Structural study of simple organic acids by small-angle x-ray scattering and monte carlo simulations

This study represents an extension of our previous research on the structural properties of simple organic liquids to the systems of organic acids. A set of simple acids from ethanoic to octanoic was modelled with the TraPPE-UA force field and configurational bias Monte Carlo (CBMC) simulations were used to obtain a number of configurations of each system. These data were subsequently used as a basis for the calculation of X-ray scattering intensities, partial radial distribution functions and statistical analysis of molecular aggregation in liquid organic acids. The comparison of simulated X-ray scattering curves with the results of SAXS measurements has shown the agreement to be overall somewhat poorer than in our previous studies on alcohols and aldehydes, although it did improve with increasing molecular length. Hydrogen bonds between the hydroxylhydrogen atom and the carbonyl oxygen have been found to have a profound effect on the structure of the liquid acids. However, the model-based results showed that the formation of intermolecular hydrogen bonds involving the hydroxyl oxygen was disproportionately scarce in these systems. Statistical evaluation of the model configurations has shown that only about 4% of acid molecules form such a type of hydrogen bonds, in contrast to 68% of molecules that form the hydrogen bond with the carbonyl oxygen. This suggests that the force field might be underestimating the hydrogen bonding via hydroxyl oxygen. The statistical analysis has also shown that the simulated molecules preferred to assemble into small molecular aggregates, particularly into double-bonded molecule pairs.