2186
Langmuir 1996, 12, 2186-2189
Surface Tension Studies of Cetyltrimethylammonium Bromide-Bile Salt Association M. Swanson-Vethamuthu, M. Almgren,* P. Hansson, and J. Zhao Department of Physical Chemistry, Uppsala University, P.O. Box 532, S-751 21 Uppsala, Sweden Received October 10, 1995. In Final Form: January 19, 1996X Surface tension measurements have been performed on solutions containing cetyltrimethylammonium bromide and a bile salt, either sodium cholate or desoxycholate, in 50 mM NaCl. For the individual ionic surfactants the interfacial area per surfactant molecule (As) and critical micelle concentration (cmc) were determined. For bile salt solutions a break in surface tension before the cmc is explained as an indication of a change in packing of the anions at the interface from a flat (As > 150 Å2) to an upright orientation (As ≈ 41-45 Å2) and is compared with results from the monolayer studies of Ekwall and Small. For systems containing binary mixtures of the oppositely charged surfactants, the experimentally determined mixed cmc (C*) was used to estimate the mixed micelle composition and the molecular interaction parameter, β, using the treatment of Rubingh for nonideal mixtures. The mixtures showed significant deviation from ideal mixing, giving an average β ) -4 and -2.7 for the CTAB-NaC and CTABNaDOC systems, respectively. For both bile salts, the mixed cmc has a minimum when the bile salt fraction in the mixed micelles is close to 0.3, suggesting that a particularly favorable packing of the micelles is obtained in this composition range. However, using the average value of the interaction parameter in the theory of Rubingh, the cmc values of the systems are predicted within the precision of the experimental findings.
Introduction Interactions in mixtures of surface active molecules at the solution/air interface and in micelles and other aggregates have practical importance and have received theoretical attention. Treatments based on regular solution theory1-5 use an interaction parameter, β, to measure the interactions between surfactants in a mixed monolayer at the interface or in a mixed micelle at the cmc. The phase behavior and aggregation in mixtures of cetyltrimethylammonium bromide (CTAB) with sodium cholate (NaC) or desoxycholate (NaDOC) in solution have been studied extensively.6-10 In earlier work we have presented results showing large differences in the behavior of the CTAB-NaC and CTAB-NaDOC systems in solution under various mixing conditions well above the cmc’s.11-14 The two systems are strongly nonideal both at and below the cmc, owing to the strong interaction between the oppositely charged surface active ions and the dis* To whom correspondence should be addressed. E-mail address:
[email protected]. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: Knoxville, 1979; pp 337-354. (2) Holland, P. M.; Rubingh, D. N. J. Phys. Chem. 1983, 87, 1984-1990. (3) Rubingh, D. N.; Holland, P. M. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker, Inc.: New York, 1991; pp 141-162. (4) Hoffmann, H.; Po¨ssnecker, G. Langmuir 1994, 10, 381-389. (5) Rosen, M. J. Prog. Colloid Polym. Sci. 1994, 95, 39-47. (6) Barry, B. W.; Gray, G. M. T. J. Colloid Interface Sci. 1975, 52, 327-339. (7) Barry, B. W.; Gray, G. M. T. J. Colloid Interface Sci. 1975, 52, 314-325. (8) La Mesa, C.; Khan, A.; Fontell, K.; Lindman, B. J. Colloid Interface Sci. 1985, 103, 373-389. (9) Jana, P. K.; Moulik, S. P. J. Phys. Chem. 1991, 95, 9525. (10) Wu, K.; McGown, L. B. J. Phys. Chem. 1994, 98, 1185-1191. (11) Vethamuthu, M. S.; Almgren, M.; Mukhtar, E.; Bahadur, P. Langmuir 1992, 8, 2396-2404. (12) Vethamuthu, M. S.; Almgren, M.; Brown, W.; Mukhtar, E. J. Colloid Interface Sci. 1995, 174, 461-479. (13) Vethamuthu, M. S.; Almgren, M.; Bergensta˚hl, B.; Mukhtar, E. J. Colloid Interface Sci., in press. (14) Vethamuthu, M. S.; Almgren, M.; Karlsson, G.; Bahadur, P. Langmuir, in press.
S0743-7463(95)00856-0 CCC: $12.00
similar molecular structures of the hydrophobic moieties in the participating components. Fluorescence quenching studies11,13,14 on the aggregation behavior have clearly demonstrated that the micelles were small for the CnTAB-NaC system over the entire concentration range studied but tended to grow into rodlike cylindrical micelles for the CnTAB-NaDOC system close to the equimolar concentrations where phase separation into two isotropic phases occurred. No coacervation region is found with the trihydroxy bile salt, NaC, thus showing a major difference in phase behavior.12 Experiments based on cryo-TEM and viscosity consistently show differences in micelle microstructure and relative viscosity between the two systems.14 Static and dynamic light scattering studies12 show the presence of strong intermicellar interactions and large structures in the CnTAB-NaDOC system but give poor scattering intensity from the corresponding CnTAB-NaC mixtures. The previous studies11-14 have thus clearly shown a difference between the two bile salts with respect to their effects on the size and shape of the mixed micelles, where NaC promotes the formation of globular aggregates with high curvature, whereas NaDOC at not too high additions induces a formation of cylinder micelles. At high concentrations of an added electrolyte, however, NaC also gives long micelles. A viscosity maximum was found for mixed micelles with a NaC fraction of 0.17. A much more dramatic growth of the micelles occurs with NaDOC, and a strong viscosity increasesat high surfactant content even in a gel regionsis found centered around a bile salt fraction of 0.30 in the mixed aggregates. We have tried to rationalize these differences in terms of the mode of insertion of the bile ions in the aggregates, the main idea being that cholate ions predominantely adsorb flat onto the hydrophobic core/water interface of the micelle, promoting highly curved aggregates, whereas desoxycholate ions, with one OH group less, have a larger tendency to be inserted into the core, and allow aggregates of less curvature to form. The head group interactions between the oppositely charged surfactants are of course strongly attractive and would allow a tight packing at the surface. © 1996 American Chemical Society
CTAB-Bile Salt Association
Langmuir, Vol. 12, No. 9, 1996 2187
The interactions between the surfactants will of course also affect the micelle formation of the mixture, in particular the cmc. In this contribution we present the results from surface tension studies, from which the mixed cmc’s have been determined for some mixtures. These data are treated by the simple model for nonideal mixed micelles by Rubingh1,2 to provide values of the interaction parameter, β, which in a manner reminiscent of regular solution theory relates the interaction between the surfactants in the micelle to the activity factors
fi ) exp{β(1 - xi)2}
(1)
where fi is the activity factor of surfactant i and xi is its mole fraction in the micellar pseudophase. It is of interest to find how well the simple model can describe these systems and how the interaction parameter varies with the bile salt and with the composition of the micelles. For the pure bile salts the surface tension was found to change in an unusual way with concentration, suggesting a change of the packing or orientation of the bile ions on the air/water interface at concentrations below the cmc. The results are confronted with the classical monolayer studies by Ekwall et al.15 and by Small16 for various cholanic acids. We present experimental results on the surface tension of aqueous solutions of individual surfactants and binary mixtures at several mixing ratios under identical conditions. Experimental Section Materials. The surfactant cetyltrimethylammonium bromide (C16TAB) was purchased from Serva, purity >99%, and used as supplied. Cetyltrimethylammonium chloride (C16TAC) was prepared from C16TAB by ion exchange on a Dowex 1-X8 resin. The product was freeze-dried and stored in a desiccator. The sodium salts of cholic acid (NaC, Sigma; purity >99%) and of desoxycholic acid (NaDOC, Fluka; purity >99%) were used without further purification. NaCl purchased from Merck was used as supplied. All solutions were prepared in millipore water. Surface Tension Measurements. The surface tensions of the surfactant aqueous solutions were measured by a drop-volume technique. The principle of the surface tension apparatus used has been described by Tornberg.17 The volume of the solution is gradually extruded by a step-motor connected to an oscillator which generates the constant pulses. The rate of drop formation is controlled by an adjustable pulse rate, which was kept constant at 100 Hz for all the present measurements. Thus the present rate of drop formation (R) is 1.64 × 10-11 m3/pulse. From the measured pulse number P, i.e., the time required to form a complete drop of the surfactant solution, the drop volume (V) can be calculated by means of V ) RP. From the calculated value of r/V1/3 (here r is the radius of the tip and is equal to 1.75 × 10-3 m for the apparatus used) the correction factor ψ(r/V1/3) is drawn from the table presented by Wilkinson.18 Finally the surface tension (γ) can be determined by means of the following equation:
γ)
r2∆Fg 2[ψ(r/V1/3)]2
(2)
Here ∆F is the difference in densities of the two phases and g is the gravitational acceleration.
Results and Discussion (A) Surface Tensions of Bile Salt Solutions. In Figure 1 the surface tension vs concentration for solutions (15) Ekwall, P.; Ekholm, R.; Norman, A. Acta Chem. Scand. 1957, 11, 693, 703. (16) Small, D. M. In The Bile Acids; Nair, P. P., Kritchevsky, D., Eds.; Plenum Press: New York, 1971; pp 272-275. (17) Tornberg, E. J. Colloid Interface Sci. 1977, 60, 50. (18) Wilkinson, M. C. J. Colloid Interface Sci. 1972, 40, 14.
Figure 1. Plots of surface tension vs log[bile salt] concentration for [A] NaC and [B] NaDOC. The two curves are shown for each bile salt; unfilled symbols are for bile salts in 50 mM NaCl while filled symbols are for the same in 45 mM NaCl and 5 mM NaOH.
of the two bile salts in 50 mM NaCl is presented, as are results under alkaline conditions (5 mM NaOH and 45 mM NaCl). Although the free bile salts should be fully ionized in the neutral aqueous solution, the molecules both in the monolayer at the interface and in the micelles are in the acidic form to an appreciable extent19 (30%), as shown by the increase of the cmc on addition of NaOH. Furthermore, in the alkaline solution the surface tension remains larger and does not display the unusual increase after the cmc observed in the neutral solutions. This increase can be understood if the aggregated bile salt molecules are partly in the acid form. Hydrogen ions would then be adsorbed onto the micelles as they form. Micellization would then increase the pH and promote ionization of the bile acids in the interfacial monolayer and thus increase the surface tension toward the values of the fully ionized state. pH measurements in solutions above the cmc substantiate this interpretation. The pH value for the NaDOC salt in 50 mM NaCl was found to increase from 6.4 at the cmc (0.79 mM) to 7.4 at 10 mM NaDOC (in pure solvent, i.e. 50 mM NaCl, the pH value was 6.3). A similar but smaller increase in pH was found for the corresponding NaC system, which varied from pH 6.6 at the cmc (2.6 mM) to pH 7 at 10 mM. The lower change in pH found in the latter system is attributed to the smaller size and lower charge of the micelles, which therefore adsorb H+ ions more weakly than the much larger NaDOC micelles. A second unusual feature of the plots is the break to a steeper decay of the surface tension with concentration before the cmc. This break point remains in the alkaline solutions and is an indication of a tighter packing at the molecules after the break. From the Gibbs adsorption equation (eq 3) the area As per surfactant molecule at the interface can be calculated from the slopes of the two linear parts.
Γs )
1 1 ∂γ )As nRT ∂ log cs
(3)
Where Γs is the surface excess of the bile salt, R is the gas constant, T is temperature, n ) 2, and cs is the concentration of free bile salt in the solution. The results are collected in Table 1, together with some other parameters (19) Carey, M. C. Hepatology 1984, 4, 66S-71S.
2188 Langmuir, Vol. 12, No. 9, 1996
Swanson-Vethamuthu et al.
Table 1. Cmc (C°) and Interfacial Area per Surfactant Molecule, As, for Pure CTAB in 50 mM NaCl and Bile Salts under Neutral (50 mM NaCl) and Alkaline (45 mM NaCl + 5 mM NaOH) Conditions at 25 °C
(104)cmc, mol dm-3 As, Å2 (1) (2) a
CTAB in 0.05 M NaCl
NaC in 0.05 M NaCl
NaC in 0.045 M NaCl + 5 mM NaOH
NaDOC in 0.05 M NaCl
NaDOC in 0.045 M NaCl + 5 mM NaOH
2.6 55 (45)a
19.9 207 (105-190)b 46 (40-44)b
60.3 288 158
7.9 149 (85-140)b 41
28.8 204 70
Area/molecule obtained from ref 21. b Area/molecule obtained from the monolayer studies cited in ref 16.
Figure 2. Plots of surface tension vs log[CTAB] for the CTABNaC system in 50 mM NaCl at 25 °C. The symbols represent curves for different mole fractions (R1) of NaC: (9) 0.1; ([) 0.3; (2) 0.5; (1) 0.7; (b) 0.9.
deduced from the surface tension results. The alkaline conditions give larger areas per molecule, as expected when the charge is increased. Before the break point the area is larger than 150 Å2 in all cases, whereas the steep part corresponds to an area of only about 46 Å2 under neutral conditions and 70 Å2 (NaDOC) or 150 Å2 (NaC) with alkali added. These results can be compared with the monolayer studies of Ekwall et al.15 and of Small, as reported in Small’s monograph.16 A very brief summary of the relevant conclusion is that in the insoluble monolayers (which means under acid conditions for desoxycholic and cholic acid) the bile salts are lying flat on the surface, and on compression beyond a minimum area per molecule of about 18 Å2 the film collapses into a multilayer. This behavior was very clearly shown by the surface pressurearea isotherm for lithocholic acid, which has only one OH group; for cholic acid the collapse point, at a surface pressure of only 14 mN/m, was less evident and was followed by a section with a nonconstant, slowly rising surface pressure. This behavior was attributed to the relatively high solubility of cholic acid. Desoxycholic acid showed a clear collapse of the monolayer at an area of about 90 Å2/molecule, followed by a flat isotherm at a surface pressure of 20 mN/m. In this case another transition is observed in the monolayer before the collapse, at 120 Å2 and 20-23 mN/m. A monolayer of cholanic acid, with no OH groups on the steroid skeleton, can be compressed to an area of 40 Å2 with a collapse pressure of 20 mN/m. A similar upright orientation of the molecules is obtained in monolayers of lithocholate, on a substrate at pH 11; the more soluble diand trihydroxy species could not be studied at high pH by this technique. We conclude that the bile ions are lying flat at the interface before the break point in the γ vs log cs plots, with access to an area per molecule much larger than at the close-packed conditions prior to film collapse. It is
Figure 3. Plots of surface tension vs log[CTAB] for the CTABNaDOC system in 50 mM NaCl at 25 °C. The symbols represent curves for different mole fractions (R1) of NaDOC: (O) 0.0; (0) 0.1; (]) 0.3; (4) 0.5. Table 2. Mixed cmc (C*), Mole Fraction of Bile Anions in Micelles X1, and Interaction Parameter β for the CTAB-NaC System in 50 mM NaCl at 25 °C CTAB-NaC R ) 0.1 R ) 0.3 R ) 0.5 R ) 0.71 R ) 0.9 (104)cmc, mol dm-3 X1 ) NaC β (avg ) -4.0)
2.3 0.15 -3.63
1.8 0.29 -4.76
2.4 0.32 -3.6
2.8 0.405 -3.95
4.5 0.51 -3.89
Table 3. Mixed Cmc (C*), Mole Fraction of Bile Anions in Micelles X1, and Interaction Parameter β for the CTAB-NaDOC System in 50 mM NaCl at 25 °C CTAB-NaDOC (104)cmc, mol dm-3 X1 ) DOCβ (avg ) -2.7)
R ) 0.1
R ) 0.3
R ) 0.5
2.22 0.17 -2.67
1.84 0.32 -3.34
2.52 0.365 -2.07
interesting to observe that the break points occur at a surface pressure close to the collapse points or transitions in the monolayers of the corresponding acids: 14 mN/m for cholate and 20 mN/m for desoxycholate. After the break points the orientation of the bile ions are probably changed to predominantly upright (at least under neutral conditions) with an area per molecule down to 41-46 Å2; the alternative that multilayers should form by the ions on adsorption appears less probable. (B) Mixed Micelles. The surface tension plots for some mixtures of CTAB with NaC and NaDOC are shown in Figures 2 and 3. The solutions contain 50 mM NaCl, and since at the cmc there is always less than 0.5 mM CTAB, there is a 100-fold excess of Cl- over Br-, and the systems should be regarded as containing CTAC rather than CTAB. The plots have been used to determine the cmc of the mixtures. In a binary mixture of surfactants with mole fraction Ri of component i, the cmc is C*. At the cmc, the concentration Ci of i in the aqueous pseudophase is RiC*. If the micellar pseudophase, mole fraction xi of i, is an
CTAB-Bile Salt Association
Langmuir, Vol. 12, No. 9, 1996 2189
Figure 4. Experimental and predicted cmc’s of mixtures of CTAB-NaC in 50 mM NaCl at 25 °C. The plotted points are experimental data, the solid line is the prediction of the nonideal mixed model with β ) -4.0, and the dashed line is the prediction for ideal mixing.
Figure 5. Experimental and predicted cmc’s of mixtures of CTAB-NaDOC in 50 mM NaCl at 25 °C. The plotted points are experimental data, the solid line is the prediction of the nonideal mixed model with β ) -2.7, and the dashed line is the prediction for ideal mixing.
ideal mixture,20 then Ci should also be given by xiCi°, where Ci° is the cmc of pure i. In a nonideal system, the latter equality is modified by the introduction of an activity factor, fi, to give
RiC* ) fixiCi°
(4)
The activity factor is related to the interaction parameter β by eq 1 in Rubingh’s model. For each component in a binary mixture of surfactants, eq 1 can be solved for β, and by suitable substitution from eq 4, one obtains
ln β)
[ ] [ R1C* x1C1°
(1 - x1)2
ln
)
R2C*
]
(1 - x1)C2° x12
(5)
With C* determined, the second equality can be used to calculate x1 iteratively and thereafter β. Tables 2 and 3 report the experimental cmc values together with calculated x1 and β. For both bile salts, the cmc and the interaction parameter have minima when the bile salt fraction in the micelles is close to 0.3, showing that this is a favorable composition. The variation of β with the composition is large; the value of this parameter depends very sensitively on the measured cmc, however, and if the average value of the interaction parameter, -4.0 and -2.7 for NaC and NaDOC, respectively, is used to recalculate the mixed cmc values, the resulting curves are seen to represent the experimental results quite well, as shown in Figures 4 and 5. The simple one-parameter theory of Rubingh thus appears to describe these nonideal micelles within the precision of the experimental results; it is possible, however, that deviations would appear for the bile rich micelles. In Figure 6 the mole fraction of bile salt in the micelle calculated from each measurement is compared to the predictions of the theory using the average values for the interaction parameter and also to the ideal mixing case.20 It is obvious that the mixed micelles have a similar (20) Clint, J. H. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1372. (21) Rijnbout, J. B. J. Colloid Interface Sci. 1977, 62, 81.
Figure 6. Mole fraction of bile salt in micelles at the cmc (X1) vs the bulk mole fraction of bile salt. The solid lines connecting unfilled symbols represent predictions and experimental points, respectively, from applying nonideal (O) and ideal (]) mixing theories for the CTAB-NaC mixtures. The corresponding broken lines connecting filled symbols represent predictions and experimental points from applying nonideal (b) and ideal ([) mixing theories for the CTAB-NaDOC mixtures.
composition for both bile salts and that the larger negative value of the interaction parameter for NaC is due to the larger cmc of this substance and is not a sign of a more favorable interaction of CTA+ with cholate than with desoxycholate ions in the micelles. Note also in Figure 6 that, at R1 < 0.3, x1 is larger than R1 in both cases. It is also at this composition (or for NaC somewhat below) that the deviation of the β value is most significant, and the previous investigations12,14 have shown a particular tendency to the formation of threadlike micelles at higher surfactant concentrations. A particularly favorable packing of the micelles seems to be obtained in this composition range. Acknowledgment. We thank Go¨ran Svensk for skillful assistance in the surface tension measurements. Financial support from the Swedish Natural Science Research Council (NFR) is gratefully acknowledged. LA950856V
