Studies on the interactions of diglycine and triglycine with polyethylene glycol 400 in aqueous solutions by density and ultrasound speed measurements

Density and ultrasound speed were measured accurately for diglycine + water, triglycine + water, diglycine + water-polyethylene glycol 400 (PEG400) and triglycine + water-PEG400 solutions at T = (293.15, 298.15, 303.15 and 308.15) K. The results were used in evaluating thermodynamic properties as apparent molar volumes (VØ) and apparent molar isentropic compressions (KSΦ) of diglycine and triglycine in water and in PEG400 solutions. Infinite dilution values of these parameters, VoØ, and KoSΦ, were obtained from their plots as a function of molality by extrapolation and have been utilized in obtaining transfer volumes and transfer compressions at infinite dilution. All transfer volumes and transfer compressions were found to increase with increasing molality of PEG400. Apparent molar isobaric expansions were derived from the temperature dependence of VØ values at infinite dilution and at finite concentrations. All the results were interpreted in terms of solute (diglycine or triglycine) and co-solute (PEG400) and solvent (H2O) interactions.

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► Di- and tri-glycine in aqueous PEG400 solutions were investigated thermodynamically.
► Density and ultrasound speed of glycine oligomer-PEG400-water systems were measured.
► Apparent molar volumes and isentropic compressions were calculated.
► Apparent molar isobaric expansions were derived.
► Results were interpreted in terms of water–glycine oligomer-PEG400 interactions.