High Temperature End of the So-Called "Koga Line": Anomalies in Temperature Derivatives of Heat Capacities

The so-called "Koga line" is a collection of the loci of anomalies in various third derivatives of the Gibbs function, G, in the temperature-mole fraction field for aqueous solutions of nonelectrolytes. This splits the H2O-rich region into two, in each of which the molecular organization and interactions-we call it mixing scheme-is qualitatively different. In this work, we attempt to locate the high temperature end of the Koga line. This is a particularly interesting range of the boundary, since its extrapolation to zero Solute concentration provides information on possible temperature induced changes in the properties of pure water. To this end, we determine semi-isobaric heat capacities of aqueous 2-butoxyethanol by adiabatic calorimetry up to a maximum of 95 degrees C. The corrections due to vaporization were not applied due to the lack of required vapor pressure and thermal expansivity data. Furthermore, we measured directly the isobaric heat capacities per molar volume for aqueous I-propanol as well as 2-butoxyethanol by differential scanning calorimetry up to 120 degrees C at 3 atm. We then took one more temperature derivative of the respective heat capacity data. The resulting third derivative quantities from the former data showed step-type anomalies, while those from the latter negative peak-type anomalies. The loci of these anomalous points seem to point to about 70 degrees C at infinite dilution.