Aggregation behaviors of alkyl ether carboxylate surfactants in water

Abstract

Two fatty alcohol polyoxyethylene ether carboxylate surfactants (NEX-n) with different oxyethylene units have been employed to investigate the effect of headgroup on their aggregation behaviors in water. Surface tension and electrical conductivity measurements were conducted to characterize the surface activities and micelle formation in NEX-n/water systems. Different from nonionic surfactant systems, the critical micelle concentration of NEX-7 surfactant with more oxyethylene units is a little lower than that of NEX-4. The thermodynamic parameters obtained on surface tension measurements suggest a stronger hydrophobic interaction in NEX-7 aqueous solution, and disclose the driving forces for micelle formation. In addition, the liquid crystalline (LC) phase behaviors have been studied by polarized optical microscopy and small angle X-ray scattering techniques. With increasing NEX-4 concentration, the hexagonal, lamellar, sponge, and reverse cubic phases have been observed to appear in sequence, whereas only hexagonal and lamellar are formed in NEX-7/H2O system. Comparison with sodium myristate and C12E4 systems further reveals that the electrostatic interaction combining with the hydrogen bonding should be the main driving forces for the LC phase formation.