First-order Nematic-isotropic Transition Research Articles

Theory of nematic-isotropic phase transitions in solutions of rodlike aggregates

ABSTRACT A mean-field theory is introduced to describe phase behaviours for solutions of rodlike aggregates. By combining the Onsager model for rodlike aggregates and the Flory-Huggins model for polydisperse polymer solutions, we derive the free energy for the solutions of the rodlike aggregates. We calculate the mean aggregation number and orientational order parameter of rodlike aggregates. The mean aggregation number jumps at the first-order nematic–isotropic transition (NIT). We also derive the Landau-de Gennes expansion of the free energy and calculate the isotropic-nematic coexistence curves on the temperature-concentration plane. The NIT curves are qualitatively consistent with the experimental results of rodlike aggregates.

Laterally attached liquid-crystalline polymers as stationary phases. Effect of temperature in RP HPLC and comparison with a commercial C18 stationary phase

Specific side-on-fixed liquid-crystalline polymers (SOLCP) have been synthesized for use in silica-modified stationary phases in high-performance liquid chromatography (HPLC). The mesogenic side group of the SOLCP is composed of three benzoate-type phenyl rings with terminal alkoxy chains and is laterally linked to a polysiloxane backbone via an alkyl ester spacer arm. The dependence of the logarithm of the retention factor on the reciprocal temperature showed that the liquid-crystalline anisotropic order was conserved in the small pores (200 A diameter) of the silica gel. The first-order nematic-isotropic transition is lost and probably becomes second-order. Adsorption enthalpies for the liquid-crystalline stationary phases have been measurement for three polycyclic aromatic hydrocarbon isomers (ortho-terphenyl, triphenylene, and chrysene) and compared with those for a commercial C18 phase. The adsorption enthalpies never exceeded 30 kJ mol−1, i.e. ten times the thermal agitation energy,RT. They were always less on the SOLCP stationary phase than on the C18 column, emphasizing the more rigid structure of the liquid crystalline phase and its mechanism based upon adsorption. Better separation of steroids, pesticides and amino acids were obtained with the LCP-coated silica than the commercial bonded C18 column. Four small peptides were successfully separated by using pure water as mobile phase.

Heat Capacity Associated with Phase Transitions in Micellar Cesium Perfluoro-Octanoate Solutions

We have carried out a study of the ordering of cesium perfluoro-octanoate (CsPFO) micelles in water using high-resolution ac calorimetric techniques and differential scanning calorimetry (DSC). Aqueous solutions of CsPFO undergo two order-disorder phase transitions: a second-order nematic-smectic A (or lamellar) transition and a very weakly first-order nematic-isotropic transition. Small excess heat capacity peaks associated with each of these transitions have been observed for solutions that are 41 wt% CsPFO and 65 wt% CsPFO. The integrated transition enthalpy \(\int{\varDelta}C_{\text{p}}{\text{d}}T\) for the N-SmA transition is about 0.14 J g -1 (Joule per gram of CsPFO solute) at 65 wt% and about 0.10 J g -1 at 41 wt%. No qualitative difference was observed in the character of these Δ C p (N-SmA) peaks in spite of a reported change in the transition from second order at 41 wt% to tricritical near 65 wt%. The integrated transition enthalpy was larger for the N-I transition: about 0.23 J g -1 at 65 wt% ...