Environmental Applications of Engineered Materials with Nanoconfinement

  • Shuo Zhang
    Shuo Zhang
    Department of Chemical and Environmental Engineering, Yale University, 17 Hillhouse Ave., New Haven, Connecticut 06511, United States
    More by Shuo Zhang
  • Tayler Hedtke
    Tayler Hedtke
    Department of Chemical and Environmental Engineering, Yale University, 17 Hillhouse Ave., New Haven, Connecticut 06511, United States
  • Xuechen Zhou
    Xuechen Zhou
    Department of Chemical and Environmental Engineering, Yale University, 17 Hillhouse Ave., New Haven, Connecticut 06511, United States
    More by Xuechen Zhou
  • Menachem Elimelech
    Menachem Elimelech
    Department of Chemical and Environmental Engineering, Yale University, 17 Hillhouse Ave., New Haven, Connecticut 06511, United States
  • , and 
  • Jae-Hong Kim*
    Jae-Hong Kim
    Department of Chemical and Environmental Engineering, Yale University, 17 Hillhouse Ave., New Haven, Connecticut 06511, United States
    *Phone: +1 (203) 432-4386. Email: [email protected]
    More by Jae-Hong Kim
Cite this: ACS EST Engg. 2021, 1, 4, 706–724
Publication Date (Web):March 10, 2021
https://doi.org/10.1021/acsestengg.1c00007
Copyright © 2021 The Authors. Published by American Chemical Society
ACS AuthorChoiceCC: Creative CommonsBY: Credit must be given to the creatorNC: Only noncommercial uses of the work are permittedND: No derivatives or adaptations of the work are permitted
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Abstract

Engineered nanoporous materials have been extensively employed in the environmental field to take advantage of increased surface area and tunable size exclusion. Beyond those benefits, recent studies have uncovered that the confinement of traditional environmental processes within several nanometer pores exerts unique nanoconfinement effects, such as enhanced adsorption capacity, reaction kinetics, and ion selectivity, compared to their analogous processes without spatial confinement. In this review, we provide a systematic discussion covering the current understanding of nanoconfinement effects reported across diverse fields using similar materials and structures as those being explored in environmental technologies. We further abstract the underlying fundamental physical and chemical principles including molecular orientation and rearrangement, reactive center creation, noncovalent binding, and partial desolvation. Finally, we establish connections between promising nanoconfinement observations and traditional environmental processes to identify challenges and opportunities for the development of innovative functional platforms for environmental applications.

Introduction

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Particles, molecules, and ions that are confined in a nanoscale space created by a porous scaffold exhibit chemical and physical properties that are substantially different from their properties in the bulk phase. These changes occur on multiple fronts, from their individual behavior (e.g., conformation, solvation, and redox capability) (1) to the overall phase properties (e.g., density and viscosity). (2,3) These phenomena are referred to as nanoconfinement effects, which is a concept that was first introduced by the pioneering work that demonstrated the change in reaction rate law by the spatial confinement of reactants. (4) The study of nanoconfinement has expanded in the past couple of decades in a wide range of fields, encompassing molecular and nanomaterial synthesis, (5,6) energy conversion and storage, (7,8) mass transport and phase segregation, (9,10) and biochemistry and biomedicine, (11−13) and inside various porous scaffolds, either natural or artificial, in the form of nanoscopic holes, cavities, or well-defined nanometric architectures, such as tubes or channels. Results from these studies show clear advances in system performance compared to bulk phase counterparts, based on kinetic and thermodynamic behaviors, such as reaction rate acceleration, (14) reaction selectivity shift or enhancement, (15) stabilization of intermediate species in favor of chain reactions, (16) and even the development of new chemical reaction pathways. (17,18)
In recent years, the study of nanoconfinement effects began to infiltrate the environmental field, with the emergence of several unique phenomena related to water, (19) air, (20) and soil (21) remediation, within the realm of prevailing environmental technologies, such as adsorption, catalysis, and membrane separation. The phenomena that occur inside the well-designed nanopores of functional materials provide additional features that are beneficial to the objectives of these processes, beyond increased surface area and size exclusion, which are expected with size reduction. Some notable examples of additional features with environmental significance include (i) polyaniline confined inside a nanoscale polystyrene scaffold achieving significantly enhanced selective removal of Cr(VI) versus Cr(III), compared to bulk polyaniline, (22) (ii) confinement of ions in pores smaller than 2 nm allowing highly selective transport of K+ ions through membranes due to changes in ion–surface interactions, (23) and (iii) confinement of heterogeneous Fenton reactions inside nanochannels significantly enhancing the gross oxidation kinetics up to 820 times greater than the same reaction in bulk solution without pore confinement. (24)
The fundamental mechanisms attributed to these effects are complex and are dependent on reported systems and application targets. For example, when considering the removal of pollutants in water treatment, a confined reaction system involves a series of interplaying factors, such as the hydration properties of a pollutant, the coordination environment of surface functional groups or metallic sites, the pollutant–surface interactions, the interference of coexisting ions, and the redox potential. The properties of the background water molecules can also be modulated by nanoconfinement, giving rise to the formation of strong hydrogen-bonded water networks, (25) reduced mobility of water molecules, (26) phase transition to ice-like crystalline structures, (27,28) reduced dielectric constant, (29) and enhanced bonding to surfaces. (30) This example illustrates the complexity and lingering uncertainties surrounding the interpretation of confinement-based environmental systems for even the most pervasive molecules.
While the concepts of nanoconfinement and its effects have been explored across vastly diverse fields of study with varying objectives, the underlying principles are rooted in the same physical and chemical fundamentals. Therefore, attempts to advance environmental technologies by exploiting nanoconfinement effects must examine basic and applied studies performed across disparate fields that use well-defined systems and advanced detection/analytical techniques. Some nanoconfinement phenomena in a remote field of study could potentially have implications for environmental applications, spurring derivative research topics. For instance, the confined perpendicular orientation of benzene molecules inside a hydroxylated silica nanopore (31) may inspire the development of silica-based mesoporous materials for high-capacity adsorptive removal of benzoic pollutants in industrial wastewaters. The ability to produce coordinatively unsaturated transition metal sites inside porous materials (32) would extend their catalytic function to attract and activate oxidant precursors, such as hydrogen peroxide and ozone, to produce reactive radicals to degrade organic pollutants in water. Consequently, linking currently documented nanoconfinement effects to environmental technologies from the dual perspectives of fundamental studies and technology applications has many potential benefits.
This review focuses on nanoconfinement effects, ranging from technological advances and relevant fundamental studies in diverse fields such as hydrogen storage, CO2 conversion, biochemical processes, and chemical synthesis to environmental applications. First, we provide a systematic discussion of the current understanding of nanoconfinement effects, including a summary of scaffolds, typical applications across a wide range of fields, and the fundamentals behind the unique phenomena discovered so far. Then, we discuss notable confinement phenomena observed from the implementation of various environmental technologies and consider their implications based on the previously described fundamentals. Lastly, we present the opportunities and roadblocks facing the study of nanoconfinement as an innovative mechanistic tool for future environmental technology development.

Nanoconfinement Architecture

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Diverse porous scaffolds with varying morphology, dimensions, and material composition, either natural or artificial, have been employed for the investigation and application of nanoconfinement. In general, scaffolds reported in confinement studies can be categorized by dimension as follows (Figure 1): (i) individual one-dimensional hollow space in nanotubes or nanofibers, (ii) two-dimensional film-like space created by layers of sheets, intertwisted fibers, or mesh-like structures, and (iii) three-dimensional voids in topological or hierarchical porous structures that appear as porous structures, particles, or permeable membranes.

Figure 1

Figure 1. Classification of nanostructures that have been reported to exert nanoconfinement effects. The structures can be categorized as one-, two-, or three-dimensional confinement architectures. Basic geometries visualize the structures and indicate the dimension that induces nanoconfinement with a red line. Nanospaces can experience wide variation in degree of uniformity and size beyond what is shown in the models depending on material composition and synthesis method. Common materials for each structure are presented beneath each image. The selected samples are from the referenced literature although other combinations are also possible.

The structure and material composition of a nanostructure are the most important factors impacting its physical properties (e.g., morphology, pore size and distribution, mechanical strength, and electron mobility), while the surface chemistry strongly affects the bonding and coordination environments of internal processes. Materials can also impart reactive functionality to the scaffold. For example, materials such as TiO2 nanotubes, (33) carbon nanotubes, (34) and nanoporous polymers (35) are used as scaffolds to enable specific catalytic functions under nanoconfinement. Other materials that are unreactive but have well-defined pore structures can create spaces for homogeneous reactions or be further functionalized to encapsulate molecular or nanoscale catalysts to perform heterogeneous catalysis. In other systems, the desired trait for the scaffold is its response to process conditions, such as reactions that require extreme conditions, which place limitations on materials. For instance, while metal-based materials cannot support highly acidic liquid-phase reactions, they can accommodate highly oxidative or high-temperature reactions that can destroy the majority of organic scaffolds. Unreactive materials are also useful for physical processes, including adsorption and membrane transport, where confinement induces altered properties demonstrated by stronger host–guest interactions. Such interactions have been found to be beneficial for enhanced adsorption selectivity of metal–organic frameworks (MOFs) (36) and water permeation across graphene-based membranes. (37) Therefore, we categorize reported scaffolds based on material composition and discuss each category separately with respect to their unique properties that contribute to confinement studies.

Carbon

Carbon-based materials account for a large fraction of scaffolds that have been associated with nanoconfinement effects in the literature. They range widely in size and morphology from small, graphitized units to amorphous, mesoporous bulk scaffolds. Among these scaffolds, carbon nanotubes have been most frequently employed as nanoreactors to confine catalytic reactions within their 1–8 nm hollow space. (38−42) Pore diameters within this range can be readily controlled during chemical vapor deposition (43) or arc-discharge (44) syntheses. Graphite’s hexagonal, covalent bonding network with delocalized π-electrons is electrically conductive, thus allowing electron transfer with guest molecules confined inside the nanotubes and enabling redox reactions. (45) The graphene wall structure is atomically smooth and intrinsically hydrophobic, which is useful for studying water molecule transport in the absence of surface interactions. (37) Carbon nanotubes also exhibit high thermal stability (up to 700 °C in air and ∼2800 °C in vacuum), enabling internal high-temperature synthesis. (46) Other graphene-based nanostructures, which have different pore sizes but retain properties similar to those of carbon nanotubes, were also employed as scaffolds for confinement studies, including tubular hollow carbon nanofibers (internal diameter of 50–60 nm) (47,48) and planar structures, such as graphite nanoplatelets (pore diameter of <50 nm) (49) and graphene oxide layers (interlayer spacing of 3.7–5 nm). (50,51)
Mesoporous carbon materials with amorphous or partially amorphous surfaces are another class of widely studied carbon scaffold. Notable examples include carbon aerogels (pore diameters of 2.8–30 nm) (52−54) with a network structure of interconnected nanovoids, templated mesoporous carbons (pore diameters of 4–10 nm), (55−57) and hierarchically porous carbon with multimodal pore size distributions (mean pore diameters of approximately 0.8–3.2 nm). (58,59) Synthesized through carbonization, they typically contain reactive carboxyl, carbonyl, and/or hydroxyl surface groups, allowing surface-mediated catalysis to occur under nanoconfinement.

Metals and Metal Oxides

Various metal or metal oxide scaffolds have been synthesized with well-defined porous structures, taking advantage of a wide range of available synthesis methods. For example, using nanoscale lithography following nanoparticle self-assembly, a nanomesh (pore diameter of 160 nm) of Ge doped with Mn was synthesized for precise control of an electric field. (60) Atomically precise protrusions can be grown on metal surfaces with periodic nanospaces among them that allow for confinement studies, such as Au(110) nanogrooves (channel width of 1.22 nm) for catalytic dehalogenation polymerization (61) and CdSe nanoribbons (channel width of 1.4 nm) for enhanced incorporation of manganese ions under nanoconfinement. (62) Metal and metal oxide materials synthesized in films were also used as scaffolds, with reactions taking place between well-defined layers, such as MoS2 stacked layers (interlayer spacing of 1.5 nm) for radical-based degradation. (63) Alternatively, nanoconfined reactions can occur in groups of nanopores, such as VO2 nanochannels (diameters between 12 and 24 nm) for controlling metal–insulator transition behaviors (64) and a Cu(111) network of hexagonal pores (diameter of 4 nm) for assembling molecular patterns at surfaces. (65) These metal or metal oxide scaffolds can be engineered at the atomic scale, in terms of composition and the relative position of the constituents, either in the bulk material or near the surface, to exert specific electric and catalytic properties. (66) Some semiconductor metal oxides, typically TiO2, have also been employed, owing to their ability to absorb light and drive photocatalysis. Examples include uniaxial TiO2 nanotube arrays (pore diameter of 50 nm) to attain anisotropic optical properties (67) and TiO2-rGO composite layers (pore diameter of 95 nm) for efficient solar energy conversion. (68)
One widely studied material is anodized aluminum oxide with a honeycomb-like structure, synthesized via electrochemical dissolution. This material contains numerous parallel cylindrical nanopores (pore diameter of 10–400 nm and pore density of 108–1010 pores cm–2). (69) These well-aligned and uniform nanochannels with precise pore diameters and lengths have been instrumental for confinement-enhanced process analysis (70−72) and the synthesis of a diverse range of nanostructures via confined nucleation and growth (e.g., nanowires, (73) nanotubes, (74) and nanobrushes (75)).

Metal–Organic Frameworks

MOFs have well-defined topological coordination between transition metal cations and multidentate organic linkers, giving rise to periodic pores that are precisely sized at generally 2 nm or less. MOFs are commonly synthesized via solvothermal methods that mix organic linkers with solutions containing inorganic salts such as calcium, zirconium, and/or zinc. (76) Depending on the type of metal center and the surrounding ligands, the morphology, dimensions, and chemistry of the pores can be readily tailored. These tailorable qualities lead to tunable selectivity for adsorption and reaction, based on various guest molecule properties, (20) which provide a wide range of applications under nanoconfinement, such as gas storage, (77) chemical synthesis reaction, (78) sensing, (79) and drug delivery. (80)
The well-defined structure has been shown to be particularly advantageous for quantitative studies, including thermodynamic and kinetic evaluations of confined chemical processes, such as H2 desorption. (81) MOFs can be also synthesized to have a chirality to enable enantioselective nanoconfinement for the catalytic Henry reaction (82) and chiral separation. (83) The structural regularity and homogeneity also make MOFs highly amenable to postsynthetic modifications, such as the addition of catalytically active assemblies (e.g., transition metal nanoclusters, (84,85) nanoparticles, (86,87) and nanosheets (1)), along with single atom catalysts, (88) or the insertion of guest molecules or weakly coordinating anions. (89) Placing catalytic materials within the nanoconfined voids of MOFs has been explored to design synergistic catalysis (90,91) and tandem reactions. (92)

Polymers

Polymers have been widely used as substrates for membranes, ranging from filters containing micrometer-scale pores to reverse osmosis membranes, in which void spaces do not follow the classical definition of pores. Polymeric structures of interest, with respect to nanoconfinement, include periodic scaffolds with well-defined pores and controllable morphology. Intrinsic periodic macromolecular structures of polymers can lead to various microphase-separated, spatially unique scaffolds. Block copolymers are exemplary materials with well-ordered nanopores. Porous scaffolds can be prepared through the preferential removal of one polymer block while leaving another polymer block as a scaffold. (93) Various scaffold structures are available, depending on the morphologies of the block copolymers, which can be readily controlled by their relative polymer block ratios, molecular weights, and chemical identities, as well as synthesis method and postsynthesis treatment. A variety of block copolymer-driven scaffolds have been employed to examine nanoconfinement effects, including cylindrical nanodomains (mean diameter of 11.5 nm) for the synthesis of intermetallic nanoalloy arrays, (94) a double-channel network (domain spacing of 13–16 nm) for enhanced charge transport and exciton separation, (95) and a vesicle nanoreactor (internal diameter of 30 nm) for trypsin-catalyzed enzymatic reactions. (96)
Other polymeric scaffolds include polymeric microgel particles that consist of three-dimensional mesh-like networks with pore diameters ranging from a few angstroms to several nanometers. They can partition solutions and restrict the mobility of adsorbed molecules, causing nanoconfinement effects, such as the selective crystallization of polymorphs in microgel with 0.7–1.5 nm sized mesh. (97) Polymeric ion exchange resins are cross-linked porous structures, which can be used as the scaffold for confined incubation of functional metal oxide nanoparticles for the removal of toxic metal ions from water, such as the confined hydrated iron(III) oxides for the sorption of As(III) (98) and zerovalent iron for the reduction of Se(VI). (99) In addition, protein cages with monodispersed, periodic structures have been explored as vessels for enzymatic reactions in nanoconfined environments, simulating the way that enzymes react inside cells. (100) When using polymer scaffolds for liquid-phase confinement study, care should be taken to avoid material swelling or deformation that is commonly caused by the insertion of liquid molecules into polymeric networks, which would impact the accuracy of observed nanoconfinement effects.

Silicon and Aluminosilicate

Mesoporous silica with parallel pores and a large surface area has been widely used for confinement studies. For example, nanoconfinement of a solid-state electrolyte within SBA-15 (pore diameter of 8 nm) was found to significantly enhance its conductivity, which is critical for the improved performance of the Li battery. (101) Other interesting phenomena, such as a significant change in the transient photophysical property of a chromophore (7-hydroxyquinoline) (102) and salt phase transition temperature (103) have been attributed to nanoconfinement within MCM-41 pores (diameter of 2.5 nm). Other silica structures with different pore sizes and morphologies have also been employed to explore nanoconfinement effects. Various structures with much larger pores than mesoporous silica have been used as synthetic templates demonstrating nanoconfinement effects on topological self-assembly of guest molecules (e.g., groove-patterned silica with 233 nm width for a growth of well-ordered copolymer nanospheres) (104) and high-temperature crystallization (e.g., layered silica thin films with mean diameters of 32–140 nm for the synthesis of nickel oxide nanoparticles). (105) Another notable example is porous silica in the form of hollow nanospheres, which have been used as a nanoreactor for various applications, including the release of hydrogen from ammonia borane used in hydrogen storage (106) and catalytic hydrogenation. (107)
Aluminosilicate, composed of tetrahedrally coordinated Si/Al units, is another widely used material to form nanoporous scaffolds. The size, arrangement, and surface acid–base property of the pores can be tailored by controlling the intermixing of aluminum and silicon atoms. Typical examples are zeolites with well-defined crystalline microporous structures (pore diameter of <1.5 nm), which, in most cases, confine reactions in its subnanometer cavities. Past studies have discovered many interesting phenomena within zeolite scaffolds, such as the deviation of methane oxidation from its intrinsic chemistry due to confinement-induced secondary effects. (108)

Application Fields

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As discussed above, nanoconfinement studies have employed a large variety of scaffold materials to utilize their unique combinations of porous structures and surface chemical properties. Accordingly, internal reactions and separation processes have shown striking results under spatial nanoconfinement, compared to the same reactions and processes occurring in the bulk phase, across a wide range of application fields. For example, under spatial nanoconfinement, synthetic reactions can occur under milder conditions when they would otherwise need harsh conditions, like high temperatures or highly acidic/basic pH in the bulk phase. Material growth can be tailored with atomic precision using nanoscale scaffolds. Moreover, separation processes can have altered phase transition points and increased selectivity based on desolvated ion properties. In this section, we highlight several notable findings in the primary fields of application that have been reported thus far.

Hydrogen Production and Storage

Light metal hydrides with high hydrogen content are promising materials for hydrogen production and storage. However, their high thermodynamic stability, sluggish desorption, and poor reversibility make them unlikely to meet all practical requirements for hydrogen production and storage. (109) Nanoconfinement of borohydride or alanate systems have been shown to overcome some of these setbacks (e.g., by accelerating hydrogen desorption and adsorption kinetics), while reducing the activation energy and required temperature. For example, confining NaAlH4 in mesoporous carbon (Figure 2a) resulted in enhanced hydrogen desorption kinetics, improved cyclic stability, and reductions in activation energy and process temperature by 58 kJ/mol and 100 °C, respectively, compared to bulk NaAlH4 without spatial confinement. (55) The confinement effect on hydrogen desorption can be further intensified by introducing dopant catalysts onto scaffolds (e.g., porous carbon with catalyst elements of Li, (110) Ni, (111) N, (112) and CoNiB). (113) Additionally, several recently emerging systems with varied scaffold architecture show advantages for hydrogen storage and release, such as the combination of hydrides with MOFs (109,114) and hydrides with low-dimensional scaffolds, such as nanotubes (115) or core–shell structure. (116)

Figure 2

Figure 2. Schematic representation of spatial confinement effects observed in various application fields. (a) Enhanced hydrogen production from NaAlH4 within mesoporous carbon (pore diameter of 4 nm) due to the decrease in activation energy. (55) (b) Rh-catalyzed conversion of CO to ethanol inside a carbon nanotube exhibiting at least an order of magnitude enhancement of product formation rate compared to the counterpart catalytic reaction outside the carbon nanotube. (40) (c) Confined pores with polystyrene-block-poly(4-vinylpyridine) forming a charge-dominant zone that exhibits excellent anion selectivity. (117) (d) Complete segregation of binary benzene-cyclohexane mixtures under spatial confinement in mesoporous silica (inner diameter ≈ 9 nm) at a temperature (153 K) below their respective freezing and phase transition points. (118) (e) Confinement of gold nanoparticles within spherical micelle particles, forming a particle-in-particle structure that significantly enhances the optical quenching and thermal energy production of gold and consequently improves contrast in computed tomography (CT) and magnetic resonance imaging (MRI). (119) (f) Facile growth of supramolecular polymeric nanostructures under nanoconfinement. (120) (g) Improved oxygen reduction reaction and catalyst durability by a multifaceted Pt catalyst confined between graphene oxide plates due to the formation of [110]-dominant dendritic multipods. (121) (h) A 100% stretchable semiconductor produced by confining high-mobility polymer semiconductors between thin films. (122)

CO/CO2 Conversion

The reduction of carbon oxides has long been pursued as an environmentally sustainable strategy for energy production and for the synthesis of commodity chemicals. Nanoconfinement has been shown to accelerate the conversion rate of CO/CO2 into organic chemicals, with several striking enhancements. For instance, the conversion of CO to ethanol by Rh-catalyzed reactions confined inside carbon nanotubes had an increase in the formation rate of ethanol (30.0 mol mol–1Rh h–1) of more than an order of magnitude greater than the counterpart reaction where the same catalyst was deposited outside the carbon nanotubes (Figure 2b). (40) For the hydrogenation of CO2 to methanol, Cu/ZnOx nanoparticles confined inside tetrahedral and octahedral cages of UiO-bpy MOFs exhibited exceptionally high activity (up to 2.6 g MeOH kg Cu1– h–1), 100% selectivity, and over 100 h stability. (123) Moreover, by confining CO inside Ag–Cu core–shell structures to achieve sufficiently high local CO concentrations, cascade C–C coupling reactions were successfully induced. (124) Cooperative catalytic synthetic processes were also found to benefit from nanoconfinement. (125,126) For example, confining two enzymatic catalysts within electrode nanopores made the light-driven, reductive CO2 carboxylation of pyruvate highly efficient, with a turnover number at least 1 order of magnitude higher than other cofactor systems. (125)

Ion and Phase Separation

Regulating molecular/ionic separation or permeation through selective membranes is crucial for many industrial applications, including drug delivery, packaging, fuel cells, and batteries. In contrast to conventional polymeric membranes with poorly defined pore structures, recent studies have explored selective membranes with precisely controlled pore morphology and surface chemistry to create molecule/ion-specific transport behavior under spatial nanoconfinement. (50) Examples of membranes exploiting confinement for enhancing selectivity include (i) a Janus copolymer membrane with an asymmetric structure and confined pores that exhibits high anion selectivity (Figure 2c) (117) and (ii) a nanostructured copolymer that achieves a molecular permeation selectivity that is 2.5 times higher than the corresponding homogeneous film. (127) Adding functional groups to the nanoconfinement structure can further enhance the selectivity of ion transport through modifying the ability of ionic species to pass through a pore entrance or their distribution within the pore. (128) Adding well-ordered reactive ionic liquids onto the surfaces confined inside boron nitride nanochannels was also found to promote selective ethylene transport. (129) A binary solvent system can acquire different phase separation behavior under nanoconfinement. For example, a benzene–cyclohexane mixture does not undergo phase changes at temperatures below their respective freezing and phase transition points (Figure 2d). (118) Tortuous nanoconfined pores were found to create a molecular jam effect that resulted in the complete rejection of