Interaction of chitosan and self-assembled distearoylphosphatidic acid molecules at liquid/liquid and air/water interfaces. Effect of temperature

Abstract

The effect of the polyelectrolyte, chitosan (CHI), on distearoylphosphatidic acid (DSPA) films was analyzed by cyclic voltammetry, surface pressure-area isotherm, and Brewster angle microscopy. Distearoylphosphatidic acid films formed at liquid/liquid interface produced a blocking effect to the electrochemical transfer of tetraethylammonium (TEA+) cation from the aqueous to the organic phase, as can be deduced from the cyclic voltammetry experiments. Monolayers of DSPA were performed at different experimental conditions, temperature, subphase solutions (LiCl or CaCl2), and CHI concentration. Both aqueous electrolytes studied exert strong interaction with the negative-charged polar head groups of phospholipids, which enhance the lateral interactions of the hydrocarbon chains, forming a very structured film. Under these conditions the presence of CHI produces a shift of TEA+ transfer potential toward more positive values because of the less availability of negative sites of the phospholipids for TEA+ adsorption, because they are occupied by the positive-charged amine groups in CHI chain. Surface pressure-molecular area isotherms for DSPA monolayers change in the presence of CHI, demonstrating that this polymer produces an expansion of the DSPA film and modifies the surface elasticity of the film. The effect of the temperature was also studied; an exothermic phase transition was observed in the surface pressure-molecular area isotherms for DSPA-CHI monolayers, which involves the rupture of interactions between them. Images of Brewster angle microscopy evidence an increase in the optical thickness of the DSPA films in presence of CHI that indicates that the polymer interacts with DSPA molecules at all molecular areas.