Interfacial properties of chitosan lactate at the liquid/air interface

Abstract

The interfacial properties (dynamic and equilibrium surface tension, viscosity and elasticity moduli) of chitosan lactate have been studied at the liquid/air interface by the oscillating drop shape method. Isotherms of dynamic surface tension of chitosan lactate are similar to dependences for other polyelectrolyte solutions, in particular for proteins. Chitosan is a weak cationic polyelectrolyte which can change its conformation from a linear rod to a chaotic and compacted coil. Therefore, the experimental dependence of the equilibrium surface tension on concentration of chitosan lactate was analyzed with the adsorption model proposed earlier for proteins. This model accounts the possibility of polyelectrolyte molecules existence in surface layer in n states with different molar surface varying from the maximum value at very low surface coverage by polyelectrolyte molecules to a minimum value at high surface coverage. Good agreement between the calculated and experimental values of surface tension was observed. The dependences of the elasticity and viscosity moduli of chitosan lactate solutions on the drop oscillations frequency are conditioned by the influence of exchange processes both between the surface layer and the bulk solution and in the surface layer itself. An increase of the solution concentration intensifies the exchange processes, and an increase of the oscillation frequency suppresses them. It is shown that the dependence of the surface viscoelasticity modulus of chitosan lactate is extreme in nature with a pronounced maximum. The reason for such behavior is the possibility of changing the molar surface area of the polyelectrolyte at the interface dependent on the amount of adsorption and its structural properties. Attempt of theoretical description of the viscoelasticity modulus within the framework of model accounting mono- or bilayer adsorption did not lead to a satisfactory result, possibly due to barrier adsorption mechanism of chitosan. But bilayer model provide qualitative description of extreme behavior of surface viscoelasticity on concentration. The values of the surface viscoelasticity modulus of chitosan lactate occupy an intermediate position in comparison with the data available in the literature for globular and flexible-chain proteins, that is consistent with their molecular structure. In addition, the work shows the applicability of the adsorption model, developed earlier for proteins in the framework of a nonideal two-dimensional solution theory, for describing the surface properties of other polyelectrolytes. This makes it possible to obtain qualitative and quantitative information about the processes occurring in the systems under study.