Small angle neutron scattering and viscosity studies of micellar solutions of bis-cationic surfactants containing hydroxyethyl methyl quaternary ammonium head groups

Abstract

The effect of head group polarity and spacer chain length of bis-cationic surfactants on shape, size and rheology of surfactant aggregates in aqueous solution has been investigated through small angle neutron scattering (SANS) and viscosity measurements. A series of bis-cationic surfactants C₁₂H₂₅N⁺(CH₃)(C₂H₄OH)–(CH₂)_s–N⁺(CH₃)(C₂H₄OH)C₁₂H₂₅ 2Br⁻, referred to as 12-s-12 MEA, where two highly polar quaternary amine centers are covalently connected through polymethylene spacers at head group level, has been synthesized and characterized. Critical micellar concentration of these surfactants in aqueous solution determined by tensiometry and conductometry was observed to be 10–100 times lower than conventional bis-cationic surfactants [C₁₂H₂₅N⁺(CH₃)₂–(CH₂)_s–N⁺(CH₃)₂C₁₂H₂₅ 2Br⁻, referred to as 12-s-12 DMA]. The extent of aggregate growths and variation in shape and size of micelles was observed to be strongly dependent upon head group polarity, spacer chain length and experimental conditions such as concentration and temperature.