Langmuir 1990,6, 1211-1216 between the hydrocarbon chains and the surrounding solvent. On the other hand, the aggregation number is limited since spherical micelles with high aggregation numbers are highly charged and have a high electrostatic free energy. It is less unfavorable for the hydrocarbon chain of the amphiphile to be in contact with formamide or ethylene glycol than with water as reflected by the interfacial tensions in Table V. The solvophobic interaction will increase when the water content of the solvent is raised, and the aggregates therefore grow in size as water is added to the system, just as we have observed. However, comparing formamide and ethylene glycol as solvents, we find that the cmc values and aggregate sizes higher interfacial tension yos are similar despite the ~ 5 0 % and almost 3 times higher dielectric constant for formamide. The general lack of correlation with the cr of the solvent suggests t h a t it is of small importance for
1211
determining the aggregate size. A quantitative explanation of these observations requires a more elaborate modeling. Since the surfactant concentration is high, both intra- and interaggregate electrostatic interactions should be taken into account. Other effects, e.g., solvent penetration and specific interactions, are possibly also of importance for a full understanding of micelle formation in nonaqueous solvents.
Acknowledgment. The work is financially supported by the Research Council a t the Swedish Board for Technical Development (STUF). Ann-Charlotte Malmvik kindly provided the deuterated surfactant. We also want to thank Peter Stilbs for giving us the opportunity to use the curve-fitting program. Registry No. a-Deuterated hexadecyltrimethylammonium bromide, 127132-58-5; formamide, 75-12-7; ethylene glycol, 10721-1; N-methylformamide, 123-39-7.
Micellar Solubilization in Strongly Interacting Binary Surfactant Systems C. Treiner’,*)+ M. Nortz,i and C. Vautioni Laboratoire d’Electrochimie, UA CNRS 430, Universit6 Pierre et Marie Curie, 4, Place Jussieu, Bat.F., Paris 75005, France, and Laboratoire de Pharmacie GalBnique, FacultB de Pharmacie de Paris-sud, rue J.B.Cl&ment,92290, Chatenay-Malabry, France Received October 5, 1989. In Final Form: January 16, 1990 The apparent partition coefficientP of barbituric acids between micelles and water has been determined in mixed binary surfactant solutions from solubilitymeasurements in the whole micellar composition range. The binary systems chosen ranged from the strongly interactingsystem dodecyltrimethylammoniumchloride + sodium dodecyl sulfate to weakly interactingsystemssuch as benzyldimethyltetradecylammoniumchloride + tetradecyltrimethylammonium chloride. In all cases studied, mixed micelle formation is unfavorable to micellar solubilization. A correlation is found between the unlike surfactants interaction energy, as measured by the regular solution parameter /3 and the solute partition coefficient change upon surfactant mixing. By use of literature data on micellar solubilization in binary surfactant solutions, it is shown that the change of P for solutes which are solubilized by surface adsorption is generally governed by the sign and amplitude of the interaction parameter 0. For solutes which are solubilized by penetration in the mixed micelle hydrocarbon core, the solubilization increase observed in all available cases may be interpreted by a Laplace pressure change due to micellar sphere-to-rod transition, which occurs in strongly interacting binary surfactant systems.
Introduction The study of solubilization of polar and nonpolar molecules by surfactants in aqueous solutions has made some progress in recent years essentially because of efforts t o rationalize the data in terms of thermodynamic quantities obtained at controlled solute activity and the use of different experimental approaches that allow a critical examination of micellar solubilization data. Finally, the number of investigated systems, particularly in the case of ionic surfactant solutions, has considerably increased,
* To whom all correspondence should be addressed. +
Laboratoire d‘Electrochimie.
* Laboratoire de Pharmacie Galhique. 0743-7463/90/2406-1211$02.50/0
permitting attempts at generalization of the behavior of polar hydrophobic molecules in these media.’ The need of finding new systems for the solubilization of scarcely soluble compounds, the decrease of the toxicity of some surfactants when mixed systems are used, and the fact that the solubilization of polar solutes is not greatly affected by the surfactant alkyl chain length have led to the study of the solubilization capabilities of mixed binary surfactant solutions. The systems investigated so far are not very numerous, and from the available data, conflicting conclusions have (1)Treiner, C.; Mannebach, M. H. J. ColloidInterface Sci. 1987,118, 243.
0 1990 American Chemical Society
Treiner et al.
1212 Langmuir, Vol. 6, No. 7, 1990
been put forward. We have recently that the regular solution approach may be used as a general framework for the understanding of such complex systems. According to this model, which has been originally devised for the prediction of nonpolar gas solubilities in mixed solvents,5 if the interaction energy between the two surfactants forming a mixed micelle is attractive, then the solubilization of a solute should be less in any mixed micelle than in either pure surfactant solutions. This should be the case for all surfactant binaries except for those for which a perfluoro surfactant is concerned. Here, the interaction is slightly repulsive, and the available data confirm that in such cases3 the micellar solubilization is larger than in either pure surfactant solutions. This approach is purely thermodynamic, and no attempt is made to take into account possible structural micellar changes upon surfactant mixing which might influence the solubilization phenomenon. Some authors have prefered to relate the variation of micellar solubilization upon mixed micelle formation to an increase of aggregation number6 or to an increase of micellar compactness?+ Although such changes may occur, it is the purpose of the present investigation to show that they are not essential to understand the micellar solubilization of polar solutes in mixed surfactant solutions. In the present report, we shall be more concerned with the effect of surfactant-surfactant interaction on micellar solubilization in strongly interacting micellar systems. With the additional use of recent results from the literature, we shall be in a position to show that the regular solution approach is the easiest and presently the most useful approach to correlate solute solubilization in binary surfactant solutions to the physicochemical properties of mixed micelles. New solubilization data will be presented essentially for two barbituric acids in binary anionic and cationic surfactant systems which strongly deviate from ideality and for which large increase of micellar size has been demonstrated. These results will supplement a previous investigation which focused on the effect of solute size on the solubilization changes of barbituric acids in two series of binary surfactants with relatively mild intermicellar interaction~.~ Experimental Section Two barbituric acids have been chosen: Butobarbital, (5ethyl-5-butylbarbituric acid) (Expandia), and Heptabarbital, (5ethyl-5- (l-cycloheptenyl) barbituric acid) (Geigy). We had previously used Reposal4 as a n example of a solute of large size: 5-ethyl-5-(bicyclo[ 3.2.l]octen-2-yl)barbituricacid. This compound is no longer available commercially, so we have replaced it with Heptabarbital, a molecule of the same size as Reposal. As the molecular surface area of the various barbituric acids differs only by their hydrophobic moiety, these have been calculated for Reposal and Heptabarbital by using the method of Herman10 and shown to be equal t o 150.0 and 156.7 Az,ll respectively. These values are reasonably similar for the present purpose as the discussion will show. (2) Treiner, C.; Bocquet, J. F.; Pommier, C. J. Phys. Chem. 1986,90, 3052. (3) Treiner, C.; Khodja, A. A.; Fromon, M. Longmuir 1987,3, 729. (4) Treiner, C.; No&, M.; Vaution, C.; Puisieux, F. J . Colloid Interface Sci. 1988, 126, 261. (5) O'Connell, J. P. AIChE J. 1971,17,658. (6) Abe, M.; Kubota, T.; Uchiyama, H.; Ogino, K. Colloid Polym. Sci. 1989,267,365. (7) Tokiwa, F. J. Colloid Interface Sci. 1968,28, 145. (8) Tokiwa, F.; Tsujii, K. Bull. Chem. SOC.Jpn. 1973, 46, 1338. (9) Nishikido, N. J. Colloid Interface Sci. 1977, 60,242. (10) Herman, R. B. J. Phys. Chem. 1972, 76, 2574. (11) Vaution, C.; Treiner, C.; Puisieux, F.; Carstensen, J. T. J.Pharm. Sci. 1981, 70, 1238.
Table I. Critical Micelle Concentration for Mixtures of Benzyldimethyltetradecylammonium Chloride (x= 1) and Tetradecyltrimethylammonium Chloride %sa
cmc
1.0
0.0019 0.0021
0.80 0.60
0.50 0.40 0.20
0.0
0.0022 0.00245 0.00264
0.0033
P -0.96
-1.13 -1.16 -0.86 -0.86
0.0053
XMb
1.0 0.87 0.73 0.67 0.61 0.44 0.0
avg -0.92 f 0.16 o Stochiometric surfactant mole fraction. * Micellar composition at the cmc. All concentrations are expressed on a molar basis.
The solubility measurements were made at 25 f 0.05 "C and the solutions analyzed a t p H 13 by using a UV spectrometer a t 256 nm. The critical micelle concentration (cmc) of the mixed surfactant solutions was determined by using an automatic conductance device from Taccussel. T h e surfactants used were sodium decyl sulfate (CloNa) and sodium dodecyl sulfate (C12Na) from Merck, trimethyldecylammonium bromide (CloBr) from Eastman Kodak, benzyldimethyltetradecylammonium chloride (C14BzC1) from Sigma, tetradecyltrimethylammonium chloride (c&1) and dodecyltrimethylammonium chloride (ClzC1) from TCI (Japan), and dodecylpoly(oxyethy1ene) (C12E23) from Aldrich. These compounds were used as received except for CloBr, which was recrystallized several times from pure acetone; their cmc values were in agreement with accepted literature values.
Results and Discussion Determination of Regular Solution Parameters. One of the purposes of the present investigation was to inquire to what extent the interaction energy between two surfactants within the mixed micelles controls the micellar solubility changes. In a previous report: binary surfactant systems with relatively small interaction coefficients (6) were used, namely, the ClzNa + C12E23 system with P = -2.5 and the C14Br + C12E23 system with P = -0.6.4 Here we have chosen CloNa + CloBr with P = -13.212 and the C12Na + ClzCl binary with P = -25.l3 In addition, we have determined the value of 0 for a cationic + cationic system (CI4BzC1 C14Cl) from cmc determinations using conductance measurements. The deviation from ideality of the cmc values of a binary mixed micelle may be represented by using the regular solution formalism with a single empirical coefficient which includes all nonideal interactions between the two surfactants and between the solute and the mixed micelle. We have used Rubingh's basic equation^.'^ The results concerning the C14BzC1 + C1&1 system are to be found in Table I. An average value of -0.92 f 0.16 is obtained, which indicates that the two different cationic headgroups interact only mildly at the mixed micellar surface. We have also calculated from the same conductance data the degree of counterion association, a, of the micelles. a is equal to 0.5 and 0.75 for C1&zC1 and c&1, respectively. The relatively small value obtained for the aromatic surfactant is not too surprising with the bulky head-group hindering the approach of the anion from the positive charge. The variation of a from one pure surfactant to the other was found linear, within experimental error. The solubilization experiments were used to calculate an apparent partition coefficient P. from the relationship
+
(12) Holland, P. M.; Rubingh, D. N. J. Phys. Chem. 1983,87, 1984. (13) Zhu, B. Y.; Rosen, M. J. J. Colloid Interface Sci. 1984,99,435. (14) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 337.
Langmuir, Vol. 6, No. 7, 1990 1213
Micellar Solubilization in Surfactant Systems = ( S M - S W ) / [ S W ( c S - c,)] (1) SMand SWare the solubilities in the presence and absence of surfactant of total concentration CS,all concentrations being expressed in the molar scale. T h e monomer concentration C, was calculated by using the following set of equations:15J6 ps
cm
xM = [-(C - A)
= x M c l f l + xMc2fZ
+ ((C - A)2 + ~ x s C A ) ” ~ ] / ~ A(2)
where
f i = exp[@xM21 C1 and CZare the cmc of each pure surfactant, C is the total surfactant concentration, XM and x s are respectively the micellar and stochiometric mixed micelle mole fraction, and 0 is the empirical coefficient of the regular solution activity coefficients fl and f2. In most cases studied, the solubility change was linear with surfactant concentration up to a t least 0.03 m. Only in either surfactant-rich mixtures were XM values somewhat different from x,. As the regular solution equations are expressed in a mole fraction basis, the final results were expressed in that concentration scale using the approximate equation P = 55.5Ps (Tables 11-V). The P coefficients will be discussed hereafter in the mole fraction basis. It must be emphasized that the present data do not represent real, but rather apparent, thermodynamic constants. However, we shall show that the same trends are observed for the change of P(x) with micellar composition whether real or apparent partition coefficients are observed. This will be sufficient for the present discussion. Micellar Solubilization Behavior. Single Surfactant Solutions. Table VI collects the available data which have been obtained from solubility experiments under the same experimental conditions for the most studied barbituric acid, Butobarbital, in single surfactant solutions. Cationic, anionic, and nonionic surfactants have been used with alkyl chain lengths ranging from CIOto C14. It is remarkable that the P(x) values are almost identical for the CIZand the c14 surfactants regardless of the charge or the size of the surfactant head-group. Only for the Clo surfactants are the P values somewhat smaller than for the other single micellar solutions. This confirms that no specific interaction takes place between the barbituric acid and any of the surfactants studied. The solubilization should be, therefore, the sole consequence of the hydrophobic properties of the solute. This is evidently an ideal case to test the applicability of the regular solution model. Solubilization in the Mixed Surfactant Solutions. We have shown before2-‘ that the partitioning of a neutral nonpolar solute between a pseudobinary micellar solution and water may be represented by the relationship
In P(x)= X M In PI + (1- xM) In Pz + x,(I - xM)B (3) PI and PZare mole fractional partition coefficients of the solute in the single surfactant solutions, and P ( x ) is the same parameter in the mixed micelles. B is an empirical parameter which, within the framework of the regular solution theory, should be equal to p. This is not even the (15) Clint, J. H.J . Chem. SOC.1976, 71, 1327. (16)Bourrel, M.; Bernard, D.; Graciaa, A. Tenside Detergents 1986,
21, 311.
Table 11. Partition Coefficient. of Butobarbital in Mixtures of TrimethyldecylammoniumBromide (x= 1) and Sodium Decyl Sulfate 1.0 0.90
0.80 0.60
1220 800 510 430
0.40 0.20 0.10 0.0
430 820 1050 1510
Mole fraction basis. Table 111. Partition Coefficient of Butobarbital in Mixtures of Benzyldimethyltetradecylammonium Chloride (x= 1) and Trimethyltetradecylammonium Chloride 1.0 0.8 0.6
2090 2080 2020
0.4 0.2 0.0
1960 2040 2240
Table IV. Partition Coefficient of Heptabarbital in Mixtures of Sodium Dodecyl Sulfate (I= 1) and Dodecylpoly(oxyethy1ene)a 1.0 0.75 0.50
4100 2380 2220
0.25 0.05 0.0
2280 2280 2880
Table V. Partition Coefficient of Heptabarbital in Mixtures of Sodium Dodecyl Sulfate (x = 1) and Trimethyldodecylammonium Chloride 1.0 0.90 0.30 0.20
4100 2200 250 300
0.10 0.05 0.0
880 1130 1470
Table VI. Partition Coefficient of Butobarbital in Different Single Micellar Surfactant Solutions surfactant Cl4BzCl CiiCl C14Br ClzNa
P(x) 20900 22400 18W4 18W4
surfactant CizEzs Cda CloBr
P(x) 21004 151W 122W
This work. Superscript 4 refers to ref 4.
case for a nonpolar gas in mixed solvents,6 where eq 3 systematically overestimates the actual solubility change. However, we may expect that for a given solute B will be correlated to p. Figures 1 and 2 present the results obtained for the systems displaying the largest deviation from ideality. Note that in the case of the C12Na + C&1 binary system, a large turbid region (noted t.rJ is observed in which no reliable solubility measurements can be performed. The turbid region is much smaller with the CloNa + CloBr system. The fitting of these data using eq 3 provided the following results in terms of the empirical B coefficient. For Butobarbital in C l d a + C&r and in C1&C1+ C&1, B is equal respectively to -4.5 f 1.2 and -0.60 f 0.12. For Heptabarbital in C12E23 + Cl2Na and C12C1+ ClZNa, the B coefficient is equal respectively to -2.1 f 0.4 and -8.8 f 2.2. The solid line in Figures 1 and 2 represents the variation of P(x)with XM according to eq 3 with the average B coefficients just calculated. In all cases, this equation represents very well the experimental results even in the extreme case of the ClzNa + C&l system for which the micellar solubilizationbecomes very small. It is interesting to note that Heptabarbital, a barbituric acid with nearly the same hydrophobic size as Reposal, shows, within experimental uncertainty, the same negative deviation from ideality in the ClzNa + &E23 mixtures ( B = -2.1 f 0.4
1214 Langmuir, Vol. 6, No. 7, 1990
Treiner et al.
-1 0
-30
Figure 1. Variation of P(n) for Butobarbital in mixed surfoctant solutions. The solid line represents the calculated P values obtained by using eq 3. t.r. represents the turbid region.
looo~
,
\
0
.5
X
1
Figure 2. Variation of P(w) for Heptabarbital in two series of
mixed surfactant solutions. The solid line represents the calculated R(r)values obtained by using eq 3. t.r. represents the turbid region for the C12Na + C12C1 system. and -2.3 f 0.4,4 respectively). This observation confirms the point raised earlier, i.e., that either solute may be used for comparison purposes. The results concerning Butobarbital (not shown) in Cl4BzCl + C14C1, with an only slightly negative 6 va!ue (@= -0.92), also display the smallest negative deviation from partitioning ideality. Figure 3 presents the correlation found between B and /3 for five different binary surfactant systems from the smallest to the strongest @ values. The results for two different barbituric acids are shown on the same graph, although we have shown before4 that there is a slight variation of B with solute size in a given binary system. This change, although real, is too small to be shown on the scale of Figure 3 and can be neglected in the present discussion. The linear correlation observed may be looked upon as a confirmation of our previous suggestion,namely, that the more negative the 6 values, the larger the decrease of micellar solubilization of polar solutes upon surfactant mixing. The linear correlation is a remarkable result because the different surfactant mixtures are defined by a single empirical coefficient. On the other hand, the micellar solubilization of the barbituric acids concerns essentially the hydrophobic moieties of these complex
p
c
0
-10
Figure 3. Correlation between the B values of eq 3 for the ternary mixed surfactant/solute systems and the @ coefficient for the binary mixed surfactant systems: 0,Heptabarbital; 0, Butobarbital. (1) C14BzCl+ C14C1; (2,3) C12Na + C12E23; (4) C14Br + C12E23; (5)CloNa + CloBr; (6) ClzNa + C&l. Present work, systems 1, 3, 5, 6; ref 4, systems 2, 4.
molecules;ll thus the polar malonylurea group plays no part in the interaction with the mixed micelles. The same trend is also found using the previously published data for 1-PeOH although only three different surfactant mixtures had been ~ t u d i e d .A~ positive B value was found in the case of a positive 8 coefficient (mixed hydrocarbon and perfluorocarbon surfactants), whereas a strong decrease of partitioning and thus a very negative B value was observed with the CloNa + CloBr system. The case of ClzNa + C12E23 displayed an intermediate behavior between the two more extreme cases. The correlation displayed in Figure 3 does not mean that any polar solute will fall on the same line. Different solutes in the same series of surfactant mixtures may present different dependences of B with 8. However, these results do show that the very large increase of micellar size as shown by Zana et al." for the anionic cationic surfactant system (C12Na + C12C1) and very recently by other a u t h ~ r s for ~ ~similar J ~ surfactant mixtures with different chain lengths does not lead to an increase of P. On the contrary, it is precisely for these surfactant mixtures that the solubilization shows the largest negative deviation from ideality. Thus, the rate of change of P ( x ) with micellar composition is essentially a function of the intramicellar interaction energies between the unlike surfactants. It is important to point out that the P values for 1-PeOH in the mixed CloNa + CloBr system have been determined by using head-space gas chromatography3 as well as microcalorimetry.20 With both techniques, the solute activity can be controlled, which is not the case with solubility measurements. Nevertheless, the same overall profile of P ( x ) versus x was observed, similar to that of Butobarbital, for example, in the same binary surfactant solutions. Thus, the fact that apparent instead of real thermodynamic constants are used in the present work does not introduce any ambiguity in the discussion with respect to the use of eq 3. Comparison w i t h o t h e r Solubilization D a t a i n Mixed Surfactant Solutions. In view of the different opinions one may find in the literature on the driving force for solubilizationin mixed micelles, we have found it useful to summarize the trends observed by different authors in
1
t.r.
0
-20
+
(17) Malliaris, A.;Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114. (18) Kato, T.;Iwai, M.; Seiyama J . Colloid Interface Sci. 1989, 130,
--".
ARQ
(19) Yu,Z. J.; Zhao, G. X. J. Colloid Interface Sci. 1989, 130, 421. (20) Bury, R.; Treiner, C. J. Solution Chem. 1989, 18,499.
Micellar S o l u b i l i z a t i o n in Surfactant Systems Table VII. Micellar Solubilization and Other Physicochemical Parameters for Binary Mixed Surfactant Systemsb solute mixed surfactant B M A P benzene NaPFO + NaDEC +1.8 (2) + (25) + (29) 1-pentanol NaPFO + NaDEC +1.8 (2) + (25) - (29) yellow OB C12& + CJ3zNa -3.5 ( a ) + (8) -15 (13) - (6) PC DPDA + C12Na -15 (13) + (6) + (6) 1-octanol DMLL + C12Na -1 (13) - (6) - (6) 1-octanol DMLL + C14Br -1.3 ( b ) - (30) 1-hexanol CISpyCl + NPEla 1-hexanol C12C1+ ClzNa -25 (13) + (17) - (28) -13.2 (12) + (19) - (3) 1-pentanol CloNa + CloBr 1-pentanol C12E23 + ClzNa -2.6 (32) 0 (e) - (3) +2.2 (g, 3) + (f) + (3) 1-pentanol LiPFO + C12Li yellow OB C12D6 + ClzNa -4.0 (33) - (33) yellow OB ClzEe + AOT -4.2 (33) - (33) yellow OB C&6 + LiFOS -4.8 (33) - (33) -2.8 (9) + (9) yellow OB C12E49 + CIZMg -2.8 (9) - (9) yellow OB C12E6 + C12Mg -0.8 ( c ) - (9) yellow OB C12E4B + ClzCl -0.8 ( c ) - (9) yellow OB ClZE6 + C12C1 B. acids Cl2EZ3+ C12Na -2.6 (32) - (4) B. acids C12E23 + C14Br -0.6 ( c ) 0 (e) - (4) -25 (13) + (17) + (22) 1-decane Cl&l + C12Na 1-hexane NPElo + c16PyC1 -1.3 ( b ) + (27) 1-hexane NPElo + ClzNa -4.8 (d) + (27) 1-hexane c l & c l + ClzNa -25 (13) + (27) "Kolp, D.; Laughlin, R.; Zimmerer, R. J. Phys. Chem. 1963, 67, 51. bNguyen, C. M.; Rathman, J. F.; Scamehorn, J. F. J. Colloid Interface Sci. 1986, 112, 438. CRosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1982, 86, 164. Carrion Fite, F. J. Tenside Detergents 1985, 22, 5. e Nagakawa, T.; Shinoda, K. In Colloidal Surfactants; Academic Press: New York, 1963; p 163. f Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162. 8 Miyagishi, S.; Ishibai, Y.; Asakawa, T.; Nishida, M. J. Colloid Interface Sci. 1985, 103, 164. NPElo, nonylphenolpoly(oxyethy1ate); AOT, sodium bis(2-ethylhexy1)sulfosuccinate; DPDA, dodecanamidopropyl(dimethy1amino)acetate. LiFOS, lithium perfluorooctanesulfonate; B. acids, barbituric acids; cl6Pyc1, hexadecylpyridinium chloride; DMLL, see text. Numbers in parentheses refer to references, italic letters refer to footnotes above.
recent years. The result of this compilation (which is by no means exhaustive but nevertheless representative) is presented in Table VI1 with some relevant physicochemical parameters for binary surfactant mixtures. The table is arranged as follows. The different column heads refer, respectively, to the solute, the binary surfactant system, the value of 0(or when this parameter is not available, the most probable value estimated from closely related systems), the aggregation number change AN with respect to micellar composition, and the corresponding partition change AP. I t is unfortunate that, in most cases, experimental solubilization results in mixed surfactant systems are presented only as graphs, preventing a quantitative analysis. However, the sign of AF' can be deduced from these graphs. A plus sign indicates a positive deviation from the simple additivity rule and a minus sign a negative deviation. The same notation holds for AN. The values in parentheses correspond to references to the original papers. It will be useful for the clarity of the discussion that the relation of the various parameters to the micellar solubilization phenomenon be analyzed separately. a. Regular Solution Parameter 8. This parameter is negative for all the systems studied except for those containing a perfluoro surfactant for which /3 is positive.1.21 Thus, a negative partition coefficient deviation from the ideal additivity rule should be observed for all cases except for surfactant systems with a perfluoro (21) Shincda, K.; Nomura, T. J. J. Phys. Chem. 1980,84, 365.
L a n g m u i r , Vol. 6, No. 7, 1990
1215
compound. Note that these cases cannot be of a great practical interest because as the repulsion between perfluoro and hydrocarbon moieties is increased (for example, by increasing the surfactant's hydrocarbon chain length), a partial demixing of mixed micelles will eventually occur,21,22limiting the solubilization increase. b. Size of the Mixed Micelles. From the available data in the whole micellar composition range, an increase of micellar size above the simple additivity rule may be found for a variety of mixed systems as can be observed from the results of Table VII. An increase of micellar aggregation number upon surfactant mixing was also found with the mixed anionic + nonionic systems: Cl2Na + Tri~ . same ~ ~ qualitative ton X-lo023and Cl2Na + C ~ B Z EThe result was obtained,from a small-angle neutron-scattering (SANS) study on mixed anionic surfactants with a perfluoro component.26 Finally, Nagarajan thermodynamic theory of mixed micelle formation predicts also an increase of aggregation number when compared to single surfactant micelles.26 Thus, only for the C12Br + C12Es system has a size decrease effect (less than the additivity rule) upon surfactant mixing been experimentally founds6 c. Solubilization i n Mixed Micelles. In order to further clarify the actual situation, one must differentiate between the nonpolar and the polar solutes. Thus it appears clearly that the solubilization of nonpolar solutes (hydrocarbons) is increased in mixed micelles over the single micellar results. This is the case with hexane, c y ~ l o h e x a n e or , ~ ~decane.27 Such results a r e in contradiction with the prediction of eq 3. In effect, the enthalpy of mixing of the mixed hydrocarbon core in which the nonpolar solute will be located should be close to zero, and thus, the solubilization should follow the ideal mixing rule. In fact, eq 3 should be applicable only to solutes which are solubilized in that portion of the mixed micelle where the interaction between the two surfactants takes place. We shall come back to this point below. Note that the solubilization of benzene in mixed hydrocarbon + fluorocarbon micelles is also larger than in either single surfactant solutions.29 This effect might be the consequence of either the positive /3 values of the regular solution parameter for these binaries as was found for 1-PeOH2s3or to the same reason for which the apolar hydrocarbons solubility is increased in mixed surfactant systems. This question will be addressed further below. The effect of polar solutes seems more difficult to analyze. Fint of all, there are a variety of solutes of widely different polarities. Benzene would be, for example, a compound on the borderline. Among the solutes for which an unambiguous negative B value can be ascertained, one may find polar solutes such as (Table VII) l - p e n t a n ~ l , ~ $ ~ l - h e x a n ~ l1-octanoP , ~ ~ ~ ~ (in the case of a mixed amphoteric + cationic system), phosphatidylcholine (PC),31baror a ~ o b e n z e n e .These ~~ bituric acids,4.a1 yellow 0B,33*34 (22) Mukerjee, P.; Yang, A. Y. C. J. Phys. Chem. 1976,80,1288.
(23) Dubin, P. L.; Principi, J. M., Smith, A.; Fallon, A. M.J . Colloid Interface Sci. 1989, 127, 558. (24) Tokiwa, F.;Aigami, K. Kolloid-2. 2. Polym. 1970,239,687. (25) Burkitt, S. J., Ottewill, R. H.; Hayter, J. B.; Lingram, B. T. Colloid Polym. Sci. 1987,265, 628. (26) Nagarajan, R. Langmuir 1985,1,331. (27) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F.J. Colloid Interface Sci. 1989,130, 254. (28) Wears, J. G . Private Communication, 1987. (29) Carlfors, J.; Stilbs, P. J. Colloid Interface Sci. 1985,103, 332. (30) Nguyen, C. M.; Scamehorn, J. F.;Christian, S. D. Colloids Surf. 1988, 30, 335. (31) Tanaka, K.; Takeda, T.; Nakamura, M.; Yamamura, S.; Miyajima, K. Colloid Polym. Sci. 1989, 267, 550. (32) Treiner, C.; Vaution, C.; Miralles, E.; Puisieux, F. colloids Surf. 1985, 14, 285.
Treiner et al.
1216 Langmuir, Vol. 6, No. 7, 1990 results are in line with the semiquantitative prediction of eq 3. Particularly noteworthy is the case of CnEs mixed with C12Na,AOT, and LiFOS (a perfluorooctanoate surfactant);33the partitioning of yellow OB in the binary surfactant solutions follows exactly the order of the /3 values; namely, the decrease of yellow OB solubilization upon surfactant mixing is + LiFOS > C12& + AOT > Cl2E6 + C12Na. A positive synergistic effect, in apparent contradiction with expectation, has been noted in some cases. These concern essentially yellow OB in different types of mixed surfactant systems.34 We shall discuss them separately. The first case concerns manganese dodecyl sulfate + nonionic surfactants with long oxyethylene chains such as C&2g or C12E49. When the anionic surfactant is mixed with a nonionic one with smaller oxyethylene chains, a negative synergistic effect is observed in accordance with eq 3. We have noted before3 that in the case of such systems the very definition of a solubilization concentration in terms of moles of solute/mole of surfactant is difficult because the whole nonionic surfactant does not participate in the solubilization process. In fact, according to the type of calculation performed, the order of increased solubilization of a particular solute in a series of nonionic surfactants may even be reversed.35 Thus the validity of the procedure adopted for the calculation of P may be questioned in this complicated situation. The second case concerns yellow OB in a mixed octyl benzenesulfonate + C12E9 system where the benzenesulfonate ring is at a terminal position.* Here a large increase of solubilization is observed. The interpretation proposed by the authors invokes a reduction of surface micellar charge upon surfactant mixing with a concomitant decrease in micellar constraint. If valid, this interpretation should also apply to the other anionic + nonionic systems of Table VII. This is not the case. In fact, another mechanism may be suggested. Dyes such as yellow OB may aggregates through the so-called stacking effect due to the presence of the aromatic rings. The effect of adding the nonionic component is to separate the benzenesulfonate rings from each other at the micellar surface and to favor a closer face-to-face approach of the dye from the surfactant aromatic head-groups. This configuration should increase the dye solubilization by a kind of stacking effect which was more difficult to achieve when the benzenesulfonate rings were in contact with each other in the single anionic micelle. Therefore, this phenomenon implies an additional effect, obviously not predictable from the regular solution approach a specific interaction (stacking) induced by a steric effect (separation of benzene rings). This leaves only one anomalous mixed surfactant system, the amphoteric Nfl-dimethyl-N-lauroyllysine(DMLL) + C12Nasystem, which apparently induces an increase of micellar solubilization for l-octanol? although the /3 value (not known) should be negative. T h e obviously complicated structure of such mixed micelles precludes any attempt of a reasonable explanation based on structural arguments. As discussed above, the alleged increase of mixed micellar size cannot be considered as a valuable explanation, as it should apply also to the other systems analyzed. An alternative interpretation might be put forward. We (33) Muto,Y.;Asada,M.;Takasawa,A.;Esumi,K.;Meguro, K. J. Colloid Interface Sci. 1988, 124, 632. (34) Nishikido, N. J. Colloid Interface Sci. 1977,60, 242. (35) Barry, B. W.; El Eini, D. I. D. J.Pharm. Sci. 1976,28,210. (36) Burdett, B. C. In Aggregation Processes in Solution; WynJones, E.,Gormally,J., Eds.;Studies in Physical and Theoretical Chemistry 26; Elsevier: New York, 1983.
have recalled above that the solubilizationof apolar solutes such as hydrocarbons in mixed micelles is systematically larger than in single micellar systems, in opposition to the behavior of polar molecules. 1-Octanol can be considered as a borderline solute displaying apolar as well as a slightly polar character. (In fact, the increase of micellar solubilization of 1-octanol is much less than that observed for hexane or cyclohexanein mixed ionic surfactant systems.) Thus, the increase of solubilization of 1-octanol might be of the same origin than that of the nonpolar solutes. The question remains then of the fundamental reason for the different solubilizationbehavior of apolar and polar solutes in mixed surfactant systems. One may simply suggest the relevance of some fundamental parameters. Most hydrocarbon solubilities were measured in strongly interacting surfactant systems. The aggregation number increase is then very large and implies a structural change from spherical to cylindrical micelles. The above analysis indicates that this change is sensitive to those solutes which solubilize by incorporation in the micellar hydrocarbon core. Here, the strict application of Laplace pressure equation, as suggested by Mukerjee for single-component micelles,37 to the mixed micelle situation predicts an increase of micellar solubilization when changing from spherical to cylindrical symmetry simply because Laplace pressure is less for a cylinder than for a sphere. Structural changes should have little effect on the micellar solubilization of solutes which interact with the micelle by surface adsorption. This concerns most, if not all, polar solutes.
Conclusions T h e regular solution approximation offers a straightforward semitheoretical background to the discussion of micellar solubilizationdata in aqueous binary surfactant solutions. I t predicts, in agreement with experimental evidences, that the formation of mixed micelles is unfavorable to the micellar solubilization of polar solutes. One general exception, also predicted by the model, is illustrated with perfluoro Surfactants, although the expected synergistic effect cannot be very large due to the possible demixing of the micelles. The other exception seems provided by the case of aromatic solutes in mixed aromatic + nonaromatic surfactants. One of the conclusions of the present study is that the increase of micelle aggregation number, which occurs for most mixed micelles, is not responsible for the micellar solubilization change of polar solutes. The situation is completely different with apolar compounds. In that case, the increase of aggregation number upon surfactant mixing with the concomitant change in structure from spherical to cylindrical symmetry favors the solubilization of those solutes which penetrate the hydrocarbon core, as predicted by the basic Laplace pressure equation. Registry No. SDS,151-21-3; DTAC, 112-00-5; TTAC, 4574benzyldimethyltetradecylammonium 04-3;C12Ez3, 9002-92-0; chloride, 139-08-2; butobarbital,77-28-1; heptabarbital, 509-864; sodium decyl sulfate, 142-87-0; decyltrimethylammonium bromide, 2082-84-0. ~
~
~
~~~
(37) Mukerjee, P. Kolloid-Z. Z. Polym. 1970, 236, 76.
