Effects of Ionic Liquid Nanoconfinement on the CO2/CH4 Separation in Poly(vinylidene fluoride)/1-Ethyl-3-methylimidazolium Thiocyanate Membranes.

A combined experimental and molecular dynamics (MD) simulation approach was used to investigate the effects of the nanoconfinement of a highly CO2/CH4-selective ionic liquid (IL), 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]), in porous poly(vinylidene fluoride) (PVDF) matrices on the gas separation performance of the resulting membranes. The observed experimental CO2/CH4 permselectivity increased by about 46% when the nominal pore diameter in PVDF, which is a measure of nanoconfinement, decreased from 450 to 100 nm, thus demonstrating nanoconfinement improvements of gas separation. MD simulations corroborated these experimental observations and indicated a suppression in the sorption of CH4 by [EMIM][SCN] when the IL nanoconfinement length decreased within the nonpolar PVDF surfaces. This is consistent with the experimental observation that the CH4 permeance through the IL confined in nonpolar PVDF is significantly less than the CH4 permeance through the IL confined in a water-wetting polar formulation of PVDF. The potential of mean force calculations further indicated that CO2 has more affinity to the nonpolar PVDF surface than CH4. Also, a charge/density distribution analysis of the IL in the PVDF-confined region revealed a layering of the IL into [EMIM]- and [SCN]-rich regions, where CH4 was preferentially distributed in the former and CO2 in the latter. These molecular insights into the nanoconfinement-driven mechanisms in polymer/IL membranes provide a framework for a better molecular design of such membranes for critical gas separation and CO2 capture applications.

Reactive Molecular Simulation of the Damage Mitigation Efficacy of POSS-, Graphene-, and Carbon Nanotube-Loaded Polyimide Coatings Exposed to Atomic Oxygen Bombardment.

Reactive molecular dynamics simulation was employed to compare the damage mitigation efficacy of pristine and polyimide (PI)-grafted polyoctahedral silsesquioxane (POSS), graphene (Gr), and carbon nanotubes (CNTs) in a PI matrix exposed to atomic oxygen (AO) bombardment. The concentration of POSS and the orientation of Gr and CNT nanoparticles were further investigated. Overall, the mass loss, erosion yield, surface damage, AO penetration depth, and temperature evolution are lower for the PI systems with randomly oriented CNTs and Gr or PI-grafted POSS compared to those of the pristine POSS or aligned CNT and Gr systems at the same nanoparticle concentration. On the basis of experimental early degradation data (before the onset of nanoparticle damage), the amount of exposed PI, which has the highest erosion yield of all material components, on the material surface is the most important parameter affecting the erosion yield of the hybrid material. Our data indicate that the PI systems with randomly oriented Gr and CNT nanoparticles have the lowest amount of exposed PI on the material surface; therefore, a lower erosion yield is obtained for these systems compared to that of the PI systems with aligned Gr and CNT nanoparticles. However, the PI/grafted-POSS system has a significantly lower erosion yield than that of the PI systems with aligned Gr and CNT nanoparticles, again due to a lower amount of exposed PI on the surface. When comparing the PI systems loaded with PI-grafted POSS versus pristine POSS at low and high nanoparticle concentrations, our data indicate that grafting the POSS and increasing the POSS concentration lower the erosion yield by a factor of about 4 and 1.5, respectively. The former is attributed to a better dispersion of PI-grafted POSS versus that of the pristine POSS in the PI matrix, as determined by the radial distribution function.

Molecular simulation of pH-dependent diffusion, loading, and release of doxorubicin in graphene and graphene oxide drug delivery systems.

In this study, the adsorption of doxorubicin (DOX), an anticancer drug, on pristine graphene (PG) and graphene oxide (GO) nanocarriers with different surface oxygen densities and in an aqueous environment with varying pH levels was investigated using molecular dynamics (MD) simulation. The drug loading and release on the GO nanocarrier was also simulated using pH as the controller mechanism. Overall, the DOX/nanocarrier interactions become stronger as the graphene surface oxygen density increases. Although pH has a negligible effect on the single-molecule drug adsorption on the GO surfaces under acidic and neutral conditions, significantly stronger DOX/nanocarrier interactions occur for the GO nanosheet with a lower surface oxygen density (GO-16, with an O/C ratio of 1 : 6) at basic pH levels. Moreover, the DOX/nanocarrier interactions are greatly weakened in the GO nanosheet with higher surface oxygen density (GO-13, with an O/C ratio of 1 : 3) under basic conditions. These observations are partly attributed to a more favorable geometry of the DOX molecule on the GO-16 surface as opposed to a loosely attached DOX molecule on the edges of the GO-13 nanosheet. When comparing the adsorption kinetics and transport properties of the DOX molecule in different GO systems, the drug diffusion coefficient increases with decreasing pH value (going from basic to neutral to acidic) due to the reduced total water-nanocarrier interactions. The latter observation is an indication of the more facilitated transport of the DOX molecule in an aqueous medium towards the nanocarrier surface at lower pH levels. Finally, we have confirmed the loading and release of the DOX molecules on the GO nanocarrier under neutral (pH = 7) and acidic (pH = 5) conditions, respectively. The former signifies the blood pH level, whereas the latter is reminiscent of the pH of a tumorous cell. The computational results presented in this work reveal the underlying mechanisms of DOX loading and release on PG and GO surfaces, which may be used to design better graphene-based nanocarriers for the DOX delivery and targeting applications.