Effect of the alkyl chain length of the alcohols on the nematic uniaxial-to-biaxial phase transitions in the potassium laurate/alcohol/K2SO4/water lyotropic mixture

TL;DR

This study investigates how the length of alcohol alkyl chains affects phase transitions in lyotropic mixtures, revealing a specific chain length window for biaxial nematic phases and their relation to micellar shape anisotropy.

Contribution

It provides new insights into the relationship between alcohol chain length and nematic phase behavior in lyotropic mixtures, highlighting the role of micellar shape anisotropy.

Findings

Biaxial nematic phase exists within a specific alkyl chain length window.

Phase transition temperatures increase with longer alkyl chains.

Mixtures show high birefringence linked to micellar shape anisotropy.

Abstract

Lyotropic liquid crystalline quaternary mixtures of potassium laurate (KL), potassium sulphate (K2SO4)/alcohol (n-OH)/water, with the alcohols having different number of carbon atoms in the alkyl chain (n), from 1 octanol to 1-hexadecanol, were investigated by optical techniques (optical microscopy and laser conoscopy). The biaxial nematic phase domain is present in a window of values of n = nKL \pm 2, where nKL = 11 is the number of carbon atoms in the alkyl chain of KL. The biaxial phase domain got smaller and the uniaxial-to-biaxial phase transition temperatures shifted to the relatively higher temperatures on going from 1-nonanol to 1-tridecanol. Moreover, these new mixtures present high values of the birefringences comparing to other lyotropic mixtures. This result is expected to be related to the micellar shape anisotropy. Our results are interpreted assuming that alcohol molecules tend to segregate in the micelles in a way that depends on the relative value of n with respect to nKL . The larger the value of n, the more alcohol molecules tend to be located in the curved parts of the micelle, favoring the uniaxial nematic calamitic phase with respect to the biaxial and uniaxial discotic nematic phases.