Practical limitations on the use of thermodynamic data from isothermal processes

Abstract

Enthalpy, entropy and volume data obtained for processes studied in aqueous solvents generally have been assumed to apply to the solute process without consideration of the coupling between the process and the two-state equilibrium of water. Walrafen's confirmation of the latter in 1983 shows that long-debated model to be correct so the enthalpy and entropy contributions to a free-energy change to give unambiguous information must be corrected for the water contribution. The situation is further complicated by differential chemical interaction of amphiphilic solutes with the two water species since experimental complications make correction difficult or impossible. A more general source of error in isothermal experiments is the linkage to the thermal-equilibrium device. That thermal problem discovered only in 1967 is not yet treated in textbooks although it is always a complication in isothermal processes and responsible for a hierarchy of thermodynamic quantities with different levels of reliability. Major consequences for several familiar thermodynamic and extra-thermodynamic methods are examined in terms of relative reliability. In most cases the thermal corrections are restricted by changes in phase state on cooling.