Adsorptions of formic and acetic acids on ice surface: Surface binding configurations and a possibility of interfacial proton transfer

Adsorptions of formic (FA) and acetic acids (AA) on I_h ice surface were studied using quantum mechanical/effective fragment potential (QM/EFP) scheme. Contrasting to the earlier studies in which trans-conformers were found as major surface configurations, our QM/EFP models found various cis-and trans-conformers on ice surfaces with the cis-conformers being more stable. The surface binding energies and configurations were largely dependent on surface heterogeneity yielding the various surface conformers. In addition, the overall binding energies of acetic acid are slightly higher as compared to formic acid, implying the stabilization effect of methyl group. Our study also found a feasible deprotonation route of adsorbed trans-formic acid. In contrast, acetic acid prefers molecular form due to the unfavorable hydrophobic methyl group. Therefore it is interesting to note that the additional methyl group of acetic acid enhances surface binding energies. But at the same time it reduces the chance of its deprotonation. Our ice model clearly demonstrated the significant effects of intrinsic surface heterogeneity on the distributions of surface binding energies and configurations, which cannot be represented by small water clusters.