Influence of the polycation architecture on the oscillatoric forces of aqueous free-standing polyelectrolyte/surfactant films

Abstract

In the present study, freestanding films were formed from aqueous solutions of surfactant and polyelectrolyte and they were investigated by a thin film pressure balance. The surfactant was either a nonionic alkylpolyglucoside or cationic hexadecyltrimethylammonium bromide. Linear poly(diallyl dimethylammonium chloride) (PDAD-MAC) and poly(ethylene imine) (PEI) of different degrees of branching were investigated. The resulting disjoining pressure isotherms show steps in film thickness owing to oscillatoric structural forces. With respect to the polyelectrolyte architecture the step size, Δh, shows different scaling behavior with the polyelectrolyte concentration, c. While Δh in the thickness of the films containing linear polyelectrolytes scales as c ^-1/2, for the branched polyelectrolytes Δh scales as c ^-1/3. The effect of electrostatic interactions on the film stratification was studied by changing the ionic strength or the polymer charge density. For this purpose the ratio between cationic and neutral monomers was varied in the case of the strong PDADMAC or in the case of weak PEI the pH was changed. Both increasing the ionic strength and decreasing the polymer charge density leads to a reduction in the oscillation amplitude of the disjoining pressure. At high ionic strengths or for neutral polymers the steps in the film thickness vanish.