Densities,ρ, speeds of sound, c, and refractive indices, n_D, have been measured for the systems N,N-dimethylformamide (DMF) + propanone, + 2-butanone, or + 2-pentanone in the temperature range from 293.15 to 303.15 K and at 298.15 K for the DMF + 2-heptanone mixture. Due to the high volatility of acetone, the corresponding n_D measurements were developed at 293.15 K and 298.15 K. The direct experimental data were used to determine the excess molar volumes,V_m^E, and the excess refractive indices, n_D^E, at the working temperatures. Values of the excess functions at 298.15 K, for the speed of sound,c^E, the isentropic compressibility, κ_S^Eand for the excess thermal expansion coefficient, α_p^E, were also calculated. The investigated systems are characterized by strong amide-ketone interactions, which become weaker when the alkanone size is increased. This is supported by negative〖V_m^E〗^values; by the dependence on temperature and pressure of V_m^E, and by positiveP_int^E(excess internal pressure) values. Analysis of the systems in terms of the Rao’s constant indicates that there is no complex formation. In addition, negative V_m^E values also reveal the existence of structural effects, which largely contribute to the excess molar enthalpy, H_m^E. V_m^E and H_m^E values increase with the chain length of the 2-alkanone. It allows conclude that the relative V_m^E variation with the ketone size is closely related to that of the interactional contribution to this excess function. Molar refraction values, R_m, show that dispersive interactions become more relevant for the systems including longer 2-alkanones.
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