Ultrasonic and Volumetric Investigation of Aqueous Solutions of Amides

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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.