Surface Structure of Organoclays

Abstract

Mica with amphiphilic cations is of extensive technological interest. The mechanical stability of various plastics and rubbers, as well as barrier and optical properties in thin films, can be substantially improved by the addition of organically modified mica.[1] Besides, mica surfaces have proved to be an excellent model system for surface phenomena because the mineral can be cleaved into regular surfaces that extend over mm. The inorganic-organic interfaces have been the subject of numerous experimental studies [2][3][4][5][6][7][8] and theoretical investigations of related systems were performed at coarse-grained [9,10] and atomistic levels. [11][12][13][14] Accurate molecular dynamics simulation at the atomistic level supports the interpretation of experiments, for example, XRD and solid-state NMR data. [7,15] With our recent simulations, [13,14] the inclination angles of the alkyl chains and basal-plane spacings of the filler particles are reproduced in quantitative agreement with experiments, and predictions of the interface structure as well as conformational analyses of the hydrocarbon chains are possible. Herein, we report monolayer phases on mica platelets with alkali ions and surfactant ions of different length, at different temperatures, and we give insight into the occurring phase transitions. [6,7,13,14] We consider that in dry mica surfaces in which 80 % of the alkali ions, most commonly lithium in the reported experimental studies, are exchanged by organic ammonium ions of different length (Scheme 1) so that 20 % of the alkali ions remain. This is a technologically realistic situation in view of the relatively difficult quantitative replacement of all interlayer cations (without intercalation). We investigate two borderline cases. The first simulated structure is a homogeneous mixture of surfactant ions and alkali ions on the mica surface. The second simulated structure refers to "phaseseparation", that is, segregation of cations at the surface. In this case, we model the surfactant islands on mica in which the alkali exchange is quantitative. In both cases, we employ a mica model that contains the upper half of 5 3 1 unit cells with realistic atomic charges. [13,16] For the homogeneous phase, twelve dialkyldimethylammonium ions (Scheme 1) and three potassium ions are attached, thus accounting for 80 % cation exchange. For the simulation of "islands", 15

Molecular Dynamics Simulations of Glycerol Monooleate Confined between Mica Surfaces

“…The basic setup is illustrated in Figure . The single-layer mica unit-cell structure (5.1918 Å × 9.0153 Å × 10.0228 Å) with stoichiometric aluminum defects was taken from the INTERFACE-PCFF force field toolkit. − The unit cell was tessellated by 10 × 6 × 1 to form a 52 Å × 54 Å × 10 Å surface in the xy plane. Each mica surface was composed of 2520 atoms.…”

“…Each mica surface was composed of 2520 atoms. All interatomic interactions were calculated using the INTERFACE-PCFF force field; − see the Supporting Information for details. The surface was equilibrated under NPT conditions for 10 6 time steps of 1 fs to remove any structural defects that would otherwise cause the surface to warp.…”

Self-assembly and friction of glycerol monooleate and its hydrolysis products in bulk and confined non-aqueous solvents

“…13 Mica was described using the INTERFACE-PCFF toolkit from the Heinz group. [49][50][51][52] For internal consistency, the INTERFACE-PCFF force field was used for both the mica surfaces and the confined fluid. As shown explicitly in ref.…”

Self-assembly and adsorption of cetyltrimethylammonium bromide and didodecyldimethylammonium bromide surfactants at the mica–water interface

“…The compositions of these systems are summarised in Table 1. The mica structure was taken from the Interface Force Field toolkit, [47][48][49][50][51] with the unit-cell dimensions being 5.1918 Å Â 9.0153 Å Â 10.0228 Å. A 10 Â 6 Â 1 surface slab was formed from the unit cell.…”