The Structure Theory of Water. I. Two State Theory

A statistical thermodynamic theory was successfully applied to liquid water, based on the two state model which consists of an ice-like structure (class I) and unbonded monomers (class II). The resulting partition function was formulated and calculated using the two quantities, the energy difference between class I and class II, ε, and the free volume per molecule, vf, as the adjustable parameters which were varied within the physically-reasonable range. The magnitudes of ε and vf were estimated to be 3 kcal/mol and 4.0×10−25 cm3 respectively to give the best fit of calculated values to the experimental data. From the partition function obtained, the Helmholtz energy, the internal energy, the entropy, and the specific heat were calculated. The agreement between the calculated values and the experimental data was found to be satisfactory. From the obtained result it may be concluded that the simple two state model is adequate to explain the thermodynamical data for liquid water.