Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases

TL;DR

This study measures the linear compressibility of two-dimensional fatty acid mesophases using grazing incidence X-ray diffraction, revealing a wide range of values linked to different molecular mechanisms and phase structures.

Contribution

It provides the first detailed microscopic measurements of linear compressibility in fatty acid mesophases, correlating mechanical responses with molecular arrangements and phase behavior.

Findings

Compressibility varies from 0.1 to 10 m/N across phases.

Largest compressibilities are in tilted phases, independent of chain length.

Negative compressibilities observed in certain phases due to molecular reorganization.

Abstract

The linear compressibility of two-dimensional fatty acid mesophases has determined by grazing incidence x-ray diffraction. Surface pressure vs molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion along a given direction for isotropic applied stress) was determined both in the sample plane and in a plane normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with different molecular mechanisms. The largest compressibilities (10m/N) are observed in the tilted phases. They are apparently independent of the chain length and could be related to the reorganization of the headgroup hydrogen-bounded network, whose role should be revalued. Intermediate compressibilities are observed in phases with quasi long-range order (directions normal to the molecular tilt in L_2 or L_2' phases, S phase), and could be related to the ordering of these phases. The lowest compressibilities are observed in the solid untilted CS phase and for 1 direction of the S and L_2'' phases. They are similar to the compressibility of crystalline polymers and correspond to the interactions between methyl groups in the crystal. Finally, negative compressibilities are observed in the transverse plane for L_2' and L_2'' phases and can be traced to subtle reorganizations upon untilting.