A THERMODYNAMIC STUDY OF THE SOLVOPHOBIC EFFECT IN FORMAMIDE SOLUTIONS OF NONPOLAR MOLECULES

The thermodynamics associated with the solvophobic effect in solutions of nonpolar molecules in formamide were examined using high-performan ce liquid chromatography (HPLC). The transfer of a series of alkylbenz enes from formamide to hydrocarbon environment was studied in the temp erature range 5-80-degrees-C in order to obtain the molar free energy, entropy, and enthalpy of transfer. The HPLC system consisted of poly( dimethylsiloxane), PDMS, coated glass beads as the stationary phase an d formamide as the mobile phase. The free energy and the enthalpy of t ransfer were found to be negative, while the entropy of transfer was p ositive. In general, the thermodynamic transfer quantities for formami de are both smaller in magnitude and less temperature dependent than t hose previously determined with water as the mobile phase. The relativ ely small DELTAC(p) of transfer in the case of formamide indicates tha t there is negligible structuring of the formamide molecules around th e nonpolar molecules, in contrast to the water case. The positive entr opy of transfer in the case of formamide is attributed to complex form ation between the alkylbenzenes and formamide. The driving force for t he solvophobic effect seems to arise from the same source as the hydro phobic effect, namely the large cohesive energy of the solvent. This i nterpretation was used to explain differences in surfactant aggregatio n in formamide and water.