Structure of Sulfuric Acid Solutions Using Pair Distribution Function Analysis

Abstract

Solvation and mesoscale ordering of sulfuric acid and other strong acid solutions leads to suppressed freezing points and strong rheological changes with varying concentration. While the solid-state structures are well-understood, studies focused on the evolving solvation structure in the solution phase have probed a limited concentration range (∼1−6 M). This study applies a total scattering approach in both the wideangle X-ray scattering (WAXS) and pair distribution function (PDF) regimes to elucidate the evolving solvation structure over its full range of acid concentration (0−18 M). The emergence of a prepeak in the WAXS regime at intermediate concentrations indicates a transition from noninteracting sulfate molecules in the dilute limit to sterically limited sulfates at concentrations near its deep eutectic point. Fits to the PDF data quantify this trend, showing a transition from octahedrally hydrated sulfates up to 6−7 M concentrations, followed by gradual dehydration, and eventually reaching a solution structure similar to that of water-in-salt electrolyte systems at high acid concentrations.

Investigating the phase diagram-ionic conductivity isotherm relationship in aqueous solutions of common acids: hydrochloric, nitric, sulfuric and phosphoric acid

“…Phase diagrams for HCl, HNO 3 , H 2 SO 4 , and H 3 PO 4 solutions in water were obtained from Boryak et al 23 , Tizek et al 24 , Kinnibrugh et al 25 , and Corti et al 26 respectively. The data from the four binary phase diagrams were digitized using Automeris software.…”