The Excess Partial Molar Heat Capacity of Water Is a Measure of Its Structure in Binary Aqueous Solvent Mixtures

The excess partial molar heat capacity of the water in binary aqueoussolvent mixtures (W + S), CPW E, provides insight into the water structure enhancement, if present. A cubic representation of CP E of binary aqueoussolvent mixtures Cp E = b0 + b1xS + b2xS 2 + b3xS 3 is valid for water-rich mixtures, xS = 0.3, hence also CPW E = -b2xS 2 - 2b3xS 3 is readily obtained. Values of CPW E(xS) were obtained for many aqueous co-solvent mixtures from literature data beyond those dealt with in the author's previous publication. Typical values of the maximal CPW E are 10 to 20 J K-1 mol-1 or 24 to 48% of the difference CP l-ig = CP(liq) - CP(id.gas), which for water at 25°C is 42 J K-1 mol-1. However, energy input into the water entails not only ordering of the water, but also input into vibrational modes. Therefore, only a fraction of the heat capacity of ideal gas water needs to be subtracted from CPW E in order that only structure (order) enhancement is reckoned. Empirically, a fraction of 0.3 appears to be satisfactory. A mixture model for water structure in terms of compact and bulky hydrogen bonded domains allows structure enhancement to be interpreted as transfer of molecules between them. Strong, small hydrogen bonding solutes do not enhance the structure, fitting well into it. Solutes with many methyl groups, though miscible with water, do enhance its structure.