Kinetics of Poly-L-lysine Adsorption on Mica and Stability of Formed Monolayers: Theoretical and Experimental Studies.

Abstract

Various physicochemical parameters of poly-L-lysine (PLL) solutions comprising the diffusion coefficient, the electrophoretic mobility, the density and the intrinsic viscosity were determined at pH range 3.0 - 9.2. This allowed to calculate derivative parameters characterizing the PLL molecule such as its zeta potential, the number of electrokinetic charges, the ionization degree, the contour length and the cross-section area. These data were exploited in theoretical calculations of PLL adsorption kinetics on solid substrates under diffusion transport. A hybrid approach was used comprising a blocking function derived from the random sequential adsorption (RSA) model. In experiments, the PLL adsorption on mica was studied using the streaming potential measurements and interpreted in terms of a general electrokinetic model. This confirmed a side-on adsorption mechanism of the macroion molecules at the examined pH range. Additionally, using this method, the stability of PLL monolayers was determined performing in situ desorption kinetic experiments. In this way, the equilibrium adsorption constant and the energy minimum depth were determined. It was confirmed that the monolayer stability decreases with pH following the decrease in the number of electrokinetic charges per molecule. This confirmed electrostatic interaction driven adsorption mechanism of PLL. It is also predicted that at pH 5.7 - 7.4 the monolayers were stable under diffusion-controlled desorption over the time exceeding 100 hours. Beside significance for basic science, the results obtained in this work can be exploited for developing procedures of preparing stable PLL monolayers of well controlled coverage and electrokinetic properties.