Intermolecular Interactions and Thermophysical Modeling of Dimethyl Sulfoxide + Tri- <i>n</i> -butyl Phosphate Mixtures

Density, viscosity, and speed of sound were measured for dimethyl sulfoxide (DMSO) + tri-n-butyl phosphate (TBP) mixtures at atmospheric pressure (0.10 MPa) over the full composition range and temperatures from 298.15 to 323.15 K. The mixtures exhibit pronounced structure-breaking behavior, with excess molar volumes reaching positive maxima of 1.05–1.07 cm3·mol–1 near a TBP mole fraction of ≈0.45. Viscosity deviations show positive peaks between 0.9 and 1.7 mPa·s, while isentropic compressibility deviations remain positive across all temperatures. Negative speed of sound deviations, reaching −300 m·s–1, further indicate weakened intermolecular networks compared to the pure components. FTIR spectra reveal composition-dependent shifts of 5–12 cm–1 in the S═O and P═O stretching bands, consistent with weak hydrogen bonding and dipole–dipole interactions. Density functional theory (DFT) calculations support formation of a stabilized TBP–DMSO adduct, showing short intermolecular contacts, charge redistribution, and reduced HOMO–LUMO gaps, consistent with enhanced polarity upon mixing. Experimental data were correlated using the Jouyban–Acree model, while the PC-SAFT equation of state reproduced liquid densities with an overall average relative deviation (ARD) of 0.08%. In addition, new interaction parameters for the (CH2)3PO–DMSO pair were computed and applied in group-contribution-based predictive models (UNIFAC-VISCO and UNIFAC-THERMO), yielding viscosity prediction ARDs of 2.7% and 2.4%, respectively.