Intercalation and Exfoliation of Kaolinite with Sodium Dodecyl Sulfate
Kaolinite (Kaol) was intercalated with dimethyl sulfoxide (DMSO) and subsequently methanol (MeOH) to prepare intercalation compounds Kaol-DMSO and Kaol-MeOH. Kaol-MeOH was used as an intermediate to synthesize Kaol-sodium dodecyl sulfate (SDS) intercalation compound (Kaol-SDS) via displacement reaction. The ultrasonic exfoliation of Kaol-SDS produced a resultant Kaol-SDS-U. The samples were characterized by X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), thermal
... (FTIR), thermal analysis, scanning electronic microscopy (SEM), transmission electron microscopy (TEM) and particle size analysis. The results revealed that the intercalation of sodium dodecyl sulfate into kaolinite layers caused an obvious increase of the basal spacing from 0.72-4.21 nm. The dehydroxylation temperature of Kaol-SDS was obviously lower than that of original kaolinite. During the intercalation process of sodium dodecyl sulfate, a few kaolinite layers were exfoliated and curled up from the edges of the kaolinite sheets. After sonication treatment, the kaolinite layers were further transformed into nanoscrolls, and the exfoliated resultant Kaol-SDS-U possessed a smaller particle size close to nanoscale. species can be intercalated directly into kaolinite, such as urea [24, 25] , potassium acetate     and dimethyl sulfoxide [30, 31] . Moreover, kaolinite would not be exfoliated even though these species were intercalated into kaolinite layers due to the strong inner hydrogen bonds between the layers. Actually, natural minerals have been widely used for the preparation of advanced materials                . Much work has been done to demonstrate that various intercalated guest species can be extended by the "displacement method". Especially the methoxy-modified kaolinite shows more versatility for further intercalation reactions. For example, Komori et al. reported that kaolinite-alkylamines intercalation compounds were prepared by using the methanol-treated kaolinite intercalation as an intermediate [47, 48] . The results showed that the basal spacing increased up to 5.75 nm when octadecylamine was used. Matusik et al. prepared kaolinite intercalation compounds with benzylalkylammonium chlorides [49, 50] . They have reported that a methoxy-modified kaolinite was used as a precursor, which had OCH 3 methoxyl groups attached to the octahedral sheet. Additionally, Gardolinski and Lagaly [51, 52] reported that n-hexylamine, n-octadecylamine and n-docosanamine were intercalated into the layers of kaolinite by the same method reported by Komori and co-workers [47, 48] . When n-docosanamine was used, the largest basal spacing of 6.42 nm was obtained, and the platy layers transformed into nanoscrolls after the deintercalation performed with toluene. Kuroda et al. also reported that some quaternary ammonium salts intercalated into the interlayers of kaolinite using methoxy-modified kaolinite as an intermediate, and most of the exfoliated kaolinite layers also transformed into nanoscrolls  . Liu et al. demonstrated kaolinite layers got curled up into one-dimensional nanoscrolls automatically when they are exfoliated in methanol after intercalation with alkyltrimethylammonium salts  . This depicted the morphological change of kaolinite layers during intercalation and exfoliation. The guest species mentioned above were defined as cationic intercalators, because they could be ionized to produce cations. In the case of intercalation, after kaolinite stacking layers reacted with cationic long chain intercalators, the layer-to-layer distance expanded, and the binding forces of adjacent layers decreased significantly. The kaolinite layers were lastly exfoliated and curled up into nanoscrolls. Therefore, it is not a good way to obtain kaolinite with a high aspect ratio. Based on the literature described above, little attention has been paid to the intercalation of kaolinite with anionic guest species. Sidheswaran et al.  firstly examined the preparation of intercalation compounds of kaolinite by a one-step displacement method with the salts of long-chain fatty acids. The results indicated that the expanded basal spacing of kaolinite was independent of the chain length of guest species, because the basal spacing of kaolinite expanded from 0.7-1.1 nm. Wang et al.  recently reported that the interlayer spacing of kaolinite was enlarged to 4.55~4.79 nm intercalated by sodium stearate using the methoxy-modified kaolinite as the precursor. In this work, as an important anionic surfactant, SDS was intercalated into kaolinite layers using Kaol-MeOH as the initial material first. Afterwards, the Kaol-SDS was treated by the sonication method. It was found that the morphology of most exfoliated kaolinite layers remained intact. The exfoliation method may provide the possibility of preparing thin kaolinite flakes with large aspect ratios.