Session Y25: Behavior of Liquids Confined on the Nanometer Scale IV

Live

In this talk, I will discuss various experimental and theoretical results that we obtained recently in the Micromégas team on the transport of fluids and ions in the ultra-confinement. I will in particular focus on the odd properties of the water carbon couple, which highlights a variety of quite exotic transport properties, that we will discuss and rationalize: ultra-low friction, specific charge adsorption, non-linear transport, mechano-sensitivity, …
I will show how these specificities can be used as building blocks to build a ionic machinery, from ion pumps to artificial neuromorphic behavior.

This work was done with N. Kavokine, A. Marcotte, T. Emmerich, P. Robin, A. Poggioli, A. Niguès, A. Siria from Micromégas team, ENS Paris.


References
[1] “Fluids at nanoscales: from continuum to sub-continuum transport”, N. Kavokine, R. Netz and L. Bocquet, Annual Review of Fluid Mechanics (2020); https://doi.org/10.1146/annurev-fluid-071320-095958
[3] “Molecular streaming and voltage-gated response in Angström scale channels” T. Mouterde, A. Keerthi, A. Poggioli, S. Dar, A. Siria, A.K. Geim, L Bocquet and R. Boya, Nature 567 87 (2019)
[4] “Mechanically activated ionic transport across single digit carbon nanotubes”, A. Marcotte, T. Mouterde, A. Nigues, A. Siria and L. Bocquet, Nature Materials, 19, 1057 (2020)
[5] “Ionic Coulomb blockade as a fractional Wien effect ” N. Kavokine, S. Marbach, A. Siria, L. Bocquet, Nature Nanotechnology (2019)   

Live

When electric double layers overlap in a charged nanopore, classical Poisson-Boltzmann theory predicts uniform screening charge of ions in equilibrium with a bulk electrolyte reservoir. The dominance of surfaces, however, foreshadows extreme sensitivity to boundary conditions, which leads to some surprising phenomena of biological relevance: (1) The sacred assumption of local electroneutrality breaks down, as ions evacuate the pore in favor of the reservoir, leaving the surface charge unscreened, if electric fields leak into the solid matrix; this phenomenon may explain the collapse of conductivity of nanopores at low salt concentration, as well as unscreened ion transport through biological ion channels; (2) If ions are unable to escape a nanopore as temperature is lowered, they can enable extreme freezing-point depression and large disjoining pressures, consistent with extreme cryotolerance in biology and freeze-thaw damage in concrete.   

Live

Nonlinear optical spectroscopies of confined liquids and sum- and difference- frequency mixing spectroscopies at interfaces are widespread methods to determine the molecular dynamics of fluids in the vicinity of solid surfaces. In this work, we introduce a new technique based on the Optical Kerr Effect Spectroscopy (OKE), carried out by the total internal reflection of both pump and probe beams at the solid/liquid interface. This new technique, Surface Time-Resolved OKE (STROKE), provides a unique tool complementary to the methods mentioned above. This ultrafast spectroscopy relies on the optical response probed after the anisotropy generated by the pump pulse, due to the induced molecular alignment in the region of its evanescent field. We report on the first results of this approach in the SF11 glass/CS₂ system and compare the observed dynamic with OKE results from confined liquids in sol-gel nanoporous. We compare the diffusive responses to evaluate confinement and wall interactions.   

Live

The microscopic behavior of Ru-based catalysts confined in nanometer sized pores in the supported ionic liquid phase (SILP) is studied using Molecular Dynamics. To this end, we have developed based on quantum mechanical calculations an atomistic force-field for the catalysts. Our system is made up of a silica pore with a functionalized inner surface including an ionic liquid (IL) and heptane in which both the catalysts and the substrates are initially placed. Our results reveal a phase separation of a nano-scale IL phase on the pore surface and a diffusive heptane phase on top of the IL phase. The catalysts are well dissolved into the IL phase within the pore and show much lower mobility than in the bulk IL. These linker free immobilization of the catalyst is expected to lead to a high selectivity. Our study aims to reveal the microscopic properties such as the configurational characteristics and dynamics of the catalysts in the pore, which are not accessible by experiments. We discuss the solvent effects, as well as the influence of the confined environment of the pore on the catalytic molecules and the relevance to novel experiments. This work is the first step towards an optimization and tuning of the setup of catalytic reactions within the framework of the SILP technology.   

Live

Nanofluidics technology has made considerable progress over the last decade. Using Van der Waals assembly, it is now possible to make atomically smooth, pristine carbon bidimensional capillaries with atomic scale precision. In this presentation we show an alternative route to fabricate bidimensional, 'activated' carbon nanochannels using electron beam irradiation and Van der Waals assembly. We discover that the 'activated' carbon conduits outperforms by orders of magnitude pristine carbon materials in terms of osmotic energy conversion, with power density values reaching a hundred of kW/m². We attribute the huge increase in osmotic currents across activated carbon nanochannels as compared to pristine channels to an optimal combination of high surface charge and low friction on the activated carbon, boosting interfacially driven, epi-osmotic transport. Our results pave the way to the development of efficient carbon-based membranes for osmotic power harvesting under salinity gradients.

T. Emmerich, K.S Vasu, A. Nigues, A. Keerthi, B. Radha, A. Siria and, L. Bocquet, submitted.   

Live

Understanding the structure and flow of water confined between surfaces is vital in lubrication, protein folding, nano-fluidics, etc. Current methods and experiments to measure nano-confined water's dissipative properties are not conclusive enough to tell whether it deviates from its bulk properties[1,2]. This work will introduce an independent optical method. Our newly developed instrument and Monte-Carlo based analyzing technique allows us to study tracer diffusion in nano-confined water with controlled confinement gaps[3]. We can measure the translational diffusion-time-scales of a fluorescent-tracer molecule and the population-fraction for each time-scale. Our preliminary experiments indicate that even in 1nm gap, the bulk-like dynamics prevail. An additional coexisting slow-moving tracer population is observed depending on the electrostatic interactions of confining interfaces with tracers. The gap dependence studies showed such slow dynamics are localized near to the confining interfaces. More control experiments are being done to understand these results and single out the effect of confinement on water's dissipative properties.
[1] Phys. Rev. Lett. 100(2008), 106102
[2] Nature 413(2001), 51
[3] Rev. Sci. Instrum. 91(2020), 013702   

Live

Confining soft matter in dimensions of a few molecular diameters impacts its physical properties, compared to its bulk counterpart. As spatial constraints approach the range of intermolecular interactions, or even the size of the molecular entities, confined materials can show dramatic changes in their structural and thermodynamical properties, which in turn can affect the rate of chemical reactions and phase transitions. In this light, polymerization dynamics are expected to change under nanoconfinement, opening new pathways for studying fundamental physical and chemical phenomena. Ηere we report on the effect of spatial confinement on free radical photopolymerization. We used monolithic silica glasses prepared by the sol-gel method as the means of confinement, with pore sizes ranging between 2 and 10 nm. Our findings showed a dramatic enhancement of the polymerization efficiency under nanoconfinement, with the rate of polymerization increasing as the spatial confinement becomes tighter. The origin of this effect will be discussed in terms of the polymerization kinetics of free radical polymerization.   

Live

Water structure, dynamics and transport in nanoconfinement have been shown to exhibit anomalous behavior. Although significant experimental and theoretical progress has been made to understand water behavior inside carbon nanotubes (CNTs), the quantum effect that water experiences due to the electronic structure of the CNT has not been investigated in detail. In this presentation, we show clear evidence on the electronic structure coupling between water and CNTs. We use first-principles simulations to compute various structure and transport properties of water inside CNTs. Our results show that water molecule behavior can depend significantly on whether the CNT is metallic, semi-metallic or semiconducting. Further, we show that the electronic coupling can lead to water exhibiting interesting trends in free energy, dipole moment and friction factor. Our results provide a fundamental understanding of water inside CNTs which is crucial to design high performance nanofluidic devices for various applications including energy harvesting, water desalination, and biomimetic devices.   

Live

Acetonitrile is an amphiphilic molecule with a large dipole moment whose complex behavior in the liquid state belies its relatively simple molecular structure. Here we present both molecular simulations of the bulk and interfacial liquids and experiments using neutron scattering and optical Kerr effect spectroscopy to probe the bulk and nanoconfined liquids. Using angularly resolved radial distribution functions, we identify a complex microscopic structure in the simulated bulk liquid in which most liquid molecules are associated with one or more neighboring molecules in antiparallel or head-to-tail configurations. This structural picture is associated with pairing times longer than the typical rotational and translation time constants. In contrast to the bulk liquid, acetonitrile is known to adopt an interdigitated, bilayer-like organization at hydroxylated silica interfaces. We present initial results of molecular simulations on such systems and spectroscopic results on nanoconfined acetonitrile, including putative signatures of hydrogen bonding at the liquid/silica interface.   

Live

Isolated, large hydrocarbon molecules optimize van der Waals bonding by lying flat on substrates. Likewise, in three-dimensional hydrocarbon crystals, the van der Waals bonding is optimized between the flat molecular planes. In the intermediate case, can a densified monolayer tilt before forming 3D crystals that dewet the substrate? We used scanning tunneling microscopy and density functional theory calculations to study molecular layers of coronene on Cu(111) to determine balance between coronene-substrate and coronene-coronene interactions. Our findings disagree with some of the microscopy observations that reported tilted coronene at the monolayer, and we proposed that configurations of tilted molecules are metastable. Our calculations also show that coronene double layers are composed of tilted molecules, providing an alternative explanation for tilted coronene. However, double layers are metastable with respect to the formation of bulk coronene crystals that dewet the substrate, so their occurrence would suggest nucleation barriers to dewetting into three-dimensional islands. Similar metastability has been observed in metal-on-metal epitaxy.   

On Demand

Liquid confinement below micrometer thicknesses is of interest to several scientific disciplines and is inherently challenging to study experimentally. More extreme confinement at the single-digit nanometer scale is possible through the use of self-assembled nanostructures such as carbon nanotubes. Bridging the gap between micro- and nanoscale liquid confinement has been achieved with two-piece liquid cells within a transmission electron microscope (TEM), but irreproducibility and difficulties maintaining uniform liquid layer thickness makes high spatial and spectroscopic resolution challenging. We report on improvements made to a monolithic, in-situ TEM liquid cell with sub 100nm liquid thickness that overcomes the common challenges encountered with two-piece TEM liquid cells.
1. Tanase, M. et al. High-Resolution Imaging and Spectroscopy at High Pressure: A Novel Liquid Cell for the Transmission Electron Microscope. Microsc. Microanal. 21, 1629–1638 (2015).