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Register a new userNanoconfined fluids: Uniqueness of water compared to other liquids
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Nanoconfinement can drastically change the behavior of liquids, puzzling us with counterintuitive properties. Moreover, it is relevant in applications, including decontamination and crystallization control. It still lacks a systematic analysis for fluids with different bulk properties. Here we fill this gap. We compare, by molecular dynamics simulations, three different liquids in a graphene slit pore: (A) A simple fluid, such as argon, described by a Lennard-Jones potential; (B) An anomalous fluid, such as a liquid metal, modeled with an isotropic core-softened potential; (C) Water, the prototypical anomalous liquid, with directional hydrogen bonds. We study how the slit-pore width affects the structure, thermodynamics, and dynamics of the fluids. We check that all the fluids, as expected, show similar oscillating properties by changing the pore size. However, the nature of the free-energy minima for the three fluids is quite different: i) only for the simple liquid all the minima are energy-driven, while their structural order increases with decreasing slit-pore width; ii) only for the isotropic core-softened potential all the minima are entropy-driven, while the energy in the minima increases with decreasing slit-pore width; iii) only the water has a changing nature of the minima: the monolayer minimum is entropy-driven, at variance with the simple liquid, while the bilayer minimum is energy-driven, at variance with the other anomalous liquid. Also, water diffusion has a large increase for sub-nm slit-pores, becoming faster than bulk. Instead, the other two fluids have diffusion oscillations much smaller than water slowing down for decreasing slit-pore width. Our results clarify that nanoconfined water is unique compared to other (simple or anomalous) fluids under similar confinement, and are possibly relevant in nanopores applications, e.g., in water purification from contaminants.
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We study in this paper the possible existence of Roskilde-simple liquids and their isomorphs in a rough-wall nanoconfinement. Isomorphs are curves in the thermodynamic phase diagram along which structure and dynamics are invariant in suitable nondimensionalized units. Two model liquids using molecular dynamics computer simulations are considered: the single-component Lennard-Jones (LJ) liquid and the Kob-Andersen binary LJ mixture, both of which in the bulk phases are known to have isomorphs. Nanoconfinement is implemented by adopting a slit-pore geometry with fcc crystalline walls; this implies inhomogenous density profiles both parallel and perpendicular to the confining walls. Despite this fact and consistent with an earlier study [Ingebrigtsen et. al, Phys. Rev. Lett. 111, 235901 (2013)] we find that these nanoconfined liquids have isomorphs to a good approximation. More specifically, we show good scaling of inhomogenous density profiles, mean-square displacements, and higher-order structures probed using the topological cluster classification algorithm along the isomorphs. From this study, we conjecture that in experiments, Roskilde-simple liquids may exhibit isomorphs if confined in a suitable manner, for example with carbon nanotubes. Our study thus provides an alternative framework for understanding nanoconfined liquids.
Water provides the driving force for the assembly and stability of many cellular components. Despite its impact on biological functions, a nanoscale understanding of the relationship between its structure and dynamics under soft confinement has remained elusive. As expected, water in contact with biological membranes recovers its bulk density and dynamics at $sim 1$ nm from phospholipid headgroups but surprisingly enhances its intermediate-range order (IRO) over a distance, at least, twice as large. Here, we explore how the IRO is related to the waters hydrogen bond network (HBN) and its coordination defects. We characterize the increased IRO by an alteration of the HBN up to more than eight coordination shells of hydration water. The HBN analysis emphasizes the existence of a bound-unbound water interface at $sim 0.8$ nm from the membrane. The unbound water has a distribution of defects intermediate between bound and bulk water, but with density and dynamics similar to bulk, while bound water has reduced thermal energy and much more HBN defects than low-temperature water. This observation could be fundamental for developing nanoscale models of biological interactions and for understanding how alteration of the water structure and topology, for example, due to changes in extracellular ions concentration, could affect diseases and signaling. More generally, it gives us a different perspective to study nanoconfined water.
We have investigated the dynamics of water confined in mesostructured porous silicas (SBA-15, MCM-41) and four periodic mesoporous organosilicas (PMOs) by dielectric relaxation spectroscopy. The influence of water-surface interaction has been controlled by the carefully designed surface chemistry of PMOs that involved organic bridges connecting silica moieties with different repetition lengths, hydrophilicity and H-bonding capability. Relaxation processes attributed to the rotational motions of non-freezable water located in the vicinity of the pore surface were studied in the temperature range from 140 K to 225 K. Two distinct situations were achieved depending on the hydration level: at low relative humidity (33% RH), water formed a non-freezable layer adsorbed on the pore surface. At 75% RH, water formed an interfacial liquid layer sandwiched between the pore surface and the ice crystallized in the pore center. In the two cases, the study revealed different water dynamics and different dependence on the surface chemistry. We infer that these findings illustrate the respective importance of water-water and water-surface interactions in determining the dynamics of the interfacial liquid-like water and the adsorbed water molecules, as well as the nature of the different H-bonding sites present on the pore surface.
We study the adsorption of ideal random lattice copolymers with correlations in the sequences on homogeneous substrates with two different methods: An analytical solution of the problem based on the constrained annealed approximation introduced by Morita in 1964 and the generating functional (GF) technique, and direct numerical simulations of lattice chains averaged over many realizations of random sequences. Both methods allow to calculate the free energy and different conformational characteristics of the adsorbed chain. The comparison of the results for random copolymers with different degree of correlations and different types of nonadsorbing monomers (neutral or repelling from the surface) shows not only qualitative but a very good quantitative agreement, especially in the cases of Bernoullian and quasi-alternating random sequences.
About a century ago, Born proposed a possible matter of state, ferroelectric fluid, might exist if the dipole moment is strong enough. The experimental realisation of such states needs magnifying molecular polar nature to macroscopic scales in liquids. Here, we report on the discovery of a novel chiral liquid matter state, dubbed chiral ferronematic, stabilized by the local ferroelectric ordering coupled to the chiral helicity. It carries the polar vector rotating helically, corresponding to a helieletric structure, analogous to the magnetic counterpart of helimagnet. The state can be retained down to room-temperature and demonstrates gigantic dielectric and nonlinear optical responses. The novel matter state opens a new chapter for exploring the material space of the diverse ferroelectric liquids.
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