Molecular Dynamics study of tetracosane monolayers adsorbed on graphite

International audience; Molecular Dynamics (MD) simulations of tetracosane (C24H50) monolayer physisorbed on graphite are carried out. C24H50 molecules are simulated with explicit hydrogens and the graphite is represented by six graphene layers. We focus our analysis on the microscopic mechanism of melting, experimentally observed at T = 340 K. We are looking for the pre-melting transformations and analyze if there is a correlation between translational disordering of molecules and their internal degrees of freedom. We analyze several order parameters and their fluctuations along the MD trajectories. We show that the all atom representation of C24H50 is much more sensitive to the model of intramolecular interactions than united atom model. In particular two components: electrostatic forces and scaled 1-4 internal non-bonded interactions can shift melting temperature by several tenths of degree. A correlation between molecular stiffness and the intermolecular forces causes molecules' footprint reduction during melting which involves a simultaneous lost of intramolecular and translational order. This cooperative mechanism is related to an abrupt increase of gauche defects within the central region of the chain.