Specific-ion effects in sulfolane–water: Hydroxide-induced phase separation versus chloride and hydronium

Sulfolane is widely used as a polar aprotic solvent in gas treating and related refining operations, including the removal of acidic components from natural gas. In the same facilities, elemental sulfur is produced in bulk via Claus recovery from H₂S, and caustic (NaOH) is employed to scrub acid gases and mercaptans. As a result, sulfolane, NaOH, and elemental sulfur may co-occur at contaminated sites, yet their coupled effects on sulfolane speciation and transport remain poorly understood. NaOH strongly perturbs water structure and competes with sulfolane for hydrogen bonding, such that increasing NaOH concentration desolvates sulfolane. A nominally miscible sulfolane–water system evolves into sulfolane-rich macro-emulsions or molecular clusters (resolved by small-angle neutron scattering), with direct implications for its migration in groundwater. Elemental sulfur, although immiscible with water, dissolves into sulfolane-rich domains and accumulates at their interfaces, enabling co-transport of reduced sulfur with sulfolane. Comparison with NaCl and HCl demonstrates that this behavior is not a generic “salting-out” effect: NaCl induces only minor changes, whereas HCl enhances sulfolane hydration through specific interactions with its sulfone oxygens, as confirmed by Fourier transform infrared spectroscopy. These results provide a mechanistic basis for predicting sulfolane fate at caustic- and sulfur co-impacted sites and for improving management of sulfolane-contaminated groundwater.