Adsorption of dodecyl- and octadecyltrimethylammonium ions on a smectite and synthetic micas

Adsorption of n-alkyltrimethylammonium ions with alkyl chain lengths of C12 and C18 on the natural Kunipia F smectite (KN) as well as on synthetic fluorotetrasilicic mica (TSM) and fluorotaeniolite (TN) whose CECs are 119, 170 and 215 meq/100 g, respectively, was studied. The C and N contents were determined and thermogravimetric and XRD analyses were carried out. When concentration of alkylammonium cations changed between 0.25 and 1.0 CEC, the maximum amount of cations adsorbed on the KN smectite was close to its CEC. In contrast, only part of inorganic (sodium or lithium) ions in the interlayer spaces of the TSM and TN fluoromicas were replaced by the organic ions, even if an excess of the surfactants was used. However, at a high concentration of the C18 salt, the salt molecules were additionally adsorbed on the silicates, resisting washing with water but being removable with ethanol. The XRD patterns of the organo-smectites showed that the differences in the charge densities of the KN layers are low. In contrast, TSM and TN are built of layers with various layer charges. Thus, the alkylammonium-exchanged micas represent polyphase systems consisting of regularly and randomly interstratified layers differing in the amount of alkylammonium cations adsorbed. A part of the interlayers presumably contain both organic and inorganic ions with spacings controlled by the arrangements of the organic ions.