Dynamics of the chemo-optical response of supported films of nematic liquid crystals

We report a study that elucidates key physicochemical phenomena that underlie the dynamics of adsorbate-induced ordering transitions in micrometer-thick films of nematic liquid crystals (LCs) supported on chemically functionalized surfaces. By flowing a gas containing dimethyl methylphosphonate (DMMP) over a film of nematic 4-cyano-4′-pentylbiphenyl (5CB) supported on a surface decorated with aluminum perchlorate, we quantify the dynamics of the optical response (intensity of transmitted light) 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. Building from prior infrared spectroscopic analysis revealing that the optical response of the LC is due to an ordering transition triggered by competitive ligand-exchange reactions at the chemically functionalized surface (i.e., between the phosphonate and nitrile groups of the DMMP and LC, respectively, with the aluminum perchlorate-decorated surface), we interpret the measurements reported herein within the framework of a simple model of mass transport. The model reveals 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. The analysis also reveals that the response of the LC is triggered once the concentration of DMMP within the LC film rises to approximately 0.25mM. By flowing a stream of gas free of DMMP over a LC film pre-exposed to DMMP, we established that the relaxation of the LC film to the initial state is also largely controlled by mass transport of the DMMP across the vapor side of the LC–vapor interface. 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 150ppb for the non-optimized system reported in this paper). These results and others reported in this paper identify key physical phenomena that dictate the dynamics of adsorbate-induced ordering transitions in LC systems, and provide important guidance to the design of LCs as chemo-responsive materials for use in sensors.