Computer simulation of ionic liquids thin films and nano-droplets on silica surfaces

M. G. DEL PÓPOLO AND P. BALLONE

London, England

Congreso; CCP5 Annual Meeting 2008; 2008

The formation of thin films and nano-droplets supported on solids plays a major role in numerous industrial and natural processes, providing ample motivation to investigate the way  intermolecular forces determine the equilibrium morphology and rheological properties of low-dimensional wetting structures.  Prominent among those interactions are Coulomb forces whose strength and long-rage character confer singular physico-chemical properties to ionic fluids, whose manipulation at the nanoscopic level has become not only feasible but increasingly important in recent years. The advent of a new generation of organic electrolytes, collectively know as room temperature ionic liquids (RTIL), offers unprecedented opportunities  to investigate the molecular organization of dense Coulomb fluids under low dimensional confinement. RTIL ultra-thin films supported on ionic, metallic and semiconducting surfaces have been characterised by atomic force microscopy,  providing valuable information on the response of these systems  to mechanical and electrical stress in terms of widely different adsorbate/substrate interactions.  In order to gain insight into the microscopic properties of these complex interfaces we have performed atomistic simulations, based on empirical force fields, of monolayers and submonolayers of a prototypical RTIL (Butylmethylimidazolium-bistriflamide ([BMIM][NTf2] ) supported on silica. We discuss the structural, dynamical and dielectric properties of the films as a function coverage degree, and report the calculation of the various interfacial energies which allow us to determine the equilibrium shape of nanometer-sized droplets. The simulation results are discussed in terms of the surface chemistry associated to the degree of hydroxilation of the surface.