The hydration of formic acid and acetic acid

Formic acid and acetic acid share the unique feature, amongst carboxylic acids, of crystallising in the form of long chains, containing both O-H⋯O and C-H⋯O hydrogen bonds. We have performed a neutron diffraction study of the pure acids and of three mixtures of acid and water (2:1, 1:1 and 1:2). The data from the SANDALS diffractometer at ISIS have been modelled using the Empirical Potential Structure Refinement code, which is able to reproduce a set of configurations compatible with the experimental data. The relative importance of the hydrogen bonds present in the solution is assessed based on geometrical criteria: bond length and directionality as well as number of bonds. At all concentrations, the carbonyl oxygen on the carboxylic group is the most active site for a strong hydrogen bond. The tendency to establish direct interactions between acid molecules in the presence of water is reduced for acetic acid by a larger degree than for formic acid. The overall tendency is for a greater number of hydrogen bonds being formed when the solution is more diluted. The availability of good quality structural data on the liquid states is of great importance for the understanding of spectroscopic experiments and for benchmarking both classic molecular dynamics and ab initio simulations. The results provide a springboard to more realistic models of aerosol formation, which is greatly needed for better understanding of cloud formation processes.