Effect of inorganic and organic counterions on interfacial properties of oleic acid-based gemini surfactants

Abstract

We studied the effect of counterion species of oleic acid-based gemini surfactants with carboxylate headgroups. Adsorption and self-aggregation properties were investigated by static surface tension, pyrene fluorescence, dynamic light scattering and phase diagram measurements. We found that, in the case of inorganic counterions (Li+, Na+, K+, and Cs+), a decreased ion size leads to a decreased critical micelle concentration (cmc) as well as a decreased area per surfactant molecule adsorbed at the air/aqueous solution interface (Acmc). The smaller inorganic counterions (hard acid) can interact with the carboxylate headgroups (hard base), and hence reduce the head-to-head electrostatic repulsion. The smaller counterions also induced the lower phase transition concentration from an optically isotropic solution (L1) phase to a hexagonal liquid crystal (H1) phase. In the case of organic counterions (monoethanolamine (MEA) and diethanolamine (DEA)), the micellization behavior was predominantly affected by the hydrophobicity of the counterions, whereas the hydrated ion size was a key parameter determining the molecular packing at the air/solution interface. In addition, we found a structural transformation from larger aggregates to smaller ones (micelles) to occur for the organic counterion systems at dilute surfactant concentrations.