Interactions between mica surfaces in sodium polyacrylate solutions containing calcium ions

• The forces acting between negatively charged muscovite mica surfaces immersed in solutions containing sodium polyacrylate (NaPAA) have been studied. No evidence for PAA adsorption in the absence of calcium ions in the solution was found. However, at a CaCl2 concentration of about 3x10-3 M a layer of PAA adsorbed on each surface. At large separations, the forces between the PAA-coated surfaces were dominated by repulsive double-layer forces. At separations below 50-80 Å, depending on the solution conditions, an attractive force in excess of the van der Waals attraction was observed. The adhesion force between the layers was 7-8 mN/m at pH 6 and increased somewhat with increasing pH to about 9 mN/m at pH 10. We argue that both the long-range attraction and the adhesion force primarily are due to COO- - Ca2+ --OOC bridges between the opposing surfaces and possibly charge correlation effects. It was found that calcium ions bind to the PAA and to the mica surface, whereby the electrostatic repulsion between the polyelectrolyte and the surface is lowered. We suggest that this is the main re ason why Calcium ions promote the adsorption of PAA onto negatively charged minerals. However, it also seems likely that calcium ions increase the attractive interactions between the surface and the PAA segments.

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