Microcalorimetric methods for direct measurement of solubility and solvent activity

New calorimetric methods for direct measurement of solubility and solvent activity are presented. The classical role of calorimetry for measurements of enthalpic parameters is abandoned and the technique is instead used for assessing thermodynamic quantities related to the chemical potential of a substance in a solution. Isothermal calorimetry is here employed as a general analytical method where the rate of an isothermal process is studied by measurement of the corresponding heat flow rate between the calorimetric cell and the surroundings. Ways of interpret the calorimetric signal in terms of solvent vapour pressure, solvent fugacity or solute solubility is presented.

A simple and efficient way of determine liquid solubility and a calorimetric gas solubility meter are described. A calorimetric apparatus for direct determination of the volatility or "the escaping tendency" of solvent in solutions has been built and the possibility of direct use of isothermal calorimetry in vapour/liquid equilibria studies is demonstrated. For measurements on solutions with a single volatile solvent the results are of high quality. For mixtures containing several volatile components the results are very promising but further work is needed to obtain optimal quality. Due to the automated functions of a modern calorimeter the measuring procedures are straightforward and simple. The concept developed here can easily be applied in any available heat-conduction calorimeter of modular design.

The enthalpic changes of dissolving hydrocarbons at infinite dilution in formamide were measured and the temperature dependence of the enthalpic values was studied by determining the corresponding heat capacity changes. The low solubility of hydrocarbons and aggregation of amphiphilic compounds in formamide is mainly due to an unfavourable enthalpic effect for the solvation of hydrocarbon groups and not, as in the case of water, to a large negative entropy change.