Nanoscale Simulation of Plastic Contaminants Migration in Packaging Materials and Potential Leaching into Model Food Fluids.

Polymers are the most commonly used packaging materials for nutrition and consumer products. The ever-growing concern over pollution and potential environmental contamination generated from single-use packaging materials has raised safety questions. Polymers used in these materials often contain impurities, including unreacted monomers and small oligomers. The characterization of transport properties, including diffusion and leaching of these molecules, is largely hampered by the long timescales involved in shelf life experiments. In this work, we employ atomistic molecular simulation techniques to explore the main mechanisms involved in the bulk and interfacial transport of monomer molecules from three polymers commonly employed as packaging materials: polyamide-6, polycarbonate, and poly(methyl methacrylate). Our simulations showed that both hopping and continuous diffusion play important roles in inbound monomer diffusion and that solvent-polymer compatibility significantly affects monomer leaching. These results provide rationalization for monomer leaching in model food formulations as well as bulky industry-relevant molecules. Through this molecular-scale characterization, we offer insights to aid in the design of polymer/consumer product interfaces with reduced risk of contamination and longer shelf life.

Molecular-Scale Exploration of Mechanical Properties and Interactions of Poly(lactic acid) with Cellulose and Chitin.

Poly(lactic acid) (PLA), one of the pillars of the current overarching displacement trend switching from fossil- to natural-based polymers, is often used in association with polysaccharides to increase its mechanical properties. However, the use of PLA/polysaccharide composites is greatly hampered by their poor miscibility, whose underlying nature is still vastly unexplored. This work aims to shed light on the interactions of PLA and two representative polysaccharide molecules (cellulose and chitin) and reveal structure-property relationships from a fundamental perspective using atomistic molecular dynamics. Our computational strategy was able to reproduce key experimental mechanical properties of pure and/or composite materials, reveal a decrease in immiscibility in PLA/chitin compared to PLA/cellulose associations, assert PLA-oriented polysaccharide reorientations, and explore how less effective PLA-polysaccharide hydrogen bonds are related to the poor PLA/polysaccharide miscibility. The connection between the detailed chemical interactions and the composite behavior found in this work is beneficial to the discovery of new biodegradable and natural polymer composite mixtures that can provide needed performance characteristics.

Exploring the Effects of Wetting and Free Fatty Acid Deposition on an Atomistic Hair Fiber Surface Model Incorporating Keratin-Associated Protein 5-1.

The complex development of cosmetic and medical formulations relies on an ever-growing accuracy of predictive models of hair surfaces. Hitherto, modeling efforts have focused on the description of 18-methyl eicosanoic acid (18-MEA), the primary fatty acid covalently attached to the hair surface, without explicit modeling of the protein layer. Herein, the molecular details of the outermost surface of the human hair fiber surface, also called the F-layer, were studied using molecular dynamics (MD) simulations. The F-layer is composed primarily of keratin-associated proteins KAP5 and KAP10, which are decorated with 18-MEA on the outer surface of a hair fiber. In our molecular model, we incorporated KAP5-1 and evaluated the surface properties of 18-MEA through MD simulations, resulting in 18-MEA surface density, layer thickness, and tilt angles in agreement with previous experimental and computational studies. Subsequent models with reduced 18-MEA surface density were also generated to mimic damaged hair surfaces. Response to wetting of virgin and damaged hair showed rearrangement of 18-MEA on the surface, allowing for water penetration into the protein layer. To demonstrate a potential use case for these atomistic models, we deposited naturally occurring fatty acids and measured 18-MEA's response in both dry and wet conditions. As fatty acids are often incorporated in shampoo formulations, this work demonstrates the ability to model the adsorption of ingredients on hair surfaces. This study illustrates, for the first time, the complex behavior of a realistic F-layer at the molecular level and opens up the possibility of studying the adsorption behavior of larger, more complex molecules and formulations.

Interfacial study of clathrates confined in reversed silica pores.

Storing methane in clathrates is one of the most promising alternatives for transporting natural gas (NG) as it offers similar gas densities to liquefied and compressed NG while offering lower safety risks. However, the practical use of clathrates is limited given the extremely low temperatures and high pressures necessary to form these structures. Therefore, it has been suggested to confine clathrates in nanoporous materials, as this can facilitate clathrate's formation conditions while preserving its CH4 volumetric storage. Yet, the choice of nanoporous materials to be employed as the clathrate growing platform is still rather arbitrary. Herein, we tackle this challenge in a systematic way by computationally exploring the stability of clathrates confined in alkyl-grafted silica materials with different pore sizes, ligand densities and ligand types. Based on our findings, we are able to propose key design criteria for nanoporous materials favoring the stability of a neighbouring clathrate phase, namely large pore sizes, high ligand densities, and smooth pore walls. We hope that the atomistic insight provided in this work will guide and facilitate the development of new nanomaterials designed to promote the formation of clathrates.

Water adsorption fingerprinting of structural defects/capping functions in Zr-fumarate MOFs: a hybrid computational-experimental approach.

Engineering structural defects in MOFs has been intensively applied to modulate their adsorption-related properties. Zr-fumarate MOF (also known as MOF-801) is a prototypical defective MOF with proven versatile adsorption/separation performances depending on the synthetic conditions, however the relationship between the nature/concentration of both structure defects/capping functions and its adsorption features is still far from being fully understood. In this work, we first present a systematic theoretical exploration of the individual contributions of linker and cluster defects as well as of the capping functions to the overall water adsorption profile of the MOF-801 framework. This computational effort based on the construction of defective structure models and the use of Grand Canonical Monte Carlo simulations further enabled the identification of the overarching defective structure for two MOF-801 samples based on their experimental adsorption isotherms reported previously. An experimental effort was then deployed to synthesize two Zr-fumarate MOF samples with controlled nature and concentration of structural defects as well as capping functions. This computational-experimental hybrid strategy revealed the water adsorption isotherm as a fingerprint of the nature and concentration of structural defect/capping groups exhibited by the MOF adsorbent. We expect this study to deliver meaningful insights to further design MOFs with target adsorption features through a rational engineering of structural defects.

Defective Zr-Fumarate MOFs Enable High-Efficiency Adsorption Heat Allocations.

Adsorption-driven heat transfer devices incorporating an efficient "adsorbent-water" working pair are attracting great attention as a green and sustainable technology to address the huge global energy demands for cooling and heating. Herein, we report the improved heat transfer performance of a defective Zr fumarate metal-organic framework (MOF) prepared in a water solvent (Zr-Fum HT). This material exhibits an S-shaped water sorption isotherm (P/P0 = 0.05-0.2), excellent working capacity (0.497 mLH2O mL-1MOF) under adsorption-driven cooling/chiller working conditions (Tadsorption(ads) = 30 °C, Tcondensation (con) = 30 °C, and Tdesorption(des) = 80 °C), very high coefficient of performances for both cooling (0.83) and heating (1.76) together with a relatively low driving temperature at 80 °C, a remarkable heat storage capacity (423.6 kW h m-3MOF), and an outstanding evaporation heat (343.8 kW h m-3MOF). The level of performance of the resultant Zr-Fum HT MOF is above those of all existing benchmark water adsorbents including MOF-801 previously synthesized in the N,N-dimethylformamide solvent under regeneration at 80 °C which is accessible from the solar source. This is coupled with many other decisive advantages including green synthesis and high proven chemical and mechanical robustness. The microscopic water adsorption mechanism of Zr-Fum HT at the origin of its excellent water adsorption performance was further explored computationally based on the construction of an atomistic defective model online with the experimental data gained from a subtle combination of characterization techniques.

Unravelling the water adsorption in a robust iron carboxylate metal-organic framework.

A Fe-MOF was obtained from aqueous solution in high yield under reflux. The water sorption properties were studied by powder X-ray diffraction, volumetric and gravimetric sorption experiments and molecular simulations. The subsequent filling of hydrophobic and hydrophilic pores as well as the stability of the material are demonstrated.

Structure of the Polymer Backbones in polyMOF Materials.

The molecular connectivity of polymer-metal-organic framework (polyMOF) hybrid materials was investigated using density functional theory calculations and solid-state NMR spectroscopy. The architectural constraints that dictate the formation of polyMOFs were assessed by examining poly(1,4-benzenedicarboxylic acid) (pbdc) polymers in two archetypical MOF lattices (UiO-66 and IRMOF-1). Modeling of the polyMOFs showed that in the IRMOF-1-type lattice, six, seven, and eight methylene (-CH2-) groups between 1,4-benzenedicarboxylate (terephthalate, bdc2-) units can be accommodated without significant distortions, while in the UiO-66-type lattice, an optimal spacing of seven methylene groups between bdc2- units is needed to minimize strain. Solid-state NMR supports these predictions and reveals pronounced spectral differences for the same polymer in the two polyMOF lattices. With seven methylene groups, polyUiO-66-7a shows 7 ± 3% of uncoordinated terephthalate linkers, while these are undetectable (<4%) in the corresponding polyIRMOF-1-7a. In addition, NMR-detected backbone mobility is significantly higher in the polyIRMOF-1-7a than in the corresponding polyUiO-66-7a, again indicative of taut chains in the latter.

Controlled Transdermal Release of Antioxidant Ferulate by a Porous Sc(III) MOF.

The Sc(III) MOF-type MFM-300(Sc) is demonstrated in this study to be stable under physiological conditions (PBS), biocompatible (to human skin cells), and an efficient drug carrier for the long-term controlled release (through human skin) of antioxidant ferulate. MFM-300(Sc) also preserves the antioxidant pharmacological effects of ferulate while enhancing the bio-preservation of dermal skin fibroblasts, during the delivery process. These discoveries pave the way toward the extended use of Sc(III)-based MOFs as drug delivery systems (DDSs).

Thermo-Responsive MOF/Polymer Composites for Temperature-Mediated Water Capture and Release.

We report an in situ polymerization strategy to incorporate a thermo-responsive polymer, poly(N-isopropylacrylamide) (PNIPAM), with controlled loadings into the cavity of a mesoporous metal-organic framework (MOF), MIL-101(Cr). The resulting MOF/polymer composites exhibit an unprecedented temperature-triggered water capture and release behavior originating from the thermo-responsive phase transition of the PNIPAM component. This result sheds light on the development of stimuli-responsive porous adsorbent materials for water capture and heat transfer applications under relatively mild operating conditions.

Porous Metal-Organic Framework CUK-1 for Adsorption Heat Allocation toward Green Applications of Natural Refrigerant Water.

The development of new water adsorbents that are hydrothermally stable and can operate more efficiently than existing materials is essential for the advancement of water adsorption-driven chillers. Most of the existing benchmark materials and related systems in this field suffer from clear limitations that must be overcome to meet global requirements for sustainable and green energy production and utilization. Here, we report the energy-efficient water sorption properties of three isostructural metal-organic frameworks (MOFs) based on the simple ligand pyridine-2,4-dicarboxylate, named M-CUK-1 [M3(µ3-OH)2(2,4-pdc)2] (where M = Co2+, Ni2+, or Mg2+). The highly hydrothermally stable CUK-1 series feature step-like water adsorption isotherms, relatively high H2O sorption capacities between P/P0 = 0.10-0.25, stable cycling, facile regeneration, and, most importantly, benchmark coefficient of performance values for cooling and heating at a low driving temperature. Furthermore, these MOFs are prepared under green hydrothermal conditions in aqueous solutions. Our joint experimental-computational approach revealed that M-CUK-1 integrates several optimal features, resulting in promising materials as advanced water adsorbents for adsorption-driven cooling and heating applications.

Humidity-induced CO2 capture enhancement in Mg-CUK-1.

Kinetic CO2 adsorption measurements in the water-stable and permanently microporous Metal-organic framework material, Mg-CUK-1, reveal a 1.8-fold increase in CO2 capture from 4.6 wt% to 8.5 wt% in the presence of 18% relative humidity. Thermodynamic CO2 uptake experiments corroborate this enhancement effect, while grand canonical Monte Carlo simulations also support the phenomenon of a humidity-induced increase in the CO2 sorption capacity in Mg-CUK-1. Molecular simulations were implemented to gain insight into the microscopic adsorption mechanism responsible for the observed CO2 sorption enhancement. These simulations indicate that the cause of increasing CO2 adsorption enthalpy in the presence of H2O is due to favorable intermolecular interactions between the co-adsorbates confined within the micropores of Mg-CUK-1.

Achieving Superprotonic Conduction with a 2D Fluorinated Metal-Organic Framework.

A hydrolytically stable metal-organic framework (MOF) material, named KAUST-7', was derived from a structural phase change of KAUST-7 upon exposure to conditions akin to protonic conduction (363 K/95% relative humidity). KAUST 7' exhibited a superprotonic conductivity as evidenced by the impedance spectroscopic measurement revealing an exceptional conductivity up to 2.0 × 10-2 S cm-1 at 363 K and under 95% RH, a performance maintained over 7 days. Ab initio molecular dynamics simulations suggested that the water-mediated proton transport mechanism is governed by water assisted reorganization of the H-bond network involving the fluorine moieties in KAUST-7' and the guest water molecules. The notable level of performances combined with a very good hydrolytic stability positions KAUST-7' as a prospective proton-exchange membrane alternative to the commercial benchmark Nafion. Furthermore, the remarkable RH sensitivity of KAUST-7' conductivity, substantially higher than previously reported MOFs, offers great opportunities for deployment as a humidity sensor.

Confined methanol within InOF-1: CO2 capture enhancement.

The CO2 capture performance of InOF-1 was optimised by confining small amounts of MeOH within its micropores (MeOH@InOF-1). In comparison with fully activated InOF-1, MeOH@InOF-1 shows a 1.30 and 4.88-fold increase in CO2 capture capacity for kinetic and static isothermal CO2 adsorption experiments respectively. Density functional theory calculations coupled with forcefield based-Monte Carlo simulations revealed that such an enhancement is assigned to an increase of the degree of confinement felt by the CO2 molecules resulting from the formation of a lump at the vicinity of the µ2-OH groups since MeOH strongly interacts with these adsorption sites and is thus highly localized in this region.

Unusual adsorption site behavior in PCN-14 metal-organic framework predicted from Monte Carlo simulation.

The adsorption equilibrium of methane in PCN-14 was simulated by the Monte Carlo technique in the grand canonical ensemble. A new force field was proposed for the methane/PCN-14 system, and the temperature dependence of the molecular siting was investigated. A detailed study of the statistics of the center of mass and potential energy showed a surprising site behavior with no energy barriers between weak and strong sites, allowing open metal sites to guide methane molecules to other neighboring sites. Moreover, this study showed that a model assuming weakly adsorbing open metal clusters in PCN-14, densely populated only at low temperatures (below 150 K), can explain published experimental data. These results also explain previously observed discrepancies between neutron diffraction experiments and Monte Carlo simulations.