Interfacial behavior of gemini surfactants with different spacer lengths in aqueous medium

A series of alkanediyl-1-s-bis(dimethyltetradecylammonium bromide) (abbr. 14-s-14,2Br⁻) has been characterized in an aqueous medium at temperatures of 288.2, 298.2, and 308.2 K. Critical micelle concentration (cmc) of the surfactants was determined by measuring the surface tension and electrical conductivity as a function of concentrations. The micelle formation was elucidated thermodynamically from the results of electrical conductivity measurement. A steady-state fluorescence quenching and dynamic light scattering were performed to determine the aggregation number and the size of micelles, respectively. Furthermore, the surface potential (ΔV) was measured against surfactant concentrations to understand the condensing state of the adsorbed layer located at the air–water interface. The ΔV value directly can catch a vertical unequal shift of electric charges in the surface region above the bulk where the ΔV value is zero or electroneutrality is held. The results of ΔV measurements indicated that the adsorbed surfactants in the surface region were saturated in terms of amounts far below the cmc and that the vertical orientation of tetradecyl chains of 14-s-14,2Br⁻ was improved by the longer spacers in a bending conformation towards the air.