Probing the influence of pH dependent citric acid towards the morphology of rock salt: a computational study

Abstract

The influence of citric acid under different pH conditions towards the morphology of sodium chloride was investigated employing DFT calculations. Prior to any analysis, conformational search of citric acid and its three dissociated forms was performed with molecular mechanics MM2* and DFT (B3LYP/6-31+G*) methods in the gas and aqueous phase. The low energy conformers generated in the aqueous phase were found to be in good agreement with the available crystal structures. The calculated higher composition of citric acid at natural pH = 0.75 i.e., α₀ = 0.99 is not effective for the habit modification of sodium chloride. Quantum chemically derived molecular electrostatic potential (MESP) showed the possible sites of citric acid and its dissociated forms with the estimated deepest V_min values and their magnitudes provide an insight for their interaction with the crystal surfaces of sodium chloride. The composition analysis and MESP derived data suggest that the dissociated forms of citric acid at higher pH values are responsible for the change in habit of sodium chloride from cubes to octahedrons. Further, slab model calculations performed to mimic the surfaces of sodium chloride with DFT methods using a conductor-like screening model in the aqueous phase (COSMO) for the interactions of citric acid and dihydrogen citrate validates that there is no preference for the {111} plane of NaCl with citric acid. However, a significant preference was seen with dihydrogen citrate, which quantifies the MESP analysis and corroborates the dependence of morphology on the pH of the solution.