Effects of inorganic cations on the steric force between polyacrylamide layer physically adsorbed on SiO2 wafer and poly (ethylene oxide) layer

Abstract

The physical adsorption of polyacrylamide on the SiO2 wafer in high salty solutions was investigated by an improved AFM steric force measurement. The colloid probe was modified by poly (ethylene oxide) layer and used to determine the steric force between adsorbed polyacrylamide and poly (ethylene oxide) layer in NaCl, KCl, MgCl2 and CaCl2 solutions. Assuming the adsorbed polyacrylamide complies with the self-similar model, the surface excesses, the thicknesses of the proximal regions, and "free energies of sticking" were estimated. Take the case of 5000 mg/L NaCl for example. The concentration profile was deduced from the corresponding steric force profile. The polyacrylamide concentration next to the SiO2 wafer was estimated to be about 147 g/L and the average concentration in the proximal region was estimated to be about 100 g/L. Based on the above estimations, interaction strength between the adsorbed cations and polyacrylamide was established, Mg2+ > Ca2+ > K+ > Na+. Moreover, the conformations of the adsorbed polyacrylamide were changed from "lying flat" to folded ones by the stronger interactions between the adsorbed cations and polyacrylamide.”