Analysis of gelatin chain aggregation in dilute aqueous solutions through viscosity data

Abstract

Viscosity data of gelatin solutions with concentrations between 10 −5 and 10 −3 g/cm 3 are studied in a horizontal capillary viscometer for a wide range of shear rates, covering the dilute and very dilute zones, where either intermolecular aggregation and intramolecular folding are possible, respectively, and the sol–gel transition is not observed. It is found that the dilute solutions behave as Newtonian fluids for the range of shear rates studied. Intrinsic viscosity values revealed that the maturation temperature has a relevant influence on the type of chain aggregations forming thermo-reversible clusters. The use of potassium thiocyanate and urea partially inhibited cluster formations. On the contrary, the aggregation process was not modified with the addition of surfactant sodium dodecylsulphate, indicating that hydrophobic interactions were not the cause of this phenomenon. The evolution of cluster formations in the dilute zone was analyzed through Smoluchowski aggregation theory and the aggregate-size distribution function was evaluated allowing one to conclude that it tends to be Gaussian and high polydisperse as the maturation time increases. Calculations show that disordered aggregates with high amount of occluded solvent are formed at low temperatures, while for higher temperatures, more compact structures are attained. Through the evaluation of the colloidal stability ratio, it is shown that the aggregation process is very slow in the dilute zone with an energetic barrier of around 25 times the Brownian thermal energy, and that the possibility of significant aggregation in the very dilute zone must be excluded.