An Ia3¯d thermotropic cubic phase from N-alkylpyridinium tetrahalocuprates

Abstract

The thermotropic liquid crystalline behaviour of two N-alkylpyridinium tetrahalocuprates was investigated. The n-pentadecyl tetrachloro and tetrabromo derivatives (CU15-CL and CU15-BR, respectively) exhibit very similar behaviour on cooling, and specifically, a high temperature smectic phase followed by a cubic phase. We fully elucidated the Ia3¯d space group for the cubic phase from X-ray diffraction on single crystals. The type of cubic phase is still under discussion, but we suggest that it is a direct type of structure based on the known phase sequence of lower and higher homologues. A model for the local organization of CU15-CL and CU15-BR in the cubic and smectic mesophases is also proposed.

Notes

^†The curvature argument is actually invoked to explain the local phase structure, with LC mean curvature values ranging from either positive or zero (‘normal’, ‘direct’ or Type I phases) to negative (‘inverse’, ‘reversed’ or Type II phases). In water–amphiphile systems, direct phases arise from the so-called oil-in-water situation. In solventless LC phases, interfaces are ideal surfaces separating polar regions from apolar ones. A more thorough discussion on direct and inverse phases, and a precise definition of interfacial curvature have been given by Seddon et al. [9].

^‡Triply periodic minimal surfaces (TPMS) are mathemathical models for bicontinuous cubic phases. A graphical representation of the simplest TPMS surfaces with cubic symmetry can be retrieved at the following URL: http://msri.org/publications/sgp/ SGP/main.html. Sadoe and Charvolin have given a crystallographic description of TPMS.

^§In the crystalline phase of CU15-CL, the total density equals ρ _tot=1.13 g cm⁻³, and the paraffinic volume fraction equals 0.67 with ρ _para=0.9 g cm⁻³ and ρ _polar=1.61 g cm⁻³. We make the following assumptions inside the mesophases: 0.7<ρ _para (g cm⁻³)<0.8 (by comparing with the known molten n-alkanes density in the temperature range of our study [24]); ρ _polar (g cm⁻³)<1.61; 2*1.13/3=0.75<ρ _tot (g cm⁻³)<1.13; ρ _para<ρ _polar.