Effect of electrolyte concentration on the dynamic surface tension and dilational viscoelasticity of adsorption layers of chitosan and dodecyl chitosan.

The effect of an external salt (AcONa) on the kinetics of adsorption and structure formation inside the adsorption layers (ALs) of chitosan (Ch) and dodecyl chitosan (C12Ch) as well as on the frequency dependence of the complex dilational elasticity modulus of these layers has been studied. The complex dilational elasticity modulus of adsorption layers of polymers has been measured on the drop tensiometer (Tracker, IT Concept, France) upon applying a small sinusoidal variation of the drop area with a given frequency, omega, in the range from 10(-2) to 0.63 rad/s and recording the variation of the surface pressure. It has been found that, in the absence of the salt, the dilational storage modulus, E'(omega), of ALs of both Ch and C12Ch is lower with regard to the loss modulus, E' '(omega), in the whole range of frequencies used, testifying for the liquidlike rheological behavior of these layers. With an increase of the salt concentration up to CAcONa > 0.1 M, the ALs become solidlike, as shown when E'(omega) > E' '(omega). Consequently, the characteristic frequency, omega c, corresponding to the intercept between the E'(omega) and E' '(omega) curves, gradually varies from omega c > 1 rad/s to omega c < 0.01 rad/s when the salt concentration is increased from zero to CAcONa = 1 M. Hydrophobically modified C12Ch, having long grafted alkyl chains, exhibited a higher sensitivity to the presence of salt than Ch: the former solidifies more readily and at lower salt concentrations than the latter. It has been found that the experimental E'(omega) and E' '(omega) curves exhibit two characteristic relaxation frequencies, omega 01 approximately 1 rad/s and omega 02 approximately 10(-3)-10(-2) rad/s, whose physical meaning and values were related to the structure of the ALs and to the competitive contribution of electrostatic and hydrophobic interactions between amino and nonpolar groups of Ch and C12Ch to the formation of a gel-like network inside the polymeric film at the interface.