Sulfolane modulates tannic acid–pyridine organization in bulk water and at a silica–water interface

Sulfolane and pyridine may coexist in impacted aquifers, where naturally occurring polyphenols could alter their apparent miscibility and interfacial association. Without sulfolane, tannic acid and pyridine form composition-dependent droplets and flocs in water. Using tetramethylene sulfoxide (TMSO) as a comparator, we examined how sulfolane modifies this association in bulk liquids and at an oxidized silicon–water interface. Phase mapping showed that increasing sulfolane expanded optically clear regions without eliminating heterogeneous states. Fourier-transform infrared microspectroscopy indicated enrichment of tannic acid and pyridine within separated domains. Small-angle X-ray scattering showed that optically clear mixtures retained composition- and temperature-dependent diffuse heterogeneity. Across the tannic-acid–sulfolane series, pyridine increased the relative contribution of electron-density variations sampled at longer distances while redistributing the fitted diffuse contributions toward finer comparative spatial scales; heating progressively weakened the longer-distance variations in all series. Density functional theory calculations supported selected dry and hydrated phenolic–pyridine–sulfolane configurations and favoured retention of explicit water over complete displacement. Qualitative sequential O and N K-edge X-ray absorption fine structure (XAFS) measurements on one oxidized silicon membrane extended this bulk picture to an interface. A pyridine-associated N K-edge feature was unresolved after tannic-acid-containing exposures and rinses but reappeared when pyridine and sulfolane were co-introduced, consistent with sulfolane-associated reorganization of a multicomponent interfacial assembly. Vacuum-optimized and COSMO-water-optimized hydroxylated silica-cluster calculations supported the electronic feasibility of selected sulfolane-containing arrangements. Sulfolane therefore modulates rather than eliminates tannic-acid–pyridine organization in bulk water and at a history-conditioned oxide–water interface.