Density Speed of sound Betaine hydrochloride drug Hepler's constant Cosphere overlap model A B S T R A C T Densities (r) and speeds of sound (u) of betaine hydrochloride (B.HCl) drug (0.01–0.06) mol kg À1 in (0.10, 0.20 and 0.30) mol kg À1 aqueous NaCl and KCl solutions have been reported as a function of temperature at T = (293.15 –313.15) K and at atmospheric pressure. Using experimental data the apparent molar volume (V 2,f), partial molar volume (V 2 1), transfer partial molar volume (D t V 2 1), apparent molar isentropic compressibility (K s,2,f), partial molar isentropic compressibility (K s,2 1), transfer partial molar isentropic compressibility (D t K s,2 1), partial molar expansion (E 2 1) and Hepler's constant (@ 2 V 2 1 /@T 2) P were calculated. Further, all these parameters are employed to understand the temperature and cosolute effects on B.HCl drug–solvent interactions. Cosphere overlap model is used to analyze the hydrophilic/ ionic-ionic interactions in (B.HCl drug + NaCl/KCl + H 2 O) ternary mixture. The sign of Hepler's constant have been used to interpret the structure making or braking tendency of B.HCl drug in aqueous NaCl and KCl solutions at probed compositions and temperatures. ã 2014 Elsevier B.V. All rights reserved.