Mass density measurements as a tool to distinguish between micelle size and shape in nematic and chiral nematic phases

Abstract

The relative alteration in micelle size and micelle shape brought about by two sets of specific solutes have been investigated by mass density measurements at constant composition and temperature. The mass density of the first set of solutes, phenol, benzene, cyclohexanol and cyclohexane, was measured in the same host phase and found to change in the order: phenol > cyclohexanol > benzene > cyclohexane. The contribution of the solutes to the mass density of the host phase was interpreted to arise from the capability of the hydrophilic group and the hydrophobic part of the solutes to interact with the corresponding region of the host micelle. The second set consisted of racemic amphiphilic mixtures and their enantiomers, decyl- and dodecyl-esters of serine and alanine. It was found that the mass density increases in the serine phases but decreases in the alanine ones as the DL-racemic amphiphiles of the nematic phases were replaced by their L-isomers. These phenomena were shown to arise from a definite arrangement of the head groups in the racemic nematic and intrinsic chiral nematic phases. Furthermore, on the basis of the mass density difference between the achiral/induced chiral nematics and the racemic/intrinsic chiral nematics, a distinction between the induced chiral nematic phases and the intrinsic chiral ones is possible.