Sound Velocity in Aqueous Non-Electrolyte Solutions in Relation to Water Structure

Abstract

An acoustic interferometer is employed to obtain the variation of sound velocity with concentration and temperature in hexamethylenetetramine (HMT), urea, thiourea, acetamide and N,N-dimethylformamide (DMF) and their aqueous solutions. The plot of velocity of aqueous solutions of these organic substances against temperature gives parabolic curves similar to one for pure water. In these curves, the maximum height of sound velocity is higher than that for pure water and the temperature at maximum point is lower than for water. The curves of sound velocity vs. concentration at various temperatures show values of maximum velocity at intermediate concentrations. The maximum value decreases with increasing temperature and finally disappears in some cases. Results show that the velocity dependency on temperature and concentration are qualitatively determined by the relative concentration of non-associated water to cluster. The sound velocity of acetamide and DMF is 1404.4 m/sec (65°C) and 1478.8 m/sec (20°C), respectively, and shows negative temperature coefficients of −2.68 m/sec/deg and −3.90 m/sec/deg, respectively.