Atomistic simulation of nafion membrane. 2. Dynamics of water molecules and hydronium ions

We have performed a detailed and comprehensive analysis of the dynamics of water molecules and hydronium ions in hydrated Nafion using classical molecular dynamics simulations with the DREIDING force field. In addition to calculating diffusion coefficients as a function of hydration level, we have also determined mean residence time of H(2)O molecules and H(3)O(+) ions in the first solvation shell of SO(3)(-) groups. The diffusion coefficient of H(2)O molecules increases with increasing hydration level and is in good agreement with experiment. The mean residence time of H(2)O molecules decreases with increasing membrane hydration from 1 ns at a low hydration level to 75 ps at the highest hydration level studied. These dynamical changes are related to the changes in membrane nanostructure reported in the first part of this work. Our results provide insights into slow proton dynamics observed in neutron scattering experiments and are consistent with the Gebel model of Nafion structure.     

A SANS determination of the influence of external conditions on the nanostructure of nafion membrane

We have studied the structure of nafion membranes by small‐angle neutron scattering in the q range of 7 10⁻³ Å⁻¹ to 0.4 Å⁻¹. External conditions were varied: counterions, electrolyte concentration, temperature, and pretreatment of membranes. The nanostructure of Nafion exhibits a sensivity that depends on the parameters considered: changing counterions and concentration leads to a reversible local reorganization of the hydrophilic cavities, whereas changing temperature and pretreament leads to a irreversible reoganization on a larger scale. The mean variation of the radius of the hydrophilic cavities versus these parameters is about 10%. The interface between hydrophobic and hydrophilic phases appears to be sharp except when the temperature exceeds 60 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 548–558, 2001     

Proton hopping in phosphoric acid solvated nafion membrane: a molecular simulation study

Molecular simulation studies of the microstructure and of the proton transport properties of phosphoric acid solvated Nafion membrane are carried out. The ab initio calculations show that the phosphoric acid is a good solvent to promote the proton ionization of the sulfonic acid group, and only two phosphoric acid molecules are necessary for the dissociation of one sulfonic acid group. A mechanism of proton hopping between phosphoric acid and protonated phosphoric acid cation in the hydrophilic subphase is also elucidated by ab initio calculations. The molecular dynamics simulations, conducted at a phosphoric acid concentration of 25.4% (wt) which is slightly lower than that of phosphoric acid swollen Nafion, show that the phosphoric acid exists in subphases and that it cannot develop into a continuous subphase. Thus, proton-hopping pathways are interrupted, and the conductivity is expected to be lower than that for pure phosphoric acid. The molecular dynamics simulations, conducted at a phosphoric acid concentration of 45.1% (wt) which corresponds to an unstable state, show that the hydrophobic poly(tetrafluoroethylene) backbones trend to gather together forming hydrophobic clusters and that the phosphoric acid forms a continuous subphase with the sulfonic acid groups located at the hydrophobic/hydrophilic interface. Thus, proton-hopping pathways can develop uninterruptedly like the pure phosphoric acid, and high conductivity is expected. The molecular dynamics study also shows that the hydrogen-bonding characteristics of phosphoric acid and sulfonate anion are similar regardless of the factor that the former can move freely while the latter is attached to Nafion backbone.     

Atomistic simulations of hydrated nafion and temperature effects on hydronium ion mobility

The effects of hydration level and temperature on the nanostructure of an atomistic model of a Nafion (DuPont) membrane and the vehicular transport of hydronium ions and water molecules were examined using classical molecular dynamics simulations. Through the determination and analysis of structural and dynamical parameters such as density, radial distribution functions, coordination numbers, mean square deviations, and diffusion coefficients, we identify that hydronium ions play an important role in modifying the hydration structure near the sulfonate groups. In the regime of low level of hydration, short hydrogen bonded linkages made of water molecules and sometimes hydronium ions alone give a more constrained structure among the sulfonate side chains. The diffusion coefficient for water was found to be in good accord with experimental data. The diffusion coefficient for the hydronium ions was determined to be much smaller (6-10 times) than that for water. Temperature was found to have a significant effect on the absolute value of the diffusion coefficients for both water and hydronium ions.     

Atomistic simulation of nanoporous layered double hydroxide materials and their properties. I. Structural modeling

An atomistic model of layered double hydroxides, an important class of nanoporous materials, is presented. These materials have wide applications, ranging from adsorbents for gases and liquid ions to nanoporous membranes and catalysts. They consist of two types of metallic cations that are accommodated by a close-packed configuration of OH- and other anions in a positively charged brucitelike layer. Water and various anions are distributed in the interlayer space for charge compensation. A modified form of the consistent-valence force field, together with energy minimization and molecular dynamics simulations, is utilized for developing an atomistic model of the materials. To test the accuracy of the model, we compare the vibrational frequencies, x-ray diffraction patterns, and the basal spacing of the material, computed using the atomistic model, with our experimental data over a wide range of temperature. Good agreement is found between the computed and measured quantities.     

Atomistic simulation of lipid and DiI dynamics in membrane bilayers under tension

Membrane tension modulates cellular processes by initiating changes in the dynamics of its molecular constituents. To quantify the precise relationship between tension, structural properties of the membrane, and the dynamics of lipids and a lipophilic reporter dye, we performed atomistic molecular dynamics (MD) simulations of DiI-labeled dipalmitoylphosphatidylcholine (DPPC) lipid bilayers under physiological lateral tensions ranging from -2.6 mN m(-1) to 15.9 mN m(-1). Simulations showed that the bilayer thickness decreased linearly with tension consistent with volume-incompressibility, and this thinning was facilitated by a significant increase in acyl chain interdigitation at the bilayer midplane and spreading of the acyl chains. Tension caused a significant drop in the bilayer's peak electrostatic potential, which correlated with the strong reordering of water and lipid dipoles. For the low tension regime, the DPPC lateral diffusion coefficient increased with increasing tension in accordance with free-area theory. For larger tensions, free area theory broke down due to tension-induced changes in molecular shape and friction. Simulated DiI rotational and lateral diffusion coefficients were lower than those of DPPC but increased with tension in a manner similar to DPPC. Direct correlation of membrane order and viscosity near the DiI chromophore, which was just under the DPPC headgroup, indicated that measured DiI fluorescence lifetime, which is reported to decrease with decreasing lipid order, is likely to be a good reporter of tension-induced decreases in lipid headgroup viscosity. Together, these results offer new molecular-level insights into membrane tension-related mechanotransduction and into the utility of DiI in characterizing tension-induced changes in lipid packing.     

Atomistic modelling of the hydration of CaSO4

Atomistic modelling techniques, using empirical potentials, have been used to simulate a range of structures formed by the hydration of γ-CaSO₄ and described as CaSO₄·nH₂O (0<n<1). The hemihydrate phase (n=0.5) is of commercial importance and has been subjected to much experimental study. These simulation studies demonstrate significant water-matrix interactions that influence the crystallography of the hydrated phase. The existence of two types of hydration site has been predicted, including one within the Ca²⁺coordination sphere. Close correlation between water molecule bonding energy, Ca²⁺-O_w bond length and unit-cell volume have been established. This shows that as the number of water molecules within the unit cell increases, the bonding energy increases and the unit cell contracts. However around n=0.5, this process reaches a turning point with the incorporation of further waters resulting in reduced binding energy and an expanding unit cell.     

Effect of nanostructure on the properties of water at the water-hydrophobic interface: a molecular dynamics simulation

The local structure of water near hydrophobic surfaces of different surface topographies has been analyzed by molecular dynamics simulation. An alkane crystal has been taken as the parent model for a hydrophobic surface. Surface structures were created by placing pits into it, which were half a nanometer deep and several nanometers wide. Around all structures, the water has a lower density, less orientational ordering, fewer water-water hydrogen bonds, and fewer surface contacts than for a flat unstructured surface. This indicates that the structured surfaces are more hydrophobic than the flat surface. Of the structures investigated, pits with a diameter of approximately 2.5 nm were effective in increasing the hydrophobic character of the surface.     

An infrared study of the effects of hydration on cation‐loaded nafion thin films

We explore the effects of hydration on the vibrational spectra of cation‐exchanged Nafion thin films. In addition to the initial H⁺ form of the membrane, the cations selected for study include: Na⁺, NH, tetramethylammonium, tetrabutylammonium, hexadecyltrimethylammonium, and tris(2,2′‐bipyridine)osmium(II). A flow cell is used to follow the changes occurring in a 1000‐Å film in response to variations in the humidity of a nitrogen purge gas flowing through the cell. The structural changes occurring due to the binding of water to these cation‐exchanged membranes is followed across a broad range of water contents with IR difference spectroscopy. Our results demonstrate a complex role for water in mediating the polyanion–cation interactions. Side‐chain reconstructions may play an important role in the ion‐binding and transport properties of the ionomer domains. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1512–1520, 2000     

Studies on polypyrrole modified nafion membrane for vanadium redox flow battery

In order to prevent the vanadium crossover and preferential water transfer in all-vanadium redox flow battery (VRFB), three methods – electrolyte soaking, oxidation polymerisation and Electrodeposition, were used to modify Nafion 117 membranes using pyrrole. The surface of the modified membranes was uniform and even, and the membranes were characterised in terms of morphology, membrane area resistance, vanadium permeability and water transfer property. The properties of all the modified membranes were improved greatly. The membranes modified by Electrodeposition showed a best combination of the membrane resistance, vanadium permeability and water transfer property, the experimental results showed that the V(IV) ion permeability of polypyrrole modified Nafion membranes by Electrodeposition at the conditions of 0.025 mA cm⁻² and 0 ^°C for 60 min reduced more than 5 times from 2.87 × 10⁻⁶ cm² min⁻¹ to 5.0 × 10⁻⁷cm² min⁻¹, and the water transfer property decreased more than 3 times from 0.72 ml/72 h cm² to 0.22 ml/72 h cm². All above properties made the modified Nafion membranes more applicative in the VRFB system. This paper also reported other methods for Nafion membrane modification and the influences of the deposition conditions on the properties of the membrane selectivity and water transfer.     

Atomistic Brownian dynamics simulation of peptide phosphorylation

We report the implementation of an all-atom Brownian dynamics simulation model of peptides using the constraint algorithm LINCS. The algorithm has been added as a part of UHBD. It uses adaptive time steps to achieve a balance between computational speed and stability. The algorithm was applied to study the effect of phosphorylation on the conformational preference of the peptide Gly-Ser-Ser-Ser. We find that the middle serine residue experiences considerable conformational change from the C(7eq) to the alpha(R) structure upon phosphorylation. NMR (3)J coupling constants were also computed from the Brownian trajectories using the Karplus equation. The calculated (3)J results agree reasonably well with experimental data for phosphorylated peptide but less so for doubly charged phosphorylated one.     

Atomistic lattice simulation of high Tc oxides

Summary A survey is outlined of some of the normal-state crystal properties of high T_c oxides that have been calculated recently by atomistic lattice simulations.     

Irreversible thermodynamics of transport across charged membranes : Part I — Macroscopic resistance coefficients for a system with nafion 120 membrane

The transport of aqueous NaCl solutions across the perfluorinated Nafion 120 membrane is studied on the base of irreversible thermodynamics. The straight resistance coefficients r_ii, partial frictions f_ik^p and diffusion indices RT/c?_ir_ii are presented and discussed. The results suggest that the main force, which impedes the flow of chloride ions across the membrane is not the friction of these ions with the negatively charged polymer network, but the friction with water. The diffusion indices RT/c?_ir_ii exceed self-diffusion coefficients found by some authors while using tracer technique. Following Meares' suggestion such results point out to the convective flow contribution to the transport of ions and water.     

Atomistic computer simulation of mechanical properties of composites

An atomistic computer simulation is performed in order to investigate structure and mechanical behaviour of materials composed of fibres embedded in a polymer matrix. A graphite-poly(propylene) system is used as a model, the energy of which has been minimized. Unlike the models of other authors the bond angles of the polymer chains are assumed as flexible. The properties found were compared with those of the bulk polymer. The results show that the presence of the graphite surface affects the structure of the polymer as well as the values of the elastic constants within a layer of 7 Å thickness. In this layer the density has a maximum and the chain segments are oriented parallel by the graphite plane. A long range order, however, cannot be observed.     

Atomistic simulation of the glass transition of di-substituted polysilanes

Molecular dynamics has been used to determine the glass transition temperature of the amorphous phase of five di-substituted polysilanes from plots of specific volume versus temperature. In each case, good agreement was obtained between the simulation values and the reported DSC results. The effect of amorphous cell dimensions and equilibration time on T_g has been investigated. The use of larger cells provides better agreement with experimental T_g and probably more accurate densities as suggested by earlier studies. The effect of pressure on the T_g of two different polysilanes was also investigated. Although experimental data for comparison is unavailable, values obtained for dT_g/dp are consistent with those reported for other polymers. Vectorial autocorrelation analysis was used to explore the mobility of the polysilane main chains and side groups relative to polyalkanes, polyphosphazenes, and polysiloxanes.     

Efficient electrocatalyst utilization: electrochemical deposition of Pt nanoparticles using nafion membrane as a template

We deposit Pt particles electrochemically on an electrode covered with a Nafion membrane. Platinum ions travel through the hydrophilic channels of the membrane, and platinum deposits are formed at the place where the channels make contact with the planar electrode. This procedure deposits the catalyst only at the end of the hydrophilic channels that cross the membrane; no catalyst is placed under the hydrophobic domains, where it would not be in contact with the electrolyte. By performing a series of cyclic voltammograms with this system, we show that deposition of the platinum through the membrane achieves better platinum utilization than deposition of platinum on the naked electrode followed by the placement of the membrane on top.     

In situ nanostructure formation of (micro-hydroxo)bis(micro-carboxylato) diruthenium units in nafion membrane and its utilization for selective reduction of nitrosonium ion in aqueous medium

Nanostructured molecular film containing the (micro-hydroxo)bis(micro-carboxylato) diruthenium(III) units, [RuIII2(micro-OH)(micro-CH3COO)2(HBpz3)2]+ ({RuIII2(micro-OH)}), was prepared by an in situ conversion of its micro-oxo precursor, [RuIII2(micro-O)(micro-CH3COO)2(HBpz3)2] ({RuIII2(micro-O)}), in a Nafion membrane matrix, where HBpz3 is hydrotris(1-pyrazolyl)borate. The conversion procedure results in fine nanoparticle aggregates of the {RuIII2(micro-OH)} units in the Nafion membrane (Nf-{RuIII2(micro-OH)}), where an average particle size (4.1 +/- 2.3 nm) is close to the Nafion's cluster dimension of approximately 4 nm. Chemically modified electrodes by using the Nafion molecular membrane films (Nf-{RuIII2(micro-OH)}-MMFEs) were further developed on ITO/glass and glassy carbon electrode (GCE) surfaces, and a selective reduction of nitrosonium ion (NO+), presumably through reaction of a {RuIIRuIII(micro-OH)} mixed-valence state with HNO2, was demonstrated without interference by molecular oxygen in an acidic aqueous solution. The Nf-{RuIII2(micro-OH)}-MMFEs are stable even in a physiological condition (pH 7), where the naked {RuIII2(-OH)} complex is readily transformed into its deprotonated {RuIII2(micro-O)} form, demonstrating an unusual stabilizing effects for the {RuIII2(micro-OH)} unit by the Nafion cluster environment.