The effect of ethanol on the phase behaviour and micro-rheology of liquid crystals

Abstract

The effect of ethanol on the phase behaviour and micro-rheology of lyotropic liquid crystals (LC) has been studied using a binary mixture of monoglyceride (MG) and aqueous ethanol. The phase behaviour study reveals the structural modulation of surfactant aggregates with increasing ethanol concentration, namely a bicontinuous cubic phase (Ia3d) transitions to the lamellar phase (Lα), at a fixed MG concentration. This behaviour is explained by considering the critical packing parameter (CPP) of the surfactant molecule. Because ethanol dehydrates the surfactant head group (a s), the CPP values increase (decreasing a s) and thus the formation of larger CPP aggregates is favoured (i.e., the Ia3d–Lα transition occurs). Cross-polarised images and X-ray scattering data support this conclusion. The structural modulation of the LC has further been investigated using a diffusing wave spectroscopy technique. The correlation and relaxation times, determined from the intersection point at short and long time scales of the mean square displacement (MSD), decrease with increasing concentrations of ethanol, indicating structural modulation of the LC. The micro-viscoelastic moduli (G′ and G′′) derived from the Laplace transformation of the MSD decrease with increasing ethanol concentrations, due to the LC modulation. The thermal effects on the micro-rheology of the LC have also been studied.