Thermodynamic and Compressibility Studies of Naphazoline Hydrochloride in Aqueous and Aqueous l-Valine Solutions: Insights into Molecular Interactions and Solvation Behavior

The molecular interactions and solvation behavior of naphazoline hydrochloride (NPH) in aqueous, 0.025 mol·kg^–1 aqueous l-valine, and 0.05 mol·kg^–1 aqueous l-valine solutions were investigated using density and speed of sound measurements within a concentration range of 0.02–0.15 mol·kg^–1 at different temperatures (T = 288.15, 293.15, 298.15, 303.15, 308.15, 313.15, and 318.15 K) and atmospheric pressure. The derived parameters, namely, the apparent molar volume of the solute, apparent molar compressibility of the solute, isentropic compressibility of the solution, limiting apparent molar volume of the solute, limiting apparent molar compressibility of the solute, thermal expansion coefficient of the solute, apparent molar expansivity of the solute at infinite dilution, and transfer properties of the solute were calculated by using density and speed of sound data. These parameters provide insights into solute–solute and solute–solvent interactions. The results indicated strong hydrogen bonding and ion–dipole interactions between NPH and water, while the presence of l-valine modulated these interactions by altering the hydration shell and compressibility of the systems. The observed derived parameter trends suggested enhanced structuring effects in aqueous solutions, which were further influenced by the zwitterionic nature of l-valine. To further understand these interactions, UV–visible spectroscopic analysis was conducted. These findings provide valuable insights into the physicochemical properties of NPH in biologically relevant environments, with potential implications for drug formulation, stability, and bioavailability in pharmaceutical applications.