6 x cm. This value was also found by Deryaguin and coworkers ’* by measuring the shear modulus of the oriented aqueous surface zone. The results for the non-polar liquids benzene and cyclohexane indicate a steep decrease of the disjoining pressure with rising temperature starting from the melting p0int.l’ THERMODYNAMIC RELATIONS FOR ORIENTED SURFACE ZONES Regarding two surface areas each of 1 cm2 and separated by an intermediate oriented liquid layer the free excess energy for the approaching of the surfaces from a distance h+ to a distance h < h f is given by f h (AFEX = -J ndh’.h + II is the disjoining pressure which is dependent on h. h+ is the maximum distance at which the disjoining pressure can no longer be detected ; it is dependent on the sensi- tivity of the experimental apparatus. During the approach of the surfaces an oriented liquid layer with the thickness h f - h gets disordered and is pressed into the outer bulk liquid. A formal case is now considered. In order to press a liquid layer having infinit- esimal thickness and lying in the medium plane between the surfaces into the bulk liquid the free excess energy a(AFE)hdh d(AFE), = - -___- ah has to be brought into the system. This case however is not a practicable one and it is convenient to replace dh by Ah the last quantity being the thickness of a mono- molecular layer.Presuming that Ah<h is always valid the free excess energy to remove the monomolecular oriented liquid layer from the medium plane between the surface areas is a(AFE)h*h. A(AF~), = - - ah (3) The total excess energy to remove this layer can be evaluated from the Gibbs-Helm- holtz relation using d(AFE)h and its temperature derivative. Since the density of the oriented surface zone is-except for water-unknown conversion into molar quantities is only possible if the surface zone density is replaced by the density pe of the bulk 1 i q ~ i d . l ~ This however may imply large errors ; for the extreme case of aqueous surface zones the densities might exceed the bulk value by 20 or even 30 %.I8 The temperature dependence of II was assumed to have the form n = Cexp(-nh) (4) with C and n being constants.K . H . ADLFINGER AND G . PESCHEL 91 Using eqn (4) and taking molar quantities the total excess energy is obtained in the form T If the temperature dependence of A(A U,") is accessible the corresponding excess heat A(AC,",,,) can be evaluated. (5) molar EXPERIMENTAL For the determination of the disjoining pressure a spherically formed and a planar fused silica surface both highly polished totally covered with hydroxyl groups and immersed in the liquid to be investigated were pressed against each other. This system was chosen to avoid the problem that dust particles in the region between the plates appreciably falsify the measurements. For two plane-parallel surfaces this problem seems to be a serious one as Hayward and Isdale l9 have pointed out.In our experimental device the planar fused-silica plate is mounted to the bottom of a container which can be filled with the liquid in question. The spherically formed plate facing the planar one is fastened to one end of a balance which can be operated electro- magnetically. Above the spherically formed plate the measuring pin of a displacement transducer is pressed upon the balance with a definite force. The displacement transducer is connected with an electronic strain gauge measuring bridge connected to a xy-recorder. All deflections of the balance can thus be registered with high acc~racy.~ lo* The distance between the plates can be determined by a rather complicated method which takes sufficient account of the surface roughness derived from the surface profilograms and interferograms of the polished fused silica plates.ll9 2o The whole apparatus is placed in a big container which can be evacuated.In this way contact of the liquid with the humidity of the surrounding air can be avoided. At lower temperatures this precaution becomes important. The distilled and carefully degassed liquid can be sucked into the evacuated container so that the formation of air bubbles can be excluded. The temperature of the liquid in the container can be registered by a special recorder. Mechanical vibrations of the apparatus due to external causes and which disturb the measurements can be reduced by a special suspension. From a theory given by the present authors the parameters n and C in eqn (4) can be evaluated. RESULTS The liquids chosen for investigation are nitrobenzene benzonitrile benzyl cyanide benzotrifluoride and acetonitrile.By applying eqn (4) for different plate distances ranging between crn and plotting II against the temperature fig. 1-3 were obtained. It is still uncertain if for all liquids investigated in this work the oriented surface zone really extends beyond the maximum distances chosen in fig. 1-3 or abruptly turns into the bulk phase at some shorter distance. This problem could not be elucidated previously since our apparatus lacks the sensitivity to yield sufficiently precise results. For the aromatic compounds benzotrifluoride shows the largest disjoining pres- sure acetonitrile an example of an aliphatic dipolar liquid exhibits a still larger effect. The most striking evidence however is the fact that the disjoining pressure seems to assume large values only in distinct temperature ranges lying about 30" above the respective melting point T,, of the compound under consideration.and 5 x 2t a //A- *\ 15 20 25 5 Y 10 15 t "C IC N e -. FIG. 2.-Temperature dependence of the disjoining pressure II for PhCF3 (a) and PhNOz (b) at different plate distances h IO-~CM 0 ; 2x10-6cm A ; 3 x 1 0 - ~ cm V. 2 2 - 4 E 5 FIG. 1 .-Temperature dependence of the disjoining pressure II for PhCN (a) and PhCH,CN (b) at different plate distances t i lo-" cm o ; 2 x cm v ; 4x cm 0. cm A ; 3 x b 35 4 0 t "C K . H . ADLFINGER AND G . PESCHEL 93 - 10 - 5 t “C FIG. 3.-Temperature dependence of the disjoining pressure II for MeCN at different plate distances h 1OV6cm 0; 2~10-~crn A; 3~10-~crn V ; 4~10-~crn 0; 5~10-~crn 0.TABLE PA PARAMETERS n AND C n x 10-6 c x 10-6 liquid temp. (“C) (cm- 1 ) (dyn/cm2) nitro benzene 36.5 38 39.5 40 benzonitrile 16 19.5 22.5 25.5 7.5 10.5 13.5 benzo trifluori de 2.7 4 7 9.5 acetonitrile - 14.5 -11.5 -8 benzyl cyanide 4.5 1.78 1.66 2.74 4.79 1.43 1.01 0.86 0.96 2.00 1.34 1.39 1.66 1.36 1.15 0.87 1.87 1.16 0.91 1.41 2.47 2.21 1.81 1.22 1.59 0.90 0.69 0.70 1.75 2.09 1.85 1.57 3.38 3.10 2.54 2.83 5.73 4.99 8.26 94 THIN FILMS OF DIPOLAR LIQUIDS In fig. 4 A(AU,") obtained by eqn (5) is plotted against the temperature difference T-T for a plate distance of 2 x For all liquids considered in this paper A(AU,E)L exhibits extraordinary properties in the temperature ranges in question ; thus it changes its sign from negative to positive with rising temperature.The corresponding mean values of the molar heat A(AC;,,) derived from the ascending and descending branches of the curves in fig. 4 respectively are given in table 2. cm. T- Tm FIG. 4.-Behaviour of the total excess energy A(AUZ)h in the reduced temperature range PhN02 x ; PhCN ; PhCH,CN 4 ; PhCF3 + ; MeCN A. TABLE 2.-A(ACvF;m)h FOR h = 2x cm liquid for the ascending for the descending branch branch cal mol-1 K- 1 nitro benzene 42 - 41 benzoni trile 2 benzyl cyanide 9 benzotrifluoride 41 acetonitrile 11 The values for A(AUE), and those for A(AC,E,,)h imply large errors which may exceed 50 %. K . H . ADLFINGER AND G . PESCHEL 95 DISCUSSION There is still no satisfactory theory to explain the magnitude of the disjoining pressure found by us and other author^.^ The disjoining pressure calculated on the basis of long-range dispersion forces extending from the solid surfaces is thought to have much smaller values.O 9 In a former paper l7 the present authors introduced the concept of molecular rotation restriction in oriented surface zones. For benzene e.g. one may imagine the molecules to be strongly adsorbed to the surface hydroxyl groups and oriented with the C,-axis perpendicular to the surface rotational movement being only pos- sible about this axis.22 This adsorption effect decreases the intermolecular distance to any vicinal molecular layer compared with the conditions in the bulk liquid in which a benzene molecule can rotate almost freely about all three axes.23 The decreased distance on the other hand gives rise to an excess dispersion interaction causing the disjoining pressure.In this way many molecular layers can be built up but with increasing distance from the surface rotational vibrations about the other two axes will become important and diminish the stabilizing excess interaction. If this concept is correct a disjoining pressure should exhibit a maximum value in a temperature range where the liquid in question is known to have a rotational transi- tion ; because for rotational freedom in the bulk liquid and rotational restriction of one molecular axis e.g. in the surface zone relatively large differences in the chemical potential i.e. A(AFE),, are expected. By raising the temperature the oriented surface zone suffers destruction and the disjoining pressure vanishes.Below the transition range rotational restriction prevails in the surface zone as well as in the bulk liquid and A(AF,"), and IT respectively should become small. Indeed our results seem to confirm the correctness of this concept since the temperature ranges found for the liquids investigated are in accordance with those revealing anomalous bulk properties which can be regarded as evidence for a rota- tional transition in liquid phase.23 Regarding the viscosity of nitrobenzene and benzonitrile respectively the plot of log q against 1 /T (fig. 5 ) shows deviations in the temperature range in question. Moreover the activation energy of flow seems to show an abrupt change in this range which likewise points to an alteration of molecular rotational behaviour. The viscosities were determined by a capillary viscometer over only small temperature steps.Our data for nitrobenzene agree with those found in l i t e r a t ~ r e ~ ~ which show the same deviations as cited above. For acetonitrile values of the molar heat C are available.25 The temperature derivative of C against T (fig. 6) indicates an anomaly in the temperature range as found in fig. 3. Work is in progress to obtain accurate evidence for the abrupt change in molecular rotational behaviour of these liquids. For halogenobenzenes we succeeded in showing that in the corresponding transition ranges likewise lying about 30" above the respective melting points the change in rotational behaviour refers to the molecular C2-axis. For these considerations we applied the method of Davies and Matheson 2 3 and cal- culated the rotational volume of a molecule in the gas phase and the space available for the same molecule in the condensed liquid phase at the characteristic temperature.2G It is difficult to explain why the disjoining pressure differs in its magnitude and its temperature dependence for the four aromatic liquids quoted in this paper.How- ever the disjoining pressure is dependent on the structure of the oriented surface zone as well as on the structure of the bulk liquid which is usually unknown. Investi- gations of other dipolar aromatic liquids to be published later revealed a tendency for inolecules with large functional groups (e.g. PhCH2CN) which give rise to steric hindrance in the molecular movement to show marked maxima of the disjoining 96 - 1*7 -1.8 F M 0 - -1.9- -29- THIN FILMS OF DIPOLAR LIQUIDS 3.2 3.4 3.6 I I I I I I I + (1) + - + + + + + (21 + + 1 ++ - + + + + + ++ + + I++$ + + + + + + + + + + + + + 1.0 I I I I I 1 I 2 4 0 2 5 0 260 2 7 0 280 2 9 0 300 T OK FIG.6.-Temperature dependence of dC,/dT for MeCN. The arrow indicates the centre of the transition range found by the disjoining pressure. K . H . ADLFINGER A N D G . PESCHEL 97 pressure whereas molecules with small functional groups (e.g. benzonitrile) exhibit less marked maxima especially at small plate distances. Regarding the excess energies A(A Ug), < RT the orientational molecular packing effect in the surface zone cannot be pronounced except perhaps for those molecules with great rotational restriction. The values of A(AC&), are rather high because of the presence of a transition range.We are indebted to Prof. Dr. G. Briegleb and the Deutsche Forschungsgemeinschaft for support of tlus work. Thanks are due to Mrs. G. NO11 for performing the precise viscosity measurements. B. V. Deryaguin 2. Phys. 1933 84,657. B. V. Deryaguin and E. Obuchov Actaphysicochim. 1936,5 1. J. C. Henniker Rev. Mod. Phys. 1949,21,322. B. V. Deryaguin Disc. FaradQy SOC. 1966,42 109. A. Sheludko Colloid Chemistry (Elsevier Publ. Co. Amsterdam 1966). B. A. Kholodnitskii Vestn. Leningrad Univ. Fiz. Khim. 1968 23 153. ' G. Peschel and K. H. Adlfinger Z. Naturforsch. 1969 Ma 11 13 ; Ber. Bunsenges. phys. Chem. 1970 74 351. M. S. Metsik and 0. S. Aidanova Research in Surface Forces ed. B. V. Deryaguin (Consultants Bureau New York 1966) vol. 2 p. 169. G. Peschel and R. Schnorrer in preparation.lo G. Peschel Z. phys. Chem. (N.F.) 1968 59,27. l 1 K. H. Adlfinger and G. Peschel 2. phys. Chem. (N.F.) in press. l2 G. Peschel and K. H. Adlfinger Naturwiss. 1967,54,614 ; Chem. Labor Betrieb 1970,21,193. l3 G. Peschel and K. H. Adlfinger Naturwiss. 1969,56 558. l4 W. Drost-Hansen Ind. Eng. Chem. 1?69 61 ( l l ) 10. l5 B. V. Deryaguin I. G. Ershova V. K. Simonova and N. V. Churayev Teor. Eksp. Khim. 1968,4 l6 S. Glasstone K. J. Laidler and H. Eyring The Theory ofRate Processes (McGraw-Hill New l7 G. Peschel and K. H. Adlfinger Z. phys. Chem. (N.F.) 1969 63 150. l9 A. T. J. Hayward and J. D. Isdale Brit. J. Appl. Phys. (J. Phys. D) 1969 2 251. 2o K. H. Adlfhger R. Schnorrer and G. Peschel Z. angew. Phys. 1970 29 136. 21 J. Frenkel Kinetic Theory of Liquids (Dover Publ. Inc. New York 1955). 22 A. V. Kiselev and D. P. Pashkus Dokl. Akad. Nauk. S.S.S.R. 1958,120,843 ; D. Michel Z. 23 D. B. Davies and A. J. Matheson Disc. Faraday SOC. 1967 43 216. 24 B. P. Nikolski Handbuch des Chemikers (VEB Verlag Technik Berlin 1956) vol. 1 . 25 W. E. Putnam D. M. McEachern Jr. and J. E. Kilpatrick J. Chem. Phys. 1965 42 749. 26 G. Peschel and R. Schnorrer Ber. Bunsenges. phys. Chem. 1969,73,917. 527. York 1961). * B. V. Deryaguin Z. M. Zorin and N. V. Churayev Dokl. Akad. Nauk. S.S.S. R. 1968 182,811. Naturforsch. 1968 23a 339. SP1-D

ISSN:0370-9302    

DOI:10.1039/SD9700100089

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Boundary Viscosity of Polydimethylsiloxane Liquids and their Binary Mixtures BY B. V. DERYAGUIN V. V. KARASEV I. A. LAVYGIN I. I. SKOROKHODOV AND E. N. KHROMOVA Institute of Physical Chemistry Academy of Sciences of U.S.S.R. 31 Lenin Prospect Moscow U.S.S.R. Received 30th April 1970 A systematic study is made of the boundary viscosity of a number of polydimethylsiloxane liquids (PMS) of molecular weights ranging from 1,000 to 40,000 by the blow-off method film thicknesses being measured by a modulation-polarimetric (elliposnietric) precedure. It is established experi- mentally that the viscosity of the liquids studied is not the same throughout the thickness of the boundary later. PMS applied to glass and steel substrates retain their bulk viscosity values down to a layer thickness of ca.150-200 A below which the viscosity increases slightly. On further decrease of the distance to 10-15 A from the substrate the viscosity becomes anomalously low amounting to about 10-20 7; of the bulk value. The existence of a layer of anomalously low viscosity may be attributed to orientation of the PMS molecules in the plane of the substrate and hence to the ease with which these oriented layers slip relative to one another. When studying the boundary viscosity of binary mixtures of PMS of different molecular weights boundary phases of elevated viscosity were found to exist at a distance of 15-30 from the substrate. This phenomenon may be attributed to a change in concentration of the mixture at the solid surface i.e. to an increase in content of the component with the higher molecular weight in the wall-adjacent layer.The importance of organosilicon liquids in present-day engineering can hardly be overestimated since they possess many outstanding properties that have found wide application as bases for greases instrument oils and other lubricants. However the further development of new lubricants based on organosilicon liquids is retarded in particular by the fact that the properties and molecular structure of these polymers in the region bounding on the solid surface have hardly been studied. We have studied the viscosity of boundary layers of polydimethyl siloxane liquids (PMS) with molecules of linear structure,l* using the blow-off the essentials of which are as follows. A thin layer of liquid (2) (see fig. 1) is applied to one of the walls of a slit formed by two plane-parallel plates (1).If a uniform current of air is then blown through this slit the tangential stress caused by the air current gives rise to a flow in the liquid film which changes its profile. If certain conditions enumerated in ref. (4) are observed this flow will be of a single-dimensional layer- by-layer nature i.e. the velocity of the particles in each layer parallel to the substrate will be strictly the same and will be a function only of the distance of the layer from the wall surface. In this case the viscosity of the liquid y can be expressed as a function of the distance l from the solid wall as 11 = (Hj2)grad p(dl/dx)t where His the slit height x is the distance from the part of the film where its thickness 98 DERYAGUIN KARASEV LAVYGIN SKOROKHODOV KHROMOVA 99 is I to the wetting boundary p is the gradient of pressure of the air along the slit and t is the duration of blowing.I - & FIG. 1 .-Blowing-off a liquid film. Thus the viscosity of the liquid is directly related to the steepness of film profile. At constant viscosity if all the other parameters are constant the film profile should be strictly linear and if the viscosity changes the slope of the line should vary so that the higher the viscosity of the liquid in any given elementary layer the steeper will be the line at layers level and vice versa. Investigations of viscosity as a function of the distance to the solid surface make it possible to elucidate not only a number of structural features in boundary layers of PMS but also their variation from the solid surface into the bulk of the liquid since viscosity is a property very sensitive to the molecular structure of a liquid.Measurements of film thickness in order to plot its profile were carried out on a modulation-polarimetric unit (fig. 2) using a LG-55 gas laser as the light ~ o u r c e ~ which considerably improves the accuracy of measurement. PUC. 2 FIG. 2.-Unit for measuring film thickness. Froin the small-size laser L the light ray passes through a quarter-wave (A/4) plate K, diaphragms D and D, polarizer PI and interrupter M on to the object (substrate with film). On passing through plate K1 the linearly polarized light of the laser acquires circular polarization. This is necessary to avoid a change in intensity of the light on rotation of polarizer PI. Diaphragm D restricts the size of the light beam D decreases the aperture angle and interrupter M modulates the light.After reflection from the substrate with the film the light passes through two plates K and K3 and a rotating Polaroid filter and is incident on the cathode of a 100 BOUNDARY VISCOSITY OF LIQUIDS AND MIXTURES photoelectric multiplier (PM) connected to an electron oscillograph 0 via a narrow- band amplifier Y with a RC filter. The film thickness can be calculated by formulae containing the optical constants of the substrate and the nature of the film the ratio of reflection coefficients of the components in the plane of incidence (vertical) and in the plane of reflection (horizontal) and the phase difference between them.6* If the amplitudes of the electrical vector of the light wave are balanced by rotating polarizer PI elliptically polarized light results with the major axes oriented at 45" to the plane of incidence.After passing through the quarter-wave plate K2 the main directions of which are also at 45" to the plane of incidence the light is linearly polarized. The azimuth of its polarization t,b is determined by means of a second quarter-wave plate K3. Its angle of rotation is estimated by measuring the phase difference. If the plate K3 is oriented appropriately the light emerging from it will be circularly polarized. In the general case the light flux intensity varies in time so that the ray on the oscillograph screen forms a straight line of varying length. At the moment of compensation circularly polarized light arises which is not affected by the rotating polarizer.Then the length of the line on the oscillograph screen stops varying. Film thicknesses I were calculated from the change in polarization azimuth $ of the light reflected from the substrate with the film by means of formulae which take account of interference on ray reflection from the thin film.6* The course of the @ = .f I I I curve depends on the coefficient of refraction of the film material and therefore we calculated the dependence of t,b on I for each liquid studied on BESM-1 and BESM-4 electronic computers. The calculated $ = f 1 I I curves were then used as calibration curves for passing from the experimentally-found polarization azimuth to the corresponding film thickness. The viscosity of a PMS series with molecular weights ranging from 1,000 to 40,000 was studied in the thin layers on glass and metal surfaces.The glass substrate was a totally reflecting prism made of " heavy flint '' glass and the steel plate (ball- bearing chrome steel containing 15 % chromium) buffed to better than the 14b class of surface finish (according to U.S.S.R. stand). Blowing-off was carried out in the apparatus described in ref. (8). The profiles of PMS films obtained on the glass substrate are shown in fig. 3. Analysis of the curves reveals that in the 200-2,OOOA thickness range the viscosity of the liquids is constant and corresponds to the bulk value found in a capillary viscometer. For example for PMS-5 and PMS-70 these values were 4.92 and 4.70 69 68 and 68.25 CP respectively. Below 200A the curves become steeper which is evidence of an increase in viscosity in this region.The common feature of the profiles of PMS films on the steel substrate (see fig. 4) is the presence of three sections differing in slope and hence in viscosity of the corre- sponding liquid interlayers. The table lists average data on measurements of the relative viscosity of these interlayers and their thickness. At a definite distance from the substrate of ca. 20-30 all the PMS studied display a sharp decrease in viscosity. Above this boundary the viscosity of the liquids is constaiit and at 150-2OOA it decreases by approximately 1.2-1.4 times sharply in most cases subsequently remaining constant up to distances of 2,200-2,500 A. The change in viscosity of a liquid at a definite distance from a solid surface had been observed earlier as was pointed out in ref.(9)-(12). The increase and decrease of viscosity in the boundary layers was attributed to phenomena related to orientation of the liquid molecules under the influence of the solid surface. Indeed the change in viscosity of a liquid as a result of orientation of its molecules in one way or another At this moment readings are taken on analyzer and polarizer. DERYAGUIN KARASEV LAVYGIN SKOROKHODOR KHROMOVA 101 has been confirmed in studies carried out by other methods. of molecules in the direction of flow of a liquid reduces its viscosity.13 For instance orientation 5 10 t5 20 h435 5 PMS-2 000. FIG. 3.-Profiles of PMS films on a glass substrate. 1 PMS-5 ; 2 PMS-15 ; 3 PMS-70 ; 4 PMS-400 ; XMM FIG. 4.-Profiies of PMS films on a steel substrate. 1 PMS-5 ; 2 PMS-15 ; 3 PMS-25 ; 4 PMS-70 ; is most probably related to a sharp disturbance of conformational equilibrium towards straightening of molecular coils and subsequent orientation of the straightened chains in the plane of the substrate.Owing to the high polarizatility of the Si-0 bond,14 each of the silicon and oxygen atoms may theoretically be a centre of attraction interaction with 5 PMS-400 ; 6 PMS-2 000. The change in viscosity of PMS observed in the layer up to 20 1 02 BOUNDARY VISCOSITY OF LIQUIDS AND MIXTURES the solid phase which is the reason why these molecules orient themselves horizontally when adsorbed. According to published data l5 for horizontal orientation of molecules the monomolecular PMS layer is 5.6A thick. This suggests that the " lower " boundary phase under consideration is made up of several monomolecular PMS layers.We therefore attribute the anomalously low viscosity to the ease with which these monolayers slip relative to each other along the slip planes bounded by methyl groups. The influence of the force field of the solid phase becomes gradually weaker in going from the substrate surface into the bulk of the liquid and the poly- molecular boundary layer breaks up. The orientation of the molecules changes resulting in an increase of viscosity which is observed at a wall distance of more than 20-30A. Above 150-2OOA the viscosity falls off owing to the molecular chains rolling up into coils and subsequently remains constant within the range of thicknesses studied corresponding to that in the bulk of the liquid. Evidently in this region the solid surface has practically no effect on the molecular structure of the liquid.As was mentioned above the centre of adsorption interaction of PMS molecules with the solid surface are not only and not so much the terminal trimethylsilyl groups as any silicon or/and oxygen atom of the main chain of the molecules. Hence it follows that there is a direct dependence between the molecular weight of linear PMS and their adsorption on the surface a conclusion which is supported by the data of Perkel and Ullman.16 If this is so the " lower " boundary phase should become enriched in time in the high-molecular-weight component and hence its viscosity should be increased. In this case the viscosity observed in the " lower " boundary phase is a consequence of two competing processes viz.horizontal orientation of the linear molecules which decreases the viscosity and increasing concentration of the high-molecular-weight component which increases it. In addition the molecules of the " lower " boundary phase being under special physical conditions in the field of surface forces of the solid phase and being oriented may form swarms bundles and other kinds of super- molecular structures owing to the statistical increase of the acts of association of molecules. This process should also increase the viscosity but taking into account the weak intermolecular interaction in liquid polydimethosiloxanes its contribution is evidently insignificant. In all the cases we have discussed (see fig. 4) the viscosity of the " lower " boundary phase was smaller than the bulk viscosity.This means that orientation of molecules is the prevailing process and the antibatic process of enrichment of the boundary phase in the high-molecular component either takes so long that it does not have time to manifest itself during the time of our experiment or cannot affect the viscosity noticeably owing to the narrow molecular-weight distribution of the samples studied. We have therefore studied the boundary viscosity of binary solutions of poly- dimethylsiloxane liquids of different molecular chain lengths which would artificially increase the coefficient of molecular-weight distribution. We studied binary mixtures of PMS of different molecular weights with the polydimethylsiloxane liquids PMS-10. These components formed clear stable solutions when mixed. The boundary viscosity was studied on a metallic substrate since the most interesting range of thicknesses in which viscosity anomalies could be expected was up to 400 A.Fig. 5 shows the film profiles obtained after blowing-off PMS-10 as such and of solutions of other PMS liquids in it on a metallic substrate. The concentration of the solutions was 10 % by weight in all cases. It is evident from fig. 5 (curve 1) that the profiles of the PNS-10 film is similar to those of the other PMS (see fig. 4) observed on a metallic substrate and that they DERYAGUIN KARASEV LAVYGIN SKOROKHODOR KHROMOVA 103 exhibit a boundary phase of low viscosity the thickness of which is as in the previous cases 15-20 A. XMM FIG. 5.-FiIm profiles of PMS solutions on a steel stubstrate. 1 PMS-10; 2 PMS-10fPMS-70; 3 PMS-lO+PMS-400 ; 4 PMS-lO+PMS-2,000 ; 5 PMS-lO+PMS-2x 10 ; 6 PMS-10fPMS-5 X lo4 ; 7 PMS-10+PMS-106.The film profiles of PMS solutions (see fig. 5 curves 2-7) in the 10-30 A range are of a distinctly stepped nature the height of the steps as well as the general shape of the curve being readily reproduceable in duplicate experiments. This step height is independent of the molecular weight of the dissolved PMS. The curve is of a stepped nature when blowing is carried out after an interval of 3 h or more from the moment of applying the liquid sample to the substrate. If the layer is blown-off immediately the steps on the film profile curve are either indistinct or absent. TABLE 1.-cHARACTERISTICS OF PMS AND THICKNESSES AND RELATIVE VISCOSITIES OF THEIR BOUNDARY LAYERS ON A METALLIC SUBSTRATE 7 2 11 A 770 12 A PMS-5 1,070 1.3992 0.15 25 1.35 235 PMS-15 1,580 1.4042 0.25 15 PMS-25 2,080 1.4050 0.11 20 1.40 240 PMS-70 2,860 1.4058 0.38 30 1.11 160 PMS-400 6,700 1.4062 0.09 10 1.35 130 PMS-2,000 36,900 1.4080 0.04 6 1.40 155 The step profile of the boundary phase is not a consequence of the instability of the film profile since on maturing the layer after blowing for 1.0-1.5 h several intermediate measurements of its thickness showed that the film profile retains its shape satisfactorily throughout this time interval.We conclude from this that the stepped profile of the film reflects phenomena at the liquid-substrate interface and is evidence of changes in viscosity and structure of the liquid localized in the thickness range up to 30A. The nature of the film profile observed in this case is proof of the existence of liquid boundary phases of changed viscosity similar to the cases discussed earlier.1° The steps formed on the curves is a result of slip of liquid inter- layers of different structure along one another.The fact that horizontal orientation of molecular chains occurs in this case too is confirmed by the absence of any relation between molecular weight and step height of the film profile for different solutions. The step-film profile characteristic of the samples held on the substrate for some time molecular refractive index rll substance weight '?D 2o l o - - 1 04 BOUNDARY VISCOSITY OF LIQUIDS AND MIXTURES before blowing off is evidence of a definite kinetics of formation of the boundary layer the study of which is beyond the scope of this paper.lnvestigation of the boundary viscosity of PMS and their binary solutions by the blow-off method has made it possible not only to substantiate the assumption made earlier l7 that the influence of the force field of the solid phase extends into the bulk of the liquid to a considerable distance changing its structure and structure-sensitive properties but also to suggest another important idea that of fractionating polymer liquids in the force field of a solid surface. qo is the bulk viscosity of PMS taken as 1; lI is the thickness of the liquid layer of decreased viscosity near the substrate ; q1 is its average viscosity ; l2 is the upper boundary of the liquid layer of increased viscosity q2. B. V. Deryaguin V. V. Karasev I. A. Lavygin I. I. Skorokhodov and E. N. Khromova Theses of Reports presented to 4th Con$ Chemistry and Applications of Organo-silicon Compounds (Moscow 1968).B. V. Deryaguin V. V. Karasev I. A. Lavygin I. I. Skorokhodov and E. N. Khromova Doklady Akad. Nauk. S.S.S.R. 1969 187 846. B. V. Deryaguin G. M. Strakhosvky and D. S . Malysheva Acta physicochim. 1944 19 541. V. V. Karasev Yu. M. Luzhnov and N. V. Churayev Zhur. Fiz. Khim. 1968,42 558. B. V. Deryaguin V. I. Gol’dansky and V. V. Karasev Doklady Akad. Nauk. S.S.S. R. 1947 57 697. ’ A. A. Vlasov Lens Coating ed. Acad. I. V. Grebenshchikov Moscow (Leningrad 1946). * B. V. Deryaguin and V. F. Pichugin Proc. 2nd U.S.S.R. Conf Friction and Wear 1949,1 103. lo V. V. Karasev and B. V. Deryaguin Kolloid. Zhur. 1953 15,366. l1 V. V. Karasev and B. V. Deryaguin Zhur. Fiz. Khim. 1959,33 100. l2 B. V. Deryaguin V. V. Karasev N. N. Zakhavayeva and V. P. Lazarev Zhur. Tekhn. Fiz 1957 l3 A. Bondy Appl. Phys. 1945 16 539. l4 C. Eaborn Organmilicon Compounds (Butterworth and Co. London 1960) chap. 8 9. l5 H. W. Fox P. M. Taylor and W. A. Zisman Ind. Eng. Chem. 1947,39 1401. l6 R. Perkel and R. Ullman J. Polymer Sci. 1961 54 127. B. V. Deryaguin Mineral’noye Syryo 1934 no. 2 33. V. V. Karasev and B. V. Deryaguin Doklady Akad. Nauk. S.S.S. R. 1948 62 762. 27 1076.

ISSN:0370-9302    

DOI:10.1039/SD9700100098

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Boundary Layers of Pure Liquids at the Graphon Surface BY S. G. ASH * AND G. H. FINDENEGG Institut fur Physikalische Chemie Universitat Wien 1090 Wien Austria. Received 14th April 1970 Measurements of the volume changes which accompany the wetting of Graphon by n-alkanes benzene and water and the heats of wetting for the same systems are analyzed. For the long-chain alkanes there is evidence for a pre-freezing phenomenon near the Graphon/liquid interface. For liquids composed of smaller molecules this effect is not found. In this paper the results of thermodynamic measurements on solid/liquid interface systems are explained in terms of the structure of the boundary layer of the liquids. Robert found that the heat of wetting of Graphon by a series of n-alkanes increases with increasing chain length.This result was confirmed by Clint et aL2 and by Everett and Findenegg.3 At 25°C the heat of wetting per unit surface area of Graphon increases by a factor of 2 from hexane to hexadecane. For tetradecane and hexadecane the heat of wetting decreases with increasing temperature.2 To explain these results for the long-chain alkanes it was postulated that close to the freezing point of the liquid the adsorbed layer has properties intermediate between those of the liquid and the ~ r y s t a l . ~ The heat of wetting would therefore include a contribution related to the heat of fusion of these alkanes. If the above concept is correct the adsorbed layer should have a higher density than the bulk liquid. Therefore the alkane + Graphon systems were further investi- gated by measuring the density of the boundary layer of the liquid.Details of these measurements will be reported elsewhere but some results are summarized here. The emphasis of the present paper is on the correlation of the volumetric results with the heats of wetting for the same systems. EXPERIMENTAL The volumetric behaviour of the liquid boundary layers was studied as a function of temperature by making precision density measurements. Known weights of Graphon were introduced into calibrated pyknometers which were then evacuated. The liquids were introduced into the evacuated pyknometers and the total weights and volumes measured at several temperatures. The results are expressed in terms of the surface excess mass of liquid me = m - p ~ v (1) where rnl is the mass and q the volume of the liquid in the pyknometer equal to the total volume of the pyknometer minus the volume of Graphon; pp is the bulk density of the liquid at the given temperature.The values of p; are determined in separate experiments. The surface concentration is the excess mass of liquid per unit surface area of solid r = ma/Au. (2) * present address Shell RmearchLtd. Thornton Researchcentre P.O. Box 1 Chester CH1 3SH England. 105 106 BOUNDARY LAYERS OF LIQUIDS The volume of Graphon in the pyknometer can be calculated from the weight of Graphon and its density. However the absolute density of Graphon is not precisely known and instead a " reference density " is used. This is determined by making the density measure- ments with a reference liquid and by equating the surface excess for this system to zero.The surface concentration is thus a relative quantity based on a chosen reference liquid for which by definition I' = 0. In the present work the reference liquid is cyclohexane. Density measurements for the Graphon+ cyclohexane system have been made at temperatures from the melting point of cyclohexane (6.5"C) to 50°C. Within the accuracy of the experi- ment a constant coefficient of expansion of Graphon was obtained. The surface area of Graphon was 87 m2 g-l based on the B.E.T. method and a molecular area of N2 = 16.2 A2. Details of the experiment will be given el~ewhere.~ RESULTS AND DISCUSSION n-ALKANES C14 C16 Cis The surface concentrations of tetradecane hexadecane and octadecane plotted against the experimental temperature minus the melting point temperature of the alkane t - t f are shown in fig.1. From these results we note (i) at a given value of t - t f the surface concentration increases with increasing chain length of the alkane ; (ii) r is strongly temperature dependent at temperatures close to the freezing point of the alkane and this effect increases with increasing chain length of the alkane. n l I I I I "0 10 20 30 40 50 FIG. 1 . T h e surface concentrations of tetradecane hexadecane and octadecane as a function of the experimental temperature minus the freezing point temperature of the alkane. 0- qN"C Qualitatively the dependence of r on the number of carbon atoms per molecule and on temperature is similar to the variation of the heat of wetting of Graphon by these liquids.2* For a quantitative correlation of the two effects we consider the definition of the surface concentration.On locating the Gibbs dividing surface at the Graphonlliquid boundary, S . G . ASH AND G . H . FINDENEGG 107 where p(x) is the actual density of the liquid as a function of the distance x normal to the boundary. The exact form of the function p(x) is not known. It is convenient to adopt a simple model of the boundary layer by making the following assumptions (i) the excess mass of liquid is distributed uniformly over a surface zone of density p and thickness h ; (ii) the properties of this surface zone are the same as the properties of the solid alkane (at the melting point). The thickness of the surface zone obtained from eqn (3) is and the number of moles of alkane contained in the surface zone is where A4 is the molecular weight of the alkane.On wetting the Graphon surface n moles of the alkane liquid form a surface zone with properties of the solid alkane. This process is accompanied by the evolution of an energy n,Hf ( H f molar heat of fusion of the alkane) in addition to the Graphon-alkane interaction energy. To calculate h and n from eqn (4) and (5) the density of the solid alkanes has been calculated from the density of the corresponding liquid alkane by assuming that at the melting point (ps-pl)/pl = 0.1100. This relationship approximately holds for a large number of n-alkane~.~ At t - t f = 0.05" the solid-like surface zones of tetradecane hexadecane and octadecane have thicknesses h = 6.8 9.0 and 11.3 A respectively. The quantity nsHf is plotted against t - t in fig. 2 for tetradecane and in fig.3 for hexadecane. The molar heats of fusion of the alkanes have been taken from the literature.6 140 r 1 (t-tf)/"C FIG. 2.-The structural contribution to the heat of wetting of Graphon by n-tetradecane as a function of the experimental temperature minus the freezing point temperature of n-tetradecane (a) 0 nsHf calculated from the surface concentration on the basis of the proposed model (b) experimental values of A,H (n-tetradecane)-A,,,H(cyclohexane) 0 Robert ; A Clint ; 0 E ~ e r e t t . ~ The observed heat of wetting AWH can be considered to be the sum of two terms (i) the heat H,, resulting only from Graphon-alkane interactions ; (ii) a heat connected with structural changes in the surface layer of the alkane. In terms of the present model A,H = H,,,+n,Hf. (6) 108 BOUNDARY LAYERS OF LIQUIDS For the wetting of Graphon by alkane liquids H is considered to be nearly constant i.e.independent of the nature of the molecules of the liquid.' is our reference liquid for which r = 0 we set H = A,H(cyclohexane) = 107 erg cm-2. Since cyclohexane (7) 0 10 20 30 40 50 (t-tf)/"C FIG. 3.-The structural contribution to the heat of wetting of Graphon by n-hexadecane as a function of the experimental temperature minus the freezing point temperature of n-hexadecane ; conventions as in fig. 2. This value is close to the estimated dispersion force component to the heat of wetting of Graphon by n-alkanes (1 12 erg cm-2).8 According to eqn (6) and (7) the quantity n,Hf should be equal to the heat of wetting difference between the given n-alkane and cyclohexane A,H(n-alkane) - A,H(cyclohexane).A comparison of these two quantities as a function of temperature is shown in fig. 2 and 3 for tetradecane and hexadecane respectively. For octadecane at the freezing point (28.2"C) n,Hf 7 235 erg/cm2 ; no heat of wetting data could be found in the literature for this liquid to compare n,Hf with the " structural " contribution to AWH. A further comparison of n,Hf and A,H(n-alkane) - A,H(cyclohexane) for four n-alkanes at a given tempera- ture is shown in fig. 4. The surface concentrations of n-hexane and dodecane and the heats of wetting have been taken from the 1iteratw1-e.~. The agreement between the experimental " structural " contribution to the heat of wetting and the correspond- ing quantity n,Hf calculated from the surface concentration on the basis of our simple model strongly indicates an ordering of the long-chain molecules near the Graphon/alkane interface at temperatures close to the freezing point of these alkanes.From measurements of adsorption from solution '* lo long-chain alkanes are strongly preferentially adsorbed onto Graphon. For dilute solutions of n-C, in n-heptane Groszek concluded that the adsorption is confined entirely to the basal planes of the graphite structure. The specific interaction between the graphite basal plane and the n-alkane molecules is attributed to the good geometrical fit of the molecules on to the hexagons of the graphite basal plane.l0 A consideration of the thickness of the surface zone of pure n-alkanes at the Graphonlalkane interface (of h above) shows that in pure long-chain alkane liquids the surface affects a much wider zone than a monolayer of molecules oriented parallel to the surface.The strong temperature dependence of the quantities A,H and r at temperatures S. G . ASH A N D G . H. FINDENEGG 109 close to the freezing point of the substances is evidence for a highly co-operative character of the forces which are responsible for the ordered structure of the zone. It is possible that the ordered structure consists of several layers of molecules adsorbed parallel to the surface. But the possibility of a different structure (e.g. a closely packed monolayer of molecules oriented normal to the surface) cannot be ruled out on the basis of the present results. 6 8 10 12 14 16 number of carbon atoms in alkane FIG. 4.-The structural contributions to the heats of wetting of Graphon by n-alkanes at 25°C (a) 0 experimental values of A,H(alkane)-A,H(cyclohexane) Everett ; (b) 0 n,Hf calculated from the surface concentration 011 the basis of the proposed model.BENZENE AND WATER The surface concentrations of benzene and water at the Graphon/liquid interface plotted against t - t f are shown in fig. 5. The situation is different from that with the chain alkanes for both liquids I? is negative over the temperature range investi- gated. For the n-alkanes we have found that a positive value of r is correlated with a positive value of A,H(n-alkane) - A,,H(cyclohexane). For benzene however r is negative while AwH is somewhat larger than the value for cyclohexane (benzene 112 erg cm-2 cyclohexane 107 erg cm-2 both values at 20°C I).This result could be the consequence of a weak specific interaction between the graphite basal plane and the benzene molecule at certain configurations of the molecules on the surface. If the surface zone is assumed to consist of a monolayer of benzene molecules oriented with the plane of the aromatic ring parallel to the solid surface the thickness of this layer is about 3.4A.11 The density of the zone (assuming r = 0 for the cyclo- hexane/Graphon interface) is then found from eqn (4) to be about 5 % less than the density of bulk benzene (at the freezing point). With increasing temperature the density of the surface zone approaches the density of the bulk liquid. At about 110 BOUNDARY LAYERS OF LIQUIDS 50°C above the freezing point r = 0. There are no data for the heat of wetting of Graphon by benzene as a function of temperature with which to correlate these density measurements.-4 1 I I I I 1 t 0 10 20 30 40 50 60 (t-t.)/"C FIG. 5.-The surface concentrations of benzene and water as a function of the experimental tempera- ture ininus the freezing point temperature of the liquid. For the water/Graphon interface r is also negative but almost independent ot' temperature (fig. 5). The heat of wetting of Graphon by water is low ; AWH = 32 erg/cm2 7u (Robert reports a value of 44 erg cm-2). There is evidence that only dispersion interactions are operative between the graphite basal plane and water.8 A positive surface concentration relative to n-heptane was reported by van Gils l 2 for water near Nylon Dacron glass fibres and glass powder. His I? values range from 0.7 x g cm-2 which is 3-4 powers of ten larger in absolute value than the (negative) value obtained by us for the Graphon/water interface.Drost-Hansen has explained the large positive surface concentration of water observed at these polar surfaces by postulating that close to the polar surface there exists an extensive zone of disordered water molecules which are more closely packed than in bulk water. Conversely the negative surface concentration of water at the non-polar surface of Graphon may be caused by the stabilization of bulky aggregates of water molecules. The distance over which significant structuring may occur is guessed by Drost-Hansen " a t somewhere between tens and hundreds (or more) of molecular diameters ". According to our surface concentration a density of the surface zone e.g.1 "/o less than that of the bulk liquid corresponds to a thickness of this zone of approximately 30A (to be compared with the nearest neighbour distance in water of less than 3 A). to 3.0 x CONCLUSION The most significant result of this work is the different behaviour near the freezing point temperature of the n-alkanes tetradecane hexadecane and octadecane on the one hand and cyclohexane benzene and water on the other hand at the Graphon/ liquid interface. For the chain alkanes there is sharp increase in the surface excess mass and in the heat of wetting when the temperature approaches the freezing point. These effects can be understood as " pre-freezing " phenomena i.e. as a cooperative S. G . ASH A N D G . H. FINDENEGG 111 ordering of the chain molecules with their long axes parallel to one another which is induced by the surface forces.For the smaller molecules of cyclohexane and benzene such an effect is not observed. The behaviour of water is similar to the behaviour of these non-polar liquids but a marked negative surface concentration is observed. We thank the Royal Society for an award to S. G. A. L. Robert Compt. rend. 1963 256 655 ; Bull. SOC. Chim. 1967 2309. D. H. Everett and G. H. Findenegg Nature 1969,223,52 ; J . Chem. Thermodynamics 1969,1 573. G. H. Findenegg J. Colloid Interface Sci. 1971 35 249. H. Sackmann and F. Sauerwald Z. phys. Chem. 1950,195,295. H. Sackmann and P. Venker. Z. phys. Chem. 1952 199 100. J. Timmermans Physico-Chemical Constants of Pure Organic Liquids (Elsevier 1950 and 1965). ’ (a) F. H. Healey J. J. Chessick A. C. Zettlemoyer and G. J. Young J. Phys. Chem. 1954 58 887. (6) F. E. Bartell and R. M. Suggitt J. Phys. Chem. 1954 58 36. (c) A. C. Zettlemoyer and K. S. Narayan The Solid-Gas Interface ed. E. A. Flood (Dekker/ Arnold New York/London 1967) chap. 6. J. A. Lavelle and A. C. Zettlemoyer J. Phys. Chem. 1967 71 414. R. Aveyaxd Trans. Faraday SOC. 1967 63,2778. lo A. J. Groszek Proc. Roy. SOC. A 1970 314,473. L. Pauling The Nature ofthe Chemical Bond (Cornell University Press New York 1960) p. 260. ’’ G. E. van Gils J. Colloid Znterface Sci. 1969 30 272. l 3 W. Drost-Hansen Ind. Eng. Chem. 1969,61 No. 11 10. ’ J. H. Clint J. S. Clunie J. F. Goodman and J. R. Tate Nature 1969 223 51.

ISSN:0370-9302    

DOI:10.1039/SD9700100105

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Contact Between a Gas Bubble and a Solid Surface and Froth Flotation BY A. SCHELUDKO SL. TSCHALJOWSKA AND A. FABRIKANT University of Sofia Dept. of Chemistry Institute of Physical Chemistry Institute of Mining and Geology Sofia Bulgaria Received 9th April 1970 The process of formation of the contact silica surface/air when pressing a liquid meniscus on to the solid/liquid interface is investigated. The kinetics of expansion of the contact area and the final states characterized by contact angles in both directions (by spreading and withdrawing of the contact) are examined. Preliminary data about the effect of surfactants on the velocity of expansion of the contact and on the contact angle and its hysteresis are presented. The new methods applied in the investigations are described. The results are compared with flotation experiment data.The purpose of this paper is to investigate the contact formed by pressing an air bubble on to a solid surface employing the method and approach used in the investi- gation of microscopic free black fi1ms.l. The data obtained are compared to flotation measurements on the basis that flotation is only possible when the film separating the particle from the gas/liquid interface breaks. ANGLES OF CONTACT A liquid meniscus is forced through a thin-walled circular silica tube of internal radius R = 0.102 cm towards a polished plate of vitreous silica until Newton’s rings appear around the tip of the meniscus i.e. at a distance of several lOOOA from the plate. The pressure upon the meniscus equal to 2Y COS ORIR + hPo9 (1) is maintained strictly constant.Its value is registered photo-electrically on a precise aqueous manometer. In eqn (l) y is the surface tension of the solution p o its density g gravitational acceleration and h the depth of immersion of the tube in the solution. The separating film formed from appropriate solution of dodecylamine hydro- chloride (1.35 x mol/l) in these conditions breaks forming an exposed contact whose radius r increases to y o . This process is observed and recorded in reflected light on a metallographic microscope with a camera. As the silica tube is much narrower than the vessel containing the solution the depth of immersion remains constant so that in all the following treatments the hydrostatic component of the pressure hpog is eliminated. After solving the vaiiational problem for the minimum surface at constant volume of a body of rotation about the axis OZ as described in ref.(l) (4) we obtain for the capillary pressure The profile of the meniscus after formation of contact is shown in fig. 1. Pr = 2y(R cos &-r sin 0,)/(R2 -r2). (2) 112 A . SCHELUDKO SL. TSCHALJOWSKA AND A . FABRIKANT 113 In the process treated here both of the angles OR and 8 form as a result of the recession of the liquid on identical surfaces of vitreous silica. Thus at r = yo we obtain 8R = 8 = O0. At constant external and hydrostatic pressure according to (1) and (2) we obtain tan O0 = ro/R. I (3) FIG. 1.-Profile of a meniscus contacting a plane surface. After establishment of ro we gradually reduce the external pressure. At first the circumference remains fixed. It begins to narrow only after a specific lowering of the pressure APM.This irreversibility (hysteresis) of the process is due to the fact that in order to start moving in the direction of wetting the angle 8 has to increase from e0 to O1. The angle OR also becomes bigger than Oo and two states are now possible. (a) OR increases more slowly than 8 and the shift of the circumference at 8 = 8 takes place at fixed position in the region 8 = 8R.l i.e. at constant H equal to Ho . (b) 8R reaches O1 before Or and the shift of Or corresponds to the condition 8 = 8R = 81. The solution of the problem for the shape of the body shown in fig. 1 for 8 = 0 = Q0 is and for the case (a) H = Ho = const. Ifo = R(l -tan O,) (4) where AE = E(A,,q2) -E(L,,q2) and AF = F(A,,q2) -F(AR,q2) (E and F are elliptic integrals of the second and first kind tabulated e.g.in ref. (5)), 114 CONTACT BETWEEN A BUBBLE AND A SURFACE From these equations the intermediate value OK,1 is calculated and from the expression cos cos 20 2A tan 8 sin 0 tan 28 sin 8 = ____ - - - ~ the angle O1 is obtained. If the latter is bigger than the solution is correct If not we have case (b) 0 = OR = 8 and calculate 01 according to (6) but with &,I = 81. TABLE 1 PH 1' dyn cm-1 ro cm x 102 APM dyn cm-2 8 61 4.6 71 0.82 - 4" 30' - 5.3 71 1.48 - 48 8" 15' -17" 6.6 71 4.28 GOO 22" 45' 51' 8.1 70 7.37 893 35" 50' 52" 11 52 9.42 2315 42" 45' 57" The values of Oo and O1 obtained for various pH regulated by ten-fold diluted universal buffer and controlled by a glass-electrode are shown in table l.* The values of y are obtained by the Wilhelmy method using a mica plate.The correc- tions for incomplete wetting of the plate are made the manner recommended in ref. (7). EXPANSION OF THE CONTACT The process of expansion of the contact r ( t ) is either recorded photographically Following when it is fast or visually read on an eye-piece micrometer when slow. ref. (2) the velocity of expansion is represented as drldt = a&. Here (7) 8 = ~(COS 0 - cos 80) (8) is the tangent to the solid-surface force acting at time t from the beginning of the process and a is the instantaneous mobility (the reciprocal coefficient of friction). Both co-factors correspond to a unit length of the circumference of wetting. As At is a function of Y through the non-equilibrium angle 8 = Q, in order to obtain the values of a for different Y the slopes dr/dt (obtained by graphical differentiation of the curves r ( t ) ) ought to be divided by Zit.The latter is easy to calculated with the aid of eqn (2) At = y[Jl-(r cos 0,/R)2-cos 8,] (9) if we assume that the angle 8 = O0 remains constant during expansion at constant pressure. * The details of the method its theory and the additional measurements ofy and floatability will be published separately. FIG. 2.-Photographs of the contact expansion frequency 12 framesis ( a ) pH 5.3 at 100 xniagnifica- tion ; (b) and ( c ) pH 8.1 at 40 x and 100 x . The appearance of the contact is marked by an asterisk. To face page 11 5.1 A . SCHELUDKO S L . TSCHALJOWSKA A N D A . FABRIKANT 115 At low pH the expansion is very rapid as is seen from the photographs in fig.2 4 and the final state yo is established roughly within a minute. On fig. 3 curve 1 the corresponding values of a(r) are given. Within the limits of scatter they are the radius of contact x 10' cm FIG. 3.-Contact mobility as a function of the contact radius. and 0 pH 5.3). Curve 1 for low pH (0 pH 4.6 Curve 2 for pH 11 (the first point 0 is obtained from -rm) ; curve 3 for pH 8.1. same at pH = 4.6 and 5.3 and of the order of cm2 s-' dyn-l. Apparently the friction here is determined by the viscosity in the volume of the liquid which is almost independent of pH. The volume mechanism of the dissipation of energy in this case is corroborated by the fact that a here coincides with a = 5 x obtained in ref. (2) for expansion of a black film of radius 7 x In that paper the volume mechanism of friction was proved by the independence of a on the film tension at given r over a wide range of values of the film tension.At high pH the situation is completely different. The time of establishment of ro is of the order of several hours. The dependence a(r) at pH = 11 (fig. 3 curve 2) shows that a in this case first reaches values which are four orders lower i.e. the friction is 10 thousand times greater than at low pH. As the volume viscosity cannot rise significantly with increase of pH the velocity of expansion is determined by surface or peripheral friction. It is interesting as well that a now decreases extremely steeply with the expansion so that the data are represented on a log scale. This effect is completely rheological-the friction increases dramatically when the deforming force diminish.The process of surmounting the rheological obstructions by big initial forces is clearly demonstrated at intermediate values of p H ~ 8 . On the photographs in fig. 2b and c it is seen that the circumference of wetting first tears and a starlike expan- sion takes place its velocity being of the same order as at low pH. With decreasing cm. I16 CONTACT BETWEEN A BUBBLE AND A SURFACE a, a regular circular circumference forms gradually (fig. 2b) which expands very slowly the values of a being close to those at pH 1 1 (fig. 3 curve 3). COMPARISON WITH FLOTATION As a basis of this comparison the curve in fig. 4 is used. The latter is obtained with quartz particles (radius R,z2 x cm) in a laboratory flotation cell under the same conditions (solution washing of the quartz) as in the measurements of contact angles.Although the shape of such curves depends on the construction and 6 7 a 9 10 PH FIG. 4.-Flotation recovery of quartz as a function of pli. regime of the flotation cell the shape of the curve in fig. 4 is sufficiently typical (see also ref (8)) to permit a comparison with the contact investigation. The decrease of floatability at high pH although the contact angles retain their high values is due to the sharp deceleration of the contact expansion. Hence our interpretation is based on the kinetics of contact expansion. According to the theory of attachment of particles to bubbles,g* lo a spherical particle under the action of a detaching force G breaks away from the bubble when the chord of the contacted part becomes equal to or smaller than 2r at * r,, = JGR,/2ny Therefore it is necessary that the contact shall succeed in expanding at least to We simplify the problem this size in order to retain the attachment or flotation.by neglecting the inertial forces and take G = G = 4xR;(P - Po)& (11) * In order to simplify the calculation the hysteresis of the contact angle [lo] is not taken into account here and later. A. SCHELUDKO SL. TSCHALJOWSKA AND A. FABRIKANT 117 where p is the density of the particles we obtain r,=2 x cm at p-po = 2 and y (see table 1) in the range 71 dyn cm-1 (rm = 1 . 7 ~ cm) to 52 dyn cm-1 (v = 2 x cm). At such small r,, the chord and the diameter of contact are almost identical and the time z for development of the contact is very short. In these conditions the integration of (7) is simple as the initial (r-0) values y(1-cos 0,) for & and a.for a can be used so that From fig. 3 curve 1 we obtain at low pH a. z 7 x and from this z = 3.3 x s for pH = 5.3 (y = 71 O0 = 8" 15'). Since at pH = 5.3 the recovery is some 10 % (fig. 4) then if we assume that it is roughly inversely proportional to the time of contact we find the latter to be of the order of a few milliseconds in good agreement with other data.ll This result confirms such an intrepretation and allows a possible estimate of a for Y = 2 x at the other end of the flotation curve at pH 1 1. Here the recovery is of the same order of magnitude but the direct determina- tion of the initial value of a is very difficult because of the steep change of a with r. Substituting in (12) z = 3.3 x y = 52 and O0 = 42" 45' according to table 1 we obtain a = 4.4 x at pH = 11 the first point of curve 2 fig.3. This result indicates that friction in the periphery of the contact which hampers the recovery at high pH is for small r = r much less than that measured for larger contacts. This first point confirms the sharp decrease of a and the rheological character of the peripheral friction. The latter is apparently closely connected with the hysteresis of the contact angle. In conclusion the present paper shows that the main part of the time of contact in this investigation of froth flotation is not the time of thinning of the separating film to its r ~ p t u r e ~ but the time of expansion of the contact after rupture to the size necessary for attachment. In other conditions e.g.for very small particles the time z strongly decreases (eqn. (12)) and it is possible that the thinning of the separating film becomes the controlling process. Possibly this is the cause of the decrease in floatability of small particles. for r = 2 x A. Scheludko B. Radoev and T. Kolarov Trans. Faraday SOC. 1968 64 2213. T. Kolarov A. Scheludko and D. Exerowa Trans. Faraday SOC. 1968,64 2864. A. Scheludko Kolloid-Z. 1963 191 52; A. Scheludko S1. Tschaljowska A. Fabrikant H. Schulze and B. Radoev Freiberger Forschung. in press. D. Exerowa I. Ivanov and A. Scheludko Ann. Universitb Sofia 1961/62 56 157. V. Beliakov R. Kravtzova and M. Rappoport Tables of Elliptic Integrals (Russ.) (Moscow 1962) vol. 1. H. Britton and R. Robinson J . Chern. SOC. 1931 1456. ' A. W. Neumann and W. Tanner Tenside 1967 4 220. R. Dean and P. Ambrose U.S. Bur. Mines. Bull. 1944,449 ; A. Gaudin Flotation (New York 1957) chap. 10 36 ; A. Gaudin and D. Fuerstenau Min. Eng. 1955 66 ; M. Eigeles and M. Volova 8th Znt. Mineral Processing Congr. (Leningrad 1968) S 12 p. 1. C . W. Nutt Chem. Eng. Sci. 1960 12 133. l o A. Scheludko B. Radoev and A. Fabrikant Ann. Unioersitb Sofia in press. I ' V. Klasscn and V. Mokrousov An Introduction f o the Theory of Flotation (Russ.) (Moscow 1959) pp. 139-144 ; H. Kirchberg and E. Topfer 7th Int. Min. Process Cong. (New York) p. 157.

ISSN:0370-9302    

DOI:10.1039/SD9700100112

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Interfacial Energies of Clean Mica and of Monomolecular Films of Fatty Acids Deposited on Mica in Aqueous and Non-Aqueous Media BY ANITA r. BAILEY,* ANDREA G. PRICE Medical Research Council Biophysics Research Unit King’s College London England AND SUSAN M. KAY Dept. of Physics Royal Holloway College Egharn Surrey England Received 3rd June 1970 The influence of a variety of media on solid/fluid interfacial energies has been measured by a cleavage technique. The solid used was mica chosen because of its near perfect cleavage and ideal bulk properties. Solid/vapour and solid/liquid interfacial energies ysv and y s ~ were measured by cleaving specimens in the form of strips first in an atmosphere of the vapour and then with the specimens completely immersed in the corresponding liquid. Samples of mica coated with mono- molecular films of fatty acids were constructed in such a way that separation took place between the oppositely oriented films.Polar and non-polar liquids and vapours were used in these experiments. The results allow an investigation into the validity of Young’s equation for contact angles which are zero or positive. For well-behaved systems in which no adsorption takes place during the cleav- age the relation is valid. For fatty acid films in an aqueous environment it is necessary to introduce an additiona1 term into the relation to preserve the equality thus The energyy~ has been associated with entropic effects occurring in the liquid phase. Its existence shows that there are measurable anisotropies in the water. The results can be explained if a single layer of the water in the immediate vicinity of the hydrophobic interface is assumed to become preferentially oriented.Model-building experiments have been used to estimate the number of water molecules involved in the interaction and a value of 213 cal/mol of oriented water was obtained for the energy involved. This allows one to predict values for the free energy of solution of small hydrophobic molecules in water which are in good agreement with those of Frank and Evans. The effect disappears in a concentrated solution of urea indicating that the structure of the liquid near the interface is in this case effectively the same as it is in bulk i.e. the urea has caused a breakdown in the ordered structure in the neighbourhood of the interface. This is consistent with current views of the denaturant action of urea on proteins and other bimolecules.YSL = YSV-YLV cos e+yH. Young’s equation describes the equilibrium of a three-phase solid-liquid-vapour system in terms of the interfacial energies between the different phases. Thus the equilibrium of a drop of liquid resting on a solid surface is described by the equation YSV = YSL+YLV cos 6 where ysv is the interfacial energy of a solid in contact with the saturated vapour of the liquid ; ysL is the interfacial energy between the solid and liquid phases ; yLv is the interfacial energy of the liquid in equilibrium with its vapour and 8 is contact angle between solid and liquid phases. has made a comprehensive theoretical study of the validity of this equation including in his analysis the effects of gravity adsorption and curvature of the liquid vapour interface.He has shown that it is in general valid provided * present address Institut fur Physik und Chemie der Grendachen Fraunhofer-Gesellschaft 7000 Stuttgart 1 Romerstr. 32. Johnson 118 A N I T A I. BAILEY ANDREA G. PRICE A N D SUSAN M. KAY 119 the phases remain homogeneous right up to the interface and adsorption effects are taken into account. We have carried out some experiments to test the validity of the equation experi- mentally. This has been done by measuring separately the energies of the solid- liquid and solid-vapour interfaces using a cleavage t e c h n i q ~ e . ~ - ~ For simple solid- liquid-vapour systems the equation holds. The experiments were then extended to include systems in which effects due to reorientation of molecules take place.’ EXPERIMENTAL Muscovite mica was used as the solid phase.This material has several important properties which enable its surface energy to be measured accurately by cleavage. Cleavage takes place along single planes in the crystal over large areas. It is thus possible to obtain specimens which are of constant thickness. Such cleavage faces are molecularly smooth and the real surface area is equal to the geometrical value. Another important property of muscovite mica is that it deforms elastically. Energy is therefore stored reversibly in the portions of the sheet which become bent during the cleavage and this quantity can be cal- culated. Hence the amount of energy to be associated with the formation of new surfaces can be determined. FIG. 1.- bottom applied -Schematic diagram of the apparatus.Cleavage of the sample is produced by lowering the clamp a known distance 6. The deflection of the top clamp gives a measure of the force to the specimen and the position of the cleavage line is located by observing the Fizeau fringes produced by the sample. A schematic diagram of the apparatus is shown in fig. 1. The specimens are in the form of narrow strips. Symmetrical cleavage is initiated at one end and vertical forces F are applied to the open ends so that the strip assumes the equilibrium shape shown with a maximum separation 6 at the point of application of the force. The total work done by the applied forces to increase the separation from 6i to 6~ is Fd6 and the amount of energy stored as elastic energy in the bent sheets during the process is increased by $A[F6][.The area of crystal cleaved is determined by examination of the high-resolution multiple-beam 1: 120 INTERFACIAL ENERGIES Fizeau fringes formed by the sheets. Cleavage was carried out slowly so that the amount of kinetic energy dissipated in the system was small. The whole cleavage mechanism was contained in a trough with optically worked faces so that the sample could be completely surrounded by the liquid or vapour under examination. Mica is also a suitable substance to act as a substrate on which to deposite monomolecular films of low-energy substances such as the fatty acids. By this means we could extend observations to include low-energy surfaces. These specimens were prepared by depositing films on the mica immediately after cleavage in a clean atmosphere.This was done by retraction from non-polar solvent a technique due to Zisman and coworkers,6 who showed that monolayers so formed consist of an oriented layer of close-packed molecules. Saturated solutions in n-decane and n-hexadecane were prepared at about 30-60°C and subsequently allowed to cool. Before use they were filtered through the finest-grade Millipore filter to remove any dust particles or small crystals which had come out of solution. After immersion 6 (mm) length cleaved LK (mm) FIG. 2.-Results obtained from cleaving a sample in water vapour. (a) Graph of the measured force against separation 6 of the open ends; (b) graph of the calculated interfacial energy term (expressed in arbitrary units) against the length of sample cleaved. The value of ysv is obtained from the slope of this line.in the saturated solution the sheets of mica emerged dry and two such sheets were then immediately placed together and allowed to seal up uniformly. The quality of the deposition was judged by examining the " sandwich " using high-dispersion Fizeau interference fringes produced between the outer surfaces by a mixture of monochromatic radiations. In this method small changes in the overall thickness of a sample can produce large variations in the intensity and hue in the interference at tern.^ Samples showing deviations from uni- formity of the transmitted light were discarded. Fig. 2 shows typical results obtained by this method for mica cleaved in water vapour at near saturation pressure. The curve (a) shows the variation of the cleavage force with increasing separation 6 of the ends of the sample.The lower graph (b) shows the total ANITA 1. BAILEY ANDREA G . PRICE A N D SUSAN M . KAY 121 interfacial energy t~ (expressed in arbitrary units) plotted against the area of the sample which has been cleaved. The value of y is obtained from the slope of this graph. Similar curves were obtained for the other cases quoted in table 1 and further examples are shown in fig. 3 and 4. 2L 22 5 20- s - o $ 18- .-.( 2 16 cleavage of stearic acid monolayer in water - - - - - - - 14- 2 I I I I 3 4 5 6 separation in mm (4 cleavage of stearic acid monolayer in water area in mm2 (@ FIG. 3.-Results obtained for cleaving hydrophobic samples in water. 122 10- 8- s E" $ 6- E E C ._ 4 - Q 3 2- INTERFACIAL ENERGIES cleavage of stearic acid monolayer in urea 1 I I I 1 2 3 4 separation in mm (4 cleavage of stearic acid monolayer in urea area in mm2 (4 Fro.4.-Similar results obtained for cleavage in a concentrated solution of urea. ANITA I . BAILEY ANDREA G . PRICE A N D SUSAN M . KAY 123 sol id new mica resealed mica lauric acid stearic acid perfluorodecanoic acid lauric acid stearic acid perfluorodecanoic acid lauric acid stearic acid TABLE 1 medium dry air (r.h. 1 %) moist air (r.h. 50-60 %) water vapour (r.h. 90 %) hexane vapour hexane liquid dry air matching sheets dry air non-matching sheets room air room air room air n-decane n-decane n-decane water water water contact angle OA - - - 0 0 - - - - - 14" 32" 74" 60" 65.5" interfacial energy ergs cm-2 308 220 183 107 271 255 260 1 20 37 25 12 15 15 5 8 13 RESULTS VERIFICATION OF YOUNG'S EQUATION Water and n-hexane both wet a clean mica surface but a finite contact angle is formed between n-decane and a mica surface coated with a monomolecular film of a fatty acid.The results give values for ysL of 107+2 ergs cm-2 for water and 255 2 ergs/cm-2 for n-hexane while those calculated from Young's equation were 1 10 and 252.6 ergs/cm-2 respectively. The contact angles formed by n-decane on the three monolayers studied were well-behaved and showed no hysteresis. Measured values of ysL for stearic acid lauric acid and perfluorodecanoic acid were 15,15 and 5 ergs/cm-2 respectively and the corresponding values calculated from Young's equation were 14.5 14.7 and 5.4 ergs/cm-2. That Young's relation holds shows that it can be valid for quite compli- cated interfaces provided the interfaces and surrounding media do not alter during the experiments.Hence although solids do not have equilibrium surfaces,8 they so nearly approach this condition that the equation may be usefully applied to them. With water we obtained quite different values for the contact angles depending on whether sessile drops or captive bubbles were used to make the measurements; with n-decane both methods yielded the same results. Again hysteresis effects were always observed with water but were absent when the liquid was n-decane. We are indebted to Mr. B. A. Levine for making independent measurements on these systems to check the results. These are shown in table 2. Results for monolayers deposited from the melt were not significantly different from those deposited by retraction from non-polar solvent.We attribute the different angles given by the two methods to the greater ease with which the small sessile drops can become contaminated. This is supported by the fact that on first contact the angles are large and comparable with the values obtained using bubbles but they rapidly fall to the values recorded above. The monolayers are not removed into the water since if a stream of clean water was allowed to flow over the surface before the measurement of contact angle was made the results were identical. The values used in the calculation of interfacial energies are shown in table 1. 1 24 INTERFACIAL ENERGIES The hysteresis of contact angle suggests that some rearrangement of the molecules constituting the monolayers must take place.Monomolecular layers formed by retraction from n-alkane solutions contain a high proportion of solvent molecules unless the samples remain immersed for a long time.g* lo Immersion times in these experiments did not usually exceed 15-30 s and one would expect under these circum- stances that only about one-third of the absorbed molecules are acid molecules. Since the films are stable these polar molecules must be uniformly distributed throughout the film allowing the solvent molecules to be anchored and oriented. A mixed film of this kind would present a surface consisting primarily of CH3 groups in air or in a non-polar environment. When on the other hand the environment is a polar liquid some of the acid molecules reorient so that the head group is exposed to the liquid.That such overturning and mobility between adjacent monolayers can take place has been adequately demonstrated by many workers e.g. Gaines.'l TABLE CONTACT ANGLE MEASUREMENTS ON MONOMOLECULAR FILMS DEPOSITED ON MICA method monolayer water decane urea OA OR OA OR OA sessile drop stearic acid 3 6" 6" 31" 36" lauric acid 10" 2" 14" 10" perfluorodecanoic acid 27" 1" 74" 27' captive bubble stearic acid 654" 35"-42" 31" 654" lauric acid 60" 42" 14" 60" perfluorodecanoic acid 79" 42" 74" 79" Since the film does not become detached only a proportion of the molecules can be in this state at any time. The surface of mica which has been exposed to moist air is covered with a strongly adsorbed monolayer of water so it is likely that on the average half of the acid molecules in a film immersed in water will be oriented towards the bulk liquid.For a gaseous environment the proportion will be lower. Advanc- ing contact angles were reproducible to 1" for measurements made with several bubbles on different parts of the same solid sample ; for successive measurements on the same region of a sample and for different samples. A spread of 5-7" was observed on measurements of the receding angle. The measured values of the monolayer/liquid interfacial energies are lower than we at first expected. If the monolayers presented a surface composed entirely of methyl groups to the liquid phase one might expect that the interfacial energy would have a value intermediate between that of the solid and the liquid.12-15 The overturning of some of the polar molecules is playing a part in the actual value of the interfacial energy with water.Similar effects are observed in the measure- ments of liquid/liquid interfacial energies. Table 3 shows results for such systems taken from the Int. Crit. Tables from which it may be seen that the polar liquids have an interfacial energy value with water which lies below that of either pure liquid with air while the nonpolar liquids produce an intermediate value. By measuring the energy of formation of hydrocarbon/water interfaces it should be possible to obtain information about hydrophobic interactions. Non-polar molecules dispersed in water show a tendency to associate. The thermodynamics of transfer of simple hydrophobic molecules from a non-polar to an aqueous medium have been discussed by several authors. Such a change is invariably accompanied by a large decrease in the entropy of the system and a decrease in the specific molar volume.16-1 * These effects are now interpreted in terms of structural changes occurring ANITA 1. BAILEY ANDREA G . PRICE A N D SUSAN M . KAY 125 in the water in the immediate neighbourhood of the non-polar molecules. Each non- polar chain is thought to be surrounded by a thin layer of water in a quasi-crystalline state and the increase in entropy consequent on the destruction of this interfacial region favours aggregation. TABLE 3.-INTERFACIAL ENERGIES FOR LIQUID IN CONTACT WITH WATER TAKEN FROM INTER. NATIONAL CRITICAL TABLES liquid/liquid liquid/air interfacial interfacial energy energy liquid ergs cm-2 ergs cm-2 benzene 35 28.9 carbon tetrachloride 45 26.9 n-hexane 51.1 18.5 n-octane 50.8 21.8 n-oct ylalcohol 8.5 27.5 diethylet her 10.7 17.0 When the experiments using monolayers were carried out in water the measured values of ysL were in general larger than those predicted by Young’s equation.The average discrepancy was 7.5 ergs/cin-’. The equilibrium of the three phases may then be expressed by adding a term yH to account for these hydrophobic effects thus where yH then has the value 7.5 ergslcm-’. In order to compare our results with the measurements of Frank and Evans,17 models of the interfacial region were constructed. C.P.K. space-filling construction models were used since the aim of the model building was to see how the clustering and orientation of water molecules in the region affects the availability of space for a molecule of water at the interface.Models of the monolayers consisted of a planar array of close-packed hydrocarbon chains one-third of the molecules havingcarboxyl end-groups. Half of the end groups oriented upwards and we allowed them to take a number of random positions in the plane so that sometimes the polar heads occurred in groups and at other times they were more uniformly distributed. The remainder of the molecules were n-alkane mole- cules aligned symmetrically along the length of the fatty acid chains. Clusters and chains of hydrogen-bonded water molecules were also assembled paying as much attention as possible to the average number of hydrogen bonds per molecule as suggested by the work of Senior and Vand.19-21 These were then placed in contact with the model interface in such a way that in the neighbourhood of a non-polar molecule the water molecules were all oriented with their hydrogen bonds towards the bulk water while near a polar end-group they were allowed to assume any orientation.This constraint limits the possible shape of the clusters. It provides hollows into which parts of the somewhat bumpy hydrocarbon surface can fit and excludes other parts from participation. These constructions were carried out many times and each may be regarded as representing a possible array at any particular instant since the process must be a dynamic one. The number of water molecules in the first layer which were oriented with their hydrogen bonds away from the monolayer was counted in each case and an average taken. One may then estimate the number of oriented water molecules per unit area of interface and associate with them the excess energy measured in the cleavage experiments.This has the value 213 cal/(mol of oriented water). In order to see what this predicts for the hydrophobic contribution in experiments such as those of Frank and Evans it was necessary to construct similar models of YSL = YSV-YSL cos O+YH, 126 INTERFACIAL ENERGIES oriented clusters to simulate the conditions in their experiments. They used n-alkanes having 1-4 carbon atoms in the chain. For C4H, the construction was carried out with the molecule either fully extended or folded as compactly as possible so as to present the least interfacial area. From these constructions we may estimate the number of moles of water which orients when one mole of hydrocarbon is dissolved in water and hence predict a value for the free energy AF involved.This assumes that the solution is so dilute that the hydrocarbon molecules exist in solution as single isolated entities. Table 4 shows our predicted values together with the results of Frank and Evans ; the agreement is remarkably good. For C4HIo the prediction indicates that most of the molecules are fully extended. This is unlikely to be the case but the presence of a small proportion of aggregated molecules would also lead to this result. TABLE 4.-PREDICTED FREE ENERGY CHANGES COMPARED WITH RESULTS OF MEASUREMENTS OF FRANK AND EVANS system (Frank and Evans) CH4 (a) benzene to water (b) ether to water (c) carbontetrachloride to water CzH6 in (a) benzene to water C3Hs liquid to water C4HIO liquid to water (6) carbontetrachloride to water AF AF predicted n cal/mol cal/moI 2,600 15.0 3,200 3,300 2,900 3,800 17.0 3,620 3,700 5,050 23.6 5,030 5,850 26.3-28 .O 5,600-5,960 n = average number of oriented water molecules/hydrocarbon molecule.Hydrophobic interactions are thought to be important in determining the tertiary structure of proteins. Urea is effective in causing denaturation of these molecules although the mechanism by which it does so is not clear. Fig. 4(a) and (b) shows curves obtained when mica covered with a monomolecular film of stearic acid was cleaved in a concentrated solution of urea. The results give a value of 4.5 ergs/cm-2 for ysL while that predicted from Young’s equation is 5 ergs/cm-2. It seems there- fore that yH = 0 in this case. DISCUSSION The results show that Young’s equation holds for a number of solid-liquid- vapour systems provided that the interfaces and the phases do not undergo any fundamental changes.With the hydrophobic monolayers in water the monolayers themselves become partially reoriented and cause a lowering of the contact angle and the interfacial energy when compared with values to be expected for a hydro- carbon/water interface. Young’s equation did not hold in this case and it was found necessary to add a term to restore the balance. This additional energy has been associated with entropic effects taking place in the immediate neighbourhood of the hydrophobic interface. By means of model- building the energy has been estimated as 213 cal/mol of oriented water and the results are in good agreement with those of other workers.The results also shed some light on processes occurring in the water when urea is added to it. Water has a fairly open structure and many authors have discussed the solubility of short-chain hydrocarbons in terms of their abilty to be accommodated in the cavities in the structure. The increased solubility of these substances in the presence of urea is thought to result from the participation of urea in the formation ANITA I . BAILEY ANDREA G . PRICE A N D SUSAN M . KAY 127 of clusters of molecules with even larger cavities between them so that more or larger solute molecules may be contained within them. In these experiments the non-polar material is in a special form. It is anchored at a solid-liquid interface and presents an extended sheet to the solution.The surface is nevertheless bumpy on a molecular scale and cavities in the water structure certainly facilitate the formation of a layer which is in good contact with the methyl groups. The disappearance of yH for urea is difficult to explain simply on the basis of increased cavity size since it is difficult to see how this can affect the ease with which the sheet can be accommodated in the structure. While this may well be a factor which determines the increased solubility of hydrocarbons in urea solutions these observations indicate that the medium is now homogeneous and implies a breakdown of the ordered structure of the water consistent with current concepts of denaturant action. We thank our colleagues at the P.C.S. Laboratory Cambridge and the M.R.C. Biophysics Research Unit King's College London for many helpful discussions in particular with Dr.D. Tabor and Dr. W Gratzer. We also thank the Medical Research Council for financial support. R. E. Johnson J. Phys. Chem. 1959,63 1655. J. W. Obreimoff Proc. Roy. SOC. A 1930 127 290. A. I. Bailey Proc. 2nd hit. Congr. Surface Activity 1957 3 4-06. A. I. Bailey and S. M. Kay Proc. Roy. SOC. A 1967 301 47. A. I. Bailey and A. G. Price J. Chem. Phys. 1970 in press. W. C. Bigelow D. L. Pickett and W. A. Zisman J. Colloid Sci. 1946 1 513. S. Tolansky Multble-beam Interferometry of Surfaces and Films (Oxford Univ. Press 19.18). R. T. Mathieson Nature 1959 183 1803. G. L. Gaines Jr. J. Colloid Sci. 1960 15 321. F. M. Fowkes Ado. Chem. Ser. 1964,43 99. ' A. W. Adamson and I. Ling Adv. Chem. Ser. 1964,43 57. l o L. 0. Brockway and R. L. Jones Adv. Chem. Ser. 1964 43 275. I 3 L. A. Girifalco and R. J. Good J. Phys. Chem. 1957 61 904. l4 R. J. Good L. A. Girifalco and G. Kraus J. Phys. Chem. 1958 62 1418. R. J. Good and L. A. Girifalco J. Phys. Chem. 1960 64 561. l6 J. A. V. Butler Trans. Faraday SOC. 1937 33 235. H. S. Frank and M. W. Evans J. Chem. Phys. 1945 13,507. W. L. Masterson 1. Chem. Phys. 1954 22 1830. W. A. Senior and V. Vand J. Chem. Phys. 1965,43 1869. 'O V. Vand and W. A. Senior J. Chem. Phys. 1965,43 1873. V. Vand and W. A. Senior J. Chem. Phys. 1965 43 1878.

ISSN:0370-9302    

DOI:10.1039/SD9700100118

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

GENERAL DISCUSSION Prof. J. Th. G. Overbeek (University of Utrecht) said What is the radius of curva- ture of the “ spherically formed plate ” used by Peschel? Is the plate distance h the shortest distance between the planar and the spherical plate? Dr. G. Peschel (Universitiit Wiirzburg) said In reply to Overbeek the curvature radius of the spherically formed plate was R = 100 cm. The plate distance h is the shortest distance between the plates i.e. it is the distance between the tops of the surface asperities which come into contact when the plates come into contact. Our theory takes account of this fact. Dr. K. J. Padday (Kodak Ltd. Harrow) said The data of fig. lb 2a and 2b of Aldfinger and Peschel seem to indicate that at some temperature either above or below that at which maxima appear the disjoining pressure decreases to very small values.Although the scale of the ordinates of these graphs do not permit accurate interpolation or extrapolation the data suggest that in the low pressure region the disjoining pressure is nearly independent of thickness of the layer. Is this a correct interpretation and if so does it mean that the function of the thickness which gives the disjoining pressure changes with temperature ? Prof. L. E. Scriven (University of Minnesota Minneapolis) said With regard to the paper by Deryaguin et al. from optical measurements by the modulation-polari- metric method profiles of thin liquid films can be calculated as the authors report here in ref. (1). One wonders if the profiles after blowing should be interpreted in terms of a simple hydrodynamic model from which the effects of van der Waals’ and electrostatic forces have been excluded.It may be asked whether the experimental and electrostatic forces have been excluded. It inay be asked whether the experi- mental measurements are of phenomena peculiar to a thin liquid film bounded by roughly parallel gas/liquid and liquid/solid interfaces rather than to the boundary region of bulk liquid in contact with solid. Have the authors considered alternative models in which the forces are appropriately accounted for (and in which viscosity could accordingly vary with film thickness without depending on distance from the solid surface)? What are the r.m.s. asperity height and aspect ratio of the “ heavy flint ” glass prism and of the USSR-14b-surface-finish steel plate used in the experi- ments? In previous publications the authors have presented film profiles but seem not to have calculated viscosities of film liquids one reason evidently being difficulty in locating precisely the “ wetting boundary ” i.e.the three-phase contact line at x = 0 in their fig. 1. It would be of some interest therefore to have available the analysis of the curves in their fig. 3 and the capillary viscometer measurement both mentioned in the paper. Of special interest would be observations of the behaviour of the wetting boundary during the blowing process. (Are the coordinate axes of fig. 3 labelled correctly?) Subtle volatility effects have been implicated for anomalous profiles of spreading films and draining films of polymethylsiloxane liquids as well as many other liquids Proc.2nd Znt. Congr. Surface Activity 1957,3 531. 128 GENERAL DISCUSSION 129 of low volatility. One therefore wonders if the authors have considered the possibility that differential depletion of slightly more volatile film fractions by the air-stream in the “ blow-off ” method might through surface tension variation or other effects contribute to anomalies in film profiles particularly when binary solutions are investigated. If surface velocity of the film does indeed increase with distance down-stream in the air-stream the gas/liquid interface suffers a little dilatation that might influence vaporization and other interfacial phenomena. Prof. B. V. Deryaguin and Prof. V. V. Karasev (Inst. Phys. Chem. U.S.S.R. Acad. Sci. Moscow U.S.S.R.) (communicated) In reply to Scriven for lion-polar liquids the linear velocity profile is observed hence electrostatic and van der Waals forces alone cannot effect the deviation from normal viscosity.Forces of attraction to the substrate do not influence directly the film flow as they are normal to the flow velocity. Indirect effect of pressure via viscosity is vanishingly small due to smallness of pressure. A curved film profile being caused by microroughness of the substrate the linear profile would not be observed also with clean liquid petrolatum. Thus microrough- ness does not influence the film profile. The dependence of the relative viscosity on the distance from the solid substrate is the main interest of this work. In several measurements presented in the report and in some previous papers we compared bulk viscosity obtained by using the capillary viscometer and by the blow-off method.Coincidence of values is obtained with accuracy to - 3 ”/o. According to the formula q = A tan 4 viscosity depends only on thc slope of the profile and does not depend on the wetting boundary position. Moreover in all cases the wetting boundary was immobile before during and after the experiment. It is not difficult to fix the wetting boundary position. For complete wetting this boundary does not change its position neither during the blow-off process nor during measurements. The plot of I is equivalent to an X plot. The last question concerning volatility and surface tension effects upon film profile can be answered in the following way all polydimethylsiloxanes were treated in vacuum to distil off all volatile components ; these polydimethylsiloxanes under normal conditions ( t = 20°C ; p = 760 mm) are non-volatile liquids.In addition evaporation being supposed to take place the steps length greatly exceeds the wetting boundary shift (which was demonstrated in Bascorn’s work). In our experiments the wetting boundary retained its position even during the blow-off process. In Bascom’s work in the preliminary vacuum treatment the spreading also stopped. The viscosity anomaly and profile distortion connected with it cannot be explained by a surface tension effect as the liquids are non-volatile and for the binary mixture the surface tensions of both components are almost the same PMS-10-19.5 dyn/cm ; PMS-2000-20.8 dyn/cm. The last remark about surface evaporation being influenced by extension is not clear.Dr. A. J. Smith and Dr. A. Cameron (Dept. Mech. Eng. Imperial College) said We have used a 100 cm3 silicone at 70°C in the apparatus described by us and investi- gated the thinning round a steel ball of American specification 52100 or British EN31. It is interesting to note that we find the same type of behaviour as Deryaguin et al. do but it occurs at a very much bigger film thickness. In view of the findings in our own papei it is interesting to note that the thicknesses at which this effect occurs are in the same order as the reactivities of the substrates. Glass gives the thinnest film. The authors’ steel whose composition we do not know but is presumably a W. D. Bascom R. L. Cottington and C. R. Singleterry Adv. Chem. Series no. 43 pp. 355-9 (Amer.Chem. SOC. Washington D.C. 1964). SPJ-E 130 GENERAL DISCUSSION martensitic stainless steel comes next and finally the steel we use is the most reactive. We would like the authors’ comments on the possibility of the dimethylsiloxane fluids containing some kind of reactive constituent which could have formed the gel- like layer described in OUT paper. 2 5 10 20 5 0 Ioc) time (min) FIG. 1 .-Thinning curve of 100 cni3 polydimethylsiloxane on steel. Prof. S. G. Mason (McGill University Canada) said As I understand Deryaguin’s blow-off method the tangential stress is taken to be the same on both sides of the air/liquid interface with the implied assumption that the surface tension 0 is indepen- dent of the film thickness down to the edge x = 0 h = 0 (fig. 1). However if (da/dh) # 0 a portion of the tangential stress developed by the air can be borne by the interface itself; this would presumably give rise to an apparent change in viscosity as the film becomes thinner even when no such change occurs.How is the effect of changing surface tension allowed for i n calculating the viscosity? Prof. B. V. Deryaguin and Prof. V. V. Karasev (Inst. Phys. Chem. U.S.S.R. Acad. Sci. Moscow U.S.S.R.) (communicated) In reply to Mason first the direct effect of surface tension ‘‘ gradient ” &/ah is extremely small or even zero in all our earlier experiments and in those under discussion. The reasons are (i) this effect is absent for one-component films and in general very weak for organic liquids because in such systems the adsorption or surface activity at the liquid air interface is very small.(ii) The effect mentioned by Mason actually depends on the derivative a0 - -- dZ - ah dl where I is tangential distance along the film surface. is of the order of magnitude of 10-3-10-4 this derivative is vanishingly small. Due to the factor dh/dl which GENERAL DISCUSSION 131 In reply to Scriven our “ model ” of boundary viscosity which depends on the wall distance independently of the distance from the film/air interface is based on the very short-range action of the latter interface in comparison with that of liquid/solid interface. The independence of our results on the heights of asperities gave no opportunity for the estimation of asperity heights and profiles of our substrates. In reply to Smith and Cameron our dimethylsiloxane fluids contained no con- stituents that would react with our stainless steel.Besides such reactivity contradicts the decreasing of viscosity of dimethylsiloxane in the immediate proximity of the substrate. Dr. S. G. Ash (Thornton Res. Centre Chester) said In the analysis of the density measurements and heats of wetting presented in the paper of Ash and Findenegg the surface excess of cyclohexane was assumed to be zero. It is possible to analyze the same results without making this assumption and so calculate the surface excess mass of cyclohexane. The surface excess mass of a pure liquid in contact with Graphon is calculated from the experimentally determined quantities using the equation where rl is the surface excess mass of liquid; S the specific surface area of the Graphon; p i the bulk liquid density; mi mf the mass of liquid and Graphon in the pyknometer v’; the volume of the pyknometer ; pg the absolute density of Graphon.Similarly for a second liquid All quantities in these expressions for r are measured except the density p g of Graphon. This can be eliminated from eqn (1) and (2) to give a relationship between rl and Tz. On the basis of the solid-like surface zone model we equate the total heat AHw of wetting of Graphon by a pure liquid to the sum of the contributions arising from the liquid-Graphon interactions H, and the structuring of the surface zone nsHf (ns = number of moles of component in surface zone ; Hf =heat of fusion of component). We identify liquid 1 with an n-alkane and liquid 2 with cyclohexane and assume that H is the same in both systems.Then (AHWh = H w + n’,(HJ)l; (3) (AHWL = H w + n W J 2 . (4) The quantity H can be eliminated from eqn (3) and (4) to give an expression relating ni and ni. The surface excess mass is related to the capacity of the solid-like surface zone by r = nsM(l - p l / p s ) ( 5 ) where M is the molecular weight and ps the solid density of the hydrocarbon. The values of ni nz (and rl I?,) can be calculated from the set of simultaneous eqn (1)-(5). This analysis is equivalent to ascribing the discrepancy between the structural contribution to the heat of wetting and the structural contribution calculated from the density measurements to the presence of a solid-like surface zone of cyclohexane (see fig. 2 3 and 4 of the paper by Ash and Findenegg). The calculated thickness of the solid-like surface zone of cyclohexane is / I = 0.5 (+0.5)& (T = 25-40°C) and rcyclohexane = 0 .4 ~ (20.4 x lo-’) g cnir2 132 GENERAL DISCUSSION The absolute values of the surface excess mass of other liquids can be calculated from the excess mass quoted in the paper (relative to rcyclohexanc = 0) by the expression The structural contribution to the total heat of wetting of Graphon by cyclohexane is which is only 1.3(+ 1.3) % of the total heat. rabs = rrel+o.5 x 10-9 pi. (n'Hdcyclohexane = 1*4(+ 1.4) erg ~ m - ~ Prof. J. Th. G. Overbeek (University of Utrecht) said With regard to the paper by Ash and Findenegg if a water-vapour interface has a contact angle on Graphon it is possible that near points where the Graphon particles touch or come very close together a vapour bubble is formed bounded by menisci with their convex sides towards the vapour.At a radius of curvature of about 1 pm such a meniscus could withstand a pressure difference of about 1 atm thus bringing the liquid phase at 1 atm and the vapour phase at a very low pressure in equilibrium with one another. This would explain the negative excess volume of water (and of benzene). Dr. G. H. Findenegg (University of Vienna Austria) (partly communicated) We agree with Overbeek that the negative surface concentration of water at the Graphon/liquid interface could be caused by small vapour bubbles. From our results the vapour volume would be 2.8 x cm3 g-l Graphon and hence there could be a very large number of small bubbles (lo7 or more per g Graphon) which would explain the good reproducibility of our results.We do not think that the surface concentration of benzene can be explained by the same effect because Graphon is completely wetted by benzene. Dr. J. W. White (Phys. Chem. Lab. Oxford) said With regard to the paper by Ash and Findenegg on the basis of recent neutron inelastic scattering measurements 1*2 I can comment on the range and magnitude of intermolecular forces likely between graphite and adsorbed alkanes and between the alkanes themselves. First as to the range for pyrolytic graphite the forces between the basal planes and their range has been measured by Dolling and Brockh~use.~ The dispersion curve for lattice vibrations perpendicular to the sheets was measured and is a strong test of the range of the interatomic p~tential.~ An almost exact sine curve was obtained which fits the theoretical model for nearest neighbour forces only.The contribution of next nearest neighbour planes to the total (attractive) force constant was only ca. 1.5 %. Our recent measurements of phonons along the [110] crystallographic direction in polyethylene and in the basal planes of hexadecane crystals indicates again that the lateral intermolecular forces are almost exclusively between nearest neighbours. The magnitude of the force also comes from the neutron experiments and may be characterized by the elastic (Young's) moduli derived from the limiting slopes of the dispersion curves near the centre of the Brillouin zone (Le. for long wavelength vibrations). To give a sense of scale the modulus of diamond ((110) planes) E = 250 x 1Olo dyn cm-2 (25 x 1Olo N m-2) is mentioned.For the direction per- pendicular to the graphite basal planes E = C33 = 39 x 1O'O dyn cm-2 (3.9 x 1Olo N m-2). From the polyethylene data we find that E = 6 x 1O1O dyn cm-2 (6 x lo9 N m-2) for paraffin chains separated by 4.1 A (0.41 nm). The lowest modulus we J. F. Twisleton and J. W. White MoZ. Phys. to be submitted. J. F. Twisleton Thesis (Oxford University 1970). G. Dolling and B. N. Brockhouse Phys. Reu. 1962,128 1120. A. J. E. Foreman and W. M. Lomer Proc. Phys. SOC. B 1957,70,1143. GENERAL DISCUSSION 133 have found so far for these systems is in the hexadecane basal plane where E = 1.5 x 1O1O dyn cm-2 (0.15 Nm-2). By assuming an additivity these numbers serve as a guide for the orders of magnitude to be expected for the adsorption force and even may be used to estimate the wetting enthalpies.It emerges that these nearest neigh- bour estimates exceed the measured enthalpies by about an order of magnitude. This suggests the need for an upwards reassessment of the strength of pairwise forces rather than to postulate long-range interactions. The neutron data also strongly suggest that a change to " head stacked " adsorption (with the hydrocarbon perpendicular to the graphite) would only occur at very high coverage because of the advantageously high " mean energy " of attachment to graphite rather than to other alkanes. Finally there are possible neutron scattering experiments which could determine the intermolecular constants on Graphon surfaces. Neutron diffraction experiments would give the average dimensions of a group of ordered molecules from the sharpness of diffraction and inelastic scattering from phonons in the adsorbed layers would give not only a measure of this (again from widths) but a measure of the modified intermolecular forces and therefore the mean intermolecular separations transverse to the chain axis.Prof. R. J. Good (Bristol University) said The results of Ash and Findenegg point to an important conclusion about the use of alkanes to determine the value of ys from contact angle data. If spreading pressure He is negligible then ys = y,(i + cos t1)~/4@,21 (1) For a non-polar liquid on a non-polar solid,2 a s 1 = (Z~~S)+/HZl+ a where Zis ionization energy. ys when determined in tlus manner using liquid alkanes has been described as the dispersion component of the surface tension of the solid.3 It is better identified as one half the free energy of cohesion across the specified crystallographic plane.In the present case it could be written yoool i.e. referring to the basal plane of graphite. When the solid is covered by an adsorbed film y s is one half the free energy of cohesion of the film-covered solid across the plane which is specified as i.e. across the surface indicated by the double vertical lines. It is a property of the solid only; and a, should be very nearly constant for a series of alkanes in a single solid. It has recently been found that for numerous low-energy surfaces' ys determined in this manner shows a trend with chain length of the alkane. attributes the trend in y s to the variation in density of the hydrocarbon with chain length. I prefer the conclusion that there is a trend in @ with chain length which is not represented by eqn (2).Now eqn (2) is based on the premise that differences in the structure of the liquid at the liquid/solid interface and at the liquid/vapour interface can be neglected. While this hypothesis can be valid for a liquid such as cyclohexane the data reported by Ash and Findenegg show that it is not so for alkanes on graphite. Consequently there should be a trend of CD with chain length for alkanes against graphite. R. J. Good and L. A. Girifalco J . Phys. Chern. 1960,64 561. R. J. Good Adv. Chem. Series 1964,43 74. F. M. Fowkes Znd. Eng. Chern. 1964,56,40. R. J. Good in Wetting (SOC. Chem. Ind. London 1967). A. W. Neumann and R. J. Good to be published. solid I monolayer (head-tail) I I monolayer (tail-head) I solid Fowkes 134 GENERAL DISCUSSION Accordingly 1 suggest that Findenegg and his coworkers perform volunietric studies on other solids for which contact angle measurements are available.Teflon would be an interesting substrate. Dr. G. Peschel (Universitat Wiirzburg) said The fact that Ash and Findenegg used cyclohexane as a reference liquid showing no surface zone anomalies might be incompatible with our results at first sight for we found a disjoining pressure in thin layers of cyclohexane between fused silica plates. In one case the plates were fully covered with hydroxyl groups in the other case the plates were free from hydroxyl groups achieved by baking out the plates at about 850°C. In the first case the disjoining pressure ranging between the melting point and about 15°C was larger than in the second case where the surfaces were less polar.It may be that Graphon surfaces only produce a negligible disjoining pressure in adjacent cyclohexane layers so that a pronounced discrepancy between his and our work is non-existent. Mr. A. J. Groszek (B.P. Co. Ltd. Sunbury) said With regard to the paper by Ash and Findenegg I have established recently that long-chain n-paraffins are adsorbed very strongly from solutions in various volatile solvents on the basal plane surface of graphite. For all the graphites investigated the long-chain n- paraffins saturate the basal planes at low solution concentration (ranging from ca. 0.001 mol % for n-C, and 10 for n-C,,) with the formation of distinct plateaux regi0ns.l From the amount of adsorption of n-C, it was inferred that the n- paraffin molecules lie flat on the basal plane surface.The data indicate that the n-C32 molecules are closely packed so that each H atom in a methylene group in contact with the surface occupies a carbon hexagon or 5.6A2. Similar results have been obtained for n-hexadecane at higher concentrations.2* The heat of displacement of n-heptane by n-hexadecane suggested strongly that at a mol fraction of about 0.1 n-hexadecane forms a monolayer composed of molecules lying flat on the basal planes as indicated by Aveyard. The heats subsequently increase unlike the amount of adsorption to the value agreeing with the high heat of immersion of Graphon in n-heptane discussed by Ash et al. The strong preferential adsorption of n-paraffins is very characteristic for graphite and has not been observed for solids having polar surfaces e.g.the high heats of immersion are not observed for silica gel and carbon blacke4 In view of the fact that the strongly adsorbed n-paraffins are orientated parallel to the surface it is most unlikely that such molecules would leave the graphite surface to produce closely- packed monolayers orientated normal to the surface. A much more energetically favourable situation would be for n-paraffin molecules to form several layers of molecules orientated parallel to the surface. The packing of n-paraffins in such multilayers would contribute to the heat of adsorption in the same way as in the film composed of vertically orientated molecules but there would be no necessity for re-orientation of the first strongly adsorbed monolayers.Another point concerns more specifically the difference in the heat of wetting of Graphon in n-heptane and n-hexadecane. We have determined this difference with the use of the flow-calorimeter for Graphon and other graphites by displacing n- heptane gradually with n-hexadecane and summing up all the heats of preferential adsorption so obtained.2 A. J. Groszek Proc. Roy. SOC. A 1970,314,473. G. I. Andrews and A. J. Groszek 3rd Con$ Industrial Carbons and Graphite (Imperial College London 1970). R. Aveyard Trans. Faraday SOC. 1967 63 2778. L. C . Robert Cornpt. rend. 1963 256 655. GENERAL DISCUSSION 135 The difference obtained for a sample of Graphon in these experiments was 93 erg/cm2 but for a more perfect graphite composed of thin flakes having a thickness of about 100& the difference was much higher at 130erg/cm2.The important point arising from these experiments is that the heat of wetting of graphite in liquid paraffins depends on the type of graphite used and that the ordering of n-hexadecane is increased when the basal plane in graphite becomes more extensive. Dr. J. K. Padday (Kodak Ltd. Harrow) said Values ysL of the systems n-decane in contact with stearic lauric or perfluorodecanoic acid monolayers of table 1 of the paper by Bailey and Price appear to be calculated from the measured values of angles of contact and ysv but with the important distinction that ysv refers to room air presumably in the absence of n-decane. Bangham and Razouk have pointed out that the Young equation for this type of system should be written in the form YLV cos 8 = Ys-YSL-n where II represents the lowering of the specific surface free energy of the solid by adsorption of vapour of the spreading liquid.The results of Bailey and Price indicate that in their system II is very small or zero. Whilst this is not surprising with n-decane spreading on a monolayer of perfluorodecanoic acid the contact angle of which is large it is surprising with the two fatty acid surfaces. The good agreement between advancing and receding contact angles of these two systems suggests that n-decane does not penetrate or otherwise alter the structure of monolayers of lauric or stearic acid on mica and that the systems behave ideally both in a thermodynamic and a physical sense. Dr. A. I. Bailey (Stuttgart) said In reply to Padday the interfacial energy terms ysv in our work always refer to the modified surface i.e.to a mica surface covered with adsorbed vapour films and/or adsorbed films of fatty acids. In the work of Bangham and Razouk ys refers to the surface energy of the bare solid. The films were deposited by refraction from solution i.e. after formation of the oriented monomolecular film the solvent has no further tendency to adsorb and II is negligible. When the adsorption takes place during the formation of the interfaces such as e.g. takes place when clean mica is cleaved in a solution of polar material in non- polar solvent then as might be expected Young’s equation is also not valid. In reply to Cameron we have not so far investigated films formed from solvents of different chain lengths. The effects described by Cameron are fascinating and we will investigate them.Dr. H. E. Ries (Chicago) said I should like to know if in any of the experiments performed by Bailey Price and Kay monolayers have been deposited on the mica by the Langmuir-Blodgett technique. I am afraid that other methods do not neces- sarily give close-packed films and that angles of contact can be somewhat misleading in this connection. By control of the surface pressures during Langmuir-Blodgett transfer the nature of the transferred film is well established.2 Dr. A. I. Bailey (Stuttgart) said In reply to Ries we made several attempts to use Langmuir-Blodgett films. Our experience was that it was almost impossible to keep the specimens absolutely dust free during the time required for the deposition. It is also relatively easy to keep a few ml of solution dust free than the entire contents D.M. Bangham and R. I. Razouk,Trans. Farday Soc. 1937 33 1459. H. E. Ries Jr. and D. C. Walker J. Colloid Sci. 1961 16 361. 136 GENERAL DISCUSSION of a trough. In this experiment dust particles between the sheets cause distortions of the mica which affect the energy conditions. Previous work by Courtney-Pratt in which the thickness of solvent deposited acid films was measured interferometrically showed that the tluckness was consistent with the molecules having extended chains oriented perpendicular to the surface. Since only a third of the molecules are acid molecules if they alone occupied the surface gaps between the molecules or tilting would result in a smaller measured thickness so it is reasonable to suppose that the remainder of the film consists of adlineated solvent molecules the whole film being close-packed or very nearly so.The contact angles with water are low but consistent with our hypothesis of partial overturning of the polar molecules. In any case the contact angles and the energy measurements apply to the same film and so are consis- tent with each other. Dr. B. A. Pethica (Unilever Res. Port Sunlight) said In the paper by Bailey Kay and Price it is proposed that for certain systems the Young equation be modified by addition of an extra term to include entropy effects. To use their attractive termin- ology “well-behaved” systems obey the Young equation. There is no thermo- dynamic justification for putting extra terms in Young’s equation and the apparent failure of the equation is to be sought in the bad behaviour of the fatty acid film+ water+mica system in which the equation is said to fail.In this system the results were variable wluch could be explained by migration of the fatty acid to the water vapour interface. The consequent reduction in the interfacial tension would also explain the apparent failure of Young’s equation. Dr. A. I. Bailey (Stuttgart) said In reply to Pethica let us consider what would happen if the fatty acid migrated away to the water/vapour interface. Each molecule which leaves would expose a region of clean mica/clean water interface which has a high interfacial energy. During measurements of ysL we are not really concerned with what happens at the water/vapour interface. This interface is several cm away from the specimen and the new solid/liquid interface is always formed in the bulk of the medium.Capillary rise measurements made on water spread with lauric acid for example give the same values as clean water. Now the absolute values of the measured interfacial energies are low and not consistent with this picture. It seems also that the use of a reduced value of the interfacial energy of water to restore the balance of Young’s equation only works if one assumes that the monolayer is both present on the solid surface and available to reduce the interfacial energy of the water. The overturning of some of the adsorbed molecules accounts for both the low values of ysL and the existence of the hysteresis of contact angle effects. Dr. A. Cameron (Mech. Eng. Dept. Imperial College) said I am always interested in the possibility of finding different methods of studying the effects of matching chain lengths of additives and carriers such as we discussed in the paper by Askwith Cameron and Cr0uch.l Have Bailey et al.any further experimental evidence when the surfactant and carrier are precisely matched?. It would be interesting if they could carry out some tests. Their method is ideally suited for such studies. Dr. K. W. Miller (Dept. of Pharmacology Oxford) said The results of Bailey’s model building are most interesting and instructive. The effect of the non-polar part of the monolayer on the adjacent water structure seems to be proportional to the area of hydrocarbon exposed. A more sensitive measure of these structural Proc. Roy. Soc. A 1966,291,500. GENERAL DISCUSSION 137 changes is the entropy but it would be most difficult to obtain this by model building.‘The entropy of dissolving lion-polar gases in water is known however with a fair degree of accuracy and this entropy is closely related to the surface aIea of the gases dissolved.’ This observation lends further credibility to Bailey’s findings. Her model building also illustrates the way that “ hydrophobic bonding ” originates not so much from the hydrophobic surface itself as from the hydrophilic nature of the water molecule ; indeed hydrophilic bonding would be a more accurate term. Bailey takes this into account by orientating her surface water molecules with their hydrogen bonds towards the bulk water. This constraint is equivalent to introducing more order into the system thus decreasing its entropy.In statistical mechanical terms this entropy decrease may be assigned to the loss at the surface of some of the many configurations that water molecules are free to take up in the bulk solvent. Present ideas about the nature of these configurations are of necessity crude but the opposing requirements of packing closely around the hydrophobic surface whilst maintaining the maximum possible degree of hydrogen bonding will evidently give rise to the decrease in entropy that is usually rather loosely ascribed to an increase in structure. Dr. A. I. Bailey (Stuttgart) said A general comment on the model building described in our paper might answer some of the questions which arose in private discussion. The models were made in order to help to visualize what might happen when molecules which form part of the normal water structure encounter a hydro- phobic entity.The only assumptions that we made with regard to the water mole- cules at the hydrocarbon interface were (i) that they should be hydrogen-bonded and participate in cluster formation and (ii) that they should occupy all the space there unless prevented from doing so by the shape of clusters in the neighbourhood. These simple assumptions led to the re-orientation of molecules at the interface and to constraints on the shape of the clusters which contain these molecules. Hence owing to the exclusion of some generally available states the system has alower entropy than for pure water. The increased order is not in the nature of a true phase change such as would result from a change in the intermolecular attraction but rather it is due to a disturbance of the normal water structure. Perhaps the term “ icebergs ” used by some workers has been rather misleading. Similar effects presumably must take place also at the surface of water. The model building helps to give an idea of how with the minimum of assumptions the entropy changes observed by other workers might arise. These ideas must represent an oversimplifi- cation however since they provide no explanation as to why some molecules are more hydrophobic than others. K. W. Miller and J. 13. Hildebrand J . Amer. Cliern. SOC. 1968,90 3001.

ISSN:0370-9302    

DOI:10.1039/SD9700100128

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Measurement of Forces between Colloidal Particles BY L. M. BARCLAY AND R. H. OTTEWILL School of Chemistry University of Bristol Bristol England Received 30th April 1970 Apparatus has been constructed for the measurement of the pressure created by disperse systems as a function of the volume concentration of the disperse phase. Experiments with sodium mont- morillonite as the colloidal system have enabled the force to be obtained as a function of the distance between the plates down to distances of the order of 108,. The results have been compared with those expected theoretically on the basis of the DLVO theory. The forces obtained at distances of less than 50 8 are much greater than those predicted by the theory and the additional force appears to arise from solvation effects in the thin liquid film between the particles.This has been confirmed by carrying out measurements in the presence of a non-ionic surface-active agent a known stabilizing agent where the repulsive forces due to solvation are enhanced. Information on the stability of disperse systems can be obtained by both kinetic le3 and equilibrium method^.^-^ In kinetic studies the electrolyte is added to the system and the rate at which the stability is lost is examined i.e. the rate of flocculation is determined.8 Although this approach has provided considerable information the interpretation of the kinetic data is limited. There are therefore considerable advantages to studying a disperse system in its own environment under essentially equilibrium conditions. One method of carrying out such a study is to measure the pressure developed in the system as a function of the distance of separation of the particles.Some swelling pressure studies 4-9 have been used hitherto to obtain information of this sort. The type of apparatus previously used has been considerably refined and the present communication describes the apparatus used and some studies carried out using sodium montmorillonite dispersions in the presence and absence of a non-ionic surface active agent. Studies carried out using monodisperse polystyrene latex particles will be described in a later publication. EXPERIMENTAL MATERIALS The distilled water used was doubly-distilled from an all-Pyrex apparatus. Sodium chloride was B.D.H. A.R. material which was roasted before use. n-Dodecyl hexaoxyethylene glycol monether (C 2E 6) was prepared by the Williamson ether synthesis.l o The critical micelle concentration determined by surface tension measure- ments was 7.25 x The montmorillonite was a sample of montmorillonite no. 26 (Bentonite) from Clay Spar Wyoming as prepared for the American Petroleum Institute Clay Mineral Standards Project no. 49. About 30 g of the dry material was dispersed in 1 1. of distilled water contain- ing 1 ml of 30 % v/v hydrogen peroxide per 100 g of clay. After standing for 24 h the sample was centrifuged to remove impurities such as silica particles. Conversion to sodium montmorillonite was achieved by redispersing the clay particles in molar sodium chloride 138 M at 25°C (compare 7.8 x lo-’ M ll). L . M . BARCLAY A N D R . H . OTTEWILL 139 solution and allowing the dispersion to stand for several days.The sodium montmorillonite was separated by centrifugation at 38,000 g for 1 h. For compression studies the centrifuged material was dispersed in M sodium chloride solution and then dialyzed against that solution for a long period. or ADSORPTION STUDIES A dispersion of ca. 1.5 % w/w sodium montmorillonite which had been extensively dialyzed against twice-distilled water was used as a stock dispersion. Various weights ranging from 0.4 to 1.5 g of this stock dispersion were added to a series of CIZE6 solutions having concentrations in the range 10-4-10-3 M and a volume of 25 ml ; these solutions uere contained in stoppered tubes. The mixtures were equilibrated at 2510.05"C for 3 days with frequent shaking. The surface tension of the mixture was used to determine the equilibriumconcentration of C12ES.A comparison of the surface tensions of the supernatants of a series of mixtures which had been centrifuged at 38,OOOg with those of the original mixtures showed them to be identical. Hence the centrifugation procedure was dispensed with in later experiments. APPARATUS FOR COMPRESSION STUDIES The cell used for pressure measurements which was constructed of stainless steel is shown diagrammatically in fig. 1. All sharp edges were rounded off to prevent damage DISPERSION CASTOR OIL MILLIPORE FILTER RUBBER D I APH RAG M FIG. 1 .-Cross-sectional diagram of compression cell. occurring to the rubber diaphragm and the Millipore filters. Similarly the end of the glass capillary which was inserted into the cell was bevelled in order to avoid chipping the tip when assembling and dismantling the cell.A PTFE O-ring placed between the capillary tip and the end of the steel orifice also minimized damage to the capillary. The capillaries were constructed from calibrated lengths of 3 mm diam. (inside) Viridia glass tubing. The stainless steel disc (porosity no. 3) fitted exactly into a recess in the top of the cell. Samples of rubber latex for the cell diaphragm were selected from a large sheet 0.007- 0.010 in. thick (Holdfast Rubber Dam) by stretching pieces in front of a light source. Only 140 FORCES BETWEEN COLLOIDAL PARTICLES sections free from bubbles within the rubber were used. A calibration curve was constructed 'for each diaphragm' of the force applied by the stretched rubber to the sol. Millipore MF filters (47 mm diam.0.1 p pore diam.) were used as membranes ; the Viskase dialysis tubing used by some authors was found to be unsuitable as a membrane and led to a very slow attainment of equilibrium. Before use approximately 4 1. of distilled water at 60°C were passed through each filter in order to remove impurities such as non-ionic surface-active agents and polyhydric alcohols. The removal of surface-active material was monitored by measuring the surface tension of the filtrate. After thorough washing the filters were dried at 7OoC between filter papers held between perforated metal plates. In order to generate pressure in the system a Budenberg hydraulic gauge tester was used with caster oil as the hydraulic fluid. The pressure applied was read directly on direct mounting Budenberg bronze tube Bourdon gauges.The gauges were used to cover the ranges 0-20 0-160 and 0-1600p.s.i. The 0-2Op.s.i. gauge was calibrated in position on the complete apparatus by connecting a mercury manometer to the hydraulic system. The other two gauges were calibrated using a dead weight tester. In order to maintain the selected pressure constant a pressure compensating mechanism was built into the apparatus so that compensation occurred as the dispersion medium was forced out of the sol compartment. For this purpose the pressure gauges were used as regulators. A knife-edge metal contact was attached to the glass face of each gauge and a platinum contact to each of the gauge needle-pointers. For each gauge the knife-edge contact and a suitable point on the chassis of the gauge were connected to an electric motor (Parvalux 2 rev/min) which by means of a chain drive mechanism could rotate the piston drive wheel and so increase the pressure.The electric motor drive spindle was connected to a drive shaft via a chain and sprocket. The rotation of this shaft was transmitted via another chain and sprocket set fixed to the threaded piston rod. The piston could be operated manually by retracting a spring-loaded pin (fixed to one sprocket on the drive shaft) from the key-way on the drive shaft so that the sprocket rotated freely about the shaft (see plate 1). The major section of the apparatus including the cell and capillary was fitted into a tight-fitt ing air-thermostat box which was maintained at a temperature of 25.0f0.05"C. In order to load the cell for measurements the diaphragm recess volume was initially adjusted to ca.6 ml by retracting the piston and approximately 5 ml of the degassed sol to be used was introduced into this. The pressure was then slowly increased in the system until the meniscus of the sol was slightly above the rim of the cell. A clean millipore filter was placed on top of the liquid surface without trapping air bubbles underneath it. The cell was then assembled. Tnitially a small pressure of less than 1 p.s.i. was applied so that the dispersion medium slowly emerged from the cell interior forcing most of the air out of the sintered disc into the capillary. More air was released by raising and lowering the pressure repeatedly over a period of about 1 h. The " dead space " above the membrane which included the pore volume of the sintered disc and the volume of the exit aperture in the top of the cell was determined from values of the initial and final solid contents in con- junction with the volume of the supernatant in the capillary.Usually at the beginning of each run a fairly large volume of dispersion medium was forced from the cell from a compara- tively dilute sol. For montmorillonite about 4 g of approximately 2.5 % w/w dispersions were used ; higher concentrations were not practicable as the material gelled at concentrations above 3 "/ w/w. Since a relatively large volume of liquid had to be removed on the first compression the first equilibrium pressure reading often took up to 24 h. The meniscus of the liquid in the capillary was observed with a cathetometer and equilibrium at a given pressure occurred when there was no further change in the position of the meniscus.The whole apparatus was mounted rigidly on a steel frame. RESULTS COMPRESSION STUDIES WITH MONTMORILLONITE The curves of equilibrium pressure against distance of plate separation for and 10-1 M sodium chloride solutions are shown in fig. 2. montmorillonite in Plate 1 Photograph of apparatus showing cell and compression mechanism. [To face page 140. L . M. BARCLAY AND R. H. OTTEWILL 141 Experimentally the first compression on each sample gave a larger repulsion at a given distance up to a pressure in the region of 20atm. After the first compression however subsequent compression and decompression experiments gave coincident results as can be seen from fig. 2. The distance between the plate surfaces H, was calculated assuming a specific surface area of 800 m2/g and using the expression Ho = 2V/mA distance between plates (A) FIG.2.-Equilibrium pressure against distance between plate surfaces for sodium moiitmorilloriite dis- persions at 25°C. In M sodium chloride solution 0 first compression ; V decompression ; 0 subsequent recompression. In 10-1 M sodium chloride solution 0 first compression ; v dc- compression ; A subsequent recompression. where V = volume of liquid in a sol containing m g of clay and A = specific surface area. A check on the interplate separation distance was obtained by carrying our a low-angle X-ray diffraction examination * of two vacuum concentrated samples. The particles in these samples had not been deliberately orientated and hence the distances obtained were probably the interparticle distances in oriented domains.The X-ray results are compared with those obtained from the surface area in table 1. TABLE 1 .-INTERPARTICLE SPACING DISTANCES % wlw of HO HO montmorillonite calc. from first calc. from in sample order X-ray pattern surface area 37.4 40.8 A 42.0 8 34.2 45.5 A 48.2 8 The X-ray results based on a plate thickness of 8.5 A are in good agreement with those calculated using a surface area of 800 m*/g. * We thank Dr. S Clunie for this determination. 1 42 FORCES BETWEEN COLLOIDAL PARTICLES ADSORPTION OF C12E6 ON MONTMORILLONITE The adsorption isotherm obtained for the adsorption of C12E6 onto mont- morillonite from water at 25°C is shown in fig. 3. The adsorption of C12E6 continued equilibrium concentration of C12E6 ( x lo4 M) FIG.3.-Adsorption of CI2E6 on sodium montmorillonite at 25"C from solution. M sodium chloride t critical 0 centrifuged at 38,000 g ; 0 centrifuged at 12,500 g ; A not centrifuged. micelle concentration. to increase above the critical micelle concentration and did not reach a steady value at or just below this concentration as has been observed with silver iodide l2 and polystyrene 1atices.l However the isotherm is essentially similar to that found by Schott l3 in studies of the adsorption of C12E14 on montmorillonites. At the critical micelle Concentration (7 x M) the adsorption of CI2E6 was 1.16 x mol/g which on the basis of a surface area of 800 m2/g corresponds to an area per adsorbed molecule of 144 A.2 This would correspond to a horizontal extended orientation of the CI2E6 molecule.COMPRESSION CURVES FOR MONTMORILLONITE IN THE PRESENCE OF C12E6 The equilibrium pressure against distance curves obtained for montmorillonite in the presence of 7 x M sodium chloride are given in fig. 4. A pronounced difference occurred between the first and second compression curves but subsequent compression and decompression points all fell on the same curve. A feature of the curves is that although the electrolyte concentration was maintained at M at pressures below 20 atm the equilibrium distance was reduced in the presence of C12E6. Above 20 atm the system was more expanded in the presence M C12E6 and of C12E6. DISCUSSION The two basic problems encountered in the present work were the accurate measurement of the equilibrium pressure and the estimation of the interparticle spacing.The use of a servo-mechanism allowed the applied pressure to be maintained accurately until the liquid expelled from the cell had reached an equilibrium height L. M. BARCLAY AND R . H. OTTEWILL 143 in the capillary. However the pressures observed in the compression of a sodium montmorillonite dispersion for the first time were always higher than those observed on subsequent compression and decompression cycles. This effect was attributed to a grain pressure in which edge-face contacts between the clay plates occurred leading to a card-house structure of the type described by~anO1phen.l~ At the high pressures presumably the plates re-aligned to a parallel arrangement although the possibility of some domains could not be excluded. The hysteresis was largest at low equilibrium pressures for both the electrolyte concentrations examined.The displacement at ~~~~ 5 0 100 150 2 0 0 250 3 0 0 distance between plates (A) FIG. 4.-Equilibrium pressure against distance between plate surfaces for sodium montmorillonite in M sodium chloride solution in the presence of an equilibrium concentration of 7 x lo-’ M CI2E6. . . . . . . first compression ; A non-equilibrium decompression ; 0 subsequent recompression. - - - curve in M sodium chloride solution. 0.1 atm pressure was 130 A in 10-1 M sodium chloride solution and 90 8 in M sodium chloride solution ; the larger electrical repulsion in M sodium chloride solution clearly aided the dispersion. In all the experiments the second and sub- sequent compression curves gave the same results and there was good agreement between the results obtained with different samples.We assume therefore that the grain pressure was largely eliminated after the first compression. Since separation of the montmorillonite layers into basic sheets occurs the surfaces can be considered as molecularly smooth. On the basis of a sheet thickness of 8 A the low angle X-ray diffraction results agreed closely with those calculated from the surface area. The agreement was sufficiently satisfactory (table 1) to conclude that although some error is involved in the determination of the interparticle spacing, 1 4 4 FORCES BETWEEN COLLOIDAL PARTICLES this is probably small. Thus the experimental evidence suggests that the system studied involved flat plate-flat plate interactions through the liquid medium.Hence it is of considerable interest to compare the results obtained with those expected on the basis of the theory of colloid stability put forward by Deryaguin and Landau l 5 and Verwey and Overbeek.16 The theory involves a consideration of the electro- static repulsion FR and the van der Waals attraction FA acting between the plates so that the total force can be written F = FR+ FA. The force of electrostatic repulsion per cm2 of plate is given by F R = 2nkT (cosh u- l) where rz = number of ions per ml and u = ve$,,/,lkT. $ H o / 2 is the potential mid-way between the plates taking Ho as the interplate distance which can be evaluated for a particular potential on the plate $o or surface charge using the integral This integral can be evaluated from tables 14* l6 taking K = Debye-Huckel reciprocal length and z = vet,h6/kT.If the potential at any distance other than at the surface or the mid-point is given by $ then y = ve$/kT. The curves for the force of repulsion against distance of plate separation calculated for conditions of both constant potential and constant surface charge density in loF4 M sodium chloride solution are shown in fig. 5. The surface charge of the clay surface was taken to be 3.53 x lo4 e.s.u./cm2. The difference between the two curves is small except at close distances. The force of attraction between flat plates of thickness 6 is given by the expression16 A 1 1 2 6n Eli (H0+26)3 ( H + C ~ ) ~ FA = -(- + The net Hamaker constant A was calculated from A = ( J A i i - JA22)' where A l l = Hamaker constant of the particle and A22 that of the medium.A was taken as 2 . 0 ~ erg the value for silica,l' and the value for water (A2') was taken as 5 . 6 ~ 10-13 erg.18 The thickness of the montmorillonite plates was taken as 8.0A. Calculations showed that the attractive energy became very small for such thin plates at interparticle distances greater than 30A and that the effect of retardation calculated using the expression developed by Hunter l9 was negligible. The curve of total force against interparticle distance is also shown in fig. 5 which shows that the force at a distance greater than 40A arises solely from electrostatic repulsion. At a distance of ca. 158 there is a maximum in the curve and hence at shorter distances than this the van der Waals force of attraction should predominate and the plates would coagulate into a primary minimum.This however was not observed experimentally. Up to the highest pressures exerted (approximately 100 atm) it was always possible to decompress and retrace the original compression curve thus indicating the reversibility of the system. The pressure continued to increase with decreasing distance and in M sodium chloride the distance between the plates at an applied pressure of 100 atm was only 17.5 A. Thus no evidence is L . M. BARCLAY AND R . H . OTTEWILL 145 found for the primary minimum effects expected and it seems therefore that an additional force of repulsion has to be considered for such systems. Deryaguin and Greene-Kelly 2o have suggested that structural boundary layers of water 30-200 8 thick exist between the silicate layers of swollen montmorillonite and contribute to the stabilisation of the particles at close distances.van Olphen 21 and Briant 22 have also indicated that hydration forces cannot be neglected in clay systems and evidence for hydration forces in stable black soap films has been given by Goodman et aZ.23 The resemblance between the present results and those obtained by Goodman and coworkers is striking and would suggest that the effects observed at high pressures distance between plates (A) FIG. 5.-Pressure against distance between plates in M sodium chloride solution. - theoretical curve for electrical repulsion at constant potential (250 mV) ; - . - . - theoretical curve for electrical repulsion at constant charge (3.53 x lo4 e.s.u./cm*) ; . . . total force between plates assuming a plate thickness of 8 A.- - - - experimental curve in M sodium chloride solution. in view of their reversibility are due to solvation forces and not to grain pressure. An alternative possibility is that in the model of the electrostatic repulsion the electrical situation at close distances has been oversimplified. At distances greater than 50A it is reasonable to postulate that the repulsion arises solely from electrostatic repulsion ; this is in agreement with previous workers.4* 9 9 24* 2 5 9 26 In the present work the predicted values of the pressure are larger than those found experimentally in M sodium chloride solutions at distances greater than 150& but are less than those measured at distances of less than 150A. In 10-1 M sodium chloride solutions the measured pressures were larger over the whole range examined than those predicted theoretically for the same 146 FORCES BETWEEN COLLOIDAL PARTICLES distance of separation ; this is not unexpected in view of the difficulties of the theory in the more concentrated electrolyte solutions.RESULTS I N THE PRESENCE OF C12E6 The results shown in fig. 4 indicate that at an applied pressure of less than 20 atm the interparticle spacing is considerably less in the presence of CI2E6. Although electrokinetic experiments were not possible with the montmorillonite plates with silver iodide sols l2 and with polystyrene latex dispersions l1 a considerable drop in the electrokinetic potential occurs on the adsorption of C12E6. This would appreci- ably reduce the electrostatic repulsion and hence allow the plates to approach more closely.At pressures greater than 20 atm the curve is displaced to larger distances than those found in M sodium chloride solution alone and at the highest pressures there is a displacement of ca. 9A. The adsorption isotherm indicates that at the concentration of C12E6 used the non-ionic inolecule is adsorbed in a horizontal orientation on the clay surface. It would seem likely that this orientation was maintained under pressure since an analysis of the liquid expelled into the capillary for C12E6 gave the expected equilibrium concentration. The flat lying orientation of the C12E6 molecule is also in agreement with the X-ray studies of Schott l3 011 the adsorption of CI2El4 on sodium and calcium montmorillonite. His results clearly indicated a flat orientation and he suggested either that the ethylene oxide groups were bound to the silica sheets by secondary valence forces or that the poly- oxyethylene glycol chain tended to pack into the holes between the oxygen atoms of the silica surface to give a close fit.The adsorbed layer of C12E6 would reduce the van der Waals attraction between the sheets 27* 28 and also act as an effective dispersing agent for the sodium montmorillonite since ethylene oxide/water contacts are preferred to ethylene oxidelethylene oxide contacts at ambient temperatures. Under these conditions card-house floc formation would be minimized and since the electrical repulsion has also been reduced the increased gradient of the pressure against distance curve at high pressures can only be a consequence of the strong repulsive forces arising from the interaction of the hydrated layers.We thank the University of Bristol for the award of a Graduate Scholarship to L. M. B. H. Reerink and J. Th. G. Overbeek Disc. Faraday SOC. 1954 18 74. R. H. Ottewill and J. N. Shaw Disc. Faruday SOC. 1966,42 154. A. Watillon and A. M. Joseph-Petit Disc. Faraday SOC. 1966 42 143. B. P. Warkentin G. H. Bolt and R. D. Miller Soil Sci. SOC. Arner. Proc. 1957 21 495. D. Tabor J. Colloid Interface Sci. 1969 31 364. A. D. Roberts and D. Tabor Nature 1968 219 1122. R. H. Ottewill and J. A. Sirs Bull. Photometric Spectr. Group 1957 10 262. G. H. Bolt Ph.D. thesis (Cornell University 1964). ’ K. J. Mysels and M. N. Jones Disc. Faraday SOC. 1966 42 42. lo J. M. Corkill J. F. Goodman and R. H. Ottewill Trans. Faraday SOC.1961 57 1627. l 1 R. H. Ottewill and T. Walker KolloidZ. Z . Polymere 1968 227 108. l 2 K. G. Mathai and R. H. Ottewill Trans. Furahy SOC. 1966 62 750 759. l3 H. Schott Kolloid-Z. 1964 199 158. l4 H. van Olphen An Introduction to Clay CoZloid Chemistry (Interscience John Wiley. New York 1963). l5 B. V. Deryaguin and L. Landau Actaphysicochim. 1941 14 633. l6 E. J. W. Verwey and J. Th. G. Overbeek Theory of the Stability of Lyophobic Colloids l7 W. Black J. G. V. de Jongh J. Th. G. Overbeek and M. G. Sparnaay Trans. Faraday Soc. (Elsevier Amsterdam 1948). 1960 56 1597. L . M. BARCLAY AND R. H . OTTEWILL l 8 H. R. Kruyt Colloid. Science (Elsevier Amsterdam) vol. 1 1952. l9 R. J. Hunter Austral. J. Chem. 1963 16 774. 2o B. V. Deryaguin and R. Greene-Kelly Trans. Faraday Soc. 1964 60,449. 21 H. van Olphen T.A.P.P.I. 1968 51 145A. 22 J. Briant Compt. rend. IZP Colloque I’A.R.T.F.P. 1968 p. 31. 23 J. S. Clunie J. F. Goodman and P. C. Symons Nature 1967 216 1203. 24 G. H. Bolt and R. D. Miller Soil Sci. SOC. Amer. Proc. 1955 19 285. 2 5 B. P. Warkentin and R. K. Schofield J. Soil. Sci. 1962 13,98. 26 R. Yong L. 0. Taylor and B. P. Warkentin Clays Clay Min. 1963 13,268. 27 M. J. Vold J. Colloid. Sci. 1961 16 1. 28 R. H. Ottewill Nonionic Surfactants ed. M. J. Schick (Marcell Dekker) 1967 1 627. 147

ISSN:0370-9302    

DOI:10.1039/SD9700100138

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Boundary Layers Between Silver Iodide and Aqueous Solutions at Low Temperatures BY B. VINCENT * AND J. LYKLEMA Laboratory for Physical and Colloid Chemistry Agricultural University de Dreijen 6 Wageningen The Netherlands Received 14th July 1970 Potentiometric titration studies have been made on AgI suspensions over the temperature range 0-20°C in the presence of LiN03 KN03 and RbN03. Points-of-zero-charge (P.z.c.) double-layer capacities and surface entropy data derived from the titration results indicate an increase in water " structure " as the temperature tends to O'C around the P.Z.C. region and particularly on the positive side. For strongly negativeIy charged surfaces the charge ordering effect on the dipoles tends to break down this structuring. The results of flocculation studies on AgI sols have been combined with the titration data to yield information about changes in double layer parameters with temperature.The results fit the suggestions made regarding changes in water structuring at the interface. During the last decades considerable progress has been made in the field of inter- facial electrochemistry especially with mercury electrodes. One of the important results has been that structural features of the liquid adjacent to the electrode e.g. the direction of orientation of water dipoles are reflected in measurable electro- chemical quantities such as the differential double-layer capacitance and the point of zero charge (P.z.c.). Hence inference on the structural properties of the interphase can be derived from these and other electrochemical measurements.Studies with non-metallic charge carriers are more scanty and as a rule less accurate but they have the advantage that for certain systems stable dispersions can be made. In those cases the double-layer measurements may be amplified by stability studies. The AgT-system has received special attention because stable AgT sols can be made as well as reproducibly operating AgI electrodes. Moreover its interfacial structural properties are of interest because AgI is a powerful cloud seeder. Some preliminary information on the boundary layer structure of this system has now been obtained by combining double-layer and stability studies as a function of temperature.' In view of the expectation that structure formation in the interface would be promoted by a decrease in temperature these measurements have now been extended to cover the temperature range down to 0°C.EXPERIMENTAL MATERIALS AgI suspensions were prepared by addition of 0.1 M AgNO to an equal volume of Surface areas were determined for each suspension used by * present address Dept. of Physical Chemistry University of Bristol Cantock's Close Bristol 0.1 M KI in the usual way.* BS8 1TS ; or I.C.I. Paints Division Ltd. Wexham Road Slough Bucks. 148 B . VINCENT AND J . LYKLEMA 149 comparison of surface charge values in C g-l from the experimental titration curve in lo-' M KNO, with surface charge values in pC cm-2 from standard curves for this system. Most of the experiments reported here were carried out on a suspension of surface area 1.34 m2 g-l. Sols were prepared by the similar addition of 2 .0 ~ M KI. Both suspensions and sols were aged at 80°C for 3 days. All salts used were of A.R. quality AgN03 KI LiNO, Union Chimique Belge ; KN03 Baker ; RbNO, Merck. The RbN0 was recrystallized twice from water. All the other salts were used as supplied. Water was doubly-distilled and passed down a AgI column before use. M AgNO to 2 . 2 ~ POTENTIOMETRIC TITRATIONS The potentiometric titration technique by which (surface charge pAg) or (surface charge PI) curves may be obtained for aqueous AgI suspensions has been extensively disc~ssed.~9 The apparatus used here was essentially similar except that the titration cell was constructed so that it could be completely immersed in a cryostat bath (Colora Ultra Cryostat KT 40 S). In this way the temperature of the cell was controlled to fO.l"C over a range 0-20°C.Potential measurements (f0.2 mV) were made using a potentiometer (Knick Praisiziono pH Metre type pH 34). The average reading of 4 or 5 Ag/AgI electrodes was taken. Calibration was carried out using standard solutions at PI 4.0 5.0 and pAg 4.0 5.0. The cell resistance was also checked periodically during a run (Phillips Bridge Gh 4249) to ensure that there was no blocking of the salt bridge capillary by AgI particles. Titration runs were performed in the following manner. A sample of concentrated suspension of known composition was weighed in a graduated flask. This gave the weight of AgI without resort to drying. The suspension was washed into the titration vessel with lo- M salt solution. A set of titrations was then carried out in the following order (1) 20"C+ -+ -addition of KI ; (2) 10°C- -++addition of AgN03 ; (3) O"C+ -+ -addition of KI ; (4) 20°C-+addition of AgN0,.At this point further inert electrolyte was added and the sequence repeated. STABILITY MEASUREMENTS These were performed using the " kinetic " method of Reerink and Overbeek.6 The change in optical density (at A = 556 nm) of the flocculating system was followed using a Vitatron spectrophotometer (Vitatron N.V. Dieren The Netherlands). This instrument incorporates a magnetic stirrer device. The output was fed to a Kipp Micrograph pen- recorder (Kipp N.V. Delft The Netherlands). The spectrophotometer cell-housing was thermostatted by pumping cooling liquid from the cryostat around its jacket. Temperature control was f0.5"C but at ambient temperature differentials of more than about 5°C it was necessary to enclose the apparatus within a dry-box to prevent condensation on the optical cell.The flocculation runs were carried out in cylindrical optical cells (0.9 ern diam.). 0.5 nil sol (AgI concentration 10 mM) were added with the aid of a syringe to 2 ml of salt solution in the cell which also contained a small glass-covered magnetic stirrer. Both the sols and salt solutions were adjusted to the necessary PI and stored at the required tempera- ture prior to use. RESULTS AND DISCUSSION SOLUBILITY PRODUCT L AND POINT OF ZERO CHARGE (P.z.c.) OF AgI pL values for AgI over the temperature range studied and at salt concentrations 10-3-100 M were calculated from the standard electrode potentials. These are shown in fig. 1 together with the values obtained by other auth0rs.l.There is an apparent continuous increase in the solubility of AgI with increase in temperature and with increase in inert electrolyte concentration. 150 AgI AT LOW TEMPERATURES The P.Z.C. (pAg") values (see table 1) was taken for each system as the mutual intersection point of the experimental "charge " (k charge with respect to an arbitrarily chosen reference point) against pAg curves. This point was generally well defined to within 0.2 pAg units except that the 100 M curves tcnded to cross the oo = 0 line at slightly higher pAg value. 1; I I d I I I 1 I I \ 10 2 0 3 0 4 0 5 0 6 0 temperature "C FIG. 1.-Solubility product (expressed as pL) of AgI as a function of temperature. I Lyklema,' lo-' M (with M ; 111 this work M K biphthalate present) ; 11 Honig and Hengst,' (a) 10-3 M (b) 10-1 M (c) 100 M.TABLE 1.-pAg VALUES FOR AgI authors ref. method salt ooc 10°C 2O0C 1 this work titration LiN03 7.10 6.40 5.80 KNO 6.95 6.20 5.65 2 Honig and Hengst 7 suspension effect KN03 5.51 5.51 5.52 3 Fairhurst 8 electrophoresis KNO - 5.54 5.40 RbN03 6.75 6.10 5.50 The three sets of data show apparent discrepancies. This work indicates a significant rise in pAg" with decreasing temperature. Those of Honig and Fairhurst are virtually independent of temperature over this range. Titration results however yield true P.Z.C. values whereas the suspension effect and electrophoresis give iso- electric points (i.e.p.). At higher temperatures the P.Z.C. values of Lyklema and the i.e.p. of Honig again diverge (e.g.,at 60°C; P.Z.C. = 5.45; i.e.p. = 4.38).* The were made in the presence of K-biphthalate and biphthalate ions tend to absorb specifically at the P.Z.C.which would in turn tend to move the P.Z.C. towards a higher pAg" value (e.g. at 20°C P.Z.C. = 5.72). This specific adsorption would presumably however decrease with increasing temperature whereas the divergence between i.e.p. and P.Z.C. increases. * Lyklema's measurements B . VINCENT A N D J . LYKLEMA 151 difference between the P.Z.C. and the i.e.p. appears to be minimal at about 20°C. As the solubility product itself shows no break around 20°C slructural changes in the solid Agl can hardly be invoked to explain the observed behaviour. A tentative explanation for the increase in pAg" on going from 20 to O"C as observed by us is as follows. At 20°C the P.Z.C.is very asymmetrical pAg" = 5.6 PI" = 10.6 pL (= pAg"+pI") = 16.2. Much less iodide is needed than silver to make the surface uncharged. At the same time the adsorption of Ag+ at the positive side of the P.Z.C. is very strong whereas positive AgI sols are rather unstable. One way of explaining these facts is to assume that at 20"CAgf absorbs largely in an associated form i.e. as AgN03. In the first place this would support the result that only relatively small amounts of I- are required to make the surface negative ; in the second place it explains the high adsorption at the positive side without concomitant rise in potential. At 0°C the P.Z.C. is less asymmetrical pAg" = 6.9 PI" = 10.5 pL = 17.4. At the same time the adsorbability of Ag+ at the positive side is reduced (see titrations).The implication is that at 0°C these AgN03 ion pairs are more dissociated than at 20°C. Perhaps therefore due to an increase in water " structuring " at the interface the NO; ions are less easily adsorbed in the Stern layer. This explanation is corroborated by the surface excess entropy calculations (see later). POTENTIOMETRIC TITRATION RESULTS The titration curves for LiN03 KN03 and RbN03 are given in fig. 2 3 and 4 respectively. At low inert electrolyte concentrations ( lov3 M,) (do,/dT),, is negative. This is expected since here the diffuse layer term dominates the total double-layer capacity. There is a tendency however noticeable at low negative surface charges and particularly so on the positive side for (do,/dT),, to reverse sign in 10-1 and 1 M salt. By comparison with the curve at higher temperatures,l (do/dT),A becomes zero around 20"C i.e.the same region where the anomalous increase in pAg" begins. The differential double layer capacity values at the P.Z.C. are presented in table 2. TABLE 2.-DIFFERENTIAL DOUBLE LAYER CAPACITIES AT P.Z.C. (pF/cm2) 20°C 10°C 0°C i.e.c. LiNO3 KN03 RbN03 LiNO3 KNO3 RbNO3 LiNO3 KNO3 RbN03 loo M 18.0 24.8 30.0 1 9 . 2 23.5 29.0 17.7 20.5 22.5 lo-' M 1 5 . 0 17.5 23.0 16.6 17.0 1 8 . 0 14.0 16.0 16.0 M 9.5 10.2 11.6 10.7 11.5 12.5 9.5 10.0 10.0 M 5.4 6.0 6.2 6.0 6.0 6.0 6.6 6.6 6.6 The apparent specificity with regard to the nature of the cation particularly at higher salt concentrations where the capacity is dominated by the Stern layer term increasing in the order Li+<K+<Rb+ indicates some absorption of cations at the P.Z.C.as well as anions. This is also reflected in the slight dependency of the P.Z.C. on the nature of the cation. The decrease in capacity with lowering of temperature particularly from 10 to O"C again points to desorption of NO;-ions. (Since the decrease is relatively smaller for LiN03 this might indicate some desorption of the strongly hydrated Lif ions also). Desorption of NO ions on lowering the temperature from 20 to 0°C was also indicated on the positive side of the zero point of charge from a preliminary components-of-charge analysis of the type suggested by Lyklema. 152 AgI AT LOW TEMPERATURES - 5 -4 n N - I FIG. 2.-Surface chargepotential curves for AgI in the presence of various concentrations of LiN03 presence of various concentrations of LiN03 A 10" M ; B 10-1 M ; C M.- 20°C ; -- 10°C ; . . . 0°C. M ; D (E- Ep.z.c.)/mV FIG. 3.-Surface charge-potential curves for AgI in the presence of various concentrations of KN03 A 100 M ; B 10-1 M ; c 10-2 M ; D 10-3 M. - 2 0 0 ~ ; -- iooc; ... ooc. The differential capacity is much less temperature dependent at high negative surface charge when any water " structuring " would be opposed by the orientating effect on the water dipoles (e.g. in 10-1 M RbN03 +o = -250 mV C = 12.8 (20°C) 13.0 (lO°C) 13.2 (OOC) ,uF/cm2). INTERFACIAL ENTROPIES The derivative of the excess entropy of the AgT/solution interface with respect to the surface potential $o can be calculated from the temperature dependence of the B . VINCENT AND J . LYKLEMA 153 surface charge using either one of two equations derived previously by Bijsterbosch and Lyklema.O (af/a$O)T',as = -s"(CI-/F) + ( a a ~ / a T ) s p A g a s - ( a ~ O / a ~ s ) p * g T ( - s ~ + In - (C/F)(-si,+ +R ln a&+) (1) ( a f / a $ 0 ) T a s = sa(CAg+IF) +(a~O/aT)pI,asf (aaO/a&)pI,T(-s,"+ In (C/F)(-s;- + R In a*-) (2) In these equations the interfacial excess entropy q" is defined by where s denotes molar entropies the superscipts a and p apply to the solid (AgI) and liquid (W) phase respectively and subscript s refers to the salt. CI- and CAg+ are the contributions of the iodide and silver ions respectively to the total differential capacitance C. The way in which one splits C up into its component parts is immaterial. O Eqn (1) and (2) are equivalent the only difference being that in (1) the Ag+ ion is taken to be the potential-determining species whereas in (2) it is the I- ion.The equivalence of the two equations was corroborated by the computations although individual terms differed sometimes by a hundredfold the final values for (af/a$o)T,as agreed within a few percent. The general behaviour of (af/i?$o)T,a is to a large extent determined by the (dao/dT)pAg,as term. Further details on the method of computation are given in ref. (10). '1" = s' - saTAgI - SBrw S" = ( S - S" - SB)/A (3) (4) - 5 - 4 n $ - 3 Y W 1 g -2 - I / .' / I + 2 A -100 - 2 0 0 - 3 0 0 FIG. 4.-Surface charge-potential curves for AgI in the presence of various concentrations of RbN03 A 100 M; B 10-1 M; c 10-2 M; D 10-3 M. - 2 0 0 ~ ; -- iooc; ... ooc. The values obtained for (~r,f'/a$o),,,s were plotted as a function of $o graphically integrated and finally plotted as a function of a.using the P.Z.C. as the reference point. The results are given in fig. 5 together with some results obtained previously at higher temperatures in the presence of lQ-3 M K biphthalate.'O The data shown 154 AgI AT LOW TEMPERATURES are for LiN03 since this would be expected to reveal structural changes in the inter- facial water more strongly than KN03 or RbN03 because the masking effect of specific adsorption on the negative side is least for the Li+ ion. (The curves for KN03 and RbN03 are in fact similar but the trends are less marked.) b F -. -_ - - - - -_____ '. . o ;5Oc 8 5 c - 0 . 5 00 pWm2 FIG. 5.-Tnterfacial excess entropy with respect to the point of zero charge as a function of surface charge for decimolar solutions of LiN03 - current work ; -- previous work lo in which K-biphthalate was also present.Interfacial entropies are composite and complex quantities. One should therefore not over-interpret the data. Nevertheless some interesting trends seem to emerge. First even around 20°C the variation of y" with o0 is more than tenfold the amount that could be accounted for by the adsorption entropies of counterions calculated from their specific adsorption energies. Its order of magnitude suggests changes in interfacial water structure. A remarkable feature is that at low temperatures y' increases with increasing negative surface charge whereas at higher temperatures the trend is the reverse. At high temperatures where hydrogen-bonded structuring will be weak this has been interpreted lo as being due to structure promotion resulting from the increased polarizing action of the field as the surface is made more negative.This is to be combined with structure breaking effects of the NO -counter-ions adsorbed strongly at the positive side. At low temperatures the situation is reversed. The decrease in entropy with increasing positive charge in combination with the marked decrease in capacitance suggest again some expulsion of NOT-ions with an increasing hydrogen-bonded structure formation. This is in agreement with the interpretation offered for the shift in the P.Z.C. Given the observed trends it follows that at a temperature around or slightly above 20°C a transition must take place where y" is largely independent of go. (The exact temperature cannot be deduced from the data given because the high temperature curves are obtained in the presence of biphthalate).At this temperature with increas- ing negative charge the two opposing trends i.e. the increase in dipole ordering and decrease in water structuring would tend to cancel each other. STABILITY The stability of a hydrophobic sol measured as the flocculation concentration (cf in mmol/l.) reflects the charge and potential distribution in the electrical double B . VINCENT AND J . LYKLEMA 155 layer. As a consequence stability data can serve as an additional source of informa- tion. However its applicability with respect to the detection of structure formation is limited because stable sols can only be made at sufficient high surface charge i.e. in the region where any effect of structure formation is relatively weak.There are two ways in which structure formation would be expected to affect cf (i) electrostatically because if there is structure formation the extent of counterion adsorption and hence the potential of the outer Helmholtz phase t+hd is changed; (ii) kinetically because the rate of counterion transfer from the diffuse part of the double layer to the Stern layer during the encounter of the particles as required by DLVO-theory is reduced. In fig. 6 and 7 flocculation concentrations under various conditions and the corresponding double layer parameters are shown. The flocculation concentration apparently decreases with decreasing temperature at PI 4.6 but the opposite is true at PI 6.6. The surface charge shows a slight maximum at around 10°C for all three cations at both PI.The trends in cf and o0 are not parallel. This is not expected a priori since cf reflects the charge distribution whereas go is the total charge. In order to analyze these data further Stern potentials specific adsorption inner layer capacities and dielectric constants have been calculated using an analysis to be discussed elsewhere,ll but which may be summarized briefly as follows. From the flocculation data and an ionic components of charge analysis on well-established potentiometric titration data for KN03 at 20°C a value of A = 2 . 4 0 ~ J was established for the Hamaker constant for AgI in aqueous systems. This value was then used to calculate the Stern potentials and hence the other double-layer parameters from the flocculation concentrations and titration data.2 i- I I I 0 10 2 0 temperature "C FIG. 6.-Experimental parameters under various conditions (a) flocculation concentrations ; (b) surface charge. counter ion type LiN03 -; KN03 ... ; RnN03 .... PI 4.6( x) ; 6.6(0). The specific adsorption curves (expressed as a percentage of the surface charge) indicate little or no dependence on temperature at PI 4.6 but at PI 6.6 there appears to be a definite fall-off in specific adsorption of cations on lowering the temperature from 20 to 0°C. Thus at highly negative surface charges (as at PI 4.6) the specific adsorption is largely governed by electrostatic forces and is therefore largely tempera- ture independent over this range but at lesser negative surface charges (as at PI 6.6) there is at least some effect from water structuring leading to some desorption of counter ions on lowering the temperature.The Stern-layer capacity falls slightly with increasing temperature and is in the order Rb+>K+>Li+ as expected but is also in the order PI 6 . 6 ~ ~ 1 4.6. Since specific adsorption increases with decreasing PI and there are probably no significant changes in inner layer thickness this could reflect changes in the inner layer dielectric 156 AgI AT LOW TEMPERATURES constant. (At such strong specific adsorptions (-50 %) the occupancy of the Stern layer is such that the polarizabilities of the adsorbed ions themselves probably contribute significantly to the dielectric constant of the inner layer.) The mean value at 20°C 7.5& 1.5 corresponds well with the value of about 6 quoted for the inner layer at the Hg+water intex-face.12 The mean value rises to about 8.0+ 1.5 at O'C which again supports the idea of increased hydrogen-bonded structuring of the inter- facial water although only slight at these negative charges.22 r 7Ol- ................. x.. ....................... x 0 ..................... ...... .... __- - 40 19- '.. -. (0) I 10 0 2 0 10 It3 2 0 0 20- - 10 ........ .... ........ ....... .... ......... . . . . . . . 9 - .... ......... ..... .... .o ... 0 '0 I& " ' >----- 'o-----_o ' I ' ' ' . . ""0 --A- -_____ -x X - I I (j. (d) 3 0 12- 2 0 10 2 0 0 10 ternperaturel'c FIG. 7.-Derived double layer parameters at the flocculation concentration (a) Stern potential ; (b) specific adsorption (expressed as a percentage of surface charge) ; (c) Stern-layer capacity ; (d) Stern-layer dielectric constant.Counter-ion type LiN03 -; KN03 ... ; RbN03 .... PI 4.6 ( X) ; 6.6 (0). CONCLUSIONS It is generally observed that adsorption increases with decreasing temperature. For the adsorption of I-ions on AgI this trend was c0nfirmed.l The more detailed study of the adsorption of potential determining ions reported in this paper shows that this trend is not followed at temperatures between 0 and 20°C. At the negative side of the P.Z.C. the adsorbed amount (and hence the surface charge a,) is almost independent of temperature whereas on the positive side a. tends to decrease with decreasing temperature. The effect is that of increased structure formation in the aqueous layer adjacent to the AgI surface at less negative or more positive charges together with consequent desorption of NOT-ions.It is fully corroborated by surface excess entropy calculations. Flocculation experiments are less informative because only data at high negative surface charges are available. In this region the structure formation effect is less pronounced. However the stability trends observed at lower negative surface charge agree well with the theory suggested above. There remains the discrepancy between p.z.c.-values from titration and i.e.p.-values from the suspension effect and B . VINCENT AND J . LYKLEMA 157 electrophoresis. The experimental data seem to be well established. Unfortunately we cannot offer a satisfactory explanation for this. We thank the Royal Society for the provision of an Overseas Fellowship to one of us (B.V.) and also thank Miss Olga van Hiele for technical assistance. ' J. Lyklema Disc. Faraday SOC. 1966 42 81. see e.g. B. H. Bijsterbosch and J. Lyklema J . Colloid Sci. 1965 20 665. Int. Crit. Tables to be published. J. Lyklema and J. Th. Overbeek J . Colloid Sci. 1961 16 595. G. L. Mackor Rec. Trav. Chim. 1951 70,763. H. Reerink and J. Th. Overbeek Disc. Farduy SOC. 1954 IS 74. ' E. P. Honig and J. H. Th. Hengst J. Colloid Interface Sci. 1969 30 109. ' A. L. Smith private communication of unpublished work by D. Fairhurst. l o B. H. Bijsterbosch and J. Lyklema J. Colloid Interface Sci. 1968 28 506. l 2 J. O'M. Bockris M. A. V. Devanathan and K. Muller Proc. Roy. SOC. A 1963 274 55. J. Lyklema Trans. Faraday SOC. 1963 59,418. B. Vincent B. H. Bijsterbosch and J. Lyklema to be published.

ISSN:0370-9302    

DOI:10.1039/SD9700100148

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Boundary Layer near the Surface of a Solid Body and Low- Frequency Dielectric Dispersion BY S. S . DUKHIN Institute of Colloid and Water Chemistry Academy of Sciences of the Ukrainian S.S.R. Kiev U.S.S.R. Received 19th March 1970 The paper deals with the possibility of studying the stagnant part of the boundary layer on the basis of effects due to polarization of the electrochemical double layer (DL). A theory is developed for polarization of the diffuse part of a thin DL of a spherical particle in a direct and alternating field allowing for the effect of ion flow through the stagnant part of the DL. Formulae are obtained on the basis of this theory for electrophoresis complicated by polarization and for the large low- frequency dielectric dispersion E(o). Calculation of the stagnant layer thickness and the ionic mobilities of this layer is shown to be possible on the basis of electrophoretic measurement on three fractions of spherical particles with identical values of $d and Z,.Using only one fraction we can obtain similar information from the low-frequency section of E(o). The advisability of investigating electrokinetic and related electro-surface pheno- mena in disperse systems in connection with the boundary layer problem has been acknowledged and substantiated by systematic studies 2-4 which have already yielded valuable results. We shall consider certain new means for studying the boundary layer by electric methods. Since terminology in this new field of research has not yet been established we shall adhere to Deryaguin’s model-system ideas and terminology.We shall call a boundary layer one in which anomaly of structure and the structural-sensitive properties of the liquid are observed The fact that within a layer of some thickness the liquid loses fluidity and does not participate in hydrodynamic processes is only one of many manifestations This layer is called stagnant. Such effects as thermo-osmosis and capillary osmosis indicate that the thickness of the boundary layer may exceed that of the stagnant layer. It is natural to compare the thickness of the stagnant layer with the distance from the surface to the slip plane a conception based on electrokinetic phenomena. The stagnant layer thickness may then be estimated by the difference in the values of the Stern $d and electrokinetic potentials. Simultaneous determination of the ( and $* potentials have been made for an oil-water i n t e r f a ~ e ~ with an absorption monolayer of ionogenic surfactant and for spherical micelles.lo In both cases the slip plane coincides with the surface enveloping the hydrated charges of the long- chain ions.This does not however affect the possibility of their being a stagnant polymolecular layer of polar liquid on a solid surface. Some difference between the [ and $d potentials may be due to the viscoelectric effect l1 and the roughness of the surface.12 Later research,l** l 3 however indicates that the viscoelectric constant was overestimated by more than one order in ref. (1 1). Hence with moderate values of t,bd and of the ionic force the viscoelectric effect of this anomaly of liquid structure. 158 S . S . DUKHIN 159 cannot cause a perceptible difference between the t,bd and c potentials and hinder the estimation of the stagnant layer thickness.Since the time of Freundlich who introduced the concept of the stagnant layer it was believed that within this layer the ions as well as the liquid were immobile so that only the mobile part of the diffuse layer contributes to the surface conduct- ance. This assumption is part of Bikerman’s l4 surface conductance theory and the Overbeek-Booth double-layer polarization theory and has persisted up to recently.16 There are no grounds for this assumption and it should be discarded. It has long been noted that normal values of ionic mobilities are retained in hydro- dynamically immobile ge1s.l’ The physical meaning of viscosity differs * depending on whether we consider the macroscopic flow of the system or the movement of small molecular particles through the same medium.The structural frame of the polymer excludes the possibility of macroscopic movement of liquid but does not hinder the thermal motion of the liquid molecules and ions,l* and consequently cannot prevent ion migration in an electric field. Similarly structure formation of a polar liquid under the effect of a surface gives rise to a hydrodynamically stagnant layer. But the intensive thermal motion of the liquid and the ion is maintained as evidenced from nuclear resonance data. Accordingly all diffuse layer ions are to be considered mobile and we shall call the diffuse-layer charge the mobile charge in distinction to the electrokinetic charge which is only part of the mobile charge.Simultaneous determination of the mobile and electrokinetic charges yields information about the ion content in the stagnant layer from which the difference between the I)d and 5 potentials and the stagnant layer thickness may be calculated with the aid of the Gouy-Chapman theory. Information about the mobile charge can be obtained from surface conductance measurements but the question then arises as to the possible deviation from the normal of ionic mobilities in the stagnant layer. The difference between the mobile and electrokinetic charges may also be due to surface roughness. This difference should not however be sensitive to temperature variations as is the case in structure formation of the liquid which determines the stagnant layer thickness. Fridrikhsberg measured three magnitudes characterizing the electric properties of BaSO microcrystal surfaces the surface conductance streaming potential and ionic adsorption.Comparing the results he concluded that the tangential electro- migration ionic currents permeate the stagnant layer as well as the mobile part of the double layer that the magnitude of ionic mobility in it is close to the bulk magnitudes of the mobilities and its thickness is approximately 15 A. The precision of Fridrikhsberg’s method of composite electro-surface measure- ments is decreased because capillary-porous systems were used. It is difficult to substantiate rigorously the possibility of calculating the specific surface conductance 7ca and the [ potential from macroscopic measurements on capillary-porous systems.Moreover when the contribution of the surface conductance to total conductance of the system becomes perceptible a necessary condition for measuring IP there is considerable double-layer polarization a substantial effect of which cannot be allowed for practically. However the effect of double-layer polarization which complicates investigation of the electrokinetic properties of capillary-porous system affords new opportunities for studying the stagnant layer in dilute dispersions. Development along these lines was apparently retarded because the cumbersome mathematical apparatus in the Overbeek-Booth theory of double-layer polarization of spherical particles made it impossible to take into account the difference between the I)d and c potentials mani- fested in the effect of the ionic stream through the stagnant layer on polarization and 160 BOUNDARY LAYERS AND DIELECTRIC DISPERSION electrophoretic movement.The mathematics of double-layer polarization of particles of regular shape has recently been considerably simplified 20* 21 in the important special case of a thin double layer. The new mathematical technique permits extension of the theory and in particular to base theory on various models of the colloid micelle taking into account the difference between the $d and 5 potentials. Under the effect of an external electric field tangential flows of diffuse layer ions arise which redistribute them over the surface; the double layer is deformed and polarized and thus departs from the original spherical symmetrical structure. Steady tangential flows of double-layer ions are maintained through ion exchange with the contiguous volume of electrolyte.The steady exchange of cations and anions is possible only when concentration differences arising beyond the double layer as well. A change in electrolyte concentration along the outer double-layer boundary causes a change in its thickness the latter becoming less when the electrolyte concentration is higher and increases with a decrease in electrolyte concentration. An essential simplification in thin double-layer theory2 is that although the double layer as a whole is in a non-equilibrium state equilibrium is maintained locally between the given double-layer area and the contiguous volume of electrolyte. For each double layer area i.e. at fixed 8 Boltzmann's formula retains its validity connecting the ionic concentration inside the double layer C* (X$) with the concentra- tion at its outer boundary c'(8) and with the change in potential across the double layer 4( X,8) where 8 is the angle with the external field E X is the distance to the surface Z* is the electrovalence.The deviation of 4(X,8) from the initial spherically symmetrical 4*(X) caused by the concentration change c*(O) can be expressed in general form through c*(O) on the basis of the local equilibrium between the given double-layer area and the contiguous volume of electrolyte. Functions are derived in ref. (20)-(21) describing the spatial potential and charge distribution in the polarized double layer and the contiguous electrolyte volume involved in the process of exchange with the double layer. This yielded a formula 22 for electrophoresis velocity complicated by double- layer polarization U,.We present the formula for dimensionless electrophoretic mobility in a symmetrical electrolyte z+ = z- = z = 1 C*(X,8) = c*(@ exp (Tzfe4(X,8)/kT) (1) 3&2 + 6m) sinh2 (f/4) + 41 + 6 In cosh (5"/4)[(2 + 6m) sinh (i"/2) - 3mf+ 2g] - - > (2) rca + (8 + 24m) sinh2 ([/4) - 24m In cosh (&I) + 4q where q = p(cosh@,/2) -cosh(i/2)) g = p(sinh(&,/2) - sinh(i/2)) &d = (e$d/kT) = (ec/kT>* ql and are the viscosity and dielectric constant of the liquid p is the ratio of the diffusion coefficients in the stagnant layer and in the bulk m is the dimensionless parameter given in ref. (15) (20) a is the radius of the sphere zc-I is the double layer thickness. The second term always negative characterizes the decrease in velocity due to polarization.Factors q and g characterize the effect of ionic currents through the S . S . DUKHIN 161 stagnant layer on polarization and electrophoresis. If the electrophoretic movement is measured on three fractions of spherical particles with surfaces of identical nature (i.e. 5 and $d is the same for all three fractions and Ica is known) we obtain three equations with three unknown from which we determine ( $d andp. Since with double-layer polarization the particle acquires an induced dipole moment the dielectric constant of the suspension should differ from that of the medium not only because of the trivial effect due to the difference between the dielectric constants of the medium and the particle. If the frequency of the alternating field is not great so that local equilibrium can be established between the given double layer area and the contiguous electrolyte volume the polarization mechanism in the alternating field is the same as in the static which enabled the author together with Shilov 23 to extend the thin double-layer polarization theory to the case of an alter- nating field of moderate frequency.Polarization of the particle proved to be associated with the ion concentration decrease along the outer double-layer boundary which varies periodically i n time with a certain lag behind the imposed field. This lag in phase is mathematically expressed in the fact that polarizability i.e. the ratio of the dipole moment to the external field cP is a complex variable if the time dependence of the alternating field is described by the exponent of the complex argument .Owing to the presence of conductance the continuous phase (electrolyte) is also polarized with a perceptible lag in phase so that its dielectric constant is also complex E = c1 -i4nK,/o being proportional to the conductance of the continuous phase K1. The dielectric increment caused by introduction into the continuous phase of colloidal particles with volume-ratio n may be represented by means of the Maxwell- Wagner theory 24 On multiplying Im EY; by Im a* a component of Re AE* of the suspension arises which increases considerably with decrease in m since the latter is accompanied by an unlimited increase in Im 8:. The complicated formula presented in ref. (25) for the static dielectric increment Re A$ i.e.Re AE* with m+O is simplified for highly charged particles when exp ( $ d / 2 ) 9 1 AE* = 3n~Ta". (3) A huge rise in Re AC-* was observed in the low-frequency band by Schwann et aZ.,26 working with monodisperse suspensions of spherical polystyrene particles in an aqueous solution of KCl. For the special case of a = 0.094pm n = 0.3 the experimentally found relationships Re E*(w) (curve 1) and Im E*(m) (curve 2) are presented in fig. 1. Assuming $d = c p = 1 Ica = 60 (in accordance with the electrolyte conductance given in ref. (26) and using the mcasured value of Re E*(m) at co-+O equal to 2370 we arrived at the conclusion that $d = 3.5 for the particles. Accepting this value of t j d we then calculated Im ?(a) (curve 3). A certain dis- crepancy between the experimental and theoretical curves of Im E*(m) may be due to the fact that at the volume-ratio of the particles used in the experiment their diffusion atmospheres greatly overlapped a complication which cannot be readily taken into consideration in the theory and hence was neglected.For the interpretation of their experimental data Schwann and collaborators used Schwarz's hypothesis 24 about the peculiar behaviour of counterions which they called " bound ". These ions are said to be capable of redistribution under the effect SPI-F 162 BOUNDARY LAYERS AND DIELECTRlC DlSPERSlON of the field along the particle surface without moving away from it i.e. the possibility of ion exchange with the disperse medium is excluded. Such behaviour of the double- layer ions has not yet been explained on double-layer theory; moreover in later papers the investigators note the inadmissibility of such an idealization.28 The double-layer model accepted by Schwarz assumes that the outer part of the double 2 0 0 0 1000 layer is formed by bound Within the frame work of mental results.- - - 3 0 0 0 1200 I ions only and there is accordingly no diffusion layer. this model Schwarz satisfactorily interprets the experi- 8 0 0 6 00 2 0 0 I 1 I I I I 0 I 10 100 I000 frequency (kHz) FIG. 1 .-Dispersion of dielectric constant of dispersion polystyrene spherical particles ; experimental data 26 and theory of polarization of diffused part of double layer 2 5 ; 1 and 2 Re Ai* and Im AE* from data of ref. (26) ; 3 Im AE* theoretical curve.25 Even if there is no diffuse atmosphere in an equilibrium double layer as assumed by Schwarz on polarization of the bound counterion layer a diffuse atmosphere appears locally compensating the polarization charge of the bound ions.Since Schwarz did not have at his disposal a theory of the polarization of the diffuse part of the double layer his theory did not take into account the potential jump in this diffuse atmosphere and its effect on the tangential transfer of the bound ions. The correction of this error in a paper by Shilov and the author 2 5 dealing with Schwarz’s model indicates that in Schwarz’s theory Re AE*(co) is over-estimated by more than one order i.e. there can be no agreement between Schwarz’s theory and Schwann’s experiment. Ions bound according to Schwarz’s theory affect A P but cannot contribute to 5. Hence experimental proof of the equality of [ and t,bd potentials by the electrophoresis data and dielectric measurements is at the same time proof of the non-existence of Schwarz’s bound ions.Chelidze 29 measured U and i*(co) for almost nionodispersed diluted latex suspensions of nairite (a = 0.13 pm 7ca - 15) and chloroprene (a = 0.45 pm ica = 60) in aqueous solutions obtaining values of 3400 and 1400 for Re A$/n. According to these experimental data cSM calculated according to Smoluchowski’s formula equals 2.3 ; 4 and T calculated from the solution of the system of equations (2) and (4) equal 3.5 and 3.5. For chloroprene SSM = 2.5 qd = 3.2 f = 3. S . S . DUKHIN 163 The difference between $d and c, and the agreement within limits of experimental error of $d and 5 calculated from eqn (2) and (4) corroborate the correctness of the theory of double-layer polarization in a direct and alternating field its effect on electrophoresis and Ai?.Along with the difference of more than one order between the experimental values 26 of Re A$ and those calculated from the refined formula for A 2 derived for the Schwarz Chelidze’s experiments 29 indicate that Schwarz’s model does not agree with the facts at least with respect to the investigated systems and that AE*(u) measurements constitute a promising niethod for measuring the $d potential. In an experimental verification of the DLVO theory 13* 30 or the electroviscous effect on monodisperse suspensions it is advisable to measure the large low-frequency dielectric dispersion rather than electrophoresis. In addition agreement of 5 and $d indicates that in the investigated systems boundary layers are either absent or so thin as compared to K - ~ that they cannot be detected with the present experimental precision.To secure detection and measurement of the boundary layer thickness by the recommended method latices should be used for which the presence of boundary layers is more probable (those studied in ref. (5) for instance) and I C - ~ should be decreased by raising the electrolyte concentration. If not only Re A$ but the entire low-frequency section of the dispersion curve is used it is possible to determine the change in ionic mobilities in the boundary layer along with 5 and $d. J. Th. G. Overbeek Pure Appl. Chem. 1965 10 359. D. A. Fridrikhsberg and V. Y . Barkovsky Kolloid Zhur. 1964 26 722. N. Bondarenko S.Nerpin Bulletin RILEM 1964 29 13 ; N. Bondarenko S. Nerpin Ztzt. Congr. Pure Appl. Chem. (Moscow 1965) thesis A and B. S. V. Nerpin and A. F. Chudnovsky Soil Phys. (Nauka Moskow 1967) chap. 13. G. A. Johnson S. M. A. Lecchini E. Y. Smith J. Clifford and B. A. Pethica Disc. Faraday Soc. 1966 42 120; B. V. Deryaguin Disc. Faraday SOC. 1966 42 109. B. V. Deryaguin and G. P. Sidorenkov Doklady A N . S.S.S.R. 1941 32 622. B. V. Deryaguin G. P. Sidorenkov E. A. Zubashchenko and E. V. Kiseleva Kolloid Zhur. 1947 9 335. ref. (4) chap. I 39. D. A. Haydon Proc. Roy. SOC. A 1960,258,319. J. Lyklema and J. Th. G. Overbeek J. Colloid Sci. 1961 16 501. l o D. Stighter J. Phys. Chenz. 1964 68 3600 ; J. Colloid Interface Sci. 1967 23 379. l Z J. J. Bikerman J . Chem. Phys. 1941 9 880. l 3 A. Wattilon and A.M. Joseph-Petit Disc. Faraday Soc. 1966 42 143. l4 J. J. Bikerman Z. phys. Chem. A 1932 163 378. l 5 J. Th. G. Overbeek Kolloid Beihefte 1943,54,287 ; F. Booth Proc. Roy. SOC. A 1950,203,514. l 6 P. H. Wiersema A. L. Loeb and J. Th. G. Overbeek J. Colloid Interface Sci. 1966 22 78. l 7 T. Graham Ann. 1862 121 1 ; R. Taft and L. Malm J . Phys. Chenz. 1939 42 499. l8 H. Moraweta Macromolecules in Solution (Wiley Interscience N.Y. chap. 2 3 A5. l 9 B. V. Deryaguin and S . S . Dukhin KolloidZhur. 1969,31 350. ’O S. S. Dukhin Sbornik issledovaniya v oblasti poverkhnostynykh sil (Nauka Moscow 1967) p. 335. S. S. Dukhin and V. N. Shilov Kolloid Zhur. 1969 31 706. 2 2 S. S. Dukhin ref. (20) p. 364 ; S. S. Dukhin and N. M. Semenichin KolloidZhur. 1970,32,360. 23 V. N. Shilov and S. S . Dukhin KoIloidZhur. 1970,32,117. 24 K. W. Wagner Arch. Electrotechn. 1914 2 371. 2s V. N. Shilov and S. S. Dukhin KolloidZhur. 1970,32 no. 2 S. S. Dukhin Dielectric Properties of Disperse System in Surface and Colloid Science ed. Egon Matijevic (Wiley N.Y. 1970) vol. 3 293. ’‘ P. H. Schwan G. Schwarz J. Maszuk and H. Pauly J. Phys. Chem. 1962,68,2626. 27 G. Schwarz J. Phys. Chem. 1962 68 2636. I. P. MacTague and J. H. Gibbs J. Chem. Phys. 1966 44,4295. 29 T. L. Chelidze and V. N. Shilov Kolloid Zhur. in press. 30 R. H. Ottewill and J. N. Shaw Disc. Faraday SOC. 1966 24 154.

ISSN:0370-9302    

DOI:10.1039/SD9700100158

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

GENERAL DISCUSSION Dr. L. M. Barclay (B.P. Ltd. Penarth) and Dr. R. H. Ottewill (University of Bristol) (communicated) In our paper we have reported mzasurements of the force of interaction between montmorillonite plates as a function of electrolyte concentra- tion and the distance of separation between the plate surfaces. The other model which is of interest theoretically is that for interaction between spherical particles. For this case the curve of potential energy Vagainst distance Ho of surface separation has the general form shown in fig. la. Three features of the curve are of particular interest the secondary minimum S the point of inflexion I and the primary maximum M. of inter- action between spherical particles leads to a curve of force (-aY/dH,) against Ho. This has the form shown in fig.lb and it is clear that the maximum force occurs at Differentiation of the appropriate expressions for potential energy I I I I FIG. 1.-Potential energy (a) and force (6) againt distance for interaction between two spherical particles (schematic). the distance at which the point of inflexion occurs on the potential energy curve; at the primary maximum and the secondary minimum the force is zero. Thus the interaction between spherical particles should lead experimentally to a force against distance curve showing these features the distances and magnitudes of the force being dependent on particle size and ionic strength. An extensive series of measure- ments have been made using the apparatus described in our paper with monodisperse E. J. W. Verwey and J. Th. G. Overbeek Theory of Stability of Lyophobic Colloids (Elsevier Amsterdam 1948).164 M GENERAL DISCUSSION 165 5 0 - n 6 8 4 0 - E u W VJ k 5 3 0 - 2 g 2 0 - .I d .I 0 10 - distance between surface of spheres in 8 diameter of particle = 1845 A. FIG. 2.-Pressure against distance between particle surfaces. Polystyrene latex at pH 10 and 25'C ; 100*0 volume % of latex FIG. 3.-Pressure against volume concentration of polystyrene latex at 25°C ; 0 1760 8 diam. ; 0,lO 990 A diam. 166 GENERAL DISCUSSION latex preparations as the disperse system. The results obtained with a latex having a modal diameter of 1845 A are shown in fig. 2. It is clear from this curve that very little pressure is exerted by the particles until the distance of surface separation is about 320A. The pressure then rises with decreasing distance and appears to reach a maximum at a separation distance of ca.230A. The distance axis on fig. 2 has been calculated on the basis of hexagonal close-packing between spherical particles. Although this assumption is supported by electron microscope examinations of highly compressed then dried latex dispersions it is not yet certain whether a com- pletely ordered hexagonal close-packed system is maintained at the various volume concentrations or whether ordered domains are formed at the lower pressures ; the packing is now being examined by optical methods. The effect of a change in diameter of the particles from 1760 to 10 990A is shown in fig. 3. The smaller particle size shows a gradual increase of pressure until a volume concentration of 64 % is reached ; then the pressure rises rapidly.The larger particle size material however shows a pressure rise at ca. 60 % volume concentration which is almost perpendicular to the volume concentration axis. The potential-energy curves for the smaller particle size latex at this ionic strength do not show a secondary minimum of any significance whereas those for the larger particles do. It therefore seems highly probable that the volume concentration at which the pressure starts to rise rapidly with distance could coincide with the ordering of the latex particles into a secondary minimum structure. From the theoretical calculations the force of repulsion rises very rapidly for particles with diameters of lO,OOOA whereas the rise for particles with diameters of the order of 1,000 A is gradual.The curves obtained bear a close resemblance to those expected theoretically. More quantitative conclusions can be reached for dispersions of spherically shaped particles once the distance of surface separation has been unequivocably established. Prof. J. Th. G. Overbeek (University of Utrecht) said One expects the best fit between theory and experiments for larger distances between the plates. According to fig. 5 (and fig. 4) of the paper by Ottewill et al. the calculated repulsion is larger than the one found in the experiments. The fit might be improved by introducing the Stern-correction which would lower the calculated repulsion. Did he try this and does it lead to reasonable values for the capacity of the Stern layer? Dr. R. H. Ottewill (University of Bristol) said In reply to Overbeek we have made some calculations of the force of interaction including an allowance for the Stern layer and this does improve the fit at long distances.Unfortunately this procedure does involve a somewhat arbitrary choice of the Stern adsorption potential. The capacity obtained for the inner layer is about 12 pF. Prof. G. M. Bell (Chelsea College) and Dr. S. Levine (Manchester University) said These comments on the work of Barclay and Ottewill may also have some relevance to other papers. Denoting the electrical van der Waals' and structural terms in the force per unit area between plane parallel surfaces by F' Fw and Fs respectively we agree with Barclay and Ottewill that Ps is not likely to be significant at separations of more than 50A and think that this limit could be considerably lower.Existing theoretical values of &+FW are far from exact and differences between these values and measured pressures are not reliable estimates of Fs. The remainder of our remarks will be concerned with F'. The DLVO evaluation of FE excludes the ionic environment (e.g. discreteness- of-charge) effects which depend on the difference between the actual situation in the GENERAL DISCUSSION 167 vicinity of an ion and the mean field situation. These include the image effect due to polarization of the dielectric interface by the ion field and the self-atmosphere effect which for a finite ion includes the “ cavity ” effect due to the displacement of mean charge by the ionic volume. The physical meaning of these omissions can be appreciated from the fact that for an uncharged air/electrolyte interface the DLVO theory would give uniform ionic concentrations and hence zero excess surface tension.Unfortunately the introduction of ionic environment and other corrections con- siderably complicates the DLVO theory and for two interacting charged interfaces adequate numerical values of FE are not so far available although calculations have been made by Sanfeld and others. Some idea as to possible effects may be obtained from the authors’ work on the force between uncharged plates in an electrolyte medium. This is based on a model like that of Onsager and Sarnara~,~ but with two interfaces instead of one and also uses local values of the Debye-Huckel constant K in the screening terms rather than the bulk value. It is found that where PI is the ideal osmotic pressure term while P2 and P3 are due directly to image-self-atmosphere effects.With a point-ion model the form of these terms is unaltered for symmetrically charged plates but the numerical values alter owing to the changed ionic concentrations. For charged or uncharged plates at separations 2h large compared with K-’ the terms P +P3 give an h-3 law repulsion. However the equivalent Hamaker constant is only 3 x 10-14 erg so that this repulsion will be cancelled by the van der Waals attraction unless retardation is appreciable. As pointed out by P e t h i ~ a ~ Pi is attractive for uncharged plates owing to the depletion of ionic concentration between the plates but for charged plates at sufficiently high potential it is repulsive as in DLVO theory where FE = P I .The sum P2+P3 is always repulsive and may account for the excess repulsion found by Barclay and Ottewill for 0.1 M electrolyte and at some separations for 0.0001 M electrolyte. Caution is necessary as to the overall effect since ionic depletion between the plates due to imaging will reduce the magnitude of the repulsive term P I . In a complete theory ionic volume and polarization effects would also have to be considered 1* 5 9 as well as the effect of the thinness of the plates on image potentials. The standard DLVO theory of F E assumes a “ reservoir ” large enough for the chemical potential of each species to remain effectively constant as the surfaces are moved. This does not seem to be the case in Barclay and Ottewill’s experiments where in the initial state nearly all the fluid is between the plates and is then “ squeezed out ”.Another point is that both the constant potential and the constant surface charge assumptions are artificial. However the use of a realistic adsorption isotherm would give results lying between the two curves so that if the latter are close together as in the calculations for 0.0001 M electrolyte the error is likely to be small. Fw = Pi +P +P3 Dr. L. Barclay (B.P. Penarth) and Dr. R. H. Ottewill (University of Bristol) said We appreciate the points made by Bell and Levine. We agree that the assumptions of constant charge and constant potential are somewhat artificial particularly as it A. Sanfeld Thermodynamics of Charged and Polarized Layers (Wiley-Interscience London 1968). G. M. Bell and S . Levine J. Chem. Phys. 1968,49 4584.L. Onsager and N. N. T. Samaras J . Chem. Phys. 1934 2 528. B. A. Pethica Expt. Cell. Res. Suppl. 1961 8 123. G. M. Bell and S . Levine Chemical Physics ofIonic Solutions B. E. Conway and R. G. Barradas ed. (John Wiley & Sons Inc. 1966). pp. 409-461. S. Levine and G. M. Bell Disc. Faraday Soc. 1966 42,69. 168 GENERAL DISCUSSION is conceivable that the extent of adsorption in the Stern layer could be a function of the distance of separation between the plates at distances as close as those studied. Prof. H. van Olphen (Amhem Netherlands) said With regard to the paper by Barclay and Ottewill the alternative approach to swelling measurements on expanding clay systems is to allow individual flakes to swell under no constraints and to obtain the equilibrium unit layer distance from X-ray diffraction patterns.1-3 As expected from double-layer theory equilibrium distances decrease with increasing electrolyte concentration of the medium. At each equilibrium distance repulsive and attractive forces must cancel each other. However the calculated double-layer repulsion at each equilibrium distance observed appears to be considerably greater than the van der Waals attraction. If a specific adsorption potential is assumed for the inter- layer counter ions to reduce the repulsion to equal the van der Waals attraction the adsorption potential would have to vary with electrolyte concentration which is unlikely. An alternative trivial explanation of the observations is that swelling is limited by cross-linking of parallel particles by non-parallel ones. Estimates of edge-to-face linking forces of bentonite particles derived from rheological observations show that relatively few cross-links would be necessary to limit the swelling as ob~erved.~ Dr.R. H. Ottewill (University of Bristol) said The procedure suggested by van Olphen is an interesting alternative to the one that we have employed but there may be some advantages in starting with a dilute system in which the particles are well dispersed. We consider that in the first compression edge-face links are broken down. However once the system has been subjected to a pressure of ca. SOatm then most of the plates would be likely to take up a parallel arrangement. This contention appears to be supported by the fact that after the first compression the pressure can be decreased to ca. 1 atm and then recompressed to a high value several times with a high degree of reproducibility in the results.It is probable that the whole system will not be composed of parallel plates ; rather that there will be domains with a parallel arrangement of plates. Negative adsorption measurements of the chloride ion gave a value of 690 m2/g which would suggest that a high degree of dispersion into fundamental plates had occurred. The possible variation of Stern potential with electrolyte concentration could be a deficiency of the theory and it would be interesting to calculate the force of interaction with allowance for the discreteness of the charges. Moreover it is conceivable that at such close distances of approach the extent of adsorption could he a function of the distance of plate separation.Dr. B. A. Pethica (Unilever Res. Port Sunlight) said Fundamental to the arguments of the paper by Barclay and Ottewill is the question as to whether the pressures measured by their apparatus reflect the characteristics of interacting plates every- where surrounded by the fluid phase or whether there is in the cell a gel-like matrix of interacting particles equivalent to a solid structure capable of resisting mechanical deformation. The question can be settled in a variety of ways the most direct being to relate the pressure-volume data directly to the vapour adsorption isotherm in the same colloid system. Similarly the general utility of the electrostatic repulsion calculations could be illuminated by a direct Donnan calculation using the same charge density assumptions as were made in the calculations given in the paper.K. Norrish Disc. Furuday SOC. 1954 18 120. K. Norrish and J. A. Rausell-Colom Clays and Clay Minerals 1962 10 123. D. E. Andrews P. W. Schmidt and H. van Olphzn Clays and Clay Min?rmls 1967,15 311. H. van Olphen J. Colloid Sci. 1962 17 660. GENERAL DISCUSSION 169 A further thermodynamic question relates to the effect of pressure on the c.m.c. of the polyoxyethylene surfactant which was used at 7 x M. The micelle point at atmospheric pressure is quoted as 7.25 x lo-’ M in one place and 7 x lo-’ M in another. In either case the concentration in the compression experiments is close to the c.m.c. and it would be necessary to know the effects of pressure on the c.m.c. and on the adsorption for a full analysis of the pressure effects in the suspensions.Dr. Th. F. Tadros (Plant Protection Ltd. Bracknell Berks) said Does the adsorp- tion of C12E6 on montmorillonite above the CMC correspond to vertically oriented chains or does Ottewill obtain adsorption of micelles? Has he measurements of equilibrium pressure in presence of C12E6 at high salt concentrations where the double layer repulsion term tends to zero leaving VA and AG,? Dr. L. M. Barclay (B.P. Penarth) and Dr. R. H. Ottewill (University of Bristol) said The first part of Pethica’s question has essentially been covered in the reply to van Olphen. Petbca is quite correct in stating that the same results should be obtained from measurements of water vapour adsorption on montmorillonite. The comparison is however not as easy as it might seem. First as observed by many author^,^'^ the water vapour adsorption isotherms on montmorillonites depend on the source of the montmorillonite.For example i n the work of Barshad a type I1 isotherm was obtained with a sodium montmorillonite from Otay California and a type 111 isotherm with a sodium montmorillonite from Clay Spur Wyoming. The results therefore are only meaningful if obtained on exactly the same material preferably on the same batch. A second problem with comparing published work on water adsorption with our data is that the pressure measurements have been made over a range which corresponds to p/po values in the range 0.9-1.0. The published isotherms are very short of data in this region and it is well known that it is a difficult range in which to measure adsorption. Conversion of published data into pressure against distance curves does give curves resembling our own but the adsorption data are not precise or extensive enough to allow a quantitative comparison.We have not carried out any Donnan calculations. Where the effects of pressure on c.m.c. have been measured the c.m.c. increases with pressure up to 1,000 atm. I am not aware of any similar measurements with non-ionics and we cannot measure this directly in our apparatus. It was possible to analyze C1&6 however in the solution expelled into the capillary. The concentra- tion of this was that expected from the isotherm and hence we concluded that the effect of pressure on the adsorption equilibrium in the range examined was negligible. Where the effects of pressure on c.m.c have been mea~ured,~.the c.m.c. increases with pressure up to 1,000 atm. Dr. L. Barclay (B.P. Penarth) and Dr. R. H. Ottewill (University of Bristol) said In reply to Tadros the adsorption isotherm for CI2E6 on montmorillonite is an interesting one since it differs in form from those found on hydrophobic substrates such as silver iodide,6 Graphon and polystyrene.* With these materials the I. Barshad 8th Nut. Conf. Clays and Clay Minerals 1959 8,84. W. A. White 3rd Nut. Con$ Clays and Clay Minerals 1955 3 186. A. C. Zettlemoyer G. J. Young and J. J. Chessick J . Phys. Chem.. 1955 59 962. R. F. Tuddenham and A. E. Alexander J. Phys. Chem. 1962 66 1839. S. D. Hamann J. Phys. Chem. 1962 66 1359. K. G. Mathai and R. H. Ottewill Trans. Faraday SOC. 1966 62 750 759. R. H. Ottewill and T. Walker Kolloid-Z. 2.Polymere 1968 227 108. ’ J. M. Corkill J. F. Goodman and J. R. Tate Trans. Faraday SOC. 1966 62 979. 170 GENERAL DISCUSSION isotherm reaches a saturation value close to the c.ni.c. With montmorillonite adsorption continues to increase above the c.m.c. despite the fact that constant activity of the solute is normally assumed to be reached at the c.m.c. In fact a limiting adsorption value of 2.5 x mol/g is only attained at about an equilibrium concentration of 3 x On the basis of a surface area of SO0 m2/g this corresponds to an area per molecule of 53 A2. It is tempting to conclude that this corresponds to a vertically oriented layer since this value compares favourably with the figure of 55A2 found for adsorption of CI2E6 of Graphon and one of 40 A2 obtained on polystyrene where it is clear that vertically orientated monolayers are formed.on the adsorption of C12E14 who used X-ray methods to determine the distance between the sheets two steps were found one at a distance of 4.4A and the other at S.4A. The first distance corre- sponded to a single layer of surface-active agent molecules lying flat on the surface and the second to a bi-layer of molecules. This evidence appeared to indicate attachment of the ethylene oxide groups to the hydrophilic silica layers and this would argue against reorientation to form a vertically orientated layer. It is probably possible for the hydrocarbon chains to associate however and thus form dimers on the surface and this would give an area per molecule not drastically different from M CI2E6. However in the work of Schott that found.64 62 6 0 58 5 6 54 volume concentration of latex (%) FIG. 1.-Equilibrium pressure against volume concentration for a latex with an adsorbed layer of dodecylhexaoxyethylene glycol monoether (CI2Es) 0 pH 10 without added sodium chloride ; 0 pH 9.6 with 0.5 M sodium chloride. We have not measured the interaction between montmorillonite sheets with However we have carried out an and the results are given in fig. 1. adsorbed CI2E6 at high salt concentrations. experiment of this type using polystyrene latices H. Schott Kolloici-Z. 1964 199 158. L. Barclay Ph.D. Thesis (University of Bristol 1970). GENERAL DISCUSSION 171 Two curves are presented one for particles having a diameter of 1,845 A and an adsorbed layer of CI2E6 at pH 10.0 without added salt and the other in the presence of 0.5 M sodium chloride.The addition of salt enables the particles to come closer together as would be expected from the decrease of electrical repulsion but there is still a very strong repulsion and there is a considerable increase in the gradient at low pressures. The fact that the system remains a stable dispersion can be attributed to “ steric stabilization ” and this is confirmed by kinetic studies of stability. However until we have precise measurements of the distances between the spherical particles we cannot tell whether overlap of the adsorbed layers actually occurs. Dr. G. Peschel (Universitdt Wiirzburg) said With regard to the paper by Ottewill et al. the disjoining pressure between colloidal particles in disperse systems might be sensibly dependent on the diameter of the particles as some evidence shows; e.g.Jura and Harkins in applying their well-known method for determining the surface area of powders condensed water from saturated water vapour in titanium dioxide particles which had a mean diameter in the ,LI region. The obtained layer thickness is of only 5 molecular diameters which cannot be regarded as a pronounced long-range orientation effect. Zorin on the other hand carried out a similar experi- ment but using a macroscopic flat glass surface. He found layer thicknesses of about 100 molecular diameters. Therefore additional information concerning the repelling forces between the particles might be gained by a variation of the particle diameters in Ottewill’s apparatus. Are such experiments possible ? Dr. A. L. Smith (Chem.Dept. Liverpool Polytechnic) said In the paper of Vincent and Lyklema it is stated that titration experiments yield true P.Z.C. values which might therefore (by implication) be expected to differ from the isoelectric points (i.e.p.) determined electrokinetically. However the (o,,E) plots from titrations will only have a common intersection point (used to determine the P.z.c.) when there is no specific adsorption in practice at low electrolyte concentrations. Under these circumstances the P.Z.C. and i.e.p. coincide. There thus seems to be a real experi- mental discrepancy between authors revealed by table 1 not explicable by specific adsorption. It does not seem necessary to invoke the adsorption of silver nitrate ion pairs of doubtful existence in bulk solution to explain the higher differential capacities observed on the positive side of the P.Z.C.and still less to explain the asymmetry of the P.Z.C. While some specific adsorption of nitrate ions is to be expected and is indeed revealed by the small shifts of the P.Z.C. to higher pAg values in 1 mol dm-3 KN03 the fluoride ion produces differential capacities on the positive side almost as high as those in the presence of nitrates even though no shifts in P.Z.C. can be detected.2 Specific adsorption of the strongly hydrated fluoride ion is more- over not expected and it is hardly likely that the fluoride and nitrate ions would have similar tendencies to form ion-pairs with the silver ion. The higher capacities on the positive side could be at least partly due to a con- tinuation on the positive side of the rise in capacity K of the Stern layer evident from all published capacity measurements as the AgT surface becomes less negatively charged.This variation of K which cannot on the negative side be an apparent effect due to specific adsorption (of cations) since this would only make the variation larger could have its origin in water dipole re-~rientation,~ variation in the thickness Z . A. M. Zorin and N. V. Churayev KolloidZhur. 1968,30 371. J. Lyklema Trans. Farahy SOC. 1963 59,418. S. Levine G. M. Bell and A. L. Smith J . Phys. Chem. 1969,73 3534. 172 GENERAL DISCUSSION and/or effective dielectric constant of the inner layer or possibly solid-state effects. The two effects cited as giving higher total differential capacities on the positive side viz. specific adsorption of anions or higher values of K will have opposite effects on the Stern plane potential +d the first decreasing t,9d and the second increasing it.This should be reflected in the electrokinetic 5 potential. Experimental electro- phoretic mobilities of positively charged AgI are not noticeably smaller than those on the negative side at the same value of I t,bo I and while not excluding some specific adsorption of nitrate ions are certainly not consistent with adsorption of Ag+ " largely in the associatcd form as AgN03 ". Dr. B. Vincent (University of Bristol) and Prof. J. Lyklema (Wageningen) said The first point Smith makes viz. that there is a real experimental discrepancy between P.Z.C. and i.e.p. measurements is virtually a restatement of our remark at the end of the paper. However we did not imply that this difference was due to specific adsorption of one of the added ions.In fact table 1 shows that there are some specific ionic effects but these are small as compared to the difference between P.Z.C. and i.e.p. In connection with the interpretation of the high capacitances on the positive side of the P.z.c. at 20°C the capacitance in KF is definitely lower than that in KNO,,' although even in KF it is much higher than on mercury. High capacities at the positive side of the P.Z.C. have also though to a lesser extent been observed by Iwasaki and De Bruyn for Ag,S (The high capacitances found on oxidic materials such as TiOz or Fe,O have a different cause and should not be considered in this connection.) Given the fact that there seems to exist a relationship between Ag+ as the potential-determining ion and high capacitances one is almost automatically led to an interpretation in terms of specific interactions in- volving Ag+ adatoms.As moreover the high surface charge does not produce a high [-potential (it is comparable to that at the negative side as confirmed by Smith in his remark) it is logical to postulate considerable counterion adsorption in the Stern-layer as well. If on the other hand an explanation is sought in terms of factors like dipole orientation and dielectric constant variation i.e. factors that are not primarily related to the presence of Ag+-adatoms it is not clear whether high capacitances are not observed on mercury as well. It was for these reasons and others given in the paper that we drew attention primarily to typical chemical effects occurring in the Stern layer thereby not excluding other but secondary possibilities.Finally the fact that we are concerned with a trend that is typical for AgI or at least for silver salts implies at the same time that we should not make recourse to bulk phenomena (like the extent of ion-pairing in solution) to interpret our experi- mental facts because if they were due to bulk phenomena they should occur in all double layers in disagreement with established facts. but not for mercury. Dr. Th. F. Tadros (Plant Protection Ltd. Bracknell Berks) said In the paper by Vincent and Lyklema they have assumed that Ag+ adsorbs at 20°C largely in an associated form i.e. as AgN03 whereas at O'C AgN03 ion-pairs are more dis- sociated. The association constant is given by the Fuoss equation KA = (4nNa3/3000) exp (e2 JDakT) E.P. Honig Tms. Furaduy Sac. 1969,65,2248. J. Lyklema and J. Th. G . Overbeek J. Colloid Sci. 1961 16 595. I. Iwasaki and P. L. de Bruyn J. Phys. Chem. 1958,62,594 fig. 8. GENERAL DISCUSSION 173 where a is the distance of closest approach between the paired ions and the other terms have their usual meaning. They mentioned in their paper that as a result of increased hydrogen-bonded structuring from 20 to O'C D increases from 7.5 1.5 to 8.0+ 1.5. However this increase in D does not outweigh the decrease in T and the final result would be higher KA at the lower temperature contrary to their explanation. Do I understand that they are speaking in terms of structure-stabilized ion-pairs Dr. B. Vincent (Bristol University) and Prof.J. Lyklema ( Wageningen) said We agree with Tadros that a decrease in ion pairing with decreasing temperature is not what one would intuitively expect. However the experimental facts do show a definite decrease in Ag+-adsorption with decreasing temperature and neither is this trend apriori expected at least not on the basis of classical energetic or electrical considerations. It was t h s trend in conjunction with a few auxiliary considerations that led us to thmk of increased structure formation with decreasing temperature. The anomalous behaviour of the association of Agi- and NO in the Stern layer as tentatively postulated by us must be looked upon in a similar fashion it is largely due to-presumably structural-causes that are not accounted for by the Fuoss equation.In this connection the slight increase of the inner layer dielectric constant with decreasing T (fig. 7d) in our paper applies to negatively charged AgI-surfaces whereas the structure formation takes place mainly at the positive side of the P.Z.C. Dr. S. Levine (Manchester University) and Dr. A. L. Smith (Liverpool College of Technology) (communicated) The tentative explanation given by Vincent and Lyklema for their increase with temperature in the pAg of the Agl suspension at the P.Z.C. is not convincing. Recently Honig has queried the neglect by colloid chemists of the diffuse layer in the solid AgT phase due to lattice (Frenkel) defects and a further study of this problem has been made by Levine et aL3 Some of the theory in these papers seems relevant to the shift with temperature in the pAg at the P.Z.C.We use the subscripts c b and s to denote the solid phasc the (surface) phase boundary and the aqueous solution phase respectively. Then the condition of uniform electrochemical potential pAg+ of the Ag+ ion yields the relations pAg+ = p,"-2.303kTpAg = ,uz+eO$b+kT In vb = ,u,"+e,\l/,+kT In n,. Here p pg and p,O are functions of pressure and temperature; $c and $b are the electrostatic potentials inside the AgI crystal and on the AgI surface (taking the potential zero in the interior of the solution) n is the volume density of Frenkel defects in the solid phase and vb the surface density of corresponding defects on the AgI surface (responsible for the excess or deficit of Ag+ ions on this surface) eo,k and T have their usual meanings. At the P.z.c. in the absence of specific adsorption of indifferent electrolyte we may assume $b = Xb (the X-potential) the potential drop at the P.Z.C.due to water dipole orientation mainly in the Stern inner region4 We note that t+bC # \l/b at the P.Z.C. because there would still be a surface charge equal in magnitude but opposite in sign to the diffuse layer charge inside the solid phase. The surface defects may have various origins e.g. vacancies in steps kinks on a step etc. and we assume such defects lead to an excess or deficit of both Agt and I- ions at the surface. Also Here nc vb and Xb will all be functions of temperature. e.g. R. M. Diamond J. Pliys. Clieni. 1963 67 2513. E. P. Honig Trans. Faraday Soc. 1969 65 224; Nature 1970 225 537. P. L. Levine S. Levine and A. L. Smith J. Colloid Interface Sci.1970 34 549. S. Levine G. M. Bell and A. L. Smith J . Phys. Clzern. 1969 73 3534. 174 GENERAL DlSCUSSION pz - p i which is partly due to the difference between the free energies of solvation of Agf ion in the solution and on the surface will depend on temperature. These considerations certainly suggest a dependence of pAg on temperature at the P.Z.C. Dr. B. Vincent (Bristol University) and Prof. J. Lyklema ( Wageningen) said We thank Levine for his suggestion to explain the observed shift in the P.Z.C. on the basis of a variation of n and v b with temperature. A quantitative study will be needed to evaluate to what extent the changes in the properties of the solid phase contribute to the observed shift. At any rate these changes must be reversible with change in teinperature because the P.Z.C.measurements were reversible as a function of T. At the same time variation of the solid state properties cannot be solely responsible for the observed trends. First there are definite lyotropic effects in the P.z.c. in the adsorption of potential-determining ions and in the interfacial excess entropy (not given in fig. 5 but mentioned in the text of our paper). This proves that at least a (great) part of the effects has to do with the properties of the Stern layer. Moreover the more or less pronounced transition around 20°C in e.g. the surface excess entropy and in the double-layer capacitance at the positive side of the P.z.c. is not reflected in the solubility product indicating that we are concerned not with a transition in the solid-phase properties but in the surface properties. In conclusion there is now ample evidence that we are concerned with interfacial effects. There is no indication for solid phase effects as well but their existence cannot be excluded a priori. Quantitative evaluation would be desirablc.

ISSN:0370-9302    

DOI:10.1039/SD9700100164

出版商:RSC    

年代:1970

数据来源: RSC