Effect of Hofmeister and Alkylcarboxylate Anionic Counterions on the Krafft Temperature and Melting Temperature of Cationic Gemini Surfactants

The effect of counterions was investigated to probe the principal ionic effects on the solubility in water and melting behavior of cationic gemini surfactants. We focused on two types of counterions: (1) small inorganic counterions that are typically taken from the Hofmeister series were studied to focus on the effect of ion type and (2) <i>n</i>-alkylcarboxylate counterions were studied to focus on the effect of the hydrophobicity of counterions. The Krafft temperature (<i>T</i>_k) and melting temperature (<i>T</i>_m) were obtained by conductivity measurements, calorimetric measurements, and optical microscopy observation. The results clearly indicate that <i>T</i>_k, which represents the solubility of surfactants, is not determined by a single parameter of ions such as the hydration free energy, as is too often assumed, but rather by the combined effects between the hydrophobicity of anions associated with other effects such as the polarizability, dehydrated ion size, and ionic morphology. In parallel, our observation demonstrated that all of the surfactants showed a transition from a crystalline phase to a thermotropic liquid-crystalline phase at around ca<i>.</i> 70 °C, which transformed to an isotropic liquid phase at around ca<i>.</i> 150 °C, and that the transition temperatures depended strongly on the counterion type. The counterion effects on the solubilization and melting behaviors were then compared with micellization properties that have been reported previously. These results provide new insight into understanding the effect of ions on the delicate balance of forces controlling the solution properties and aggregate morphology of charged amphiphilic molecules. Specifically, the solubilization properties of these cationic surfactants with various counterions were determined mainly by the subtle interplay between the hydration of counterions and the dissociation energies (stability of crystallinity) of the ion pair

Physicochemical Properties and Supernucleophilicity of Oxime-Functionalized Surfactants: Hydrolytic Catalysts toward Dephosphorylation of Di- and Triphosphate Esters

Aggregation and kinetic studies have been performed to understand the hydrolytic potencies of the series of oxime-functionalized surfactants, viz., 3- hydroxyiminomethyl-1-alkylpyridinium bromide (alkyl = C_<i>n</i>H_2<i>n</i>+1, <i>n</i> = 10, 12, 14, 16, 18) in the cleavage of phosphate esters, <i>p</i>-nitrophenyl diphenyl phosphate (PNPDPP) and bis­(2,4-dinitrophenyl) phosphate (BNDPP), in mixed micelles with cetylpyridinium bromide (CPB). Micellization and surface properties of mixed micelles functional surfactants with CPB were studied by conductivity and surface tension measurements. Acid dissociation constants (p<i>K</i>_a) were determined, the effect of functional surfactant alkyl chain length and pH on the observed rate constant (<i>k</i>_obs) for phosphate ester cleavage has been discussed, and the effect of substrate on the supernucleophilicities of the studied oximes was monitored. Functionalized oxime-based surfactants were proved to be supernucleophiles to attack on the PO center of tri- and diphosphate esters. Oximes with hexadecyl alkyl chain length (3-C₁₆) showed maximum micellar effect on the rate constants toward PNPDPP. Micellar effects were analyzed in terms of the pseudophase model