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.