Influence of alkyl group and temperature on thermophysical properties of carboxylic acid and their binary mixtures

• Densities (ρ), sound velocities (u) and refractive index (n) were measured for carboxylic acid mixture.
• Derived properties were calculated from measured ρ, u and n, respectively.
• Redlich–Kister polynomial was used to fit the results.
• Lorentz–Lorenz equation was used to predict the density or refractive index.
• Sound velocity mixing rules were applied to the experimental sound velocity.

In this work, volumetric, acoustic and refractive index methods have been used to study the interactions between carboxylic acids mixtures as a function of temperature and concentration. The density (ρ), sound velocity (u), refractive index (n) of butanoic acid, pentanoic acid and heptanoic acid and their binary systems (butanoic or heptanoic acid + pentanoic acid) have been measured at 293.15, 298.15, 303.15, 308.15 and 313.15 K and at p = 0.1 MPa. The Lorentz–Lorenz approximation and sound velocity mixing rules were used to test the accuracy of the experimental data. The derived properties such as excess molar volumes, VmE, isentropic compressibilities, κs, excess isentropic compressibilities, κsE, and deviation in refractive indices, Δn, were also calculated. The Redlich–Kister polynomial equation was used to fit the excess/deviation properties. These results are useful for describing the intermolecular interactions that exist between the components in mixtures. This work also tests various sound velocity mixing rules to calculate the sound velocity of the binary mixture from pure component data, as well as examine the use of the Lorentz–Lorenz approximation to predict density from refractive index and vice versa.