Structural Study of Simple Organic Acids by Small-Angle X-Ray Scattering and Monte Carlo Simulations

This study represents an ex tension of our previous re search on the structural pro per ties of simple organic liquids to the systems of organic acids. A set of simple acids from ethanoic to octanoic was model led with the TraPPE-UA force field and configurational bias Monte Carlo (CBMC) simulations were used to obtain a number of configurations of each system. The se data we re subsequently used as a basis for the calculation of X-ray scattering in ten si ties, 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 so mew hat poorer than in our previous studies on alcohols and aldehydes, although it did improve with in creasing molecular length. Hydrogen bonds between the hydroxylhydrogen atom and the carbonyl oxygen have been found to have a pro found 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.