Simple model for DNA adsorption onto a mica surface in 1:1 and 2:1 electrolyte solutions

We propose a simple theory of interactions between like-charged polyelectrolyte and a surface based on a mean-field Derjaguin-Landau-Verwey-Overbeek approach. It predicts that the van der Waals attractive interactions are responsible for irreversible physisorption of polyelectrolytes onto charged surfaces. We show that monovalent salts contribute significantly to repulsive interactions, while enhancing the attraction very slightly. The effect of the divalent counterions is reverse. Therefore, to achieve the adsorption, the overall repulsion due to 1:1 electrolyte should be counterbalanced by the stronger van der Waals attraction due to the presence of doubly charged counterions in solution. The theory has been validated experimentally against its ability to predict the minimum polymer/surface interaction energy required for the adsorption using DNA/mica in NaCl, MgCl2, and NiCl2 solutions as a test system. The theory explains the mechanism of linear DNA adsorption to a mica surface for different solvent compositions and can be used as a tool for predicting the optimum conditions for AFM experiments on linear polymer systems. The model can also be used to make general conclusions on the conformation of polymer molecules on a surface. We have shown for the DNA/mica surface system that when the adsorption of DNA is mostly governed by long-range van der Waals forces the molecule adopts an ideal 2D conformation. When the adsorption is mostly due to short-range ion-correlation forces, DNA will appear 3D --> 2D projected in agreement with experimental data.