A small-angle X-ray scattering study of the structure of aqueous laponite dispersions

Two Laponite clays, of general formula [Mn+](x/n) [Li-x Mg6-x Si-8 O-20 F-y (OH)(4-y)], were prepared with different Li+ levels (0.57 and 0.70% w/w) a nd F- levels (9.5 and 9.9% w/w). The clays were characterized by measuremen ts of nitrogen adsorption isotherms, cation exchange capacity, and conducto metric titration data. The clay with higher Lif and F- levels had a higher (more negative) face charge and a lower (less negative) edge charge. Small- angle X-ray scattering (SAXS) data obtained from dilute aqueous dispersions indicate that both clays are of similar particle size (diameter ca. 40 nm and thickness ca. 3.5 nm). Experimental structure factors for concentrated Laponite dispersions were calculated using the ratio of scattered X-ray int ensities from concentrated and dilute dispersions. Maxima in the experiment al structure factors were observed at scattering angles of 0.14-0.2 nm(-1) under favorable solution conditions (pH greater than or equal to 9 and ioni c strength ca. 5 x 10(-3) mol dm(-3)) for one of the Laponite clays. These results are interpreted in terms of an ordered,aligned structure where the particles' face and edge electrical double layers are expanded and negative in sign. This is in contrast to a "house-of-cards" structure which results when particle aggregation is promoted by lower pH, higher ionic strength, or a lower particle edge charge.