Drug–bile salt interactions in aqueous media: molecular insights from volumetric, viscometric and thermoacoustic properties

This work explores the interaction behaviour of the drugs Diphenhydramine Hydrochloride (DPHH)and Doxycycline Hyclate (DCH) in aqueous Sodium Cholate (NaC) solutions over the concentration range of 0.01, 0.015, and 0.02 mol·kg⁻¹ and within the temperature interval of 298.15 K–318.15 K under atmospheric pressure. Experimental determinations and observations of density (ρ), viscosity (η), and ultrasonic velocity (u) were used to evaluate several physicochemical and acoustic parameters, namely apparent molar volume ( V 2 . ϕ ), apparent molar isentropic compression ( K 2 , ϕ , S ), and their limiting values ( V ϕ 0 and K ɸ , S 0 ). Viscosity data were interpreted through the Jones–Dole relation to determine the A- and B-coefficients. In addition, transfer parameters [ Δ tr V ɸ 0 , Δ tr B , and Δ tr K ɸ , S 0 ], hydration number ( n H ), limiting molar expansibility ( E 2 , ɸ 0 ), and Hepler constant [ ∂ E 2 , ɸ , v 0 / ∂ T) p ] were determined to elucidate the intermolecular interactions occurring between the dissolved species and the surrounding solvent molecules. The experimental findings demonstrate that the interaction behaviour and hydration characteristics of the investigated drug systems are strongly governed by molecular architecture, sodium cholate concentration, and temperature. The findings help clarify how the drug interacts with bile salts and how these species are solvated in aqueous media of pharmaceutical and biological relevance.