Phase behavior and structure of a biocompatible microemulsion based on Tween 20, 2-ethylhexylglycerine and isopropyl palmitate in water

Abstract

Microemulsions are thermodynamically stable mixtures of water and oil. For many applications, such as in cosmetics, it is important that the components are biocompatible. For the formulation of a new biocompatible microemulsion based on Tween 20 (Tw20) surfactant, the glycerol ether 2-ethylhexylglycerin (EHG) is employed as an alternative to commonly used short- to medium chain alcohols as cosurfactant. Tw20 itself is not capable of any significant solubilization but the addition of EHG allows to enhance the solubilization capacity for the polar oil isopropyl palmitate (IPP) largely. At a Tw20 to EHG molar ratio of 1:12, i.e., very large excess of cosurfactant, the solubilization capacity is increased by nearly three orders of magnitude compared to a pure Tw20 solution. The resulting oil-in-water (O/W) microemulsion is studied using static and dynamic light scattering (SLS/DLS) and small-angle neutron scattering (SANS). The SANS spectra are analyzed with a core–shell ellipsoid model, which indicates formation of oblate droplets at higher EHG content. Dynamic light scattering experiments confirm the presence of microemulsion droplets with hydrodynamic radii increasing from 3.8 to 19.8 nm upon addition of cosurfactant. In summary, this study shows that EHG is a cosurfactant that enhances the solubilization properties of a biocompatible surfactant largely and thereby renders it very useful for formulations.