Swelling and elongation of tetradecyltrimethylammonium bromide micelles induced by anionic sodium laurate

Abstract

Natural fatty acid salts such as sodium laurate (SL), which is sparingly soluble in water at room temperature due to its high Kraft point, can undergo cooperative self-assembly in the presence of tetraalkylammonium surfactants and make them attractive candidates for potential applications. Here, we report microstructural changes in tetradecyltrimethylammonium bromide (TTAB) micelles induced by the addition of SL and compare it to that of cetyltrimethylammonium bromide (CTAB)–SL mixed micelles, employing small angle neutron scattering, dynamic light scattering and rheological measurements. The addition of sodium laurate induces both axial and equatorial growth of TTAB micelles while primarily axial growth is observed in CTAB micelles. The roles of electrostatic and steric contributions in controlling the packing of surfactant molecules within the aggregate have been identified. Rheological studies show that at small mole fraction of sodium laurate, the mixed micelles behave like Newtonian fluids while at high mole fraction of laurate salt, long polymer-like micelles are formed which show shear thinning behavior. The predominant swelling of mixed micelles formed by TTAB—SL mixtures has been explained on the basis of a decrease in the asymmetry of the chain lengths in the cationic–anionic pair. This study highlights the role of biocompatible and naturally occurring fatty acids such as SL in inducing swelling and elongation of cationic micelles.