The influence of pH and SDS concentrations on the ethanolic coacervation of dilute fish skin gelatin solutions

Abstract

The properties of collagen-derived gelatin have found use in the production of nanoparticles as vehicles for drug delivery. One such method involves the fractionation or coacervation of the gelatin protein by the addition of non-solvents. The molecular weight profile of gelatin in solution determines its behaviour and the protein structure of gelatin may be affected and altered by a change in pH, addition of salt or surfactant and by changes in temperature. The response of gelatin to a non-solvent is affected by the net charge on the gelatin molecule which is in tum determined by the pH of the solution. Interaction of aqueous gelatin with SDS affects the overall behaviour of gelatin in solution in response to addition of a non-solvent by association with gelatin through hydrophobic and ionic interactions with the charged gelatin molecules. This causes unfolding of the protein and formation of a gelatin-surfactant complex. The response to fish-skin gelatin to progressively increasing concentration of ethanol was investigated under various conditions of pH and SDS concentration. This study was further extended by testing out an alternative method for surfactant delivery which involved introducing the surfactant in the ethanol/water mixtures and not in the initial gelatin stock solution. An indication from the results obtained is that the iso-electric point of the fish-skin gelatin sample solutions may be close to pH 9. Although it was clear that the behaviour of fish gelatin solutions in the presence of non-solvent is complex especially compared to that of B-type gelatin, certain behaviour may still be interpreted in the same manner. It was observed that gelatin solutions below the iso-electric point (IEP), where gelatin molecules bear a net positive charge, showed a primary and secondary desolvation process whereas gelatin solutions around the IEP showed only one desolvation process. This confirms the hypothesis that the degree and mechanism of SDS binding to gelatin is affected by the pH of the solution. Results for secondary desolvation (pH values from 6 to 9) showed that precipitation behaviour of gelatin as well as the response to the non-solvent were irrespective of the surfactant concentration. Also, no significant difference was observed for secondary desolvation values on variation of the surfactant delivery method. This means that curves for the two different methods, at both low and high concentrations can be fitted to the same curve. This suggests that the method used for precipitation and then for production of nanoparticles from them does is rather robust with respect to surfactant addition.