J. Phys. Chem. 1989, 93, 7441-7445
7441
Physicochemical Properties of Aqueous Mixtures of Tetrabutylammonium Bromide and Anionic Surfactants. 1. Temperature-Induced Micellar Growth and Cloud Point Phenomenon Zhi-Jian Yu* and Guangzhi Xu National Laboratory for Structure Chemistry of Unstable and Stable Species, Institute of Chemistry, Academia Sinica. Beijing, 100080, China (Received: December 20, 1988; In Final Form: April 26, 1989)
The solution behavior of the tetrabutylammonium tetradecyl sulfate surfactant has been studied by viscosity, surface tension, and conductivity measurements. A new type of micellar size change, that is, temperature-induced micellar growth, is obtained and interpreted qualitatively in terms of the conformational energy of the micellar surface layer, which is brought about from the increase of the gauche conformation of the butyl chains at the micellar surface layer. The micellar solutions exhibit a cloud point phenomenon. Above the cloud point, there is a surfactant phase with a small amount of water which is separated from the solution. The cloud point increases with the decrease of micellar concentration, the addition of inorganic salt, and the reduction of the ratio of tetrabutylammonium bromide to sodium tetradecyl sulfate. The Occurrence of the cloud point phenomenon is accounted for in terms of the penetration of the butyl chains at the surface layer of one micelle into another due to the hydrophobic effect.
Introduction The knowledge of micellar behavior and cloud point phenomena is of both practical and theoretical interest, since the solubilizing power of surfactants depends on their state of aggregation,' and homogeneous solutions are always demanded in the most practical use of surfactants. One may find from the literature that there are two classes of surfactants, the solution behaviors of which are in violent contrast. The first class is the nonionic surfactants, the micellar size of which increases (or at least does not decrease) with the increase of temperature. Such systems always have cloud point properties; Le., on heating the aqueous solution to a certain temperature, the homogeneous solution separates into two coexisting aqueous phases.2-12 This behavior has been attributed to a dehydration of the hydrophilic group of the surfactant with increasing temperature.I2 The ionic surfactants belong to the second class in which the cloud point phenomenon is rarely observed.13 It is generally believed that the size of the ionic micelles reduces when the temperature is r a i ~ e d . ' ~ -Missel ~ ~ et al. have proposed a ladder model to describe successfully the sphere-to-rod transition in the sodium alkyl sulfate micellar systems.I5J6 They deduced from their model that a large enthalpy decrease due to the reduction in area of the micellar core/water interface is completely responsible for the growth of micelles as a function of temperature. Such enthalpic contribution to the hydrophobic effect has been suggested in other s t ~ d i e s .The ~ ~exothermic ~~~ heat of reaction of micellar growth is found in other single ionic surfactant systems's-20 and in the mixed systems of cationic and anionic surfactants.*I In this paper, we report a new micellar growth behavior, Le., micellar growth with increasing temperature, and to report a cloud point phenomenon for solutions of the ionic surfactant tetrabutylammonium tetradecyl sulfate. Experimental Section The sodium tetradecyl sulfate (C14SNa) was a commercial research grade product from Beijing Chemical Co. and was recrystallized from ethanol four times. A surface tension measurement gave a sharp break at a critical micelle concentration M. No surface tension minimum was found, which of 2.0 X implies that no surface active impurities were contained in C14SNa. Reagent grade tetrabutylammonium bromide (B4NBr) from Beijing Chemical Co. was twice recrystallized from petroleum ether-acetone mixtures. The tetrabutylammonium tetradecyl *To whom correspondence should be addressed. Present address: Department of Chemical Engineering, Auburn University, Auburn, AL 36849.
0022-3654/89/2093-7441$01.50/0
sulfate (B,NC14S) was obtained by the following procedure: equimolar amounts of B4NBr and C14SNawere mixed in water, the B4NC,4Swas extracted from water by benzene, the benzene phase was washed with water three times, and after evaporation of the benzene, the product was dried under vacuum for 1 day. The B4NCI4Sthus obtained was an oily liquid at room temperature and a crystalline white powder at 0 "C. Reagent grade NaBr from Beijing Chemical Co. was heated on an evaporating dish for 6 h and stored in a desiccator until use. Water used to prepare all solutions was twice distilled from alkaline permanganate. Surface tension was measured by means of a drop weight technique. Viscosity was measured with a Ubbelohde viscometer and a Controvers 30 low-shear viscometer which permits the measurement in .a shear rate range from 0.02 to 130 s-I. The (1) Winson, P. A. Solvent Properties of Amphiphilic Compounds; Butterworths: London, 1954. (2) Balmbra, R. R.; Clunie, J. S.; Corkill, J. M.; Goodman, J. F. Trans. Faraday SOC.1962, 58, 1661; 1964,60, 979. (3) Corkill, J. M.; Goodman, J. F.; Walker, T. Trans. Faraday Soc. 1967, 63, 759. (4) Ottewill, R. H.; Storer, C. C.; Walker, T. Trans. Faraday SOC.1967, 63, 2796. (5) Robson, R. J.; Dennis, E. A. J . Phys. Chem. 1977,81, 1075. (6) Paradies, H. H. J . Phys. Chem. 1980, 84, 599. (7) Triolo, R.; Magid, L. J.; Johnson, J. S.;Child, H. R. J . Phys. Chem. 1982,86, 3689. (8) Corti, M.; Minero, C.; Degiorgio, V. J . Phys. Chem. 1984, 88, 309. (9) Cebula, D. J.; Ottewill, R. H. Colloid Polym. Sci. 1982, 260, 1118. (10) Ravey, J. C. J. Colloid Interface Sci. 1983, 94, 289. (1 1) Brown, W.; Johnsen, R.; Stilbs, P.; Lindman, B. J . Phys. Chem. 1983, 87, 4548. (12) Hayter, J. B; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (13) Appell, J.; Porte, G. J. Phys. Lett. 1983, 44, 689. (14) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1976, 80, 1076. (15) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Young, C. Y.;Carey, M. C. J . Phys. Chem. 1980, 84, 1044. (16) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M.C. J . Phys. Chem. 1983, 87, 1264. (17) Porte, G.; Appell, J.; Poggl, Y. J . Phys. Chem. 1980, 84, 3105. (18) Porte, G.; Appell, J. J. Phys. Chem. 1981, 85, 2511. (19) Dorshow, R.; Briggs, J.; Bunton, C. A,; Nicoli, D. F. J . Phys. Chem. 1982,86, 2388. (20) Zhou, Z.-K.; Yu, Z.-J. Acta Physico-Chim. Sin. 1985, 1, 141. (21) Yu, Z.-J.; Zhao, G.-X. J. Colloid Interface Sci. 1989, 130, 414, 421. (22) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 711. (23) Rohde, A,; Sackmann, E. J. Colloid Interface Sci. 1979, 70, 494. (24) Tanford, C. The Hydrophobic Effect: Formation of Micelles & Biological Membranes; Wiley: New York, 1980. (25) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J . Phys. Chem. 1976, 80, 905.
0 1989 American Chemical Society
7442 The Journal of Physical Chemistry, Vol. 93, No. 21, I989 50
b
Yu and Xu
\
-3
-4 log c
Figure 1. Surface tension of B4NC14Ssolutions as a function of logarithm concentration: (1) 20 OC, (2) 40 OC without NaBr; (3) 25 O C with the addition of 0.2 M NaBr.
temperature in the measurements was controlled and measured to within 0.05 O C . The cloud point temperatures were determined by a sudden appearance of turbidity in the solution by the naked eye on heating the solution at a rate of 0.05 OC-min-l.
Results and Discussion Surface Adsorption and Cmc. Plots of the surface tension versus the logarithm of concentration of the aqueous solution of B4NCI4Sat two different temperatures are shown in Figure 1. The cmc values taken from the breaking points of the curves are 0.256 and 0.260 mM for 40 and 20 "C, respectively. These data are essentially equal within experimental error. Figure 1 also shows the sutface tension curve for an equimolar mixture of B4NBr and C14SNain a constant NaBr concentration of 0.2 M at 25 OC. The cmc value obtained from this curve is 0.089 mM. According to the Gibbs adsorption equation for dilute solution,26 the total amount of the surface excess of tetrabutylammonium ion B4N+ (r+)and tetradecyl sulfate ion CI4S- (r-)at the cmc can be calculated from
r++ r- = - R T d-f d In C where y is the surface tension, C is the surfactant concentration, R is the universal gas constant, and Tis the absolute temperature. r+and r- are equal for the system without NaBr. By fitting the curves of Figure 1 to eq 1 we get I'+ = 1.96 X and I'+ + r- = 5.43 X mol.cm-2 for the systems without NaBr and with 0.2 M NaBr, respectively. The surface area occupied per B4N+ion deduced from r+is 84 A2 for the system without NaBr, which is considerably smaller than that of 154 A2 obtained from a geometrical calculation for an entirely stretched conformation of B4N+, and that of 176 A2 obtained from the experimental measurement of octyldecyltributylammonium bromide.27 If the conformation of B4N+ ion in the surface adsorption layer is such that the butyl chains on B4N+ are bent in directions either toward the bulk phase or toward the surface, the surface area occupied by one ion is about 64 AZfor B4N+ and about 39 A2 for the -SO4group from a geometrical calculation. Thus the limiting area occupied by one B4N+ ion in the mixed surface layer of B4N+ and CI4S-is about 103 A2, which is quite consistent with the experimental value. Therefore, such conformation of B4N+ at the surface adsorption layer is rational; in other words, the butyl chains on B4N+ on the surface adsorption layer must be bent to a high degree. The average surface area occupied per ion (B4N+ and C14S-) calculated from the total amounts of the surface excess r++ ris 3 1 A2 for the system with 0.2 M NaBr. This value is too small to be allowed for the geometrical packing of equimolar amounts (26) Lang, H. Kolloid-2. 1957, 153, 155. ( 2 7 ) Brashier, G . K.; Thornhill, C. K. Proc. La.Acad. Sci. 1968, 31, 102.
35
25
45
1 ('CC)
Figure 2. Plot of the relative viscosity of B4NCI4Ssolutions against temperature. The micellar concentrations in curves 1-5 are 60, 50, 40, 30, and 20 mM, respectively. The arrow positions indicate the cloud points of the corresponding solutions. The dotted line is calculated from curve 1 by using Debye's equation in the absence of the viscosity anomaly term.
of B4N+ and CI4S- in the surface adsorption layer. It may, therefore, be certain that some of the B4N+ in the surface adsorption layer is replaced by the inorganic ion Na+. Viscosities of the Micellar Solutions. Figure 2 shows the dependence of the relative viscosity vrelupon the temperature at various micellar concentrations determined by a Ubbelohde viscometer. Since the critical micelle concentration is very low for the solutions investigated, the viscosity at the cmc is replaced by that of pure water in calculating the relative viscosity. The 60 mM solution was also measured by a Controvers 30 low-shear viscometer. It is found that the viscosity of this solution has no shear rate dependence in the shear rate range from 0.02 to 130 s-* and is consistent with that measured by the Ubbelohde viscometer within experimental error. Therefore, the system studied behaves as a Newtonian fluid, and the data determined by the Ubbelohde viscometer represent the true values of the viscosities. From Figure 2 it can be seen that vrel increases with increasing temperature and the curves a t high concentration are concave upward. The slope of vrel versus the temperature at a fixed temperature decreases with a decrease in micellar concentration, and at the lowest concentration studied (20 mM) no r],, change may be perceived over the whole temperature range. We did not try to measure the viscosities in the concentration range below 20 mM because the low ionic strength probably induces electroviscosity effects. Several authors have suggested that the viscosity of a binary liquid mixtures should show a strong characteristic temperature dependence as the critical solution temperature is approached.2g30 It is clear from their data that the viscosity anomaly also exists in a concentration range below critical concentration when the temperature approaches the phase separation temperature boundary. As will be seen below, our ionic surfactant system possasses a cloud point phenomenon. If the micellar solution could be regarded as a binary liquid mixture, then there will be two possibilities for the increase of relative viscosity of a given micellar solution as the temperature approaches the cloud point. One is the increase of micellar size, and the other is the micellar concentration fluctuation. (28) Debye, P.; Chu, B.; Woermann, D. J . Polym. Sci. 1963, IA, 249. (29) Rigler, J. K.; Wolf, B. A.; Breitenbach, J. W. Angew. Mukromol. Chem. 1975, 57, 15. (30) Izumi, Y . ;Miyake, Y. Phys. Reu. A 1977, 16, 2120.
B,NBr-Surfactant
The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 1443
Mixtures
In order to make a crude estimate of the possible effect of the viscosity anomaly on qd, we assume that Debye’s equation,28which was tested in lower consolute temperature can be used in our system, using the cloud point instead of the critical temperature T, - T T, - T log q = A 7B log 7 +C (2)
30
,
+
1,
1,
where q is the absolute viscosity in cP, T, is the critical demixing temperature, and A , B, and C are constants. The term A(T, 7‘)/ T, represents the normal temperature dependence of the viscosity and the term B log ( ( T , - T)/T,) is the contribution of the viscosity anomaly due to the critical phenomenon. Fitting the viscosity data of the solution a t 60 mM, where the temperature dependence of the viscosity is the largest among the samples studied, to eq 2 by the method of least squares, we obtained A = 0.45, B = -0.008, and C = 0.36. By subtracting the viscosity anomaly term from the measured viscosity data, we obtained the normal viscosities which are plotted in Figure 2 as the dotted curve. It may be seen from Figure 2 that the contribution of the viscosity anomaly to the total value of viscosity is rather small and that the relative viscosity after subtracting this viscosity anomaly term still increases with an increase of temperature. This result does not mean that the viscosity anomaly exists in our concentration range, which is far from the critical concentration (as will be seen below), but it does mean that the effect of the viscosity anomaly is unimportant even if it exists. Thus it may be concluded that the increase of qrrlwith increase of temperature reflects the growth of micellar size. The arrows in Figure 2 indicate the cloud point temperatures of the corresponding solutions. The viscosities were measured in temperature ranges from 20 OC up to temperatures 0.3 OC below the cloud point. It is found that qreldoes not diverge logarithmically as the temperature approaches the cloud point. For the solution at 20 mM, qrel is essentially unchanged, even at the temperature 0.3 OC below the cloud point. In the companion paper3I it is shown that the viscosity of the micellar solution decreases when the temperature approaches the cloud point. Therefore, the problems encountered in interpreting light-scattering data due to critical-like behavior are not seriously present in interpreting the viscosity data. We use the Huggins3’ and Kraemer33equations to calculate the intrinsic viscosities of the micelles VSP/(C-
cmc) = [SI
+ K[7I2(c- cmc)
In qrel/(c- cmc) = [q] - K’[qI2(c- cmc)
(3) (4)
where qsp is the specific viscosity, [ q ] is the intrinsic viscosity, K and K’ are two hydrodynamic interaction coefficients known as the Huggins and Kraemer constants, respectively. When qsp < 1, In qrel can be expanded in a series to yield In
9,el
= In (1
+ Vsp)
=
qsp - VSp2/2
(5)
Substituting eq 3 into eq 5, we find In qrel = [ql(c - cmc)
+ ( K - f/2)[q12(c- cmc)’
(6)
Comparing eq 4 with eq 6, we have K
+ K‘=
1/2
(7)
From eq 3, 4, and 7 we finally obtain
[TI’
= 2(~sp- In qrcl)/(c - c ~ c ) ’
(8)
As can be seen, most of the viscosity data in Figure 2 satisfy the condition of qsp < 1, and we calculated [q] from these data in Figure 2 using eq 8. Figure 3 shows the dependence of [q] on the micellar concentration at four different temperatures. The (31) Yu,Z.-J.; Zhou, Z.-K.; Xu, G.-Z. J . Phys. Chem., following paper in this issue. (32) Huggins, M. L. J . Am. Chem. SOC.1942,64, 2716. (33) Kraemer, E. 0.Ind. Eng. Chem. 1938,30, 1200.
15
a-om
25
35
(mg/ml)
Figure 3. Dependence of intrinsic viscosity on the micellar concentrations of B4NCI4Ssurfactant. The temperatures for curves 1-4 are 20,25, 30, and 35 OC, respectively.
Huggins constant K can thus be deduced from [ q ] by using eq 3. It is found that K is independent of the temperature and of the micellar concentration, even for the two data points in Figure 2 where qsp is slightly larger than 1. The average value of K obtained from all the experimental data in Figure 2 is 0.334 f 0.012. Maron et al. have proposed an equation for calculation of [q] in the viscosity range qrcl = 1.1-1.9 (eq 14 in ref 34). It is interesting to note that eq 8 is approximately equal to their equation as K i= 1/3 for our system. From Figure 3 it can be seen that [q]increases with increasing micellar concentration at constant temperature. In ref 31 it is shown that the micellar shape of B4NCI4Sin the presence of 0.2 M NaBr is rodlike. This kind of micellar shape is likely to hold in the absence of NaBr. Since the micellar size is proportional to [q] for a given shape of nonspherical micelle, it may be said that the micellar size increases with increasing micellar concentration. Such type of concentration dependence of micellar size is common in the l i t e r a t ~ r e . ’ ~Figure ’ ~ 3 shows that on increasing the temperature or decreasing the micellar concentration, [ q ] approaches a limiting value of 9 mL/g. This minimal [q] value is larger than [ q ] = 2.5 for spherical particles predicted by Einstein’s equation, indicating a nonspherical shape of the micelles. Interpretation of the Temperature-Induced Micellar Growth. Since this type of micellar size change with temperature runs counter to that of ionic surfactants reported in the literature where an enthalpy reduction in the micellar growth process is evidenced,14-23there must exist another enthalpy contribution in addition to the hydrophobic one, and the change of this enthalpy in the micellar growth process must be positive. The difference between our system and an ordinary ionic surfactant system is that the counterion of the micelles in our system, B4N+, possesses four short hydrocarbon chains with a symmetry structure. Such an organic ion having 16 hydrocarbons has large hydrophobic, and some of these short chains may penetrate into the micellar core due to the hydrophobic effect. On the other hand, the geometric restriction makes it infeasible for all four chains to penetrate into the micellar core. At least two more chains should be located at the micellar surface layer in contact with water, which forms an iceberg structure around the chains. When the micelle grows, the surface area of the micelle per surfactant ion and B4N+ occupied is reduced, which makes the possibility of a fully extended conformation of the hydrocarbon chains on B4N+ at the micellar surface layer more and more difficult; hence, the degree of bending on these short chains increases. Figure 4 is a simplified diagram of B4NCI4Smicelles. Such a conformation of B4N+ is qualitatively consistent with that in the water-gas surface adsorption layer. Generally, two directions may be chosen for bending the butyl chains: one is toward the water phase and the other penetrates the micellar core. In (34) Maron, S. H.; Reznik, R. B. J . Polym. Sci. 1969, 2 4 309.
7444
The Journal of Physical Chemistry, Vol. 93, No. 21, 1989
Yu and Xu
50
t
40 0 0
v
30 oore
Figure 4. Representation of B4NC14Smicelles.
the latter case, the icebergs on the butyl chains penetrating into the micellar core will break down, and thus the entropy of the system increases. This entropy increase is a driving force for the micellar growth. On the other hand, the bending of the butyl chains will decrease the trans conformation and increase the gauche conformations of higher energy. We call this enhanced energy due to the bending of hydrophobic chains at the micellar surface layer the conformation energy of the micellar surface layer. Obviously, the existence of this energy is a factor which acts against micellar growth. The endothermic nature of the micellar growth of our system may be interpreted as due to the fact that the contribution of the conformation energy of the micellar surface layer is larger than the contribution of hydrophobic enthalpy brought about from the shrinking of the micellar core-water interface. When the micellar surface layer is more crowded with B4N+ at higher surfactant concentration (see below), the degree of bending of the butyl chains in the micellar growth process should be larger, leading to a higher conformation energy of the micellar surface layer. Consequently, the micellar size should grow more rapidly with an increase of temperature, which is the case shown in Figure 2 . Though the micellar growth pattern of our ionic surfactant is similar to that of nonionic surfactants, the physical origins of the two processes are different. In nonionic surfactant systems the micellar growth with temperature is due to the breaking down of structured water around the hydrophilic part of the surfactants. In our system some of the iceberglike water structure surrounding the butyl chains is also broken down during micellar growth, but this process is probably energetically favorable. It is evidenced calorimetricallyz4that the transfer of alcohol molecules from water to pure alcohol liquid is endothermic, while the transfer of large hydrocarbon molecules from water to pure liquid hydrocarbon is exothermic. The exotherm in the latter case is due to the formation of hydrogen bonds by the water released from the iceberglike structure, which is consistent with the enthalpy decrease from the reduction of the area of the micellar surface layer when the micelle grows. Cloud Point Phenomenon. B4NC14Scan be solubilized in water to form a homogeneous transparent solution at low temperature. The solution becomes opalescent on heating and then separates into two transparent liquid phases with a clear phase boundary between them on standing for several days. The critical phase separation temperature is called the cloud point. Curve 1 in Figure 5 shows the experimentally determined cloud point Tdas a function of the concentration of pure B4NC14Ssurfactant. It is found that Td decreases with increasing surfactant concentration. We have measured the surfactant concentration in both the upper and lower phases by taking out given amounts of the two phases separately, evaporating off the water, drying under vacuum for 2 days, and then weighting the remaining surfactant. The data points of the lower phase are represented by the cloud point curve. The result of the upper phase is plotted as curve 2 in Figure 5. It shows that the surfactant concentration of the upper phase is about 91%, which indicates that the upper phase essentially consists of the surfactant with only a small amount of water solubilized in or solvated to the surfactant. To examine whether such a phase separation phenomenon is the property of B4NC14Sor is due to B4N+ (or C,4S-) ion alone, we measured the concentrated solutions of pure B4NBr or CI4SNa. We did not find the phase separation phenomenon in the tem-
m
I"
z
20
10
20
40
Conoentmtlon
60
80
k by weight
Figure 5. Phase diagram of B4NC14Sin water: curve 1, cloud point, above which is a two-phase region; curve 2, surfactant-rich phase
boundary.
2o
t I
I
I
I
1
20
40
60
80
Figure 6. Cloud point of B4NCI4Ssolutions as a function of B4N+ion concentration at various salt concentrations: curve 1,0.2 M NaBr; curve 3, no additional salt; curve 2, 0.2 M Nat and 25 m M CI4S-.
perature range from 0 to 100 OC. Thus, it is clear that the cloud point phenomenon is due to the behavior of B4NC14S. Figure 6 shows the cloud point as a function of B4N+ concentration for various inorganic salt solution concentrations. The solutions represented by curve 1 were prepared by mixing equimolar amounts of B4NBr and C14SNawith the total concentration of NaBr kept constant at 0.2 M by adding NaBr. The solutions for curve 2 were prepared by dissolving B4NBr in a solution of 25 mM CI4SNaand 175 mM NaBr and diluting the result with the solution. Curve 3 was free of NaBr. It is found from curve 2 that the cloud point Td decreases with increasing B4N+ concentration. Since the surfactant concentration is constant, the content of B4N+ in the micelles will be increased with the increase of B4N+concentration. Thus, Td is inversely proportional to the content of B4N+ in the micelles. Curve 1 gives further evidence for this point. It intersects with curve 2 at 25 mM B4N+, above which Td is higher than for curve 2, while below which the situation is just the opposite. It is clear that, under the same concentration of the counter ions Na+ and B4N+, the larger the concentration of C14S- is (curve 1 on the right-hand side of the intersection point), the lower will be the content of B4N+ in the micelles. From curve 1 and curve 3 one may see that the addition of NaBr invariably increases Td, and this increase in Td becomes larger at lower micellar concentration. Such a behavior of Tdwith NaBr may be attributed to the competition for adsorption on the micellar structure between the two counterions B4N+ and Na+. When
The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 7445
B4NBr-Surfactant Mixtures
I
I
’
10
20
30
c (“1
Figure 7. Specific conductivity as a function of concentration of B4NC14S surfactant a t 30 OC without NaBr.
NaBr is added, the inorganic counterion Na+ is adsorbed onto the micellar surface. This decreases the surface charge density of the micelles and consequently reduces the attraction force between B4N+ and the micelles. As a result, the content of B4N+ in the micelles is decreased. We have performed conductivity measurements on the B4NC14S solutions at 30 OC without NaBr to examine whether the micelles are charged. Figure 7 shows the dependence of the specific conductivity K on the surfactant concentration. The cmc value taken from the break point of the curve is about 0.21 mM which is quite consistent with that obtained from the surface tension measurement within experimental error. From Figure 7 it can be seen that K increases with increasing concentration above the cmc, which clearly indicates that the micelles are charged. The slope of K versus micellar concentration, which is a measure of the degree of ionization of the micelles, decreases progressively with increasing micellar concentration. For example, the slopes at the initial micellar concentration range and at about 30 mM are 29.2 and 14.7 mQ cm-’ M-I, respectively. It may be concluded, therefore, that the content of B4N+ in the micelles increases with the increase of micellar Concentration. Since the micellization ability of CI4S- is greater than that of B4N+ (if existing), it may be expected that the micelles are negatively charged. Thus the monomer concentration of B4N+ in the water phase increases with increasing micellar concentration due to the nonequimolar proportion of B4N+ and CI4S-in the micelles. In other words, the chemical potential of the monomer B4N+ (or the tendency of B4N+ to enter the micelles) is increased, resulting in an increase of the content of B4N+ in the micelles. Interpretation of Cloud Point Phenomenon. Hayter et al. have explained the origin of the cloud point phenomenon in nonionic surfactant systems.I2 They suggested that the phase separation of the micellar solution is due to the van der Waals attractive energy between the micelles, and there exists a barrier between the micelles which is brought about by the presence of at least a monolayer of highly structured water at each micellar surface, which is weakened with increasing temperature. In the ionic surfactant systems the situation is more complex. Since the micelles are charged, there must be an electrostatic repulsion between the micelles in addition to the van der Waals attraction force. If one considers these two forces alone, the cloud point would be decreased on adding the electrolyte, since the electric repulsion force is weakened by the electrolyte. The experimental result shows (Figure 6 ) that the cloud point is changed in just the contrary direction. Moreover, in the systems of equimolar mixture of cationic and anionic surfactants, the micelles are essentially neutral, giving a very much lower electrical repulsion force between the micelles;*I yet, in this case no cloud point phenomenon was found. The above results strongly suggest that the change of electric repulsion force is not very important for the occurrence of the cloud point phenomenon and there must be other forces
between the micelles besides these two forces. Mikulich reported35 that the degree of solvation increases with increasing surfactant head-group size, and the adsorption capacity of each alkyltributylammonium salt molecule could be up to 20-30 water molecules. The solvation which is a universal phenomenon for all ionic micellar systems is therefore a factor acting against the Occurrence of the cloud point phenomenon. We suggest that, in addition to the van der Waals attraction, the electrical repulsion, and the solvation layer, there may exist another interaction factor between micelles which may be critical for the Occurrence of the cloud point phenomenon. Since there are free butyl chains on the micellar surface which do not penetrate into the micellar core, they may have the chance to penetrate into another micelle. In other words, B4N+ may act like a bridge linking two micelles together. If the penetration length of the butyl chain is confined to the first two hydrocarbons, the free energy reduction will be about 3 k~a1.mol-l.~~ This value is about half of the typical hydrogen bond energy. As a result of this hydrophobic effect, the micelles may experience more close contact. The above four factors affecting the intermicellar interaction may be divided into two classes. One includes van der Waals attraction and the penetration effect of the butyl chains, the effect of which is to attract two micelles together, inducing a collapse of the micelles. The other includes the electrostatic repulsion and the structured water layer around the ionic head groups at the micellar surface, which prevents the micelles from closely contacting and makes the micelles stable in solution. A combination of these two classes which oppose each other results in an energy barrier between the micelles. As the temperature is raised, the barrier is lowered due to the progressive decrease of the structured water on the micellar surface layer. At a cloud point temperature the barrier has become sufficiently lower that a collapse of the micelles has occurred. Since the number of free butyl chains on the micellar surface is proportional to the content of B4N+ in the micelles, the number of butyl “bridges” between two micelles is increased with an increase in the content of B4N+ in the micelles. As a result, the hydrophobic effect of the butyl chains between two micelles becomes stronger; i.e., the energy barrier between the micelles becomes lower. This is in favor of the occurrence of the cloud point phenomenon, as can be seen in Figures 5 and 6.
Conclusions We have reported from viscosity measurements a new type of micellar growth, i.e., micellar size increase with increasing temperature for the tetrabutylammonium tetradecyl sulfate surfactant without additional inorganic salt. This unusual micellar behavior is interpreted in terms of the conformation energy of the micellar surface layer which is brought about by an increase of gauche conformation of the butyl chains at the micellar surface layer when the micelles grow. The viscosity measurement also shows that the micelles grow with increasing micellar concentration. The micellar solutions exhibit a cloud point phenomenon. Above the cloud point a surfactant phase with a small amount of water in it is separated from the solution. The cloud point is increased with a decrease of micellar concentration, the addition of inorganic salt, or the reduction of the ratio of tetrabutylammonium bromide to sodium tetradecyl sulfate. The change of the content of tetrabutylammonium in the micelles is responsible for the variation of the cloud point. The Occurrence of the cloud point phenomenon is accounted for in terms of the penetration of the butyl chains of a given B4N+ ion into the surface layers of two neighboring micelles due to the hydrophobic effect. Acknowledgment. We thank Dr. Z.-K. Zhou for helpful discussions and valuable suggestions for this work. Registry No. B4NBr, 1643-19-2; B4NC14S, 122271-88-9; C14SNa, 1191-50-0.
(35) Mikulich, A. V.; Popov, A. V.; Hoegfelt, E.; Soldatov, V. S. Dokl. Akad. Nauk BSSR 1980, 24, 610.
