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)(10)-SH are found to be orthogonal to those measured on SAMs of the corresponding sodium carboxylate salt. Conversion of the ca rboxylic acid to its sodium salt is amplified into a change in orientation of 10(4)-10(5) mesogens per surface-confined acid group. The change in orie ntation of the LC is easily transduced into an optical output by forming tw ist distortions within the LC upon conversion of the carboxylic acid to its sodium carboxylate sail. The threshold conversion of acid to salt that dri ves the formation of the twist distortion within the supported LC can be sy stematically shifted between 8% and 100% by using mixed SAMs formed from HO OC(CH2)(10)SH and H3C(CH2)(11)SH, and by pretreating these SAMs in aqueous solutions buffered between pH 1.7 and 14. We also demonstrate that region-s pecific conversion of the carboxylic acid to its sodium carboxylate salt ca n 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 da ys. We conclude that images of the reactants and products of this surface-c onfined chemical transformation can be conveniently recorded in the orienta tions of supported LCs.