Permeation mechanism of gas molecules through polyimide barrier coatings with freeze- and oven-dried modified layered silicates

We report the first atomistic simulation study to characterize poly(ionic liquid) (PIL) membranes and examine their capability for post-combustion CO(2) capture. Four PILs based on 1-vinyl-3-butylimidazolium ([VBIM](+)) are examined with four different anions, namely bis(trifluoromethylsulfonyl)imide ([TF(2)N](-)), thiocyanate ([SCN](-)), hexafluorophosphate ([PF(6)](-)) and chloride ([Cl](-)). Gas molecules (CO(2) and N(2)) in [VBIM](+)-based PILs interact with polycations more strongly than with anions. Therefore, the role of anions in gas solubility is insignificant, which is in remarkable contrast to monomeric ILs. The solubilities predicted in the four PILs are close and in good agreement with available experimental data. The sorption, diffusion and permeation selectivities of CO(2)/N(2) predicted from simulation are consistent with experiment. Particularly, the diffusion selectivities are approximately equal to one, implying that CO(2)/N(2) separation is governed by sorption. This study provides atomistic insight into the mechanisms of gas sorption, diffusion and permeation in [VBIM](+)-based PILs and reveals that polycations play a dominant role in determining gas-membrane interaction and separation.

Gas solubility in glasses – principles and structural implications

Aquaporins (AQPs) are a family of integral membrane proteins, which facilitate the rapid and yet highly selective flux of water and other small solutes across biological membranes. Molecular dynamics (MD) simulations contributed substantially to the understanding of the molecular mechanisms that underlie this remarkable efficiency and selectivity of aquaporin channels. This chapter reviews the current state of MD simulations of aquaporins and related aquaglyceroporins as well as the insights these simulations have provided. The mechanism of water permeation through AQPs and methods to determine channel permeabilities from simulations are described. Protons are strictly excluded from AQPs by a large electrostatic barrier and not by an interruption of the Grotthuss mechanism inside the pore. Both the protein's electric field and desolvation effects contribute to this barrier. Permeation of apolar gas molecules such as CO(2) through AQPs is accompanied by a large energetic barrier and thus can only be expected in membranes with a low intrinsic gas permeability. Additionally, the insights from simulations into the mechanism of glycerol permeation through the glycerol facilitator GlpF from E. coli are summarized. Finally, MD simulations are discussed that revealed that the aro-matic/arginine constriction region is generally the filter for uncharged solutes, and that AQP selectivity is controlled by a hydrophobic effect and steric restraints.

Synthesis, characterization, and gas permeation properties of a hydrogen permeable silica membrane supported on porous alumina

The possibilities for transport of sulfur through preformed oxide scales by both solution‐diffusion and molecular gas (24) permeation mechanisms are examined thermodynamically to establish the limiting conditions under which each is viable.The results are tested, using nickel and cobalt specimens, and it is concluded that, although both mechanisms may operate in parallel, the permeation of gas molecules is the more dangerous since it can operate over wider ranges of gas atmosphere composition. The permeation of SO2 molecules throu through oxide scales growing on cobalt is clearly demonstrated.

Preliminary investigation of gas transport mechanism in a H+ irradiated polyimide-ceramic composite membrane

Enhanced reliability of phosphor-converted white light-emitting diodes based on a laser-cured silicone encapsulant layer

MoS2 nanosheet as a promising nanostructure membrane for gas separation

Reinforced thermoplastic composite pipes (RTPs) have been widely used for oil and gas gathering and transportation. Polyvinylidene fluoride (PVDF) has the greatest potential as a thermoplastic liner of RTPs due to its excellent thermal and mechanical properties. However, permeation of gases is inevitable in the thermoplastic liner, which may lead to blister failure of the liner and damage the safe operation of the RTPs. In order to clarify the permeation behavior and obtain the permeation mechanism of the mixture gas (CH4/CO2/H2S) in PVDF at the normal service conditions, molecular simulations were carried out by combining the Grand Canonical Monte Carlo (GCMC) method and the Molecular Dynamics (MD) method. The simulated results showed that the solubility coefficients of gases increased with the decrease in temperature and the increase in pressure. The adsorption isotherms of all gases were consistent with the Langmuir model. The order of the adsorption concentration for different gases was H2S > CO2> CH4. The isosteric heats of gases at all the actual service conditions were much less than 42 kJ/mol, which indicated that the adsorption for all the gases belonged to the physical adsorption. Both of the diffusion and permeation coefficients increased with the increase in temperature and pressure. The diffusion belonged to Einstein diffusion and the diffusion coefficients of each gas followed the order of CH4 > CO2 > H2S. During the permeation process, the adsorption of gas molecules in PVDF exhibited selective aggregation, and most of them were adsorbed in the low potential energy region of PVDF cell. The mixed-gas molecules vibrated within the hole of PVDF at relatively low temperature and pressure. As the temperature and pressure increase, the gas molecules jumped into the neighboring holes occasionally and then dwelled in the holes, moving around their equilibrium positions.

Aquaporins (AQPs) are protein channels which facilitate rapid water permeation across cell membrane. The AQPs are very vital for biological organs, as their malfunction causes severe diseases in human body. A particular family of AQPs, that is AQP5, has a significant role in lung fluid transport due to submucosal glands structure. However, it has not been yet well understood whether these protein channels can conduct gas molecules. Here, Molecular Dynamics (MD) simulations are used to investigate the CO2 permeability and diffusion in AQP5 during a 40-nanosecond period. For the first time, equilibrium and Steered MD (SMD) are used to simulate self and force-induced diffusion of CO2 molecules across AQP5 and POPE lipid bilayer. According to PMFs profile associated to CO2 permeation, the hydrophobic central pore provides a more suitable pathway for gas molecules compared to other AQP5 channels. Although CO2 molecules can also permeate across AQP5 water channels, the rate of CO2 permeation through four channels of the AQP5 monomers is much lower than the central pore. The rate of CO2 permeation through four AQP5 water channels is even lower than CO2 diffusion through POPE lipid membrane. The results reported in this investigation demonstrate that MD simulations of human AQP5 provide valuable insights into the gas permeation mechanism for both the equilibrium self-diffusion, and quasi-equilibrium condition.

Carbon molecular sieve membrane (CMSM) is the promising candidate for natural gas purification because of its excellent stability, per-selectivity and permeability. However, morphological design of CMSM is very important for the specific application. In this project, two CMSM samples were synthesized at different pyrolytic conditions and examined for separation of N2/CH4 gas pair. Adsorption and permeation experiments were conducted to examine the separation performance of each membrane sample. At the ambient conditions, a perm-selectivity of ~ 6 was found for (N2/CH4) pair while it is ~30 for (CO2/N2) pair on the membrane pyrolyzed at the lower pyrolytic temperature of 800°C. When pyrolytic temperature is increased to 1000°C, however, these two selectivity values are changed to ~1.5 (decrease) and ~ 100 (increase), respectively. Analysis revealed that both surface diffusion and molecular sieving play important roles in the overall gas permeation mechanisms, which result in the abnormal behaviors in the selectivities of different gas molecules. It is concluded that the variation of pore size and thickness are critical for the design of surface flow selective carbon molecular sieve membranes.

A simulation study of methane-hydrogen gas mixture permeation through nanoporous palladium membrane using molecular dynamics

Microporous silica membranes have silica polymer network voids smaller than 3 Å where only small gas molecules such as helium (2.6 Å) and hydrogen (2.89 Å) can be transported. These silica membranes are highly expected to be available for H2 separation. In order to examine gas permeation mechanisms in the silica polymer network voids, factors such as membrane porous structures, gas diffusivity, and gas permeability were studied via membrane permeation molecular dynamics simulation. The thermal motions of silica membrane constituent atoms were examined according to classic harmonic oscillation potential using a suitable amorphous silica structure and non-equilibrium molecular dynamics (NEMD) simulations of gas permeation. The dynamic model successfully simulated the gas permeation characteristics in an amorphous silica membrane with a suitable Hooke’s potential parameter. The introduction of the oscillative thermal motion of the membrane atoms enhanced gas diffusivity. Helium and hydrogen diffusivity and permeability were analyzed using gas translation (GT) and solid vibration (SV) models. The diffusion distance of gas molecules between adsorption sites was around 5.5–7 Å. The solid-type vibration frequencies of gas molecules in the site were on the order of 1013 and were reasonably smaller for heavier helium than for hydrogen. Both the GT and SV models could explain the temperature dependency of helium and hydrogen gas diffusivities, but the SV model provided a more realistic geometrical representation of the silica membrane. The SV model also successfully explained gas permeability in an actual silica membrane as well as the virtual amorphous silica membrane.

Molecular dynamics simulations are particularly useful in providing details that permit understanding of phenomena occurring at surfaces, phenomena characterised by surface-sensitive experimental methods yielding average properties. The methods and examples that we consider here reveal how the characteristics of the surface and the permeant affect permeation events at the surface of soft matter, in particular, lipid bilayers. We choose permeation of lipid membranes as our example of the ability of molecular dynamics simulations to provide molecular-level mechanisms that are otherwise not available via other means. Molecular permeation through lipid membranes is a fundamental biological process that is important for small molecules such as therapeutics as well as nanoparticles that have become important modes of drug delivery. We describe methods applicable to such large systems and use examples where the permeants are uncharged particles: small molecules (Xe, O2, CO2), bare gold nanocrystals, gold-core nanoparticles with hydrophobic ligands (alkane thiols of various lengths) and gold-core nanoparticles with hydrophilic ligands (methyl-terminated polyethylene glycol of various lengths). In each example, we have previously validated our findings by comparisons with experimental data, using such information as is available from X-ray diffraction, electron paramagnetic resonance, nuclear magnetic resonance, atomic force microscopy, various imaging methods, diffusion measurements, dynamic light scattering. In addition to spherical core nanoparticles, we also examine the characteristic permeation mechanisms of gold nanorods with polyethylene glycol ligands, where the aspect ratio different from 1 makes the permeation event dependent on the angle of the rod axis relative to the membrane surface. This review examines the phenomena associated with the interaction of various permeants with the lipid bilayer that serves as our model membrane. We consider adsorption at the interface, the permeant within the top lipid leaflet, in the middle of the membrane within the lipid tail region, within the bottom lipid leaflet and finally exiting the membrane on the way to recovery, for various permeants: gas molecules, bare gold nanocrystal, gold nanoparticles with alkane thiol ligands, PEGylated gold nanoparticles and PEGylated gold nanorods. We observe formation of a water pore, occasional transport of ions, lipid flip-flops, lipid displacement from the membrane, and rotational behaviour of PEGylated nanorods during the permeation process. These events differ depending on the chemical nature (hydrophobic or hydrophilic) of the ligands, their length, the coverage density on the gold surface and the aspect ratio of the gold core. Direct comparisons across the board are possible by using identical interaction models (MARTINI coarse grain), molecular dynamics methods, simulation set-ups and analyses of MD results, thereby permitting generalisations to be made about mechanisms for the various events and how they are affected by these factors.

Gas transport through nano and micro composites of natural rubber (NR) and their blends with carboxylated styrene butadiene rubber (XSBR) latex membranes