Anomalies in the Third Derivatives of Gibbs Energy and Their Temperature Dependence in Aqueous 2-Butoxyethanol and Glycerol: On the so Called Koga Lines

By using a home-made differential pressure perturbation calorimeter, we directly determined the partial molar entropy-volume cross fluctuation density of solute B, (SV) delta (I'), for aqueous solutions of 2-butoxyethanol (BE) and glycerol (Gly). (SV) delta (I') is one of the third derivatives of the Gibbs energy, G, and shows anomalous behavior, the mole fraction dependence of which depends on the nature of the solute. The hydrophobic BE shows a peak-type anomaly and the hydrophilic Gly a more subtle bend-type one in their mole fraction dependences of (SV) delta (I'). The peak top for the former and the break point for the latter are regarded as singular. They, together with those of other third derivative quantities, form a single curve as a function of temperature for a given solute. We call the curve the ``Koga Line{''}. It approaches to about what appears to be a universal temperature, about 70-80 A degrees C, when extrapolated to the infinite dilution. This temperature seems to be independent of the class and the individual identity of solute. This hints that even for pure H2O there is a subtle change in the property at about 70-80 A degrees C. We speculate this is related with the change in the bond-percolation nature of the hydrogen bond network of H2O, following the site-correlated percolation model of H2O advanced by Stanley et al.