Physicochemical characterization of cationic gemini surfactants and their effect on reaction kinetics in ethylene glycol–water medium

Abstract

Micellar and surface properties of two cationic gemini surfactants viz. alkanediyl-α,ω-bis(hydroxyethylmethylhexadecylammonium bromide) (C16-s-C16, MEA 2Br−, where s=4, 6) in aqueous and ethylene glycol–water medium (0–20%, v/v) have been investigated by conductivity and surface tension measurements. The values of critical micellar concentration (CMC) and degree of micellar ionization (α) increase by increasing the concentration of ethylene glycol and the spacer chain length of gemini surfactants. The standard Gibbs free energy changes (ΔGm°), enthalpies (ΔHm°) and entropies (ΔSm°) of micellization of gemini surfactants, determined by studying the variation of critical micelle concentration with temperature, revealed spontaneous and exothermic micellization behavior. The large rate enhancement for the hydrolysis of p-nitrophenyl acetate (PNPA) and p-nitrophenyl diphenyl phosphate (PNPDPP) by α-nucleophiles such as, benzohydroxamic acid (BHA), acetohydroxamic acid (AHA), salicylhydroxamic acid (SHA) and butane-2,3,-dione monooxime (BDMO) were obtained in the presence of gemini surfactants over the monomeric surfactant i.e. cetyltrimethylammonium bromide (CTAB). Kinetic constants were observed to be dependent on spacer chain length of gemini surfactants as well as on the nature of nucleophiles used. Ethylene glycol retarded the rate of reaction and C16-6-C16, MEA 2Br− showed better catalytic effect for phosphate ester hydrolysis.