Adsorption of polyelectrolytes onto oxides - the influence of ionic strength, molar mass, and Ca2+ ions

The paper presents results concerning the adsorbate structure and the adsor ption mechanism of polyelectrolytes (polyacrylic acid, polymethacrylic acid and humic acid) onto the oxides gibbsite, goethite and alumina. The invest igations deal with the influence of different parameters such as ionic stre ngth, divalent cations and molar mass of the polyacids. The observed adsorp tion isotherms of every polyacid all show the same high affinity type typic al of polymer adsorption. The maximum adsorbed amount is correlated linearl y with the proton charge density sigma(H) of the oxides at low ionic streng th 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 o r the addition of divalent cations causes an increase of the maximum adsorb ed amount of polyacid. Due to higher ionic strength or divalent cations the repulsion of the negatively charged polyelectrolyte segments is reduced. T herefore loops and tails in the adsorption layer are stabilized. Additional ly, the increase of the maximum adsorbed amount with ionic strength indicat es 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 ar e better adsorbed than lower ones. Under these conditions the polyacrylic a cid adsorbs in a coiled conformation. (C) 2000 Elsevier Science B.V. All ri ghts reserved.