Translocation dynamics of freely jointed Lennard-Jones chains into adsorbing pores

Polymer translocation into adsorbing nanopores is studied by using the Fokker-Planck equation of chain diffusion along the energy landscape calculated with Monte Carlo simulations using the incremental gauge cell method. The free energy profile of a translocating chain was found by combining two independent sub-chains, one free but tethered to a hard wall, and the other tethered inside an adsorbing pore. Translocation dynamics were revealed by application of the Fokker-Planck equation for normal diffusion. Adsorption of polymer chains into nanopores involves a competition of attractive adsorption and repulsive steric hindrance contributions to the free energy. Translocation times fell into two regimes depending on the strength of the adsorbing pore. In addition, we found a non-monotonic dependence of translocation times with increasing adsorption strength, with sharp peak associated with local free energy minima along the translocation coordinate

Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel microand mesoporous silicas

We report results of nitrogen and argon adsorption experiments performed at 77.4 and 87.3 K on novel micro/ mesoporous silica materials with morphologically different networks of mesopores embedded into microporous matrixes: SE3030 silica with wormlike cylindrical channels of mode diameter of ∼95 Å, KLE silica with cagelike spheroidal pores of ca. 140 Å, KLE/IL silica with spheroidal pores of ∼140 Å connected by cylindrical channels of ∼26 Å, and, also for a comparison, on Vycor glass with a disordered network of pores of mode diameter of ∼70 Å. We show that the type of hysteresis loop formed by adsorption/desorption isotherms is determined by different mechanisms of condensation and evaporation and depends upon the shape and size of pores. We demonstrate that adsorption experiments performed with different adsorptives allow for detecting and separating the effects of pore blocking/percolation and cavitation in the course of evaporation. The results confirm that cavitation-controlled evaporation occurs in ink-bottle pores with the neck size smaller than a certain critical value. In this case, the pressure of evaporation does not depend upon the neck size. In pores with larger necks, percolation-controlled evaporation occurs, as observed for nitrogen (at 77.4 K) and argon (at 87.3 K) on porous Vycor glass. We elaborate a novel hybrid nonlocal density functional theory (NLDFT) method for calculations of pore size distributions from adsorption isotherms in the entire range of micro-and mesopores. The NLDFT method, applied to the adsorption branch of the isotherm, takes into account the effect of delayed capillary condensation in pores of different geometries. The pore size data obtained by the NLDFT method for SE3030, KLE, and KLE/IL silicas agree with the data of SANS/SAXS techniques

Calculating Surface Fractal Dimensions of Adsorbents

A critical analysis of different methods for determining surface fractal dimensions using adsorption measurements is presented. A new method for calculating surface fractal dimensions from capillary condensation or mercury porosimetry data is proposed. This method does not use any model of adsorption on the fractal surface. It is based on the thermodynamic relationship between the surface area of the interface and the amount of the adsorbate. The formulae obtained for surface fractal dimensions do not contain any additional parameters except the experimental data. A typical example of calculating the surface fractal dimensions of an active carbon sample is presented

Understanding the Origins of Reversible and Hysteretic Pathways of Adsorption Phase Transitions by Mesocanonical Ensemble Monte Carlo Simulations

Phase behavior of nanoconfined fluids adsorbed in metal-organic frameworks is of paramount importance for the design of advanced materials for energy and gas storage, separations, electrochemical devices, sensors, and drug delivery, as well as for the pore structure characterization. Phase transformations in adsorbed fluids often involve long-lasting metastable states and hysteresis that has been well-documented in gas adsorption-desorption and nonwetting fluid intrusion-extrusion experiments. However, theoretical prediction of the observed nanophase behavior remains a challenging problem. The mesoscopic canonical, or mesocanonical, ensemble (MCE) is devised to study the nanophase behavior under conditions of controlled fluctuations to stabilize metastable and labile states. Here, we implement the MCE Monte Carlo (MCEMC) simulation scheme, called the gauge cell method, which allows to produce van der Waals type isotherms with distinctive swings around the phase transitions regions. The constructed isotherms determine the positions of phase equilibrium and spinodals, as well as the nucleation barriers separating metastable states. The MCEMC method is applied to predict the origins of reversible and hysteric adsorption phase transitions in a series of practical MOF materials, including IRMOF-1, ZIF-412, UiO-66, Cu-BTC, IRMOF-74-V, VII, and IX. We demonstrate the unique capabilities of the MCEMC method in quantitative predictions of experimental observations compared with the conventional grand canonical and canonical ensemble simulations. The MCEMC method is implemented in the open-source RASPA software and recommended for studies of adsorption behavior and pore structure characterization of MOFs and other nanoporous materials

Molecular Dynamics Simulation of Nafion Oligomer Solvation in Equimolar Methanol-Water Mixture

The solvation of Nafion oligomer in equimolar water-methanol solution was studied by means of molecular dynamics simulations. The skeleton in water-methanol system was found to be substantially folded; its geometry was close to that in pure water. Pronounced flexibility of the skeleton was observed. At the same time, the side chains turned out to be very stiff. No conformation transitions in the side chains were monitored. The skeleton was mostly solvated by methanol. A minor preference for water in the vicinity of the sulfate group was observed. The lifetime of the hydrogen bonds of water and methanol were estimated. The lifetimes for the two components were very close to each other and several times longer than rotational correlation times of individual solute molecules in the bulk