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Abstract: Pyrene fluorescence intensity and conductivity measurements were used to determine the cmc of two monomeric surfactants namely, dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide as well as two gemini surfactants, 16-3-16 2Br and 16-4-16 2Br of the alkanediyl-α,ω-bis(alkyldimethyl ammonium bromide) type. Cmc measurements were confirmed using fluorescence anisotropy measurements employing fluorescein as probe. Anisotropy–concentration plots showed sigmoidal behaviour and the obtained values for cmc were in good agreement with those determined using conductivity and pyrene fluorescence intensity. Also, the anisotropy of both fluorescein and another fluorescent probe, perylene, were used to provide information on the micellar structure. [Copyright &y& Elsevier]

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 (), enthalpies () and entropies () 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. [Copyright &y& Elsevier]

Abstract: The surface properties and mixed micellization behavior of binary combinations of cationic gemini (C16-4-C16, 2Br−) tetramethylene-1,4-bis(cetyldimethylammonium bromide) with cationic monomeric surfactants viz. cetyltriphenylphosphonium bromide (CTPB), tetradecyltriphenylphosphonium bromide (TTPB), cetyltributylphosphonium bromide (CTBuPB), cetyldiethylethanolammonium bromide (CDEEAB) and tetradecyldiethylethanolammonium bromide (TDEEAB) have been investigated by surface tension and conductivity measurements in (10%, v/v) ethylene glycol mixture at 300K. The critical micelle concentration (cmc), surface properties, Γ max (maximum surface excess), A min (minimum surface area per molecule) and π cmc (surface pressure at the cmc) have been determined. The deviation from the ideal of the mixed micelles has been discussed on the basis of Clint theory. The results of the mixed systems were analyzed in the light of the regular solution theory (Rubingh) and Rosen''s theory to evaluate the composition of the mixed micelle, the activity coefficient, the interfacial mole fraction of the surfactant at the mixed adsorbed film and the corresponding interaction parameters (β and β σ). The negative values of interaction parameters (β and β σ) for mixed micelle formation indicate the attractive interaction for all the system. The standard Gibbs free energies of adsorption (ΔG°ads), standard Gibbs free energies of micellization (ΔG°m), the excess free energies of micellization (ΔG ex) and minimum free energy of the surface (ΔG (s) min) of gemini with cationic monomeric surfactants were also evaluated. The ΔG°ads, ΔG°m and ΔG ex values are negative indicating the spontaneity and stability of the mixed micelle. [Copyright &y& Elsevier]