Microscopical structure of the uniaxial and biaxial lyotropic nematics

Abstract

Optical birefringence and temperature range of the biaxial nematic phase in a lyotropic liquid crystal High-resolution x-ray measurements have been performed at the synchrotron of LURE (Orsay) to study both uniaxial (discotic and calami tic ) and biaxial nematic lyomesophases of the mixture K-laurate, I-decanol, and 0 2 0. The variations of the densitometric profiles of the diffracted bands are analyzed vs temperature. They are shown to be consistent with the model of similar micellar aggregates in the three nematic phases, undergoing different orientational fluctuations. It is moreover discussed that the micelles have, on the average, the shape of biaxial platelets with a (1/2/3) anisotropy, and that they build the same pseudolamellar structure in each nematic phase.

On a new topology in the phase diagram of biaxial nematic liquid crystals

“…The N U + -N B and N U − -N B transitions appear to be second order followed by a first order uniaxial-isotropic phase transition. Further experiments 5,6 on lyotropic systems also confirmed these observations. A few micellar systems are known for which phase diagrams involving uniaxial and biaxial nematic phases suggest the existence of the Landau point on the nematic-isotropic transition line.…”

Investigation of the interaction of alkali ions with surfactant head groups for the formation of lyotropic biaxial nematic phase via optical birefringence measurements

“…They assumed the hypothesis that the micelles in the N D and N C phases are discs and cylinders, respectively. Despite the fact that these assumptions are not true, in the framework of the IBM model 14,15,20 , their results of typical dimensions may give a reasonable approximation for the micellar anisotropy. They reported that, in SDS (KL) system, the axial ratios were calculated as 2.8 (2.8) in N D phase and > 2.3 (1.75) in N C phase.…”

“…The large value of the birefringence in the N D phase of the Na 2 SO 4 mixture (~8×10 -3 at 25°C) indicates the existence of more anisometric micelles (or reveals a larger molecular segregation in the micelle). 15,18,19,36 The reason why we could not obtain nematic phases with Li 2 SO 4 may be related to the crystallization process, since Li+ highly binds to the head group of KL. This situation was also reported for LiBr in the mixture KL/dodecyltrimethylammonium bromide/alkali bromide/water.…”

“…14,15 In the IBM model, the driving force for the existence of three nematic phases, the two uniaxial N D and N C and the biaxial N B , is related to characteristic orientational fluctuations of the same type of (orthorhombic) micelles in these nematic phases. The investigation of the uniaxial-to-biaxial phase transition requires precise measurements of an order parameter in the vicinity of the transition temperature.…”

Ferrofluids: properties and applications

“…The collective behavior of the liquid crystalline matrices was observed with magnetic fields of about 20 G. The use of ferrofluids to orient lyotropics was essential in experiments in which the reciprocal structure of the biaxial nematic phase was determined [63].…”