Adsorption properties of crystalline silicas: (II) Adsorption of anionic surfactants and delamination

Lamellar crystalline silicas (crystalline silicic acids, chemical composition SiO₂·xH₂O; examples: H₄Si₁₄O₃₀·xH₂O, H₄Si₂₀O₄₂·xH₂O) are distinguished from the amorphous forms by their layered structure and exceptional adsorption properties. One outstanding example is the reaction with anionic surfactants. Several types of crystalline silicas (typical H₄Si₂₀O₄₂·xH₂O) can intercalate ionic pairs consisting of surfactant anion and gegen ion into the interlayer space. The saturation value of SDS adsorption is 0.475 mmol SDS/g H₄Si₂₀O₄₂·3H₂O. The acid H₄Si₁₄O₃₀·xH₂O adsorbs anionic surfactants at the external surfaces only (saturation value 0.04 mmol/g H₄Si₁₄O₃₀·0.8 H₂O). When anionic surfactants are adsorbed in the interlayer space, the layer separation increases to such an extent that the crystals disarticulate in a fan-like manner or delaminate into thinner packets of layers or smaller aggregates. Washing-out the SDS ionic pairs or drying reconstitutes the parallel layer orientation and leads to re-aggregation of the packets and fragments.