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DIFFERENCES IN QUASICRYSTALS OF SMECTITE-CATIONIC SURFACTANT COMPLEXES DUE TO HEAD GROUP STRUCTURE

Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology 807-1, Shuku-machi, Tosu, Saga 841-0052, Japan

^* E-mail address of corresponding author: y-imai{at}aist.go.jp

The organoclay quasicrystals formed by exchange reactions of fluoromagnesian smectite (FMS) and two kinds of cationic surfactants were investigated by X-ray diffraction (XRD) of both aqueous suspensions and dried powders. potential measurements were also carried out. The two cationic surfactants employed in this study, dodecyltriphenylphosphonium bromide (C₁₂PBr) and dodecyltrimethylammonium bromide (C₁₂NBr), have the same alkyl chain length but different head group structures. In aqueous suspensions of C₁₂N/FMS, regular stacking with a basal spacing of 2.28 nm was observed within the range 0.4 to 1.0 of the cation exchange capacity (CEC) of the C₁₂N/FMS ratios. The basal spacing was constant in this range. The most intense diffraction for the C₁₂N/FMS suspension was observed at 0.6 CEC, and a further increase in the C₁₂N/FMS ratio resulted in a decline of the diffraction intensity and a slight increase in the peak width. On the other hand, in C₁₂P/FMS suspensions with the surfactant/FMS ratios <0.6 CEC, only weak, broad XRD peaks were observed. With higher C₁₂P/FMS ratios (0.8 and 1.0 CEC), strong diffraction peaks appeared with a basal spacing of 3.20 nm. The differencein the observed basal spacings of C₁₂N/FMS and C₁₂P/FMS tactoids was ~0.9 nm, which did not agree with the difference in the estimated height of the two surfactants (~0.1 nm). These differences of the tactoid structures between C₁₂N/FMS and C₁₂P/FMS suspensions could be explained in terms of the difference in the adsorption manner of the surfactants on the silicate surface and in the size of the surfactant head groups.

Key Words: Adsorption • Dodecyltrimethylammonium Bromide • Dodecyltriphenylphosphonium Bromide • Fluoromagnesian Smectite • Hemimicelle • Intercalation Compounds • Stacking • Tactoids • X-ray Diffraction • Zeta Potential