Effect of Phenoxyethanol and Sodium benzoate on the liquid crystal phase behavior of mixed surfactant system

Abstract

Surfactants in a mixed concentrated (∼30 wt% and higher)-surfactant system self-assemble to form a variety of liquid crystal (lc) types depending the lc curvature determined by the surfactant chemistries and their relative compositions. Preservatives, fragrances and other minor additives present in an industrial formulation can either incorporate into the lc and/or influence the aqueous phase ionic strength to alter the spatial arrangements of the surfactants and the lc curvature. These mechanisms could additively or competitively determine the lc curvature and thereby the lc types formed. We report on the mechanisms for preservatives Phenoxyethanol (PE) and Sodium benzoate (NaB) governing the lc phase behavior for a mixed concentrated-surfactant system: 16 wt% zwitterionic Cocoamidopropyl betaine (CAPB), and 7 wt% each of anionic Sodium Lauroyl isethionate (SLI), Sodium Methyl Lauroyl Taurate (SMLT), and 6 wt% Lauric acid. The lc type(s), the lc dimensions including inter-lc distances were derived from SAXS and polarized microscopy images. The predominant lc phase without preservatives was H1lc, while anionic SLI and SLMT present in excess were insoluble to form dispersed crystals. The addition of PE molecules resulted in their incorporation in the lc surfactant film to cause a transition to micelles, and Lα lc due to incorporation-induced flattening of the film. NaB molecules prevented micelles formation and preserved the H1lc by serving as a hydrotropic agent. The NOESY spectra for the lc systems with PE or NaB at 0–3 wt% confirmed incorporation of PE in surfactant film as opposed to NaB. For both PE and NaB present in a lc system, the dominance of hydrotropic nature of NaB causing film bending rather than the film flattening effect of PE at low (∼1 wt%) PE levels resulted in H1lc as the predominant phase, and reduced dominance of film bending effect at a high PE level (≥ 2 wt%) led to the formation of Lα lc.