Structure and Properties of Alkylammonium Monolayers Self-Assembled on Montmorillonite Platelets

Abstract

Monolayers of mono-, di-, tri-, and tetraalkylammonium cations of varying chain length (C4, C8, and C18) were self-assembled on montmorillonite platelets. The structure and chain dynamics of these SAMs were probed by infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Depending on the cross-sectional area, the available area/cation, and the alkyl chain length, the molecules adopt a two-dimensional order or a disordered state at ambient temperatures. Short alkyl chains lie flat, disordered on the substrate surface as long as there is enough space. With increasing volume of the organic layer between two silicate layers facing each other, the chains force the layers to enlarge their basal-plane spacing but remain disordered. At a certain length and number of chains, the molecules adopt an ordered state due to increasing chain interactions and packing density. To minimize their conformational entropy and maximize their packing density, the chains attached to platelets facing each other interdigitate. The average molecular axis in the organic thin film is inclined to the montmorillonite surface normal by an angle, which depends on the packing environment and the geometry of the molecules. In the ordered state, the alkyl chains preferentially assume an all-trans conformation. With increasing temperature, conformational transformation of the chains takes place, leading to a dynamically disordered phase (liquidlike). Although the translational freedom of the chains is restricted by the electrostatic binding of the headgroups to the substrate, the conformational transformation leads to chains with random conformation and destroys the two-dimensional order. The phase transition manifests itself in an increase in basal-plane spacing as well as in IR absorption frequency and carbon resonance shifts accompanied by an entropy change. The density of the organic ultrathin film confined between two silicate layers seems to decrease on heating across the phase transition, leading to an increase in volume and consequently in the organic layer thickness and in d-spacing, respectively. The basal-plane spacing of 3C18 and 4C18 is appreciably larger than that of C18 and 2C18, which is advantageous for exfoliation in the synthesis of polymer nanocomposites.