Dynamic surface tension and dilational viscoelasticity of adsorption layers of a hydrophobically modified chitosan

The kinetics of the adsorption at the air-water interface and the processes of the structure formation inside the adsorption layers of the alkylated chitosan (the cationic polysoap) have been studied in function of its bulk concentration and the ageing time of the adsorption layers. It has been shown that the dynamic surface tension y(t) of this polysoap aqueous solution measured by the axisymmetric rising bubble shape analysis is characterized by several stages with different characteristic relaxation times. During induction and post-induction times the adsorption is the diffusion-controlled process of macromolecules from the bulk of the solution to the interface. At the final stage the adsorption is characterized by much lower rate due to the steric hindrance exerted by the yet formed adsorption layer on newly adsorbing macromolecules. The structure (physical gel) formation within the adsorption layers of the alkylated chitosan via intermolecular hydrophobic interaction or by hydrogen bonding in function of the ageing time (up to t(f) similar to 5 x 10(4) s) has been studied with the help of the two-dimensional dilational rheology by applying sinusoidal deformation to the bubble area within the frequency range of w = 0.05-0.4 rad/s. One characteristic relaxation frequency rheological model of Maxwell has been applied to determine two-dimensional "storage" E' and "loss" E" elasticity moduli, as well as the intrinsic elasticity module E-0 and the characteristic relaxation frequency w(0) (or the intrinsic viscosity eta(0) =E(0)w(0)/2 pi) of the adsorption layer in function of the bulk polymer concentration C-p. It has been shown that the parameters E-0 and eta(0) sharply decrease with increasing C-p. This finding is interpreted on the basis of proposed mechanism explaining the effect of the conformational state of the macro-ions inside the adsorption layers on the Gibbs elasticity and the characteristic relaxation time. (c) 2004 Elsevier B.V. All rights reserved.