Adsorption of polyelectrolytes onto oxides — the influence of ionic strength, molar mass, and Ca 2+ ions

Abstract

The paper presents results concerning the adsorbate structure and the adsorption mechanism of polyelectrolytes (polyacrylic acid, polymethacrylic acid and humic acid) onto the oxides gibbsite, goethite and alumina. The investigations deal with the influence of different parameters such as ionic strength, divalent cations and molar mass of the polyacids. The observed adsorption isotherms of every polyacid all show the same high affinity type typical of polymer adsorption. The maximum adsorbed amount is correlated linearly with the proton charge density σ H of the oxides at low ionic strength of 0.01 M NaCl. During the adsorption process the surface charge of the oxides turns from positive to negative. An increase in the ionic strength or the addition of divalent cations causes an increase of the maximum adsorbed amount of polyacid. Due to higher ionic strength or divalent cations the repulsion of the negatively charged polyelectrolyte segments is reduced. Therefore loops and tails in the adsorption layer are stabilized. Additionally, the increase of the maximum adsorbed amount with ionic strength indicates a chemical bond between polyacid and surface. Irrespective of molar mass, polyacrylic acid adsorbs in a flat conformation if the ionic strength is low. Above a certain threshold of the ionic strength higher molar masses are better adsorbed than lower ones. Under these conditions the polyacrylic acid adsorbs in a coiled conformation.