Enhanced ordering reduces electric susceptibility of liquids confined to graphene slit pores

Abstract

The behaviours of a range of polar and non-polar organic liquids (acetone, ethanol, methanol, N-methyl-2-pyrrolidone (NMP), carbon tetrachloride and water) confined to 2D graphene nanochannels with thicknesses in the range of 4.5 Å to 40 Å were studied using classical molecular dynamics and hybrid density functional theory. All liquids were found to organise spontaneously into ordered layers parallel to the confining surfaces, with those containing polar molecules having their electric dipoles aligned parallel to such surfaces. In particular, monolayers of NMP showed remarkable in-plane ordering and low molecular mobility, suggesting the existence of a previously unknown 2D solid-like phase. Calculations for polar liquids showed dramatically reduced static permittivities normal to the confining surfaces; these changes are expected to improve electron tunnelling across the liquid films, modifying the DC electrical properties of immersed assemblies of carbon nanomaterials.

Universal Reduction in Dielectric Response of Confined Fluids

“…As depicted in Figure a–d, for all the fluids considered, compared to the bulk, the in-plane dipole–dipole correlation, h Δ ( r ) (see the Methods section for definition) is enhanced, showing the tendency of the dipoles to lay parallel to the graphene surface. Such a preferred orientation was reported for polar liquids on graphene . This preferred orientation can be understood by analyzing the in-plane dipole–dipole electrostatic energy of the interfacial layer, which can be written as where h D represents the angular dependence of the dipole–dipole interaction energy (see the Methods section for mathematical definition), r ∥ is the in-plane separation distance between the dipoles, L IFL is the width of the interfacial layer, and ρ IFL is the density of the molecules in the interfacial layer.…”

“…50 However, our results indicate that not only protic fluids such as water or methanol (hydrogen-bonding fluids) but also aprotic fluids exhibit a low out-of-plane dielectric permittivity under confinement, suggesting that the underlying mechanism for the reduction in permittivity is not just due to the hydrogen bond network effect in the interfacial layer (IFL). Since the main contribution to the static dielectric permittivity of polar fluids is the orientational polarizability (compared to the electronic polarizability), 51 and the dominant term is the dipole polarization, we investigate the in-plane dipole orientational correlations in the first fluid density layer (interfacial layer) adjacent to the wall (see Supplementary Figure S5). As depicted in Figure 3a− d, for all the fluids considered, compared to the bulk, the inplane dipole−dipole correlation, h Δ (r) (see the Methods section for definition) is enhanced, showing the tendency of the dipoles to lay parallel to the graphene surface.…”

“…Such a preferred orientation was reported for polar liquids on graphene. 51 This preferred orientation can be understood by analyzing the in-plane dipole−dipole electrostatic energy of the interfacial layer, which can be written as dichloromethane (d, h). The top row shows the dipole−dipole pair correlation function, and the bottom row shows the angular dependence of the dipole−dipole interaction energy in bulk (red color) and in the interfacial layer next to the graphene surface (black color) as a function of the separation distance r. It is important to note that in IFL the distance between the dipoles lies in the xy plane (parallel to the surface).…”

Impedance response of ionic liquids in long slit pores

“…The model for the ionic liquid can be generalised with ease to incorporate non-constant permittivities and diffusivities; molecular dynamics and mean-field simulations have shown that confinement can cause significant variations in these quantities [Kondrat et al, 2014, Terrones et al, 2016. Relevant DDFT studies include the work of Qing et al [2020] incorporating density-dependent permittivities, and the theoretical investigation of packing fraction-dependent diffusivities by Stopper et al [2018].…”

Electrocrystallization of Supercooled Water Confined between Graphene Layer

“…Particular attention is focused on the study of the effect of external fields (electric, magnetic, ultrasonic, etc.) on the crystallization of confined water [29][30][31][32]. The crystallization of water in an applied electric field -electrocrystallization -is a key phenomenon for such processes as the formation of atmospheric precipitation, formation and growth of ice on the wings of aircraft, and cryoconservation of biological materials [33,34].The aim of this work is to study the effect of the strength and direction of a uniform electric field on the process of crystallization of spatially confined supercooled water.…”