Cleavage Energy of Alkylammonium-Modified Montmorillonite and Relation to Exfoliation in Nanocomposites: Influence of Cation Density, Head Group Structure, and Chain Length

Cohesion between layers of organically modified aluminosilicates creates a barrier to exfoliation in polymer matrices and can be thermodynamically described by the cleavage energy, While the measurement of cleavage energies at the nanometer scale is experimentally challenging, molecular simulation indicates a tunable range between 25 and 210 mJ/m(2) upon variation of the cation exchange capacity (CECs of 91 meq/100 g and 143 meq/100 g), headgroup chemistry (primary and quaternary ammonium), kind chain length (C-2 to C-22) in a series of 44 alkylammonium-modified montmorillonites. Designed organoclays of low cleavage energy can support exfoliation when the surface tension, which relates to reconstructed surfaces after cleavage, is comparatively higher. The difference between high and low cleavage energies is associated with Coulomb interactions depending on the position of the positively charged surfactant head groups in the interlayer with respect to the two clay layers at equilibrium separation and further with the interlayer density depending on volume packing of the surfactants in the solvent-free minerals. Surfactants with NH3 head groups exhibit low cleavage energies between 25 and 55 mJ/m(2). Surfactants with NMe3 head groups exhibit cleavage energies up to 210 mJ/m(2) for short chain length and approach a range between 30 and 65 mJ/m(2) for longer alkyl chains. The cleavage energy as a function of chain length indicates local minima for interlayer structures comprised of loosely packed, flat-on layers of alkyl chains and local maxima for interlayer structures comprised of densely packed, flat-on layers of alkyl chains, convergent to 45 +/- 5 mJ/m(2) for longer chains (> C-20). We employed molecular dynamics simulation in full atomistic detail using an accurate, extensively validated force field [Heinz et al. Chem. Mater. 2005, 17, 5658] and advanced sampling techniques to obtain equilibrium surfactant conformations.