The preferential solvation (PS) viscosity model was used to correlate binary liquid viscosities of aqueous, nonaqueous, nonpolar-polar, molecular solvent-ionic liquid (IL), and molecular solvent-deep eutectic solvent mixtures for the purpose of determining local composition in the chemical systems and analysis of chemical phenomena. Lignin solubility in aqueous systems was directly proportional to the population of (1-2) complex molecules. Cellulose solubility in molecular solvent-IL mixtures was directly proportional to the population of (2-2) associated molecules. Local compositions determined from the PS viscosity model were able to be used to estimate mixture solvent polarity and the variation of carbazole selectivity in crude anthracene separations. The PS viscosity model applied to chromatography allowed estimation of appropriate bulk composition regions for separations. Local compositions determined from the PS viscosity model can be used to analyze a wide variety of chemical phenomena in liquid solvent mixtures.