The surface tension of water calculated from a random network model.

The surface tension (excess surface free energy) of water is an important factor in many biological and chemical systems. It is often invoked to correlate or explain the low solubility of nonpolar compounds in water as well as the so-called hydrophobic effect of nonpolar molecules aggregating in water. While water's surface tension is not easily obtained from theory, the random network model (RNM) of Henn and Kauzmann (H-K), which described water's well-known, anomalous thermodynamic properties, can be used successfully to calculate the surface tension of water at atmospheric pressure between temperatures of 0 and 100 degrees C. Agreement with observed values, particularly at lower temperatures, is good, although the H-K RNM is too ice-like in nature to allow for the rapid drop off of surface tension with increasing temperature. Suggestions are offered for improving the model. In addition, an alternative explanation for the preferred orientation of surface water molecules is proposed based on water's H-bonding and attendant intramolecular, zero-point distortion energy.