Reversed-phase retention thermodynamics of pure-water mobile phases at ambient and elevated temperature

The use of pure water at superheated temperatures, between 100 and 200 °C, as a mobile phase for reversed-phase separations is explored. The thermodynamics of the retention process at low temperature (15–55 °C) are compared to the thermodynamics at elevated temperature (125–175 °C). Significant differences in the enthalpy of the retention process are observed between the two temperature ranges. This is possibly due to changes in the hydrogen-bond network of the pure-water mobile phase, which would change the solvation, and therefore retention, of non-polar solutes. The change in thermodynamic values between the two temperature regions invalidates extrapolation of retention as a function of temperature between the two temperature regions for the prediction of room-temperature pure-water retention factors. The thermodynamic changes observed as the temperature is increased are similar to those seen when mobile phase composition is changed (by adding organic modifier) at constant temperature.