Intercalation of alkylammonium cations into expandable fluorine mica and its application for the evaluation of heterogeneous charge distribution

Abstract

The interstratified layer nano-hybrids between n-alkylammonium and fluorine mica are prepared by ion-exchanging the interlayer Na+ ions in the synthetic fluorine mica, Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96, with n-alkylammonium cations, CH3(CH2)n − 1NH3+ (Cn; n = 6, 8, 10, 12, 14, 16, and 18). According to the X-ray diffraction profiles of nano-hybrids and their computer simulation results, their interstratified structural features could be clearly distinguished depending upon the number of carbon atoms in the alkyl chain (Cn). The C6–C8 and C14–C16 derivatives exhibit normal intercalation phases with the basal spacings of ∼13.8 A and ∼18.0 A, indicating the parallel mono- and bilayer arrangements of the intercalants between silicate layers, respectively. On the other hand, the C10–C12 and C18 hybrids show distinct superlattice lines in the X-ray diffraction patterns due to the interstratification between parallel monolayer–bilayer (d001 = 31.6 A) and parallel bilayer–pseudotriple layer (d001 = 39.6 A) of alkylammonium molecules, respectively, in the interlayer space of mica. The origin of such interstratification is found to be due to the charge heterogeneity of silicate interlayers in the Na+-fluorine mica; a high layer charge with 0.37 e−/Si4O10 and a low one with 0.28 e−/Si4O10 charge densities, respectively. The charge heterogeneity of silicate layers is also confirmed through the step-wise deintercalation of intercalants during their thermolysis under a nitrogen atmosphere.