Results for HR solute in dilute solutions of the primitive cluster model

Results for HR solute in dilute solutions of the primitive cluster model 

We begin with some results on the effect of the solute on the distribution of cluster size. In all of the following discussions, the HB solvent refers to the primitive cluster model for water. 

For some intermediate n 2 (Fig. 3.28), there is an initial decrease in X as we add s, then X increases but eventually decreases to zero as ys approaches unity. For larger clusters, say, n 20, the mole fraction X, initially increases but eventually decreases as the concentration of the solute increases. The details of which cluster size is stabilized or destabilized by the solute depends on the temperature and pressure, but the typical behavior of X for small n and X for large n is the same for the entire range of temperatures and pressures we are interested in. 

To understand the unusually large and negative entropy and enthalpy of solvation, it is not necessary to examine the detailed variation of each of the X . Instead, we examine the average number of HBs per water molecule, which we denote by (Xhb). This is a measure of the overall structure of the system. Variation of this quantity with ys determines the value of the entropy and the enthalpy of solvation of the solute s. 

Average number of HB e per molecule Average number of HB s per molecule 

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