Salting-out effect in polypropylene glycol-amino acid aqueous solutions revealed by vapor pressure osmometry

• Vapor-liquid equilibrium measurements for dilute PPG-amino acid aqueous solutions.
• Evaluation of departures from the semi-ideal behavior.
• Study of salting-out phenomenon in PPG-amino acid aqueous systems.
• Effect of temperature and amino acid structure on the extent of salting-out effect.

Precise vapor pressure osmometry measurements at T   = 303.15, 308.15 and 313.15 K have been performed for low concentrations of different binary and ternary aqueous solutions including: amino acids (glycine, alanine, serine and proline) in pure water, PPG725 in pure water and amino acids in aqueous solutions of 0.01 w/w PPG725. In order to study the salting effects occurring in polymer-amino acid aqueous systems, deviations from the semi-ideal behavior have been evaluated. The results indicate that because of unfavorable interactions between PPG and amino acids, they are preferentially hydrated in aqueous media. Therefore, the amount of free water molecules in the PPG + amino acid + water solutions is less than that expected based on the semi-ideal behavior. Hence, under condition that solutes molalities in the ternary solutions are the same as those in the corresponding binary solutions, the values of aw+1−awp°+awa° and Δp−(Δpp°+Δpa°) are negative and become more negative by increasing temperature and hydrophilicity of amino acids side chain.

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