Investigation of liquid crystalline phases using microscopy, differential scanning calorimetry and rheological techniques

Abstract

Systematic studies on the phase diagrams of a series of nonionic surfactants namely Synperonic A7, A9, A11 and A20 are reported. The phase diagrams were first established using polarising microscopy, as a function of both concentration and temperature. For A7, a hexagonal phase was produced above 30% surfactant concentration which extended up to 55%. Above the latter concentration, a lamellar phase appeared. With A9, the appearance of the first liquid crystalline phase was shifted to higher concentrations but the phase diagram was qualitatively similar to that of A7. With A11, however, a cubic phase appeared between the hexagonal and lamellar phase. A20 showed a cubic phase as the first liquid crystalline phase (above 25%) followed by a hexagonal phase, but no lamellar structures could be detected. These transitions could also be followed using differential scanning calorimetry (DSC) and rheology. Viscoelastic measurements showed these transitions very clearly. For example, above the transition temperature, the liquid crystalline system changed from predominantly viscous to predominantly elastic response. The phase diagrams were established using microscopy, DSC and rheology, and good agreement was obtained between the three methods.