Temperature-Dependent Physicochemical Properties of L-alanine and L-isoleucine in Aqueous Diphenhydramine hydrochloride

The present study investigates the temperature-dependent physicochemical properties of L-alanine and L-isoleucine in pure water and aqueous diphenhydramine hydrochloride solutions of varying molal concentrations (0.05, 0.10, and 0.15 m o l k g − 1 ) over the temperature range 293.15-318.15 K under a constant pressure of 101.3 kPa. Measurements of density and viscosity served as the basis for calculating a range of volumetric and viscometric parameters, such as apparent molar volume ( V ϕ ), partial molar volume at infinite dilution ( V ϕ 0 ), limiting apparent molar expansibility ( E ϕ o ), transfer parameters and viscosity B -coefficients. Analysis of these thermodynamic properties enabled the assessment of the dominant molecular interactions between L-alanine and diphenhydramine hydrochloride, as well as between L-isoleucine and diphenhydramine hydrochloride, in aqueous media. The collective findings indicate the prevalence of hydrophilic-hydrophilic interactions, highlighting the significant influence of diphenhydramine hydrochloride on solute–solvent interactions, hydration behaviour, and the structural organization of water around the solutes. The combined volumetric and viscometric results indicate an overall structure-breaking tendency under the investigated conditions.