The structure of trimethylsulfoxonium-exchanged vermiculite has been examined to compare it with other onium-exchanged structures, such as tetramethylammonium- and tetramethylphosphonium-exchanged vermiculite. The three organic cations are tetrahedral in shape, but trimethylsulfoxonium [(CH3)3SO+] has an oxygen atom replacing a methyl group at one apex. This study describes the effect this substitution and the larger S atom have on the site location in the interlayer and the effect on the vermiculite 2:1 layer. These clay minerals may be commercially useful as adsorbents.
Na-exchanged crystals of vermiculite from Santa Olalla, Spain, were intercalated with trimethylsulfoxonium [Me3SO+ = (CH3)3SO+] molecules by refluxing in an aqueous 0.25 M trimethylsulfoxonium iodide solution at 80ºC for 14 days. The resulting Me3SO+-exchanged vermiculite crystals were studied by single-crystal, X-ray diffraction methods and by computer modeling (density functional theory). Cell parameters are a = 5.349(2), b = 9.270(3), c = 13.825(8) Å, and β = 97.40(4)º, the space group is C2/m, and the polytype is 1M. Refinement results (R = 0.073, wR = 0.080) show that in the average structure of C2/m, the S atoms of the Me3SO+ molecules form two partially occupied planes [2.066(2) Å from each basal oxygen plane] between the 2:1 layers, and the S atoms show considerable positional disorder. The O atom of the Me3SO+ molecule occurs in the central plane of the interlayer, as far away from each 2:1 layer as possible. In projection down the c* axis, the Me3SO+ molecule resides within the center of the silicate rings from each adjacent 2:1 layer. In the ideal (static) model of the Me3SO+-exchanged vermiculite structure, the Me3SO+ molecule is oriented such that two methyl groups point toward charge-deficient bridging oxygen atoms of the basal plane; thus, the organic pillars charge compensate the bridging oxygen atoms of the 2:1 layer that are charge deficient. In projection, the oxygen atom of the Me3SO+ molecule projects over a tetrahedron containing Si. Computer modeling showed that if H2O is not included in the model, the Me3SO+ molecule (S and O atoms) is in the center of the interlayer, but with the addition of randomly placed H2O, two partially occupied planes similar to the X-ray derived model are formed.