(536h) Microstructure of Room-Temperature Ionic Liquids at Charged Surfaces Revealed By Integrated Modeling and Experimental Approaches

Authors: 
Feng, G., Vanderbilt University
Black, J., Oak Ridge Naitonal Laboratory
Balke, N., Oak Ridge Naitonal Laboratory
Cummings, P. T., Vanderbilt University
The interfaces of room-temperature ionic liquids (RTILs) at charged surfaces are of great importance for many applications, such as electrical energy storage and lubrication. Many nanoscale techniques (e.g., atomic force microscopy, AFM) are developed to reveal the microstructure of electrical double layers (EDLs) formed by RTILs at charged solid surface, and with the help of molecular dynamics (MD) simulation one can obtain detailed information of EDLs at molecular level and have a better understanding of the interfacial phenomena.

In this talk, firstly we report direct observation of structure and properties of the topological defects formed in ordered ionic liquid layers at carbon interfaces. The observation of layer structure at structural defects such as step edges reinforced by MD simulations define spatial resolution of the method. We have observed the internal structure of topological and structural defects on the ionic-liquid surface interfaces using 3D force mapping. The structural defects were found to result only in short range changes in ionic liquid ordering, with the changes in 3D force fields consistent with rigid shift of the layers with narrow disordered region between the two, consistent with MD simulations. At the same time, formation of serendipitous dislocation-type topological defects were observed on planar surfaces. The decrease in the ordering was observed within the defect, consistent with behavior expected for classical liquid crystals.

Secondly, further MD work was done to investigate the influence of the thickness of the electrode surface step on the microstructure of interfacial ILs. A strong correlation was observed between the interfacial IL structure and the step thickness in electrode surface as well as the ion size. Specifically, when the step thickness is commensurate with ion size, the interfacial layering of cation/anion is more evident; whereas, the layering tends to be less defined when the step thickness is close to the half of ion size. Furthermore, two-dimensional microstructure of ion layers exhibits different patterns and alignments of counterion/co-ion lattice at neutral and charged electrodes.

Thirdly, beyond carbon surfaces, we then explore the ILs at charged mica surfaces. By comparing the measured force profiles from AFM to ion density profiles from MD simulations, we were able to determine which ion of the ionic liquid the AFM tips are sensitive to during force measurements, and this is determined by ion larger ion volume and/or weight. By varying the anion or cation of the ionic liquids we are able to move from a regime where we detect the position of the anions (e.g. [emim][Tf2N]) to one where we detect the position of the cations (e.g. [emim][BF4]). Using AFM data alone we cannot identify which ion we are sensitive to, and through a direct comparison of experiment data with molecular dynamics simulation, we are able to gain a better understanding of the mechanisms leading to the measured force profiles, therefore making future interpretation of force-curves at ionic-liquid solid interfaces easier. We will also show the interfacial structure of humid ILs at charged mica surfaces, in particular, the water accumulation will differ from that at carbon surfaces.