摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 849-853 Study of the Structure-breaking Effect in Aqueous CsCl Solutions Based on H20/D,0 Isotope Effects on Transport Coefficients and Microdynamical Properties Antonio Sacco* and Hermann Weingartnert Dipartimento di Chimica, Universita degli Studi di Bari,4, Trav.200 Re David, 1-70126 Bari, Italy Bernd M. Braun and Manfred Holz lnstitut fur Physikalische Chemie und Elektrochemie der Universitat Karlsruhe, Kaiserstr. 12, 0-76128 Karisruhe, Germany We have studied the effect of H,O/D,O isotopic substitution upon various transport and microdynamical proper- ties in aqueous solutions of structure-breaking salts, using aqueous CsCl in the concentration range up to 6 mol kg-' as a representative example. We report on isotope effects upon the self-diffusion coefficients of water and of Cs+ and CI- ions, upon the reorientational correlation times of water deduced from 2H magnetic relaxation rates, and upon the magnetic relaxation rates of the quadrupolar relaxing ionic nuclei 133Cs+ and 35CI- in these solutions. Comparison is made with the isotope effectupon the viscosity, and some subtle differences are outlined.The most substantial one is related to the isotope effects upon 13'Cs+ and 35CI- relaxation which selectively probe motions in the first cosphere of the ions. These are substantially lower than those observed with the other dynamical quantities, showing that dynamical isotope effects in the first cosphere may differ substantially from those in the bulk. It is concluded that the structure-breaking effect extends to the first co-sphere of the ions.This paper is intended to contribute to our understanding of the 'structure-breaking ' effect' or 'negative hydration'2 of salts like KCl or CsCl in aqueous solutions. These terms are now widely used for describing the effect of large ions on the water structure, although we have little knowledge on the actual structural changes. In particular, there is a profound influence of structure-breaking ions on transport coefficients, evidenced by a decrease in viscosity, and described by nega- tive B-coefficients in the Jones-Dole expansion3 (tf/tfo) -1 = AC''2 + BC + (1) where tfo is the viscosity of pure water and C the salt concen- tration.By some convention on the splitting of B, the same notion is used for single-ion properties. Although there are cases with B < 0 in non-aqueous solvents: negative B-coeficients are above all characteristic for water. Transport theories which treat the solvent as a structure- less medium characterized by its macroscopic viscosity and relative permittivity' are presently incapable of predicting negative B-coefficients.6 Hence, any explanation has to account for the molecular nature of the solvent. In fact, it is possible to examine the molecular basis of the structure- breaking effect by studying quantities which specifically monitor translational or rotational processes in solution on the molecular level, or probe the local dynamics of solvent molecules near ions: 'H and 'H magnetic relaxation probe reorientational motions of water spin-echo or radiotracer self-diffusion experiments monitor the trans-lational motions of water molecules7 and of ions,* magnetic relaxation of quadrupolar ionic nuclei probes fluctuating electric field gradients at these nuclei, thereby providing information on the local dynamics of solvent molecules near ion^.^*'^ Here, we adopt a new approach for investigating the structure-breaking phenomenon by monitoring the effect of H20/D,0 substitution upon the above-mentioned dynami- cal quantities.Again, continuum models are doomed to fail On leave from the Institut fur Physikalische Chemie und Elektro- chemie der Universitat Karlsruhe, Germany.in the explanation of such isotope effects. One reason is that the relative permittivities of H20 and D20 are almost the same, so that in continuum theories both solvents give rise to the same long-range electrostatic effects on dynamcal proper- ties like the B-coefficient.6 Differences are then inadvertently related to the molecular nature of the solvent. To address the problem in more detail, we note that for pure water at 298 K the isotope effect upon the viscosity defined by Vr = tf(D,O)/tf(H20) (24 is q, = 1.228.'' Defining analogous quantities for the self- diffusion coefficient and reorientational correlation time of water by Dr, w = Dw(D2O)/Dw(H20) (2b) and 7Zr = ~2(D20)/72(H20) (24 we have DrIk m 72r E v,,'~-'~i.e. the factor 1.228 is the same for translational and rotational motions.Moreover, it exceeds by far what is expected from the so-called 'square- root-of-mass law','' which predicts Dr;t and tf, to be equal to [m(D,O)/m(H,O)] 'I2 = 1.05, where rn is the molecular mass. It is intriguing to explain this observation by a strong coupling of translational and rotational motions, which implies that D,,is closer to the ratio of the square roots of the moments of inertia, i.e. Dr;k x 1.38, rather than to m1/2.15*16We have recently confirmed this interpretation on a broader basis by considering transport processes in meth- anol,' 7*1* where an unambiguous correlation between the self-diffusion coefficients and moments of inertia of eight iso- topic species was found.The question is then how structure-breaking ions affect this coupling. Interestingly, for structure-breaking salts viscosity B-coefficients are more negative in D20 than in H,O, while for structure-formers they are almost In other words, the H20/D20 isotope effect decreases upon addition of structure-breakers, while for structure-formers it remains almost constant. However, in general, one could expect 850 dynamical isotope effects to differ both locally and with respect to the different modes of motion. This is the hypothe- sis to be tested in the present work. Some evidence for such a behaviour comes from the limiting ionic mobilities, which for structure-breakers exhibit a smaller isotope effect than pre- dicted from the bulk viscosity." Moreover, one of the present authorsi3 has reported magnetic relaxation rates of ionic nuclei, specifically of 87Rb+, 23Na+ and "Br- in water, finding a substantial decrease of isotope effects with increas- ing salt concentration.To investigate these questions in more detail, we have per- formed a systematic study of isotope effects upon trans-lational and rotational motions in aqueous CsCl solutions at salt concentrations from 0.1 to 6 mol kg-'. The properties of the '33Cs+ and 35Cl- nuclei make this salt particularly suited to magnetic resonance studies. We report in detail on the influence of CsCl on H20/D20 isotope effects upon the self-diffusion coefficient and reorientational correlation time of water.Moreover, we have monitored isotope effects on the self-diffusion of the Cs' and C1- ions. We define these isotope effects in analogy to eqn. (2a)-(2c) by Dr, cs = DCs(D2O)/Dcs(H20) (24 and D,c1 = DCl(D2O)/DC,(H2O) (24 where Dc, and Dc, denote the self-diffusion coefficients of the Cs' and C1- ions, respectively. Finally, we will report on H20/D20 isotope effects upon the magnetic relaxation rates l/Tl of the quadrupolar nuclei 133Cs+ and 35Cl-. These are defined by (1/T1)r, cs = (I/7'1)cs(D20)/( 1/T1 )Cs(H20) (2f1 and As will be seen below, the latter quantities reflect the local dynamics of solvent molecules near ions. Although intrinsic dificulties connected with some of these experiments do not allow measurements of isotope effects with the accuracy typical for some other quantities, say elec- trical conductances,' ' these experiments will provide impor- tant new information on the molecular basis of the structure-breaking phenomenon. Experimenta1 CsCl of 'suprapur' quality (Merck, Darmstadt) was dried at 200°C.Solutions in H20 (deionized and distilled) and D20 (Aldrich, D-content >99.8 at.%) were made up by weight. Samples used in relaxation time measurements were degassed by several freeze-pumpthaw cycles. The equipment and experimental procedures for obtaining highly accurate data are described in detail elsewhere : self-diffusion measurements by 'H,l7 and 2H and 133Cs 20,21 NMR spin-echo tech- niques; relaxation time measurements of 2H and of quadru-polar ionic nuclei;22 diaphragm cell experiments on C1-self-diffusion using the radiotracer 36Cl-.23 We draw particu- lar attention to the methods for obtaining highly accurate data, described in these papers. Results We first consider isotope effects upon the self-diffusion coeffi- cient of water in aqueous CsCl solutions.Two methods were applied to obtain such data. First, we have supplemented conventional 'H NMR spin-echo measurements of H20 self- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 diffusion by analogous experiments using the 2H resonance of D20. Secondly, in a new type of experiment, we have per- formed 'H spin-echo measurements in H20-D20 mixtures of different isotopic compositions, followed by linear e~trapolation~~to zero proton concentration, as shown for a typical example in Fig. 1.This procedure yields the tracer diffusion coefficient of HDO in D20, which in pure water differs by a factor 1.015 from the true self-diffusion coefficient of D20, owing to the different tracer mass.12 The same cor- rection is then also applied for aqueous solutions. Fig. 2(a) summarizes the resulting isotope effects upon the self-diffusion coefficients of water obtained by the two methods. Note that for a significant comparison of data in H20 and D20 one has to refer to equal 'aquamolalities', m*, defined by the number of mol of salt per 55.5 mol of solvent. Moreover, in order to enable a direct comparison with vis- cosity data, Dr;: rather than D,has been plotted. Every data point in Fig.2(a) is an average over frequent experiments with an estimated uncertainty of f0.015. For brevity, we do not quote the absolute values for the self-diffusion coefficients in H20 or D20, but refer to the data for CsCl in H20 obtained by Hertz and Mills25 which were reproduced by us to better than f1%. Values for D20 can then be recalculat- ed in conjunction with the isotope effects shown in Fig. 2(a). In order to obtain information on water reorientation in CsCl solutions we have monitored 2H relaxation. The spin- lattice relaxation rate, 1/Tl, is related to the reorientational correlation time z2 of the water molecules by l/Tl = 3/h~~(e~Qq/h)~z, (3) In pure D,O the nuclear quadrupole coupling constant (e2Qq/h) is near 250 kHz, so that z2 = 2.5 ps'4*26 [an addi- tional factor in eqn.(3) due to the asymmetry of the electric field gradient is negligible in water]. The isotope effect upon water reorientation is then obtained by comparing data for pure D20 with data for D20 (HDO) dissolved at trace con- centrations in H20. In practice, the latter figure is obtained by performing experiments at various mole fractions of D20 in H20-D20 mixtures, followed by extrapolation to zero deuteron concentration. Assuming that the quadrupole coup- ling constant does not depend on the isotopic composition, this allows the determination of isotope effects upon 72, regardless of an exact knowledge of the coupling constant. In fact, with pure water we have obtained a value of 1.223,suffi-2.o 0 0.2 0.4 0.6 0.0 t Xn Fig.1 Self-diffusion coefficient of protonated species in a 2 rnol (55.5 mol solvent)-' CsCl solution as a function of the isotopic com- position of water expressed by the mole fraction of protons. The lim- iting value at xH-+0 yields the tracer diffusion coefficient of HDO in D,O. After correction for the tracer mass (dashed line) this limiting value is equal to the self-diffusion coefficient of neat D,O in the CsCl solution. J. CHEM. SOC. FARADAY TRANS., 1994. VOL.. 90 85 1 I l.15t 1.10--2 4 6 6 h1.10 LJ 110 2 ll0lv 0 ' I 1.10 ' I 1.051 1.0511 !I 10L-' I ' ' 1 1.0-J m* m* Fig. 2 Dynamical H20/D20 isotope effects <, upon various tram- port coeficients and microdynamical quantities in aqueous CsCl solutions as a function of salt concentration. tr corresponds to (4 0,;;(6)~2r;(c)0,t.s; (d)DLh!; (l/T1)r+Cs; (f)(1/Tl)r,c1*The solid lines represent qr .All quantities are defined in eqn. (2a)-(2g).The salt concentration is in aquamolality units, i.e. rnol CsCl per 55.56 rnol of solvent. ciently close to the value 1.228 obtained for the bulk viscosity and inverse self-diffusion coefficient to justify this procedure. Results obtained in the same way for CsCl solutions are shown in Fig. 2(b).The estimated uncertainty is 0.01 5. Fig. 2(c) and (6) show isotope effects upon the self-diffusion coefficients of the Cs' and C1-ions. For absolute values obtained in H20 we refer to an earlier publication containing data for Cs' 2o and to the work of Hertz and Mills2' containing data for C1-.Again, for easy comparison with other quantities, inverse ratios 0,' have been plotted. The estimated accuracy is & 0.02 for Cs + and & 0.01 for CI -. respectively. The corresponding limiting values at infinite dilution of the ions can be calculated from the accuratelj known limiting ionic conductances ikl'means of the by Nernst-Einstein relation D, = F2/RTj., . Note that the resulting isotope effects = 1.199 and DrIhl= 1.215 at infinite dilution do not correspond to what is expected from the bulk viscosity.' ' Data for the magnetic spin-lattice relaxation rates of 133Cs+and 35Cl- dissolved in H,O have been measured in the context of a general research program of our joint labor- atories dealing with the fundamentals of the quadrupolar relaxation of ionic nuclei in aqueous electrolyte solutions.The results of these studies will be reported in detail else- here.^' We have supplemented these data by measurements of CsCl solutions in D,O. The resulting isotope effects are summarized in Fig. 2(e) and (f).They should possess an accuracy of & 0.02. While in all cases considered above, data for the limit of infinite dilution are available, this is not so for the ionic relax- 1.a 1 m u ", 1.10k. cv I I1.0 0.1 0.2 m* Fig. 3 Isotope effects ( l/Tl)r.cs upon 133Cs'relaxation rates at low salt concentrations. m* is measured in aquamolality units.ation rates. We have therefore attempted to extend the relax- ation time measurements to salt concentrations as low as possible to estimate relaxation rates at infinite dilution. In the course of these experiments sufficiently accurate results for isotope effects could be obtained for Cs' relaxation down to about 0.01 rnol (55.5 rnol solvent)-' in salt concentration, while the results for 35Cl -relaxation in this concentration range did not allow the extraction of isotope effects with the desired accuracy. Fig. 3 shows these low-concentration results for '33Cscrelaxation. The data should possess an accuracy of k0.03. Discussion It seems appropriate to consider first some aspects of dynamical isotope effects in pure water. The higher viscosity of D20 as compared with H20 is usually ascribed to a stronger 'structuredness' of D20due to stronger hydrogen bonding. In fact, 'HfH substitution changes the moments of inertia which affect the intermolecular vibrations.28 In partic- ular.the infrared librational band of pure H,O at 685 cm-' is shifted to 505 cm-' in D20,29which corresponds to the square root of the moments of inertia. This mode directly reflects the restraints produced by hydrogen bonds, which are, hence, stronger in D20 than in H20. Computer simula- tions show3' that such librational and also hindered trans- lational modes associated with the bending of bonds contribute significantly to the power spectrum of the velocity autocorrelation function which determines the self-diffusion coeficient, thus yielding a natural explanation for the translation-rotation coupling evidenced by the self-diffusion coefticien ts.As added salts change the hydrogen-bonded structure of water, one would expect the dynamical isotope effects to change also. This can indeed be extracted from viscosity data for alkali-metal chloride^.'^.^' These data show a substantial reduction of qr upon addition of structure-breakers. In the case of CsCl this amounts to a decrease from 1.228 in pure water to about 1.150 at 6 rnol (55.5 rnol solvent)-'. Fig. 2(a)-(f) show the resulting curve for qr for comparison with the data for the other dynamical quantities considered in this work. In contrast, the isotope effect decreases little upon the addition of structure-forming LiCI." No change or even a slight increase is observed with hydrophobic tetra-alkylammonium ions.6 In other words, the structure-breaking effect is larger in D,O than in H20, while structure formers show about the same effect in both solvents.The common explanation is that owing to the greater 'structuredness' of D20,the breaking effect is also more pronounced.6 How is this behaviour reflected by the microdynamical quantities considered in this work? To this end, we discuss first the isotope effects upon the self-diffusion coefficient and reorientational correlation time of water in Fig. 2(u) and (b). The major result is that the isotope effect upon water reorien- tation coincides with the behaviour of the viscosity, while at high salt concentrations the self-diffusion data lie systemati- cally above the viscosity curve.Based on the estimated uncer- tainties, the latter effect is near the limits of experimental error, but its systematic appearance for data obtained by two different methods seems to ensure that it is real. In other words, the isotope effect upon self-diffusion of water is less affected by addition of a structure-breaker than that upon the viscosity and water reorientation. In this sense, a presumed breaking of hydrogen bonds also decouples the rotational and translational motions. Let us next consider the isotope effects upon the self- diffusion coefficients of the Cs+ cations in Fig. 2(c) and (4, respectively. As noted, in these cases the infinite dilution values do not coincide with what is predicted from the bulk viscosity.As discussed in detail by Kay and Evans,'' this indicates that in the cosphere of ions the isotope effect is dif- ferent from that in the bulk. It is then interesting to see what happens at finite salt concentrations, and above all in the high-concentration regime, where all water molecules are in the cosphere of ions. Our results indicate that and Dr.&approach qr. This is particularly obvious from the data for Cs' diffusion, as in this case at low concentrations the difference is sufficiently large to show up in our measurements, and its disappearance with rising m* is clearly visible. This is more difficult to prove for CI-, where at zero salt concentration the difference is smaller, and possibly not beyond the limits of uncertainty of our experiments.In any case, within the limits of uncertainty of our data no difference between qr and Dr;kl is seen at high salt concentrations. +Finally, we consider 33Cs and "Cl- spin-lattice relax- ation. We note that apart from the application in the study of isotope effects, these data are important in their own right, as they give significant information on ion-solvent and ion-ion interactions.lo The latter aspects are treated el~ewhere.~' For quadrupolar nuclei in monoatomic species, the relaxation rate l/Tl may be written as" (4) where A is a constant for the given nucleus, (VL) is the mean square electric field gradient q at the nucleus and z, is the correlation time for fluctuations of q.Electrostatic theory assumes that the q arises from the electrostatic moments of solvent molecules and from the charges of counter ions. The detailed expressions are comple~,~*~~*'~ but we can use here the simple result that, at the level of accuracy needed here, (VL) is the same in H20 and D20? Hence, (l/Tl)r reflects isotope effects on the correlation time z,. In fact, zq is a particularly interesting probe for local dynamics, as theory predicts (V;) to exhibit a very short- range R-' distance dependence for the ion-solvent inter-action. Also, we know from computer simulations of ionic" and uncharged" monoatomic solutes, that zq reflects at least three modes of motion, namely short-time librational motions of water molecules, lateral motions which disturb the symmetry of the first cosphere and reorientational motions of water molecules in the first cosphere.Hence, zq probes pro- cesses which are quite different from those reflected by the dynamical quantities discussed above. As a complication, there appears, however, in eqn. (4) an additional term (l/7'l)remfor the background contribution due to the remaining relaxation mechanisms. Owing to its large quadrupole moment, 35Cl- relaxes exclusively by quad- rupolar interaction. For '33Cs the quadrupolar interaction + is weaker, but the common belief is that, nonetheless, '33Cs+ J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 relaxes only by the quadrupolar mechanism." This may be justified for most applications, but in the context of the very subtle effects considered here, we cannot disregard the possi- bility of a small 133C~-1H dipolar contribution in H20, say of the order of 3%.Owing to the lower magnetic moment of 2H, this dipolar contribution would be absent in D,O, so that the data could indicate an isotope effect up to 3% smaller than actually present for zq. The major result of the data in Fig. 2(e) and (f)is that in the entire concentration range the isotope effects upon the quadrupolar relaxation rates are substantially smaller than that upon the viscosity. Moreover, the curve for the 13'Cs+ cation lies below that for 35Cl-. Qualitatively similar obser- vations have been made in our previous work on 2'Na+, 87Rb+ and 'lBr- relaxation in aqueous alkali bromide and iodide sol~tions,~~*~~ but as CsCl is a stronger structure- breaker, all effects are more pronounced, and from an experi- mental point of view, these larger effects enable a more detailed investigation.Let us consider first the isotope effects at high dilution, where ion-solvent interactions will prevail. While it is impos- sible to perform measurements at concentrations low enough to extract accurate infinite dilution values, the data in Fig. 2(e) and (f)and the low-concentration data in Fig. 3 show that neither the viscosity ratio of 1.228 nor the somewhat lower ratios of the limiting mobilities, 1.199 and 1.215 for Cs' and C1-, are approached.This is also true, if a small dipolar contribution to "'Cs relaxation is taken into account. Hence, in the direct neighbourhood of an ion, the isotope effect upon quadrupolar relaxation rates is quite dif- ferent from that in the bulk. This is to a minor degree already evidenced by the limiting ionic mobilities," but is obviously reflected in a more pronounced manner by quadrupolar relaxation, where due to the R-' dependence of the mean- square field gradient the nearest neighbourhood of the ion is probed more selectively. In view of these results, it is hard to escape the conclusion that the structure-breaking effect extends to the first cosphere of the ions. The question whether the structure-breaking effect has its origin in the first or second cosphere has been debated for some time,2*35-39 starting with the well-known model of Frank and Wen" which assumes the existence of a first hydration sphere with rather immobilized water mol- ecules, surrounded by a second region, where structure-breaking takes place.While it appears to be impossible to test this model by direct experiments, molecular dynamics simulations of the translational and reorientational dynamics of water mol-ecules near iodide ions" have indicated that molecular motions are also accelerated in the first hydration shell. This corresponds to an earlier interpretation of magnetic relax- ation data by Engel and Hertz2 and of diffusion data by Sam- 0il0v.~~The picture is" that, for large ions, the ordering influence exerted on water molecules in the first hydration shell by ions becomes comparable to the influence of the sur- rounding water molecules.In this case the energy barriers will be flattened out, and the thermal motions will break up the local structure. With respect to cationic hydration, this is also confirmed by recent neutron scattering data for the water structure around K ions.40 These have yielded a com- + paratively broad and featureless first hydration shell around K+ ions, at contrast with well resolved and ordered shells for the smaller Li+ and Na+ ions. Finally, let us consider the behaviour at high salt concen- trations. From Fig. 2(e) and (f) we find little concentration dependence.As at the same time qr decreases, the isotope effects on the ion relaxation approach those on the other dynamical quantities. Whether eventually they will become J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 853 the same depends largely on whether 133Cs+relaxation is 4 K. Crickard and J. F. Skinner, J. Phys. Chem., 1969,73,2060. purely quadrupolar. Unfortunately, the latter question is difficult to answer. Theoretical estimates of the '33C~-1H dipolar interaction give some support to such a contribution, but owing to the RP6dependence of the dipolar interaction much rests on the Cs'-0 distance used in these calculations. It appears also to 6 7 8 K. Ibuki and M. Nakahara, J. Chem. Phys., 1985,85,7312. K. Ibuki and M. Nakahara, 2. Naturforsch., Teil A, 1991, 46, 127.L. Endom, H. G. Hertz, B. Thull and M. D. Zeidler, Ber. Bun- senges. Phys. Chem., 1967, 71, 1008. H. G. Hertz, M. Holz and R. Mills, J. Chim. Phys., 1974, 71, 1355. be impossible to prove its existence experimentally by NOE measurements. However, there is an interesting indirect argu- ment in favour of a small dipolar contribution: One could expect that when hydration spheres overlap, isotope effects upon cationic and anionic relaxation become the same.l3 This has indeed been found in earlier work on s7Rb+ and 9 11 12 13 14 H. G. Hertz, Ber. Bunsenges. Phys. Chem., 1973, 77,477. M. Holz, Progr. N.M.R. Spectrosc., 1986, 18, 327. R. L. Kay and D. F. Evans, J. Phys. Chem., 1965,69,4216. R. Mills, J. Phys. Chem., 1973, 77, 685, M. Holz, J.Chem. SOC., Faraday Trans. 1, 1978,74,644. D. Lankhorst, J. Schriever and J. C. Leyte, Ber. Bunsenges. Phys. Chem., 1982,96,215. 'Br -relaxation in aqueous RbBr.' The present results for CsCl can only be reconciled with this expectation by assuming a 133C~-1Hdipolar contribution of about 3% over the entire concentration range. In any case, there remains the fact that even at 6 mol (55.5 mol solvent)-', where each water molecule is in the solvation 16 17 18 H. J. Tyrrell and K. R. Harris, Difusion in Liquids, Butterworths, London, 1984. H. L. Friedman, in Molecular Motions in Liquids, ed. J. Lascombe, Reidel, Dordrecht, 1974, p. 87. H. Weingartner, M. Holz, A. Sacco and M. Trotta, J. Chem. Phys., 1989,91, 2568. M. Holz, H. Weingartner and A. Sacco, Ber.Bunsenges. Phys. sphere of an ion, and where hydration spheres begin to overlap, the isotope effects are still substantially higher than one would predict from the square-root-of-mass law. In other words, the translation-rotation coupling evidenced by such high values still persists in concentrated solutions, although to a lower extent than observed in the pure solvent. How 19 21 22 Chem., 1990,94,332. A. G. Ostroff, B. S. Snowdon Jr. and D. E. Woessner, J. Phys. Chem., 1969,73,2784. B. M. Braun and H. Weingartner, J. Phys. Chem., 1988,92, 1342. M. Holz and H. Weingartner, J. Magn. Reson., 1991,92, 115. M. Holz, A. Sacco and M. Trotta, J. Solution Chem., 1990, 19, 193. these effects are quantitatively related to the different hydro- gen bonding in H,O and D20 remains to be investigated by theory or simulations.We note, however, that in contrast to the common belief, large H/D isotope effects upon transport coefficients are not limited to water or to hydrogen-bonded protons in non-aqueous solvents. Isotope effects which 23 24 26 27 28 H. Weingartner, B. M. Braun and J. M. Schmoll, J. Phys. Chem., 1987,91, 979. H. Weingartner, Ber. Bunsenges. Phys. Chem., 1984,88,47. H. G. Hertz and R. Mills, J. Chim. Phys., 1976,73,499. B. C. Gordalla and M. D. Zeidler, Mol. Phys., 1986, 59, 817. M. Holz, X. Mao and A. Sacco, to be published. G. E. Walrafen in Water. A Comprehensive Treatise, ed. F. exceed by far what is predicted from the rn''2-law have also been found for other molecules with strongly directional interactions, we quote dimethyl sulf~xide/[~H,]dimethyl sulf- oxide with 0,' = 1.12 as opposed to rn'" = 1.038 as a typical example.' 29 31 Franks, Plenum, New York, 1972, vol.1, ch. 5. D. A. Draegert, N. W. B. Stone, B. Curnutte and D. S. Williams, J. Opt. SOC. Am., 1966, 56, 64. G. Szasz and K. Heinzinger, J. Chem. Phys., 1983,79, 3467. A. Selecki, B. Tyminski and A. G. Chmielewski, J. Chem. Eng. Data, 1970, 15, 127. 32 H. G. Hertz, M. Holz and A. Sacco, Chem. Scr., 1989,29,291. The authors are grateful to the Ministry of University and of Scientific and Technological Research (MURST), Italy, for financial support. H.W. thanks the Consiglio Nazionale delle Ricerche (CNR), Italy, for a visiting fellowship at the Uni- versity of Bari. 33 34 J. Schnitker and A. Geiger, 2. Phys. Chem. (Munich), 1987, 155, 29. M. Holz and C. K. Rau, J. Chem. SOC., Faraday Trans. 1, 1982, 78, 1899. H. S. Frank and W-Y. Wen, Discuss. Faraday SOC., 1957, 24, 133. 36 H. L. Friedman, in Water. A Comprehensive Treatise, ed. F. References 37 Franks, Plenum, New York, 1973, vol. 3, ch. 1. Faraday Discuss. Chem. SOC., 1977,64;general discussions. 1 H. S. Frank and M. W. Evans, J. Chem. Phys., 1945,13, 507. 2 G. Engel and H. G. Hertz, Ber. Bunsenges. Phys. Chem., 1968.72. 808. 38 39 A. Geiger, Ber. Bunsenges. Phys. Chem., 1981,85, 52. 0.Ya Samoilov, Discuss. Faraday SOC.,1957,24, 141. G. W. Neilson, Z. Naturforsch., Teil A, 1991,46, 100. 3 R. A. Robinson and R. A. Stokes, Electrolyte Solutions, Butter-worths, London, 1969. Paper 3/07041G; Received 29th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000849

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 855-858 Transfer Gibbs Energies for ClOi, BrOi, lo;, ClOi and 10: Anions for Water-Acetonitrile and Water-fed-Butyl Alcohol Mixtures JBn Benko and Olga Vollarova Department of Physical Chemistry, Faculty of Science, Comenius University, 842 75 Bratislava, Slovakia The transfer Gibbs energies, AtrSGe, of KCIO, , KBrO, , KIO,, KCIO,, KIO, and the corresponding caesium salts have been obtained through the gravimetric measurement of solubility in aqueous mixtures with acetonitrile (AN) and tert-butyl alcohol (ButOH). Single-ion values of Atr,Ge have been calculated using the TATB assump-tion. The trends observed for Atr,Ge are discussed in terms of specific ion-solvent interactions and the structur- al effect of the solvent mixtures.Single-ion thermodynamic transfer functions of solutes from one solvent (often water) to another find applications in many areas of chemistry. Much work has been done on a great variety of electrolytes in water mixtures with aceto- nitrile as well as with tert-butyl alcohol, but little attention has been paid to oxyanions of halogens except 10; and ClO, 'in H20-ButOH and ClO, in H,O-AN., Oxyanions generally show stronger specific interactions in aqueous mix- tures; this might be one reason why ClO, 'or NO, differ from monatomic anions such as halide anions5 in H,O-AN mixtures. We have recently reported the kinetics of 10; oxi-dation of some Co"' complex in water-organic mix- tures and as a by-product of these investigations we have obtained A,,G* for 10, in H,O-Bu'OH, H20-(CH3)2C0 and H20-CH30Hmixtures. We report now AwsGe for 10, in H'O-AN and other oxyanions of halogens in aqueous mixtures with AN and Bu'OH.The data for selected K+ and Cs' salts provide a basis for analysis of the extrathermo- dynamic assumption [AtrsG*(Ph,As +) = A,, G*(BPh;)] wed in the calculation of single-ion thermodynamic proper- ties. Experimental Materials The compounds NaBrO, ( >99.8%), CsNO, ( >99.0%) Lachema Brno and HJO, (>99.5%), HClO,, HIO, (>97%), KClO, (>99.5%), KI04 (>99.8%), KBrO, (>!MI%),KI03 (>99.5%), Bu'OH (>99.5%, H20 < 0.1%) and AN (>99.8%, H20c0.05%) Merck (stated % purities in parentheses) were of analytical grade.Caesium salts (CsIO,, CSIO, and CsClO,) were prepared from CsN0, and the appropriate acid. Addition of CsNO, to saturated solu- tions of NaClO, and NaBrO, resulted in precipitation of CsClO, and CsBrO,, respectively. All salts were re-crystallized from water prior to use and their purity was checked by elemental analysis (halogens titrimetrically and alkali metals using flame photometry). All solvents were redistilled before use. Solubilities Solubilities of salts in water and in aqueous-organic mixtures were determined by agitating an excess of the solid salt with the solvent at 298.2 K. The concentrations were determined gravimetrically. Evaporation of the solvent was performed carefully and slowly under an IR lamp to prevent any loss in salt weight. Solubility values were averages of three indepen- dent measurements.The standard error in the solubility determinations was & 1%. Results Measured solubilities of potassium and caesium salts in H20 and in aqueous-organic mixtures containing Bu'OH and AN are given in Tables 1 and 2. The solubilities of the salts in water S, and in the solvent mixtures S, are related to the transfer Gibbs energy of the salt by Atn Ge = ~RTIn(S,y,f/S,y:) (1) The solubilities were corrected to infinite dilution using the activity coefficients, y *, calculated from the Davies equa- tion:' log y* = -A[I"'/(l + Ill') + 0.3a (2) where A is the Debye-Huckel parameter and I is the ionic strength. A may be calculated from the known relative per- mittivities of H,O-AN and H,O-Bu'OH mixtures.* Transfer Gibbs energies of anions (Tables 3 and 4) can be calculated using published estimates for the corresponding K+and Cs+ ions in aqueous Bu'OH and aqueous ACN.Table 1 Solubilities of the salts investigated in water and H,O-Bu*OH mixtures at 298.2 K S/10-, mol dm-, ~(Bu'OH) KClO, KIO, KClO, 0 15.1 2.26 68.5 0 15.08" 2.26" 70.17" 0.010 12.6 1.91 57.1 0.02 1 11.2 1.70 48.0 0.033 9.91 1.52 41.3 0.046 9.15 1.45 36.1 0.076 7.88 1.39 27.8 0.113 7.10 1.32 22.6 x2 CsClO, CsIO, CsClO, ~ 0 8.89 5.67 35.5 0,010 7.52 4.86 30.3 0.021 6.77 4.37 25.9 0.033 6.15 4.04 22.7 0.046 5.85 3.85 20.0 0.076 5.16 3.75 15.8 0.113 4.56 3.56 13.0 " Values according to ref.11. KBrO, KIO, 47.9 42.6 47.3" 42.6" 38.3 30.8 31.0 22.7 25.6 16.9 20.9 12.7 14.6 7.66 11.0 5.23 CsBrO, CsIO, ~ ~~ 14.1 8.25 11.6 6.37 9.5 4.74 7.8 3.64 6.5 2.80 4.6 1.73 3.5 1.18 Table 2 Solubilities of the salts investigated in H,O-AN mixtures at 298.2 K S/lO-, mol dm-, x(AN) KC10, KIO, KC10, KBrO, KIO, 0.017 15.5 2.32 64.2 43.5 34.0 0.036 16.4 2.37 61.5 39.3 27.6 0.057 17.7 2.61 59.3 36.5 23.1 0.077 19.1 2.69 56.2 32.1 18.1 0.126 21.9 2.99 50.4 25.7 11.5 0.182 23.7 3.09 42.5 18.4 7.0 x2 CsClO, CsIO, CsClO, CsBrO, CsIO, 0.017 9.45 6.09 34.5 13.2 7.04 0.036 10.3 6.14 33.4 12.3 5.94 0.057 11.2 6.85 32.8 11.5 5.09 0.077 12.5 7.22 31.6 10.4 4.13 0.126 14.6 8.2 1 29.0 8.3 2.80 0.182 16.1 8.66 24.6 6.2 1.75 Discussion The results in Table 1 show that all the 1 :1 electrolytes investigated are more soluble in water than in H,O-Bu'OH mixtures. KClO,, KIO,, CsCIO, and CsIO, are, however, more soluble in H20-ACN than in H20 (Table 2).Solu-bilities of all potassium salts in water are in close agreement with those in the literature" (Table 1). The solubilities of KC104 (Table 2) are somewhat higher than those published3 but the trend with increasing AN concentration is the same. The solubility decreases with increasing molecular weight of the salts while KIO,, CsClO, and KClO, deviate from this trend in H20.In H20-AN (40% V), the effect of electrolyte size is less significant than in water, but somewhat greater than in the H,O-Bu'OH (40%V) system. Those observations are related to changes in the solvent structure due to solvent mixing. Introduction of the solutes into the solvent causes a change in the solvent structure which is greater with increas- ingly large solutes. Cs', C1-, Br-, I- and ClO, act as net structure breakers in water and K+ is a borderline case, but all the ions mentioned above are structure makers in non- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 aqueous solvents such as MeOH, DMSO and ACN.12 The different solubility behaviour of salts cannot be explained simply by structure-breaking or structure-making mecha-nisms.More informative, however, is the way in which the Gibbs energies vary with solvent composition in the mixtures. The transfer of all anions from H20 to H,O-Bu'OH mixtures is non-spontaneous, as indicated by the positive Gibbs energy of transfer (Table 3). Positive AimGe values have been inter- preted in terms of a stronger interaction of the anions with water than with Bu'OH. Preferential solvation by water is also expected to be reduced by the strong interaction of the water molecules with Bu'OH. The irregularities in AtrsG* us. composition around x2 x 0.05 (Fig. 1) are smaller than the experimental uncertainty and their existence is therefore doubtful. Nevertheless, these irregularities occur at the cosolvent concentration where the cosolvent reinforces the structure of water and this position does not depend on the nature of the ion but reflects a property of the mixture.AlrsGe values (Fig. 1) become increasingly positive in the order At, G*(I-) < At= G*(Br -) < A', Ge(Cl-)' and A,,, G*(IO;) < AtmG*(C106) but for halate anions the reverse order Air, G*(ClO;) < Alrs G*(BrO,) < A,, Ge(IO,) is observed. We compare the order of AlrsGe values with partial molal volumes, V'&, which have frequently been used as probes of ion-solvent interaction. In water, the V6 values of halide and halate anions13 are in the same order as the AtrsGe values. The partial molal volumes of C1-and Br- ions14 go through a shallow minimum and then increase as the Bu'OH content increases.This change of partial molal volume coincides with the well documented change of solvent structure occurring in H,O-Bu'OH mixtures.' 5,1 At higher cosolvent concentrations, the A,rs Ge values are somewhat more sensitive to the nature of the anion than those in the water-rich region. The ion size, the distribution of charge on the surface of the ions and the geometry are important factors in A,,Ge values. The shape of ions probably plays a role during their accommodation in the solvent cavity. The symmetrical charge distribution on the surface of X-and XO, ions in contrast to XO, and the shape of XO, being similar to the spherical symmetry of X-may explain the observed order of Airs Ge.The change in A,, G* of the anions Table 3 Transfer Gibbs energies for investigated anions evaluated from potassium and caesium salts in H,O-Bu'OH mixtures corrected to infinite dilution at 298.2 K Atn G*/kJ mol -' x(Bu'0H) ClO;(K+) ClO;(Cs+) IO;(K+) IO;(Cs+) ClO;(K+) ClO;(Cs+) BrO;(K+) BrO;(Cs+) IO;(K+) IO;(Cs+) 0.010 0.18 0.07 0.08 0.00 0.17 0.10 0.47 0.22 0.02 1 -0.17 -0.16 -0.31 -0.48 0.35 0.13 0.64 0.39 0.033 0.05 0.04 -0.13 -0.13 0.82 0.60 1.25 1-10 0.046 0.56 0.56 0.25 0.37 1.80 1.53 2.5 1 2.2 1 0.076 2.18 1.94 1.13 1.28 3.97 3.5 1 5.02 4.57 0.113 3.51 2.90 2.04 1.90 5.95 4.94 7.19 6.17 Table 4 Transfer Gibbs energies for investigated anions evaluated from potassium and caesium salts in H,O-AN dilution at 298.2 K At" G*/kJ mol-' 0.96 0.49 1.54 1.08 2.56 2.08 4.18 3.53 7.27 6.47 9.88 8.48 mixtures corrected to infinite x(AN) ClO;(K+) ClO;(Cs+) IO;(K+) IO;(Cs+) ClO;(K+) ClO;(Cs+) BrO;(K+) BrO;(Cs+) IO;(K+) IO;(Cs+) 0.017 -0.27 -0.36 -0.27 -0.40 0.21 0.07 0.36 0.23 0.99 0.65 0.036 -0.60 -0.90 -0.46 -0.50 0.38 0.10 0.80 0.43 1.94 1.29 0.057 -1.31 -1.50 -1.32 -1.39 0.13 0.04 0.78 0.49 2.50 1.75 0.077 -2.03 -2.17 -1.82 -1.74 0.13 0.01 1.06 0.79 3.19 2.53 0.126 -2.43 -2.22 -2.13 -1.96 0.98 0.80 2.26 2.18 5.47 4.65 0.182 -2.24 -1.89 -1.81 -1.17 2.37 2.65 4.47 4.5 1 8.22 7.79 CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 I 1 1 0.05 0.10 x( Bu'OH) Fig. 1 Gibbs energles of transfer to water-Bu'OH mixtures at 298.2 K. Data for Cl-, Br-and I-ions were recalculated from the published A,rsGe values of potassium salts.' (a) IO,, (b)BrO; , (c) CIO;, (4CI-, (e)Br-, (f) I-, (9)ClO,, (h)10; with cosolvent composition is largely a reflection of the H,O-Bu'OH interaction which is modified, of course, from that in pure binary mixtures by the solute ions. AN is considerably different from Bu'OH in its interaction with water in view of its very different size, shape and elec- tronic characteristics. In particular, the shape and size of AY us. Bu'OH will result in a considerably smaller hydrophobic effect. The course of the Atr,G* us.AN concentration plot 1s less dramatic than is observed in H,O-Bu'OH. The variation and order of AtrsGe values in AN-H,O are analogous to those in H,O-Bu'OH for halide and halate anions but for perhalate anions negative AIrsGe values, in contrast to the positive ones in H,O-Bu'OH, were found (compare Fig. 1 with Fig. 2). The anions studied are preferentially solvated by I 'I 6 -k4 E 73u2 d 0 -2 0.1 0.2 x(AN) Fig. 2 Gibbs energies of transfer to water-AN mixtures at 298.2 K. Data for C1-, Br-and I-ions are from published (0) lo;, (b)BrO;, (c) C1-, (d)Br-, (e)ClO;, (f)I-, (9)IO,, (h)ClO,. 857 I I I 1 0.1 0.2 x(org) Fig. 3 Transfer Gibbs energies for ClO; (0)and 10, (0)to water-rich binary solvent mixtures at 298.2 K. Data for 10, and for ClO, 'in H,O-MeOH and H,O-Me,CO are published values. (a)Bu'OH, (b)Me,CO, (c) MeOH, (d)AN.water in both water-organic cosolvent mixtures and the anomalous transfer Gibbs energies for perhalates on going from water to a variety of AN mixtures is a result of a specific effect of the anion on the solvent-solvent interaction. Sodium perchlorate' 'sl * is almost completely dissociated in AN solvent mixtures of composition up to x2 = 0.305. The irregu- larities around x2 = 0.05 in A,,,G* us. AN concentration are a reflection of changes in the solvent-solvent interactions. '' At this AN concentration the extrema in A,,, V+ for halide ions were also observed.,' Fig. 3 shows that addition of AN actually results in stabili- sation of perhalate anions but the behaviour of Bu'OH and Me,CO (acetone) is different from that of AN, MeOH adopts an intermediate position.This cosolvent difference can be understood in terms of the different Raoult's law and water activity behaviour for the respective solvent mixtures. The results obtained for AtrsGe of the anions investigated calculated from solubilities of K+ and Cs' salts (Tables 3 and 4) suggest that the TATB method in both investigated mixtures is in this case suitable for evaluation of the proper- ties of individual ions in solution at low cosolvent concentra- tions. A similar observation was made for AlrsHe of electrolytes with common cations (Na' or K') in water- methanol mixtures., ' The compilation of ionic enthalpies and entropies of transfer for the anions investigated throws more light on ion-solvent interactions than can be obtained from Gibbs energy data alone. References J.Burgess, 0. Vollarova and J. Benko, Transition Met. Chem., 1987, 12, 238. 0.Vollarova and J. Benko, J. Chem. SOC.,Dalton Trans., 1983, 2359. B. G. Cox, C. Guminski and H. Schneider, J. Am. Chem. SOC., 1982,104, 3789. P. Singh, I. D. MacLeod and A. J. Parker, J. Solution Chem., 1984, 13, 103. B. G. Cox, R. Natarajan and W. E. Waghorne, J. Chem. SOC., Faraday Trans., 1979,7586. 858 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 9 10 11 12 0.Vollarova and J. Benko, J. Chem. SOC.,Faraday Trans., 1993, 89,1745. C. W. Dawies, Zon Association, Butterworth, London, 1962, eqn. (3.14). Ja. Ju. Achadov, DielektriEeskije Svojstva Binarnych Rastvorov, Nauka, Moscow, 1977, pp. 272-287. J. Pointud, J. Juillard, J-P. Morel and L. Avedikian, Electrochim. Acta, 1974, 19, 229; J. Juillard and C. Tissier, Electrochim. Acta, 1982, 27, 123. B. G. Cox and W. E. Waghorne, Chem. SOC. Rev., 1980,9,381. M. Broul, J. Njlvlt and 0. Sohnel, Tabulky Rozpustnosti Anorganickjlch Lutek Ve Vode, Academia, Praha, 1979. M. H. Abraham, J. Lizsy and E. Papp, J. Chem. SOC., Faraday Trans. 1, 1982,78, 197. 15 16 17 18 19 20 21 Y. Koga, W. W. Y. Siu and T. Y.H. Wong, J. Phys. Chem., 1990, 94,7700. K. Nakanishi, Chem. SOC. Rev., 1993,177. R. L. Benoit and S. Y. Lam, J. Am. Chem. SOC., 1974,99,7385. M. S. K. Niad and A. Khan, J. Chem Eng. Data, 1993,38,98. E. Kamieilska-Piotrowicz and H. Inerowicz, J. Chem. Soc., Faraday Trans., 1990,86,3391. G. T. Hefter, J-P. E. Grolier E. H. Roux and G. Roux-Desgranges, J. Solution Chem., 1990,19,207. J. Benko and 0. Vollarova, Collect. Czech. Chem. Commun., 1992,57,2227. 13 14 B. E. Conway, J. SoZution Chem., 1978,7, 721. M. Dollet, J. Juillard and R. Zana, J. Solution Chem., 1980, 9, 827; G. T. Hefter, J-P. E. Grolier and A. H. Roux, J. Solution Chem., 1989,18,229. Paper 3/06564B;Received 2nd November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000855

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6). 859-864 2-Methoxyethanol-Water Solvent System :Static Relative Permittivity from -10 to +80 OC Fulvio Corradini, Luigi Marcheselli, Andrea Marchetti, Mara Tagliazucchi, Lorenzo Tassi and Giuseppe Tosi Department of Chemistry, University of Modena, via G. Campi, 183,41100 Modena, Italy A detailed dielectric study of 2-methoxyethanol (ME)-water (W) mixtures has been carried out as a function of temperature in the range -10 to +80"C and over the entire binary composition range (0 dx d 1). The experi- mental data, obtained by the heterodyne beat method at 2 MHz, were used to test some empirical relations of the type E = s(T), E = E(X) and E = E(T,x), in order to assess the empirical performances in dielectric behaviour of these mixtures, including the experimental conditions which are able to modify such patterns.The data reported here for ME-W binary mixtures were useful trying to understand the relative discriminating ability of both com- ponents towards cooperative intermolecular interactions in the liquid state, the quantitative similarities and differences between the chosen pure species, the intermolecular phenomena and interactions influencing the dielectric properties of the mixtures and the usefulness of a qualitative description of the possible formation of solventsosolvent complex species involving hydrogen-bonding, dipole-dipole and other interactions. 2-Methoxyethanol (ME) has been widely investigated as a non-aqueous solvent in the past' and it is an important potentially acidic solvent for analytical and industrial pur-poses. The study of physico-chemical properties of binary mixtures of ME with other solvents helps to clarify its struc- tural arrangements at the molecular level.In order to investi- gate the molecular orientations and interactions occurring between the unlike species of the binary mixtures, static rela- tive permittivities at different mole fractions of ME in water (W) at 19 temperatures ranging from -10 to +80 "C hake been determined. Note that ME differs from W in its steric, hindrance, quad- rupolar charge distributions and number of proton-donating and -accepting sites; furthermore, its hydrogen-bond donat- ing tendency seems to be partially overshadowed by its quasi-aprotic nature (Kautoprot at 20'C),' and by the fact = that ME can exist in two conformations, gauche or anti with respect to the two substituent groups in the alkyl chain, -OH and -OCH,.In the gauche conformation, a strong intramolecular hydrogen bond is formed between the two groups, reducing the possibility of interaction with neigh- bouring molecules ;under these conditions only intermolecu- lar dipolar interactions or hydrogen-bond behaviour can OCCU~.~ Taking into account the bifunctional nature of the ME component, dielectric studies of its binary mixtures with water are of interest in elucidating the structural arrange- ments and molecular interactions between unlike species. Furthermore, it is possible to estimate the magnitude of the deviation of the dielectric behaviour for ideal mixtures bv applying a general relationship between the pure-componen t properties and the binary composition, thus tentatively derik -ing the stoichiometry of solvent-cosolvent complex moieties.Experimental Materials ME (containing <0.05% of water by weight, found by Karl-Fischer titrations) was Carlo Erba (Milan) high-purity grade reagent. The solvent was preserved over 3 %i molecular sieves for several days before use, and the final purity was checked by gas chromatography (99.7%), confirming the absence of other significant organic components. Water. utilized for the preparation of the binary mixtures and as the pure solvent, was doubly distilled over KMnO, in a quartz apparatus and had a specific conductance <55 nS cm-'.Apparatus and Procedure The solvent mixtures were prepared by weight through a Mettler PM 4800 A-range, operating in a dry box to avoid contact with atmospheric moisture. The probable error in the ME mole fraction (xl) is estimated to <1.5 x lo-,. The equipment and the experimental procedures for stan- dardization of cells and relative permittivity measurements have been described elsewhere., The relative permittivities for the standards were taken from ref. 5. The experiments were generally repeated at least 10 times for each composition and at each temperature, with a con- fidence interval of 95%, and the results were averaged. The reproducibility of measurements expressed as the standard deviation a(&)was ca.& 0.2%. The thermostatted measuring cell was encased in a poly- urethane protective jacket, and the temperature control was provided by a Lauda K2R thermostat bath maintained to & 0.02"C.The temperature was measured by a Pt 100 ther-moresistor (Tersid, Milan) immersed in the measuring cell, and resistances were measured with a Wayne Kerr 6425 Pre-cision Component Analyzer. Karl-Fischer titrations were performed with an automatic titration system (Crison model KF 431) equipped with a digital burette (Crison model 738). Results and Discussion The experimental relative permittivities measured for these binary solvent mixtures (A, B, . . . M) are presented in Table 1, where x1 indicates the mole fraction of ME and where some values are absent owing to phase separation.Furthermore, for the two mixtures H (xl = 0.0892) and I (xl = 0.0545) it was impossible to determine the static relative permittivity at temperatures higher than 40 and 30 "C, respectively, even though a high degree of accuracy was achieved for the prep- aration of different sample mixtures and the cells were refilled J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Experimental relative permittivities (E) for 2-methoxyethanol(l) water (2) binary mixtures at various temperatures t/T 1.oooO 0.6756 0.4792 0.3494 0.2564 A B C D E -10 20.03 30.10 40.14 47.20 55.59 -5 19.55 29.09 39.05 46.08 53.96 0 19.08 28.13 38.03 44.90 52.62 5 18.63 27.28 37.10 43.68 51.32 10 18.19 26.46 36.1 1 42.5 1 49.79 15 17.75 25.7 1 35.01 41.30 48.64 20 17.35 24.97 34.17 40.18 47.40 25 16.94 24.28 33.19 39.12 46.16 30 16.54 23.52 32.18 38.08 44.76 35 16.17 22.89 31.39 37.26 43.69 40 15.76 22.2 1 30.60 36.21 42.42 45 15.38 21.52 29.68 35.16 41.35 50 15.02 20.77 28.81 34.23 40.22 55 14.68 20.18 28.12 33.49 39.34 60 14.32 19.56 27.40 32.43 38.19 65 14.04 19.02 26.58 31.59 37.13 70 13.71 18.52 25.76 30.92 36.12 75 13.34 17.80 25.14 30.10 35.34 80 13.01 17.24 24.43 29.15 34.45 many times.This probably arises from the high specific con- ductance of these solutions which is dependent on the tem- perature and the composition of the mixture.The relative permittivity in its expanded form e =E' -id' contains a real (E') and an imaginary (d') contribution; probably for the two abovementioned mixtures E" is always greater than E', making it impossible to determine the relative permittivity by the het- erodyne beat method.6 A comparison between the data of the present work and those of literature for pure ME at various temperatures and for its binary mixtures with water at 25"C, has been made. Our values agree very well with those of ref. 7 for binary mixtures, while the contrary is true for pure ME as reported in ref. 8, although the difference is small in the range 30- 40 "C. The dependence of our relative permittivities on the tem- perature has been checked by using the equation' In E = a.+alT where T is in K and the ai coefficients are empirical fitting parameters, listed in Table 2 along with the standard devi- ation a(ln E) for each mixture. Eqn. (1) appears to be ade- quate to represent the experimental measurements as the average difference dE is evaluated as follows : CNI%,lc -Eexp IA&= N where Nis the number of experimental points (186) of Table 1. Using this equation dE = k0.06. Table 2 Coefficients aiand standard deviations a(ln E) of eqn. (1) for the 2-methoxyethanol(l)-water(2) solvent system 1.m 4.250 92 -4.766 90 1.3 0.6756 5.004 89 -6.094 83 3.2 0.4792 5.149 77 -5.529 44 2.0 0.3494 5.262 50 -5.345 76 2.3 0.2564 5.421 31 -5.337 77 2.0 0.1865 5.592 14 -5.494 30 1.6 0.1327 5.548 44 -4.974 87 1.3 0.0892 5.978 32 -6.242 82 1.2 0.0545 5.857 88 -5.564 04 1.4 0.0249 5.896 23 -5.403 30 1.3 O.oo00 5.697 8 1 -4.487 67 0.8 0.1865 0.1327 0.0892 0.0545 0.0249 O.oo00 F G H I L M 63.09 69.28 76.28 61.40 67.55 73.95 78.67 59.74 65.9 1 71.67 76.5 1 83.00 87.45 58.20 64.34 69.63 74.39 80.80 85.48 56.72 62.80 67.5 1 72.54 78.64 83.67 55.11 6 1.28 65.41 70.52 76.7 1 81.82 53.67 59.82 63.43 68.58 74.63 80.1 1 52.19 58.3 1 61.36 66.62 72.64 78.3 1 50.81 56.96 59.42 64.66 70.76 76.56 49.40 55.42 57.65 -68.86 74.80 48.12 54.20 55.84 -67.01 73.23 -46.68 52.77 -65.26 7 1.46 -45.36 51.56 -63.55 70.00 -44.30 50.20 -61.82 68.45 42.96 49.00 --60.16 66.85 41.79 47.75 --58.55 65.42 40.83 46.50 --56.87 63.87 39.56 45.34 --55.33 62.47 38.49 44.30 --53.84 61.10 In order to investigate the E =E(xl) correlation, we plotted in Fig.1 the trend of E us. water mole fraction (xJ. The trend is non-linear and a polynomial expansion of the typeg (3) has been employed to fit the isothermal experimental relative permittivity data. The flj coefficients (for j =4) are sum-marized in Table 3, along with the standard deviations at each temperature. The usefulness of this equation is shown by the average uncertainty = f0.46 over all the experimental data. Obviously, it should not be employed when reliable experimental data are not available, i.e. in the range 0.0249 <x1 <0.1327 at temperatures 240"C,in order to avoid obtaining extrapolated values with no physical signifi- cance.In order to investigate the possibility of representing the dielectric properties of this solvent system through a single function of the type E =E(T,xl), the experimental values were fitted by the equation ij In E = yij ~'x', (4)00 obtained by combining the eqn. (1) and (3). This three- dimensional correlation model appears to be very useful in 80i -10°C A 60 E 40 20 1 I I I I 0.0 0.2 0.4 0.6 0.8 1.o x2 Fig. 1 Experimental trend E us. x2 for the 2-methoxyethanol (1)-water (2) solvent system from -10to +80 "C J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 861 Table 3 Coefficients Bj of eqn. (3)for the 2-methoxyethanol(l)-water(2)solvent system at different temperatures tl”c Bo B1 82 83 84 -10 4.561 45 -2.829 12 3.66403 -4.153 14 1.75400 -5 4.496 58 -2.420 23 2.33255 -2.525 88 1.089 67 0 4.473 52 -2.495 37 2.753 10 -3.322 77 1.54015 5 4.448 7 1 -2.537 03 3.005 58 -3.810 81 1.81811 10 4.425 35 -2.601 02 3.297 10 -4.304 99 2.084 09 15 4.400 8 1 -2.637 52 3.441 86 -4.547 52 2.218 48 20 4.37691 -2.706 04 3.816 34 -5.214 74 2.581 14 25 4.35155 -2.758 48 4.075 78 -5.650 25 2.81 1 08 30 4.326 56 -2.80464 4.282 97 -6.022 85 3.023 61 35 4.306 02 -2.872 23 4.598 47 -6.553 01 3.303 69 40 4.282 41 -2.931 76 4.870 18 -7.006 45 3.543 16 45 4.260 85 -2.775 88 3.841 80 -5.250 77 2.656 99 50 4.238 52 -2.834 03 4.109 78 -5.768 14 2.962 66 55 4.213 96 -2.869 78 4.401 70 -6.377 20 3.317 25 60 4.189 80 -2.921 87 4.593 82 -6.669 14 3.468 60 65 4.16611 -2.95300 4.68355 -6.80521 3.55049 70 4.13945 -2.933 07 4.601 97 -6.71465 3.523 98 75 4.11 6 21 -3.038 54 5.265 30 -8.02030 4.267 99 80 4.09205 -3.041 89 5.21049 -7.938 73 4.24344 ofIn E) x lo2 1.2 1.1 0.9 1.1 1.2 1.2 1.5 1.6 1.8 1.9 2.2 1.7 1.8 1.9 2.2 2.2 2.2 2.4 2.7 interpolating the static relative permittivity for any pair of independent variable quantities T and xl.The use of this model equation must be avoided in the abovementioned con- ditions because the limitations of eqn. (3) are reflected in eqn. (4). This fitting equation, whose coefficients yii, as evaluated by the TSP” statistical package, are reported in Table 4, provides a set of calculated values that are in very good agreement with the experimental values with an average uncertainty L\E = kO.47.Excess Functions When dealing with completely miscible binary liquids, it seems very useful to examine how their excess properties depend on the composition of the mixture. This fact requires the choice of a suitable criterion to establish the significance of ideal mixing behaviour and the ideal composition depen- dence. For thermodynamic properties, the ideal composition dependence may be defined within the widely accepted gener- alization of Raoult’s law.ll Since ideal behaviour has not been theoretically established for non-thermodynamic proper- ties such as E, many models and theories have been sug- gested.I2-l6 Even if the various definitions of ideality appear to be sufficiently consistent, many ambiguities regarding the detailed interpretations of the different excess relative permit- Table 4 Coefficients yij of eqn.(4)for the 2-methoxyethanol-water solvent system and standard deviation ofln E) variable ij ~~~ quantity Yij 00 5.76600 01 -0.527 57 02 -6.008 31 03 12.937 36 04 -7.91675 10 -4.73687 x lo-’ 11 -7.26200 x lo-’ 12 3.24082 x 13 -5.99401 x 14 3.47638 x 1.3 x tivity curves are present. As a consequence, in order to avoid the misleading use of insuficiently tested and proved theories, it is more prudent to apply an intuitive, but reliable, approach that translates non-thermodynamic quantities into quasi-thermodynamic quantities.Therefore, following sugges- tions in the literature,” the deviation of the dielectric behav- iour of the real systems from ideality can be evaluated by the equation : & = EE + EIXl + E2X2 (5) where cE is the excess function, and and c2 are the values for the two pure species at each temperature. Note (Fig. 2) that cE appears always to be very large and negative at each temperature investigated. In order to consider also the variation of cE with composi- tion, the excess data were isothermally fitted by a smoothing equation of the type’* 5 EE = x1x2 1 U&l -(6)k=O whose uk coefficients are listed in Table 5, along with the standard deviations g(cE).Note that the curves of Fig. 2 exhibit a pronounced, broad, minimum centred around x2 x 0.65, i.e. ME :W = 1 :2; the cE value corresponding to this 0 EE -1 0 -20 0.0 0.2 0.4 0.6 0.8 1 .o x2 Fig. 2 Isothermal excess relative permittivity curves for the 2-methoxyethanol (1)-water (2)solvent system calculate by eqn. (6) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 5 Coefficientsakof eqn. (6) for the 2-methoxyethanol-water solvent system at various temperatures tpC 00 a1 a2 0 -63.04 -89.69 21.89 5 -62.56 -82.23 3.95 10 -62.07 -75.00 -13.32 15 -61.56 -68.04 -0.39 20 -61.04 -61.41 -45.53 25 -60.52 -54.87 -60.78 30 -59.98 -48.70 -74.99 35 -59.43 -42.77 -88.75 40 -58.93 -35.73 -105.84 45 -60.06 11.44 -202.58 50 -59.63 20.34 -222.92 55 -59.19 28.90 -242.36 60 -58.73 36.80 -260.25 65 -58.26 44.47 -277.43 70 -57.78 51.62 -293.35 75 -57.28 58.43 -308.47 80 -56.78 64.87 -322.53 minimum becomes more negative as the temperature is decreased, so that it is -18.8 at 0“C.In many theories explaining the behaviour of non-electrolyte solutions, the major contribution to the deviation from ideal mixtures is attributed to the hydrogen-bonding tendency between components, the dipole-dipole interactions and specific interactions such as dispersion forces.lg In general, when negative deviations from ideal behaviour of thermomechanical properties (p, q, E ...) occur in mixtures of components whose molecules are very different in shape and size, these deviations can be accounted for by geometric effects.” In this light, note that both components of these mixtures show considerable proton-donating and proton- accepting ability, although the number of active sites is differ- ent for the two pure species.Furthermore, the W molecules are smaller than those of ME, the molar volumes being Vl = 79.237 cm3 mol-’ and V2 = 18.068 cm3 mol-’ at 25°C.21 Hence, the large difference in the shape and molecular size of these two components can reasonably be invoked as an important factor in arranging unlike species in the final liquid structure of the mixtures. Taking into account all these con- siderations, the negative deviations from ideality for the system under study can therefore be related to intermolecular hydrogen bonding and other interactions” between the com- ponents of the mixture.According to suggestions in the literat~re,~~ the formation of a complex species could be supposed, having an approx- imating composition corresponding to the minima of Fig. 2. Therefore, the minimum in the excess relative permittivity could indicate a maximum in the structuredness between dif- ferent components in mixtures. After the above considerations, note that the trend of E~ us. temperature (Fig. 3) suggests that the mixtures studied may be separated into three distinct composition groups. In the absence of substantiated explanations in the literature we expect that phenomena that depend on the molecular and supramolecular composition could occur to induce this dis- tinction. By studying the thermodynamic properties of binary mix- tures with water, other authorsz4 have argued that the addi- tion of a cosolvent progressively inhibits cooperative fluctuations in the hydrogen-bonding connectivity of the water molecules, these network interactions being progres- sively replaced by a water-cosolvent mixed connectivity.In a3 a4 a5 o@E) x 10 387.18 -782.17 383.27 4.0 349.25 -623.93 254.34 3.8 312.33 -471.03 130.27 4.0 276.78 -324.35 11.58 4.4 243.02 -185.31 -100.90 4.9 209.45 -49.03 -210.31 5.5 177.74 78.76 -3 12.77 6.2 147.44 201.97 -41 1.67 7.7 110.68 355.14 -534.40 8.4 -205.38 1254.22 -1048.94 4.5 -256.06 1438.72 -1183.69 5.0 -304.99 16 16.01 -1312.90 5.4 -349.76 1779.15 -1432.22 5.8 -393.37 1936.29 -1546.49 6.2 -433.93 2082.78 -1653.29 6.5 -472.35 2221.59 -1754.43 6.8 -508.79 2351.86 -1849.04 7.1 pentamers (as the minimal units) and more complex cluster units which fluctuate rapidly between low-density-low-entropy forms and high-density-high-entropyforrn~.~~*’~Fur-thermore, it was hypothesized that the average size of the fluctuating units should increase with decreasing tem-perature, or upon hydration of partially hydrophobic species.This is the case, for example, of water-hydrazine mixtures, whose excess thermodynamic properties could be separated into three regions. A brief examination of the excess relative permittivities of the ME-W mixtures could lead to similar observations.Obviously, the pattern that will be described requires that the dielectric properties are very sensitive both to hydrogen- bonding interactions and to cooperative fluctuations. (i) Region MI, 0 < x1 z0.1 (Mixtures H, I and L) In this composition range, cE exhibits a clear dependence on composition, reflecting a breaking off of the cooperative fluc- tuation units. Furthermore, as deE/dT < 0, the trend of the curves is influenced strongly by the hydrophobic character of ME. An explanation for this is that in dilute aqueous solution ME may exist in the predominant gauche cyclic conformationz7 and in this arrangement strong intramolecu- lar hydrogen bonding would reduce the ME-W hydrogen-bonding tendency and increase the hydrophobicity of ME.Note that an increase in temperature should favour ME anti conformers (without the intramolecular hydrogen bond) rather than the gauche conformers. As a consequence, in this I -20 t particular, an explanation of the magnitude and the trends of 0 20 40 60 the thermodynamic properties in the aqueous mixtures con- t/”csidered, could lie in cooperative fluctuation phenomena Fig. 3 Excess relative permittivity 11s. temperature for the 2-which are characteristic of water, involving tetramers and methoxyethanol (1)-water (2) solvent system J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 configuration the ME molecules may interact better with water, providing a more reliable and ordered mixing struc- ture of the medium, whose constituent units could be hetero- aggregated entities of the type ME 2W and fluctuating water units; the large deviation from ideality for these mixtures with increasing the temperature is indicative of molecular aggregation of the cosolvent.28 A further description in terms of structural models could be made taking into account the fact that the upper limit for MI corresponds to the cosolvent concentration beyond which the solvated heterocluster structures cannot exist, as dcE/dT is negative in this region and positive for greater ME con- tents.On this basis, we could suggest that the binary mixture having dcE/dT = 0 could be taken as the limiting composi- tion of the existence of solvated heteroaggregated clusters.On the other hand, the solvent properties of these mixtures could also be described in terms of fluctuating behaviour, since the MI composition range seems to suggest fluctuations in the homocooperative water units. (ii) Region MI,,0.1 < x1 x 0.3 (Mixtures E, F and G) The increasing trend of cE with temperature (Fig. 3) reflects the progressive loss of W-W hydrogen bonding, clearly showing a tendency to quasi-ideal behaviour in the mixture sequence E -= F < G. At the molecular level and on the basis of the preceding remarks, we can describe the curves of this region as follows. Starting from the MI limit region, the mixing properties are characterized by the W-W inter- and intra-connectivities ; with increasing the organic cosolvent ratio the homecoopera- tive interactions decrease, and are progressively replaced by hetero ME-W connectivities. Therefore, the upper composi- tion limit of the M, region may be considered as that com- position where the solvent-cosolvent hydrogen-bonding interactions become predominant.This limit could corre-spond to the formation of the complex solventxosolvent moieties of stoichiometric ratio ME : W = 1 :2, x1 2 0.33. Furthermore, at this composition limit all the ME and W molecules should be involved in the heteroaggregated ME 2W clusters, which corresponds to the maximum devi- ation from ideality and the maximum structuredness of the pure species, as shown in Fig.2 by the relatively smooth variation of cE within a broad minimum. From Fig. 2 it is possible to see the strong dependence of cE on the composi- tion of the ME-W mixture both in the upper MII limit region and over the entire MIregion. (iii) Region MIII,0.3 x x1 < 1 (Mixtures B, C and D) In this region, the variation of the excess property reflects a molecular situation in which mixed hydrogen-bonding con- nectivities replace those of the pure organic solvent. Increas- ing temperature causes little deviations from ideality, while the contrary is true for increasing water content, in the sequence B c C c D. Since at the molecular level the hydrogen-bonding interactions in pure ME display no signifi- cant cooperativity compared with W, starting from the upper limit of the MII region (xl x 0.3), the increasing ME-ME homoconnectivity should lead to the disappearance of het- eroclusters; consequently, one-dimensional and successively two- and three-dimensionally ordered structures due to the ME-ME interactions should take place up to the pure ME species.Conclusions The theories and empirical treatments used here to analyse the experimental relative permittivity data of ME-W mix-tures appear to be the most simple and are adequate for the present system, even if for a few mixtures it was impossible to obtain the experimental E values and, as a consequence, the calculating procedures must be used ‘with a grain of salt’. However, in connection with our previous work, we have tried to justify some experimental evidence through conjec- tures regarding the formation of adduct species whose pres- ence, however, may be revealed only by indirectly measurable properties. It seems very likely that by mixing the two components, ME acts as a structure breaker with respect to pure water.Furthermore, such a breaking off and the large change in intermolecular forces on passing from pure to mixed species, would have an appreciable effect on the properties of the molecules and, as a consequence, on the macroscopic experi- mental behaviour of the systems. Following the approach of Payne and Theodorou,’ attempts have been made to explain the behaviour of these liquid mixtures on the basis of the sign and the magnitude of excess quantity cEthat, as it is always negative and very large, could be mainly associated with the formation of solvent- cosolvent complex moieties with a lower dipole moment.The stoichiometric ratio of these complex species corresponds to the maximum deviation of the excess quantity cE from ideali- ty, i.e. ME : W x 1 : 2 mole ratio at all the temperatures investigated. The excess relative permittivity investigated here, cE, and the excess molar volumes VE studied previously2’ provide similar evidence : both functions are negative, and the magni- tude of the deviation increases with the temperature. The authors are thankful to Prof. C. Preti for providing helpful suggestions and encouragement to carry out this work.L. M. is grateful to Hospal-Dasco s.p.a. (Modena, Italy) for the award of a Junior Research Fellowship. The Minister0 dell’universita e della Ricerca Scientifica e Tecnol- ogica (M.U.R.S.T.) of Italy is gratefully acknowledged for financial support. References 1 W. H. Byers, J. Chem.Phys., 1939,7, 175. 2 A. P. Kreshkov, M. T. Smolova, A. Veveris and B. Spince, Zh. Fiz. Khim., 1977, 51, 1827. 3 R. Iwamoto, Spectrochim. Acta, 1971,27,2385. 4 G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi, J. Chem. SOC.,Faraday Trans. I, 1988,84,4427. 5 A. A. Maryott and E. R. Smith, Table ofDielectric Constants of Pure Liquids, Natl. Bur. Stand., Circ. No. 514, 1951. 6 A. R. Van Hippel, in Dielectrics and Waues, Wiley, New York, 3rd edn., 1962. 7 G.DouhCret and A. Pal, J. Chem. Eng. Data, 1988,33,40. 8 V. Viti and P. Zampetti, Chem. Phys., 1973,2,233. 9 G. Ritzoulis, N. Papadopoulos and D. Jannakoudakis, J. Chem. Eng. Data, 1986,31, 146. 10 TSP-Time Series Processor-User’s Guide, ed. B. H. Hall, TSP International, Stanford, CA, 1987. 11 B. F. Lovelace, W. H. Battlke and J. C. W. Frazer, J. Am. Chem. SOC.,1923,45, 2930. 12 L. Onsager, J. Am. Chem. SOC.,1936,58,1486. 13 J. G. Kirkwood, J. Chem. Phys., 1939,7,911. 14 0.Dusart, J. P. E. Grolier and A. Viallard, Bull. SOC. Chim. Fr., 1977,787. 15 M. I. Davis and G. Douheret, Thermochim. Acta, 1986,104,203. 16 S. Oswal, Can. J. Chem., 1988,66, 111. 17 R. Payne and I. Theodorou, J. Phys. Chem., 1972,76,2892. 18 0.Redlich and A. T. Kister, Znd. Eng. Chem., 1948,40,341. 19 J. P. Hirschfelder, C. F. Curtis and R. B. Bird, in Molecular Theory of Gases and Liquids, Wiley, New York, 1954. 20 E. A. Guggenhim, in Mixtures, Oxford University Press, Oxford, 1952. 21 F. Corradini, G. C. Franchini, L. Marcheselli, L. Tassi and G. Tosi, Aust. J. Chem., 1992,45, 1109. 22 R. J. Fort and W. R. Moore, Trans. Paruday SOC., 1966, 62, 1112. 23 Yu. Ya. Fialkov, Zh. Fiz. Khim., 1963,37, 1051. 864 J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 24 R. Lumry,E. Battistel and C. Jolicoeur, Fataday Symp. Chem. SOC.,1982, 17,93. 28 1981,85,733. G. Atkinson, S. Rajagopalan and B. L. Atkinson, J. Phys. Chem., 25 26 27 C. A. Angell, J. Phys. Chem., 1971,75,3698. R. Speedy, J. Phys. Chem., 1984,88,3364. L.P. Kuhn and R. A. Wires, J. Am. Chem. SOC., 1964,86,2161. Paper 31049461; Received 16th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000859

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6),865-868 Preferential Solvation of a p-Sensitive Dye in Binary Mixtures of a Non-protic and a Hydroxylic Solvent Marivania Scremin, Sandra Patricia Zanotto, Vanderlei Gageiro Machado and Marcos Caroli Rezende” Departamento de Quimica ,Universidade Federal de S. Catarina, Norianopolis, SC 88040-970,Brasil The prefer enti aI soIvation of the soIvat oc h rom i c dye N,N,N’,N’-tetramethyleth yIened iami noacety I-acetonatocopper(ii) perchlorate (l),[Cu(tmen)(aca)]CIO, , in binary solvent mixtures comprising one hydroxylic component (water, methanol, ethanol or propan-2-01) and a non-protic co-solvent (acetone, acetonitrile or dimethylformamide) is discussed and interpreted in terms of hydrogen-bonding effects which alter the ‘intrinsic’ electron-donating ability of the hydroxylic solvent. Solvatochromic dyes have often been used in the investiga- tion of the properties of solvents and solvent mixtures.The realization that their behaviour in solution ultimately reflects specific interactions with their microenvironment has made them ideal probes for studies of preferential solvation in binary mixtures. Research in this area is often concerned with protic-aprotic solvent mixtures, owing not only to the wide use of such media in chemical processes, but also to their complexity, with strong dipole-dipole attractions occurring side by side with equally important hydrogen-bonding inter- actions. Dyes may be classified according to their sensitivity to dif- ferent properties of the medium.Compounds which are pre- dominantly sensitive to the electron-accepting properties of a solvent may be characterized as r-sensitive, whereas p-sensitive dyes mainly reflect the donor properties of the medium. The variety of solvatochromic probes employed in such studies have in common the fact that they are, in most cases. strongly sensitive to the hydrogen-bond accepting andjor donating ability of the medium.’-’ Thus, deviations from an ideal behaviour in mixtures comprising protic solvents are generally ascribed to hydrogen bonding of the solvent to the dye. Studies utilizing dyes which are sensitive to the electron- donating ability of the medium are much more scarce. Among these dyes, the series of salts of ethylene-diaminoacetylacetonatocopper(n), first prepared by Fukuda and Sone‘ are unique in that they are not sensitive to the electron-accepting ability of the polarizability of the medium, being considered as exclusive probes for the solvent donating ability.’ Thus, Marcus and Migron have employed com-pound 1 (Scheme 1) in the determination of Kamlet and Taft’s parameter /3 for quite a few solvents and solvent mix- tures.’~~ ’CH:, ICH: 1 J Scheme 1 A study on the solvatochromism of dye 1 in dimethyl-formamide (DMF)-nitromethane mixtures has been published.” We decided in the present work to extend this study to protic-aprotic solvent mixtures for various reasons. Besides the aforementioned scarcity of such investigations, we were interested in detecting anv Deculiar behaviour in these mixtures, as frequently happens when an r-sensitive dye is In addition, and contrary to the expected behaviour of DMF-nitromethane solutions of 1, where the electron-donating abilities of the two components are well established and are very different, the donating behaviour of solvent mixtures comprising protic components is in principle unpredictable.This stems from the fact that the values for electron-donating ability of protic solvents vary widely according to the method utilized for their obtention, an effect which may reflect the indirect action of hydrogen bonding in solution. Accordingly, one may encounter values of ‘bulk’ electron-donating ability, which differ appreciably from donor numbers of ‘isolated’ molecules.Such variations should, in principle, be found in binary mixtures of variable composition, where the degree of hydrogen bonding of the protic co-solvent changes with its mole fraction in the mixture. Thus, in spite of the simplification introduced in a system with a pure @-sensitive probe (no direct hydrogen- bonding interaction between the dye and the protic co-solvent), deviations from the ‘normal’ behaviour found in non-protic solvent mixtures may still occur, an anticipation which justifies the present work. Experimental The spectra of dye 1 in all binary mixtures were recorded on a Beckman DU-65 spectrophotometer. The copper dye 1 was prepared following a reported pro- cedure.6 Redistilled water and analytically pure alcohols were employed in all solutions.In addition, pure dimethyl-formamide and redistilled acetone and acetonitrile were dried over molecular sieves prior to use. Results and Discussion The solvatochromic behaviour of dye 1 in binary mixtures comprising one hydroxylic component is shown for acetone- ROH (Fig. l), acetonitrile-ROH (Fig. 2) and DMF-ROH (Fig. 3). The hydroxylic solvents employed were water, meth- anol, ethanol and propan-2-01. The data plotted in Fig. 1-3, which give the variations of the wavenumber, V,,,, of the dye in mixtures of variable molar composition, X, were fitted to third-order polynomials by means of a least-squares method. With the exception of the water-acetone plot, the general equation Vmax = a +bX +cX2 +dX3 was found to reproduce all data rather accu- rately, as shown in the figures.The values of the coefficients u, b, c and d for each binary mixture are given in Table 1. Values of Vmax in acetone-water mixtures were constant and equal to 16.86 x loM3cm-’ for water mole fractions ~0.13. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 X X 0.2 0.4 0.6 0.8 1 1 1 1 1 1 ' 1 1 17.4 17.4 17.2 7 17.0 5 m 16.8 t t 16.8 1 16.8 1 II 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 X X Fig. 1 Variations of V,,, of dye 1 in binary mixtures with mole frac- Fig. 2 Variations of Vmax of dye 1in binary mixtures with mole frac- tion of the hydroxylic component: (a) acetone-water; (b) acetone-tion of the hydroxylic component: (a) acetonitrile-water; (b) methanol; (c)acetone-ethanol; (d) acetone-propan-2-01 acetonitrile-methanol; (c) acetonitrile-ethanol; (6) acetonitrile-propan-2-01 X For binary mixtures richer in acetone, the observed G,,, values were as follows 5,,, (X)]: 17.51(0),17.12(0.03), 17.01(0.05), 16.92 (0.075), 16.89 (0.1).A first distinction may be drawn between binary mixtures 16.9 1containing the relatively weak, non-protic donor solvents acetone and acetonitrile and those containing the strong donor DMF. Whereas in mixtures of the latter, the dye is in all cases preferentially solvated by DMF, in the acetone- Table 1 Values of the coefficients a, b, c and d in the polynomial Vmrx = a + bX + cX2+ dX3for each binary mixture 3 16.9coefficients/10-binary mixture a b C d ~ acetone-MeOH 17.51 -1.37 1.64 -0.84 Iacetone-EtOH 17.51 -0.30 -0.68 0.39 16-7* acetone-Pr'OH 17.51 -0.55 0.8 1 -0.69 acetonit rile-H 2O 17.30 -1.56 2.22 -1.16 acetonitrile-MeOH 17.30 -0.24 0.01 -0.22 0.2 0.4 0.6 0.8 acetonitrile-EtOH 17.30 -0.53 0.74 -0.71 X acetonitrile-Pr'OH 17.30 -0.21 0.89 -1.04 DMF-H 20 16.58 -0.03 0.49 -0.19 Fig.3 Variations of Cmnx of dye 1 in binary mixtures with mole frac- DMF-EtOH 16.58 0.12 -0.56 0.72 tion of the hydroxylic component: (a) DMF-water; (b) DMF-ethanol J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Estimated molar percentage of the hydroxylic component. ROH, in the solvation shell of dye 1, for a binary mixture with a 1 1 bulk molar composition non-protic co-solvent (%) - ~~ ROH acetone acetonitrile DMF ~ H2O ca. 100 74 30 MeOH 66 33 - EtOH 46 34 4 Pr’OH 37 3 - ROH and acetonitrile-ROH systems, preferential solvation IS shifted from the hydroxylic component to the non-protic co-solvent as we change from water to propan-2-01. The degree of solvation of the dye by the hydroxylic co- solvent in each binary mixture may be more readily grasped by a comparison of the values of Table 2.These values, obtained from the curves drawn in Fig. 1-3, are estimated percentages of ROH in the solvation shell of 1 when the bulk composition of the solvent mixture is 1 : 1. By employing the third-order polynomials given in Table 1, the percentages of ROH may be obtained from the relationship ROH(%) = (ru -Vo)/(V1 -V,), where the subscripts 0.5, 0 and 1 refer to the calculated Vmax values for solutions where X = 0.5, 0 and 1, respectively. Thus, in a 1 : 1 molar acetone-water mixture, the dye is exclusively surrounded by water molecules, whereas, in acetone-propan-2-01, the solvation shell of the dye comprises about 37% of alcohol molecules.A comparison of the acetone and acetonitrile systems indi- cates a greater effectiveness of the latter solvent in competing with the hydroxylic co-solvent for the solvation of the dye. Preferential solvation by acetonitrile is verified in all binarq alcoholic mixtures, a trend which is reversed only when water is the hydroxylic co-solvent (Fig.2). In the case of acetone mixtures, water and methanol solvate the dye preferentially. whereas ethanol-acetone mixtures exhibit a nearly ideal behaviour, with bulk compositions very similar to local solvent distributions around the dye (Fig. 1). This greater effectiveness of acetonitrile, as compared with acetone, in solvating the copper dye probably reflects a greater association of the former with the soft Cu” ion. because of the stronger TC interactions of the -CN group with the planar dye.’0*’2 The preferential solvation of [Cu(tmen)(aca)]ClO, bq DMF in nitromethaneedimethylformamide mixtures was ascribed to the greater electron-donating ability of the latter solvent.” However, when one of the components is a hydroxylic solvent, the picture that emerges is more complex.The value of Vmax for 1 in a pure solvent may be assumed to be a measure of the donating ability of the medium. A rea-sonably good correlation was obtained betweenV,,, and Gutmann’s donor numbers (DN) for a series of solvents.6 interactions is facilitated by the assumption of different electron-donating abilities for the hydroxylic co-3olvent. Its ‘bulk’ donating ability reflects and incorporates the effects of extensive hydrogen bonding, which alters the ‘intrinsic’ donating ability of isolated molecules. Fig. 2 shows that the copper dye 1 is better solvated by acetonitrile than by any alcohol, in spite of the larger Gmax of the former. This may be due to the soft nature of the copper cation mentioned above, which associates better with aceton-itrile than with any alcohol ROH.This, however, suggests that the ‘intrinsic’ electron-donating abilities of these alco- hols are in fact smaller than their ‘bulk’ values, obtained in pure solvents. This situation is reversed when water is the hydroxylic co- solvent. Comparison of Fig. l(a) and (d), or of Fig. 2(aj and (d), shows that water solvates 1 in binary mixtures much more effectively than propan-2-01. The reason for this is not, according to our view, that water is intrinsically a better donor solvent than propan-2-01. The opposite should, in fact, be true. The alkyl groups in an alcohol should render the hydroxy group more basic than in water. The greater degree of dye solvation by water, than by propan-2-01 rather reflects differences in hydrogen-bond donation of these two solvents.In binary mixtures a strong hydrogen-bonding solvent may act as a ‘solvent scavenger’, binding and sequestering the non-protic co-solvent from the solvation shell of the dye. It may also stabilize and reinforce, through hydrogen bonds, other hydroxylic molecules already present in the solvation shell of the dye. A hydrophilic net is woven around the dye, with the more hydrophobic co-solvent being gradually expelled from its solvation shell. This is accompanied by enhanced electron-donating ability of the axial ROH ligands, because of hydrogen bonding to other ROH molecules, as shown in Scheme 2. H H,o$”------0I F+ ‘” Scheme 2 These effects oppose the ‘intrinsic’ weak electron-donating ability of the ROH molecules vis-Ci-vis the soft copper complex, and, in the case of the strongest hydrogen-bonding donor solvent water, even surpass the greater affinity of the dye for acetonitrile [Fig.2(a)]. It is interesting, at this stage, to compare our observations with similar studies published previously involving binary aqueous mixtures. Our data agree quite well with the report- ed values of Vmax for compound 1 in pure solvent^.^ Values Following this argument, the electron-donating abilities of ,!3. for the electron-pair donation tendency of the medium, water and the small aliphatic alcohols methanol, ethanol and propan-2-01 do not differ appreciably and are all greater than that of acetone or acetonitrile.This is apparent from the bathochromic shifts observed for the longest wavelength band of 1 in acetone or acetonitrile as a hydroxylic solvent is added. Nevertheless, preferential solvation of 1 by the better donor co-solvent is not always observed, as seen in Fig. l(cl, (4,Fig. 2(b), (c) and (4.Since direct solute-solvent hydrogen- bonding interactions may be excluded from our systems, the observed deviations and the progressive ‘take-over’ of the solvation shell by the hydroxylic component, as one changes from propan-2-01 to ethanol, methanol and water, must reflect indirect solvent interactions. The analysis of these have been reported previously for various aqueous mix- ture~.~.’.’ Migron and Marcus employed compound 1 as a p indica-tor and their data for water-acetonitrile mixtures’ agree quite well with our results, if the relationship Vmax = 18.76 -2.7938, derived by the same authors,’ is used to convert 3,,, into p values.Thus, we obtained a value of 16.86 x cm ‘ for the absorption wavenumber of 1 in water, a value very similar to the corresponding values in methanol (16.89 x lW3), ethanol (16.87 x and propan-2-01 (17.00 x lop3 cm-I). This corresponds to @ = 0.68, essen- tially the same as that reported by the authors (0.67) in their study of water-acetonitrile mixtures. Krygowski et al. studied various aqueous binary mixtures, using as solvatochromic probes 4-nitroaniline and N,N-diethyl-4-nitroaniline,' and arrived at results which are at variance with our observations.This may be due to the use of indicators which are not purely p-sen~itive.~ In fact, their results yield a very low electron-donating ability value for water (BKTor #I = 0.19), much smaller than that for methanol (0.62), ethanol (0.77) or propan-2-01 (0.88). These values were essentially the same as those reported previously by Taft and co-workers,14 who listed them between brackets as 'less certain'. Accordingly, in all aqueous mixtures studied by the authors, the #I indicators were prefentially solvated by the organic co-solvent, an observation which departs from our results with dye 1. In conclusion, the present study brings to light the ambi- guity of the electron-donating ability concept, when applied to hydroxylic solvents.Spectroscopic measurements of the pure &sensitive dye 1 yield similar values of electron-donating ability for water and small aliphatic alcohols. This observation cannot explain their different behaviour in sol-vating compound 1 in binary mixtures. Indirect hydrogen- bonding effects explain the relative order of effectiveness of the hydroxylic co-solvent in the preferential solvation of the dye. This effectiveness increases in the order Pr'OH < EtOH < MeOH < HzO, and parallels the strength of these compounds as hydrogen-bonding donor solvents. Grants by the Brazilian Conselho Nacional de Pesquisa Cientifica e Tecnologica (CNPq) are gratefully acknowledged.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 References 1 J. G. Dawber, J. Ward and R. A. Williams, J. Chem. SOC., Faraday Trans. I, 1988,84,713. 2 (a) P. Chatterjee and S. Bagchi, J. Chem. SOC., Faraday Trans., 1991,87,587; (b)1992,88,1675. 3 E. Bosch and M. Roses, J. Chem. SOC., Faraday Trans., 1992,88, 3541. 4 C. Lerfand P. Suppan, J. Chem. SOC., Faraday Trans., 1992,88, 963. 5 E. Dutkiewicz, A. Jakubowska and M. Dutkiewicz, Spectrochim. Acta, Part A, 1992,48,1409. Y. Fukuda and K. Sone, Bull. Chem. SOC. Jpn., 1972,45,465. Y. Migron and Y. Marcus, J. Phys. Org. Chem., 1991,4,310. Y. Migron and Y. Marcus, J. Phys. Chem., 1991,95,400. Y. Migron and Y. Marcus, J. Chem. SOC., Faraday Trans., 1991, 87, 1339. 10 D. Bourdin, D. Lavabre, J. P. Beteille, G. Levy and J. C. Micheau, Bull. Chem. SOC.Jpn., 1990,63,2985. 11 Y. Marcus, J. Solution Chem., 1984,13,599. 12 M. C. Rezende, C. Machado, S. P. Zanotto and M. Scremin, J. Phys. Org. Chem., 1993,6,637. 13 T. M. Krygowski, P. W. Krona, U. Zielkowska and C. Rei- chardt, Tetrahedron, 1985,41,4519. 14 M. J. Kamlet, J. L. M. Abboud, M. H. Abraham and R.W. Taft, J. Org. Chem., 1983,48,2877. Paper 3/06092F; Received 12th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000865

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 869-873 Cationic Micellar Effect on the Kinetics of the Protolysis of Aromatic Carboxylic Acids studied by the Ultrasonic Absorption Method Teruyo Isoda,t Miyuki Yamasaki and Hiroshige Yano* Daiichi College of Pharmaceutical Sciences, 22-1,Tamaga wa-cho, Minami-ku, Fukuoka 815,Japan Takayuki Sano Department of Materials Science, Faculty of Science, Hiroshima University, Higashi-hiroshima 724, Japan Shoji Harada Hiroshima Bunkyo Women 3 College, Kabehigashi, Asakita-ku, Hiroshima 731-02,Japan The protolysis of carboxylic acids has been kinetically studied by the ultrasonic absorption method in the pres-ence of tetradecyltrimethylammonium bromide (TTAB) micelles in aqueous solution. The carboxylic acids studied were classified into two categories, one capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form the bond, namely the benzoic acid deriv- atives (BAD). The rate constant (y2k,, kb),the apparent dissociation constant (Ka),and the volume change of the reaction (AV) were obtained.Different K, dependences of the rate constants observed for SAD and BAD are discussed in relation to the effect of intramolecular hydrogen bond. pK, dependences were also observed for AV of SAD and BAD. These dependences are larger than those in aqueous solution. This result was attributed to the change of arrangement of water molecules around the solute in micellar solution and aqueous solution. A number of intramolecular hydrogen bonds in the protein are contributing not only to the stabilization of its structure, but also to the control of the reactions of the dissociative groups.We can generally admit that the intramolecular hydrogen bond increases the dissociation of acid' and decreases the volume change of the reaction2 (AV) in aqueous solution. By the ultrasonic absorption method, we have reported that the micelles promote the dissociation of acids and bases, and catalyse their rapid ionization reactions3 -6 in the same manner as in the slow reactions, e.g. the hydrolysis of esters, amides, and Schiff In the present work, we measured the ultrasonic relaxation absorption based on the protolysis of BAD and SAD in TTAB micellar solutions and aimed to examine the effect of intramolecular hydrogen bonds on the rate constants and AV in the micellar solution.Experimental Benzoic acid (BA), m-nitrobenzoic acid (m-NO,BA), m-chlorobenzoic acid (m-ClBA), p-hydroxybenzoic acid (p-OHBA), p-toluic acid (p-CH,BA), salicylic acid (SA), 4-methylsalicylic acid (4-CH3SA), 5-methylsalicylic acid (S-CH ,SA), 5-chlorosalicylic acid (5-ClSA) and 5-bromosalicylic acid (5-BrSA) purchased from Nakarai and 3-methylsalicylic acid (3-CH3SA) and 3-hydroxy-4-methylbenzoic acid (3-OH-4-CH3BA) from Aldrich, and TTAB from Tokyo Kasei were all reagent grade and used without further purification. Ultrasonic absorption measurements were performed with the pulse technique over the frequency range 5-105 MHz.Details of the apparatus have been described elsewhere.' The velocity of sound was measured by the sing-around even in the saturated solution. When the acids were solu- bilized in the TTAB micelles, however, the ultrasonic absorp- tion was observed in spite of their low concentrations. All of the spectra were characterized by a single relaxation equation' '-I3 M-' = A/{1 + (flfJ') + B (1) where s( is the absorption coefficient,fis the frequency,S, is the relaxation frequency, and A and B are the relaxation and non-relaxation absorptions, respectively. The absorption parameters,f,, A and B, were determined by a least-squares fit of the experimental data to eqn. (1). Experimental errors were within 9, 6 and 6% of the corresponding values, respec- tively.Representative ultrasonic absorption spectra are shown in Fig. 1. Experimental conditions and the absorption parameters obtained are summarized in Table 1. The concen- tration, acidity of acids and pH in the solution, greatly affect the ultrasonic absorption ; similar dependences have been observed for the protolysis of carboxylic acids in cationic micellar systems in our previous ~orks.~,~ All of the results 6 100-'6100nr c -----------0 T, N method at 1.92 MHz. Density was measured by a pyc-nometer. The pH and ultrasonic measurements for the solu-tions were carried out under a dry nitrogen gas atmosphere. All the measurements were performed at 30.0 "C. Results and Discussion The acids used in this work are only slightly soluble in water and the ultrasonic relaxation absorption was not observed ~ t Nee Yamashita.V 1 I I 1 5 10 50 100 f/M Hz Fig. 1 Representative ultrasonic absorption spectra of acid aqueous solution (0.05 mol dm-3) in the presence of TTAB (0.30 mol dm-3) at 30.0"C : 3-methylsalicylic acid (0,pH = 1.93) and p-methylbenzoic acid (0,pH = 2.78) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Ultrasonic absorption parameters for various pH of acids (0.05 mol drn-') in the presence of TTAB (0.30 mol dm-j) at 30.0 "C pH A/lO-" s2 cm-' B/10-17 s2 cm-' f,/MHz m-N0,BA1.88 45 31 15.0 2.04 71 30 11.3 2.17 76 30 10.8 2.36 63 31 11.6 2.57 42 31 13.2 rn-ClBA 1.89 28 30 13.5 2.11 44 29 10.5 2.36 68 29 8.3 2.66 48 30 9.7 2.82 29 30 12.7 BA 1.99 17 27 18.0 2.23 32 28 13.0 2.63 46 28 10.3 3.01 36 28 12.0 3.30 16 29 16.8 3-OH-4-CH3BA 2.10 17 32 20.0 2.67 45 31 11.4 2.99 44 31 10.5 3.30 25 31 14.6 3.54 12 31 21.7 p-OHBA2.09 11 30 20.0 2.23 27 29 12.5 2.99 46 29 9.4 3.19 26 30 12.6 3.39 15 30 16.8 p-CH3BA 2.28 16 29 15.5 2.49 28 29 11.2 2.78 48 29 8.5 3.14 26 28 11.2 3.39 18 29 13.4 mentioned above suggest that the relaxation absorption can be ascribed to the protolysis of carboxylic acid solubilized in the TTAB micelles. kt RCO; + H+ SRC02H (1) kb where R is incorporated into the micelle core.For reaction (I), the relaxation frequency and the maximum relaxation absorption per wavelength, are expressed by the fol- lowing equations' '-' 7cp u2 (a'4rnax = -(A V)2 -(3)2RT with r = [RCO,]-' + [Hf]-' + [RCO,H]-' (4) -K, 'C,[H+]-K,-'C, + (K,-'[H+] + 1)2 where C, is the total concentration of acid, a' is the excess absorption coefficient, R the wavelength, p the density, U the sound velocity, AV the volume change of the reaction, and the subscript max means the maximum value. When K,-'C, > 1 is satisfied, as in the present experiments, these equations predict that the minimum and the maximum of PH A/10-17 s2 cm-' B/lO-" s2 cm-' f,/MHz 5-BrSA 1.20 29 32 35.6 1.38 37 31 30.3 1.60 47 31 26.7 1.92 44 30 23.5 2.52 15 30 25.4 5-C1SA 1.19 20 33 35.6 1.29 29 33 30.0 1.42 36 33 25.2 1.64 45 32 22.3 1.94 33 32 20.0 2.11 26 32 21.4 2.35 16 32 23.0 3-CH3SA 1.11 14 29 43.1 1.50 53 29 21.0 1.93 102 31 13.9 2.33 62 29 14.8 2.54 36 31 16.8 5-CH3SA 1.07 18 31 30.7 1.54 42 30 20.0 1.92 89 31 13.4 2.27 44 31 15.9 2.74 27 30 18.0 4-CH3SA 1.15 13 31 38.4 1.57 50 31 20.0 2.04 94 31 14.5 2.42 50 30 18.0 3.21 14 30 25.0 SA 1.23 17 32 33.7 1.61 41 32 21.8 1.97 88 31 14.6 2.44 45 30 17.8 2.72 28 30 , 20.0 (27cf,)and r-[then (C~'A)~.J, respectively, appear at the defi- nite value of pH, i.e. pH*; pH* = -(log Co + log K,)/2 (6) Since C, is known, the value of K, can be determined from pH*.As shown representatively in Fig. 2 and 3, experimental data are in agreement with eqn. (2)and (3).The values of K, , y2k,, kb and AV were determined so as to give best fits for the experimental values of (27cf,) and (a'A)mm. All the values obtained are summarized in Table 2. As an index of the catalysis, the differences of the pK, values in aqueous solution and micellar solutions, denoted by ApK,, are listed in Table 2 together with those in the DAC3 and DPC solution^.^ In order to evaluate this, the electro- static effect and the polarity effect of the micelle should be examined.Since the cationic atmosphere of the micelle stabil- izes the carboxylate anion, the electrostatic effect promotes the dissociation of the acids. The polarity of the micelle was evaluated from the ratio of ketonic and enolic forms of benz~ylacetoanilide'~solubilized in the micelles. Fig. 4 shows the UV spectra of benzoylacetoanilide in the micellar solu- tions together with that in the aqueous solution. This figure shows that the polarity of the micelle is lower than that in water, and the TTAB micelle provides the most hydrophobic field. The decrease of polarity induces the depression of the dissociation, then the two opposite factors affect the pK, of acid in the TTAB micellar solution. All the ApK, values are J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 87 1 positive and indicate that the dissociation is promoted in micellar solution, which means that the electrostatic effect is 1II superior to the hydrophobic effect. The largest value of ApK,I in DAC solution is easy to understand from a considerationI II of the largest electrostatic effect induced by the exposure of I I, the charged centre outside of the Stern layer. The equivalentI I value of ApK, in the TTAB and DPC micellar solutions andI2 I I the superior hydrophobicity in the former, known from the5 UV spectra, indicates that the electrostatic effect is larger in0 the TTAB solution. cI( The acids in this work can be divided into two groups, one3 capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form a bond, namely the benzoic acid deriv-atives (BAD).The values of ApK, of SAD are about two times larger than those of BAD; this can be attributed to the stabilization of COY by intramolecular hydrogen bond for-:mation in the hydrophobic environment of the TTAB O1 2 3 4 PH Fs2 Plots of 2nf, us. pH for acid aqueous solution (0.05 rnol dm-j) in the presence of 'TTAB (0.30 rnol dm-j) at 30.0"C:3-methylsalicylic acid (0)and p-methylbenzoic acid (a) O01234 PH Fig. 3 Plots of (a'A)-us. pH for acid aqueous solution (0.05 mol dm-3) in the presence of TTAB (0.30 mol dme3) at 30.0"C:3-methylsalicylic acid (0)and p-methylbenzoic acid (0) micelle.15.16 Acidity dependences of the rate constants for BAD are plotted in Fig.5A and the relationships are expressed by eqn.(7)and (8) for the aqueous solution, and eqn. (9) and (10) for the micellar solution. y2kf/dm3mol-'s-' = 10*o*6 (7) k&-' = 10'0.6Ka (8) y2kf/dm3 mol- s-' = 108*3K-0*44 (9) a kds-1 = 108.JK0.56 (10) In the micellar solution, the rate constants at pK, = 0 are more than two orders of magnitude smaller than that in aqueous solution. This indicates that the TTAB micelle causes the reaction mechanism to vary widely from that in aqueous solution. Another feature is the lack of dependenceof ;t2kf on K, in aqueous solution while both rate constants are dependent on K,in micellar solution. The protolysis of carboxylic acids has often been inter-preted by the followingmechanism (iii) Table 2 Kinetic parameters for the protolysis of acids (0.05mol drn-') in the presence of TTAB micelle at 30.0"C y2kf/109 dm3 mol-' s-l W1O6s-I AV/cm3 mol-' pK, TTAB DAC DPC BAD m-N0,BA 5.1 4.7 11.8 3.03 0.5 1.2 0.4 m-C1BA 6.0 2.4 12.1 3.40 0.4 1.2 0.3 BA 12 1.2 17.3 4.01 0.2 1.2 0.2 3-OH, 4-CHSBA 16 1.o 1 7.0 4.19 0.2 p-OHBA p-CH,BA 14 17 0.93 0.85 17.8 17.0 4.21 4.31 0.4 0.1 SAD SBrSA 2.9 36 9.4 1.90 0.7 5-ClSA 2.7 30 8.1 1.94 0.7 3-CH3SA 3.2 11 12.2 2.44 0.6 SCH3SA 3.3 10 11.6 2.49 0.7 CCH3SA 3.7 10 12.6 2.56 0.6 SA 3.9 9.9 12.5 2.60 0.4 200 300 400 A/nm Fig.4 Absorption spectra of benzoylacetoanilide in aqueous solu- tion (-), in the solutions of TTAB (-* -), DAC (---) and DPC (. .. ..) where (i) ==(ii) is a diffusion process and (ii) e(iii) is a proton-transfer process and (ii) is a steady-state intermediate. Since the index of K, in eqn. (7)-(10) indicates the degree of proton transfer at a transition state," it is concluded that the rate- determining step is the diffusion process in aqueous solution and the proton-transfer process in micellar solution. These results are similar to those of BAD in DAC and DPC solu-tion~,~,~but the catalytic effect is much larger in TTAB solu-tion. In the case of SAD, the situation is a little different. Sum- marizing a small amount of kinetic information on the intra- molecular hydrogen bonding acids in aqueous solution,' 9-20 following K, dependences of the rate constant of SAD were obtained and shown by the dotted line in Fig.5B. y2kf/dm3 mol-' s-' = loge6 (11) kds-l= 109.6~~ (12) 5 I I I 1 0 1 2 3 4 PK, J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 The smaller k, of SAD compared with eqn. (8) is explained in the following way. The reaction species of the protolysis is the non-bonding COY group (open form) which is in rapid equilibrium with the bonding one (closed form) as expressed by the following equation. where (i') is the closed form and (i) is the open form. Since (i)' s(i) is much faster than (i) e(ii), the relaxation equation for the overall reaction is given by + [i] + [i']) + k, = Y,(~)([H+]1 + K43 Here, the value of k, might be similar to those of ordinary acids and of the order of 10" dm3 mol-' s-'.Since K,, is much larger than 1, however, the apparent rate constant y2k; becomes much smaller than the rate constant of protolysis of ordinary carboxylic acids. K, dependences of the rate constants of SAD in the TTAB micellar solution are plotted in Fig. 5B and given by y2k,/dm3 mol-' s-1 = 109.1Ka-0.18 (14) As seen in the figure, the values of the intercept and the slopes are very close to those in aqueous solution. These results indicate that the reaction mechanism and the energy levels of the protolysis of SAD are not much different from those in aqueous solution; then, in reaction (11), (i)e(ii) 11 B 10 9 8 7 6 I E 5 J 0 1 2 3 4 PK, Fig.5 A, pK, dependence of the rate constants of the protolysis of acid in the presence of 0.30 mol dm-3 TTAB (large Circles, data of BAD) and in the absence of micelle (small circles, data from ref. 17). B, pK, dependence of the rate constants of the protolysis of acid in the presence of 0.30 mol dm-3 TTAB (large Circles, data of SAD) and in the absence of micelle (A, SAD; 0,lactic acid). (a)log(y2k,) and (b)log k,. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20 I 1 1 t-0 ,E 10 E I-d 0012345 PK, Fig. 6 pK, dependence of AV of the ionization of acid in the pres-ence of 0.30 mol dm-3 TTAB (0,SAD; a,BAD system) and in the absence of micelle (a,data from ref.21-23) might be much slower than (ii)e(iii). If this is the case, the K, dependences of the overall rate constants are expected to be the same as those in the aqueous solution, and the index K, will be 0 and 1 in eqn. (14) and (15), respectively. Detailed discussion on this problem has been developed in our pre- vious ~aper.~.~ As seen in Table 2, AV is larger in the micellar solution and dependent on K, as shown in Fig. 6. Similar tendencies have been observed for BAD in DAC and DPC solutions. With increase of pK,, localization of charge density of the carboxylate anion increases and hydration is promoted. While the molar volume might not be greatly affected in the non-ionic form, then AV increases with pK,. Hepler studied the protolysis of acids and amines in aqueous solution and observed a linear relationship between AV and AS.24If this is the case, a linear relationship is expected between AV and pK, and the volume change is mainly attributed to the change of the arrangement of water molecules around the reaction species.The slope of the linear relationship between AV and pK, in TTAB solution is larger than that in aqueous solution ; this indicates that the compactness of water mol-ecules around the non-ionic form in the micellar solution decreases compared to it in the aqueous solution. References 1 CRC Handbook of Chemistry and Physics, ed. R. C. Weast, CRC Press, Cleveland, 1989. 2 R. J. H. Clark and A. J. Ellis, J. Chem. SOC., 1960, 247. 3 S.Harada, T. Yamashita, H. Yano, N. Higa and T. Yasunaga, J. Phys. Chem., 1984,88, 5406. 4 S. Harada, H. Yano, T. Yamashita, S. Nishioka and T. Yasu-naga, J. Colloid Interface Sci., 1986, 110, 272. 5 S. Harada, H. Okada, T. Sano, T. Yamashita and H. Yano, J. Phys. Chem., 1990, 94, 7648. 6 T. Yamashita, K. Tanaka, H. Yano and S. Harada, J. Chem. SOC.,Faraday Trans., 1991,87, 1857. 7 J. Fendler and E. Fendler, Catalysis in Micellar and Macro-molecular systems, Academic Press, New York, 1973. 8 Reaction Kinetics in Micelles, ed. E. H. Cordes, Plenum Press, New York, 1975. 9 H. Hoffman, H. Nusslein and W. Ulbrich, Micellization, Solu-bilization and Microemulsion, ed. K. L. Mittal and B. Linman, Plenum Press, New York, 1977, p. 263. 10 N.Tatsumoto, J. Chem. Phys., 1967,47,4561. 11 M. J. Blandamer, Introduction to Chemical Ultrasonics, Aca-demic Press, New York, 1973. 12 C. Bernasconic, Relaxation Kinetics, Academic Press, New York, 1976. 13 G. G. Hammes, Techniques of Chemistry, ed. A. Weisberger, Wiley, New York, 1974, part 2, vol. 6. 14 K. Meguro, K. Muto, M. Ueno, Chem. SOC.Jpn., 1980,3, 394. 15 K. Bowden and G. E. Manser, Can. J. Chem., 1968,46,2941. 16 G. E. Dunn and T. L. Penner, Can. J.Chem., 1967,45, 1699. 17 H. W. Nurnberg and H. W. Durbeck, 2.Anal. Chem., 1964, 205, 217. 18 B. Breslow, Organic Reaction Mechanisms, Benjamin, New York, 1969. 19 E. Grunwald in Progress in Physical Organic Chemistry, ed. S. G. Cohen, A. Streitwieser and R. W. Taft, Wiiey Interscience, New York, 1965, vol. 3. 20 T. Sano and T. Yasunaga, J. Phys. Chem., 1973,77,2031. 21 W. Kauzmann, A. Bodanszki and J. Rasper, J. Am. Chem. SOC., 1962,84, 1772. 22 A. J. Begala and U. P. Strauss, J. Phys. Chern., 1972, 76,254. 23 A. Fisher, B. R. Mann and J. Vaughham, J. Chem. SOC. 1961, 1093. 24 L. G. Hepler, J. Phys. Chem., 1965,69,965. Paper 3/0587 1I ;Received 29th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000869

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 875-878 Forces of Inertia acting on the Aqueous Pore Fluid of Anionic Polyelectrolyte Gels Ngoc-Ty Dang and D. Woermann Institute of Physical Chemistry, University of Cologne, Luxemburger Strape 116, 0-50939Koln, Ger- many Pulses of an inertia electromotive force are generated during deceleration of stiff rods of polyelectrolyte gels with negatively charged ionic groups covalently bound to the matrix of the gel. The gels are loaded with Lit, Cs', Ag+, H' and Ba2+, respectively, and are in swelling equilibrium with water. The values of (m, mass and q, charge of one counterion in the pore field) calculated from the recorded voltage pulse U dt (U, inertia electromotive force and t, time) during deceleration (time constant T z 1 ms) are larger by a factor of at least five than that calculated from the mass and charge of a 'naked' single counterion.This is attributed to an effective mass of the counterions in the pore fluid of the gels which has a larger value than the mass of a naked single counterion. During deceleration the hydrated counterions, together with the mobile water molecules in the pore fluid, are shifted relative to the ionic groups fixed to the stiff matrix of the gels. This generates an electric field causing the observed voltage pulse. The value of of the H' counterion is smaller by a factor of about five than that of the other counterion species (Lit, Cs+, Ag'). It is assumed that this is caused by the chain mechanism of proton migration found in aqueous solutions.Measurements of the electrical conduc- tivity of gels loaded with different counterion species, including H+ ions, reveal that the ratios of the counterion conductivity in the gel phase to that in free solution at infinite dilution have approximately the same value. The inertia electromotive force and the electrical conductivity measurements indicate that the mechanism of H' ion transport in the pore fluid is not modified considerably by the matrix of the gel. In a recent publication' it was demonstrated that pulses of an inertia electromotive force can be generated during the decel- eration of stiff rods of cation-exchange gels loaded with silver ions. The gels were in swelling equilibrium with water. The value of the ratio (rn/q)expcalculated from the experimentally determined voltage pulse U dt [U, inertia-electromotive force, electrical potential difference measured at zero electric current flow with two reversible electrodes (e.g.Ag wires reversible to Ag+ ions) between the ends of a gel rod during deceleration; t time]. The axis of the rod parallel to the vector of deceleration was larger by a factor of four for the SO, gels (larger by a factor of 20 for the -CH,-SO, gels! than the value found by Betsch, Rickert and Wagner' in cor- responding experiments with solid ionic conductors (e.g RbAg,I,). The duration of the voltage pulse is of the order of 1 ms. A detailed description of the physical concept to inter- pret the inertia electromotive force observed in ionic conduc- tors is given in ref.2(a). This concept was adapted to polyelectrolyte gels : During deceleration the force of inertia acts on the matrix of the gel and its aqueous pore fluid in which the counterions (q.Ag+) are dissolved. It is assumed that the matrix of the gels is so stiff that it can follow the force of inertia instantaneously. This is not true for the pore fluid. It is mobile and during deceleration is shifted slightly relative to the matrix in the direction opposite to the vector of deceleration. The Ag+ counterions move with the pore fluid. The slight shift of the pore fluid with the counterions generates an electric field acting along the axis of the gel rod. Its change with time is reflected by the observed voltage pulse. It can be assumed that at each instant of time during deceleration the electric field in the gel phase is just sufficient to decelerate the Ag+ ions at the same rate as the matrix.The time constant, T, for establishment of this stationary state is estimated to be ca. s. [T = rn/(6nqr);rn, r, mass and radius of a single ion, q, viscosity of the pore fluid]. Results of experiments are reported in which forces of inertia act on the aqueous pore fluid between highly cross- linked cation-exchange rods loaded with a mixture of two counterion species ([Li+/Ag+], [Cs+/Ag'], [Ba2 '/Agf] and [H' Ag']). The experiments are carried out as a function of the mole fraction, ZAg+, of the Ag' counterions in the gel in the range 0.05 < tAg+< 1.The presence of Ag+ ion in the gel phase is necessary to be able to use silver wires as reversible electrodes to monitor the electric voltage pulse of the inertia electromotive force. Experimental Polyelectrolyte Gels Two types of cation-exchange gels (type 1, abbreviated SO; and type 2, abbreviated CH,-SO,) in the form of rods, length, 50 mm, radius, 2.5 mm were used. The methods of preparation are given in ref. 3 and 4, respectively. They were loaded with two counterion species ([Ag+/H"], [Ag'/Li'], [Ag+/Cs'], [Ag'/Ba2']) by treatment with aqueous solu- tions containing the nitrates of the corresponding cations and washed free of electrolyte by extended treatment with distilled water. The mole fraction tiof H', Li', Cs' and Ba" in the gel phase was determined analytically C0.05 < ,ti < 1; ii= fii/(n", + fiAg+);fii, amount of substance of ionic species i in the membrane phase].The methods of characterization of the gels are given in ref. 5. The results are given in Table 1. The two gels differ mainly in their water content and fixed ion concentration. The specific electrical conductivity, i;-( =l(Z/A$)c= o, Ap = (i, electric current density passing through the rod; A4, electric potential difference; I, distance between the Ag electrodes in contact with the gel rod) of the gel phase in the absence of co-ions was measured to obtain information about the elec- trical mobility of the counterion species within the gels. rZ-was measured as described in ref.5. The experiments were carried out under the condition cjX < 1 (c,electrolyte concentration; X, fixed ion concentration). The specific electrical conductivi- ty i;-of a gel containing counterions only is related to the molar ion conductivity of the counterions im, by the relation i;-= lm,counter Zcounter-+ F2X2d;,(F, Faraday constant; X, fixed ion concentration ;d, ,hydrodynamic permeability of 8 76 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Characterization of the gels gel counterion fiH2O) Y*(H,O) xll0-j mol (~m-~ pore fluid) cm5 J-' s-' so; Li + H+ 0.573 1 0.5660 0.5768 0.5665 1.56 1.58 2.53 2.65 CH,-SO; cs + Ag + Ba2+ Li + H+ 0.5155 0.4642 0.5068 0.77 16 0.77 13 0.5853 0.5139 0.5780 0.78 15 0.7833 1.74 1.93 1.77 0.427 0.430 2.09 1.32 1.68 9.15 9.07 cs+ 0.7304 0.7643 0.457 6.63 Ag+Ba2+ 0.7106 0.7270 0.7372 0.7619 0.470 0.459 6.05 6.52 yHz0, ygz0, mass fraction of water taken up by the gel; yHzO= m,,dm; m = mge, + mHzo+ mcouncer;ygzo = mHzd(mge,+ mHzo);X,analytically determined fixed ion concentration; J,,, specific mechanical permeability; & = L(jv/AP)A4=o,k=o;L, length of the rod;jv, volume flow density; AP, applied hydrostatic pressure difference.the gel matrix; d;, = 4j,/AP),4=0,dE=o ; j,, volume flow density; AP, hydrostatic press_ure difference; L, length of the rod).6 The term Econ= F2X2d,,describes the convection con- ductivity caused by an electro-osmotic volume flow. The con- tribution of the electric convection conductivity to the total conductivity (kco,,/k) is estimated to be <0.10 for the CH,SOJ gel and to be <0.15 for the SO, gel.For a gel loaded with H+ ions the ratio (kco,,/k) is smaller than 0.04. The results of the conductivity measurements are given in Table 2. Measurement of Inertia Electromotive Force A rod of the swollen gel, free from adherent water, was intro- duced into a Plexiglas tube and was sealed airtight with a screw cap. The position of the rod was fixed by two Plexiglas screws pressing against the top and the bottom of the rod. Two silver wires were introduced through the wall of the Plexiglas tube and connected to silver electrodes. The elec- trodes were tightly pressed against the gel rod by Plexiglas screws at a distance, I = 40 mm, symmetrical to the centre of the rod (L/2).They acted as reversible electrodes for the Ag+counterions present in the gel.The electrodes were con- nected to an amplifier and digital memory-scope to record the voltage pulse, 5 U dt, during deceleration. During decel- eration the axis of the rod is parallel to the vector of deceler- ation. The Plexiglas tube containing the gel rod was placed into a cavity in a cylindrical aluminium block. The alu- minium block was earthed, thereby acting as an electric shield. For the experiment the aluminium block was dropped Table 2 counterion Ag + H+ Li + Na+ K+ cs+ Ba2+ H /Ag+ + Electrical conductivity, k, of the cation exchange rods zi so; CH,-SO; 1 1.16 f0.02 0.65 f0.02 1 8.89 f0.25 5.24 f0.05 1 0.99 &-0.02 0.65 f0.02 1 1.58 f0.02 0.95 f0.02 1 2.06 f0.02 1.21 f0.03 1 1.71 & 0.06 1.00 & 0.04 1 0.65 f0.03 0.45 & 0.02 1,+ SO; CH,-SO; 0.10 0.16 7.70 f0.30 4.56 f0.08 0.40 0.37 5.93 f0.14 3.49 f0.04 0.62 0.60 3.59 & 0.10 2.14 &-0.05 0.9 1 0.78 1.99 f0.05 0.78 f0.04 The data refer to gels washed free of electrolyte. T x: 25 "C.2i,molt fraction of counterion species i in the gel phase. onto a plastic support (for further details see ref. 1). The velocity, urnax,of the aluminium block before hitting the bottom plate is given by u,,, = (2~h)"~(9,standard, acceler- ation of free fall; h, drop height; 5 < h/cm < 30; 100 < uma,/cm s -< 250).Results and Discussion The deceleration of a gel rod from a velocity, urnax,to the velocity, u = 0, generates an electric voltage pulse measured by the two silver electrodes in contact with the rod. Accord- ing to ref. 2 this voltage pulse is given by: after strike u dt = (4q)exp lumax (1)before strike U = {&Ag, x = 0) -&(Ag, x = I)) > 0; x, coordinate along the axis of the rod; x = 0, L, coordinate of the lower and upper end of the rod in vertical position; L, length of the rod, 50 mm; I = 36 mm. The value of 5 U dt is obtained by planimetry o----:U(t) curve. The values of (m/q)exp are calculated from the slope of the straight line of the 5 U dt us. u,,, plot. The slope of the 1U dt us.urnaxline [i.e. the value of (n~/q)~~~]depends slightly (i.e. less than +5%) on the pressure applied between the top and the bottom of the gel rod to fix its position in the Plexi- glas tube used for the drop experiments (see Fig. 1). A similar 1 I I 1 15 c Iv) c l a a m 10 0 F1: n v$5 +-+ --+----------i I 1 I I0 0 200 400 600 A4m Fig. 1 Effect of the mechanical force (measured as AL, the change in length of the rod) on the ratio (m/q)expobtained with a gel rod loaded with Ag+ ions. Lo = 50 mm. (a),SO; gel and (m), CH,SO; gel. The Young's modulus of the CH,-SO; gel is smaller than that of the SO, gel. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 small change in the value of the slope is introduced by reversing the position of the rods in the Plexiglas tube.Care has to be taken to adjust properly the force with which the two silver electrodes are pressed against the surface of the gel rod by Plexiglas screws. Experimentally determined values of (rn/q)expobtained with rods of the SO, and the CH,-SO, gel containing only counterions and no co-ions are shown in Fig. 2. The data are plotted as function of the mole fraction RA,+[ =fiAg+/(iil+ fiAg+)] of the Ag' counterions in the gel phase of the ion pairs [Li+/Ag+], [Cs'/Ag'], [Ba2'/Ag'] and [H+/Ag']. The data show that: The values of the experimentally determined ratio (m/q)expare larger by at least a factor of five than that calculated from the mass and the charge of the naked counterions [(m/q)c,lc/10-3 g A-' s-': H'.1.04 x lo-,; Li', 7.18 x loV2;Cs', 1.38; Ag+, 1.12; Ba2+, 0.71; rn, mass of the naked counterion] ; it is independent of zA,+to a first approximation. The value of (rn/q)expof Ba" ions is smaller, by a factor of two, because each barium ion carries two elementary charges. The values of (rn/q)expobtained with the CH,-SO, gel are larger by a factor of ca. two than that of the SO; gel.? The ratio (m/q)exphas approximately the same value for the counterion species Li', Cs' and Ag' (SO; gel: mean value (rn/q)exp = 5 x lop3 g A-' s-'; CH,-SO, gel: mean value (m/q)expz 11 x lop3 g A-' s-'). For RH+ > 0.5 the value of (m/4)exp for the counterion species Hf is smaller by a factor of ca.five (SO; gel: mean value (rn/q)expx 1 x g A-' s-'; CH,-SO; gel: mean value(m/q),,, x 2 x g A-' s-'). These findings are interpreted by assuming that the hydrated counterions dissolved in the pore fluid of the geis are distributed homogeneously over the cross-section of the pores by the thermal motion of the counterions.'*6 An electri- cal space charge density ?, is generated having a sign opposite to that of the fixed ionic groups (?, = -oFX; o,sign of the fixed ionic groups; w = -1 for gel with cation exchange properties). During deceleration the mobile hydrated counter- ions and the water molecules in the pore fluid are moved relative to the fixed charges bound covalently to the matrix. This process generates an electric field which causes the hydrated counterions to be decelerated at the same rate as the matrix.Therefore, the mass which determines the value of (m/q)expis larger than that of the naked counterion. rn is con- sidered as an effective mass to which the counterions and a certain number of water molecules contribute. With this assumption the data obtained with the systems [Li'jAg'] and [Cs+/Ag+] can be understood. Values of the effective mass meff calculated from the ratio (rn/q)exp obtained from the experiments with the systems [Ag'], [Li+/Ag+] and [Cs+/Ag+] are given in Table 3 using a mean value of (m/4)exp = 5 x lop3g A-' s-l for the SO, gel and a mean value of (rn/q)exp= 11 x 10-g A-' s -' for the CH,-SO; gel. The contribution of the mass of the naked counterions to the effective mass meff can be neglected to a first approximation.This reflects the finding that the value of (m/q)expis independent of the nature of the counter- ion species. Values of the ratio of the number of water mol- ecules NHzOto the number of mobile counterions NcOunterin the pore fluid [i.e. (NHzO/Ncounter)exp]calculated from meff are also given in Table 3. The values of (NHzO/Ncoun~er)expcan be 6- I 45-c I -I-* I I--_I I EJ, m *I 04- F\ 2 F-2-:3 h - A A--A - ,/ ' /,i'/ _- I 1 1 I 14 l2 I I . m10 4 // 8)-/ 01 / / I /'.4' ' 0' I I 1 1 1 1 0 0.2 0.4 0.6 0.8 1 RA, f Fig. 2 Experimentally determined values of (m/q)expobtained with rods of (a) the SO; gel and (b) the CH,-SO; gel loaded with a pair of counterions: (m), [Li+/Ag']; (+), [Cs+/Ag+], (A),[Ba2+/Ag'] and (@), [H+/Ag+]).The gels contain no co-ions. TAg+is the mole fraction of Ag+ counterions [TAg-= fiA8-,'(fii + fiAgt)]. tration and the water content of the gels [see Table 3; y$,o = mH20/(mHz0+ rnge,)]. It turns out that CQ. 70% of the water molecules taken up by the SO, gel contribute to the rneff of the mobile counterions. The corresponding value for the CH,-SO; gel is ca. 50%. It can be expected that the water molecules present in the gel do not contribute to meff. A fraction of them will adhere to the hydrophilic pore wall (boundary layer). They can be considered immobile on the timescale of deceleration.The data obtained with the counterion pair [H'/Ag+] for RH+ > 0.5 indicate that the H+ counterions behave differ- ently, during deceleration, to the Li+, Cs' and Ag+ counter- ions. For both kinds of gels the ratio (rn/q)exp has values smaller by a factor of ca. five (see Fig. 2). The number of water molecules contributing to the effective mass of a counterion is smaller by a factor of ca. 10. It is assumed that compared with the corresponding values (NH20/Ncounter)analthis effect is caused by the chain mechanism of proton migra- calculated from the analytically determined fixed ion concen- t The (rr~/q)~~~value of the CH,-SO; gel reported for Ag-counterions' is larger. It turned out that this is an artefact caused by an inadequate fixation of the softer gel rod in the Plexiglas tube in the drop experiments.tion in water. Measurements of the direct current electrical conductivity, E, of the gels reveal that the ratio of the counterion conductivities in the gel phase (I,.,,, H+/jm,i) neglecting the small contribution of the convection conduc- tivity [(ilcon/k)< 0.151 and that in free solutions at infinite dilution (i;, i}H+/lLg,(i = Li', Na', K', Cs+, Agf) have J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Results of the evoluation of the inertia electromotive force so; Ag + Li+/Ag+ Cs+/Ag+ H+/Ag+,gAg+< 0.5 CH,-SO; Ag + Li+/Ag+ Cs+/Ag+ Hi/Ag+, ZAg+ -= 0.5 (0.80) (0.80) (0.80) (0.16) (1.76) (1.76) (1.76) (0.32) (m/q)exp,ratio of the effective mass of a counterion and its charge; NH20,number of water molecules; NcOunter,number of counterions; gAg+, mole fraction of Ag+ counterions in the gel phase; ( ), mean values, see Fig.2. mH20= 0.3 x g. approximately the same value: (Irn,H+/&,,, i), (A:, H+/A:, i), [H+/Li+]: (9.0) (9.0); [H+/Na+]: (5.6) (6.9); [H+/K+]: (4.3) (4.7); [H+/Cs+]: (5.2) (4.5); [H+/Ag+]: (7.7) (5.65). This indicates that the mechanism of the H+ ion transport in the pore fluid is not modified considerably by the matrix of the gels. The inertia electromotive force measurements give the same information as the measurements of the electrical conductivity of the gels. The observed effect of the action of forces of inertia on the pore fluid of a polyelectrolyte gel could be used in the construction of a sensor for acceleration and deceleration.This device would be more sensitive than a device already developed2 for this purpose using an ionic superconductor as sensor. It is assumed that an H30+ ion in the pore fluid of the gels is strongly connected to three H20 molecules by hydrogen bonds forming a H90: complex. This assumption is based on the accepted structure of the primary hydration shell of a H30+ ion in free The mechanism of charge motion consists of proton transfers within the hydrogen bonds of the H,Ol complex which is accompanied by associ- ations and dissociations of hydrogen bonds at the periphery of the complex. The proton oscillates continuously and very quickly (time constant of the order of s) within the complex. Motion over larger distances is determined by the migration of the whole hydrogen bond complex (rotation of water molecules into orientations in which they can accept or donate protons).The time constant of this 'structural diffusion' process over the distance of a hydrogen bond is estimated to be s.' This special mechanism of charge motion of the H'ions in aqueous solutions leads to the assumption that during decel- eration the H+ ions escape the HgOf complexes moved by the forces of inertia together with the mobile water molecules of the pore fluid at time intervals that are short compared with the time constant of deceleration (1 ms). They migrate by structural diffusion in the direction opposite to that of the electric field generated by the force of inertia.Therefore, the electric field generated by the forces of inertia in a gel loaded with H+ ions is smaller than that generated in a gel loaded with Ag+, Li+ and Cs+ ions, respectively, under the same conditions. The measured pulse of the inertia electromotive force is smaller. Structural inhomogeneities in the two types of gels, formed by regions with locally higher and lower values of the fixed ion concentration (cross-linking), could explain the depen- dence of (rn/~&~in the [H+/Ag+] system for ZAg+ > 0.5 (see Fig. 2). Such inhomogeneities show up in other types of experiments with the same gels.'0." On the basis of these experiments it is expected that the Ag+ and H+ are not dis- tributed uniformly in the gel phase. The Ag+ counterions will be located preferentially in the highly cross-linked regions of the gels.12 Conclusion The experiments demonstrate that forces of inertia acting on the aqueous pore fluid of anionic polyelectrolyte gels loaded with different counterion species (Li+, Cs', Ag', H+ and Ba2+) during deceleration produce a voltage pulse.It is larger than that calculated from the mass and charge of a single counterion. The measured voltage pulses allow us to determine an effective number of water molecules per mobile counterion in the pore fluid causing this effect. The experi- ments reveal that the mechanism of H+ ion transport in the pore fluid is not modified considerably by the matrix of the gel, confirming measurements of the electrical conductivity of the gels.We thank H. Rottger for his help in the early stages of the experiments. References 1 H. Rottger and D. Woermann, Ber. Bunsenges. Phys. Chem., 1992, %, 623. 2 (a) M. Betsch, H. Rickert and R. Wagner, Ber. Bunsenges. Phys. Chem., 1985, 89, 113; (b) M. Betsch, H.Rickert and R. Wagner, Solid State Zonics, 1986, 18,19, 1193; (c) W. Koch and H. Rickert, Solid State Zonics, 1988,28-30, 1664; (d) W. Koch and H. Ricker in High Conductivity Solid Ionic Conductors, Recent Trends and Applications, ed. T. Takahashi, World Scientific, New Jersey, 1989, p. 64. 3 G. Manecke, 2. Phys. Chem., 1952,201,193. 4 E. Phillipsen and D. Woermann, J. Membrane Sci., 1984, 17, 139. 5 G. Wiedner and D. Woermann, Ber. Bunsenges. Phys. Chem., 1975,19,868. 6 R. Schlogl, Stofltransport durch Membranen, Steinkopff Darm- stadt, 1964, p. 75. 7 M. Eigen and L. DeMaeyer, in The Structure of Electrolyte Solu- tions, ed. W. J. Hamer, Wiley, New York, 1959, p. 64. 8 T. G. Fillingim, N. Luo, J. Lee and G. W. Robinson, J. Phys. Chem., 1990,94,6368. 9 G.W. Robinson, J. Phys. Chem., 1991,9!5, 10386. 10 H. Rottger and D. Woermann, Langmuir, 1993,9, 1370. 11 C. Richter and D. Woermann, to be published. 12 F. Helfferich, Zonenaustauscher, Verlag Chemie, Weinheim, Bergstrak, (1959), p. 173. Paper 3/06094B; Received 12th October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000875

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 879-882 IR and Submillimetre-wave Spectra of Doped Poly(p-phenylene vinylene) Samir El-Atawyt and Keith Davidson" School of Physics and Materials, Lancaster University, Lancaster, UK LA I 4YA Poly(p-phenylene vinylene) (PPV) films of varying thickness have been prepared using standard precursor routes. A new oxidative doping technique, using degassed concentrated H,SO, under low pressure, was used. Precise levels of doping were achieved, these levels being dependent on the time of exposure and the thickness of the sample. A very wide range of spectral absorption measurements, covering the range 20-4000 cm-', were carried out on doped and undoped samples and on the precursor polymer. Large variations in absorption peaks throughout the entire range were noticed upon doping.New assignments of the unknown IR peaks together with the observed peaks in the submillimetre range are proposed. For several years the study of conjugated polymers has been of major interest to chemists, physicists and electronic engi- neers. A variety of investigations have been done on the semi- conducting and conducting properties of intrinsic and doped materials. Conjugated polymers in general are considered to be medium to large bandgap semiconductors with highly anisotropic properties. The relatively weak p, orbital overlap between chains compared with the potentially delocalised n bonding along the chains gives these materials a quasi-one- dimensional electronic structure that differs strongly from more conventional conductive materials.Most non-degenerate conjugated polymers are insulators in the ground state. However, creation of self-localised charges, such as polarons, bipolarons and solitons upon doping, results in lattice distortions. These particle-like entities have energy levels much smaller than the intrinsic bandgap between conduction and valence bands. Many attempts have been made to dope conjugated polymers; both p-type and n-type doping can be achieved using chemical, electrochemical or vapour-phase doping.'-7 Doping of conju- gated polymers greatly increases the number of localised polarons and bipolarons (or solitons in doped trans-polyacetylene) thereby increasing the rate of inter- and intra- chain transfer causing a large increase in electrical conduc- tivity.8-'1 The optical properties of conjugated polymers have also been of interest to many workers.First, the non-linear electro-optical properties, such as the dependence of optical transmission on electric field strength, are of potential use in electro-optic modulation devices' ,*' and in active and passive integrated optic wave guides. This has been domons- trated in trans-polyacetylene MIS and MISFET device struc- tures where the optical absorption was spectrally resolved as a function of applied electric fields.' 2314 Electro- and photo-luminescence has been extensively studied by a number of researchers. Much of the work has been aimed at developing high-efficiency light-emitting diodes (LEDs) as display devices.Several attempts have been made to increase the efficiency and to control the emission wave- length, and chemical tuning through co-polymerization has been shown to produce a blue shift in the spectral emission of PPV.15-17 PPV is amongst the most investigated of conjugated poly- mers, particularly its potential for use in electro-optical devices.15-17 Spectral studies in the UV, VIS and IR ranges have been reported for films of PPV and virtually full assign- t Present address : Institute of Graduate Studies and Research, University of Alexandria, Alexandria, Egypt. ments of absorption peaks have been made down to fre- quencies as low as 400 cm-' (25 pm wa~elength).~*'~.'~ A major breakthrough in the study of PPV and other polymers, is the synthetic route, via soluble polyelectrolyte precursors, that allows the formation of thin films of the otherwise non- processible PPV.The increase in the processibility provided by this technique is largely responsible for the success of electro-optical and electronic devices based on PPV.20 Both oxidative and reductive dopants have been used with PPV. H2S04, AsF, and I, have been used for p-type doping and sodium naphthalide for n-type d~ping.~ Conductivities of 100 and 10 S cm-I have been reported for H,SO, and AsF, dopants, respectively. Experimenta1 Free-standing films of PPV were prepared by solution casting of the precursor polyelectrolyte and thermal conversion to conjugated PPV.'' The materials were prepared following the established synthetic route shown in Scheme 1. p-Phenylenedimethylene-1,l-bis(tetrahydrothi0phen-1-ium)di-chloride was prepared by reacting &,a'-dichloro-p-xylene (0.75 mol dmP3) with tetrahydrothiophene (2.2 mol dm-3) in 80 :20 methanol : water mixture at 50°C. The product was then precipitated in cold acetone followed by filtration and extensive drying in vacuum. The precursor polymer, poly {p-phenylener 1-(tetrahydro thiophen- 1-io)et hy lene chloride] f , was obtained by reacting the bis-sulfonium salt with an equi- molar quantity of NaOH (0.2 mol dm-3) in distilled water at 0°C for 1 h under a nitrogen atmosphere. This reaction was quenched by slowly adding HCI (1 mol dm-3) until the solu- tion became slightly acidic (pH 6.8).The product was purified by dialysis against distilled water for five days. r 1 220"C vacuum L Jn U Scheme 1 The samples examined in this report were obtained by solution casting from methanol and prepared to various thicknesses ranging from 5 to 110 pm. Conversion to PPV was done by heating to 220°C for 18 h under dynamic vacuum. The resultant homogeneous, dense films were brownish yellow in colour. A new technique for doping was tried and found to be suc- cessful. The samples were immersed in a flask containing con- centrated H2S04 that had been degassed by a freeze-thaw process. The pressure inside the flask was then lowered until outgassing of the sample started occur.The sample changes colour very rapidly to deep blue and eventually to almost black. Outgassing occurs at reduced pressure indicating the onset of the ionic transfer interaction of the sulfate group inside the PPV film. The doping level and depth of doping are controlled by immersion time. To stop the process the vacuum is released, the film removed and washed thoroughly with acetone. IR spectra of the free-standing films were measured in the range 400-4000 cm-' using a Nicolet FT 205 spectrometer. The submillimetre measurements were performed using a millimetre-wave spectrometer and a liquid-helium-cooled composite germanium detector, covering the spectral range 15-600 cm-'. Each submillimetre spectrum is the average of two runs ratioed against three background runs.The detector response was continuously monitored and the post elec- tronics gain adjusted throughout the measurements. Conductivity was measured using the van der Pauw, four- probe technique.21 The conductivity was calculated from the following equation : where I is the applied current, V is the measured voltage and d is the sample thickness. Results and Discussion High-quality, free-standing films of precursor polymer and fully converted PPV of different thicknesses and dopant content were prepared and their absorption spectra and elec- trical properties examined. IR and Submillimetre AbsorptionMeasurements The IR spectra of films of the precursor polymer and the fully converted PPV are shown in Fig.1. The spectrum of PPV is identical to the spectra published by other workers4'' and ,,Y<...10.20.1 bL ;i 4000 3200 2400 1400 1800 1000 600 wavenumber/cm-' Fig. 1 IR absorbance spectrum of 5 pm thick film of fully converted PPV (-) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 virtually all of the absorption features have previously been Itas~igned.'~-'~ has also been shown by the work of Bradley," that all of the assigned peaks are categorised as having a dichroic ratio >1. That is, the peaks are electric field polarization dependent.18 The dichroic ratio is AJA , where A, and All are the magnitudes of absorbance of radi- ation with electric fields polarised perpendicular and parallel to the chain direction.This anisotropy ratio defines whether the absorption mechanism is due to in-plane or out-of-plane bending and stretching modes. The in-plane modes are essen- tially activated by parallel polarisation, while the out-of- plane modes are due to interchain activation caused by per- pendicular fields. Also, for unpolarized fields, the ratio between the in-plane and out-of-plane amplitudes Aip/Aopfor a specific mechanism and location on the polymer skeleton defines the degree of n electron delocalization caused by the wavefunction overlap along the polymer backbone. In Fig. 1 the absorbances at 1013 and 837 cm-' have been assigned to in-plane and out-of-plane p-phenylene CH bending modes, respectively, and the ratio of these values is: (Aip/Aop)p-phenylene= 0.06.While for the vinylene CH, the in- plane bending mode at 1267 cm-' and the out-of-plane mode =at 965 cm-' gave: (Aip/Aop)vinylene0.05. As these ratios decrease, the n electron delocalization increases, indicating an increase in the uninterrupted conjugation length. Also from the various values of these ratios, it is clear that both mecha- nisms of the vinylene and p-phenylene can express the degree of electron delocalization, in contrast to the disagreement found by Bradley and co-workers' about the trans-vinylene .~~moiety used by Zerbi et ~1 as a measure for delocalization. The peaks at 429 and 784 cm-' have not previously been assigned. It is postulated here that both peaks are associated with the trans-vinylene moiety.The reason being the absence of these peaks in the precursor polymer spectrum shown in Fig. 1. In that figure, nearly all the large peaks are assigned to the p-phenylene ring which is present in both precursor polymer and fully converted PPV (except the broad peak at 3375 cm-' due to the solvent OH-stretch band). For example, peaks at 555,837, 1423 and 1519 cm-' in Fig. 1 are assigned to p-phenylene out-of-plane ring bend, CH out-of- plane bend and semicircular ring stretch (1423 and 1519 an-'), respectively, and can be clearly identified in the figure. Traces of peaks assigned to the trans-vinylene group can be observed in the precursor polymer spectrum; however, these are attributed to PPV formed by partial conversion at room temperature.Submillimetre transmission spectra of the precursor polymer and fully converted PPV are shown in Fig. 2. A broad band, around 100 an-', is seen in the precursor 100 90 80 h W 70 .-6 60 v) .I50E 40!! c. 30 20 10 0 100 200 300 480 500 600 wavenumber/cm-' and precursor polymer (---). The latter is shifted Fig. 2 Submillimetre-wave transmission spectrum of 12 pm thick (-) PPVfully converted offilmupwards for clarity. and precursor polymer (---) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 polymer spectrum which is not present in that of the PPV. It is believed that both the thiophene ring and trapped solvent contribute to this peak. Note the presence of interference fringes in the PPV spectrum.These confirm the thickness uniformity of the PPV film within the interferometer throughput area of 2 mm2. The precursor polymer spectrum shown in Fig. 2 exhibits absorption bands at 170, 497, 523 and 555 cm-'. The peak at 555 cm-' has already been assigned whilst the peaks at 497 and 523 cm-' are not observed in the PPV spectrum and are probably associated with the thiophene ring. The peak at 170 cm-' is present in both spectra in Fig. 2 (with a slight shift in the PPV spectrum due to modulation with the dielectric visibility fringes). This suggests that this peak is related to a group frequency charac- terising the unaltered p-phenylene ring. This is also confirmed by the absence of this peak in the spectra of fully doped samples as explained later.Doping of the PPV polymers by H2S04 was achieved by the technique described earlier. Accurate monitoring of the exposure time for successful doping is essential. Over- exposure leads to embrittlement of the sample whilst short exposure results only in surface doping of the sample. The exposure time is strongly temperature dependent, being significantly reduced at higher temperatures. A large number of samples were studied under different doping conditions and the optimum conditions determined. The optimum conditions in this instance are those leading to the maximum level of electrical conductivity with no deterio- ration in the mechanical properties of the sample. Contin- uous measurements of the submillimetre-wave spectrum for the doped samples have shown that the optimum doping time is reached when the peak at 170 cm-' disappears.Dif- ferent thicknesses have been examined and the optimum exposure times recorded. For example, samples of thicknesses 5, 12,25 and 39 pm required exposure times of 4,7,25 and 50 min, respectively. This thickness dependence is shown graphi- cally in Fig. 3. The submillimetre spectra of three doped PPV samples of different thicknesses are shown in Fig. 4.Two of the samples, of thickness 5 and 12 pm, were doped to optimum conditions, whereas the third sample, of 110 pm thickness, was immersed in the acid for only 30 min, resulting in reduced doping levels. Thus the peak at 170 cm-' can still be observed in the 110 pm sample.All three spectra are dramatically changed from the spectrum of undoped PPV shown in Fig. 2; only the peaks at 429 cm-',associated with the trans-vinylene moiety, and 555 cm-', associated with the p-phenylene group, can be seen in all spectra. Fig. 5 shows the IR spectrum of a 5 pm thick doped sample with exposure time of 5 min. There are a number of changes in the absorption peaks when compared with the 50 40 5 30 v) tY ,.o 20 .10 881 100, I I .. , ,I,, ,, f, ,, , ,,-*,,, , , , , , , , , , . 0 100 200 300 400 500 600 wavenumber/cm-' Fig. 4 Submillimetre-wave transmission spectra of three sulfate- doped PPV samples. From top to bottom, 5, 12 and 110 pm thick, respectively. The 110 pm thick film was not allowed to reach optimum doping conditions.spectrum of the undoped sample shown in Fig. 1. One impor- tant difference between the two spectra is the change in the unsymmetrical semicircular ring stretch at 1594 cm-I. It can be seen that this peak increases in intensity after doping, indi- cating an increase in the loss of centre of symmetry in the phenylene ring. Our observations lead us to propose the fol- lowing mechanism for the oxidative doping of PPV by con- centrated sulfuric acid: 2PPV -2e--,2PPV+ H2S04 + 2e-+H2 + SO:-The bubbles of gas seen when the polymer outgasses are hydrogen (along with small amounts of trapped air). The dopant anions become incorported into the regimes between the oxidised polymer chains.However, the physical attach- ment of the sulfate group to the PPV is weak and washing with a proton-containing solvent (such as water) removes the dopant and the polymer returns to its pre-oxidised, non- conducting state. Solvents such as acetone, used in this study for washing the doped samples, have no easily dissociated protons and thus do not lead to undoping of the sample. Although this is a two-electron reduction of sulfuric acid, the electrons could come from widely separated sites on a chain (or even from different chains) leaving individual, positively charged polarons and changing the electronic configuration from the aromatic structure to the conducting quinoid struc- ture, as shown in Scheme 2. The resulting increase of conju- gation is responsible for the observed change in colour.1.4 1 .-1-21 5 1.0 4000 3200 2400 1800 1400 1000 600 wavenum ber/cm -' Fig. 5 IR spectrum of a 5 pm thick film of fully converted PPV allowed to reach optimum doping conditions aromatic -e H,SO,1 1 Scheme 2 This mechanism is in agreement with the observed spectra. That is, the change in the unsymmetrical semicircular ring stretch at 1594 cm-' indicates enhanced loss of centre of symmetry in the phenylene rings after doping. The loss of mechanical integrity, observed in our experi- ments, upon excessive doping is due to the production of large numbers of positively charged polaron sites, resulting in electronic rearrangment and the rupture of bonds along the polymer chain.Conductivity The maximum conductivity attained using this simple doping technique is 1 S cm-l, when measured soon after doping. This value drops to 0.5 S cm-' after exposure to ambient conditions for one week and remains constant thereafter. This reduction in conductivity is attributed to a redistri-bution of dopant by diffusion to reach a uniform level thoughout the sample rather than a high concentration near the sample surfaces. Gagnon et aL4 have reported a much higher value for PPV doped by continuous exposure to H2S04 vapour by a cryogenic distillation technique. However, these workers did not comment on the long-term stability and conductivity levels of their sample. The work done in this study suggests that a conductivity value of 100 S cm-' will be achieved only by excessive sulfate oxidation resulting in the degradation of the polymer itself. Conclusion In this report a simple and reliable technique was developed for doping fully converted, free-standing PPV films of thick- nesses up to 110 pm.Precise control of the doping time results in the production of robust films suitable for practical applications. Optimum doping times can be determined by visual monitoring of the outgassing rate of the immersed sample, something not possible in vapour-phase doping tech- niques. The conductivity of the samples becomes constant after one week owing to the dopant mobility giving a homo- geneous distribution of sulfate throughout the polymer. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Excessive doping yields higher conductivities but also results in a large drop in the mechanical properties of the PPV. Assignments have been made for previously unassigned peaks in both the IR and submillimetre absorption spectra. The authors would like to thank the British Council for financial support. They also express their gratitude to the Astrophysics Group, QMC, London University, in particular to Dr. P. R. Ade and Prof. P. E. Clegg, for help in performing the submillimetre-wave spectra and conductivity measure-ments. References 1 P. J. Nigrey, A. G. MacDiarmid and A. J. Heeger, J. Chem. SOC., Chem. Commun., 1979,593. 2 A. J. Heeger, Science and Applications of Conducting Polymers, ed.W. R. Salaneck, D. T. Clark and E. J. Samuelsen, Adam Hilger, Bristol, 1991, p. 1. 3 J. I. Jin, S. H. Yu and H. K. Shim, J. Polym. Sci., Part B, Polym. Phys., 1993, 31, 87. 4 D. R. Gagnon, J. D. Capistran, F. E. Karasz, R. W. Lenz and S. Antoun, Polymer, 1987,28,567. 5 D. R. Gagnon, J. D. Capistran, F. E. Karasz and R. W. Lenz, Polym. Bull., 1984, 12,293. 6 C. C. Han, F. E. Karasz and R. W. Lenz, Polym. Commun., 1987, 28,261. 7 J. I. Jin, S. H. Yu and H. K. Shim, J. Polym. Sci., Polym. Chem., 1991,29,93. 8 N. F. Moh, Conduction in Non-Crystalline Materials, Oxford University Press, Oxford, 1987. 9 P. D. Townsend and R. H. Friend, Phys. Reo. B, 1989,40,3112. 10 R. H. Friend, J. H. Burroughs and K. E. Ziemelis, Science and Applications of Conducting Polymers, ed.W. R. Salaneck, D. T. Clark and E. J. Samuelsen, Adam Hilger, Bristol, 1991, p. 35. 11 C. S. Brown, M. E. Vickers, P. J. Foot, P. D. Calvery and N. C. Billingham, Polymer, 1986,27, 1719. 12 J. H. Burroughs, C. A. Jones and R. H. Friend, Nature (London), 1988,355,137. 13 D. D. C. Bradley, Polym. Int., 1991,26, 3. 14 J. H. Burroughs, C. A. Jones, R. A. Lawrence and R. H. Friend, NATO AS1 Ser. E: Appl. Sci., 1990,182,221. 15 J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Bums and A. B. Holmes, Nature (London),1988,347,539. 16 R. H. Friend and R. W. Gymer, Nature (London), 1992,356,47. 17 R. H. Friend, D. D. C. Bradley and A. B. Holmes, Phys. World, 1992,42. 18 D. D. C. Bradley, J. Phys. D: Appl. Phys., 1987,20, 1389. 19 I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polym. Commun., 1984,25, 327. 20 D. Bloor, Science and Applications of Conducting Polymers, ed. W. R. Salaneck, D. T. Clark and E. J. Samuelsen, Adam Hilger, Bristol, 1991, p. 23. 21 H. H. Wieder, Laboratory Notes on Electrical and Galvano- magnetic Measurements, Materials Science Monographs, Elsevier, Amsterdam, 1979, vol. 2. 22 G. Zerbi, C. Castiglioni, S. Sala and M. Gussoni, Synth. Met., 1987, 17,293. Paper 3/05609K; Received 17th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000879

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 883-888 Dielectric and Steric Hindrance Effects on Step-polymerization of a Diepoxide with Monoamines G. P. Johari Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L 7 The step-polymerization reaction between monoamines and diepoxide has been investigated by measuring the permittivity and dielectric loss as a function of reaction time, t,,, at different temperatures. Hexylamine and cyclohexylamine were used both in stoichiometric and non-stoichiometric (amine-rich) compositions with the diglycidyl ether of bisphenol-A. The dielectric consequences of increase in the growth of the macromolecule during the step-polymerization are analogous to the frequency dependence of a chemically invariant sub- stances.The reaction-time dependence of &* has the functional form: @(t)= exp -[t/z(t,,)lr where Y = 0.3-0.4. Steric hindrance of the NH, group in cyclohexylamine slowed the rate of polymerization and the E* features were shifted towards longer t,, . Excess amine accelerated the polymerization process, but shifted the dielectric features towards longer reaction time. Although the reaction reached completion, the mixture did not vitrify. In this respect the dielectric behaviour differs from that of network polymerization. Reaction temperature, time, composition and chemical structure of the monoamine, all affected the strength of the sub-glass transition tem- perature (T,) relaxations of the polymerized product. The rate of chemical reactions in a polymerization process becomes controlled by the diffusion coefficient of the reacting species, especially as the reactions approach completion or viscosity becomes high.However, this diffusion coefficient decreases as a macromolecule grows, thus slowing the rate of the very chemical reaction that allows its growth. In those polymerization processes where the groups remain reactive, the extent of polymerization itself slows the rate of its progress, an occurrence referred to as negative feedback between a chemical (reaction) and physical (molecular diffusion) process.’ Cal~rirnetric,’.~ andultrasoni~~~’~ Brillouin light scattering’ studies of macromolecular network formation have shown that relax- ation characteristics of a reacting mixture change with time in a manner analogous to that observed on supercooling a chemically stable substance.Thus, for each relaxation state of the chemically stable, unreacted or partly reacted liquid (with van der Waals and other weak molecular interactions) char- acterized by its relaxation time alone, there is a correspond- ing state of its polymeric structure (containing now a certain number of intramolecular bonds) with the same relaxation time but at a higher temperature. Since molecular diffusion time is determined by EJk, T,where Ei represents the activa- tion energy for a diffusion process, and k, the Boltzmann constant, the same diffusion time can be achieved by (i) an increase in Ei at a constant temperature, T, as during the growth of a macromolecule, or (ii) a decrease in T for a con- stant Ei as on supercooling a chemically stable liquid.This means that there is a phenomenological equivalence in terms of k, T,for molecular diffusion time in the same composition, with and without covalent bonds. For dielectric studies of the growth of a macromolecule, we, as others,12-19 had chosen substances which formed a network structure on polymerization, mainly because of their well known technological importance as thermosetting resins. Nevertheless, one study of free-radical polymerization of methyl methacrylate was carried out, but without success in achieving vitrification.20 Here we report a dielectric study of linear-chain polymerization between monoamine and di-epoxide, and examine the effects of steric hindrance of the NH, group and non-stoichiometry on the rate of poly-merization ; we also report the dielectric properties of the polymerization product.This is a first report of our studies on a series of substances from which hindrance for molecular diffusion caused by a network topology is absent, because the reaction of an epoxide with a difunctional amine does not normally produce a cross-linked network. Experimental Hexylamine and cyclohexylamine of 99% purity were pur- chased from Aldrich Chemicals and diglycidyl ether of bisphenol-A (DGEBA), M, = 380 and functionality = 2, was donated by Shell Chemicals as Epon 828. All chemicals were used without further purification.Accurately weighed amounts were mixed and transferred to a 10 mm diameter glass container. A ten-plate miniature tunable capacitor was immersed in the liquid mixture, and a thermocouple was kept immersed 2 mm above the capacitor. The details of the dielectric measurement assembly, temperature controls and source of errors have been described in earlier papers.’ Results The permittivity, d, and dielectric loss, E”, were measured for a fixed frequency of 1 kHz at suitable intervals as poly-merization of stoichiometric (1 : 1 molar ratio) and monoamine-rich (2 : 1 molar ratio) mixtures of hexylamine- DGEBA and cyclohexylamine-DGEBA at different tem-peratures progressed. These are plotted against the reaction time, t,,, in Fig.1 and 2. E’ decreases at different rates in different regions, but the manner by which the decrease occurs in a given region of t,, varies with the composition and the temperature. [Note that the discontinuity of curves (e) near t,, = 3 ks in Fig. 2 was caused by an accidental failure of the temperature control unit, and is not a property of the material.] E“ decreases initially towards a local minimum followed by a peak from where it ultimately decreases to a low value. This decrease varies with the tem- perature and composition of the reacting mixture. The plots also show that on increasing the reaction temperature, T,, , d(t,, -,0) and the &”-peak height, decrease, dr(tre-+ 0) increases and the curves shift towards shorter tre.These fea- tures are qualitatively similar to those observed previously.6 E’ and E” for all monoamine4iepoxide mixtures are plotted in a complex plane in Fig. 3, where each point refers to a 884 8.51 8.04 I 7.5-7.0->. c '5 6.5-.-c *g 6.0-h 5.5-5.0-4.5-4.0-3.5 I I I I I I I l l 1 I I I I I Ill1 1 1 I I I Ill 10: 1: 0.1: 0.01 ! I I I I I IIII I I I I I IIII I I I I I (Ij 100 1000 10 000 10G lr00 t re/S Fig. 1 Permittivity and dielectric loss of hexylamine-DGEBA mix-tures measured at 1 kHz us. tre. Data shown by filled rectangles are for 2 : 1 molar ratio; all others for 1 : 1 (stoichiometric) molar ratio. (a), (b) and (c) refer to the reaction temperatures given in Table 1.certain t,, . The shape of the plot is skewed more at the long-time than the short-time end, except for the stoichiometric hexylamine mixture at 316.2 K and its amine-rich mixture at 309.5 [Fig. l(c)]. As in earlier st~dies,~-~the E* data were converted to M*(=~/E*)and the real and imaginary com-ponents of M* are plotted in a complex plane in Fig. 4. These show two relaxation processes : (i) conductivity relaxation, which appears as a semicircle with centre on the axis and (ii) Table 1 Dielectric parameters measured at 1 kHz during the poly-merization reaction of the two monoamines and DGEBA hexylamine-DGEBA 306.2 (a) 14.4 0.525 3.8 (3.9) 6.6 0.35 316.2 (b) 10.6 0.474 3.8 (3.9) 6.3 0.35 309.5" (c) ->0.024 5.4 cyclohexylamine-DGEB A 300.2 (6) 37.2 0.649 3.5 (4.0) 7.5 0.34 306.2 (e) 23.3 0.580 3.8 (3.9) 7.1 0.32 312.2 (f) 16.9 0.548 3.8 (3.9) 6.8 0.34 316.2 (g) 11.8 0.526 3.8 (4.0) 6.7 0.34 309.5" (h) 26.6 0.430 3.5 (3.6) 5.9 0.34 Notations in brackets refer to the plots in Fig.1-7. Values in brackets are those used for calculating Y and z. &'(tre+0) was obtained from the data fits shown by triangles in Fig. 3, and differs from those in Fig. 1 and 2. These values represent the dipolar contri-bution. " Monoamine saturated composition molar ratio (2 : 1). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 8.5 I 8.0-7.5-7.0->..z 6.5-.-Z 6.0-E g 5.5-4.0 3.5 0.001 / I I I I1 Ill1 I I I I IIIII I I I IIIII 100 1000 10 000 100 000 t re/S Fig.2 Permittivity and dielectric loss of cyclohexylamine-DGEBA mixtures measured at 1 kHz us. tre. Data shown by dots are for 2 : 1 molar ratio; all others for 1 : 1 (stoichiometric) molar ratio. (4,(e), (f),(9)and (h) refer to the reaction temperatures given in Table 1. dipolar relaxation which appears as a skewed arc (t,, increases from left to right in Fig. 4). After a certain period of reaction (at which the plots in Fig. 1 and 2 terminate), the product was cooled in situ to 77 K and its E' and E" for 1 kHz measured as a function of tem-perature up to near T,, .Plots of tan 6( =E"/E') us. temperature of the polymerized product in Fig. 5 show relaxations in the glassy state. (The change in the cell constant with change in temperature is not known accurately and therefore tan 6 instead of E" plots are given.) These appear as a peak at about 250 K and a shoulder to this peak in the range 100-175 K.The height and the temperature of both vary with the nature and amount of reactants. Earlier studies have shown that for a given reactant and T,, they also vary with the reac-tion time,21*22particularly during the early period of chemi-cal reactions." The data determined from Fig. 3 are listed in Table 1. Here, ~'(t,,-+0) differs from those in Fig. 1 and 2 for the fol-lowing reasons. Values in Fig. 1 and 2 contain contributions from interfacial impedance which decreases as the reaction progresses (see ref. 6 for details), leading thus to a minimum in E" at the long-time end of the plots in Fig.3. &'(ire-+ 0) in Fig. 1 and 2 thus differs from that in Fig. 3. The values corre-spond to the dipolar process. Discussion The dielectric properties during the polymerization are deter-mined by the changing composition of the mixture in which a J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 3.01 n A 2.5 uI 2.0 -81 Iu 0 1.5.-LU $ 1.0 .--0 0, I I 1 I I I I I -.-I CB 4.5 4.0 I 3.5 v) -8 3.0 .-0 2.5 -a a 5 2.0 1.5 1 .o 0.5 0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 perrnittivity Fig. 3 Cole-Cole plots of A, hexylamine-DGEBA and B, cyclohexylamine-DGEBA mixtures measured at 1 kHz. (a)-(h) refer to the reaction temperature, the composition and the monoamines given in Table 1.The reaction time increases from right to left. Tri- angles are the calculated values from the data in Table 1. (c) and (h)are on the scale, the axis for the others is shifted upwards, as shown. macromolecule grows.In chemical terms, it means a decrease in the number of monomers and their dipole moments, in physical terms an increase in the configurational restriction to dipolar diffusion and in the intermolecular vibrational fre- quencies. The changes in the dielectric properties during the poly- merization process are caused by at least seven effects:'e6 (i) A general decrease in the dc conductivity as the diffusion coeffi- cient of impurity ions present and zwitter ions22 formed in the reacting mixture decreases with increase in its viscosity and as any proton transfer along the H-bonded network in the mixture is virtually eliminated by the consumption of the H bonds between the monomers.(ii) An increase in the molecular diffusion or relaxation times as a result of which the permittivity measured at a fixed frequency decreases monotonically toward the value corresponding to the IR region. (iii) A change in the number of dipoles per unit volume and, therefore, in the contribution to permittivity from orientation polarization, AE, as a result of both the chemical reactions that alter the dipole moment and the volume contraction that raises the number density of dipoles. (iv) The splitting of a unimodal relaxation function into a bimodal relaxation function.' (v) A change in the dielectric relaxation function as the chemical structure of the liquid changes and its viscosity and density increase.(vi) A change in the contribution to permittivity due to IR-polarization, as the vibrational frequencies of the various 885 0.20 j 1 ............... 01 I -1. 0.30 BI t/;."---m..........*.*,ib...............1 0.20 ................. M" 0.15 21 qb .............*'......\I0.10 /.. ........ .....0 I I 4 I I 0 0.05 0.10 0.15 0.20 0.25 0.30 M' Fig. 4 Complex plane plots of M" and M' for A, hexylamine-DGEBA and B, cyclohexylamine-DGEBA measured at 1 kHz. (a)-(h) refer to the reaction temperature, the composition and the monoamines given in Table 1.(a) and (d) are on the scale, the axis for the others is shifted upwards, as shown. The reaction time increases from left to right. 0.1 *fly-I it 0.001 I I I I 0.001 100 150 200 250 300 . i0 T/K Fig. 5 Loss tangent of the polymer formed on reaction of mono- amines with diepoxide measured at 1 kHz 0s. T. A, Hexylamine and B, cyclohexylamine. (a)-(c), (e), (9) and (h) refer to the reaction tern-perature and time given in Fig. 1. modes in the structure change on polymerization and densifi- cation. (vii) A change in the optical refractive index or optical polarization as the structure densifies on polymerization. All these effects can be taken into account in eqn. (1). However, their relative magnitudes vary for a given instant reaction.Generally, the change in dielectric properties is dominated by (i), (ii) and (iv), for which substantial evidence has been provided by our earlier st~dies.~-~ As before,6 we write: Eqn. (1) is derived from time-frequency interrelations when the thermodynamic properties of a material remain constant with time. So, for a fixed t,,, the equation is valid as in most cases. It is also valid when the rate of change of thermodyna-mic properties with time is much slower than the rate of polarization and depolarization or when T is much smaller than the inverse of the reaction rate constant. For such cases : However, when the two approach each other, eqn. (1) is no longer valid, and an evaluation of the function [@(t)lt,, becomes complicated, because now two timescales are involved.This evaluation remains to be done. For convenience of analysis, we assume that (doldt), E~ and E, do not change with t,,, and replace /? by T, the reaction parameter for time-variant chemical and physical states. [It has a parallel in the physical ageing beha~iour,,~ where a similar form to eqn. (2) is used with 7 increasing irreversibly with the ageing time.] This does not imply that a direct Fourier transform of eqn. (2), now containing Y instead of j?(t,,) into a frequency domain is meaningful. It does have a technical importance for following the progress of reaction by measuring the dielectric and ultrasonic proper tie^.^,' /? (for a time-invariant state) is usually higher than T and, for network formation on polymerization, the two have opposite temperature dependence.’ Eqn.(1) is invariant on the choice of o or T as a variable, and features of E‘ and E” in Fig. 1-4 result from an irreversible increase of 7,which increases oz from a value less than one to one greater than one. When or = 1, &“ = &Lax. As in earlier the relaxation time was calcu- lated from the data in Fig. 1-4. Its value corresponds to the characteristic time 7(t,,) of eqn. (2), and not to the E” peak or average value of 7. It is plotted logarithmically against the reaction time of monoaminediepoxide mixtures in Fig. 6 and 7. The sigmoid-shaped plots shift towards longer t,, with increase in T,, ,and increase in the monoamine content of the mixture.The first effect is expected for virtually all poly- merizing reactions which are accelerated by increasing the temperature, and the second depends upon the amine used. Y and other dielectric parameters are summarized in Table 1. We now discuss the above observations in terms of the growth of a macromolecule and the steric hindrance effect of the amine on this growth. In contrast with our earlier studies of network polymers formed by the reaction of a diamine with a diep~xide,~-* the reaction product here should be a linear-chain polymer, for here the -NH, group of the mono- amine reacts with the epoxy groups to form -CH( 0H)- CH, -N( R)-CH, -CH( 0H)-linkage between two DGEBA molecules (a less probable reaction between one J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Or--1 I log (trels) Fig. 6 log 7 us. log t,,, as polymerization in hexylamine-DGEBA progresses. (a) and (6) refer to the reaction temperature given in Table 1. NH, group and two epoxide groups of the same DGEBA molecule will produce a highly strained molecular ring but not a network structure). Thus, the alkyl group, R, of the amine becomes a pendant in a linear chain of DGEBA mol- ecules linked by nitrogen atoms. Calorimetric studies of the reaction products showed that their endotherms are narrow, similar to those for linear-chain polymers. (Endotherms for network polymers tend to be broad.) A com-parison of the various plots in Fig. 1 and 2, and the data in Table 1 show that the E” peak during polymerization at the 0 -1 -2 h $ -3 Y CT,--4 -5 -6 -71 I I 3.8 4.0 4.2 4.4 4.6 4.8 log (treis) Fig.7 As for Fig. 6, but for polymerization of cyclohexylamine- DGEBA mixtures. (4-(h) refer to the conditions listed in Table 1. Data shown by dots are for 2 : 1 molar ratio; all others are for 1 : 1 molar ratio. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 same T,, appears at a shorter reaction time for hexylamine than for cyclohexylamine. This means that the composition of the polymer (and molecular weight) reach the same relax- ation time (=w-’ or 159 ps) after 14.4 ks of reaction with hexylamine and 23.3 ks with cyclohexylamine at 306.2 K. A corresponding difference is also observed at 316.2 K in Table 1.The rate of growth of the polymers is more easily examined by a comparison of the increase in the relaxation time with the reaction time given in Fig. 6 and 7, which are drawn on the same scales. Polymerization is more rapid with hexyla- mine than with cyclohexylamine. Since the reaction mecha- nism between a diepoxide and monoamine is expected to be independent of the shape of the monoamine, this implies that the probability of mutual diffusion of the reacting epoxide and -NH, groups is greater in linear-chain hexylamine than in cyclohexylamine. This is particularly important because the rate of chemical reaction is seen to be viscosity indepen- dent when the viscosity of the reaction medium is low, but viscosity dependent when it is high and molecular diffusion controls the reaction rate.Since the viscosities of the unre- acted mixtures here are not greatly different, the slower rate of polymerization must be attributed to a greater steric hin- drance of the -NH, group in cyclohexylamine than in hexylamine, which decreases the probability of mutual approach of NH, and epoxide groups. We now discuss the effect of increasing monoamine (amine- rich non-stoichiometry) on the observed dielectric behaviour. Although the E” peak shifts towards longer t,, for both mono- amines (Fig. 1 and 2), this shift is not clearly discernible for hexylamine. Both amines are less viscous than DGEBA, so an increase in the amine concentration lowers the viscosity of the unreacted mixture.Both an increase in concentration and a higher diffusion coefficient increase the polymerization rate of an amine-rich composition relative to the stoichiometric composition. Therefore, one expects that the E” peak for the amine-rich composition will appear sooner than for stoichio- metric Composition. However, even with the faster poly-merization rate, there is the requirement that for the same relaxation time, the size and molecular weight of the macro- molecule be greater in a low-viscosity solvent than in a high- viscosity solvent. This requirement is fulfilled at long t,, in a low-viscosity solvent, i.e. t,, to reach wz = 1 is greater as the increase in the polymerization rate is outweighed by the requirement for a larger macromolecular size.The relatively smaller decrease of E’ from 6.2to 5.3,the low E” as t,, 4co in Fig. l(c), and the absence of a dipolar relaxation process in Fig. 3(c),all show that although polymerization in amine-rich hexylamine-DGEBA has reached completion, the relaxation time of the solution (1 mol polymer in 1 mol unreacted hexy- lamine, or oligomers containing different numbers of -CH(OH)-CH,-NH-R linkages) at 309.5 K does not reach 159 ps (=o-’)as t --+ m. The shift in the E” peak towards longer t,, in amine-rich mixtures may therefore be interpreted either as a solvent effect on the relaxation time, or as an indication of the formation of a variety of oligomers, or a combination of both. This observation is supported by the tan 6 plots in Fig.5 which show that the a-relaxation peak of the amine-rich hexylamine-DGEBA mixture appears at ca. 285 K; for all others, it appears above t,,. Measurements, after pumping to remove the excess amine from the reaction product, will ascertain whether the effect is entirely due to the unreacted amine acting as solvent or due to the formation of oligomers as well. For the cyclohexylamine-rich mixture, wz does reach unity as the reaction progresses and an E” peak is reached (Fig. 2 and 4).As discussed, t,, for the peak is expected to be longer here than for the stoichiometric mixture, but z does not become high enough to cause vitrification (Fig. 7). Our studies of network polymerization have shown the opposite, i.e.chemical reaction in a 4,4’-diaminodiphenylmethane-rich DGEBA mixture occurred faster and the E‘’ peak appeared at a shorter time than in a stoichiometric mixture, i.e. excess diamine accelerated the reaction.24 It is also found 25*26 that when the reactants are dissolved in an inert solvent, the reac- tion is retarded and the polymer formed has a lower cross- link density and % than that formed without a solvent. Thus, network polymerization differs from step-polymerization in this respect. For a detailed understanding of these effects, we are investigating linear-chain polymerization of monoamine-diepoxide mixtures in inert non-polar solvents. When the reaction temperature is increased, the plot of 7 us. t,, for the stoichiometric cyclohexylamine-DGEBA mixture shifts towards the left, but such plots at different T,, do not cross each other, as seen in Fig.7. This occurs also for the hexylamine mixture in Fig. 6,but the plots for the two temperatures show that one crosses the other at t,, w 15.9 ks. Strictly interpreted, it means that z of the products and reac- tants mixture at t,, w 15.9 ks at 316.2K is the same as that at the same time but at 306.2 K.Thereafter, 7 at 316.2 K remains less than at 306.2K as the reaction continues further. This, of course, is also apparent in Fig. l(a) and (6) where E‘ and E“ at 316.2K (6) do not decrease to as low values, and oz does not increase to as high values as for 306.2 K.It seems that there are at least three reasons for such an occurrence.First is the extent of chemical reaction itself which, at limiting long times (when equilibrium concentration of the reactant and product is reached), is less at a high temperature than at low temperatures, because the equilibrium constant of exo-thermic reactions decreases rapidly with increase in the tem- perature. Thus, the ultimate concentration of the polymerized product is less at 316.2 K than at 306.2K,and hence r is less, although it increases with time initially more rapidly at a higher K, than at a lower T,, . Secondly, there are more oligo- mers formed at a higher than at a lower T,, and hence z remains lower and, thirdly, z of the reactant-product mixture is extremely sensitive to temperature, so that for the same composition of the reactant-product mixture a change from 306.2 K to 316.2 K decreases z by as much as two decades. The effect observed in Fig.6 may be due to all three, but if the first is the predominant cause, then all reacting mixtures which do not vitrify on polymerizing, and thereby allow the concentration of reactants and products to remain at chemi- cal equilibrium after a reasonable observation period, should show the crossover of z as in Fig. 6.We plan to examine this by further studies. The relaxation in the glassy state of fully and partially polymerized mixtures is of interest in determining the extent to which localized motions contribute to the electrical properties. In pure DGEBA, there are two such relaxation processes appearing at 156 K as a peak of height 0.025, and at 200 to 225 K,a shoulder of height 0.008 (T,= 293 K).,, In the polymerized states, both the magnitude of the sub-? relaxation processes and the shape of the features at the two temperatures are reversed.The low-temperature process appears in about the same temperature range as in pure DGEBA,,, but its strength is decreased and it appears as a shoulder; the high-temperature process also appears in the same temperature range but its strength is increased and it appears as a peak except for the hexylamine-rich mixture where the peak vanishes. It seems that changes in all the four variables namely: (i) T,, , (ii) t,, , (iii) composition, and (iv) structure of the amine, affect the sub-T, relaxations.Increasing T,, and t,, raises the height of the high-temperature peak and decreases that of the low-temperature shoulder. Substitution of hexylamine by 888 cyclohexylamine increases the strength of both the low- and high-temperature relaxations and shifts the peak towards a higher temperature. Increasing amine content lowers the height of the high-temperature, but raises that of the low- temperature relaxations. In our detailed studies of the effect of reaction temperature and time on the sub-% relaxations in a network polymer formed by the reaction of 4,4'-diaminodiphenylmethanewith DGEBA,2'*27 and pure DGEBA catalysed with N,N'-dimethylbutylamine,22 an increase in T,, increased the height of high-temperature relaxation and decreased that of the low- temperature relaxation for a fixed reaction time, as did the increase in t,, for a fixed Te.On the basis of these earlier observations we concluded that sub-T, relaxations are more appropriately understood in general physical terms without assigning, on an ad hoc basis, a particular diffusion mode or group of molecules for each relaxati~n.~~*~~ In the unreacted state there is only one, the low-temperature (145 K) relax-ation peak in the diamine-DGEBA mixtures20*21*27(there are two in pure DGEBA).What seems remarkable is that in the limits of monomeric and almost fully polymerized states, there is also only one sub-T, relaxation process. In the mono- meric state it appears at 145 K, in the polymerized state at 262 K.In the intermediate states of incomplete poly- merization both occur, and there is a gradual attrition of the first and emergence of the second as polymerization approaches completion. This evolution of sub-% relaxations is evidently a reflection of the development of structures with molecular-packing inhomogeneity as weak van der Wads, H-bonding, and dipolar interactions lose their predominance to topologically well defined, covalent bonded structures. Conclusions Dielectric measurements of a liquid in which chemical reac- tions occur to cause linear-chain polymerization provide useful information on both the irreversible change in the relaxation time and its dependence on the reaction time. In particular, the study leads to the following conclusions: (1)The irreversible decrease in the permittivity measured for a fixed frequency as the polymerization continues is phenomenologically similar to that observed by increasing the frequency under isothermal conditions of a chemically stable liquid.(2) Several effects associated with steric hindrance of the reacting group slow the rate of polymerization. (3) Excess amount of the more fluid component in the reacting mixture accelerates the process, but vitrification does not occur. (4) Reaction time, temperature, composition and chemical structure all affect the strength of sub-% relaxations of the polymerization product. I am grateful to Mr. W. Pascheto and Mrs. M. Wang for their assistance in data collection and analysis.This research J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. References 1 G. P. Johari and M. B. M. Mangion, J. Non-Cryst. Solids, 1991, 131-133,921. 2 M. Cassettari, G. Salvetti, E. Tombari, S. Veronesi and G. P. Johari, I1 Nuovo Cimento, 1992,14D, 763; J. Polym. Sci., Part B: Polym. Phys., 1993,31, 199; Physica A, in the press. 3 G. P. Johari, Dynamics of Irreversibly Forming Macromolecules, in Disorder Effects on Relaxation Processes, ed. R. Richert and A. Blumen, Springer-Verlag, 1994, in the press. 4 M. B. M. Mangion and G. P. Johari, J. Polym. Sci., Part B: Polym. Phys., 1991,29,1117; 1127. 5 M.G. Parthun and G. P. Johari, Macromolecules, 1992, 25, 3149; 3254; J. Polym. Sci., Part B: Polym. Phys., 1992,30,655. 6 G. P. Johari, in Chemistry and Technology of Epoxy Resins, ed. B. Ellis, Chapman and Hall, London, 1993, ch. 6, p. 175; J. Mol. Liquids, 1993,56, 153. 7 E. Tombari and G. P. Johari, J. Chem. Phys., 1992,97,6677. 8 M. Cassettari, G. Salvetti, E. Tombari, S. Veronesi and G. P. Johari, J. Mol. Liq., 1993,56, 141. 9 I. Alig, D. Lellinger and G. P. Johari, J. Polym. Sci., Part B: Polym. Phys., 1992,30,791. 10 I. Alig, K. Nancke and G. P. Johari, J. Polym. Sci., Part B: Polym. Phys., in the press. 11 M. B. M. Mangion, J. J. Vanderwal, D. Walton and G. P. Johari, J. Polym. Sci., Part B: Polym. Phys., 1991,29,723. 12 C. Huraux and A.Sellaimia, C.R. Acud. Sci. Paris, Part B, 1973, 277, 691. 13 A. Soualimia, C. Huraux and B. Dispax, Mucromol. Chem., 1982, 183,1803. 14 S. D. Senturia and N. F. Sheppard Jr., Ado. Polym. Sci., 1986, SO, 1. 15 D. R. Day, Polym. Eng. Sci., 1986,26,362. 16 See references cited in the reviews, ref. 6 and 14. 17 D. E. Kranbuehl, J. Non-Cryst. Solids, 1991,131-134,930. 18 G. M. Maistros, H. Block, C. D. Bucknall and I. K. Partridge, Polymer, 1992,33,4470. 19 C. G. Delides, D. Hayward, R. A. Pethrick and A. S. Vatalis, J. Appl. Polym. Sci., 1993,47,2037. 20 E. Tombari and G. P. Johari, J. Chem. Soc., Faruduy Trans., 1993,89,2477. 21 M. B. M. Mangion and G. P. Johari, J. Polym. Sci., Part B: Polym. Phys., 1991,29,437. 22 I. Alig and G. P. Johari, J. Polym. Sci., Part B: Polym. Phys., 1993,31,299. 23 I. M. Hodge and A. R. Berens, Macromolecules, 1981,15,762. 24 M. B. M. Mangion, M. Wang and G. P. Johari, Macromolecules, 1992,30,433. 25 K. Hofer and G. P. Johari, Macromolecules, 1991,24,4978. 26 G. Mikolajczak, J. Y.Cavaille and G. P. Johari, Polymer, 1987, 28,2023. 27 M. B. M. Mangion, M. Wang and G. P. Johari, J. Polym. Sci., Part B: Polym. Phys., 1992,30,445. 28 J. D. Keenan and J. C. Seferis, J. Appl. Polym. Sci., 1979, 2.4, 2375. 29 M. Ochi, M. Yoshizumi and J. Shimbo, J. Polym. Sci., Part B: Polym. Phys., 1987,25,1871. Paper 3/055461; Received 14th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000883

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 889-893 Theory of Monolayers of Non-Gaussian Polymer Chains grafted onto a Surface Part 1.-General Theory Victor M. Amoskov and Victor A. Pryamitsyn Institute of the Problems of the Mechanical Engineering, 61 Bolshoy pr., V.O., 199178 St.-Petersburg, Russia A theory describing layers of polymer chains grafted to a flat surface (polymer 'brush') is developed. We consider a brush of chains with arbitrary extensibility and, within the framework of molecular field theory, we suggest a general scheme for calculating the selfconsistent pseudo-potential, which determines the structure of the brush. This potential appears to be defined only by the mechanism of extensibility and is independent of the interactions between chains in the brush.A model potential for freely jointed chains (FJC) was calculated and used for describing an FJC polymer brush in good solvent conditions. We found good agreement between our results and the numerical calculations of Skvortsov eta/. The properties of polymers adsorbed at solid/liquid or liquid/ liquid interfaces are important in areas such as tribology and biophysics. One type of adsorption is 'grafting' or end adsorption, in which polymer chains are attached by one end to a surface. This may be achieved in several ways, for example, by constructing a block-copolymer in which one block is strongly adsorbed onto the surface. Similar struc- tures appear in the superstructures of block-copolymers. The resulting structure is usually called a 'brush'.The theory of brushes has been developed by many authors. I-' Initially, polymer brushes were analysed using the scaling method by Alexander,' De Gennes2 and Birshtein and Zh~lina.~ The general result of these theories was that the polymer chains in the brush are strongly stretched. To explain this result, let us consider a system consisting of long polymer chains grafted, by one end, to a flat surface. The number of segments in the chain, N, is one of the major parameters in the theory: N + 1. It is known that under the conditions of negligible inter- action between segments, the distance between the free ends of the chain is (hi) = Nu2, where u is the length of the Khun segment. In the presence of repulsion, with a finite radius of interaction between the segments (good solvent conditions), (h,) K N', where v z0.6." Let us estimate the size of the chains in the 'brush' in the presence of repulsion.Since the density of segments in a brush of long chains has to be finite, the height of the brush, Hb, has to be proportional to N if the grafting density per unit area, 0, is constant. This dependence for the planar 'brush' is universal in any solvent or in a melt. Hence the chains in a planar brush are strongly stretched with respect to their ideal size, Hdh, a No.', and their size in a good solvent Hdh, a This stretching of chains in the brush is caused by the repulsion between chain segments. Scheutjens and Fleer4 employed self-consistent field (SCF) theory for the lattice model of the polymer brush.They calcu- lated numerically the structured details of brushes of finite N. Later Skvortsov et ul.' used the same model and combined it with the original procedure for extrapolation of the results of numerical calculations for finite N to N + a. The analytical SCF theory of polymer brushes was elabo- rated by Semenov6 for the case of brushes without solvent; it was developed for brushes with solvent by Pryamit~yn,~ and Zhulina et uL8 and independently by Milner et u1.9*10This theory uses the Gaussian model of polymer chains," and, as is shown in ref. 5, it appears to be an asymptote of the Scheutjens-Fleer model for the case where N -+ 00 and Hb 4 Nu. (contour length of polymer chains).Unfortunately, the Gaussian model of polymer chains is inadequate for strongly stretched chains, when Hb < Nu. Such strong stretching of polymer chains can appear in the charged brushes of polyelectrolyte chains, or in very dense grafted brushes, for example, in block-copolymer super-structures, when one of the blocks is a flexible polymer and the other is a rod-like mesogenic polymer. Shim and Cates constructed a very elegant approach for polymer-chain brushes with the ad hoc approximation of finite extensibility of the polymer chains.12 They found qual- itative agreement with Scheutjens and Fleer's numerical cal- culations, but no quantitative agreement. In our opinion, this is due to the fact that Shim and Cates' (SC)ad hoc approx-imation of finite extensibility cannot be realized in a physical model of the polymer chain (see below).In this paper, we discuss general SCF theory applied to brushes with an arbitrary mechanism of chain extensibility and apply this theory to the model of freely jointed polymer chains, which seemed to be adequate for polymer chains with finite extensibility. To compare our theory with the results of numerical calculations we examine dense grafted polymer brushes in good solvent conditions. Brushes of polyelectrolyte polymers and mesogenic brushes will be discussed in the future papers. Model In the SCF (or molecular field) model an assembly of inter- acting chains is described as a group of non-interacting chains in the self-consistent pseudo-potential, pLcp(r)],where p(r) is the local density of polymer-chain segments and p@) is their chemical potential.In thermodynamically stable systems, p(p) is an increasing function of p. If the grafting density is relatively high, NuZ% u-l, one can assume that the brush is homogeneous along the grafting surface and p(r) = p(x), where x is the distance from the graft- ing surface. Thus the problem of describing an assembly of chains in the brush is reduced to that of one chain in a one-dimensional, self-consistent stretching field. A long polymer chain in a non-uniform stretching field can be described as a spring in the stretching field." The polymer-chain conformations with maximum statistical weight correspond to the state of static equilibrium of the spring in the stretching field, whereas the extensibility of the spring corresponds to the local extensibility of the chain.It is significant that to produce chain stretching, p(x) has to be a decreasing function, consequently p(x) also has to be a decreasing function. It is reasonable to use the average elon- gation (6x(f)) of a chain of N segments under a stretching force,f, applied to the ends of the chain for the definition of the extensibility of a polymer chain, e(f): where F(f) is the Gibbs energy of a chain of N segments under a stretching force,f, applied to the ends of the chain. It can be proved that for any physical model of polymer chains, F(f)has to be an analytical even function off: Note that the SC model does not meet this condition.The system of equations describing the static equilibrium of the nth segment of a polymer chain of N segments in a field of stretching force,f(x), via the approach of a non-linear spring in a one-dimensional stretching field with potential p(x),in dimensionless form is: where 1 = n/N, z = x/Na, p = uf/kT (dimensionless stretching force),f(z) = aV(z)/az, V(z) = -p/kT (dimensionless increas- ing function of z), e@) is the non-linear extensibility of the chain, k is the Bolzmann constant and Tis the temperature. The boundary conditions are: Z(~)I~=~ = 0; p(I)(l=l= 0. The first integral of eqn. (2) is E(p) = V(ZJ -V(Z) (3) where E(p) = SpO e(p’)dp’ and z, is the height of the free end of the spring.The value of z, is determined by the equation: dzltA[V(z,) -V(z)] = (4) where A(V) = e[@(V)], E[qV)] = V. Eqn. (3) and (4) give solutions to eqn. (2), if the potential, V(z), and extensibility e(p), are known. For models of freely jointed polymer chains in a three- dimensional continuum and on a simple cubic lattice, the non-linear extensibilities, Ej(p),ej(p) and Ejl(p),ej,(p), are : sinh pEj@) = log( F), ej(p) = coth p -p-1 Selfconsistency Equations for the Polymer Brush To calculate the self-consistent potential we have to calculate the local density of polymer segments in the brush. From eqn. (2) we can define the local density of segments for a spring in stretching field, V(z): Qualitative analysis shows that p(z, 2,) is a monotonic increasing function of z for any stretching field, V(z),and has a singularity at the point z = z,. As mentioned above, to produce stretching of chains in the brush the local density of J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 chain segments in the brush has to be a decreasing function of z. How can we increase p(z, z,) for each chain in the brush and maintain the necessity of decreasing the brush density p(z)? Let us consider a polydisperse brush of chains with all possible lengths 0 < L < Lo. Let w(9)(9= L/Lo) be the distribution function of the length of the chains and g[z,(9)] the distribution function of the height of the chain ends, where ze(9)= x,/Lo is the reduced height of the end of the chain with length 3,the function Y(z,) being the length of the chain with end height z,.They are connected by the equation : (7) From eqn. (4) we have : If p(p), w(9) and cr are known, eqn. (7)-(9) completely determine the brush parameters. The selection of the distribu- tion z,(9) provides self-consistency for the potential V(z). It is evident that z,(9) has to be an increasing function of 9. To produce decreasing p(z) the free ends of the polydisperse chains have to be distributed from z = 0 to z = z,(I). It is easy to prove from eqn. (8) that if an excluded zone, where the free ends of the chains are absent, exists between z = 0 and z,(l), the function p(z) would be increasing in this zone. Evidently we have a similar situation for monodisperse brushes.The only possibility for providing a continuous dis- tribution of the free ends of the monodisperse chains in the brush is to use the potential in which the spring is in a state of indifferent equilibrium. The equation for this potential is : If the dependence of the chemical potential, p(p), is known we can calculate the distribution of the density of polymer segments in the brush: -p[p(z)] = kTV(z) + constant } (11)p(z) dz = O/Ulmax where z,, is the maximum possible height of the chain seg- ments. For the free brush with local interactions, without external restrictions, the value of zmax is determined by the =requirement ap/ap Iz=zrmx0 and by the normalization requirement for the density, eqn.(1 1). Eqn. (8) is transformed into the equation for self-consistency : p[ V(z)] = c Izmaxp(z,z’)g(zf) dz’ g(z’) dz’ where g(z) is the distribution function of monodisperse free chain ends, which is independent of the description of mono- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 891 disperse brushes, and p(V) is determined from the equation for the chemical potential, V = -p(p)/kT. If V(z) is the solution of eqn. (lo), we can invert eqn. (12) (see Appendix): g(2) = --b{V[~(Z)] -V(Z'))dz' (13) where V[Z(z)] = V(z,,.J -V(z), $(v) = (dp(v)/du). For small z,,, where [V(z,) -V(z)] is also small, the Gaussian approach is correct, i.e. &I) zp/3, A[V(z,) -V(z)] z J[V(z,) -V(z)] and eqn. (10) and (12) are trans- formed to: This is the well known system of equations for a brush of Gaussian The pseudo-potential for indifferent equilibrium of a Gaussian chain is V,(z)= 3n2/8 z2 = 3/2(nx/ ~QN)~.Self-consistent Potential for the Model of Freely Jointed Chains We expand y(z) and Vjl(z) in a power series of z: q(z) = IF=Vf)z2& and vjl(z) = c;= V~')Z~~,where Vy)= (3/8)z2, Vt)= (9/320)n4, Vg)= (153/56000)~~, Vt)= (3321/12544000)~~; Vt)= (1535643/60368000000)n'O, vp = (849O22533/3453O496O)n12, and so on; V?') = (3/8)n2, V(Zjl)= 0, VC,jl)= (9/2560)n6, Vv" = ( -81/114688)n8, V:') = (5769/22937600)n'O and so on (see Appendix); the first 30 values of Vf) are presented in Table 1. It is significant that Vg')= 0; this is because Ejl(p)= p2/ 6 + O(p6),i.e. the model of freely jointed polymer chains on a cubic lattice at light extension is closely allied to the Gauss- ian model.In general, except for the simple cubic lattice, E(p) = p2/6 + O(p4)and V(z) = (3n2/8)z2+ O(z4). Table 1 First 30 values of Vf)and B, 1 3.701 101 650 3.701 101 650 2 2.739 630 685 -0.961 470 965 3 2.626 652 618 -0.112 978 067 4 2.51 2 070 432 -0.114582 186 5 2.382 221 849 -0.129 848 584 6 2.272 557457 -0.109 664 392 7 2.200499 532 -0.072 057 925 8 2.162403637 -0.038 095 895 9 2.144 171 977 -0.018231 660 10 2.131 963 619 -0.012208358 11 2.1 18 021 735 -0.013941 884 12 2.101 168 777 -0.016 852957 13 2.084 057 663 -0.017111 114 14 2.069 843 538 -0.014214 125 15 2.060 092 646 -0.009750 892 16 2.054 408 293 -0.005 684 353 17 2.05 1 234 597 -0.003 173 696 18 2.048 948 605 -0.002 285 992 19 2.046 572 565 -0.002 376 040 20 2.043 897 907 -0.002 674 658 21 2.041 191 961 -0.002 705 947 22 2.038 801 424 -0.002 390 5 37 23 2.036 894 352 -0.001907 072 24 2.035 415 417 -0.001 478 935 25 2.034 189 192 -0.001 226 224 26 2.033 054 638 -0.001 134555 27 2.03 1 943 401 -0.001 11 1237 28 2.030 878 752 -0.001 064 649 29 2.029 924 035 -0.000954717 30 2.029 126 557 -0.000 797 477 1 I 20.00 15.00 ! 5.00--$ ,,,*' ,,,' Z Fig.1 Self-consistent pseudo-potential, q(z),for a polymer brush of FJCs in a good solvent (-); self-consistent pseudo-potential, VJz), for a polymer brush of FJCs on cubic lattice (---); parabolic self- consistent pseudo-potential, V,(z), for the Gaussian brush (-.-); pseudo-potential for Shim and Cates' model' (-- - - -) We analyse the continuum model of FJCs in more detail because it seems to be more realistic for the applications of this theory. c(z) is singular when z + 1. An analysis of this expansion shows that the main part of this singularity is: vj(z)Oc 2(1 -Z2)+ + 0[(1 -z2)-'] This allows us to estimate V,(z): 30 2z2(1 -Zz)-l + c A&z2& 30 < y(z) < (1 -Z2)-l 1Bkz2& &= 1 &= 1 (14) where A, = V, -2 and B, = V, -V,-'. The coefficients V, and B, are presented in Table 1. The expansion of eqn. (14) converges quite well.The accuracy of this estimation is <0.001% on z = 0.9 and 0.4% on z = 0.99. We use this expansion for the calculation of the V,(z) dependence, which is shown in Fig. 1. Dense Grafted Brush in a Good Solvent The usual model for the description of polymers in solutions is the Flory-Huggins lattice model.' ' For an athermal good solvent the chemical potential, pFH, for this model is : The density of polymer segments in the brush for this model is: The value of zmax is determined from the normalization con- dition : Zmax -lmXexp[q(z)-Y(z,,,)] dz = ou2 (1 7) The dependences of p(z) for different au2 and z,,,(m2) are presented in Fig. 2 and 3. One can see that if ou' < 0.1, the Gaussian approach is quite adequate, the density profile is parabolic; if m2> 0.5, the density profile is flattened; and if au2 > 0.8, the density profile is step-like and z,,, x au2.Eqn. (13) allows us to cal- 1 .oo 0.80 0.60 b.. h h(v P 0.40 0.20 Z Fig. 2 Density profiles for the polymer brush of FJCs in a good solvent (lattice model) for ou2 =0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9 culate the distribution of ends, g(z): 1 d5g(z) = --exp[vj(z) -vj(zmBz) exp[vj(z')] dz' (18)au2 dz where V(2) = V(z,) -V(z); the function, g(z), for different au2 is presented in Fig. 4. One can see that in the region au2 x 0.5 the behaviour of g(z) is changed. If auZ< 0.5, g(z) is a convex function and the maximum value of g(z) decreases with increasing aa2; if nuz >0.5, the maximum value of g(z) increases with increasing au2.Comparison with Other Models Skvortsov et a1.' used the SCF theory of Scheutjens and Fleer for a brush of FJCs on a three-dimensional cubic lattice. They calculated numerically the structural details of brushes of finite N and applied a special procedure for the extrapolation of these results to N +co.The data of Skvort- sov et a1.' for the dependence z,,(aa2) and our calculations 1.o 0.8 0.6 0.4 0.2 I0.0 I"~'"'~~I""""'I"""'~'l""''~''I''~''"''I 0.0 0.2 0.4 0.6 0.8 1 .o oa Fig. 3 Maximum height, z, vs. surface fraction of polymer ou': (-) polymer brush of FJCs in a good solvent; (..) polymer brush of FJCs on a cubic lattice; (--) Gaussian brush in a good solvent, (-.-) asymptotic z,, = [40u2/7r2-j1'3; (---) Shim and Cates model;12(A) data of Skvortsov et uf.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 30 -h-3 20-b: 10i, z Fig. 4 Distribution function of free ends: (-) g(z) for a polymer brush of FJCs in a good solvent (lattice model) for oa2 = 0.1,0.2, 0.3, 0.4,0.5,0.6,0.7,0.8,0.9; (---) g(z) for a Gaussian brush, au2 = 0.77 for the cubic lattice model are shown in Fig. 3. One can see a good agreement between these results and our theory. Note that if au2 c 0.5, the results for this model and the Gaussian model are closely allied. To compare our results with these of Shim and Cates we reduced their self-consistent potential' to the form : z(V) = 1 -exp -erfc371 Fig. 1 shows this potential.One can see that this potential is not an even function of z: 3n2 3n3VAZ) = -z2 i--z3 4-* * 8 16 The dependence of zm,(au2) for the SC model is presented in Fig. 3. The presence of the unphysical term z3 in the potential causes a strong divergence of the SC results from the results of other models. This compelled Shim and Cates12 to use dif- ferent phenomenological parameters in order to compare their results for brushes of chains with finite extensibility with the results of the more detailed self-consistent Scheutjens- Fleer model4 for different grafting densities. We thank July Lyatskaya, Ekaterina Zhulina, Sergey Frid- rikh and Alexander Skvortsov for very helpful discussions. This work was supported, in part, by a Soros Humanitarian Foundation Award by the American Physical Society.Appendix To solve eqn. (10) we introduce new variables: u = V(z'), u = V(z), U = V(zmax), dz =(dz/du) do. We have: To solve eqn. (Al) we can use the Laplace transformation: P(o)= rexp( -oz)F(z) dz After applying the Laplace transformation to eqn. (Al) and returning to the independent variable z in the first integral we J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 have : exp[ --uV(z)]dz J exp[ -oE(p)] dp = (A2) 0 0 As with the usual models of polymer chains with finite extensibility, E(p) = tion of z, V(z) = Lrn=Ekp2‘, then V(z)is an even func- V, z”. To calculate V, we expand both the integral in eqn. (A2) by the pass method in the power series on U-’/~-”,n 2 0 and multiply both series.After comparing the corresponding coef- ficients we can calculate V,. Since polymer chains must exhibit Gaussian behaviour under very small extensions, E, = 6, V, = in2.We have written a computer program for calculating these coefficients. To solve eqn. (12) we introduce new variables, u = V(z’), u = V(z), U = V(z,.,-), u = V(z’), dz = (dz/du) du. We have: q(u) dup(u) = IU-A(u-U) where 4(u)= g[x(u)] (dz/du). To solve eqn. (A3) we introduce w = (U-u), R(w) = p(U -u) and Q(u) = q(U -u): After applying the Laplace transformation to eqn. (A4) and using eqn. (A3) we have: dz d(w)[exp( -mu) -du du = Q(o) 045) After inverting the Laplace transformation, eqn. (A6) follows: dR(w -U) dz -duQ(w) = du du and by returning to initial variables we obtain eqn.(13). References 1 S. Alexander, J. Phys. (Paris), 1977,38,977. 2 P-G. DeGennes, Macromolecules, 1980,13, 1069. 3 T. M. Birshtein and E. B. Zhulina, Vysokomol. Soedin., Ser A, 1983,25,1862. 4 J. M. Scheutjens and G. J. Fleer, J. Phys. Chern., 1979,83,1619. 5 A. M. Skvortsov, A. A. Gorbunov, I. V. Pavlushkov, E. B. Zhulina, 0.V. Borisov and V. A. Pryamitsyn, Polym. Sci. USSR, 1988,30,1706. 6 A. N. Semenov, Sou. Phys. JETP, 1985,61,733. 7 V. A. Pryamitsyn, The Orientational Ordering of Polymer Systems, Candidate Thesis, Institute of Macromolecular Com- pounds, Leningrad, 1987. 8 E. B. Zhulina, V. A. Pryamitsyn and 0.V. Borisov, Polym. Sci. USSR, 1989,30,205. 9 S. T. Milner, T. A. Witten and M. E. Cates, Macromolecules, 1988,21,2610. 10 S. T. Milner, Science, 1991, 251,905, and references therein. 11 P-G. DeGennes, Scaling Concepts in Polymer Physics, Cornell University Press, Ithaca, NY, 1979. 12 D. F. K. Shim and M. E. Cates, J.Phys. (Paris), 1989,50,3535. 13 T. M. Birshtein and 0. V. Ptitsyn, Conformations of Macro-molecules, Wiley-Interscience, New York, 1966. Paper 3/04708C; Received 4th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000889

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 895-898 Effect of Potassium on the Surface Potential of Titania Dorninique Courcot, Leon Gengembre, Michel Guelton and Yolande Barbaux Laboratoire de Catalyse Heterogene et Homogene, URA C.N.R.S.402, Ba^t.C3, U.S.T.LiIIe. F-59655, Villeneuve d'Ascq, France Barbara Grzybowska" Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krako w 30239, Poland The surface potential, x, of several commercial titania powders has been measured by the vibrating condenser method in the temperature range 100-450"C in air. The K impurity detected by XPS on some of titanias lowers the x value of TiO, considerably. A series of K-doped TiO, anatase samples containing from 1.1 to 11 K atoms nm-* was prepared and examined by surface potential and XPS techniques.The surface potential of anatase decreases linearly with increasing potassium content up to ca. 2.4 K atoms nm-2, and then remains constant. The surface potential of K-doped samples varies to a smaller extent with temperature compared with undoped anatase. On the basis of the surface potential and XPS data obtained, it is suggested that the mode of K deposition on the surface of anatase changes at a coverage of about 2 K atoms nm-2. Using potassium as an additive to TiO, ,which is essential in the production of white titania pigments, can have an unfa- vourable effect on catalytic performance when commercial titanias, containing potassium as an impurity, are used as supports for vanadia catalysts for oxidation of hydrocar-bon~.'-~On the other hand, potassium is often added as pro- moter to industrial metal or oxide catalysts dispersed on support^.^*^ Characterization of surface properties of TiO, with added potassium is of great interest in catalytic studies.Recent work on this subject has shown considerable modifi- cation of the acidic properties of K+-doped anata~e~9~ and of the electronic properties when K atoms are adsorbed on a monocrystalline TiO,( 100)(r~tile).~ The effect of K addition on the reactivity of anatase in chlorination reactions has been also rep~rted.~ In the present work, surface potential and XPS measure-ments have been applied to characterize various commercial TiO, supports and to study the effect of K addition on the surface properties of anatase powders.Experimental Several commercial TiO, preparations were used in the pre- liminary experiments. Their characteristics are given in Table 1. TiO, Tioxide (Tiox-1), pure anatase (within the accuracy of the XRD and Raman methods) and without surface impu- rities, as verified by XPS, was selected for further studies on the effect of potassium. Before the introduction of potassium onto its surface, the sample was calcined for 4 h at 700°C. Table 1 Characteristics of some commercial titanias specific surface rutile content" producer area/m2 g-' (%) P-25 ET-1 (Eurotitania) Tiox.-1 Degussa Tioxide Tioxide 53.2 47 27.4 39.6 10.4 0 AN-PO AN-PI Chemical Works 8.4 6.9 4.7 trace RT-P Police, Poland 7.4 96 Ti-Pro1 Prolabo 8.7 4.7 Calculated from XRD data using the formula:26 rutile = [I1 -1/ (1 + 1.261,/IA)] x 100%, where I, and I, are the intensities of the diffraction peaks at d = 3.53 A and 3.26 A in diffractograms of anatase and rutile, respectively.The specific surface area decreased slightly to 23.8 m2 g-'; however, no rutile was observed after the calcination. Different amounts of K, varying from 1.1 to 11 K atoms nm-(which corresponds, respectively, to surface coverage of 1 K atom 100 A-2 to 1 K atom 10 A-2) were introduced by adding appropriate quantities of KHCO, (Fluka purissima) solution to a suspension of the support in water, followed by evaporation under vacuum at 50 "C,drying at 95 "C and cal- cination for 4 h at 500°C in a stream of air.The specific area of the K-doped samples did not vary within 10%from that of pure Tiox-1 after additional calcination. The K-promoted samples are denoted in the text by symbol K-X, where X indicates the calculated number of K atoms nrn-,. XP spectra of the samples were recorded with an AEI ES 200B spectrometer. The atomic ratio of the elements on the surface, n,/nB, was calculated from the intensity ratio IJIB with the formula: values of CT being taken after Scofield." The surface potential was measured by the vibrating con- denser method.' '-14 The method involves recording the Volta potential difference AV between two plates of a con- denser made of the sample and the reference electrode.AV is equal to the difference in the work functions of the solids which form the two plates of the condenser. As the work function of the reference electrode is constant under the experimental conditions adopted, the variations in A V reflect the changes in the work function of the sample. In the cases in which the structure of the bulk of the solids is not changed by chemisorption of gases or surface doping, the work func- tion is a measure of the surface potential, defined as the dif- ference between the electrostatic potential at the surface immediately outside a solid and the internal electric potential. The surface potential represents a potential barrier which has to be surmounted by an electron distribution in the surface layer and depends on the dipole structure of the solid surface.The apparatus used in the present work consisted of a cell made of stainless steel containing two electrodes mounted vertically; the cell was connected to a controlled gas-flow system. The sample electrode was constructed of a stainless- steel plate 20 mm x 30 mm and 1.5 mm thick, covered on one side with gold foil, on which a sample of the powder under study was deposited from a suspension in amyl alcohol. The heating Thermocoax element and thermocouple for recording the sample temperature were placed inside the steel plate. The vibrating reference electrode facing the sample electrode at a distance of 1 mm was made of a graph- ite plate, 3 mm thick. The choice of the reference electrode was a subject of a separate paper.The work function of the reference electrode was constant under measurement condi- tions. The measurements were performed under a flow of 20 vol% 0, in Ar in the temperature range 50-450°C. The values of surface potential, x, reported in the text are relative to the graphite electrode, an increase in x indicating that the surface becomes more negatively charged. The surface poten- tials were reproducible for a given sample within 5 mV. Results and Discussion Commercial Titanias Table 2 gives the surface potential data for commercial TiO, powders and compares them with the impurity contents on the surface as determined by XPS. The x values at 450°C, ~450,are much lower (by ca.1 V) for those titanias which contain impurities than for the preparations for which no surface contaminants were detected. At the same time, x varies only slightly with temperature for the contaminated samples, as indicated by the difference Ax450-200 (column 3 in Table 2). Washing one of the samples (AN P1) with water for 12 h, which removed most of the potassium from the surface, brought about an increase in x and Ax. This suggests that the differences in the x values observed between different titanias are due in the first place to the K impurity. Addition of small amounts of P fin the form of H(NH4),P04] to the sample of Tiox-1, free of contaminants, did not lead to any changes in the values of x450. Kdoped Anatase Tiox-1 XPS Measurements Table 3 lists binding energies (fib) of different elements in the series of K-doped TiO,-Tioxide samples, the C 1s signal with E, = 285 eV being taken as the reference.All the samples exhibited a small peak at Eb = 289 k0.5 eV, most probably due to carbonates; its intensity, however, did not vary signifi- cantly with increasing K content and was similar to that of adventitious carbon observed on pure Ti0,-Tiox-1. The binding energies of the Ti 2p and 0 1s levels are slightly lower for K-containing samples than for undoped TiO, , indicating some electron transfer from potassium to the support, which leads to an increase in the anionic charac- ter of oxygen and a decrease in the positive charge on tita- nium atoms. A similar effect was observed in ref.9. The binding energy of K 2p decreases when the amount of pot- Table 2 Surface potential values and surface impurities for com- mercial titanias impurity (atoms per 100 Ti atoms) 1450 Ax450-200 /mV /mV K P Ca P-25 1840 400 ET-1 (Eurotitania) 1740 440 ---Tiox.-1 1990 540 -AN-P1 310 130 9 3 -AN-P 1 washed 1100 250 2 3 AN-PO 210 70 9.2 6 1.1 RT-P 40 0 9.2 4.9 0.5 -Ti-Pro1 360 60 9 7 J. CHEM. SOC. FARADAY 'TRANS., 1994, VOL. 90 Table 3 Binding energies in Kdoped anatase Tiox-1 EdeV K content" /atoms nm-2 Ti 2p K 2p 0 1s ~ Tiox.-1 0 459.1 - 530.4 K-1 1.1 459.2 293.2 530.2 K-2 2.1 458.5 292.8 529.9 K-2 washedb 459.3 293.3 530.4 K-4 4.0 458.7 293.1 530 K-11 11.5 458.8 292.6 529.8 ~ ~ " Introduced onto the Tiox-1 surface. * Sample K-2 after washing with doubly distilled water at 25 "C.assium on the surface increases, suggesting some loss of its cationic character at higher potassium coverage. The variation in the surface atomic ratio K: Ti, derived from the XPS intensity data, with the total amount of pot- assium introduced is presented in Fig. 1. A break in the curve observed at a K content of ca. 2 K atom nm-2 suggests that the mode of potassium dispersion may change in this region. A similar course of changes in surface composition was observed when oxides (e.g. V,05, MOO,) were deposited on supports and has been ascribed to the change from mono- layer two-dimensional VO, or MOO, species (at low contents of the deposited oxide) to three-dimensional microcrystals of V205 or MOO^.'^-'^ It cannot be excluded that at higher K content the atomic dispersion of this additive changes to the dispersion in form of the K,O phase.Note that Busca and Ramis' ascribed the complete modification of the titania surface, observed in an FTIR study for the sample containing one K+ per 0.11 nmz to the formation of a 'surface K20 phase '. Surface Potential Studies Fig. 2 shows the change in the surface potential in air with temperature for the K-doped anatase Tioxide samples. The surface potentials are lower for K-containing samples than for pure TiO, over the whole temperature range studied. For pure TiO, the x values increase significantly with tem-perature, indicating an increase in the negative charge on the surface.Such an increase has previously been ascribed to transformation of the chemisorbed oxygen species from less to more negatively charged forms, e.g. 0, + e--+0; or ~0-+ e-02-,.19,20 it could be also due to an increase in the total amount of chemisorbed oxygen with temperature. 2 K atoms nniv2 Fig. 1 Variation of the surface atomic ratio nK : nTi with total pot- assium content for K-doped Tiox-1 anatase J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1750 *---*-*--*---+----+ /*' Fig. 2 Change of the surface potential with temperature for a series of K-doped Tiox-1 anatase samples: A, Tiox-1; 0,K-2 washed; a, K-1; +, K-2; 0,K-4; A, K-11 The changes of x with temperature are smaller for K-containing samples; for the sample K-11, which has the highest potassium content, a small decrease in x at higher temperatures is observed.It appears that the presence of potassium also affects the chemisorption of oxygen on the anatase surface: it could block the oxygen adsorption centres or hinder the transform- ation of the species shown above. Separate studies on the type of oxygen species and on oxygen chemisorptive proper- ties are in progress to decide between these possibilities. The lowering of the surface potential on addition of K can be explained by the change of sign of the dipole in the topmost layer of Ti02, potassium ions K+ covering the negatively charged surface oxygen anions.Injection of elec- trons from potassium to titania, changing the density of current carriers in the space-charge region of TiO, and hence the value of potential barrier (considered as the cause of work function changes when K atoms are adsorbed on semi-conductors8) seems less probable in our case since potassium was introduced in a cationic form. Electron transfer cannot be completely excluded in view of the changes in binding energies for Ti4+ and 02-ions described above. In Fig. 3, the values of x at 450°C are plotted as a function of the K content. At low doping levels of potassium, the potential decreases linearly with increasing K coverage up to ca. 2.5 K atoms nm-2 and changes only slightly with further increase in the K content.This corresponds to about 18% of the total surface occupied by terminal oxygen atoms on the anatase surface, ca. 14.2 atoms nm-2 for the 001 cleavage plane or 16.1 atoms nmV2 for the 110 plane. Note that a break in the nK :nTi us. K content plot (Fig. 1) is observed at similar value of potassium coverage. It appears that potassium ions are adsorbed on certain particular centres on the anatase surface which contribute considerably to the surface potential. Once these centres are filled, the mode of dispersion of K changes. Identification and localization of these centres remains a matter of speculation. They can be ascribed for instance to 897 1600 1200 >E 800 \::x" 400 0 -400 0246810' K atoms nm-2 Fig.3 Dependence of surface potential on total K-content for K-doped anatase Tiox-1 (T = 45OoC, stream of 20 vol0/0 0,-Ar, 10 dm3 h-') unsaturated 0 atoms on the 001 plane, coordinated to two Ti atoms, carrying a charge of -& and located 0.041 nm above this plane.21 Localization of potassium atoms on the OH groups of anatase, replacing an H atom, can be also envis- aged; some of the many hydroxy groups of Ti02 remain on the surface even after it is heated at temperatures >250 0C.22-24 It is interesting to note that a similar value was found for the number of sites on an anatase surface, which is capable of binding a vanadia phase strongly in V,O,/TiO, catalysts. Strongly bound VO, species (2.6 VO, nm-2) exhibits only dehydrogenating properties in isopropyl alcohol decomposi- tion and is the only VO, species that remains after washing the soluble V,O, with aqueous NH, .References 1 A. V. van Hengstum, J. G. van Ommen, H. Bosch and P. J. Gellings,Appl. Catal., 1983,8, 369. 2 A. V. van Hengstum, J. Pranger, J. G. van Ommen and P. J. Gellings,Appl. Catal., 1984, 11, 317. 3 S. L. T. Anderson, J. Chem. SOC., Faraday Trans. 1, 1986, 82, 1537. 4 W-D. Mross, Catal. Rev.-Sci. Eng. 1983,2!5,591. 5 Thornton, Vacuum,1992,43,1133. 6 C. Morterra, A. Chiorino and G. Ghiotti, J. Chem. SOC., Faraday Trans. I, 1982,78,2649. 7 G. Busca and G. Ramis, Appl. Sur$ Sci., 1986,27,114. 8 R. Casanova, K. Prabhakaran and G. Thornton, J. Phys. Condens. Matter, 1991,3, S91. 9 G.Mink, I. Bertoti, I. S. Pap, M. Mohai, T. Szekely, T. M. Duc and E. Karmazsin, Reactivity of Solids, 1987,4, 251. 10 J. H. Scofield,J. Electron Spectrosc. Relat. Phenom., 1976,8, 129. 11 J. C. Riviere, Solid State Surface Science, ed. M. Green, ZRRC London, 1969, pp. 179-289. 12 J. Nowotny and M. Destriau, Bull. SOC. Chim. Fr., 1976, 1-2,91. 13 Y. Barbaux, Doctoral Thesis, University of Lille, 1978. 14 I. D. Bailcie and E. Venderbosch, Rev. Sci. Instrum., 1991, 62, 725. 15 Y. Barbaux, J. P. Bonnelle and J. P. Beaufils, J. Chim. Phys., 1976,73,25. 16 G. C. Bond, J. P. Zurita and S. Flamerz, Appl. Catal., 1986, 27, 353. 17 J. Mendiadua, Y. Barbaux, L. Gengembre, J. P. Bonnelle, B. Grzybowska and M. Gasior, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 1987,35, 513. 18 Y. Barbaux, A. R. Elamrani, E. Payen, L. Gengembre, J. P. Bonnelle and B. Grzybowska,Appl. Catal., 1988,44, 117. 898 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 19 20 21 22 B. Grzybowska, Y. Barbaux and J. P. Bonnelle, J. Chem. Res., 1981, (S) 48, (M)0650. Y. Barbaux, J. P. Bonnelle and J. P. Beaufils, J. Chem. Res., 1979, (S),48, (M)0556. M. A. Enriquez, C. Domerieux-Morin and J. Fraissard, J. Solid State Chem., 1981,40,233. G.D. Parfitt, Prog. SurJ Membrane Sci., 1976, 11, 181, and refer- ences therein. 23 24 25 26 G. Busca, H. Saussey, 0.Saur, J. C. Lavalley and V. Lorenzelli Appl. Catal., 1985, 14,245. K. I. Khadzhiivanov, A. A. Davydov and D. G. Klissurski, Kinet. Katal., 1988,29, 161. B. Grzybowska, to be submitted. R. A. Spurr and M. Myers, Anal. Chem., 1957,29,760. Paper 3/06045D;Received 1 lth October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000895

出版商:RSC    

年代:1994

数据来源: RSC