1590
Langmuir 1998, 14, 1590-1596
Salt-Induced Sphere-to-Disk Transition of Octadecyltrimethylammonium Bromide Micelles Martin Swanson-Vethamuthu,*,† Eloi Feitosa,‡ and Wyn Brown Department of Physical Chemistry, University of Uppsala, Box 532, 751 21 Uppsala, Sweden Received August 16, 1996. In Final Form: December 29, 1997
We have used surface tension measurements, differential scanning calorimetry (DSC), dynamic light scattering (DLS), and cryo-transmission electron microscopy (cryo-TEM) to investigate the dynamic and structural behavior of octadecyltrimethylammonium bromide (C18TAB) micelles in water and NaBr solution. The surface tension data for fixed C18TAB concentrations of 25 mM and varied NaBr additions (0-50 mM) shows that the critical micelle concentration (cmc) increases after an initial decrease at 0.5 mM NaBr. This unusual effect has been explained using results from DSC and DLS. At low salt concentrations (below ca. 25 mM) the relaxation time distribution is bimodal with a dominant fast mode due to spherical micelles. Above ca. 35 mM NaBr disklike structures are favored and the relaxation time distribution is more closely unimodal. The postulated sphere-to-disk transition is supported by cryo-TEM micrographs. A pronounced increase in the micellar effective hydrodynamic radius (RH) is observed as the NaBr concentration is increased above about 35 mM; below 35 mM the RH of the spherical micelles changes little with ionic strength.
Introduction The effect of adding monovalent electrolyte to ionic surfactants in solution is well-known to provide the driving force for microstructural transitions from micelles of high curvature in salt-free solutions to larger structures with cylindrical and planar geometries. In addition, there are accompanying effects on the cmc and the phase behavior. A transition to from spherical to rod-like micelles occurs when simple salts are added to the initially salt-free aqueous surfactant solution due to the screening of electrostatic repulsions between the charged headgroups. The salt-induced transition from globular micelles to cylindrical aggregates and larger planar microstructures on further salt addition is mainly an electrostatic effect.1-4 The properties of long-chain derivatives (n g 18) of monoalkyl cationic surfactants such as octadecyltrimethylammonium halides (C18TAX) in water or salt solutions have been little studied owing to their low solubilities in water and their high Krafft boundaries. Kodama et al.5,6 studied thermal transitions in the C18TAX/water system as a function of counterion type and investigated the thermodynamic stability of the gel and coagel phases existing below the Krafft transition temperature. Some studies7,8 have dealt with dilute micellar solutions close to the cmc, and others have dealt with the phase behavior † Present address: Department of Chemical Engineering, University of Delaware, Newark, DE 19716. ‡ Permanent address: Departamento de Fisica, IBILCE/UNESP, Sa˜o Jose´ do Rio Preto, SP, Brazil.
(1) Ekwall, P.; Mandell, L.; Solyom, P. J. Colloid Interface Sci. 1971, 35, 519. (2) Porte, G.; Appell, J.; Poggi, Y. J. Phys. Chem. 1980, 84, 3105. (3) Cates, M. E.; Candau, S. J. Phys.: Condens. Matter 1990, 1990, 6869. (4) Khatory, K.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993, 9, 1456. (5) Kodama, M.; Seki, S. J. Colloid Interface Sci. 1987, 117, 485. (6) Kodama, M.; Tsujii, K.; Seki, S. J. Phys. Chem. 1990, 94, 815. (7) Barry, B. W.; Russel, G. E. J. J. Colloid Interface Sci. 1972, 40, 174. (8) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993, 97, 6024.
and the liquid-crystalline phases.9 Increasing the alkyl chain length (nc) of the cationic surfactant in water results in an increased micelle size-shape polydispersity, which in turn can lead to differences in phase behavior and formation of new phases within a homologous series of binary systems. At high surfactant concentrations, the formation of intermediate liquid-crystalline phases instead of the bicontinuous cubic phases found for short alkyl chains is observed. For example, studies9,10 on the C16TAX/water system (X ) Cl, Br, I) have shown that the length of the alkyl chain and the type and location of counterions are important variables influencing the occurrence and stability of intermediate liquid-crystalline phases formed between the normal hexagonal (H1) and lamellar (LR) phases. In a recent study11 of alkyltrimethylammonium bromides in water and salt solutions as a function of alkyl chain length (even numbers of carbons from 12 to 18), the C18 derivative attracted our attention because at high ionic strengths a transition from globular micelles to lamellar aggregates was found, apparently without intermediate growth into cylindrical micelles. Only a sphere-to-rod transition was found for the corresponding C16TA+ system under similar conditions. This remarkable difference between the two surfactants differing only by two methylene segments prompted us to investigate the present system in more detail to elucidate this behavior. The results of surface tension and DSC measurements at varied electrolyte levels are presented, followed by dynamic light scattering results. Evidence from cryo-TEM is provided which supports our conclusions. Materials and Methods Octadecyltrimethylammonium bromide (C18TAB) was purchased from Fluka and used without further purification; the (9) Blackmore, E. S.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1115. (10) Henriksson, U.; Blackmore, E. S.; Tiddy, G. J. T.; So¨derman, O. J. Phys. Chem. 1992, 96, 3894. (11) Swanson-Vethamuthu, M.; Almgren, M.; Karlsson, G.; Bahadur, P. Langmuir 1996, 12, 2173.
S0743-7463(96)00816-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/24/1998
Salt-Induced Sphere-to-Disk Transition of C18TAB Micelles NaBr was analytical grade. Highly pure (MilliQ-plus) water was used to prepare the solutions. The surface tensions of surfactant aqueous solutions were measured by a drop-volume technique. The procedure has been described earlier.12,13 The purity of the surfactant was tested by repeating cmc measurements for two different batches of C18TAB. No surface-active impurities were detected, and the results were found to be reproducible. DSC measurements were carried out using a DuPont Model 9900 Thermal Analyzer to monitor enthalpic changes occurring due to transition from a biphasic mixture to a micellar solution. A dry nitrogen purge was maintained throughout the experiments. DSC samples of 5-10 mg were prepared in special pans available for liquid compositions and placed in a high-pressure cubicle after properly crimping the pans to minimize weight loss. The heat flow obtained from DSC curves depended on the rate at which temperature was ramped and the amount of salt in the sample. Temperature ramps were run from 20 to 70 °C at a heating rate of 0.5 °C/min. Faster temperature ramps and higher salt content in samples gave sharper transition temperature peaks. However, a slow rate of heating was used in all cases to obtain more accurate ∆H values. All samples were preheated above the transition temperature to ensure homogenous mixing and later cooled to about 5 °C and maintained for 2 days to complete the nucleation of the coagel phase before performing any DSC. The coagel phase is the biphasic hydrated crystalline state in excess water or NaBr solution. The transition to the micellar phase can be direct or indirect, the latter involving an additional metastable gel phase6 (Lβ) which appears homogenous and semitransparent at 34 °C upto the Krafft Temperature6,9 (Tc ) 38 °C). The Lβ phase is probably composed of stiff and tilted monolayers of interdigitated alkyl chains that precipitate out with time. Care was taken to ensure that all ∆H changes recorded reflected the total phase transition from fully equilibrated mixtures to clear isotropic micellar solutions. Dynamic light scattering (DLS) measurements were made using the same apparatus described previously;14 a 633-nm He/ Ne laser was used as light source and the detector system consisted of an ITT FW 130 photomultiplier connected to an ALV-Langen Co. multibit, multitau, autocorrelator through a digitalized amplifier/discriminator system. The inverse Laplace transform (ILT) analysis of the normalized intensity autocorrelation functions, g2(t), was performed using the algorithm REPES15 to obtain the relaxation time distribution, τA(τ), according to
g1(t) )
∫τA(τ) exp(t/τ) d ln τ
(1)
Langmuir, Vol. 14, No. 7, 1998 1591
Figure 1. (a) Plots of surface tension versus log [C18TAB] at 40 °C for selected NaBr concentrations as shown. (b) Effect of NaBr concentration on the cmc of C18TAB in solution at 40 °C. the concentration18 (C)
D ) D0(1 + kDC +...)
(2)
where kD is the hydrodynamic virial coefficient which characterizes the thermodynamic and hydrodynamic interactions between the particles in solution, given by
kD ) 2A2M - kf - 2v2
(3)
where g1(t) is the normalized electric field autocorrelation function, g2(t) - 1 ) β|g1(t)|2. β is a factor (e1) taking into account deviations from ideal coherence,16 and τ is the relaxation time. The relaxation time distribution is presented as a τA(τ) versus log τ profile, with τA(τ) in artitrary units, providing an equal area representation.17 The average relaxation rates and amplitudes are obtained from the moment of each mode (Γ ) 1/τ). The smoothing parameter (P) was normally selected as 0.5. The diffusion coefficient for each mode is calculated from the relation D ) Γ/q2, in the limit of zero scattering angle, where q ) (4πn0/λ) sin(θ/2) is the scattering vector modulus, λ is the wavelength, n0 is the solvent refractive index, and θ is the scattering angle. The diffusion coefficient for small particles is linearly dependent on
where A2 is the second virial coefficient, M is the molecular weight, kf is the friction factor, and v2 is the partial specific volume. D0 is the diffusion coefficient at infinite dilution, which is related to the hydrodynamic radius (RH) through the Stokes-Einstein equation,
(12) Tornberg, E. J. Colloid Interface Sci. 1977, 60, 50. (13) Swanson-Vethamuthu, M.; Almgren, M.; Hansson, P.; Zhao, J. Langmuir 1996, 12, 2186. (14) Johnsen, R. M.; Brown, W. In Laser Light Scattering in Biochemistry; Harding, S. E., Sattele, D. B., Bloomfield, V. A., Eds.; Royal Society of Chemistry: Cambridge, U.K., 1992; p 77. (15) Jakes, J. Czech. J. Phys. 1988, B38, 1305. (16) Zhou, B.; Chu, B. J. Colloid Interface Sci. 1988, 126, 171. (17) Nicolai, T.; Brown, W.; Johnsen, R. M.; Ste`pa`nek, P. Macromolecules 1990, 23, 1165.
Surface Tension and DSC. In Figure 1a the surface tension (γ) versus C18TAB concentration (C) plots in water and in increasing amounts of NaBr (5, 35, and 75 mM) at
D0 ) kBT/6πη0RH
(4)
where kB is the Boltzmann constant, T is the absolute temperature, and η0 is the solvent viscosity. The samples for cryo-TEM were vitrified from 40 °C according to a procedure described earlier.19
Results and Discussion
(18) Brown, W.; Nicolai, T. In Dynamic Light Scattering: The Method and Some Applications; Brown, W., Ed.; Clarendon Press: Oxford, 1993; p 272. (19) Edwards, K.; Gustafsson, J.; Almgren, M.; Karlsson, G. J. Colloid Interface Sci. 1993, 161, 299.
1592 Langmuir, Vol. 14, No. 7, 1998
Swanson-Vethamuthu et al.
Figure 2. Variation of enthalpy change (∆H) as a function of increasing NaBr concentration.
40 °C are presented. The reductions in γ due to C from the surface tension of pure water (69 mN m-1 at 40 °C) for the salt-free case and the different NaBr compositions are similar, about 40 mN m-1 at the cmc. The cmc is taken as the surfactant concentration where a distinct break occurs in the two linear sections of the γ versus C plots shown in Figure 1a. The surface tension remains constant above the cmc only for the NaBr-free composition because all excess surfactant monomers are incorporated into micelles of approximately similar size.11 In contrast, all compositions with added NaBr show a lower surface tension after the cmc has been reached. This apparent decrease of tension after the cmc could be due to incorporation of excess surfactant monomers into micelles with increasing size polydispersity or due to formation of a gel monolayer at the interface.20,21 By applying the simplified Gibbs adsorption equation (eq 5) to surface tension data, the area As per surfactant molecule at the air/water interface can be calculated from the slope of the linear part of the γ versus C plots in Figure 1a.
Γs ) 1/As ) -(1/nRT)(dγ/(d log csurf)
(5)
Where Γs is the surface excess of the cation, C18TA+, R is the gas constant, T is the temperature, n ) 2, and csurf is the concentration of free C18TAB monomers in solution. The difference in slope for C18TAB/NaBr compositions compared to the salt-free solution indicates the change in packing at the air/water interface. When no salt is present, the area per molecule is 54 ( 2 Å2 while additions of salt result in a significant decrease in As, which lies between 40 and 44 Å2. The decrease in As agrees well with the screening effect postulates and with the area per molecule obtained from neutron reflectivity data.8 Figure 1b shows a Corkill-Harkins-type representation of cmc as a function of NaBr concentration (cs), that is (log cmc versus log(cs + cmc)). The cmc for C18TAB in pure water is 0.34 ( 0.02 mM at 40 °C, in good agreement with literature values.8,22 Addition of 0.5 mM NaBr resulted in an expected lowering of the cmc to 0.25 mM. For salt compositions greater than 0.5 mM, the cmc does not (20) Wade, W. H.; Morgan, J. C.; Schechter, R. S.; Jacoson, J. K.; Salager, J. L. Soc. Pet. Eng. J. 1978, 18, 242. (21) Puig, J. E.; Franses, E. I.; Davis, H. T.; Miller, W. G.; Scriven, L. E. Soc. Pet. Eng J. 1979, 71. (22) Phillips, J. N. Trans. Faraday Soc. 1955, 51, 561.
Figure 3. Intensity autocorrelation functions, g2(t) - 1, for a solution of C18TAB, 50 mM at 40 °C, at increasing concentration of NaBr, as shown in the legend. Measurements at θ ) 90°.
decrease monotonically but instead increases to 0.30, 0.32, and 0.4 ( 0.02 mM for solutions containing 5, 35, and 75 mM of NaBr, respectively. The decrease in cmc (although only initially) is expected because, for longer chain surfactants in pure water, the free electrolyte level is small and with no added salt the electrostatic free energy contribution, Gel (to the total free energy), to be overcome for micellization is relatively high. Upon NaBr addition, a decrease in the cmc is obtained because the increased counterion concentration lowers the repulsion between charged headgroups and reduces Gel, thus favoring micellization at a lower concentration (cmc). The dependence of cmc on electrolyte concentration is well-known for ionic micellar systems22 in general and for lower chain homologues of alkyltrimethylammonium halides7 (C16TAB-C10TAB) with NaBr in particular. However, such an effect is not found above 0.5 mM additions of NaBr in the C18TAB/NaBr mixtures. To understand this observation, complementary experiments using DSC are discussed below. DSC was performed on compositions identical to those used for surface tension, DLS, and cryo-TEM measurements to monitor the enthalphic change associated with the phase change within a temperature range from 20 to 70 °C. Figure 2 shows variation of the enthalpy change (∆H) associated with the total coagel-micelle phase transition of the C18TAB/water system with increasing NaBr content. An enthalpy change, ∆H, of 74 kJ/mol is observed for the salt-free composition. A large ∆H is expected when the hydrated crystals dissolve completely to form stable micelles above Tc; the literature6 value is 64 kJ/mol. Adding small amounts of NaBr (5-35 mM) lowers the ∆H to a value between 27 and 29 kJ/mol, close to the ∆H measured for the metastable gel-micelle transition at Tc* ) 34 °C in salt-free compositions (∆H ) 27.1 kJ/mol).6 The lowering of ∆H with NaBr additions can be understood by combining results from DLS and cryo-TEM (shown later). The combined results indicate that all isotropic micellar compositions studied at 40 °C consist of coexisting spherical aggregates and planar structures in equilibrium. In salt-free solutions the dominant structures are monodisperse spherical micelles in equilibrium with a few disklike objects11 that are probably remnants of the gel phase existing below Tc. Spherical micelles and planar structures continue to coexist in compositions with NaBr additions above 0.5-35 mM, where the ∆H remains
Salt-Induced Sphere-to-Disk Transition of C18TAB Micelles
Figure 4. Angular dependence of the relaxation rates of the fast and slow modes obtained with DLS from an ILT of the autocorrelation functions for a sample with 40 mM C18TAB in 5 mM NaBr at 40 °C.
a
b
Figure 5. (a) Relaxation time distributions for the C18TAB solution in a salt-free solution and in different NaBr (5 and 25 mM) concentrations as a function of increasing surfactant concentrations up to 50 mM. Measurements were performed at θ ) 90° and 40 °C. (b) Relaxation time distribution for the C18TAB solution in different NaBr (37.5, 50, and 75 mM) concentrations as a function of increasing surfactant concentrations up to 50 mM.
nearly constant (27-29 kJ/mol). However, it is apparent that the structures with planar geometry are increasingly favored at higher NaBr content, as concluded from the DLS results shown below and geometrical packing arguments.23 The increase in cmc on addition of NaBr above 0.5 mM is now expected because one must further increase the free surfactant concentration in order to form less
Langmuir, Vol. 14, No. 7, 1998 1593
Figure 6. Effect of salt additions on the total scattering intensity for different C18TAB/NaBr/water compositions. Measurements at θ ) 90° and 40 °C.
favored micelles of higher curvature. Increasing the NaBr concentration from 35 to 50 mM lowers the ∆H from about 27 to about 9 kJ/mol, in line with the large planar microstructures observed by cryo-TEM and DLS. The presence of undetected non-surface-active impurities promoting bilayer formation cannot be ruled out. However, before an interpretation based on nonequilibrium effects can be made, systematic dynamic surface tension data need to be acquired. Dynamic Light Scattering. The scattering measurements were performed at 40 °C (at this temperature the micellar solution is clear and homogeneous), but the salt-free solution has also been measured at 50 °C to check that the close proximity to the Krafft temperature range (ca. 34-38 °C) has no significant effect on the relaxation time distribution. At 40 and 50 °C the distributions are essentially the same (results not shown). In presence of NaBr the Tc is marginally lowered but the possible effect of temperature on the diffusion coefficient was not examined. The autocorrelation function, g2(t), for C18TAB in the presence of some selected NaBr concentrations up to 75 mM is depicted in Figure 3. For salt-free and low-salt compositions, the distribution is at least double-exponential with a slowly decaying long-time portion. This behavior is typical for polyelectrolytes at zero or low ionic strength. Further additions of NaBr increase the total scattering intensity dramatically and result in a strong reduction in the contribution of the long-time tail of the correlogram. The correlograms illustrated in Figure 3 have a “tight” log scale and therefore appear at first sight to be grossly similar. Both the correlograms and the inversion results are provided because they are the primary data forms underlying the distributions in subsequent figures (e.g. Figure 5A). Figure 4 shows a typical linear dependence of the fast and slow mode relaxation rates (Γ) on the square of the scattering vector (q2) for a composition of 50 mM C18TAB in 5 mM NaBr. This shows that the two modes are diffusive. The Stokes-Einstein equation (eq 4) may be used to evaluate the hydrodynamic radius at infinite dilution. The relaxation time distributions for C18TAB in the absence and presence of salt, NaBr (0-75 mM), are (23) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
1594 Langmuir, Vol. 14, No. 7, 1998
Swanson-Vethamuthu et al.
Figure 8. Hydrodynamic radius (RH) at infinite dilution as a function of the NaBr concentration obtained from extrapolation in Figure 5.
Figure 7. (a) Variation of the fast mode diffusion coefficient as a function of C18TAB concentration in the presence of increasing NaBr concentrations, as shown. Measurements at θ ) 90° and 40 °C. (b) Variation of the slow mode diffusion coefficient as a function of C18TAB concentration in the presence of increasing NaBr concentrations, as shown. Measurements at θ ) 90° and 40 °C.
shown in Figure 5 with progressively increasing surfactant concentrations at a fixed ionic strength within a given series. The first column of Figure 5a shows ILT traces for the salt-free case. In this sample the contribution of the dominant spherical micelles to the total scattered intensity is weak, since the slower mode which arises from the scattered intensity of a few large clusters dominates the intrinsically low total scattering from the samples. The inverse Laplace transform used provides highly reproducible decay time distributions for multiple decays, as has been earlier established by analysis of simulated distributions. Even the presence of large birefringent particles will not exert an influence on the decay times of other components which are well-separated on the time scale, although they may dominate the total scattered intensity. Fluorescence quenching and cryo-TEM results presented in an earlier publication11 for these compositions demonstrated the presence of globular micelles with an aggregation number of 230 ( 3 monomers. In the presence of 5 mM NaBr (column 2, Figure 5a) the fast diffusive mode attributed to globular micelles clearly dominates the distribution of relaxation times. The interand intraaggregate screening of the repulsive interactions causes the slower mode attributed to the motion of a few
Figure 9. Dependence of the NaBr concentration on the hydrodynamic virial coefficient for the C18TAB solution. Measurements at θ ) 90° and 40 °C.
larger aggregates to diminish in relative intensity. Column 3 shows the corresponding relaxation time distributions for the system with 25 mM NaBr. The faster mode is now completely dominant but shifts to longer times when compared to those for the previous two columns, indicating a gradual growth of the globular micelles. The relative amplitude of the slower mode has decreased, but it progressively increases as the surfactant concentration is increased within the series. Figure 5b, column 1, represents a composition (37.5 mM NaBr) where the total scattering intensity dramatically increases, indicating some type of transition to much larger aggregates. The dominant diffusive mode is shifted to even longer relaxation times. Within the series of surfactant concentrations the main peak is rather broad at low concentrations and shifts to longer times as the surfactant concentration is increased. The composition probably represents a highly polydisperse sample in size and shape of the aggregates. At 50 mM NaBr the transition is already complete and the distribution is rather well-defined and unimodal. Higher salt concentrations (75 mM in this diagram) show mainly two diffusive modes with very large aggregates. In general the main mode tends to shift to lower frequency (longer relaxation times) as the salt concentration is successively increased. We intend to show that this shift of the main diffusive
Salt-Induced Sphere-to-Disk Transition of C18TAB Micelles
Langmuir, Vol. 14, No. 7, 1998 1595
Figure 10. Cryo-TEM micrographs taken at 40 °C from a 25 mM C18TAB sample in the presence of (A) 5, (B) 37.5, and (C) 50 mM NaBr. Bar ) 100 nm.
1596 Langmuir, Vol. 14, No. 7, 1998
peak indicates a growth of micelles from initial spherical symmetry possibly to larger disklike micelles and lamellar structures of planar geometry, as is also suggested by the cryo-TEM data presented below. Figure 6 shows the dependence of the normalized total scattering intensity as a function of C18TAB concentration with increasing concentration of NaBr. Up to 25 mM NaBr, a gradual stepwise increase in intensity is observed. At 37.5 mM NaBr and above a dramatic increase in the normalized scattered intensity is observed. The dependence of the scattered intensity on the ionic strength supports growth of the micelles to large aggregates. The variations of the diffusion coefficient calculated from the dominant diffusive mode and the slow mode (present in the salt free compositions and those with 5 mM NaBr) as a function of total surfactant concentration at different NaBr concentrations are shown in parts a and b of Figure 7, respectively. Figure 7a shows that, except for the compositions with 37.5 mM NaBr, D increases with increasing surfactant concentration. This general behavior of the diffusion coefficient differs in 37.5 mM NaBr, which shows instead a nonlinear decrease in D with increasing surfactant concentration. Figure 7b shows the nonlinear dependence of D for the slower mode. The intercepts of the curves in Figure 7 allow estimation of the hydrodynamic radius (RH) using eq 4. The resulting dependence of RH on salt concentration is shown in Figure 8. Below about 35 mM of NaBr, RH increases only slowly and the micelles are predominantly spherical, coexisting with a few larger disklike micelles; above 35 mM NaBr, there is a sharp increase of RH with the NaBr concentration, indicating that the micelles grow. At high salt concentrations the disklike lamellar aggregates seen in the cryo-TEM pictures are favored. A similar extrapolation for the slower mode gives RH ) 328 and 300 Å, at zero and 5 mM NaBr, respectively. Figure 9 shows the variation of the hydrodynamic virial coefficient (kD) as a function of increasing ionic strength. Small additions of salt decrease kD due to the screening of the electrostatic repulsion between the charged headgroups. Further addition of salt decreases kD up to the composition with 37.5 mM NaBr, above which a sharp increase in kD is observed. This behavior supports the proposed transition from small globular micelles to large planar or disklike micelles, which are characterized by different interaction parameters. The bimodal distribution of diffusive relaxation times suggests that the large and small aggregates coexist in salt solution. As the concentration of salt increases, growth into the larger disklike aggregates is favored. The transition to larger structures takes place at about 35 mM NaBr. At 50 mM salt and above, the dominant structures are probably disklike lamellar aggregates seen in the cryo-TEM pictures. Cryo-TEM. The cryo-TEM micrographs present twodimensional projections of the microstructures adsorbed on a porous polymer film vitrified rapidly at 40 °C. The
Swanson-Vethamuthu et al.
disklike structures appear as thick lines when observed from the top (A) or irregular cuplike structures when observed from the side (B), depending on the viewing angle. When larger lameller structures are seen, they can appear twisted, showing both the top and side views of the same microstructure. Figure 10a shows that the predominant structures at low salt concentration (25 mM C18TAB in 5 mM NaBr) are globular micelles (C) which coexist with a few larger disklike micelles (A). At intermediate salt concentration (37.5 mM), shown in Figure 10b, the disklike micelles (A and B) are the dominant structures and these coexist with globular micelles (C). The micrograph shows a large polydispersity in sizes and shapes and some twisted bilayer structures. Figure 10c shows large bilayer structures (top and side views). Increasing salt concentrations to 50 mM and above seems to shift the equilibrium to the much larger disklike structures and lamellar aggregates, which are increasingly favored; intermediate cylindrical rodlike micelles were not observed. Conclusions Direct imaging of the C18TAB/water mixtures at fixed surfactant concentration and temperature provides a qualitative sequence of microstructures formed as a function of increasing NaBr concentration. The dominating microstructure in a particular composition is a result of a complex interplay between hydrophobic, electrostatic, and ionic strength effects. Surface tension measurements show that, with increasing NaBr concentration, the area per headgroup at the air/water interface decreases from 54 ( 2 Å to about 42 ( 2 Å, which provides a consistent packing argument for a favored transition from spherical micelles to larger disklike structures. The enthalpy change ∆H, for the coagel to micelle transition is significantly lowered in the presence of NaBr. The increase in cmc can be linked to the lower ∆H observed on NaBr additions. The formation and stability of low-curvature aggregates in solution is favored, and consequently a higher free surfactant concentration is necessary to form less favored micelles of higher curvature. From DLS, the inverse Laplace transform analysis shows that at low concentrations of NaBr the relaxation time distribution is predominantly bimodal, owing to the coexistence of globular and disklike microstructures. At higher salt concentrations the distribution is more closely singleexponential, owing to the predominance of the planar structures. Acknowledgment. E.F. thanks the Conselho Nacional de Desenvolvimento Cientı´fı´co e Tecnolo´gico (CNPq) for a stipend (Grant 201720/93-0). We are grateful to Goran Karlsson for the cryo-TEM measurements and Samrat Dua for the DSC measurements. This work has been supported in part by the Swedish Technical Research Council (TFR). LA9608167
