Formulating additives in thermoresponsive surfactant-based nematic liquid crystals

Abstract

Bicelles can be formed by mixing in given mole fractions two ethoxylated alkyl ether carboxylic acid surfactants of very different HLB in water. We determine the effect of adding three of the most used additives in formulation in health- and home care: propylene glycol, glycerol, and ethanol. The effects of additives are determined and compared in a concentrated isotropic phase above the LCST, a pseudo-lamellar phase, and a discotic nematic phase. The two latter are birefringent, and the nematic phase is viscoelastic. Propylene glycol acts as a co-solvent, improving the temperature stability of the nematic phase up to 20 wt% propylene glycol. Further addition of propylene glycol reduces the phase transition temperatures, inducing microstructural changes due to headgroup dehydration and preferential solubilization of the hydrophilic short chain surfactant. Glycerol acts as an anti-solvent, progressively decreasing phase transition temperatures by dehydration of headgroups. Ethanol is a good co-solvent for the surfactant-mixture. Adding up to 5 wt% ethanol increases the temperature stability of the nematic phase. Higher concentrations of ethanol lead to a single isotropic phase with increasingly molecular dissolution of the surfactants. The effect of the considered additives on molecular packing is followed by high resolution X-ray scattering.