Solvation Structures

Abstract

The dynamics of liquid molecules near solid interfaces have important implications in studies such as the interaction of biomolecules in water, or the energy-storage capability of ionic liquids. In this experiment, we access volumes small enough for direct comparison with simulations in attempt to refine our understanding of the dynamics occurring at these liquid-solid interfaces.

Project

The accuracy with which simulations can predict dynamics of liquid molecules near solid interfaces is gauged by their ability to predict and explain experimental observations. Due to limited computing power, meaningful comparisons can only be made with experiments that have probed volumes within ~(3nm)³. This research project aims to access such small volumes by using atomically sharp tips on atomically flat solid surfaces.

We use dedicated instrumentation to accurately measure the conservative and dissipative interactions between the tip and the solvation structures near a surface as a function of tip-surface distance, and compare them to predictions made by simulations. Two separate experiments are performed, each with very different scientific goals, listed below. The main experimental challenge for the study of hydration structures above the mica surface is the need for 3D imaging, whereas the study of 1D solvation profiles of ionic liquids above the Au(111) surface requires electrochemical control of the surface in an anaerobic environment.

The hydration structures of water molecules play a crucial role in the interaction of biomolecules. Using a well-defined model system - water on mica - allows to compare experiment to simulations with the goal of refining modelling parameters which to date carry unsatisfactory levels of uncertainty. The long-range order of the highly structured hydration layers above the mica surface requires the use of 3D imaging methods. We use 3D phase-and-frequency modulation atomic force microscopy (3D-FPM-AFM) to acquire hydration force maps, along with dissipation measurements, to study the dynamics and distribution of water molecules around the highly structured mica surface.

3D frequency shift map of water on mica, where the long range order of the distribution of water molecules can be clearly observed. This dataset was acquired at Kyoto University, during a collaboration with the Yamada research group.

Ionic liquids carry promising potential for the use in super-capacitors necessary for high-density energy storage where the supply and demand of power generation do not overlap in time. Cost-effective development of new high-capacity ionic liquids relies on accurate predictive power made by simulations. In this experiment, the interest lies in changes of the solvation profile as a function of the electrochemical potential of the Au(111) surface. A comparative study between experimental and simulated solvation profiles as a function of electrochemical potential should allow the tuning of modelling parameters necessary to accurately predict the performance of ionic liquids for charge storage applications.

Electrochemical liquid cell used for ionic liquid solvation experiment. The electrodes (RE, WE, CE) allow for full electrochemical control of the Au(111) sample while approaching the cantilever tip towards the sample