Investigations on the role of edge activator upon structural transitions in Span vesicles

Abstract

Spanlastics (viscoelastic vesicles) have evolved from nonionic surfactant vesicles, and are composed of vesicle builder (VB) and edge activator (EA). EA engages with the bilayer of vesicles and modulates its fluidity, elasticity and integrity. In this study, we hypothesized to modulate the microstructural characteristics of Span 80 (sorbitan monooleate) vesicles using Tween 80 (a hydrophilic micelle forming non-ionic surfactant, polyoxyethylated Span 80) as EA. The experiments were designed to investigate the effect of EA-VB ratio (2:1, 1:1 and 1:2) and total surfactant concentration (0.5%, 1.0%, 2.0%, 5.0% and 10% v/v) upon formulation attributes. Formulations were prepared using injection method and structural transitions were studied using dynamic light scattering (DLS), small angle neutron scattering (SANS) and electron microscopy. Scattering results show that large vesicles were formed as (a) proportion of EA became low and, (b) total surfactant concentration increased from 0.5% to 10%. At a fixed EA-VB ratio (2:1), profound microstructural changes were noticed while we shifted towards higher surfactant concentration. Here, mixed micelle-rich aggregates (0.5%) transformed to a vesicle-micelle mixture along with an intervening phase dominated by ellipsoidal micelles. These structures could be verified from the electron microscopic data and changes in the viscosity of samples during application of controlled shear. Altogether, we show that morphology of vesicle and fluidity of its bilayer can be tuned through a careful optimization of EA concentration.