New Structural Anomaly Induced by Nanoconfinement

Core-softened colloid under extreme geometrical confinement

Geometrical constraints offer a promising strategy for assembling colloidal crystal structures that are not typically observed in bulk or under 2D conditions. Core-softened colloids, in particular, have emerged as versatile chemical building blocks with applications across various scientific and technological areas. In this study, we investigate the behavior of a core-softened model confined between two parallel walls. Employing molecular dynamics simulations, we analyze the system's response under extreme confinement, where only one or two layers of colloids are permitted. The system comprises particles modeled by a ramp-like potential confined within slit nanoslits created by two flat, purely repulsive walls with a lateral side L separated by a distance Lz. Through a systematic analysis of the phase behavior as Lz increases, or as the system undergoes decompression, for different values of L, we identified a mono-to-bilayer transition associated with changes in the colloidal structure. In the monolayer regime, we observed solid phases at lower densities than those observed in the 2D case. Importantly, we demonstrated that confinement at specific Lz values, allowing particle arrangement into two layers, can lead to the emergence of the square phase, which was not observed under monolayer or 2D conditions. By correlating thermodynamic, translational, and orientational ordering, as well as the dynamics of this confined colloidal system, our findings offer valuable insights into the utilization of geometrical constraints to induce and manipulate structural changes.

Characterization of Water Structure and Phase Behavior within Metal-Organic Nanotubes

Water behavior under nanoconfinement varies significantly from that in the bulk but also depends on the nature of the pore walls. Hybrid compound offers the ideal system to explore water behavior in complex materials, so a model metal-organic nanotube (UMONT) material was utilized to explore the behavior of water between 100 and 293 K. Single-crystal X-ray and neutron diffraction revealed the formation of a filled Ice-I arrangement that was previously predicted to only occur under high pressures. 17O NMR spectra suggest that the onset of melting for the water in the UMONT channels occurs at 98 K and the presence of ice-like water up to 293 K, indicating that the complete ice-water transition does not occur before dehydration of the material. Overall, the water behavior differs significantly from hydrophobic single-walled carbon nanotubes indicating precise control over water can be achieved through rational design of hybrid materials.

Impacts of Surface Adsorption on Water Uptake within a Metal Organic Nanotube Material

The confinement-dependent properties of solvents, particularly water, within nanoporous spaces impart unique physical and chemical behavior compared to those of the bulk. This has previously been demonstrated for a U(VI)-based metal organic nanotube that displays ice-like arrays of water molecules within the 1-D pore space and complete selectivity to H₂O over all other solvents and isotopologues. Based upon our previous work on D₂O and HTO adsorption processes, we suggested that the water uptake was controlled by a two-step process: (1) surface adsorption via hydrogen bonding to hydrophilic amine and carboxylic groups and (2) diffusion of the water into the hydrophobic 1-D nanochannels. The current study seeks to evaluate this hypothesis and expand our existing kinetic model for the water diffusion step to account for the initial surface adsorption process. Vapor sorption experiments, paired with thermogravimetric and Fourier-transform infrared analyses, yielded uptake data that were fit using a Langmuir model for the surface-adsorption step of the mechanism. The water adsorption curve was designated a type IV Brunauer-Emmett-Teller isotherm, which indicated that our original hypothesis was correct. Additional work with binary solvent systems enabled us to evaluate the uptake in a range of conditions and determine that the uptake is not controlled by the vapor pressure but is instead completely dependent on the relative humidity of the system.

Tracer diffusion in crowded solutions of sticky polymers

Macromolecular diffusion in strongly confined geometries and crowded environments is still to a large extent an open subject in soft matter physics and biology. In this paper, we employ large-scale Langevin dynamics simulations to investigate how the diffusion of a tracer is influenced by the combined action of excluded-volume and weak attractive crowder-tracer interactions. We consider two species of tracers, standard hard-core particles described by the Weeks-Chandler-Andersen (WCA) repulsive potential and core-softened (CS) particles, which model, e.g., globular proteins, charged colloids, and nanoparticles covered by polymeric brushes. These systems are characterized by the presence of two length scales in the interaction and can show waterlike anomalies in their diffusion, stemming from the inherent competition between different length scales. Here we report a comprehensive study of both diffusion and structure of these two tracer species in an environment crowded by quenched configurations of polymers at increasing density. We analyze in detail how the tracer-polymer affinity and the system density affect transport as compared to the emergence of specific static spatial correlations. In particular, we find that, while hardly any differences emerge in the diffusion properties of WCA and CS particles, the propensity to develop structural order for large crowding is strongly frustrated for CS particles. Surprisingly, for large enough affinity for the crowding matrix, the diffusion coefficient of WCA tracers display a nonmonotonic trend as their density is increased when compared to the zero affinity scenario. This waterlike anomaly turns out to be even larger than what observed for CS particle and appears to be rooted in a similar competition between excluded-volume and affinity effects.

Impact of nanoconfinement on acetylacetone Equilibria in Ordered Mesoporous Silicates

Nanoconfinement is one of the most intriguing nanoscale effects and affects several physical and chemical properties of molecules and materials, including viscosity, reaction kinetics, and glass transition temperature. In this work, liquid nuclear magnetic resonance (NMR) was used to analyze the behavior of 2,4-pentadienone in ordered mesoporous materials with a pore diameter of between 3 and 10 nm. The liquid NMR results showed meaningful changes in the hydrogen chemical shift and the keto-enol chemical equilibrium, which were associated with the pore diameter, allowing the authors to observe the effects of nanoconfinement. An interesting phenomenon was observed where the chemical equilibria of 2,4-pentadienone confined in a mesoporous material with a pore diameter of 3.5 nm was similar to that obtained with free (bulk) 2,4-pentadienone in larger pore materials. Another interesting result was observed for the enthalpy and entropy of the tautomeric equilibria of 2,4-pentadienone confined in mesoporous materials with a 5.5 nm pore diameter being -7.9 kJ mol^-1 and -15.9 J mol^-1.K. These values are similar to those obtained by dimethyl sulfoxide. This phenomenon indicates the possible use of ordered mesoporous materials as a reaction substitute in organic solvents. It was further observed that while the values of enthalpy (ΔH) and entropy (ΔS) had been modified by confinement, the Gibbs free energy (ΔG) value remained closer to that observed in free (bulk) 2,4-pentadienone. It is expected that this study will help in understanding the effects of nanoconfinement and provide a simple method to employ NMR techniques to analyze these phenomena.

Controlling water structure and behavior: design principles from metal organic nanotubular materials

Water exhibits unique and unexpected behavioral and structural changes when confined to the nanoscale, notably within the pores of metal–organic nanotubes.

Influence of Cholesterol on the Dynamics of Hydration in Phospholipid Bilayers

We investigate the dynamics of interfacial waters in dipalmitoylphosphatidylcholine (DPPC) bilayers upon the addition of cholesterol, by molecular dynamics simulations. Our data reveal that the inclusion of cholesterol modifies the membrane aqueous interfacial dynamics: waters diffuse faster, their rotational decay time is shorter, and the DPPC/water hydrogen bond dynamics relaxes faster than in the pure DPPC membrane. The observed acceleration of the translational water dynamics agrees with recent experimental results, in which, by means of NMR techniques, an increment of the surface water diffusivity is measured upon the addition of cholesterol. A microscopic analysis of the lipid/water hydrogen bond network at the interfacial region suggests that the mechanism underlying the observed water mobility enhancement is given by the rupture of a fraction of interlipid water bridge hydrogen bonds connecting two different DPPC molecules, concomitant to the formation of new lipid/solvent bonds, whose dynamics is faster than that of the former. The consideration of a simple two-state model for the decay of the hydrogen bond correlation function yielded excellent results, obtaining two well-separated characteristic time scales: a slow one (∼250 ps) associated with bonds linking two DPPC molecules, and a fast one (∼15 ps), related to DPPC/solvent bonds.

Waterlike anomalies in a two-dimensional core-softened potential

We investigate the structural, thermodynamic, and dynamic behavior of a two-dimensional (2D) core-corona system using Langevin dynamics simulations. The particles are modeled by employing a core-softened potential which exhibits waterlike anomalies in three dimensions. In previous studies in a quasi-2D system a new region in the pressure versus temperature phase diagram of structural anomalies was observed. Here we show that for the two-dimensional case two regions in the pressure versus temperature phase diagram with structural, density, and diffusion anomalies are observed. Our findings indicate that, while the anomalous region at lower densities is due the competition between the two length scales in the potential at higher densities, the anomalous region is related to the reentrance of the melting line.

Wettability effect on nanoconfined water flow

Understanding and controlling the flow of water confined in nanopores has tremendous implications in theoretical studies and industrial applications. Here, we propose a simple model for the confined water flow based on the concept of effective slip, which is a linear sum of true slip, depending on a contact angle, and apparent slip, caused by a spatial variation of the confined water viscosity as a function of wettability as well as the nanopore dimension. Results from this model show that the flow capacity of confined water is 10^-1∼10⁷ times that calculated by the no-slip Hagen-Poiseuille equation for nanopores with various contact angles and dimensions, in agreement with the majority of 53 different study cases from the literature. This work further sheds light on a controversy over an increase or decrease in flow capacity from molecular dynamics simulations and experiments.

Anomalous diffusion and diffusion anomaly in confined Janus dumbbells

Self-assembly and dynamical properties of Janus nanoparticles have been studied by molecular dynamic simulations. The nanoparticles are modeled as dimers and they are confined between two flat parallel plates to simulate a thin film. One monomer from the dumbbells interacts by a standard Lennard-Jones potential and the other by a two-length scales shoulder potential, typically used for anomalous fluids. Here, we study the effects of removing the Brownian effects, typical from colloidal systems immersed in aqueous solution, and consider a molecular system, without the drag force and the random collisions from the Brownian motion. Self-assembly and diffusion anomaly are preserved in relation to the Brownian system. Additionally, a superdiffusive regime associated to a collective reorientation in a highly structured phase is observed. Diffusion anomaly and anomalous diffusion are explained in the two length scale framework.

Confinement effects on the properties of Janus dimers

We show how the confinement between two parallel walls affects the self-assembly, and dynamic and thermodynamic properties of Janus dumbbells.

Random pinning changes the melting scenario of a two-dimensional core-softened potential system

In experiments two-dimensional systems are realized mainly on solid substrates, which introduce quenched disorder due to some inherent defects. The defects of substrates influence the melting scenario of the systems and have to be taken into account in the interpretation of experimental results. We present the results of molecular dynamics simulations of a two-dimensional system with a core-softened potential in which a small fraction of the particles is pinned, inducing quenched disorder. Ppotentials of this type are widely used for the qualitative description of systems with waterlike anomalies. In our previous publications it was shown that the system demonstrates an anomalous melting scenario: at low densities the system melts through two continuous transitions in accordance with the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory with an intermediate hexatic phase, while at high densities the conventional first-order melting transition takes place. We find that the well-known disorder-induced widening of the hexatic phase occurs at low densities, while in the high-density part of the phase diagram random pinning transforms the first-order melting into two transitions: a continuous KTHNY-like solid-hexatic transition and a first-order hexatic-isotropic liquid transition.