Sulfolane clustering in aqueous saline solutions

Abstract

Sulfolane is a groundwater pollutant. While sulfolane is miscible in pure water, its miscibility in the presence of ions has not been widely investigated. This aspect is relevant to predict sulfolane migration in groundwater. Na2SO4 separates sulfolane from water, yielding bulk separation, emulsions seen by optical microscopy or molecular sulfolane clusters. We study these clusters with fluorescence spectroscopy, small-angle x ray (SAXS) and neutron (SANS) scattering, and x-ray absorption fine structure (XAFS). Fluorescence spectroscopy reveals non-monotonic changes in the local viscosity of the water phase with 10% sulfolane and 0.25-1M Na2SO4, likely resulting from the interplay between sulfolane clustering and enhanced interactions between water molecules. NaCl affects the micro-viscosity of water similarly to Na2SO4, but at higher concentrations. At low sulfolane percentages, Cl− decreases the activity coefficients of water and sulfolane, suggesting stronger sulfolane–sulfolane and water–water interactions. SAXS confirms that Na2SO4 induces sulfolane clustering. SAXS data modeled with a power law and a Gaussian reveal a correlation length ξ = 16.3 Å, which we view as the radius of a sulfolane cluster surrounded by water and Na+ ions. SANS also shows that 2 mol. % of sulfate and chloride salts induce sulfolane clustering, with sulfates having a more marked effect. Furthermore, XAFS reveals that sulfates affect sulfolane sorption onto Si3N4 surfaces. Without Na2SO4, sulfolane directly sorbs onto Si3N4 surfaces. Conversely, with Na2SO4, water is directly sorbed onto Si3N4, likely because it surrounds sulfolane clusters. Also, hydrated Na+ ions are in Si3N4 surface proximity.