On De Gennes narrowing of fluids confined at the molecular scale in nanoporous materials
Wanda Kellouai, Jean-Louis Barrat, Patrick Judeinstein, Marie Plazanet, Benoit Coasne, J. Chem. Phys. 160, 024113 (2024).

Beyond well-documented confinement and surface effects arising from the large internal surface and severely confining porosity of nanoporous hosts, the transport of nanoconfined fluids remains puzzling in many aspects. With striking examples such as memory, i.e., non-viscous effects, intermittent dynamics, and surface barriers, the dynamics of fluids in nanoconfinement challenge classical formalisms (e.g., random walk, viscous/advective transport)—especially for molecular pore sizes. In this context, while molecular frameworks such as intermittent Brownian motion, free volume theory, and surface diffusion are available to describe the self-diffusion of a molecularly confined fluid, a microscopic theory for collective diffusion (i.e., permeability), which characterizes the flow induced by a thermodynamic gradient, is lacking. Here, to fill this knowledge gap, we invoke the concept of “De Gennes narrowing,” which relates the wavevector-dependent collective diffusivity D₀(q) to the fluid structure factor S(q). First, using molecular simulation for a simple yet representative fluid confined in a prototypical solid (zeolite), we unravel an essential coupling between the wavevector-dependent collective diffusivity and the structural ordering imposed on the fluid by the crystalline nanoporous host. Second, despite this complex interplay with marked Bragg peaks in the fluid structure, the fluid collective dynamics is shown to be accurately described through De Gennes narrowing. Moreover, in contrast to the bulk fluid, the departure from De Gennes narrowing for the confined fluid in the macroscopic limit remains small as the fluid/solid interactions in severe confinement screen collective effects and, hence, weaken the wavevector dependence of collective transport.

Anne-Charlotte Le Gulluche, Guylaine Ducouret, Ludovic Olanier, Annie Brûlet, Olivier Sanseau, Paul Sotta, and Alba Marcellan

Model hybrid hydrogels reinforced by silica nanoparticles were designed by polymerizing and cross-linking the gels in situ. The polymer–particle interactions were tuned by using either poly(dimethylacrylamide) (PDMA), which adsorbs on silica, or poly(acrylamide) (PAAm), which does not. Besides, the dispersion state of silica nanoparticles was tuned from well-dispersed to aggregated by changing the pH from 9, which ensures repulsive interactions between nanoparticles and good dispersion state, to about 6, which affects the surface chemistry of silica and promotes aggregation. The dispersion states were characterized by small-angle X-ray scattering (SAXS). The mechanical behavior of hybrid gels with aggregated nanoparticles is markedly different from those where silica is well-dispersed within the matrix. PDMA-based hybrid gels display pronounced nonlinear behavior, somehow similar to those observed in filled elastomers. The nonlinearities are even more pronounced in gels with aggregated particles, with strong strain stiffening along with large dissipation. For those samples, reinforcement can be attributed to the combination of both reversible interactions between PDMA and silica nanoparticles, which provide strain stiffening and recovery, and the response of the silica network. Recovery processes observed in hybrid gels with dispersed particles are preserved when silica particles are aggregated, but the characteristic time needed to fully recover the mechanical response is extended from a few seconds to several hours. In PAAm-based hybrid gels with aggregated silica nanoparticles, no recovery processes are observed. This implies that the properties, namely, the very high linear tensile modulus and high dissipated energy, are driven by the rigid network formed by nanoparticle aggregation, which provides high dissipative capabilities, especially when compared to PAAm-based hybrid gels with dispersed silica, that remain soft and fragile. These gels exhibit a quite inhomogeneous structure, with permanent damage under elongation. The nonlinear dynamical behavior of hybrid gels was investigated by large amplitude oscillatory shear (LAOS) experiments. While unfilled gels show no nonlinearity up to very large strain amplitude, marked nonlinear effects combining a drop in modulus (similar to the Payne effect) and strain stiffening for increasing strain amplitude are observed in PDMA-based hybrid gels, certainly due to polymer adsorption onto nanoparticles. PAAm-based hybrid gels also show nonlinearity, with a drop in modulus for increasing strain but no strain stiffening, indicating that the presence of fillers alone can induce nonlinearity in the absence of strong, reversible polymer–particle interactions. PAAm-based hybrid gels with aggregated silica show very high stiffness and high dissipative properties at the expense of stretchability, though. Also, the structure seems to be permanently damaged under stress, revealing the importance of silica–polymer interactions for permanent mechanical reinforcement. Altogether, the analysis of the nonlinear behavior indicates the importance of combining dynamic adsorption of polymer chains on silica nanoparticles with mechanical reinforcement provided by the silica network.

https://dx.doi.org/10.1021/acs.macromol.3c01440

H. P. Khanh Ngo, E. Planes, C. Iojoiu, P. Soudant, A.-L. Rollet, P. Judeinstein

The increasing need of portable electrical resources requires to develop post-Li batteries, in which redox reactions are then based on the different alkali or earth alkaline ions. Keeping in mind the specific advantages of electrolytes based on ionic liquid (electrochemical properties, safety, stability …), this paper focuses on the specific properties of those obtained by mixing alkali or earth alkaline salts (Li+, Na+, K+, Cs+ and Mg2+) in a prototypical ionic liquid, the 1-butyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl) imide (BMImTFSI). Transport properties of these electrolytes are deciphered from viscosity, ionic conductivity and individual self-diffusion coefficients. At room temperature, change of electrolyte composition (nature of ion, concentration) induces some large variations of these transport properties. However, if these measurements are scaled towards glass transition temperatures, master curves are obtained with only slight differences between monovalent alkali and divalent earth alkaline ions. Further information is obtained from the Walden approach and evidences that these electrolytes are ’good ionic conductors’. These results are confirmed from self-diffusion coefficients which also allows to retrieve contributions of each species to ionic transport and then dissociation ratio inside these mixtures. Slight differences between alkali and earth alkali ions may be related to solvation mechanisms, as proposed from infrared spectroscopy measurements.

https://doi.org/10.1016/j.jil.2022.100044

E. Kume and L. Noirez

We show that a confned viscous liquid emits a dynamic thermal response upon applying a low frequency (∼1 Hz) shear excitation. Hot and cold thermal waves are observed in situ at atmospheric pressure and room temperature, in a viscous liquid (polypropylene glycol) at various thicknesses ranging from 100 µm up to 340 µm, upon applying a mechanical oscillatory shear strain. The observed thermal efects, synchronous with the mechanical excitation, are inconsistent with a viscous behaviour. It indicates that mesoscopic liquids are able to (partly) convert mechanical shear energy in non-equilibrium thermodynamic states. This efect called thermo-elasticity is well known in solid materials. The observation of a thermal coupling to the mechanical shear deformation reinforces the assumption of elastically correlated liquid molecules. The amplitude of the thermo-elastic waves increases linearly by increasing the shear strain amplitude up to a transition to a non-linear thermal behavior, similar to a transition from an elastic to plastic regime. The thermo-elastic efects do not give rise to any change in stress measurements and thus the dynamic thermal analysis provides unique information about dynamic liquid properties.
 

https://doi.org/10.1515/jnet-2021-0091

Tiago Outerelo Corvo, Antoine Jourdain, Shona O’Brien, Frédéric Restagno, Eric Drockenmuller, and Alexis Chennevière

Poly(ionic liquid)s (PILs), similar to their ionic liquid (IL) analogues, present a nanostructure arising from local interactions. The influence of this nanostructure on the macromolecular conformation of polymer chains is investigated for the first time by means of an extensive use of small-angle neutron scattering on a series of poly(1-vinyl-3-alkylimidazolium)s with varying alkyl side-chain length and counter-anion, both in bulk and in dilute solutions. Radii of gyration are found to increase with the side-chain length in solution as a consequence of crowding interactions between neighboring monomers. In bulk, however, a nonmonotonic evolution of the radius of gyration reflects a change in chain flexibility and a potential screening of electrostatic interactions. Additionally, at a smaller scale, SANS provides an experimental estimation of both the chain diameter and the correlation length between neighboring chains, comparison of which unveils clear evidence of interdigitation of the alkyl side chains. These structural features bring precious insights into the understanding of the dynamic properties of PILs.

https://doi.org/10.1021/acs.macromol.2c00290

Sumit Mehan, Laure Herrmann, Jean-Paul Chapel, Jacques Jestin, Jean-Francois Berret and Fabrice Cousin

We investigate the formation/re-dissociation mechanisms of hybrid complexes made from negatively charged PAA2k coated g-Fe₂O₃ nanoparticles (NP) and positively charged polycations (PDADMAC) in aqueous solution in the regime of very high ionic strength (I). When the building blocks are mixed at large ionic strength (1 M NH₄Cl), the electrostatic interaction is screened and complexation does not occur. If the ionic strength is then lowered down to a targeted ionic strength Itarget, there is a critical threshold Ic = 0.62 M at which complexation occurs, that is independent of the charge ratio Z and the pathway used to reduce salinity (drop-by-drop mixing or fast mixing). If salt is added back up to 1 M, the transition is not reversible and persistent out-of-equilibrium aggregates are formed. The lifetimes of such aggregates depends on Itarget: the closer Itarget to Ic is, the more difficult it is to dissolve the aggregates. Such peculiar behavior is driven by the inner structure of the complexes that are formed after desalting. When Itarget is far below Ic, strong electrostatic interactions induce the formation of dense, compact and frozen aggregates. Such aggregates can only poorly reorganize further on with time, which makes their dissolution upon resalting almost reversible. Conversely, when Itarget is close to Ic more open aggregates are formed due to weaker electrostatic interactions upon desalting. The system can thus rearrange with time to lower its free energy and reach more stable out-of-equilibrium states which are very difficult to dissociate back upon resalting, even at very high ionic strength.

Contact LLB : Fabrice Cousin (LLB/MMB)

"The desalting/salting pathway: a route to form metastable aggregates with tuneable morphologies and lifetimes"
S.Mehan, L. Herrmann, J.-P. Chapel, J. Jestin, J.-F. Berret and  F. Cousin, Soft Matter 17 (2021) 8496-8505.

Michal Swierczewski, Plinio Maroni, Alexis Chenneviere, Mohammad M. Dadras, Lay-Theng Lee, Thomas Bürgi

Nanoscale particles attract much attention due to their size-dependent optical, electrical and chemical properties. Of particular interest are ultrasmall metal nanoclusters which experience strong quantum confinement effect leading to profound changes in the atomic packing structure. The synthesis of these atomically precise metal clusters, typically with metal cores smaller than 2 nm in diameter, makes use of stabilizing functional molecules such as thiol ligands, hence deriving the common name – monolayer-protected clusters (MPCs). The next stage toward applications and at the same time a challenge in the field, is the manipulation and controlled organization of MPCs into two dimensional (2D) superlattices which would exhibit a collective response of the desired kind. Multiple examples of deposition techniques have been extensively studied, including droplet evaporation, spin-coating and chemical vapor deposition. However, a common drawback of all these methods is the failure to form large-scale structures of closely packed particles. Here, we study the formation and deposition of extended thin films of Au₃₈(SC₂H₄Ph)₂₄ nanoclusters onto solid supports by the Langmuir-Blodgett (LB) method. A combination of techniques, atomic force microscopy (AFM), high magnification transmission electron microscopy (TEM), X-ray reflectivity (XRR), and grazing incidence wide-angle X-ray scattering (GIWAXS) is applied to reveal the morphology and the degree of vertical and in-plane ordering of the transferred films. We find that while a degree of order is initially obtained between the clusters, temporal annealing of the compressed films successfully removes mesoscopic defects between islands of nanoclusters but that it does so at the cost of reducing the local order within the domains. To our knowledge, this is the first reported example of the deposition on an extended scale (several cm²) of ordered gold nanoclusters in the small size regime of 1 – 2 nm.

https://doi.org/10.1002/smll.202005954

Liu Chang; Hu Wenxian; Jiang Hanqiu; Liu Guoming; Han Charles; Sirringhaus Henning; Boué François; Wang Dujin

The determination of intrinsic chain stiffness of conjugated polymers is challenging, in particular, for scattering techniques because of their strong light absorption and structural instability due to the complicated intra-/intermolecular interactions. In this work, the chain conformation and aggregation formation of a high charge mobility donor–acceptor polymer (DPPDTT) are systematically investigated by using small-angle neutron scattering (SANS) and static/dynamic light scattering (SLS/DLS). On the one hand, chloroform was chosen as a good solvent, in which SANS reveals a rod-like geometry with a radius of ∼15 Å. Once the absorption effect is properly accounted for, SLS shows a power law of 1 between the radius of gyration (R_g) and molecular weight (M_w) and a negative second virial coefficient (A₂). On the other hand, 1,2-dichlorobenzene was chosen as a poor solvent, in which SANS, SLS/DLS, and atomic force microscopy (AFM) reveal a strong temperature-/concentration-dependent assembling behavior. The results provide a general picture of the multiscale assembly process of conjugated polymers.

Chang Liu, Wenxian Hu, Hanqiu Jiang, Guoming Liu*, Charles C. Han, Henning Sirringhaus, François Boué, and Dujin Wang

The determination of intrinsic chain stiffness of conjugated polymers is challenging, in particular, for scattering techniques because of their strong light absorption and structural instability due to the complicated intra-/intermolecular interactions. In this work, the chain conformation and aggregation formation of a high charge mobility donor–acceptor polymer (DPPDTT) are systematically investigated by using small-angle neutron scattering (SANS) and static/dynamic light scattering (SLS/DLS). On the one hand, chloroform was chosen as a good solvent, in which SANS reveals a rod-like geometry with a radius of ∼15 Å. Once the absorption effect is properly accounted for, SLS shows a power law of 1 between the radius of gyration (R_g) and molecular weight (M_w) and a negative second virial coefficient (A₂). On the other hand, 1,2-dichlorobenzene was chosen as a poor solvent, in which SANS, SLS/DLS, and atomic force microscopy (AFM) reveal a strong temperature-/concentration-dependent assembling behavior. The results provide a general picture of the multiscale assembly process of conjugated polymers.

https://doi.org/10.1021/acs.macromol.0c01646

A. K. Nayak, V. Kumar, T. Ma, P. Werner, E. Pippel, R. Sahoo, F. Damay, U. K. Rößler, C. Felser and S. S. P. Parkin,  Nature 548, 561–566.

Magnetic skyrmions are topologically stable, vortex-like objects surrounded by chiral boundaries that separate a region of reversed magnetization from the surrounding magnetized material. They are closely related to nanoscopic chiral magnetic domain walls, which could be used as memory and logic elements for conventional and neuromorphic computing applications that go beyond Moore’s law. Of particular interest is ‘racetrack memory’, which is composed of vertical magnetic nanowires, each accommodating of the order of 100 domain walls, and that shows promise as a solid state, non-volatile memory with exceptional capacity and performance. Its performance is derived from the very high speeds (up to one kilometre per second) at which chiral domain walls can be moved with nanosecond current pulses in synthetic antiferromagnet racetracks. Because skyrmions are essentially composed of a pair of chiral domain walls closed in on themselves, but are, in principle, more stable to perturbations than the component domain walls themselves, they are attractive for use in spintronic applications, notably racetrack memory. Stabilization of skyrmions has generally been achieved in systems with broken inversion symmetry, in which the asymmetric Dzyaloshinskii–Moriya interaction modifies the uniform magnetic state to a swirling state. Depending on the crystal symmetry, two distinct types of skyrmions have been observed experimentally, namely, Bloch and Néel skyrmions. Here we present the experimental manifestation of another type of skyrmion—the magnetic antiskyrmion—in acentric tetragonal Heusler compounds with D_2d crystal symmetry. Antiskyrmions are characterized by boundary walls that have alternating Bloch and Néel type as one traces around the boundary. A spiral magnetic ground-state, which propagates in the tetragonal basal plane, is transformed into an antiskyrmion lattice state under magnetic fields applied along the tetragonal axis over a wide range of temperatures. Direct imaging by Lorentz transmission electron microscopy shows field-stabilized antiskyrmion lattices and isolated antiskyrmions from 100 kelvin to well beyond room temperature, and zero-field metastable antiskyrmions at low temperatures. These results enlarge the family of magnetic skyrmions and pave the way to the engineering of complex bespoke designed skyrmionic structures.