Structures of Nanoconfined Liquids Determined by Synchrotron X-ray Diffraction

Area of Science:

• Materials Science
• Physical Chemistry
• Nanotechnology

Background:

• Understanding liquid behavior under confinement is crucial for nanoscale applications.
• Previous studies suggest altered molecular structures and properties in confined liquids.
• The role of molecular shape and confinement distance on liquid ordering remains an active research area.

Purpose of the Study:

• To investigate the structural ordering of octamethylcyclotetrasiloxane (OMCTS) and n-hexadecane under nanoconfinement.
• To determine the influence of surface separation distances on the molecular arrangement and intermolecular spacing.
• To correlate structural ordering with changes in viscosity under confinement.

Main Methods:

• X-ray diffraction measurements were performed on OMCTS and n-hexadecane confined between mica surfaces.
• Surface separation distances (D's) ranged from 500 nm down to the hard-wall thickness (1.9 nm for OMCTS, 1.0 nm for hexadecane).
• Resonance shear measurements were used to obtain viscous parameters.

Main Results:

• Diffraction peaks indicating mean intermolecular spacing were observed for both liquids at all studied distances.
• A significant increase in molecular order was observed for OMCTS and n-hexadecane below confinement distances of approximately 50 nm, with peak intensity increasing sharply at D < 50 nm.
• Normalized peak intensities (I_normalized) increased significantly with decreasing D, reaching 120 for OMCTS and 160 for n-hexadecane at the hard wall, while full width at half maximum (fwhm) showed no significant change until small distances.

Conclusions:

• Nanoconfinement, particularly below 100 nm, significantly enhances the structural order of OMCTS and n-hexadecane.
• The observed increase in structural order is primarily due to the ordering of molecules in the parallel plane within an enhanced layered structure.
• A certain degree of molecular ordering is a prerequisite for the observable increase in viscosity under nanoconfinement, as evidenced by the lack of viscosity increase down to ~7 nm.