Modification of surface energies of layered silicates through surfactants of different chain length
The cleavage energy between two organically modified layered silicate sheets is a significant factor for the exfoliation of layered silicates in polymer matrices. Molecular dynamics simulation of model systems with full atomistic detail shows substantial differences upon variation of the cation exchange capacity (CEC of 90 and 145 meq/100g) and the chain length of the surfactants due to layering effects of the surfactants within the galleries. Cleavage energies and relative contributions of Coulomb and dispersive interactions are presented for several systems, consistent with experimentally measured surface tensions for selected clay minerals. The cleavage energy decreases with higher chain length of the surfactants, however, the progression is not linear and related to the density of the interlayer of self-assembled surfactant chains. For example, computed cleavage energies are 56 mJ/m2 for C14 (tetradecylammonium) at high CEC and 45 mJ/m2 for C14 at low CEC.