A volumetric study of two related amphiphilic beta-blockers as a function of temperature and electrolyte concentration

The effects of electrolyte concentration and temperature on aqueous solutions of propranolol and acebutolol hydrochlorides have been investigated using density and ultrasound velocity measurements. The electrolyte range was 0.0–0.5 and 0.4–1.0 m for propranolol and acebutolol, respectively. For each electrolyte concentration the temperature range was 288.15–313.15 K. Critical concentrations were obtained from plots of ultrasound velocity against drug concentration. Experimental results yielded the apparent molar volume and the apparent molar adiabatic compressibility for both beta-blockers, measured over a wide concentration range. Negative deviations of the apparent molar volume from the Debye–Hückel limiting law in dilute solutions indicate the absence of premicellar aggregation. A negative slope was found for ΔV_m against temperature for both drugs. This negative value suggests that the expansibility of the surfactant in the micellar state is less than that in the aqueous phase. Changes in molar volume and adiabatic compressibility accompanying aggregate formation were smaller than those of typical surfactants, suggesting a more tightly packed aggregate.