Impact of the solvent properties on molecular interactions and phase behaviour of alginate-gelatin systems

Abstract

Alginate-gelatin systems are used in a wide range of applications, e.g., foods, cosmetics, drugs, and medicine. To target the techno-functionalities of alginate-gelatin systems such as texture, stability, or appearance it is necessary to understand the structure formation of gel systems, which is influenced by the molecular interactions and the phase behaviour. Therefore, the aim of this study was to characterise the molecular interactions and phase behaviour of dilute alginate-gelatin systems. For this purpose, we analysed alginate, gelatin A and their mixtures at pH 7. First, we characterised the effect of the sodium chloride concentration on the single polymer solutions by measuring the critical overlap concentration, the zeta potential, intrinsic viscosity, the Huggins coefficient as well as the molecular weight and the second virial coefficient. To gain a better understanding of the molecular interactions between alginate and gelatin, we determined the Huggins coefficient and the interaction parameter by Wolf as a function of mixing ratio, NaCl and urea concentration. Additionally, we calculated the second cross-virial coefficient. Lastly, we characterised the influence of the interactions on the secondary structure of gelatin by Fourier-transform-infrared-spectroscopy. The attractive interactions between alginate and gelatin potentially enabling the formation of soluble complexes, were demonstrated as deviations from additivity of intrinsic viscosity and negative cross-virial coefficients. Interestingly, the Wolf interaction parameter was affected by the NaCl concentration and the mixing ratio, whereas the presence of urea showed no impact. This indicates that the attractive interactions of alginate and gelatin A at pH 7 is rather caused by electrostatic interactions than hydrogen bonds. The present study clarifies the relevance of hydrogen bonds and electrostatic interactions as well as the impact of mixing ratio and solvent properties on the phase behaviour of alginate-gelatin systems. The insights contribute to a better understanding of the structure formation of concentrated alginate-gelatin systems, levelling the path to specific modifications of functionality.