The influence of pH, temperature and salt on the ethanolic desolvation of dilute gelation solutions

Abstract

The response of aqueous solutions of type B gelatin (Bloom strength 225) to progressively increasing concentrations of ethanol was investigated under various conditions of pH, temperature and the sodium chloride concentration. The behaviour of gelatin solutions in the absence of salt was observed to be highly dependent on the pH of the solution, with gelatin being most sensitive to ethanol addition at pH's close to the iso-electric point (IEP), which for type B Gelatin is around pH 5. In the absence of salt, the pH of the solution affects the net charge of the protein, effecting the degree of electrostatic repulsions between gelatin molecules, and hence the response of the protein to non-solvent. The addition of salt was seen to dramatically alter the behaviour of gelatin solutions towards ethanol at pH's close to the IEP, and less but still predominantly, at pH's away from the IEP. Salt exerted a predominant effect on the behaviour of gelatin, which masked any effect on gelatin caused by pH variation, which was the case in the absence of salt. The presence of salt is thought to give rise to two antagonistic effects; (i) zwitterionic decoupling, namely, ionised carboxyl groups are coupled with sodium ions, while ionised amino groups are coupled with chloride ions, hence disrupting intramolecular attractions which keep the molecule compact and resulting in a more open structure which interacts more and better with the solvent thus increasing solubility; and (ii) a decrease in the double layer surrounding the localised charges on gelatin molecules, which makes it possible for different molecules to come closer to each other and hence aggregate, leading to a decrease in solubility. The latter is only effective when the pH of the solution is removed from the IEP.