Structure and Dynamics of Confined Liquids: Challenges and Perspectives for the X-ray Surface Forces Apparatus.

Henning Weiss,Hailong Li,Markus Mezger,Veijo Honkimäki,Hsiu-Wei Cheng,Frank Uwe Renner,Julian Mars,Claudia Merola,Markus Valtiner

doi:10.1021/acs.langmuir.9b01215

Abstract

The molecular-scale structure and dynamics of confined liquids has increasingly gained relevance for applications in nanotechnology. Thus, a detailed knowledge of the structure of confined liquids on molecular length scales is of great interest for fundamental and applied sciences. To study confined structures under dynamic conditions, we constructed an in situ X-ray surface forces apparatus (X-SFA). This novel device can create a precisely controlled slit-pore confinement down to dimensions on the 10 nm scale by using a cylinder-on-flat geometry for the first time. Complementary structural information can be obtained by simultaneous force measurements and X-ray scattering experiments. The in-plane structure of liquids parallel to the slit pore and density profiles perpendicular to the confining interfaces are studied by X-ray scattering and reflectivity. The normal load between the opposing interfaces can be modulated to study the structural dynamics of confined liquids. The confinement gap distance is tracked simultaneously with nanometer precision by analyzing optical interference fringes of equal chromatic order. Relaxation processes can be studied by driving the system out of equilibrium by shear stress or compression/decompression cycles of the slit pore. The capability of the new device is demonstrated on the liquid crystal 4′-octyl-4-cyano-biphenyl (8CB) in its smectic A (SmA) mesophase. Its molecular-scale structure and orientation confined in 100 nm to 1.7 μm slit pores was studied under static and dynamic nonequilibrium conditions.

Highlights

Confined liquids play an important role in many technical applications and processes
The dynamics of electrolytes in nanoscale confinement are relevant to the development of novel electric double-layer capacitors and electrochemical processes.[1−4] In heterogeneous catalysis, the performance of a reaction has been shown to be sensitive to the molecular-scale liquid structure in a nanoporous material.[5]
We describe a novel design for an X-ray surface forces apparatus (X-SFA) in cylinder-on-flat geometry combining the capabilities of an SFA setup with X-ray scattering techniques

Summary

■ INTRODUCTION

Confined liquids play an important role in many technical applications and processes. When confinement approaches the molecular dimensions of liquids, structural and dynamical properties can differ significantly from their bulk behavior[11−13] (Figure 1a−d). It has been shown that, depending on the intermolecular forces, soft matter adjacent to a solid and near surfaces can exhibit interfacial regions of reduced density,[14−16] interfacial freezing,[17−19] interfacial melting,[20−23] molecular layering,[24−31] molecular orientation,[32] or specific lateral molecular arrangements.[33] the phase diagram of confined liquids can differ significantly from the bulk.[34,35] Such interface-induced structures can strongly affect the dynamics in a confined system.

Invited Feature Article

■ ACKNOWLEDGMENTS

Findings

■ REFERENCES