Structural and dynamical properties of a confined room temperature ionic liquid

C. PINILLA, M. G. DEL PÓPOLO , R. M. LYNDEN-BELL AND J. KOHANOFF

Barcelona, Espana

Congreso; Liquid systems under extreme conditions; 2006

Room temperature ionic liquids (RTIL) have drawn significant attention during the past years due to their potential applications as solvents for chemical synthesis, or in the construction of electrochemical devices. Two well documented examples are the uses of RTILs as fuel cells materials or as electrolytes for dye sensitized solar cells (DSSC). In a DSSC a dye-coated semiconductor (TiO2) layer forming a nanoporous network collects electrons from the photoexcited dye molecules. The dye is then regenerated via electrons donated by a redox couple (I- / I3-), which is dissolved in a RTIL. This redox-electrolyte system has to penetrate the porous matrix in order to accomplish the dye regeneration process. It is known experimentally that this process is fast despite the confinement and the significant size of the RTIL components. In this work we study, using molecular dynamics simulations, the behaviour of the RTIL dymethilimidazolium chloride confined between to flat walls at different confinement widths. In addition, the response of this confined liquid to an external electric field, such as that created by the charged TiO2 nanoparticles, is presented. The response of the liquid is analyzed in terms of mass and charge density fluctuations. As a result, we find that the dynamics of the confined RTIL can be described in terms of two regimes: a fast process (faster than 1ps) that involves mainly a small re-accommodation of the anions, and a much slower process corresponding to structural relaxation. We compare linear response calculations with non-equilibrium molecular dynamics simulations of the confined liquid when an applied external electric field is suddenly switched off.