水-尿素-吉草酸系の音速度と溶液構造 : (付)水-尿素-アセトン系の音速度

抄録

Using a 5 MHz crystal-controlled ultrasonic interferometer and a pycnometer, the sound velocity and density are obtained and the adiabatic compressibility is computed in the systems of water-urea, water-urea-valeric acid, and water-urea-acetone. The water-urea-system exhibits a rather common behavior in the sound velocity vs. concentration curves (Fig. 1), having a common intercept for curves of different temperatures. The concentration dependence of the density is monotonous (Fig. 2), and the behavior of the adiabatic compressibility curve with a common intercept is also similar to many of the aqueous systems (Fig. 3). As different from usual aqueous solutions, the dependence on mol fraction is completely linear for both Rao's molecular sound velocity and Wada's molecular compressibility (Fig. 4), presumably indicating that the urea molecules freely replace the water molecules in the framework of the ice-I like structure of water and the single water molecules in cavity sites. The solution obtained by dissolving valeric acid in a 6 Mol urea aqueous solution is not so different from usual aqueous systems in its sound velocity behavior: The peak-sound-velocity temperature (T_p) of pure water shifting to the lower side under the influence of both urea and the valeric acid (Fig. 5). The sound velocity in the solution obtained by dissolving valeric acid in a 10 Mol urea solution, however, is rather peculiar (Fig. 6), being sensibly constant up to 4 wt-% urea, and decreasing sharply beyond this concentration. The dependence of density on the concentration being monotonous and nearly linear (Fig. 7), and the behavior of the adiabatic compressibility curves (Fig. 8) are nearly the inversion of sound velocity curves. The molecular sound velocity, on the other hand, is quite linear in its dependence on the mol fraction of valeric acid as indicated in Fig. 9. The temperature dependence of the sound velocity is independent of concentration up to 4 wt-% valeric acid, but increases in its absolute value) with concentration beyond 4% (Fig. 10). By applying the empirical formula of Lagemann et al. on the dependence of dv/&ltdT&gt on the molecular weight for this system, we can obtain the mean molecular weight in dependence on the concentration. Circles in Fig. 11 show this result. The solid curve in this figure, on the other hand, corresponds to the hypothesis that a molecular cluster of size 21×(3 water molecules+1 urea molecule) persists in the solution up 4wt-% valeric acid, each cluster adapting up to 1 voleric acid molecule. At 4%, all clusters adopt valeric acid molecules each. Beyond this concentration, however, further addition of the valeric acid increases the number of clusters, each valeric acid forming a new cluster around it by depriving water and urea molecules from existing clusters. Water-urea-valeric acid system is known to produce urea-aduct as precipitation and we may expect some cluster structure in the aqueous solution system, too. On the other hand, water-urea-acetone system does not show this peculiar sound velocity behavior (Fig. 12). The density behavior is nearly linear also in this case (Fig. 13), and the adiabatic compressibility (Fig. 14) behaves itself quite monotonously. As this system does not produce ureaaduct, the absence of specific sound velocity-behavior is to be expected.