A thermodynamic study of aqueous acetonitrile: excess chemical potentials,partial molar enthalpies, entropies and volumes, and fluctuations

We measured vapour pressures of aqueous acetonitrile (abbreviated as ACN) a t 6, 20, and 37 degreesC, from which excess chemical potentials of ACN (mu (E)(ACN)) were calculated. We also determined excess partial molar enthalpi es of ACN (H-ACN(E)) at 6, 20, 30, 37, and 45 degreesC. From these data, ex cess partial molar entropies of ACN (S-ACN(E)) were calculated at 6, 20, an d 37 degreesC. Using density data by Benson's group, excess partial molar v olumes of ACN (V-ACN(E)) were evaluated. The response function data by the same group were also used to evaluate amplitude and wavelength of mean-squa re fluctuations in terms of volume, entropy, and cross between volume and e ntropy. All the above quantities and their dependence on the mol fraction o f solute, i.e., the effect of additional solute on the above quantities wer e used to study the effect of acetonitrile on the molecular organization of H2O. It was found that acetonitrile works as a stronger structure-making s olute than methanol. Rather its effect on H2O is about the same as that of propan-1-ol.