The low-pressure compressibilities of acetonitrile, acetaldehyde, and their mixtures were measured in the temperature range 40° to 100°C. From these data second virial coefficients were determined for the pure components and for the mixture. The absolute value of the coefficient B12, characteristic of interaction between dissimilar molecules, was considerably larger than that for B11 or B22, which are characteristic of interactions between similar molecules. As a result it was not possible to predict the properties of the mixture with only data for the pure components. Calculations based on the Stockmayer potential and with conventional mixing rules gave seriously incorrect results for the second virial coefficient B12.
The data were interpreted with the aid of an association theory. Dimerization equilibrium constants and enthalpies and entropies of formation were computed for the pure-component dimers and for the complex. The heat of formation for the complex was significantly larger than that for the dimers; comparison with energy terms calculated from electrostatic theory suggest that the structure of the complex is different from that of the pure-component dimers.