(42a) The Design of Two Approaches to Confinement Combined with X-Ray Reflectivity for Structural and Force Studies of Soft Matter at Interfaces

Soft matter systems, such as liquid crystals and lipid bilayers, that are confined between solids (at nanometer thicknesses) exhibit changes in molecular ordering that are of great importance to the fields of friction, lubrication or energy conversion. Thus, a detailed knowledge of the structure on the molecular scale of confined soft matter is of significant interest to both fundamental and applied sciences. How the structuring of confined material reacts to external triggers, such as changes of applied confining stress, sliding of surfaces against each other, temperature or humidity is notoriously hard to measure experimentally. To overcome the challenges, the design of a novel apparatus for in situ measurements of structure with external stimuli is required.

Scattering techniques, particularly reflectivity with X-rays and neutrons, are well known to provide direct measurement of the structure of soft matter at buried interfaces. Here, we will describe and compare two newly developed methods that we have used to combine nanoscale confinement, in situ, with X-ray reflectivity. The Surface Force Apparatus (SFA) is now well established for measurements of the force response of material to a confining stress, at the interface between two solid surfaces. Applied forces are quantified within the SFA using strain gauges, with well-controlled gaps between the two surfaces established for nano-confinement and optical interference used to achieve distance measurements of the gap sensitive to less than a nanometer. We have therefore combined an SFA with X-ray reflectivity (X-SFA) in order to measure the structure of confined material simultaneously with force. Where many SFAs employ a crossed cylinder configuration, a cylinder vs. flat geometry has been used for the X-SFA.

The second, alternative apparatus that we will discuss is a confinement cell, which uses a flexible Melinex sheet to provide one of the two solid surfaces. A version of the cell for neutron reflectivity has already proved useful for the investigation of lipids and polymer brushes, amongst other soft matter systems. This approach has then been applied for use with X-ray reflectivity with significant changes to the design of the apparatus. The inflated sheet provides a similar contact geometry to that used for the X-SFA, aiding comparison.

A model liquid crystal, 8CB (4’-n-octyl-4-cyano-biphenyl), has been tested in both types of apparatus in situ with reflectivity. For the X-SFA, a pore with a mm² area and a precisely controlled height of only several 100s of nanometers of 8CB was realized by our recently developed X-SFA. Dynamic forces were applied to the 8CB through compression and decompression cycles of the two opposing surfaces. Characteristically, we observed the 8CB Bragg peak at 2 nm^-1in the smectic phase for both types of apparatus. Instead of dynamic confinement changes the flexible membrane cell is well designed for temperature changes and to access a variety of molecular orientations through surface modifications. Therefore the temperature control feature was used to explore the behavior of the smectic liquid crystal close to the phase transition, however, little to no enhancement of the smectic wetting (perperdicular layering of molecules at the interface within the otherwise bulk nematic phase) was observed.

As a hydrated, more complex, liquid crystal analogue lipid bilayers have also been tested using these types of apparatus and we will be presenting our most recent findings. It was experiments with these lipid systems that ensure the X-ray confinement cell using the flexible sheet approach provides data consistent with its more established neutron counterpart. Results from experiments in situ using the flexible membrane cell and ex situ with the cylinder on flat SFA geometry will be discussed for phosphatidylcholine lipids.

Importantly, we have been able to demonstrate the feasibility of this X-SFA in tracing the anisotropy of confined 8CB molecules under dynamic compression. An alternative design of confinement apparatus has also been successfully commissioned, with the potential to jointly access a wider range of samples and conditions between the two approaches. Thus, making these novel setups suitable for studying confined soft matter under a range of conditions, particularly during dynamic compression, dynamic sliding and temperature variation.