Electric Field Effect on Condensed-Phase Molecular Systems: V. Acid–Base Proton Transfer at the Interface of Molecular Films

In this work, we studied the effect of an applied electric field on the dissociation of acetic acid (AA) at the interface of the acid and ammonia molecular layers using the ice film capacitor method. The dissociation of AA was monitored by reflection absorption infrared spectroscopy, which measured the changes in the intensities of the vibrational modes of AA and the acetate anion. The extent of field-induced dissociation was found to be dependent on the direction of the applied electric field. Interestingly, the extent of acid dissociation was greater when the field was applied in the opposite direction to that of the proton-transfer motion. This suggested that the reaction was aided by the reorientation of the reagent molecules rather than by electrostatic stabilization of the charge-transfer energy. The effect of molecular size was studied by comparing the results for AA with those of formic acid and propionic acid. The dissociation of smaller acids was more strongly enhanced than that of larger acids, supporting the interpretation that the field-induced dissociation occurred via molecular reorientation.