Clay adsorbed dyes: methylene blue on Laponite

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Abstract
The adsorption of methylene blue (MB) from aqueous solution on Laponite (a synthetic 2:1 layer-silicate) exchanged with Na+, Cs+ and tetraethylammonium ions has been studied. At low loadings the monomer form predominates, but as the loading increases dimers and trimers (or higher aggregates) are formed, the latter being the most important species when the loading is equal to, or larger than, the Na+-CEC. The distribution of the molecules over the surface is time-dependent due to changes in the aggregation of the clay particles. On Cs+-Laponite, and to a lesser extent on Na+-Laponite, a small amount of MB is protonated by cation-coordinated water molecules. In similar work where adsorption was from ethanolic solution, MB is adsorbed as monomers and dimers only at small loadings and precipitates on the surface of the clay aggregates at high loadings. These precipitates can be broken up by sonication.