THERMODYNAMIC BEHAVIOR OF SOME UNCHARGED ORGANIC-MOLECULES IN CONCENTRATED AQUEOUS UREA SOLUTIONS AND OTHER POLAR-SOLVENTS

A comparison has been made between the values of the enthalpic pairwise interaction coefficients of several organic molecules (peptides, amides and alcohols) in water, in concentrated aqueous solutions of urea, in N,N-dimethylformamide and in liquid N-methylacetamide. The second viral coefficients of the excess enthalpies are found to be positive for all the systems studied in water-urea mixtures. A preliminary analysis, carried out using the Savage and Wood group additivity approach, suggests that, in concentrated aqueous solutions of urea, this arises from the peptide-peptide or hydroxyl-hydroxyl, and the apolar-apolar group contributions, all being positive, and these overwhelm the negative contributions from the polar-apolar group interactions. A remarkable feature is the inversion of the signs of the peptide-peptide, hydroxyl-hydroxyl, peptide-methylene and hydroxyl-methylene, group interactions when compared with those which prevail in pure water. This suggests a completely different solvation and interaction mechanism in concentrated urea solutions. The enthalpic contributions from apolar-apolar group interactions in the mixed solvent, in turn, are higher than the values found in water. Some comments are made on the behaviour of some of the above solutes, in the liquid amide solvents N,N-dimethylformamide and N-methylacetamide.