Orientation order of a nonpolar molecular fluid compressed into a nanosmall space.

The ordering structures of non-polar carbon tetrachloride liquid compressed to nano-scales between parallel substrates is studied in this work. The theoretical considerations show that the potential well formed by the confined parallel substrates induces orientational ordering of non-polar molecules. Through molecular dynamic (MD) simulations, the relations between various ordered structures of a non-polar liquid (carbon tetrachloride) and the confined gap size are demonstrated. The density distribution shows that the confinement does affect the ordering modes and induces an orientational ordering of molecules at the solid-liquid interface under extreme confinement conditions. This molecular orientation suggested from the theoretical model and MD simulation is directly supported by the experimental studies for the first time. The X-ray reflectivity data reveal a strong layering effect with splitting of the density profile in C and Cl-rich sublayers. The investigation shows that the liquid structure factor in confinement has a characteristic length similar to the short-range ordering in bulk, but the confined structure is strongly influenced by the surface potential and the interface properties. This introduces preferred molecular orientation and ordering which are not favorable in the bulk phase. As the orientational ordering is closely related to crystallization, our results provide a new perspective to control the crystallization in nano-confined space by compression.

Impact of confinement and polarizability on dynamics of ionic liquids.

Polarizability is a key factor when it comes to an accurate description of different ionic systems. The general importance of including polarizability into molecular dynamics simulations was shown in various recent studies for a wide range of materials, ranging from proteins to water to complex ionic liquids and for solid-liquid interfaces. While most previous studies focused on bulk properties or static structure factors, this study investigates in more detail the importance of polarizable surfaces on the dynamics of a confined ionic liquid in graphitic slit pores, as evident in modern electrochemical capacitors or in catalytic processes. A recently developed polarizable force field using Drude oscillators is modified in order to describe a particular room temperature ionic liquid accurately and in agreement with recently published experimental results. Using the modified parameters, various confinements are investigated and differences between non-polarizable and polarizable surfaces are discussed. Upon introduction of surface polarizability, changes in the dipole orientation and in the density distribution of the anions and cations at the interface are observed and are also accompanied with a dramatic increase in the molecular diffusivity in the contact layer. Our results thus clearly underline the importance of considering not only the polarizability of the ionic liquid but also that of the surface.