Effect of Sodium Chloride and Varied Alkyl Chain ... - ACS Publications

Langmuir 1996, 12, 2173-2185

2173

Effect of Sodium Chloride and Varied Alkyl Chain Length on Aqueous Cationic Surfactant-Bile Salt Systems. Cryo-TEM and Fluorescence Quenching Studies M. Swanson-Vethamuthu, M. Almgren,* and G. Karlsson Department of Physical Chemistry, Uppsala University, Box 532, S-75121 Uppsala, Sweden

P. Bahadur Department of Chemistry, South Gujarat University, Surat 395007, India Received July 3, 1995. In Final Form: November 21, 1995X The changes in size and microstructure of alkyltrimethylammonium halide (CnTAX) micelles as a function of alkyl chain length (even number of carbons between 12 and 18) and NaCl concentration (0-2 M) in aqueous mixtures with and without trihydroxy or dihydroxy bile salts, sodium cholate or sodium desoxycholate (NaC or NaDOC), have been investigated by cryo-TEM, time-resolved fluorescence quenching, and relative viscosity measurements. Without additions, dilute solutions (25 mM) of all the surfactants form globular micelles, with aggregation numbers increasing with the chain length (to 230 ( 3 in C18TAB at 40 °C). Addition of NaCl results in a growth of the micelles. For C16TA+ a transition to long threadlike micelles occurred in 2 M NaCl, whereas the micelles remain globular at shorter chain lengths. For C18TA+ a mixture of globular micelles and large structures was observed at 0.5 and 1. 0M NaCl. The pseudoternary CnTAB/NaC/NaCl systems showed results dependent on the alkyl chain length. On addition of NaC to C12TA+ and C14TA+, in the presence of salt, a monotonous decrease in aggregation numbers with increasing concentration of NaC is found. For the longer alkyl chains, a micellar growth resulting in a transition to threadlike cylindrical micelles occurs first, giving a peak in the viscosity. The transition is most pronounced in the C18TA+ case. Further addition of NaC give smaller micelles. The various stages in the transition are seen from cryo-TEM results. In the pseudoternary C16TAB/NaDOC/NaCl system, the viscosity was orders of magnitude higher than in the corresponding compositions of the NaC system. The structure of the cylindrical micelles depend on the composition and mole fraction of NaDOC. The change in size and structure on a molecular level is discussed in terms of the structure of bile salt anions and its preferred modes of orientation in the mixed micelle under different conditions.

Introduction The effects of ionic strength, temperature, and alkyl chain length on the aggregation of ionic surfactants have been much studied and are in general well rationalized using simple concepts, such as the change of the value of the (average) surfactant parameter,1 v/a0lc (v is the volume of the hydrophobic tail of the surfactant, lc the length of the tail, and a0 the effective headgroup area). Increasing temperature shortens the tail and favors aggregates with less curvature; increasing ionic strength reduces the effective area, and also favors aggregates with less curvature. The effect of the number of carbons, nc, in the tail is more subtle. Since both the volume and the tail length are approximately proportional to nc, the value of the surfactant parameter would be mainly unaffected by nc. The number of surfactants in the largest spherical micelle, however, is proportional to nc2 (and the area per headgroup is independent of nc)2. If rodlike micelles are looked upon as cylinders with spherical endcaps, then under specified conditions there is a certain distribution of the surfactant between the two regions. Even if this distribution is not changed, the number of micelles and thus their average length will strongly depend on the number of carbons in the tail, so that longer tails will favor rodlike micelles. For mixtures of anionic and cationic surfactants, some type of average surfactant parameter can be used. The attraction between the headgroups will reduce a0 comX

Abstract published in Advance ACS Abstracts, April 1, 1996.

(1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (2) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1983, 87, 1264.

S0743-7463(95)00964-4 CCC: $12.00

pared to the values of each of the surfactants individually. Both surfactants contribute to the volume. In spherical and cylindrical aggregates, the longer of the surfactants will be most important for lc, since only some of the surfactants have to reach the center of the structures, whereas in bilayer structures an average lc will dictate the thickness of the hydrophobic layer. It is well-known that the addition of anionic surfactants to cationic micelles favors a transition from spheres via rods to bilayer structures.3-5 In earlier papers6-8 we have reported on studies of the phase behavior and aggregation structures in alkyltrimethylammonium halide-bile salt mixtures. Bile salts with their inflexible ring structures are in many respects unusual surfactants, and are usually described as having a hydrophilic and a hydrophobic face. We have mainly used sodium cholate (NaC), with three hydroxy groups on the hydrophilic face, and sodium desoxycholate (NaDOC) with two hydroxy groups. When added to C16TAB or C16TAC, NaC gives mixed micelles that decrease monotonously in size, whereas NaDOC induces a transition to rodlike micelles and, at close to equimolar concentration, a separation into two phases, one surfactant rich and the other very dilute. On the bile-rich side of this coacervation (3) Kaler, E. W.; Herrington, K. L.; Murthy, K. A.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698-9707. (4) Marques, E.; Khan, A.; Miguel, M. d. G.; Lindman, B. J. Phys. Chem. 1993, 97, 4729-4736. (5) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792-13802. (6) Vethamuthu, M. S.; Almgren, M.; Mukhtar, E.; Bahadur, P. Langmuir 1992, 8, 2396-2404. (7) Vethamuthu, M. S.; Almgren, M.; Brown, W.; Mukhtar, E. J. Colloid Interface Sci. 1995, 174, 461-479. (8) Vethamuthu, M. S.; Almgren, M.; Bergenståhl, B.; Mukhtar, E. J. Colloid Interface Sci., in press.

© 1996 American Chemical Society

2174 Langmuir, Vol. 12, No. 9, 1996

area there are still rodlike structures in the solution, but at high NaDOC excess, small spherical micelles are finally formed again. Guided by results reported for the interaction of these bile salts with lecithins,9-12 we rationalized this behavior by assuming that NaC in the mixed micelles is placed flat on the micelle surface, between the headgropus of neighboring cationic surfactants and with the hydrophilic face toward the aqueous environment. With this way of insertion, the bile salt molecule would clearly require a highly curved interface, favoring small globular micelles. NaDOC, on the other hand, was assumed to become inserted in a “normal” surfactant way, despite the fact that two hydroxy groups would become imbedded in the hydrophobic interior (to some extent this may be counteracted by hydrogen bonding between two adjacent bile salt molecules). Only in this way can we understand the transition to less curved aggregates. The investigation of the hexagonal phase of the system CTAB/NaDOC/water by X-ray diffraction8 showed a complication. The X-ray data indicated a small decrease of the cylinder radius with the bile salt concentration; fluorescence quenching results for the cylinder micelles at lower concentrations in the L1 phase were compatible with such a change. We rationalized these observations by referring to the screening of the electrostatic effects at high surfactant concentrations; this is in the first place the reason that CTAB itself forms rodlike micelles at high concentrations, which then, at still higher concentration, pack in the hexagonal phase. The surfactant parameter would then be larger in the presence of salt than without, and the addition of DOC- could in the first case result in a lowering and in the second in an increase of the surfactant parameter. This explanation, however, is not fully compatible with another observation and its rationalization. At relatively high concentrations of CTAB in the L1 phase, addition of NaDOC preserves the cylinder structure, and a gel region, probably with entangled threadlike micelles is found in the L1 area. On further addition of NaDOC, the gel breaks and a fluid solution re-forms. We suggested the structural changes from the gel region to be related to those that transform the hexagonal phase into a cubic phase (with still unknown structure), which is present in the center of the phase diagram,7 and that in both cases the transition was into structures with less curvature than the cylinders: branched rods, which in the case of the cubic phase probably are connected into interpenetrating networks (the ICR model first suggested by Mariani et al.13), and a molten, disorder version of it in the fluid solution. Obviously, a decrease of curvature from the cylinders in the hexagonal phase or in the gel area by addition of NaDOC is not compatible with the decrease of the cylinder radius of the rods within the hexagonal phase and not with the final increase of curvature when even more NaDOC is added. The problems with the suggested way of rationalizing the findings became even more apparent when some results from viscosity measurements of dilute solutions with added salt were considered. Addition of NaDOC gave a viscosity maximum at a composition which corresponds to the gel region at higher surfactant concentrations but also NaC gave a small maximum in the viscosity, at lower addition of the bile salt. The results from those studies are reported here and have been followed up by time(9) Saito, H.; et al. J. Biochem. (Tokyo) 1983, 94, 1877. (10) Ulmius, J., et al. Biochemistry 1982, 21, 1553. (11) Hjelm, R. P. J.; Thiyagarajan, P.; Alkan-Onyuksel, H. J. Phys. Chem. 1992, 96, 8653-8661. (12) Egelhaaf, S. U.; Schurtenberger, P. J. Phys. Chem. 1994, 98, 8560-8573. (13) Mariani, P.; Luzzati, V.; Delacroix, H. J. Mol. Biol. 1988, 204, 165-189.

Swanson-Vethamuthu et al.

resolved fluorescence quenching (TRFQ) and cryo-TEM studies. The TRFQ measurements were used to measure the size of the micelles or indicate the transition to rodlike structures. In the cryo-TEM method a thin film of the aqueous solution is rapidly vitrified in fluid ethane just above its freezing point. The vitrified sample is examined in a cold stage transmission electron microscope. In this way even rather labile structures may be captured and examined directly without staining or other treatments. Most of the cationic surfactants were the bromides, used at a concentration of usually 25 mM, but since the added salt was NaCl at a concentrations of 0.5 M or more (the TRFQ measurements would be less precise if high concentrations of the quenching bromide ion had been added), it is in reality the chloride forms of the surfactants that have been investigated. Experimental Section Materials. The surfactants octadecyltrimethylammonium bromide (C18TAB), from Fluka (purum grade), cetyltrimethylammonium bromide (C16TAB), purchased from Serva, purity >99%, and tetradecyltrimethylammonium bromide (C14TAB) and dodecyltrimethylammonium bromide (C12TAB), from Sigma (purity approximately 99%), were used as supplied. Cetyltrimethylammonium chloride (C16TAC) was prepared from C16TAB by ion exchange on a Dowex 1-X8 resin. The product was freezedried 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. Pyrene (Aldrich) was recrystallized twice from ethanol, and dimethylbenzophenone (DMBP, Aldrich), with purity >99%, was used as supplied. All solutions were prepared in distilled water. Methods. Time-resolved fluorescence (TRFQ) decay curves were recorded with the single photon counting technique with an experimental setup as described in an earlier publication.6 Measurements were generally performed at 25 °C and the samples were not degassed. All data analysis was performed on a personal computer with programs developed for two separate models: firstly according to a model for fluorescence deactivation in spherical micelles proposed by Infelta et al.14 and Tachiya,15 and secondly using a model for deactivation of excited states in infinite rodlike micelles.16-18 The theory and the equations used in the data analysis with the first model are the same as that described in detail earlier.6 In the second model the fluorescence decay curve after excitation with a δ-pulse is given by18

ln

(

)

F(t) t ) -k0t - [rhc]3cqQ hrhc, τq F(0)

(1a)

with

( ) {x( )

Q hrhc,

t 4π ) τq hrhc

hrhc

xτq

2

t + π

[ {( ) } {(x )x } ]}

3 exp 4

hrhc xτq

2

4t erfc 9

hrhc τq

4t -1 9

(1b)

where k0 is the natural decay constant (determined separately as the inverse of the fluorescence lifetime of pyrene in the micelles without quenchers), rhc is the radius of the hydrophobic cylinder (i.e., the cylindrical volume in which the probes and quenchers are contained), cq is the number density of quenchers in the hydrophobic volume, and τq ) rhc2/D. The parameter h ) kqrhc/D weights reaction against diffusion. Since the cylinder is not a truly one-dimensional system, all three dimensions are important when the reactants are close. This condition is handled by the (14) Infelta, P. P.; Gra¨tzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 190. (15) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (16) Almgren, M.; Alsins, J.; Mukhtar, E.; van Stam, J. J. Phys. Chem. 1988, 92, 4479. (17) Alsins, J.; Almgren, M. J. Phys. Chem. 1990, 94, 3062. (18) Medhage, B.; Almgren, M. J. Fluoresc. 1992, 2, 7-21.

Surfactant-Bile Salt Systems

Langmuir, Vol. 12, No. 9, 1996 2175

introduction of a reaction zone around the probe, with axial length arbitrarily chosen as 2rhc. A probe-quencher pair in this zone is assumed to react at a first-order rate constant, kq, similar to the constant that characterizes the reaction between a probe and a quencher in a micelle.16 The radius, rhc, of the hydrophobic cylinder plays a double role; it defines a reaction distance as the length of the reaction zone, and it determines the density of quenchers per length unit of the cylinder, by defining the cylinder thickness. With the radius known, fitting of experimental data to eq 1 yields values for D and kq; these values depend strongly on the chosen rhc value. In the fitting procedure the primarily determined parameters18 are hrhc/τq1/2 and (h/[rhc]2), which implies that Drhc4 and kqrhc3 are invariant for a given experimental curve. The hydrophobic radius was taken as rhc ) 19.6 and 21.8 Å for the C16TAB and C18TAB systems, which is about 0.9lc (lc calculated form Tanfords formula19), close to what could be expected for a cylindrical structure.20 In order to determine the number density of quenchers in the hydrophobic core of the cylindrical micelles, we estimated the total hydrophobic volume by adding the hydrophobic volumes from the cationic surfactants and the bile salts making up the cylinder micelles. A formula presented by Tanford19 gave the molecular volumes of the alkyl chains of the cationic surfactants. We estimated the contribution from the trihydroxy bile salt, 450 Å3, by reducing the apparent anhydrous molecular volume of the salt in water, 539 Å3,21 with the volumes of the head and polar groups. The samples for cryo-TEM were vitrified from 25 °C, in some cases 40 °C according to a procedure described earlier.22 The relative viscosities of solutions were measured using a Ubbelhode capillary viscometer. The flow time always exceeded 150 s, and no kinetic energy correction was necessary.

Results Viscosity, cryo-TEM, and fluorescence quenching measurements were performed on solutions containing a constant concentration, 25 mM, of CnTAB, and varied amounts (2-25 mM) of trihydroxy/dihydroxy bile salts; NaCl concentrations ranging from 0 to 2 M. All cryoTEM and fluorescence measurements were made at 25 °C. Most of the viscosity measurements were performed at 30 °C and in some cases at 25, 40 and 50 °C. Viscosity Measurements. Parts A and B of Figure 1 show the variation in relative viscosity over a concentration range from 0 to 25 mM for two different bile salts, NaC and NaDOC, respectively. In the case of the NaC/ CnTAB system (Figure 1A) it was observed that the viscosity initially decreased upon addition of NaC up to 10-15 mM and then gradually increased in all cases except for the C12TAB/NaC system where the relative viscosity increased gradually from the start. The viscosities of the mixed samples were only slightly larger than that of the solvent, water. The initial decrease in viscosity in the first four cases described above is due to the effect of added NaC which at first acts as a normal salt by screening electrostatic interactions and thus reducing the viscosity. The gradual increase observed later is simply a concentration effect. The different behavior of C12TAB is due to that it has a much higher critical micelle concentration (cmc) in water. Addition of NaC decreases the cmc and increases the amount of mixed micelles; the increase in screening is smaller and increase in aggregate concentration larger in this case. Figure 1B shows the corresponding results for the NaDOC/CnTAB system. It is evident that the viscosities (19) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley & Sons: New York, 1973; pp 1-52. (20) Gruen, W. R. D. J. Phys. Chem. 1985, 89, 153-163. (21) Small, D. M. In The Bile Acids; Nair, P. P., Kritchevsky, D., Eds.; Plenum Press: New York, 1971; p 274. (22) Edwards, K.; Gustafsson, J.; Almgren, M.; Karlsson, G. J. Colloid Interface Sci. 1993, 161, 299-309.

Figure 1. (A) The variation in relative viscosity of cationic surfactant (CnTAB) with varied NaC concentration. The concentration of cationic surfactant was kept fixed at 25 mM. The symbols represent (]) C12TAB, (4) C14TAB, (0) C16TAB, and (3) C18TAB systems. All measurements were performed at 30 °C except for the C18TAB system which was kept at 40 °C due to the high Krafft point. (B) The variation in relative viscosity of cationic surfactant (CnTAB) with varied NaDOC concentration at 30 °C. Symbols represent (]) C12TAB, (4) C14TAB, and (0) C16TAB. C16TAB represents values taken from ref 6.

are much higher than at the same compositions in the previous system. Here too there is a slight decrease at first but from 10 mM NaDOC and onward the viscosity increases rapidly as the equimolar composition is approached. The trend is strongest for the C16TAB/NaDOC system followed by the C14TAB and C12TAB systems with NaDOC, respectively. The results presented show the difference in aggregation behavior of the dihydroxy and trihydroxy bile salts in mixtures with CnTAB. The increase in viscosity on addition of NaDOC follows from a transition from spherical micelles to cylindrical micelles.6 The cylindrical micelles appear in the C16TAB and C14TAB systems at a mole fraction of 0.29 NaDOC and in the C12TAB system at about 0.38 mole fraction of NaDOC. The results show that counterion type and the alkyl chain length also influences the onset of the sphere-to-rod transition in these systems. In an earlier study we have characterized the rodlike micelles as a function of alkyl chain length and temperature and found no clear difference in the properties of the cylindrical micelles except in the fluidity of the interior.8 Figure 2 shows the effect of varied NaCl concentration on the relative viscosity of pure C16TAB solution (25 mM) at 25 °C. At low salt concentration there is a decrease in the viscosity up to 0.5M and then a dramatic increase somewhere above 1 M NaCl. As in the case of the C16TAB/NaDOC system, Figure 1B, the initial decrease is

2176 Langmuir, Vol. 12, No. 9, 1996


Figure 2. Variation in the relative viscosity of 25 mM C16TAB as a function of added NaCl at 25 °C.

due to the electrostatic screening effect. If we include the fluorescence quenching and cryo-TEM results, presented below, we can conclude that above a certain salt concentration there is a transition from globular to rodlike micelles. Earlier Porte et al.23 using QELS measurements on C16TAB solutions found that addition of Cl- ions up to 0.2 M NaCl did not induce micellar elongation and concluded that Cl- ions were inefficient in promoting this transition in agreement with our observation that 2 M NaCl is necessary to induce elongation. On the other hand Br- binds much stronger than Cl-, and Imae et al.24 showed that in aqueous CTAB solutions the salt-induced transition from spherical to rodlike micelles takes place at NaBr concentrations of 0.06 M, while in KBr solutions it appears at 0.1 M. Figure 3A shows the effect of varied NaCl on the relative viscosity of the mixed C16TAB/NaC system as a function of increasing NaC at 30 °C. The results for solutions without salt are shown by the dashed line. Already at 0.5 M NaCl there is a peak in the viscosity at 5 mM NaC indicating a change in structure of the micelles. Further additions of NaC lead to a rapid decrease in viscosity, and beyond 10 mM no significant change is observed. The position of the maximum remained independent of the NaCl concentration and temperature (figure not shown). Figure 3B shows the corresponding results for the C16TAB/NaDOC system with varied NaCl at 40 °C. Once again the viscosities in this system are much larger, by approximately a factor 10, than for the corresponding compositions in the previous system. The other important difference is that the viscosity peak is shifted from 5 mM in the NaC case to 10 mM in the NaDOC system. If C16TAB is replaced by C14TAB at 1 M NaCl, the viscosity is greatly reduced but still well above the values observed in the corresponding NaC system. The viscosity peak remains at 10 mM NaC after which a gradual decrease is observed. Once again the peak position is independent of the salt concentration and temperature (figure not shown). The dotted line represents the viscosity for samples without added NaCl at 30 °C. A similar peak in the viscosity at a particular composition has been observed in other viscoelastic surfactant solutions.25 The change in viscosity observed in the C16TAB and C18TAB systems with NaC is due to a change in the micelle structure, that was further investigated by cryo-TEM and fluorescence quenching studies. The viscosity measurements allowed us to qualitatively observe a transition from (23) Porte, G.; Appell, J. In Surfactants in Solution 2; Mittal, K. L., Lindman, B. Eds.; Plenum Press: New York, 1984; pp 805-823. (24) Imae, T.; Kamiya, R.; Ikeda, S. J. Colloid Interface Sci. 1985, 108, 215. (25) Clausen, P. M.; et al. J. Phys. Chem. 1992, 96, 474-484.

Figure 3. (A) Influence of varied NaCl on relative viscosity in the C16TAB/NaC system at 30 °C. Symbols represent the following concentrations of NaCl: (open box) 0 (values from ref 6) (crossed lines in box) 0.5 M; (diagonal line in box) 0.75 M; (solid box) 1 M. (B) Influence of varied NaCl on relative viscosity in the C14TAB/NaDOC (2) and C16TAB/NaDOC system at 40 °C. Symbols represent the following concentrations of NaCl: (open box) 0 (values taken from ref 6 at 30 °C); (crossed lines in box) 0.5 M; (diagonal line in box) 0.75 M; (9,2) 1 M.

globular to cylindrical micelles and thus to identify regions suitable for further investigations by the latter techniques. In the NaC system we could expect cylindrical micelles only in a narrow region at rather high concentrations of NaCl; however, in the corresponding NaDOC system cylindrical micelles are present in a much larger region. Figure 4A shows the effect of varied alkyl chain length for the NaC/CnTAB system in 1 M NaCl at 30 °C. The viscosity peak is most pronounced for the system with C18TAB. We observe that the viscosity of pure C18TAB in 1 M NaCl is twice that of pure water. Small additions of NaC increase the viscosity by about a factor 8 followed by a sharp decrease on increasing NaC up to 10 mM NaC; for compositions lying above 10 mM the relative viscosity is independent of the alkyl chain length. Figure 3A shows the corresponding data for C16TAB/NaC system in 1 M NaCl. The effect is the same except for that the peak is a factor 6 times lower here than in the C18TAB/NaC case. With shorter chain lengths as in C14TAB and C12TAB under similar conditions, no dramatic increase in the viscosities is observed; the viscosities remain close to that of pure water at all compositions. It is evident from these results that only the long chain surfactants transform into cylindrical aggregates in the presence of NaC. The viscosities are independent of chain length above a certain concentration of NaC, where the micelles have returned to globular shape. In an earlier paper7 we presented some rheology data for compositions with CTAB fixed at 100 mM and varied NaDOC without NaCl. Figure 4B shows the log of the relative viscosity


Figure 4. (A) Influence of varied alkyl chain length, nc, on relative viscosity in the CnTAB/NaC system in the presence of 1 M NaCl at 30 °C. Symbols represent the following systems: ([) C12TAB; (2) C14TAB; (9) C16TAB; (1) C18TAB. (B) Variation of the relative viscosity as a function of the mole fraction of NaDOC: (O) concentration of CTAB fixed at 0.1 M and with no added NaCl (taken from ref 7 at 25 °C); (9) concentration of CTAB fixed at 25 mM with 1 M NaCl at 30 °C.

as a function of mole fraction of NaDOC in order to compare the earlier results with the viscosity in a range of compositions with 25 mM CTAB in the presence of NaCl. In both cases the maximum in viscosity occurs between 0.25 and 0.30 mole fraction of NaDOC although the viscosities differ by more than a decade in magnitude. The composition at the viscosity maximum further coincides with the center of a gel-like region in the more concentrated part (10-25 wt %) of the phase diagram published earlier.7 This effect of added salt is probably a result of both reduced repulsive interactions between aggregates and promoted growth at lower surfactant concentrations. In summary, the viscosity results allow us to qualitatively observe and differentiate the aggregation behavior in the systems studied, in particular the large effects from the different structures of the two bile salts (dihydroxy and trihydroxy), and more generally small effects due to varied counterions (Cl- and Br-) and varied nonpolar alkyl chain lengths of the cationic surfactants. In a given system the relative viscosity peaks at a composition independent of salt concentrations, but the peaks are more intense at higher salt concentrations and lower temperatures in all cases. Results from Fluorescence Quenching. Figure 5 illustrates the two basic types of pyrene fluorescence decay curves observed depending on whether spherical micelles or cylindrical micelles are present in the sample. Figure 5A is fluorescence decay characteristic of small confinements, with a rapid quenching of fluorescent probes confined to the same small micelles as the quenchers,

Langmuir, Vol. 12, No. 9, 1996 2177

Figure 5. (A) Typical pyrene fluorescence decay curves for C18TAB (25 mM)/NaC (10 mM) globular mixed micelles in 1 M NaCl at 25 °C freed from the influence of the natural decay by multiplication of the measured intensities with the appropriate factor of exp(k0t). The concentration of pyrene in the samples is 1 µM and the quencher concentrations (DMBP) are as follows: UQ ) 0; Q1 ) 0.15; Q2 ) 0.3; Q3 ) 0.6 mM. (B) Typical pyrene fluorescence decay curves for C18TAB (25 mM)/NaC (5 mM) cylindrical mixed micelles in 1 M NaCl at 25 °C produced in a similar way to those in Figure 5A indicative of onedimensional decays. The concentration of pyrene is 1 µM and the quencher concentrations in the samples are the following: UQ ) 0; Q1 ) 0.15; Q2 ) 0.3; Q3 ) 0.6 mM.

followed by a tail from probes in micelles without quenchers. There is evidently no migration of quencher molecules during the lifetime of the excited probe. The compositions, which gave decay curves similar to those in Figure 5A, were analyzed using the Infelta-Tachiya model. The fitting of the decay curves to the model gives the average number of quenchers per micelle, which finally, from a knowledge of the concentration of the cationic surfactant and quencher in the micellar subphase, gives the average aggregation number, Nagg(CnTA+), in the mixed micelle. The primary results from the above analysis are the values of Nagg(CnTA+), the first-order quenching rate constant, kq, and the pyrene unquenched lifetimes, τ0, presented in Table 1A for the C12TAB and C14TAB surfactants with NaC in the presence and absence of 1M NaCl. Parts B and C of Table 1 present the corresponding results for the C16TAB and C18TAB systems with NaC. The weighted residuals and the autocorrelation function were used in addition to the χ2 values presented in the table to check the goodness of the fitting procedure. We first examine the variation of Nagg(CnTA+) with varied mole fraction of NaC (χNaC) for the mixed micelles of C12TAB and C14TAB. The aggregation number of 25 mM C12TAB without salt was found to be 63 ( 3 monomers per micelle in close agreement with literature values, i.e.,

2178 Langmuir, Vol. 12, No. 9, 1996

Swanson-Vethamuthu et al. Table 1

A. Results of Fitting the Decay Curves Obtained for the C12TAB/NaC and C14TAB/NaC Systems at 25 °C, to the Infelta Modela 0 M NaCl

1 M NaCl

C12TAB χNaC

τ0/ns

Nagg

kq × 10-7, s-1

χ2

τ0/ns

Nagg

kq × 10-7, s-1

χ2

0 0.17 0.29 0.37 0.44 0.50

116 158 187 212 232 247

63 56 48 38 29 22

4.3 2.9 4.0 4.3 3.9 3.6

1.2 1.15 1.06 1.16 1.03 1.08

177 198 219 240 258 265

72 64 53 42 34 27

3.7 3.3 3.9 4.2 3.8 3.7

1.03 1.10 1.08 1.12 0.96 1.07

C14TAB χNaC

τ0/ns

Nagg

kq × 10-7, s-1

χ2

τ0/ns

Nagg

kq × 10-7, s-1

χ2

0 0.11 0.17 0.24 0.29 0.37 0.44 0.50

115

82

1.9

1.19

141

74

1.6

0.98

173 205 231 249

55 40 34 26

1.7 2.1 2.7 2.8

1.23 0.99 1.05 1.03

172 184 200 213 218 240 260 269

97 93 85 71 58 41 33 24

1.7 1.5 1.3 1.3 1.5 2.0 2.2 2.6

1.12 1.02 1.08 1.03 1.07 0.99 1.01 1.05

0 M NaCl

2 M NaCl

B. Results of Fitting the TRFQ Decay Curves for the C16TAX/NaC System Obtained at 25 °C, to the Infelta Model C16TACb

no NaCl C16

TABb

kq ×

τ0/ns

Nagg

kq × 10-7, s-1

χ2

1.10 1.13 1.09 1.09 1.10 1.11

156 181 205 226 240 244

82 68 54 35 26 18

1.5 1.0 0.9 1.2 1.6 1.65

0.99 1.04 1.17 0.97 1.07 1.03

Nagg

114 142 181 213 229 241

108 85 66 43 34 26

C16TAB χNaC

τ0/ns

Nagg

kq × 10-7, s-1

χ2

τ0/ns

Nagg

kq × 10-7, s-1

χ2

0 0.07 0.14 0.17 0.19 0.24 0.28 0.29 0.37 0.44 0.50

153 163 174 180 184 195 208

134 120 116 114 108 89 67

0.9 0.8 0.7 0.6 0.58 0.67 0.83

1.11 1.12 1.05 1.08 0.98 1.03 1.04

165 177

143 184

0.83 0.55

1.06 1.00

221 242 260 270

87 59 45 37

0.76 1.15 1.44 1.75

1.02 1.09 1.05 1.09

0 0.17 0.29 0.37 0.44 0.50

s-1

χ2

τ0/ns

χNaC

10-7, 1.1 0.8 0.7 1.0 1.8 2.1

0.5 M NaCl

1 M NaCl

C. Results of Fitting the TRFQ Decay Curves for the C18TAX/NaC System Obtained at 25 °C, to the Infelta Model 0 M NaCl C18TAB χNaC 0c 0.17d 0.29 0.37 0.44 0.50

1 M NaCl

τ0/ns

Nagg

kq × 10-7, s-1

χ2

τ0/ns

Nagg

kq × 10-7, s-1

χ2

107 135 175 209 234 251

230 134 89 69 48 37

1.1 0.7 0.65 0.91 1.33 1.56

1.19 1.10 1.20 1.06 1.12 0.96

215 235 247 264

110 74 47 36

0.51 0.75 0.99 1.05

1.03 0.99 1.05 1.02

a The table provides the pyrene fluorescence lifetime (τ ), aggregation number (N + 0 agg) with respect to CnTA with an uncertainty )(3, quenching rate constant (kq), and the goodness of the fitting procedure expressed as χ2 values. b The values for the C16TAB/NaC and C16TAC/NaC systems without NaCl are taken from ref 6 for comparison. c TRFQ decay points obtained at 40 °C. d TRFQ decay points obtained at 30 °C.

compared to the weight averaged aggregation numbers 65 and 57 found in 50 mM C12TAB and C12TAC.26 The addition of 1 M NaCl increases the aggregation number to 72 ( 3, a moderate increase when compared to values reported for C12TAC micelles in water and for C12TAC micelles, 68, in 0.5 M NaCl.27 These values are lower than those found in 0.5 M NaBr, 88 ( 4, determined from (26) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (27) Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967, 71, 18981907.

light scattering measurements.27,28 The addition of NaC to solutions with and without salt resulted in a monotonous decrease in Nagg(C12TA+). The total aggregation number, i.e., Nagg(C12TA++Cholate-) showed a moderate increase at first, but above a χNaC of 0.2 also the total aggregation number decreased. The corresponding samples with salt showed marginally higher aggregation numbers than those without. This type of monotonous decrease found in NaC mixed systems has been observed before, both (28) Anacker, E. W.; Ghose, H. M. J. Phys. Chem. 1963, 67, 17131715.


with cationic surfactant (C16TAB and C16TAC)6 and in studies of NaC with nonionic surfactant (C10E8)29 and with sodium glychocholate (NaGC) and C10E8.30 The variation in Nagg(C14TA+) is similar to that reported above for samples with and without salt. The corresponding aggregation number for micelles in 25 mM of C14TAB was 82 ( 3 without salt which increased to 97 ( 3 monomers on addition of 1 M NaCl. A lower value of 68 ( 5 for micelles has been reported by Lianos and Zana31 from pyrene excimer quenching measurements in 0.01 M C14TAB. Roelants et al.32 reported the aggregation numbers for C14TAC (20 mM) micelles of 75 ( 6 and 67 ( 6 from fluorescence quenching of 1-methyl pyrene fluorescence by two different quenchers, I- and n-methylN-decylaniline, respectively. The results for the C16TAX/NaC system is presented in Table 1B for NaCl concentration ranging from 0 to 1 M. Without salt there is a monotonous decrease of Nagg; these results have been reported and discussed earlier6 and are used here only for comparison. At 0.5 M NaCl the micelles in 25 mM C16TAB showed an aggregation number of 134 ( 3, comparable to 141 monomers reported for 16 mM CTAC in 0.55 M NaCl.33 The Nagg(C16TA-) values for the mixed micelles are larger than the corresponding compositions without salt. Above χNaC of 0.3 the Nagg(C16TA-) is independent of salt concentration. The growth is much more pronounced in 1 M NaCl where Nagg(C16TA+) first increases from 143 ( 3 to 184 ( 3 and then there is a transition from spherical to cylindrical micelles at 5 mM of NaC (χNaC ) 0.17). For compositions at and above a mole fraction 0.3 NaC, the effect of NaCl is only marginal and the micelles remain small and globular. At 2 M NaCl C16TAB (25 mM) a transformation into long threadlike cylindrical micelles is seen (results discussed later) and on addition of NaC the cylindrical micelles remain below 0.3χNaC. At and above 0.3χNaC the size decreases to small, globular micelles. Results for the C18TAB/NaC system at 0 and 1 M NaCl are presented in Table 1C. Due to the increase in chain length, the Krafft point shifts to higher temperatures,34 38 °C, and Nagg(C18TA+) is large, i.e. 230 ( 3, for the micelles in 25 mM C18TAB without NaCl at 40 °C; there is a decrease to about 215 monomers at 45 °C. There is evidence (presented later) to suggest that pure C18TAB in 0.5M NaCl represents a dispersion of lamellar structures in a solution of spheroidal micelles. No cylindrical micelles were found in this system in the absence of bile salts even at high ionic strengths (1 M NaCl), whereas already small additions of NaC transformed the disklike aggregates to cylindrical aggregates, which then exist up to a mole fraction of 0.3 NaC. The pyrene lifetimes (τ0) in micelles determined from solutions without quenchers and the first-order quenching rate constant kq for all compositions discussed above are presented in parts A and B of Figure 6, respectively. The τ0 in all salt-free compositions is represented by the line (a) and is lower than that in the corresponding samples with added NaCl. This is because of the quenching of pyrene fluorescence by the Br- anions in addition to the oxygen quenching present in all aerated samples. The Br- ion quenching and therefore its influence on τ0 decreases with increasing χNaC. The τ0 obtained from corresponding compositions with 1 M NaCl (c) is much (29) Ueno, M.; Kimoto, Y.; Ikeda, Y.; Momose, H.; Zana, R. J. Colloid Interface Sci. 1987, 117, 179-185. (30) Asano, K.; Aki, K.; Ueno, M. Colloid Polym. Sci. 1989, 267, 935. (31) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1982, 88, 594-598. (32) Roelants, E.; Gelade´, E.; Smid, J.; De Schryver, F. C. J. Colloid Interface Sci. 1985, 107, 337-344. (33) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (34) Blackmore, E. S.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1115-1127.

Langmuir, Vol. 12, No. 9, 1996 2179

Figure 6. (A) Variation in the pyrene fluorescence lifetimes (τ0) as a function of increasing χNaC and varied even alkyl chain lengths from 12 to 18. Solutions without NaCl are represented by unfilled symbols and those with 1 M NaCl by filled symbols: C12TAB/NaC (],[); C14TAB/NaC (4,2); C16TAB/NaC (0,9); C18TAB/NaC (3,1). The crossed lines in box symbol represents C16TAB/NaC mixtures in 0.5 M NaCl. (B) Variation in the first-order quenching rate constant (kq) as a function of increasing χNaC, varied even alkyl chain length (from nc ) 12 to 18) and NaCl concentration. All symbols are the same as given in Figure 6A.

higher than (a) and moderately higher than those observed in CTAC/NaC mixtures and CTAB/NaC mixtures with 0.5 M NaCl (b). In all cases the effect of NaCl on τ0 is small close to equimolar compositions. The first reason for the increase in τ0 is the expulsion of Br- by Cl-, and the second reason is the stepwise exchange of the halide ions by the cholate anion, which has a strong shielding effect against both Br- ions and oxygen quenching.6 The variation of kq with χNaC, alkyl chain length, and NaCl (0-1 M) is shown in Figure 6B. In all cases there is an initial decrease in kq followed by an increase with increasing NaC concentration. This is because there is an increase in the total aggregation number initially, followed by a monotonous decrease up to equimolar compositions. The C12TAB/NaC system (represented by the unfilled and filled (]) symbol for samples without and with NaCl) gives the largest rate constant because the globular micelles formed are the smallest. It appears that the kq values decrease again close to equimolar compositions. Figure 5b represents a typical fluorescence decay obtained from a sample containing cylindrical micelles. The decay curves were analyzed using the model for deactivation of excited states in infinite rodlike micelles, with results as presented in Figure 7 and Table 2. The decay curves could be very well fitted to the model of eq 1 with χ2 values close to unity. The results from the analysis are the values of D, the sum of the diffusion coefficients for the excited probe and quencher in the mixed

2180 Langmuir, Vol. 12, No. 9, 1996


Table 2. Results from Analysis of TRFQ Decay Curves for the CnTAB/NaC System to the One-Dimensional Rod Model at 25 °Ca [nc] conc NaCl

χNaC

cq × 105, Å-3

τ0, ns

rhc, Å

hrhc

1M 2M

0.17 0 0.074 0.17

1.09 1.3 1.2 1.09

199 178 189 207

19.6 19.6 19.6 19.6

3.3 2.4 4.2 4.3

1M

0b 0.107 0.17 0.24

1.17 1.06 0.99 0.92

165 191 192 199

21.8 21.8 21.8 21.8

1.2 1.8 2.0 2.6

kq cyl × 10-7, s-1

D × 1011, m2 s-1

kq*rhc3 × 1020, m3 s-1

χ2

C16 123 68 104 155

2.6 3.6 2.9 2.1

3.1 6.5 4.5 2.6

20 28 22 16

1.2 1.3 1.15 1.04

C18 39 100 125 216

3.1 1.3 1.4 1.2

11.2 4.6 3.6 2.2

32 14 14 12

1.11 1.07 1.02 1.24

τq, ns

a The parameters obtained are functions of the alkyl chain length, concentration of NaCl and mole fraction of NaC (χ NaC). The number density of quenchers in the hydrophobic volume is cq, the fluorescence lifetime of pyrene in micelles is τ0, and the hydrocarbon radius used is rhc. The results obtained are the one-dimensional diffusion coefficient, D, the first order quenching rate constant, kq (cyl) and the goodness of the fitting procedure expressed as χ2 values. b Obtained at 30 °C.

Figure 7. Variation of the relative diffusion coefficient of the probe quencher pair, DRel, and the parameter kq(cyl)*rhc3 (righthand Y-axis) with increasing χNaC. Symbols represent D and kq*rhc3 for the C16TAB/NaC (2 M NaCl), (9,0), C18TAB/NaC (1 M NaCl) (1,3), and C16TAB/NaC (1 M NaCl) (boxes) systems, respectively.

Figure 8. Threadlike cylindrical micelles in a sample with 25 mM C16TAB in 2 M NaCl. Bar ) 100 nm. T ) 25 °C.

micelles, and the values of kq the pseudo-first-order rate constant in the reaction zone. kq varies with increasing NaC concentration. Since kq ≈ k2/V ∝ k2/rhc3, where k2 is a second-order constant for the reaction in the “reaction volume”, V, the parameter kq rhc3 should be correlated to D. Figure 7 shows the variation of D and the parameter kq rhc3, with composition. In the C16TAB/NaC system the mixed micelles were cylindrical in a narrow composition range. At 1 M NaCl only the composition containing 5 mM NaC (χNaC ) 0.17), and at 2 M NaCl three compositions with 0, 2, and 5 mM of NaC (χNaC ) 0, 0.074, 0.17), gave quenching indicating long cylindrical micelles for which the model of eq 1 was applicable. For the first composition the diffusion coefficient was 3.1 × 10-11 m2 s-1, slightly larger than for the same composition with 2 M NaCl (2.6 × 10-11 m2 s-1). Pure C16TAB (25 mM) in 2 M NaCl gave D ) 6.5 × 10-11 m2 s-1 in close agreement with the results obtained by Alsins and Almgren17 from fluorescence measurements on the CTAC/NaClO3 system. The diffusion coefficient decreases with increasing concentration of NaC, suggesting that the interior (hydrocarbon core) of the cylindrical micelles becomes less fluid or that the incorporated cholate anion hinders the free diffusion of the probe-quencher pair, a result which closely agrees with the results for the CnTAB/NaDOC system.8 The values of the parameter kqrhc3 lies between (16 and 28) × 10-20 m3 s-1 and varies in the same way as the relative diffusion coefficient, D. A similar trend is observed for the variation of D and kqrhc3 in the C18TAB/NaC/NaCl system as in the previous system. The only discrepancy arises in the composition C18TAB (25 mM) in 1 M NaCl. The cryo-TEM results presented below shows that this sample does not contain

cylindrical micelles but instead consists of globular micelles coexisting with disklike structures. The deactivation model of eq 1 is not applicable in this case and results in unreasonably high D and kqrhc3. However, the addition of NaC spontaneously transforms the coexisting lamellar aggregates and micelles into flexible cylindrical micelles. The results for the mixed cylindrical micelles indicate that D decreases with increasing concentration of NaC for the same reasons as discussed earlier for the C16TAB/NaC/NaCl system. The parameter kqrhc3 varies moderately like D and lies between (12 and 16) × 10-20 m3 s-1 for these compositions. The values obtained for the parameter hrhc, which is a parameter that weights reaction control to diffusion control, was in all cases larger than unity and below 10, indicating that a quencher in the reaction zone around an excited probe was more probable to react than to diffuse out from the reaction zone. Results from Cryo-TEM. In the interpretation of the micrographs one must bear in mind that projections of the systems are presented. It would not be possible, therefore, to identify a true x-junction between chains (if any where present); it would appear just as a crossing of two chains at different depths. We do regard y-junctions and loops as being usually real, however. The probability would be rather low that a chain terminates at a different depth, so that the projection appears as a y-junction or a loop. A few apparent loops or y-junctions could be expected, but not several in the same micrograph. Figure 8 shows giant threadlike cylindrical micelles present in 25 mM C16TAB/2 M NaCl. The elongated structures appear long and rigid and have probably some branching points occurring after long distances. Figure


Langmuir, Vol. 12, No. 9, 1996 2181

Figure 9. (a) Monodisperse globular micelles present in 25 mM C14TAB + 5 mM NaC in 1 M NaCl. (b) Cylindrical threadlike micelles present in 25 mM C16TAB + 5 mM NaC in 1 M NaCl. T ) 25 °C. Bar ) 100 nm.

9 shows micrographs of two samples in 1 M NaCl and 5 mM NaC added to (a) 25 mM C14TAB and (b) 25 mM C16TAB at 25 °C. Figure 9a clearly shows the presence of small globular micelles with no elongation into cylindrical micelles. Under the same conditions threadlike cylindrical micelles are formed in the C16TAB system (b). Figure 10 presents three micrographs of the micelles in 25 mM C18TAB with and without added NaCl, at 40 °C: (a) 0, (b) 0.5 M NaCl, and (c) 1.0 M NaCl. Globular micelles are seen in Figure 10a. Figure 10b shows a distribution of globular micelles (C) coexisting with disklike structures. The irregular disklike structure viewed as projections from the top (A) and side (B) are shown by the arrows. To make sure that the structures were not artifacts produced from the sample preparation procedure in the cryo-TEM studies, dynamic light scattering studies were made on the same compositions at 40 °C. The results clearly showed at least two distributions of relaxation times indicating that both large and small aggregates were present in the solution. The fast relaxation peak corresponded to a hydrodynamic radius (RH) of 28 Å and the slow to 500 Å.35 Figure 10c shows mostly lamellar aggregates (A,B) present in 25 mM C18TAB and 1 M NaCl. Figure 11 shows four micrographs of C18TAB in 0.5 M NaCl with the following NaC concentrations at 25 °C unless otherwise specified: (a) 3 mM (30 °C); (b) 5 mM; (c) 7.7 mM; (d) 11 mM. The C18TAB concentration was (35) Swanson-Vethamuthu, M.; Feitosa, E.; Almgren, M.; Brown, W. Unpublished results.

Figure 10. (a) Monodisperse globular micelles present in 25 mM C18TAB at 40 °C. Bar ) 100 nm. G points out the polymer film boundary. (b) The same composition as in (a) but in 0.5 M NaCl. A points out a disklike structure observed from the top, B points out the same structure lying on its side, and C points out a globular micelle. (c) The same composition in 1 M NaCl. The arrows point to similar structures A, B as in Figure 10(b).

fixed at 25 mM. The series of micrographs portrays the effect of increased additions of NaC to a composition originally consisting of globular and disklike structures at 40 °C (see Figure 10b). In Figure 11a the original globular micelles and disklike structures have disappeared and large cylindrical micelles, many of them branched with Y-junctions, are clearly seen. Figure 11b shows that the addition of more NaC makes the cylindrical micelles slightly more contorted and increases the frequency of

2182 Langmuir, Vol. 12, No. 9, 1996


Figure 11. A sequence of cryo-TEM micrographs taken from the C18TAB/NaC system: 25 mM C18TAB and varied NaC concentrations in the presence of 0.5 M NaCl. (a) 3 mM: Presence of large cylindrical micelles with some branching. T ) 30 °C, Bar ) 100 nm. (b) 5 mM: The cylindrical micelles are longer and show branched forks or Y-junctions (E) and some loops (D), T ) 25 °C. (c) 7.7 mM: The cylindrical micelles have shrunk in size, i.e., are less extended but more contorted and probably branching with Y-junctions (E). T ) 25 °C. The contrast in this micrograph has diminished which makes interpretation difficult. (d) 11 mM: Small globular micelles exist.


branching with Y-junctions (E); looping of micelles are also found (D). Figure 11c shows the effect of further increasing NaC, now to the foothill of the viscosity peak shown in Figure 3A. Highly contorted structures with greatly reduced length are found in many cases with branching (E). Figure 11d represents a distribution of small globular mixed micelles transformed from the cylindrical structures present in the previous composition. Figure 12 presents four micrographs of samples in 0.5 M NaCl with compositions (a) 2.5 mM, (b) 5 mM, (c) 10 mM, and (d) 20 mM NaDOC, keeping C16TAB fixed at 25 mM. The micrograph (a) shows the presence of threadlike cylindrical micelles found in a region where the viscosity starts to increase (see Figure 3B). The spherical C16TAB micelles (Nagg ) 134 ( 3) have grown into threadlike cylindrical micelles. The micrograph clearly shows the presence of Y-junctions (E) and some loops (D). Figure 12b represents a composition with 5 mM NaC corresponding to higher viscosity than (a). Here very long cylindrical micelles are present, which overlap. Some branching points and loops can be seen here too. Figure 12c shows cylindrical micelles which are more contorted and shorter than in the earlier picture. The frequencies of Y-junctions have increased (E) and loops still exist (D). Figure 12d shows cylindrical micelles that are even shorter and highly contorted. The viscosity is here comparable to that in (a) but with much more NaDOC. There are many more Y-junctions (E) and loops (D) although it is much more difficult to identify individual junctions unambiguously. Discussion CnTAB/NaCl Systems. The aggregation and phase behavior of the cationic surfactants is well described.34,36,37 Our results are in good agreement with that presented earlier and provide a good starting point for the discussion of the effects in CTAB/bile/NaCl systems. The results for C18TA+ in NaCl need some comments. No cylinder structures were found in this system; the cryo-TEM results indicate instead some type of flakes, reminiscent of the open bilayer structures that are often found in systems of unstable liposomes.38,39 Since light scattering studies also clearly show that large structures are present, in coexistence with small micelles, and the TRFQ results deviate from those for long cylinder micelles of the C16 compound, or cylinders of C18 mixed with some NaC, we cannot disregard these structures as artifacts from the sample preparation and vitrification. As the Krafft point is high for the C18 surfactant, one could otherwise suspect that the structures found in the cryo-TEM examination represented suspended microcrystals of the surfactant. CnTA+/NaC/NaCl Systems. The normal effect from addition of NaC is to decrease the size of the micelles, so that structures with larger curvature are obtained. This is the effect under all conditions when the alkyl chain length is 14 or shorter. With longer alkyl chains the results are more varied, but in all cases globular micelles are found when the fraction of NaC in the structure is larger than 0.29. The normal effect is also observed for all the surfactants in solutions without added salt. In the case of C18TA+ at 0.5 M and 1.0 M NaCl, the peculiar flake structures and the coexisting globular micelles are converted to cylinders on a small addition of NaC. This (36) Rubingh, D. N.; Holland, P. M. Cationic Surfactants: Physical Chemistry; Marcel Dekker, Inc.: New York, 1991. (37) Henriksson, U.; Blackmore, E. S.; Tiddy, G. J. T.; So¨derman, O. J. Phys. Chem. 1992, 96, 3894-3902. (38) Hammarstro¨m, L.; Velikian, I.; Karlsson, G.; Edwards, K. Langmuir 1995, 11, 408. (39) Andersson, M.; Hammarstro¨m, L.; Edwards, K. J. Phys. Chem. 1995, 99, 14531-14538.

Langmuir, Vol. 12, No. 9, 1996 2183

is also a type of normal behavior, since the transformation is in the direction of increasing curvature; the abnormal or surprising finding is that lamellar structures are present from the beginning. A clear anomalous effect from a low concentration of NaC is the transition from globular micelles to cylinder structures for C16TA+ in 1.0 M NaCl and, in the case of 2.0 M NaCl where rodlike micelles exist already without bile salt, the increase in length of the cylinders toward a viscosity maximum of the solution. Here NaC seems to decrease the curvature of the structures, and it is hard to see how this could be achieved without the bile salt molecule becoming inserted in the structure to some extent, in a similar way as we have assumed for NaDOC. The rationale for this to occur at high salt concentrations would then be the “salting out” of the hydroxy groups; they are less hydrophilic at high salt.21 It may also be possible to explain why the tendency for insertion would be less important with shorter alkyl chains (although we do not see how) but it cannot simultaneously be less important also for the long-tailed C18 compound. It is possible, however, that NaC is inserted into the C18 structures in this way, but in this case, when the starting point is the tightly packed lamellar structure, the effect is to increase the curvature. We used a similar argument to explain the thinning of the cylinders, observed from X-ray studies of the hexagonal phase, when C16TAB was mixed with NaDOC.8 Another explanation of the packing which favors the cylindrical structures is suggested from the recent SANS study by Hjelm et al.11 of the cylindrical mixed micelles that are formed when lecithin bilayers are dissolved by bile salts. These authors suggest that the phosphatidylcholine lipids are arranged radially and that the bile salt molecules are inserted between the PC headgroups with the axis parallel to the axis of the cylinder, and with the hydrophobic face toward the interior of the micelle. This model is consistent with the SANS data and with the composition of the mixed micelles, with three PC per bile salt molecule. It is also shown that very similar results are obtained with different PC-lipids, and both with dihydroxy and trihydroxy bile salts, suggesting a similar arrangement in all cases. At high salt concentration when the repulsion between the charged headgroups is reduced, one could imagine a similar packing in a cylinder micelle containing long chain CTA+ and bile salt. Two surfactants would roughly correspond to one lipid, and the favorable packing would be expected with bile salt/surfactant ) 1/6, which is close to the composition (1/5) giving the maximum viscosity for NaC. Obviously, this model could not explain the large difference between NaC and NaDOC in these systems, and we retain the proposition that the body of the dihydroxy bile salt is inserted in the interior. Effects of NaC and NaDOC on the Long Cylindrical Micelles. The sequence of electron micrographs in Figure 12 shows the morphological changes in the cylindrical micelles with increasing addition of NaDOC up to just before coacervation. The micelles appear particularily long in Figure 12b, before the composition giving viscosity maximum which corresponds to Figure 12c. In all the micrographs there are some loops and Y-junctions, but it cannot be claimed that the frequency of such structures changes with the composition. The main impression is that shorter and more contorted structures appear in Figure 12c and in particular in Figure 12d, which represents a solution with rather low viscosity just before coacervation. Qualitatively the evolution is similar to that shown in Figure 11 for C18TA+ on addition of NaC, although in this case the micelles are not as long and are broken down to

2184 Langmuir, Vol. 12, No. 9, 1996


Figure 12. A sequence of cryo-TEM micrographs taken from the C16TAB/NaDOC system: 25 mM C16TAB and varied NaDOC concentration, in the presence of 0.5 M NaCl at 25 °C. (a) 2.5 mM: The cylindrical micelles show branched forks or Y-junctions (E) and some looping (D). (b) 5 mM: The cylindrical micelles appear very elongated and flexible. A lot of entanglement, interconnections, and some looping of the micelles is seen. The arrow points out an ice crystal (F). (c) 10 mM: The cylinders are more curved and irregular, have comparitively shorter extensions than in Figure 12(b) but probably more frequent branching with Y-junctions (E). Several micelles forming loops (D) are seen. (d) 20 mM: Similar type of structures (E) and (D) as in Figure 12(c) but considerably reduced in size and highly contorted. Bar ) 100 nm.


globular micelles at a still low bile salt concentration. The two bile salts seem to affect the structuring in a similar way, that cannot be fully rationalized from a simple consideration of a uniform effect on a type of average surfactant parameter. In retrospect, this is not surprising, for if an average surfactant parameter should be relevant, then the two surfactants must be more or less randomly distributed in the structure. In the present case strongly nonideal mixing is expected. In particular, if two main modes of association of the bile salts in the structure are possible, one superfacial that we have assumed preferred by the cholate ion and one deep insertion between the surfactant molecules, that would be more prevalent for desoxycholate; then the fraction of molecules associated with the structure in one way or the other would depend both on the amount of bile salt in the micelle and on the packing in the micelle without bile salt. The accommodation of large molecules flat on the surface, between the surfactant head groups, would give a very large local disturbance that would more easily be accepted by a spherical micelle with large curvature and large area available per head group, than by a closely packed cylindrical micelle at high salt concentration. It is possible, therefore, that the first bile salt molecules added to the micelles at high salt concentration are inserted between the CTA+ molecules, even in the case of cholate, in spite of the hydrophilic OH groups, since otherwise they would disrupt the micelle structure in a too costly way. Inserted in this way the bile salt molecules could promote the transition to long cylinder structures. On the other hand, several bile salt molecules in the flat position, together with some surfactants, could assemble to form a stabilized cylinder end-cap, thus promoting the breakdown of the long threads into shorter rods, and finally spherical mixed micelles. Since several added molecules have to cooperate in this act, it would not be important until a certain fraction of bile salt molecules is present in the micelles. The critical bile salt content would be lower for cholate than for desoxycholate, since the former is more apt to adsorb in the flat position. This proposal is highly hypothetical. It contains some strong statements about the packing of the bile salt molecules in the micelles under different conditions that should be possible to test, e.g., with the powerful NMR relaxation techniques. Most of the cryo-TEM micrographs

Langmuir, Vol. 12, No. 9, 1996 2185

have too low a contrast to allow a detailed study of the structures. Occasionally, however, some examples with very good contrast are obtained, such as shown in Figures 11b and 12a,b. In these, there are some examples of branched cylinders (and some loops), but in no case does it appear as branching would be a normal and common feature. In the case of cholate, the breakdown of the threads to globular micelles is quite evident and branching would not be expected, but with desoxycholate, globular micelles do not form until an excess of the bile salt is present and the evolution into structures related to those of the cubic phase must still be regarded as a viable alternative to the formation of end-capped short rods. The crucial region is that represented by the micrograph in Figure 12d, where it is impossible to decide to what extent branched structures are present. Conclusion This investigation was prompted by the realization that the rather different effects of the bile salts sodium cholate and sodium desoxycholate were not always possible to rationalize from the assumption that the former is adsorbed flat on the micelle interface, promoting highly curved small structures, whereas the latter could be inserted in the micelles and promote a transition to less curved, larger structures. The results presented have led us to propose, tentatively, that the behavior can be understood if both bile salts, to different degrees, may associate in both ways in the mixed micelles, and in particular that above a certain fraction of bile salt in the micelles (higher for NaDOC than for NaC) these molecules may cooperatively stabilize the ends of cylindrical micelles and thus promote a breakdown to smaller structures. This would explain the observed viscosity maximum at a certain proportion of bile salt in the micelles (smaller for NaC than for NaDOC) and the decreasing size of the structures at higher bile salt content, shown in cryo-TEM micrographs. Acknowledgment. This work was supported by grants from the Swedish Natural Science Research Council (NFR), the Swedish National Board for Industrial and Technical Development (NUTEK), and the Knut and Alice Wallenberg Foundation. LA950964H

Effect of Sodium Chloride and Varied Alkyl Chain ... - ACS Publications

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