Interfacial composition and characterization of a quaternary water-in-oil mixed surfactant (cationic of different alkyl chain lengths + polyoxyethylene type nonionic) microemulsions in absence and presence of inorganic salts

Abstract

In this contribution, phase behavior and the Schulman's cosurfactant titration of a quaternary water-in-oil microemulsion, formed from equimolar cationic [alkyltrimethylammonium bromide (CnTAB)] and nonionic [polyoxyethylene (20) cetyl ether (C16E20)] surfactants, n-pentanol (Pn) and n-heptane or isopropyl myristate, have been studied along with the variation in alkyl chain length of the cationic surfactant, Cn (n=12, 14, 16 and 18) at a fixed water content (ω=25) and temperature (303K). The synergies in single-phase microemulsion zone, interfacial composition and the spontaneity of formation of mixed surfactant microemulsions have been observed with increase in Cn (n=12→18). Further, the effect of the variation of Cn (n=12→18) on the interface vis-à-vis interdroplet interaction, morphology and the dynamics of confined water of these systems have been examined by means of viscosity, dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR) measurements. The increase in the Cn (n=12→18) leads to shrink the droplet size as well as increase in the droplet numbers. Subsequently, the change in droplet size affects the states of water organization (bulk and bound) inside the pool. Additionally, the influence of different inorganic salts [normal salt (NaCl), precursor salt for nanoparticle synthesis (NiCl2, FeCl3), water structure affective salt (KF, KI)] on the transfer process of Pn (oil→ interface) at equimolar composition (of both surfactants) has been investigated. FTIR measurement (via D2O probing) indicates that the presence of salts significantly modifies the water structure in confined environment.