Using Liquid Crystals To Image Reactants and Products of Acid−Base Reactions on Surfaces with Micrometer Resolution

We report that nematic liquid crystals (LCs) of 4-cyano-4‘-pentylbiphenyl (5CB) and p-methoxybenzylidene-p-n-butylaniline (MBBA) can be used to image reactants and products of reversible, acid−base reactions on surfaces with micrometer resolution. When supported on obliquely deposited films of gold, azimuthal orientations of LCs measured on self-assembled monolayers (SAMs) formed from HOOC(CH2)10SH are found to be orthogonal to those measured on SAMs of the corresponding sodium carboxylate salt. Conversion of the carboxylic acid to its sodium salt is amplified into a change in orientation of 104−105 mesogens per surface-confined acid group. The change in orientation of the LC is easily transduced into an optical output by forming twist distortions within the LC upon conversion of the carboxylic acid to its sodium carboxylate salt. The threshold conversion of acid to salt that drives the formation of the twist distortion within the supported LC can be systematically shifted between 8% and 100% by using mixed SAMs formed from HOOC(CH2)10SH and H3C(CH2)11SH, and by pretreating these SAMs in aqueous solutions buffered between pH 1.7 and 14. We also demonstrate that region-specific conversion of the carboxylic acid to its sodium carboxylate salt can be readily imaged on micrometer scales by using LCs. Observations of the boundary region between acid and salt show no evidence of lateral transport of products and reactants over distances of micrometers over periods of days. We conclude that images of the reactants and products of this surface-confined chemical transformation can be conveniently recorded in the orientations of supported LCs.