Studies of heterogeneous interactions between N-acetyl-N′-methyl-l-α-amino acid amides and urea molecules in water at 298.15 K

The enthalpies of solution of N-acetyl-N′-methyl-l-α-threoninamide, N-acetyl-N′-methyl-l-α-tyrosinamide, N-acetyl-N′-methyl-l-α-histidinamide, N-acetyl-N′-methyl-l-α-tryptophanamide have been measured in water and in aqueous urea solutions at 298.15 K. The mixing enthalpies of aqueous urea solution with aqueous N-acetyl-N′-methyl-l-α-valinamide solution and their respective enthalpies of dilution by water have been determined at 298.15 K. The experimental results were used to obtain the enthalpic coefficients of the interaction between amino acid amides and urea molecules in water based on McMillan–Mayer's model. The values of the interaction parameters were interpreted in terms of the hydrophobic or hydrophilic properties of the amino acid radicals in the examined amide molecules and the influence on their interactions with a urea molecule in water.