For the first time, the intercalation properties of acid-activated montmorillonites treated at different acid/clay (w/w) ratios with a cationic surfactant cetyltrimethylammonium (C16TMA) hydroxide are reported. The acid activation causes a reduction in the number of cation exchange sides and, hence improves the exfoliation of the silicate sheets at higher pH values. The basal spacing increases significantly from 1.54 to 3.80 nm, and is related to the acid activation extent. The acid activated clays with acid/clay ratios above 0.2 intercalated significant amounts of C16TMA cations with a basal spacing of 3.8 nm compared to the non acid activated montmorillonite with a basal spacing of 2.10 nm. The 13C CP/MAS NMR indicates that the intercalated surfactants exhibit a significant degree of gauche conformation in the acid-activated clays. According to in-situ powder XRD, an increase of the basal spacing to 4.08 nm is observed at intermediate temperatures of 50-150 degrees C for organoclay with basal spacing of 3.80 nm, at higher temperatures above 300 degrees C, the decomposition of the surfactant occurs and the basal spacing decreases to a value of about 1.4 nm, with the persistence of a reflection at 3.8 nm for clay at a higher acid/clay ratio of 0.5.
Single crystal X-ray studies of ( 6)?(tetrahydrofuran) 2 and ( 6)?(benzene) 1.5 reveal the presence of two distinct types of dimeric edge-edge C-H … N packing motifs. All of the racemic dibromides 2-4 and 6 form clathrate compounds where their opposite enantiomers are joined edge-edge by means of C-H … N dimers. Hence this interaction is robust and of considerable value in crystal engineering. Modification of the benchmark centrosymmetric aryl-H … N dimer can result, however, in more complex modifications which better suit a given host-guest combination. Therefore a range of different C-H … N dimers is observed, with the new motif present in ( 6)?(benzene) 1.5 completing a sub-set of these variants. This structural adaptability increases the likelihood of inclusion because competition between many weak intermolecular contacts is a central part of the host design philosophy.
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