Paranematic-to-nematic ordering of a binary mixture of rod-like liquid crystals confined in cylindrical nanochannels

TL;DR

This study investigates how binary mixtures of rod-like liquid crystals transition from paranematic to nematic phases within cylindrical nanochannels, revealing size-dependent transition behaviors and the preservation of bulk composition in confinement.

Contribution

It demonstrates the size-dependent nature of paranematic-to-nematic transitions and shows that binary mixtures maintain their bulk composition in nanoconfinement for channels larger than 7 nm.

Findings

Transition can be continuous or discontinuous depending on pore size.

The ordering field scales inversely with pore radius and linearly with mixture composition.

Binary mixtures retain their bulk stoichiometry in nanoconfinement for larger pores.

Abstract

We explore the optical birefringence of the nematic binary mixtures 6CB$_{1-x}$7CB$_x$ ($0~\le~x~\le~1$) imbibed into parallel-aligned nanochannels of mesoporous alumina and silica membranes for channel radii of $3.4~\le~R~\le 21.0$ nm. The results are compared with the bulk behavior and analyzed with a Landau-de-Gennes model. Depending on the channel radius the nematic ordering in the cylindrical nanochannels evolves either discontinuously (subcritical regime, nematic ordering field $\sigma<1/2$) or continuously (overcritical regime, $\sigma>1/2$), but in both cases with a characteristic paranematic precursor behavior. The strength of the ordering field, imposed by the channel walls, and the magnitude of quenched disorder varies linearly with the mole fraction $x$ and scales inversely proportionally with $R$ for channel radii larger than 4 nm. The critical pore radius, $R_c$, separating a continuous from a discontinuous paranematic-to-nematic evolution, varies linearly with $x$ and differs negligibly between the silica and alumina membranes. We find no hints of preferred adsorption of one species at the channels walls. By contrast, a linear variation of the nematic-to-paranematic transition point $T_{\rm PN}$ and of the nematic ordering field $\sigma$ vs. $x$ suggest that the binary mixtures of cyanobiphenyls 6CB and 7CB keep their homogeneous bulk stoichiometry also in nanoconfinement, at least for channel diameters larger than $\sim$7 nm.