Effects of cationic counterions and surfactant on viscosity of an amino acid-based surfactant system

Abstract

Many sulfate-free and amino acid-based surfactant systems for personal care formulations do not respond to thickening with salt and/or amphoteric cosurfactants as readily as traditional sulfate-based formulations. Although this phenomenon is generally understood to be associated with large headgroups, the mechanistic basis for the difference is poorly understood. To address this question we have combined an empirical approach involving traditional determination of low shear viscoelasticity with a state-of-the-art mesoscopic analysis of micelle dynamics enabled by high frequency rheological measurements obtained optically (Zou and Larson, Journal of Rheology 2014, 58 (3), 681-721) We studied the impact of various organic and inorganic counterions as well as a cationic surfactant as alternative thickening agents for a model formulation of sodium lauroyl sarcosinate (SLSar) and cocamidopropyl hydroxysultaine (CAHS). These additives include lithium chloride (LiCl), sodium chloride (NaCl), L-arginine (Arg), triethanolamine (TEA), trimethylphenylammonium chloride (TMPAC), and cetyltrimethylammonium chloride (CTAC). The zero-shear viscosity for all systems studied was obtained using steady-state rheology at varying concentration and pH. The viscoelastic properties of systems containing an effective thickener (CTAC 1%) and an ineffective one (LiCl 1%) were obtained over a frequency range of 1–105 rad/s and subjected to the Zou and Larson analysis. The analysis showed that the high viscosity in the CTAC 1% system was associated with the combination of a high plateau modulus and a long relaxation time. The low viscosity in LiCl 1% system was attributable to the fast breakage time and high breakage rate constant. The latter result was consistent with findings from a previous study comparing the SLSar/CAHS system to a sodium lauryl ether sulfate/cocamidopropyl betaine (SLES/CAPB) formulation.