Langmuir 1992,8, 23-26
23
Association of Alcohol with Cationic Micelles C. Gamboa,* A. Olea, H. Rios, and M. Henriquez Departamento de Quimica, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Casilla 653, Santiago, Chile Received November 2,1990. In Final Form: July 25, 1991 Partition constants for several aliphatic alcohols between water and micellar phases were determined using ultrafiltration and dialysis. Results indicated that the micellized concentrations of the long-chain alcohol followed a Langmuir adsorption isotherm type of profile. Association of pentanol and hexanol with cylindrical micelles induces an increase in size and in dissociation degree (a),followed by a micellar breakdown. These changes can be explained in terms of the number of alcohol molecules per micelle.
Introduction The effect of alcohols on the micellar properties of surfactants in aqueous solution is a matter of current intereut1-12mainly in order to gain a better understanding of the role of alcohols in microemulsions. It is well established that short-chain alcohols have a small effect on the critical micelle concentration (cmc) or upon the micellar dissociation degree (a).2The effect of alcohols on several physicochemical properties becomes important for butanol and longer chain al~ohols.'~ For instance, Vikholm et al.14 discussed the effect of hexanol in hexadecyltrhethylammonium bromide (CTAB) solutions. The solution viscosity increases abruptly above a certain hexanol content and then decreases a t higher hexanol contents. This fact was interpreted as a shape transition from spherical to larger rod or disklike micelles, followed by a breakdown from large aggregates to spherical swollen micelles. For tetradecyltrimethylammonium bromide (TTAB) in brine (0.1 M KBr) in the presence of increasing concentrations of 1-pentanol, Zana et ala2% reported an increase of the micelle hydrodynamic radius ( R H )and in the viscosity of the solution, followed by a micellar breakdown as well. Zana explained these changes by suggesting that the amount of alcohol dissolved in the micellar phase decreases. However, there are only a few papers in the literature regarding the partitioning of alcohols between these phases. The usual approach is to determine the increase of the amount of alcohol dissolved in the presence of detergent as compared with that in water.15 Thus, for (1) Abuin, E.; Lissi, E. J. Colloid Interface Sci. 1983, 95, 198. (2) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J.ColloidInterface Sci. 1981. - - ,80., 208. (3) Yiv, S.; Zana, R.; Ulbricht, W.; Hoffmann, H. J. Colloid Interface Sci. 1981, 80,224. (4) Candau, R.; Zana, R. J. Colloid Interface Sci. 1981,84,206. (5) Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 76, 62. (6) Hirsch, E.; Candau, S.: Zana, R. J. Colloid Interface Sci. 1984.97. 318. (7) Zana, R.; Picot, C.; Duplessix, R. J. Colloid Interface Sci. 1983,93, 43. (8)Marignan, J.; Basserau, P.;Delord, P. J.Phys. Chem. 1986,90,645. (9) Gomati, R.; Appell, J.; Basseteau, P.; Marignan, J.; Porte, G. J. Phys. Chem. 1987,91, 6203. (10) Marignan, J.; Gauthier-Foumier,F.; Appell, J.;Akoum,F. J.Phys. Chem. 1988,92, 440. (11) Guerin, G.; Bellocq, A. M. J. Phys. Chem. 1988, 92, 2550. (12) Blokhus, A. M.; Hoiland, H.; Gilje, E.; Backlund, S. J. Colloid Interface Sci. 1988, 124, 125. (13) Rao, I. V.; Ruckenstein, E. J.ColloidInterface Sci. 1987,119,211. (14) Vikholm, I.; Douheret, G.; Backlund, S.; Hoiland, H. J. Colloid Interface Sci. 1987, 116, 582. (15) Muto, Y.; Yoda, K.; Yoshida, N.; Esumi, K.; Meguse, K.; BinanaLimbele, W.; Zana, R. J. Colloid Interface Sci. 1989, 130, 165. ~
~
CTAB in salt-free solutions, GettinslGobtained the following partition constants: 55.5 for butanol, 205 for pentanol, and 566 for hexanol. On the other hand, Abuin and Lissil demonstrated that the partition constant values determined in sodium dodecyl sulfate solutions saturated with hexanol and heptanol are lower than the corresponding values a t low alcohol concentration measured by fluorescence methods. Other researchers have developed an empirical equation relating the ability of an additive to depress the cmc and its distribution coefficient between micelles and water.17 The partition constant for a solute around the cmc can be obtained using that equation. For phenol in nonionic micelles of polyethoxylated nonylphenol, the association constant was determined by several methodP and the results were interpreted in terms of a Langmuir isotherm. The knowledge of the micellar alcohol concentration seems to be a very important parameter which determines several physicochemical properties. The aim of this work is to study the solubilization pattern of alcohols in cylindrical cationic micelles by determining the respective partition constants. Two systems were choosen for this study: CTAB in brine (0.16 M NaBr) and cetyltrimethylammonium tosylate (CTATOS). In the first case, solutions consist of cylindrical type mi~e1les.l~ CTATOS was chosen because it presents a well-defined second cmc attributed to a sphere-rod transition, in the absence of additives.20
Materials and Methods Viscosity measurements were carried out by extrapolating to zero flow rate, as previously described.19 Free tosylate (TOS-)concentration was measured using a tosylate ion selective electrode with a calomel reference electrode and an Orion 701 A potentiometer. The tosylate electrode was constructed with a plastic membrane containing methyltricaprylammonium tosylate.21 The electrode calibration was performed by measuring the potential (mV) of NaTOS solutions and 5 x 10+ M. The electrode response ranging between 5 x was Nernstian in this range. (16) Gettins, J.; Hall, D. J. Chem. SOC.,Faraday Trans. 2 1978, 74, 1957. (17) Treiner, C. J.ColloidInterfaceSci. 1983,93,33. Abu-Hamdiyyah, M.; Rahman, I. J. Phys. Chem. 1987, 91, 1530. (18) Kandori, K.; Greevy, R. Mc.; Schechter, R. J. Colloid Interface Sci. 1989, 132, 395. (19) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, 566. (20) Gamboa, I. C.; Rios, H.; Sepulveda, L. J. Phys. Chem. 1989, 93, 5540. (21) Ion-Selectiue Electrodes in Analytical Chemistry; Freiser, H., Ed.; Plenum Press: New York, Vol. 1.
0743-7463/92/240S-0023$03.00/00 1992 American Chemical Society
Gamboa et a!.
24 Langmuir, Vol. 8, No. 1, 1992 Partition constants (K,) for pentanol and hexanol between water and the micellar phase were obtained by two phase separation methods: (a) ultrafiltration22 and (b) dialysis equilibri~m.*~ Alcohol concentration was measured by gas chromatography using an Alltech liquid-phase column DC-200. The ultrafiltration method was employed for low-viscositysolutions. Dialysis equilibrium was preferred for highly viscous solutions because ultrafiltration filtrates gave products similar in composition to the original solution, suggesting that cylindrical micelles, oriented by pressure, might be passing through the ultrafiltration membrane. In dialysis experiments, an aliquot of detergent-alcohol solution was added to a dialysis membrane bag which was immersed in 100 mL of solvent (water, cmc, and/or brine); the systems were thermostated at 25 O C for 48 h. Aliquots of the solvent phase were analyzed chromatographically. The mole fraction scale partition constant K, is defined asz4 K , = X,iX, (1) where X , and X , are the mole fractions of alcohol in the micellar and water phases, respectively. X, and X, are related to the alcohol concentration in water (S,) and the micellar alcohol concentration (S,) through the following equations:
X, = S,/(S,
+ cmc + 55.5 + S,)
(3)
where S, is the NaBr concentration. For X, it has became customary to express the mole fractionof the micelle-incorporated solute in terms of its mole fraction in a "dry" micelle, i.e., to ignore water as a possible third micellar comp~nent.~~ Activity coefficients are assumed to be unity. K , can be related to the most common form of expressing the solute incorporation (K,) which is defined in terms of D, by This definition of K , in essence reduces the solubilization process to a pseudophase equilibrium of the solute between a micellized surfactant phase and the aqueous phase represented by the following equation: K,
S, -4-
D,
*-c
S,
K , can be related to K, through K, = 55.5K8/(1+ K,S,)
(6) CTAB and CTATOS were from Sigma Chemical Co. CTAB was recrystallized from ethanol-ether mixtures. All other reagents used were analytical grade.
Results and Discussion Partition constants of different alcohols in the presence of 0.09 M CTAB in 0.16 M NaBr were determined, and the results are shown in Table I. As expected, longer chain alcohols are more associated with the micellar phase. For pentanol and higher alcohols, partition constants increase with alcohol concentration, and pass through a maximum, prior to the onset of phase separation. K , values obtained are similar to those of Gettinls for butanol and pentanol at low alcohol concentrations. Differences can be attributed mainly to the presence of added salts that could induce alcohol association. Indeed, H ~ i l a n dreported ~~ the following partition coefficients in sodium dodecyl sulfate (SDS): (22) Gtunboa, C.; Sepulveda, L.; Soto, R. J. Phys. Chem. 1981, 85, 1429. (23) Higazy, W.; Mahmoud, F.; Taha, A.; Christian, S. J . Solution Chem. 1988, 17, 191. (24) Sepulveda,L.; Lissi, E.; Quina, F. Adu. Colloid Interface Sci. 1986, 25, 1. (25) Hoiland, H.; Ljosland, E.; Backlund, S. J . Colloid Interface Sci. 1984, 101, 467.
0
5
[Alcohol]
10
total
102
15
Figure 1. A, Hexanol in 0.09 M CTAB-0.16 M NaBr; 0, pentan01 in 0.09 M CTAB-O.16 M NaBr; 0,pentanol in 0.01 M CTAB-0.16 M NaBr. Table I. Partition Constants of Alcohols K. between Micellar Phase and Water in 0.09 M CTAB in 0.16 M NaBr at Different Alcohol Concentrations alcohol concn, M propanol butanol pentanol hexanol 0.0021 990 0.0025 23P 0.0050 2400 1020 25P 0.0075 0.0080 1050 52" 0.0090 0.0128 280 55 0.0160 0.0183 7P 625 lo00 0.0320 50n 190" 1050 2190 0.0350 220 0.037 1340 0.0480 1560 0.0640 1340 1340 0.070 0.1100 430 0.1500 260 Values obtained by ultrafiltration.
190 for pentanol, 690 for hexanol, and 1750 for heptanol. These values increase in the presence of 0.2 M NaCl to 220, 1300, and 9000, respectively. Pentanol and hexanol K , values reported in Table I indicate that the micellar alcohol concentration increases with total alcohol concentration in a certain region as shown in Figure 1. For phenol in CTAB, a K , value of 4062 was found. In 0.08 M NaBr and 0.16 M NaBr, K , increases to 4380 and 4820, respectively. Thus, added salts produce an increase in micellar phenol concentration. This effect must lead to a growth in micellar size, reflected as a relative viscosity increase in these systems. As can be seen in Figure 1, for high detergent concentration the micellized pentanol reaches a constant value before phase separation. For this same detergent concentration range, the micellized hexanol values increase linearly. However, at low detergent concentrations, micellized pentanol is lower than that at higher detergent concentration. This behavior indicates that the more concentrated micellar phase solubilizes more alcohol, but only in a definite range of alcohol concentration, Le., just below the phase separation. As already pointed the increase in the micellar alcohol concentration also induces an increase in the micellar size and, consequently, in the relative viscosity.
Langmuir, Vol. 8, No. 1, 1992 25
Association of Alcohol with Cationic Micelles
Table IV. Free Tosylate Concentration [TOS-]
Table 11. Relative Viscosity of 0.09 M CTAB in 0.16 M NaBr in the Presence of Alcohols hexhep- octaprobuta- penethaconcn, no1 pan01 no1 tanol anol tanol no1 nln, nln, nln, nln, nln, nln, M nln,
At Different CTATOS Concentrations [CTATOS], xlO4M
[TOS-I, x103 M
2.2 3.1 3.4 4.4 4.9 5.4 6.0 7.0
0.9 1.7 2.3 2.8 3.3 3.7 4.4 5.0
~
0.00 0.002 0.003 0.008 0.009 0.010 0.015 0.020 0.025 0.030 0.050 0.060 0.080 0.110 0.160 0.210 0.250 0.270 0.320 a
32
32
32
32
32 73
28 24
20
34 35
114 221
39 39 39 37
389 417 263 92
32 22
28 14
287
198
625
4270
1432 2280 3125
6945 1812 490 52
1740 410 28 12
a
32 110 500 1736 4032 3730 3330 a
32 117 249 1938 3968 2525 2350 a
a
Two-phase separation.
Table 111. Relative Viscosity nln,, K., and Micellar Hexanol (Hexanol(m))Concentration for CTATOS, 0.018 M, and CTATOS. 0.04 M hexanol total concn, M nln, hexanol(m) concn, M K, 0.018 M CTATOS 10 O.OO0 ~~
0.010 0.020 0.024 0.030 0.040 0.000 0.005 0.011 0.016 0.020 0.030 0.037 0.040
~
14
3.5 x 10-3 9.5 x 10-3
1370 1810
11 9 2
1.6 X 1.9 x 10-2
1800 1410
11
0.04 M CTATOS 236 243 344 5.8 x 10-3 718 1580 1.1x 10-2 3840 1.7 x lo-* 5900 4470 2.0 x 10-2
1650 1310 1310 925
In Table I1 data are presented for the relative viscosity of 0.09 M CTAB in 0.16 M NaBr a t different alcohol concentrations. As can be seen, these results also depend on the alcohol chain length. In fact, a high ethanol concentration produces only a slight reduction in relative viscosity, because ethanol, being highly water soluble, has little effect on micellar size. As Zana pointed out for a similar system,2ethanol is solubilized mainly in water and in the micellar palisade layer, inducing a surface charge separation which brings about a reduction in micellar size. For propanol a slight viscosity increase appears. For longer chain alcohols this effect is, as expected, even more pronounced; viscosity values pass through a maximum, just prior to the point where phase separation is reached. Table I11 presents K,,relative viscosity, and micellar hexanol concentrations at two CTATOS concentrations: 0.018 M, below the second cmc, and a t 0.04 M. At low surfactant concentration, K , values increase with hexanol concentration and then go through a maximum, but relative viscosity values are practically constant. Above the second cmc, a t 0.04 M CTATOS, K, decreases with hexanol concentration. However, the micellar hexanol concentration always increases with total alcohol concentration. Relative viscosity data increase and pass through a maximum. Thus, hexanol behaves similarly in those systems which consist mainly of cylindrical micelles.
In 0.036 M CTATOS and Different Hexanol Concentrations
[C~HI~OH], X102 M
[TOS-I, x103 M
0.54 1.0 2.2 3.0 3.8
6.2 6.5 7.3 7.9 8.5
Data for the concentration of free tosylate, measured with an ion selective electrode, are presented in Table IV for two cases: (a) a t different CTATOS concentrations and (b) a t constant 0.036 M CTATOS and different hexanol concentrations. From these results, the dissociation degree Q for CTATOS is 0.11, a value similar to that previously found20by other methods. A t constant 0.036 M CTATOS, free TOS- increases with added hexanol. This fact obviously reflects an increase in the micellar dissociation degree. In summary, above the second cmc, inclusion of hexanol in the micellar phase produces an increase in the micellar size and also in dissociation degree. At higher alcohol concentration, this latter effect will dominate, leading to a decrease in micellar size as reflected by the reduction in relative viscosity values. All of the above results suggest that the number of alcohol molecules incorporated per micelle (Na) is a very important parameter. This number can be determined if the aggregation numbers (N) for CTAB in 0.16 M NaBr and for CTATOS are known. N can be estimated in a approximate way, from the Vlu ratio, where Vis the volume of the cylindrical micelle and u the monomer volume. V can be obtained from the total length (L)of the cylindrical micelleel5 by the following equation:
V = (26.1)2nL
(7)
where 26.1 8( is the monomer length. In a similar way, the monomer volume u can be obtained by multiplying the area per ionic head grou (40 A2 given by Zana26)by the monomer length: 26.1 &l9 In this way, we estimate a value of N of 4000 for CTAB in 0.16 M NaBr, and of 8000 for CTATOS. Ikeda27gives a value of 10 000 for CTAB in 0.5 M NaBr, but he states that the molecular weight of rodlike micelles increases rapidly with increasing NaBr concentration. Assuming a constant micellar aggregation number, N , can be calculated from the micellized alcohol concentration. For hexanol and pentanol, results indicate that N , becomes so high that the micelles become "mixed micelles", bringing about a change in the dissociation degree and size. On the other hand, for 0.0018 M CTATOS, below the second cmc, where N is about 60-80, N , is also smaller than in 0.04 M CTATOS. This would explain the differences in viscosity behavior shown in Table IV. ~~~~
~
~
~~
(26) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (27) Imae, T.;Kamiya, R.; Ikeda, S.J. Colloid Interface Sci. 1985,108,
215.
26 Langmuir, Vol. 8, No. 1, 1992
It can be concluded that, in spherical micelles, incorporated hexanol does not induce as great an increase in size as when it is incorporated into cylindrical micelles. The micellar phases have a maximum in the long-chain alcohol solubilization ability near the point of a phase separation.
Acknowledgment. This work was ypported by the Fondo Nacional de Investigaci6n Cientifica y Tecnol6g-
Gamboa et al. ica (FONDECYT) Grant 0991, by the Departamento de Investigaci6n de la Universidad de Chile, and also by TWAS grants. We thank M. Luz Pefia for technical assistance. Regietry No. CTAB,57-09-0;CTATOS, 138-32-9;NaBr,764715-6;pentanol, 71-41-0;hexanol, 111-27-3;propanol,71-23-8;butanol, 71-36-3; heptanol, 111-70-6;octanol, 111-87-5.
