Free Energy of Exfoliation Between Layered Silicate Sheets
Exfoliation of polymer-layered-silicate nanocomposites can thermodynamically be considered as a two-step process in which isolated clay sheets exfoliate and polymer solvates the newly formed surfaces. The first step, exfoliation of clay sheets, is difficult to study experimentally so that we resort to molecular dynamics simulation. We employ an atomistic force field for phyllosilicates, validated with respect to surface energies, to calculate the free energy of interaction between montmorillonite and mica sheets as a function of distance, both for unmodified clays with alkaline cations and ion-exchanged clays with octadecylammonium ions (NH3+–C18H37). The free cleavage energies are 375 mJ/m2 for unmodified mica, 137 mJ/m2 for unmodified montmorillonite, 32 mJ/m2 for C18-mica, and 41 mJ/m2 for C18-montmorillonite. Electrostatic attraction is the largest contribution in the unmodified minerals, but diminishes rapidly within a distance of 0.5 nm and turns into a known small repulsion (1–5 mJ/m2) over the next few nm . Attractive van-der-Waals interactions are noticeable over a range of roughly 1 nm. Presence of an organic layer of >1 nm thickness between the clay sheets reduces electrostatic interactions to less than 1 mJ/m2. Octadecylammonium modified clays are thus held together almost exclusively through dispersive van-der-Waals forces. When the separation of unmodified or modified clay sheets exceeds 1 nm relative to their equilibrium distance, less than 2 mJ/m2 interaction energy per clay surface remain toward infinite separation and, therefore, long-range interactions between clay sheets are not significant in the exfoliation process.