Competing interactions in ionic liquids: from dipolar chains to the structure of thin films

M. G. DEL PÓPOLO

Dublin, Republic of Ireland

Congreso; CECAM meeting: Computational models of room temperature ionic liquids; 2009

The close examination of the structure, dynamics, and phase behavior of organic ionic liquids (ILs) have revealed a complex picture where Coulomb and short range forces often compete with each other. Manifestations of this phenomenon include the formation of nanoscopic ionic/neutral domains in bulk ILs, the presence of lamelar patterns in organic-ionic crystals and the formation of hydrogen-bonded networks in protic ionic liquids. In this talk we will discuss two further examples: i) the formation of dipolar chains in ILs/organic-solvent mixtures and ii) the molecular organisation of ILs films supported on hydroxylated surfaces. In the first case we investigate the ionic character of [bmim][PF6]/naphthalenemixtures using molecular dynamics simulations based on an empirical force-field. Simulations cover the full range of concentrations from pure ionic liquid to pure naphthalene. The composition dependence of structural and dynamical properties reveal a sharp transition in the bonding pattern of the fluid occurring at 20% IL concentration, and corresponding to the condensation of ion pairs into filaments bound by dipolar forces. In the second case, we discuss the structure of [BMIM][NTf2] films supported on hydroxylated silica highlighting the role of the different intermolecular forces. In well agreement with AFM experiments, simulations show that at room temperature the IL forms a solid-like deposit with a clear arrangement of layers along the surface normal.