Binary and Ternary Phase Behaviors of Short Double-Chain Quaternary Ammonium Amphiphiles: Surface Tension, Polarized Optical Microscopy, and SAXS Investigations

摘要

A series of short chain dialkyldimethylammonium amphiphiles ([DiC_n][X] and [DiC_n]2[X] with n = 6, 8, or 10) has been investigated as a function of the nature of the counteranion [X] (Cl~ and Br~-as classical references and MoO4~(2-) and WO4~92-) as catalytic anions). Critical aggregation concentrations (CACs) in water were determined by surface tension measurements, and the binary phase diagrams of the surfactant/water systems were established using polarized optical microscopy (POM) and small angle X-ray scattering (SAXS). The evolution of microstructures and lyotropic phases were discussed in terms of surfactant chain length and counterion effect. A cubic phase region was observed for the divalent counteranions associated with the intermediate chain length, i.e., [DiC8]2[MoO4] and. [DiC8]2[WO4]. Finally, the fish diagrams reflecting the polyphasic microemulsion phase behavior of the water/ amphiphile/oil ternary systems were established using the carbon number of mono-and dichlorinated n-alkanes as a scan variable. The amphiphile efficiency was also discussed according to the chain length and the nature of the counterions. The hydrophilic—lipophilic behavior of the amphiphiles was evaluated on the basis of the fish diagrams allowing their classification according to.the Hofmeister anion sequence. The unusual behavior of [DiC8j[Cl] and [DiC8]2[MO4] (M =.W or Mo) is highlighted by all experiments: these compounds are clearly intermediates between hydrotropes, [DiC6] cation, and surfactants, [DiC_(10)] cation.