J. Phys. Chem. 1981, 85,1037-1042
(260).% The decrease of the transition cooperativity of the multilamellar liposomes in the present work may be attributed to the existence of a trace of DSOCC in DPPC. With regard to the single-lamellar liposomes, if we tentatively use a value of 6.4 kcal/mol for AHcd,the cooperative unit of the largest-size liposomes in the present work is estimated as 44. This value, however, may be an overestimation, because 6.4kcal/mol is an average value for AHd determined in the unfractionated liposomes, and AHcalhas been suggested to increase with increasing liposome size.26 Thus, with regard to the value of 90 obtained in the multilamellar liposomes, we may conclude that the cooperative unit of the single-lamellar liposomes does not exceed a half of that of the corresponding multilamellar liposomes, even if they have a molecular weight as large as 3 X 10’. This decrease of the cooperativity in the single-lamellar liposomes cannot be interpreted in terms of the size effect, such as the packing constraint due to the strong curvature associated with the single-lamellar
1037
liposomes, because the effect was excluded by extraporating the molecular weight to infinity. We suppose that the decrease of the cooperativity in the single-lamellar liposomes is due to the lack of the interactions among the bilayers in each liposomes; conversely, the interactions among many concentric bilayers in each multilamellar liposome are responsible for their high cooperativity. Finally, we just mention that the rate constant of the phase transition of the single-lamellarliposomes has been found to depend on their sizes dramatically, by means of the fluorescence temperature-jump method. The experimental results on the kinetic behavior of the phase transition in the single-lamellar liposome systems and a kinetic model based on a two-dimensional Ising lattice shall be presented in succeeding papers.
Acknowledgment. This work was partially supported by Scientific Research Grants (421321 and 447129) from the Ministry of Education of Japan.
Adsorption on Electrodes and Micellization of Some Alkyl Sulfates Ghoiem Naficy, Department of Chemistty, Universlv of Teheran, Teheran, Iran
Pierre Vanel, Daniel Schuhmann, * Reni Bennes, and Emmanuel Tronel-Peyror Groupe de Recherche de Physicochimie des Interfaces, C.N.R.S., E.P. 5051, 34033 Montpelller Caex, France (Received: April 14, 1980)
The adsorption of alkyl sulfates from aqueous solution at a mercury electrode at concentrationsbelow the cmc has been studied. A comparison with the adsorption of neutral substances and mineral ions shows the probable existence of a coadsorption of the counterions. A very sudden desorption is observed for a charge of the electrode which depends on the surfactant but which in all cases is identical with the superficial charge of the micelle. The results obtained are discussed against the background of the known properties of micelles and vice versa, leading to a description of the probable contribution of the hydrophobic and electrostatic interactions to the formation of micelles.
Introduction The studies on aqueous solutions of sodium dodecyl sulfate (SDS) found in the literature deal essentially14 with the aspect of the formation of micelles which can be described as being due to the competition between the hydrophobic bond between the alkyl chains and the repulsion between the polar groups of the surfactant. The major remaining problem3area concerns the electrostatic interactions, but at present we are far from being able to give a well-founded description of counterion binding phenomena. It is a natural starting point4 to analyze counterion binding in terms of the general electric double-layer theory of charged interfaces, which has been adapted to the particular case of micelles by Stigter.5-9 (1) G. C. Kresheck, Water: Compr. Treatrise, 4, 95 (1975). (2) 51. Shinoda, T. Nakagawa, B. I. Tamamushi, and T. Isemura in
The superficial part of the micelle is constituted of the polar heads and the interstitial counterions and is termed the Stern layer by analogy with the inner compact layer associated with the diffuse layer in the classical doublelayer model of electrodes.1° However, the structure of the water in this region in micelles is less icelike than at an electrode, the counterions remaining hydrated,11J2which means that they do not form covalent linkages with the polar heads. It would also seem that the discrete charge effects are more important for micelles than in the case of a metal like mercury, while imaging effects must also be different for these two systems. In our group, we have studied the adsorption of different types of surfactants at a mercury electrode,13and a comparison of the results obtained with those for other in-
“Colloid Surfactants: Some Physicochemical Properties”, Academic Press, New York, 1963. (3) H. Wennerstrom and B. Lindman, Phys. Rep., 52, 1 (1979). (4) B. Ljndman, G. Lindblom, H. Wennerstrom, and G. Gustavsson, Micellizatzon, Solubilization,Microemulsions, [Proc. Int. Symp.],1976,
(8) D. Stigter, J . Phys. Chem., 79, 1008 (1975). (9) D. Stigter, J. Phys. Chem., 79, 1015 (1975). (10) 0. Stern, 2. Elecktrochem, 30, 508 (1924). 97, 3923 (11) H. Gustavsson and B. Lindman, J. Am. Chem. SOC., (1975). (12) M. J. Rosen, J. Colloid Interface Sci., 56, 320 (1976). (13) D. Schuhmann, R. Bennes, M. Privat, E. Tronel-Peyroz, and P.
195 (1977). (5) D. Stigter, J . Phys. Chem., 68, 3603 (1964). (6) D. Stigter, J. Colloid Interface Sci., 47, 473 (1974). (7) D. Stigter, J . Phys. Chem., 78, 2480 (1974).
Vanel, “Adsorption de tensioactifs I l’6lectrode de mercure. L’Blectrode comme modgle d’interfaces”. Communication to “Journ6es d’Etude de la Socidt6 de Chimie Physique: L’Apport de l’6iectrochimie 5. la physicochimie des tensioactifs”, Thiais, Nov 1979.
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terfacial systems has shed new light on the general competition between the electrostatic and hydrophobic forces during adsorption independently of the nature of the substratum. In particular, results bearing on the adsorption of SDS seem to indicate that there exists a correlation between these and the phenomena associated with micelle formation. This was shown by the curves for the variation of the relative surface excess with the charge of the electrode deduced from the electrocapillary curves, a procedure which is less tedious and uncertain than the measurements of the interfacial capacity.14 For binary aqueous solutions of SDS,15the results reveal a complete desorption of the anion when the charge of the electrode is more negative than -8 pC cm-2, a value identical with the superficial charge of an SDS micelle calculated from the data found in the literature, i.e., the number of aggregates n, the degree of ionization a,and the length of the alkyl chain L. The study of the simultaneous adsorption16of SDS and 2-methyl-2-propanol gives weight to the idea that the competition between the hydrophobic attractive and repulsive electrostatic forces plays an important part in the adsorbability of anions. I t would seem further that the alcohol acts on the adsorption of the lauryl sulfate ion essentially as a result of its influence on the structure of the solution. In the case of cationic1’ and nonionic surfactants,ls the adsorption characteristics are different from those of anionic surfactants because of the electrostatic interactions. However, the properties common to the three types of surfactants can be explained, we believe, if one notes that, in the absence of specific interactions between the surfactant and the substratum, their behavior at an interface largely depends on their properties in solution. This analogy results from the identical nature of the forces at the interface and in solution between solute and solute and between solute and solvent. This is on the basis of the theoretical approach due to Stigter forg for ionic micelles, and one of us (Benneslg) has shown that, in the case of the adsorption of neutral substances on an electrode, an important part of the hydrophobic interactions can be linked to their behavior in solution. For a CH2 group in a surface-active monomer, the free energies of transfer from an aqueous phase to a micelle or an adsorbed layer are very close to one another,12and the electric field as well as the geometrical constraint due to the existence of a surface would not appear to be particularly specific. Hence it is normal to expect, even for differing systems, interfacial structures showing striking analogies. According to Stigters “the Stern layer of a micelle resembles a concentrated electrolyte solution”. More specifically this means that the double layer is analogous to that of such a solution in a very strong electrical field.20 These resemblances support the idea that a study of the double layer in the presence of Surfactants can constitute a useful method of defining the part played by the forces which come into play, as has been done in the case of biological surfactants in an endeavor to throw some light (14) B. I. Tamamushi, ref 2, p 179. (15) J. P. Trujillo and R. Bennes, C. R. Hebd. Seances Acad. Sci., Ser. C, 288, 355 (1979). (16) G. H. Naficy, D. Schuhmann, P. Vanel, and E. Verdier, C. R. Hebd. Seances Acad. Sci., Ser. C , 289, 169 (1979). (17) E. Verdier, G. H. Naficy, and P. Vanel, J. Chim.Phys. Phys.Chim.Biol., 70, 160 (1973). (18) E. Bou Karam, R. Bennes, and J. Piro, J . Chim.Phys. Phys.Chim. Biol., 72, 9 (1975). (19) R. Bennes, J. Electroanal. Chem., 105, 85 (1979). (20) L. Neel, Ed., Proc. Int. Meet. SOC.Chim.Phys., 31st, 1978 (1979); see particularly p 251.
Naficy et al.
on their interfacial properties.21 It is only with electrodes, and more generally at interfaces between two conductors, of which one must be electronic, that one can vary at will the electrical variable for a constant composition in solution. Therefore, through the intermediary of electrodes one can verify the validity of the theories which concern the influence of the interfacial electrical field. Thus, according to Healy and “the classical electrochemistry of the electrical double layer appropriate to materials that operate as electrodes is shown to be a limiting description for nonelectrode materials such as clays, inorganics oxides, insolubles salts, latex, colloids, and biosurfaces”. The degree of analogy and the possibility of using an electrode as an electrochemical model for the specific behavior of another system must necessarily be carefully weighed for each specific case, while at the same time one endeavors to analyze the process considered. When one compares micelles and electrodes from the point of view of the competition between the electrostatic and hydrophobic interactions, the resemblances seem more marked than the differences, and the present paper describes a study of the adsorption on a mercury electrode of sodium alkyl sulfates undertaken with this idea. The latter may appear as evident, especially considering the results given below. However the influence of the hydrophobic interactions on the double-layer behavior has not, thus far, been analyzed from this point of view, while available studies on the adsorption of surfactants on electrodes are scarce and not particularly elaborate. We felt, therefore, that it would be useful to present an analysis based on this analogy. If the resulting inferences are indirect and can possibly be considered as speculative, they are felt to constitute nevertheless a reasonable approach to the problem.
Experimental Section The study of the aqueous solutions of decyl and octyl sulfates of sodium were undertaken by measuring the variation of the interfacial tension with the potential, by the drop time method and a specialized apparatus previously describedSz3The anionic relative surface excess, r, and the charge of the electrode, uM, were obtained directly from the electrocapillary curves, r ( E ) ,where E is the potential of the electrode with respect to a reference electrode. One uses respectively the Gibbs formula adapted to the electrodes2*and that of Lippmann: where p is the chemical potential of the salt in solution and E+ the potential with respect to a reference electrode reversible to the cations. The experiments were carried out by using a calomel 0.1 M KC1 electrode, connected to the M in sodium alkyl cell by a bridge containing a 5 X sulfate solution. The experimental values were corrected, by using the Grahame t e c h n i q ~ eto , ~obtain ~ a value corresponding, save for a constant, to E+. The salts studied were Eastman products which were washed with ether dissolved in hot ethanol and recrystallized before use. The results concerning the SDS were (21) R. T. Runbeck, D. M. Mohilner, and T. N. Solie, Electrochim. Acta, 20, 691 (1975). (22) T. H. Healy and L. R. White, Adu. Colloid Interface Sci., 9, 303 (1978). (23) E. Verdier and P. Vanel, J. Chim.Phys. Phys.-Chirn. Biol., 67, 1412 (1970). (24) D. M. Mohilner in “Electroanalytical Chemistry”, J. Bard, Ed., Marcel Dekker, New York, 1966. (25) D. C. Grahame and B. A. Soderberg, J. Chem. Phys., 22, 449 (1954). ~
Adsorption on Electrodes and Micellization
The Journal of Physical Chemistry, Vol. 85, No. 8, 1981 1039
1
2
3
- $9
c (mole/ I)
Figure 1. Variation of the interfacial tension with the logarithm of the sodium dodecyl sulfate concentration, for a few values of E+.
Y(mN/m)
420
400
38(
Flgure 2. Variation of the interfacial tension with the logarithm of the sodium decyl sulfate concentration, for a few values of E+.
those which had been obtained in a previous study.15
Results Figures 1-3 for the dodecyl, decyl, and octyl sulfates, respectively, represent, for a few values of E+,the variation of the interfacial tension with the logarithm of the salt concentration, which for the first two salts attains values
- log c(moIe/ I)
Flgure 3. Variation of the interfacial tension with the logarithm of the sodium octyl sulfate concentration, for a few values of E+.
greater than the cmc. One observes that the accepted values for this critical concentration, viz., 8.3 X and 3.3 X lo-' M, respectively, correspond to the inflection points in the first two figures. Surface excesses have been calculated for concentrations below the cmc by using the Gibbs formula. In the case of the octyl sulfate, the cmc has been determined by various techniques, including surface-tension measurements,26 with good agreement between the different results. The determinations of the interfacial tension in the present work have hence been limited to concentrations below the cmc. The breaks in the curves in Figure 2 are more marked for certain values of the potential than for others. As previously reported,21electrocapillary measurements can hence provide a very accurate and quite convenient method of evaluating the cmc. In view of relation 1,the shape of the curves would seem to indicate that, for a constant composition, the adsorption is maximal for potentials of -0.8, -0.6, and -0.4 V, respectively, for the dodecyl, decyl, and octyl sulfates. As the curves tend toward a horizontal value in all regions of concentrations for negative potentials, this indicates a desorption in these regions. This is even more clearly borne out by the curves for the relative surface excess. The corresponding values were calculated by using relation 1 and replacing activities by concentrations. A new evaluation based on the activity values given in ref 27 for SDS shows that the differences between the two sets of results were not significant. The relative surface excess is plotted against the electrode charge in Figures 4-6 and the potential in Figures 7-9. (26) K. Tajima, Nippon Kugaku Kuishi, 5, 883 (1973). (27) L.Shedlovsky, C. W. Jacob, and P.M. Epstein, J.Phys. Chem., 67, 2075 (1963).
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The Journal of Physical Chemistry, Vol. 85, No. 8, 1981
Naficy et
I
10
-1
0
o (pc/cm2)
81.
E Wok)
/ -io
10
20
Figure 4. Variation of the superficial char e of dodecyl sulfate anions with the electrode charge for (1)2 X 10- , (2)5 X (4) (3) (5)3 X and (6) 5 X M solutions; with respect to 2X the value of the cmc, these concentrations are respectively equal to 0.024,0.06, 0.12,0.24,0.36, and 0.60.
(pC/cm')
B
I
10
-10
Figure 7. Varlation of the superficial charge of dodecyl sulfate anions wlth the potential for (1) (2)2 X and (3) 5 X M solutions. -1
0
1 Figure 5. Variation of the superficial charge of decyl sulfate anions and with the electrode charge for (1)3 X lo-', (2) (3) 3 X (4) M solutions; with respect to the value of the cmc, these concentrations are respectively equal to 0.009, 0.03, 0.09, and 0.30. 10
I
E (Volt)
-rFr- (pC/cm')
Figure 8. Variation of the superficlai charge of decyl sulfate anions with the potential for (1) 3 X and (3) lo-' M (2)3 X solutions. -1
0
E (Volt)
-10
-zFT-(pC/cm')
Flgure 6, Variation of the superficial charge of octyl sulfate anions with the electrode charge for (1) 3 X loT4,(2) (4) (3)3 X and (5)3 X M solutions; with respect to the value of the cmc, these concentrations are respectively equal to 0.0023,0.008, 0.023, 0.088,and 0.23.
Errors could be introduced by applying relation 1 for surface-excess determination if associations are significant in solution below the cmc. It seems generally accepted that aggregates of small dimensions do exist in solution,%33but (28) P. Mukerjee, K. J. Mysels, and C. I. Dulin, J . Phys. Chem., 62, 1390 (1958). (29) P. Murkerjee, J. Phys. Chem., 62, 1397 (1958). (30) F. Franks and H. T. Smith, J . Phys. Chem., 68, 3581 (1964). (31) B. D. Flockhart, J . Colloid Sci., 17, 305 (1962).
Figure 9. Variation of the superficial charge of octyl sulfate anions (2)3 X lo3, (3)lo-', and (4)3 X with the potential for (1) 3 X lo-' M solutions.
there is considerable disagreement as to their size and stability constants. A kinetic model based on experimental results for relaxation timesMindicates that small aggregates could indeed be present as intermediates between mono(32) N. Kamenka, B. Lindman, and B. Brun, Colloid Polym. Sci., 262, 144 (1974). (33) R. Zana and E. Yeager, J. Chirn. Phys. Phys.-Chim. Biol., 66,252 (1969). (34) E. A. G. Aniansson, S. H. Wall, M. Almgrem, H. Hoffmann, L.
Kielmann, W. Ulbricht, R. Zana, J. Long, and C. Tondre, J. Phys. Chem., 80, 905 (1976).
Adsorption on Electrodes and Micellization
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mers or oligomers and micelles but at very small constant potentials for the SDS and decyl and octyl sulfates are respectively -1.5, -1.47, and -1.37 V; i.e., the sequence is concentration. It is hence reasonable to neglect their the inverse of that of the charge of desorption. Although presence in solution insofar as surfactant adsorption is there is no sudden desorption in the case of alcohols, concerned. The choice of the potential or the charge in the study of isotherms is open to disc~ssion.~"~'In our nevertheless one also observes a similar inversion judging from some of our previous results.ls case, where we are comparing results with those obtained for micelles, the charge appears to be the better variable. Again, as was done in the case of SDS,15the superficial For SDS, one observes (Figure 4) for each concentration charge of the micelle has been calculated, by using a maximum adsorption15which occurs at somewhat difavailable data, for the octyl and decyl sulfates. Taking for fering values of the charge of the electrode, while Figure the a g g r e g a t i ~ nnumber, ~~ respectively, 41 and 27, for the 7 shows that the variation of the maximum position with degree of association a,0.7 and 0.62, and for the radii of concentration is less when the potential is chosen as the the micelles,4O 12.9 and 10.3 A, one obtains -9.5 and -12.5 electrical variable. The maximum occurs when E+ is near pC/cm2, which exactly correspond to the values of the -0.8 V. These types of curves are characteristic of the charge of the electrode at desorption. Hence, the presence behavior of an electrically neutral ~ u b s t a n c eas , ~has ~ ~ ~ ~ of an additional CH2grouping in the alkyl sulfate molecule been observed in the case of L-a-dipalmitoylinduces a variation of some -2 pC/cm2 in the value of the phosphatidylcholine in 97 % methanol-water solutions superficial charge and of the characteristic desorption with 0.01 M NH4N03.21For cationic surfactants such as charge. The direction of variation is explained by the tetraalkylammoniumcations, one often observes a different previous remark which discards any interpretation based phenomenon: a limiting value with no minimum for on the intervening of small charged aggregates at the incathodic charge^.^^^^^ One could expect a symmetrical terfaces. behavior for anionic surfactants, and this is what is obDiscussion served for the decyl sulfate (Figures 5 and 8). With octyl This study of the variation of the adsorption of alkyl sulfate it was not possible to reach sufficiently anodic sulfates with the length of the aliphatic carbon chain shows charges to attain the maximum. that there exists a striking correlation between the charge These results seem to show that the longer the hydroof the electrode at which desorption takes place and the carbon chain the more closely does the behavior of anionic corresponding superficial charge of the micelles. surfactants, at anodic charges, approach that of uncharged Other studies on the adsorption of amphiphatic subsubstances. stances on mercury, as related to the length of carbon The most striking result in ref 15 is the existence for aliphatic chain and the nature of the polar head, have SDS of a desorption charge at -8 pC/cm2. If one represhown that the corresponding variations of the apparent sents the variation of the relative surface excess with the energy of adsorption reflect, for an important part, the potential, the curves obtained tend to converge on the x hydrophobic interactions within the solution. Thus these axis at -1.5 V, a result already suggested by the results in apparent energies of adsorption as well as the cmc can be Figure 1. Similarly, for the other two substances, one also deduced from the HLB (hydrophilic-lipophilic balance). finds characteristic charges, -9.5 pC/cm2 for the decyl and Such analogies between several quantities characteristic -12 pC/cm2 for the octyl sulfate. For the more dilute of the adsorption and the micellization strongly lend solutions the behavior is simply that of normally charged support to the notion whereby the inclusion of a monomer substances. in a micelle is determined by the same forces as those This is something quite special, as such a phenomenon which govern the adsorption of the same substance. The is not observed in the case of quaternary ammonium salt initial state in solution is the same and the final states are cations or of neutral substances like aliphatic alcohols characterized by similar compositions. The differences where the desorption is less sudden, nor is it observed in reside in the curvature of the micelle and the planar nature the case of very adsorbable inorganic anions like I-. In the of the adsorbed layer at the electrode and on the absence case of solutions of tetrabutylammonium halides, it has of substratum in the first case and the possibility of inbeen shown that the formation of superficial associations teraction between the substratum and the water in the with counterions occurs especially in the charge range second. The fact that the characteristic parameters are where desorption of the cations is expected. The associidentical for both systems means that the quantities ation increases when the anion is changed according to the studied are independent of these specific properties. Our sequence C1-, Br-, I-?9 This result suggests that, with such studies on the adsorption of surface-active substances, in solutions, the anions and the cations have the same effect particular, indicate that the determining forces are the on the structure of water. As opposed to this they have hydrophobic-hydrophilicinteractions on the one hand and an opposite effect in the case of anionic surfactants, and the electrostatic forces on the other. The importance of it seems that ion pairs form only under favorable electhese forces in the formation of micelles is also known. trostatic conditions and consequently do not perturb the Hence it is interesting to analyze the results obtained for phenomenon under considerations of negatives charges. one system in the light of the information derived from It is surprising to observe that the greater the surfacethe other. active properties of the organic anion the smaller is the The behavior of the three anions in the region of positive charge of the electrode necessary to achieve desorption. charges suggests that the more they are hydrophobic, the However, at the same time, the corresponding desorption more their adsorption is similar to that of uncharged substances. It is well-known that the occurrence of a (35) B. B. Damaskin, 0. A. Petrii, and V. V. Batrakov, "Adsorption of Organic Compounds on Electrodes" English translation Plenum Press, maximum in the variations of adsorption with the electrical New York. 1971. variable in the case of a polar nonionic organic species is (36) E. Dukiewicz, J. D. Garnish, and R. Parsons, J . Electroanal. explained by the influence of the polar moment of the Chem., 16, 505 (1968). molecule.35 It seems evident that, with neutralization by (37) R. Parsons, Rev. Pure Appl. Chem., 18, 91 (1958). (38) J. Piro, R. Bennes, and E. Bou Karam, J.Electroanab Chem., 57, an inorganic cation, a surfactant anion could be apparently 399 f1974). ~~. \ - -
-,-
(39) M. Hamdi, R. Bennes, D. Schuhmann, and P. Vanel, J. Electroanal. Chem., 108, 255 (1980).
(40) C. Tanford, J. Phys. Chem., 78, 2469 (1974).
J. Phys. Chem. 1981, 85, 1042-1046
1042
changed into an apparently polar molecule. A recent analysis of the adsorption of halides on metals from alkaline halide solutions shows that the partial neutralization of the adsorbed layer by cations must be a generalized It is hence normal that the neutralization should be more complete in the case of the adsorption of hydrophobic anions which is accentuated by the length of the hydrocarbon chain. Similarly, the degree of association of the counterions in the corresponding micelles tends to diminish. NMR measurements have shown that, if one varies the nature of the counterion, the strength of the binding of the counterion to the micelle surface is proportional to the ion-pairing ability of the ion in the bulk solution.42 This latter property can also be considered as resulting from the interaction of electrostatic and hydrophobic interactions. Finally, two features in the behavior of the studied surfactants in the adsorption and micellar states appear as very similar. The first is the existence of similar characteristic charges, and the second the partial neutralization of the species when conditions to very high concentration prevail. Both show clearly the competition of hydrophobic and electrostatic forces in adsorption and in micellization. Their influence appears compatible with the growth of aggregates. When the number of the latter increases, the insertion of hydrated counterions is limited by steric factors and the density of the cathodic charge of the aggregate must increase. When the micelle attains an aggregation number such that its superficial charge is equal to the desorption charge, it must cease increasing. If such (41) B. B. Damaskin, L. Kutnetsova, V. Palm, M. Vaarinov, and M. Salve, J. Electroanal. Chem., 100, 365 (1979). (42) J. D.Robb, J. Colloid Interface Sci., 37, 521 (1971).
a picture is speculative, it appears to be a direct and reasonable consequence of the results on adsorption and micellization. I t would be interesting to multiply such parallel experiments in order to verify whether in all cases the results agree with the model suggested above. One knows, for example, that the addition of a cosurfactant such as an aliphatic alcohol modifies the properties of micelles, and we have shown16 that, in the case of 2methyl-2-propanol, if this is added in quantities large enough to modify the structure of the solvent (water), the apparent desorption charge is considerably modified.
Conclusion A comparison of the results obtained here on the adsorption of alkyl sulfates with those found in the literature concerning micelle formation in the same solutions of surfactants at concentration greater than the cmc has led to a better understanding of the former and in turn to a clearer picture of the manner in which the competition between the hydrophobic and electrostatic forces serve to fix the equilibrium properties of micelles. Analogies between the adsorption phenomena and micellization have been observed and analyzed. The results of this analysis offer encouragement to proceed further with the type of approach which has been initiated here and which may lead to a clearer understanding of the properties of micelles from measurements carried out on electrodes. It would be interesting in future studies to verify whether the model proposed can be used to forecast the influence of the various parameters considered.
Acknowledgment. We express our gratitude to the Iranian Ministry of Universities and Sciences for granting one of us (G.H.N.) a research bursary and to Professor Brun and Madame Kamenka for most helpful discussions.
Thermodynamics of Bolaform Electrolytes. 6. Partial Molal Adiabatic Compressibilities in H20 and D20 at 25 O C Hlroyasu Nomura Department of Chemical Engineering, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya-shi, 464 Japan
and R. E. Verrall’ Department of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, S7N OW0 Canada (Received: July 22, 1980)
Adiabatic compressibilitiesof several bis(tetraalky1ammonium)bromide (bolaform) salts have been measured in HzO and DzO.Infinite dilution values of +Qo have been obtained by extrapolation based on results determined to low concentrations. The change in compressibility at infinite dilution is not monotonic within the series although it decreases with increasing mass of the salt. Values in DzO are more negative than those in HzO. These observations are discussed in terms of the effect of the charged hydrophobic cations on the local solvent structure.
Introduction A number of thermodynamic ~ t u d i e s l have - ~ been carried out on aqueous and nonaqueous bis(tetraalky1ammonium) bromide (bolaform salt) solutions with the aim of assessing the solvent effects on solution properties in (1) Verrall, R. E.; Dickson, L. W. J . Solution Chern. 1976, 5, 203. (2) Burns, J. A.; Verrall, R. E. J. Solution Chern. 1975, 4 , 369. (3) Burns, J. A,; Verrall, R. E. J. Solution Chern. 1974, 3, 39. (4) Burns, J. A.; Verrall, R. E. J . Solution Chern. 1974, 3, 289. (5) Burns, J. A.; Verrall, R. E. J . Solution Chern. 1973, 2, 489. 0022-3654/81/2085-1042$01.25/0
relation to structure of the solvent and solvation of the ionic solute. The partial molal properties of dissolved solutes can vary significantly from solvent to solvent so that the solution properties of individual solutes must be considered with respect to each solvent. The situation is simplified somewhat if one considers the solvent media HzO and DzO. Under these circumstances solvent isotope effects in the solute properties can provide information on “structural” differences between HzO and D 2 0 as well as differences 0 1981 American Chemical Society
