Thermodynamics of micellization from heat-capacity measurements.

Differential scanning calorimetry (DSC), the most important technique for studying the thermodynamics of structural transitions of biological macromolecules, is seldom used in quantitative thermodynamic studies of surfactant micellization/demicellization. The reason for this could be ascribed to an insufficient understanding of the temperature dependence of the heat capacity of surfactant solutions (DSC data) in terms of thermodynamics, which leads to problems with the design of experiments and interpretation of the output signals. We address these issues by careful design of DSC experiments performed with solutions of ionic and nonionic surfactants at various surfactant concentrations, and individual and global mass-action model analysis of the obtained DSC data. Our approach leads to reliable thermodynamic parameters of micellization for all types of surfactants, comparable with those obtained by using isothermal titration calorimetry (ITC). In summary, we demonstrate that DSC can be successfully used as an independent method to obtain temperature-dependent thermodynamic parameters for micellization.

Salicylate isomer-specific effect on the micellization of dodecyltrimethylammonium chloride: large effects from small changes.

Specific effects of the sodium salts of m- and p-hydroxybenzoates (m-HB and p-HB) on the aggregation process of dodecyltrimethylammonium chloride have been investigated by isothermal titration calorimetry, electrical conductivity, and (1)H NMR and compared with already reported data for the sodium salt of o-hydroxybenzoate (o-HB). For p-HB, it has been found that the aggregate is only formed by spherical micelles at all p-HB concentrations. On the other side, the situation is more complex for o-HB, where two distinct states of aggregation can be involved, depending on the concentration of o-HB. At high salt concentration, rodlike micelles are formed, whereas at lower concentration spherical aggregates are predominant. The transition from the cylinder to the sphere increases the mobility of the surfactant because the core of the rodlike micelles is more closely packed due to the expulsion of water from the interior of the aggregate. m-HB exhibits an intermediate behavior between these two extreme situations. The effect of the position of hydrophilic substituents on the aromatic ring on the insertion of the hydroxybenzoate anion in the micellar aggregate has been discussed.

Thermodynamic characterization of 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) micellization using isothermal titration calorimetry: temperature, salt, and pH dependence.

A systematic investigation of the micellization process of a biocompatible zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) has been carried out by isothermal titration calorimetry (ITC) at temperatures between 278.15 K and 328.15 K in water, aqueous NaCl (0.1, 0.5, and 1 M), and buffer solutions (pH = 3.0, 6.8, and 7.8). The effect of different cations and anions on the micellization of CHAPS surfactant has been also examined in LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. It turned out that the critical micelle concentration, cmc, is only slightly shifted toward lower values in salt solutions, whereas in buffer media it remains similar to its value in water. From the results obtained, it could be assumed that CHAPS behaves as a weakly charged cationic surfactant in salt solutions and as a nonionic surfactant in water and buffer medium. Conventional surfactants alike, CHAPS micellization is endothermic at low and exothermic at high temperatures, but the estimated enthalpy of micellization, ΔHM0, is considerably lower in comparison with that obtained for ionic surfactants in water and NaCl solutions. The standard Gibbs free energy, ΔGM0, and entropy, ΔSM0, of micellization were estimated by fitting the model equation based on the mass action model to the experimental data. The aggregation numbers of CHAPS surfactant around cmc, obtained by the fitting procedure also, are considerably low (nagg ≈ 5 ± 1). Furthermore, some predictions about the hydration of the micelle interior based on the correlation between heat capacity change, Δcp,M0, and changes in solvent-accessible surface upon micelle formation were made. CHAPS molecules are believed to stay in contact with water upon aggregation, which is somehow similar to the micellization process of short alkyl chain cationic surfactants.

What affects the degree of micelle ionization: conductivity study of alkyltrimethylammonium chlorides.

The critical micelle concentration, cmc, and the degree of micelle ionization, ß, of decyltrimethylammonium chloride (DeTAC), dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TTAC) in water, 0.01 M, and 0.1 M NaCl solution were determined from the electrical conductivity data in the temperature range from 278.15 to 328.15 K. It has been found that cmc is decreasing with increasing surfactant chain length and increasing concentration of added NaCl, whereas the temperature dependence of cmc shows the typical U-shaped form with a minimum around (306 ± 3) K. Further, ß is decreasing with lengthening the alkyl chain of surfactant and increasing with increasing temperature, but no distinct influence of added salt has been found actually. It can be concluded that the effect of electrolyte on micelle ionization is quite complex.

Temperature and salt-induced micellization of dodecyltrimethylammonium chloride in aqueous solution: a thermodynamic study.

Thermodynamics of micelle formation of the cationic surfactant dodecyltrimethylammonium chloride (DTAC) in water and aqueous NaCl solutions were investigated. Isothermal titration calorimetry (ITC) has been used to study the effect of the added NaCl on the critical micelle concentration, cmc, and enthalpy of micellization, Delta(mic)H(o), between 278.15 and 318.15K. Gibbs free energy, Delta(mic)G(o), and entropy, Delta(mic)S(o), were deduced by taking into account the counterion binding. From the temperature dependence of Delta(mic)H(o) the heat capacities of micellization, Delta(mic)c(p)(o), were determined. NaCl shifts cmc strongly towards lower values, indicating the screening of the repulsions of the polar head groups by counterions; but it influences Delta(mic)H(o) at higher concentrations of salt only. Delta(mic)G(o) is always negative and slightly temperature dependent. The temperature dependence of Delta(mic)S(o) indicates that the process of micellization is entropically driven. Delta(mic)H(o) decreases strongly with increasing temperature and passes through zero (endothermic to exothermic processes). The temperature dependence of the critical micelle concentration exhibits a minimum characterized by Delta(mic)H(o)=0, where Delta(mic)G(o) is of purely entropic contribution in all solutions. Delta(mic)c(p)(o) are strongly negative in all solvents, relating directly to the removal of water accessible non-polar surface of DTAC in the presence of excess counterions also.