No abstract

No abstract

The ultrasonic velocity and density of aqueous solutions of N-methylformamide, N,N-dimethylformamide, and N,N-dimethyIacetamide have been measured at 25 and 35 °C over the entire concentration range. The compressibility, excess compressibility, partial molar compressibility, partial molar volume, and pressure dependence of the partial molar volume have been calculated. The concentration dependences of the velocity and density are highly nonlinear, and the velocity maxima are observed in all the amide solutions at ca. 20 mol%. Minima in the partial molar volume exist in solutions of DMF and DMAA, but not in that of MFA. On taking account of the results for viscosity, the velocity maxima are ascribed to the complex formation, and the minima in the partial molar volume, to the competition between the volume increase due to the complex formation and the volume decrease due to the breaking-down of the water structure. The pressure dependence of the partial molar volume shows that amides and water have strong interactions, even in the low-mole-fraction regions for each component.

The ultrasonic absorption coefficients of aqueous solutions of dextran and some saccharides have been measured in a frequency range from 0.8 to 130 MHz and over a temperature range from 10 to 35 "C. In all solutions of dextran two relaxation processes are observed within the frequency range examined, which have relaxation frequencies at about 1 and 12-15 MHz. They are likely independent of molecular weight and temperature. However, the aqueous solutions of saccharides do not exhibit any relaxation. The relaxation processes observed in aqueous solutions of dextran are considered to be volume relaxations due to the segmental motion of the polymer chain. The most likely origin of the volume relaxations is the exchange process of the hydrated water molecules in the hydration sphere of the polymer.

No abstract