Influence of concentration and ionic strength on the adsorption kinetics of gelatin at the air/water interface

The adsorption kinetics of gelatin to the air/water interface is still not fully understood. We investigated two samples of gelatin having different gel strength (65 Bloom and 280 Bloom), named, respectively gelatin 65B and 280B. The gelatin samples in solution were characterised by viscosimetric methods. We evidence a chain expansion of gelatin upon dilution due to the polyelectrolyte effect. The measurement of the intrinsic viscosity in water gave values of 22 mL/g for the sample 65B and 70 mL/g for the sample 280B. Although gelatin is a highly polymolecular polymer containing branched chains, the normalisation of the concentration by the intrinsic viscosity allowed the construction of a master curve of viscosity, giving the reduced critical overlap concentrations c* ∼ 0.6 and c** ∼ 5. All concentrations of the gelatin solutions used for the determination of the adsorption behaviour were within the range of the dilute regime. The analysis of the adsorption behaviour leads us to the hypothesis that there are two different phenomena: first, a step governed by diffusion to the interface which lasts until full coverage of the interfacial film. The length of this induction period seemed to be dependent on the gelatin chain length, with larger chains taking more time. Second, the adsorption of the chains to the air/water interface leads to the formation of an encumbered subsurface layer which opposes steric hindrance to newly arriving chains. This causes a continuous slow down of the rate of surface pressure increase, which was linear on the logarithmic time scale and showed a slope of −1. The adsorption kinetics was independent of the salt concentration of the solvent and of the gelatin chain size measured by the intrinsic viscosity. The gelatin sample 65B shows furthermore higher equilibrium surface pressures at high bulk concentrations (>0.01 wt%) than sample 280B, while the contrary was observed at small concentrations.