Adsorbate-induced ordering transitions of nematic liquid crystals

The research described in this thesis is focused on surface-driven anchoring transitions of liquid crystals (LC) that are triggered by the presence of targeted vapor analytes. Thin films of nitrile containing LCs align preferentially at metal salt-decorated surfaces via coordination interactions between the nitrile group of the mesogens and the metal cations. A vapor analyte that coordinates with the metal cations more strongly than the nitrile groups of the LCs will triggers an anchoring transition of the LCs. The first section of this thesis is focused on an investigation of interfacial physicochemical phenomena underlying adsorbate-induced ordering transitions in nitrile-containing LCs supported on surfaces coated with metal perchlorate salts. The results reveal that the ordering of the LC on the metal salt-decorated surfaces is strongly dependent on the loading of metal salt, with key interfacial physicochemical processes being metal ion-nitrile coordination interactions, dissolution of salt into the LC, and formation of electrical double layers. The second section of my thesis focuses on processes that control the dynamics of the anchoring transition of the LC to the targeted vapor analyte. By flowing a gas containing dimethyl methylphosphonate (DMMP) over a film of nematic 5CB supported on a surface decorated with aluminum perchlorate, we quantify the dynamics of the optical response of the LC film as a function of key experimental parameters, including linear flow rate of the gas over the surface of the film and the concentration of the DMMP within the gas. The measurements are interpreted within the framework of a simple model of mass transport to reveal that the rate-limiting process underlying the response of the LC is the mass transport of the DMMP across the concentration boundary layer formed on the vapor side of the LC-vapor interface, and not the mass transport of DMMP across the LC film nor the intrinsic ligand exchange kinetics. Analysis of the relaxation of the system leads to an estimate of the sensitivity of the supported LC film to the concentration of DMMP in the gas phase (approximately 150 ppb for the non-optimized system reported in this thesis). The third section is focused on optimization of the experimental system in order to speed up the dynamic response of the LC sensor system to a targeted analyte. We accomplish this by studying two parts of the system. First, we report that metal salts composed of mixtures of anions of differing coordination strength can be used to increase the sensitivity and selectivity of adsorbate-induced anchoring transitions of LCs supported on surfaces decorated with the metal salts. Second, we manipulate the dynamic response of the LC by changing the thickness of the LC film. We determine that the elasticity of the LC plays a key role in determining the sensitivity of the LC films to analyte when the LC films approach thicknesses of a few micrometers. The role of elastic strain in the LC is explored further through the addition of a chiral dopant to the LC and formation of cholesteric LC phases.