No abstract

A systematic investigation of influence of the length of the side alkyl chain and the counter ions on the thermodynamic of the micellization process at imidazolium based surface active ionic liquids (SAILs) in aqueous solution was carried out by isothermal titration calorimetry (ITC) in a broad temperature range. The effect of alkyl chain length on the micellization process in water has been investigated on 1decyl-3-methylimidazolium ([C 10 mim]Cl) 1-dodecyl-3-methylimidazolium ([C 12 mim]Cl), 1tetradecyl-3-methylimidazolium ([C 14 mim]Cl) and 1-hexadecyl-3-methylimidazolium ([C 16 mim]Cl) chlorides, whereas the influence of counterion was studied at the micellization of 1-dodecyl-3-methylimidazolium chloride ([C 12 mim]Cl), bromide ([C 12 mim]Br), iodide ([C 12 mim]I), acetate ([C 12 mim]OAc), methanesulfonate ([C 12 mim]OMs), toluensulfonate (([C 12 mim]OTs), trifluromethane sulfonate ([C 12 mim]OTf), trifluoro acetate (([C 12 mim]TFA) and salicylate ([C 12 mim]Sal) in water.

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.

No abstract

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.