The Forces Between Water Molecules and the Second Virial Coefficient for Water

Abstract

In previous work on this subject, the second virial coefficient has been calculated with numerical success, but on the assumption of intermolecular forces which take inadequate cognizance of the known structure of the water molecule. In the present paper, all long range forces are computed in a semi-empirical way from dipole moment, quadrupole moment in accordance with the Bernal-Fowler analysis, and optical dispersion data. Interactions caused by quadrupole moments cannot be neglected. Short range repulsive forces are thus left undetermined. These are chosen in such a way that the second virial coefficient, in its dependence on temperature, approximates its measured values as closely as possible. It is found that a reasonable fit cannot be obtained with the use of the hard sphere model, nor with any model which permits the polar component of the force to be effective at arbitrarily small distances of separation. More satisfactory agreement results if the polar forces are eliminated at all distances smaller than the potential minimum. The best potential satisfying these various requirements is given at the end of the paper [formula (22)].