Translational diffusion and isomerization reaction of a liquid crystal molecule at solid–liquid interface of ionic liquids studied by total internal reflection-transient grating spectroscopy

Abstract

The translational diffusion coefficients and isomerization reaction rates of N-(4-methoxybenzylidene)-4-butylaniline (MBBA) dissolved in ionic liquids (ILs) were measured at the sapphire interface using total internal reflection-transient grating (TIR-TG) spectroscopy. 1-Alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amides ([C_nmim][NTf₂]) were used as ILs with different alkyl chain lengths. The diffusion coefficient of MBBA in the bulk did not monotonically decrease with an increase in the solvent viscosity, and it was maximum in [C₆mim][NTf₂]. Based on the radial distribution functions (RDFs) calculated from molecular dynamics (MD) simulations, we interpreted that the non-monotonicity of diffusion was due to the fact that MBBA behaves as an amphiphilic surfactant at the interfaces between the polar and nonpolar domains of the ILs and diffuses selectively along the surfaces of the nonpolar domains. At the sapphire interface, the diffusion coefficient of MBBA was almost the same as that in the bulk, whereas the isomerization reaction rate was significantly accelerated, irrespective of the alkyl chain length. The contact angle measurement revealed that MBBA was significantly concentrated near the sapphire interface. The above experimental results were discussed in relation to the bulk and interfacial structures of ionic-liquid solutions of a liquid crystal molecule.