Branched zwitterionic gemini surfactants micellization and interaction with ionic surfactants

Abstract

Gemini surfactants with two hydrocarbon chains, where one is branched, differing in length and with an ethylene spacer separating the two non-identical head groups, have been investigated. Their physicochemical properties were studied by equilibrium and dynamic surface tension (DST), as well as by steady-state fluorescence. These surfactants are composed of hydrophilic polar heads that carry both a positive (a quaternary ammonium salt) and a negative (a phosphodiester) charge. They are referred to as zwitterionic heterogemini surfactants and are denoted ZWHG's. The aqueous phase critical micelle concentrations (cmc's) of the ZWHG surfactants have been determined by equilibrium surface tension and by fluorescence probing. A good agreement was found between the two methods. The ZWHG's are shown to exhibit improved performance both over conventional surfactants and over a reference ZWHG surfactant based on two linear alkyl chains both in terms of micellization and of surface tension lowering. DST studies were performed to investigate air–water adsorption mechanisms using a maximum bubble pressure tensiometer (MPT2). A model for a diffusion-controlled mechanism was used to analyze the DST data and it was found that closer to the equilibrium the DST data are inconsistent with a diffusion-only mechanism. The ZWHG's show a large deviation from a diffusion-controlled process, which is a signature of slower adsorption kinetics. In addition, mixed micellization between one ZWHG surfactant (C8(−)C10(+)) and sodium dodecyl sulfate (SDS) has been investigated. The cmc of the mixtures was measured by fluorescence probing, using pyrene as probe. The variation of cmc with composition revealed considerable synergism in micelle formation. This behavior was confirmed by DST measurements, where a clear decrease in surface tension was observed upon addition of SDS to C8(−)C10(+). The synergism is believed to be due to a micellization-induced protonation of the zwitterionic gemini surfactant, giving rise to mixed micelles of anionic and cationic amphiphiles.