EFFECT OF ELECTROLYTES ON ADSORPTION OF CATIONIC POLYACRYLAMIDE ON SILICA - ELLIPSOMETRIC STUDY AND THEORETICAL MODELING

Adsorption of cationic low-charge polyacrylamide on silica from LiCl, KCl, and CsCl solutions was studied by in situ null ellipsometry. The experimentally measured adsorbed amount (Gamma), being essentially constant at low electrolyte concentrations, decreases at high salt contents. The onset of Gamma decline depends on the type of counterion. Ellipsometric thickness (d(f)) increases gradually with increase of electrolyte concentration (c(salt)) and levels off at high c(salt). The experimental results are compared with model calculations within the framework of a self-consistent-field theory of polyelectrolyte adsorption. A constant surface potential condition and a random distribution of charge on the polymer were used throughout the calculations. A proper choice of realistic surface potential and Flory-Huggins chi parameters for polymer-surface and counterion-surface nonelectrostatic interaction provides accurate description of the experimentally observed Gamma vs c(salt) and d(f) vs c(salt) dependencies. The difference between counterions is reflected by assuming their attractive interaction with the surface to become stronger in the sequence Li+ < K+ < Cs+. Structural characteristics of the polyelectrolyte adsorbed layers (size of, and fraction of segments in, trains, loops, and tails) obtained from the model calculations revealed that the molecular mechanisms behind the change of ellipsometric thickness with electrolyte concentration are the formation of larger loops and longer tails due to an increase of the ion-segment competition. For shorter polymer chains the theory predicts a qualitatively different behavior: insensitivity of ellipsometric thickness to the change in electrolyte concentration.