Alberto Striolo

Water in carbon nanotubes: Adsorption isotherms and thermodynamic properties from molecular simulation

Grand canonical Monte Carlo simulations are performed to study the adsorption of water in single-walled (6:6), (8:8), (10:10), (12:12), and (20:20) carbon nanotubes in the 248-548 K temperature range. At room temperature the resulting adsorption isotherms in (10:10) and wider single-walled carbon nanotubes (SWCNs) are characterized by negligible water uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption/desorption hysteresis loops. The width of the hysteresis loops decreases as pore diameter narrows and it becomes negligible for water adsorption in (8:8) and (6:6) SWCNs. Results for the isosteric heat of adsorption, density profiles along the pore axis and across the pore radii, order parameter across the pore radii, and x-ray diffraction patterns are presented. Layered structures are observed when the internal diameter of the nanotubes is commensurate to the establishment of a hydrogen-bonded network. The structure of water in (8:8) and (10:10) SWCNs is ordered when the temperature is 298 and 248 K, respectively. By simulating adsorption isotherms at various temperatures, the hysteresis critical temperature, e.g., the lowest temperature at which no hysteresis can be detected, is determined for water adsorbed in (20:20), (12:12), and (10:10) SWCNs. The hysteresis critical temperature is lower than the vapor-liquid critical temperature for bulk Simple Point Charge-Extended (SPCE) water model.

Simulated water adsorption isotherms in carbon nanopores

Water adsorption isotherms are calculated by grand canonical Monte Carlo simulations for the SPC/E water model in carbon nanopores at 298 K. The pores are of slit or cylindrical morphology. Carbon-slit pores are of widths 0.8, 1.0 and 1.6 nm. The simulated single-walled carbon nanotubes are of 1.4 and 2.7 nm diameter ((10:10) and (20:20) respectively). In all cases considered, the adsorption isotherms are characterized by negligible adsorption at low pressures, pore filling by a capillary-condensation-like mechanism and adsorption–desorption hysteresis loops. For both pore morphologies considered, the relative pressures at which pore filling occurs, and the width of the adsorption–desorption hysteresis loop decrease with decreasing pore size. Adsorption isotherms simulated for water in carbon nanotubes show pore filling at lower relative pressures and narrower adsorption–desorption hysteresis loops when compared to adsorption isotherms simulated in carbon-slit pores of similar sizes. By using representative simulation snapshots, the mechanisms of pore filling and pore emptying are discussed. Pore filling happens by growth of hydrogen-bonded clusters of adsorbed water molecules, without the formation of monolayers as observed in the adsorption of simple fluids. Pore emptying occurs by the formation of bubbles, often in contact with the hydrophobic surface, followed by the coalescence and growth of these bubbles.

The depletion attraction between Pairs of colloid particles in polymer solution

NVT Monte Carlo simulations were used to assess the effective interaction between pairs of colloid particles dissolved in non-adsorbing polymer solutions. The polymers were represented as freely-jointed-hard-sphere chains composed of 10, 20, or 30 segments. The size of the interacting colloid particles was similar to or smaller than the radius of gyration of the polymers. Results show a short-range colloid–colloid depletion attraction. At low polymer concentration, this attraction slowly decays to zero at increasing separations. At higher polymer concentration, the depletion attraction is coupled to a mid-range repulsion, especially for solutions of short, stiff polymers. From the simulated forces, osmotic second virial coefficients were computed for colloids as a function of polymer concentration. The calculated osmotic second virial coefficients exhibit a non-monotonic dependence on polymer concentration, in qualitative agreement with experimental results. The simulated colloid–colloid potentials of mean force were used, within a perturbation theory, to calculate fluid–fluid and fluid–solid coexistence curves. The colloids are treated as a pseudo one-component system, and the polymers in solution are considered only through the effective pair potential between the dissolved colloids. When long flexible polymers are dissolved in solution, the phase diagram for small colloid particles shows a fluid–fluid coexistence curve at low colloid packing fraction, and a fluid–solid coexistence curve at higher packing fraction. As the size of the colloid particles increases, the molecular weight of the polymer decreases, or the polymer concentration in solution increases, the fluid–fluid coexistence curve becomes metastable with respect to the fluid–solid coexistence curve.

Coarse-grained force field for simulating polymer-tethered silsesquioxane self-assembly in solution

A coarse-grained model has been developed for simulating the self-assembly of nonyl-tethered polyhedral oligomeric silsesquioxane (POSS) nanoparticles in solution. A mapping scheme for groups of atoms in the atomistic molecule onto beads in the coarse-grained model was established. The coarse-grained force field consists of solvent-mediated effective interaction potentials that were derived via a structural-based coarse-graining numerical iteration scheme. The force field was obtained from initial guesses that were refined through two different iteration algorithms. The coarse-graining scheme was validated by comparing the aggregation of POSS molecules observed in simulations of the coarse-grained model to that observed in all-atom simulations containing explicit solvent. At 300 K the effective coarse-grained potentials obtained from different initial guesses are comparable to each other. At 400 K the differences between the force fields obtained from different initial guesses, although small, are noticeable. The use of a different iteration algorithm employing identical initial guesses resulted in the same overall effective potentials for bare cube corner bead sites. In both the coarse-grained and all-atom simulations, small aggregates of POSS molecules were observed with similar local packings of the silsesquioxane cages and tether conformations. The coarse-grained model afforded a savings in computing time of roughly two orders of magnitude. Further comparisons were made between the coarse-grained monotethered POSS model developed here and a minimal model developed in earlier work. The results suggest that the interactions between POSS cages are long ranged and are captured by the coarse-grained model developed here. The minimal model is suitable for capturing the local intermolecular packing of POSS cubes at short separation distances.

Adsorption of comb copolymers on weakly attractive solid surfaces

In this work continuum and lattice Monte Carlo simulation methods are used to study the adsorption of linear and comb polymers on flat surfaces. Selected polymer segments, located at the tips of the side chains in comb polymers or equally spaced along the linear polymers, are attracted to each other and to the surface via square-well potentials. The rest of the polymer segments are modeled as tangent hard spheres in the continuum model and as self-avoiding random walks in the lattice model. Results are presented in terms of segment-density profiles, distribution functions, and radii of gyration of the adsorbed polymers. At infinite dilution the presence of short side chains promotes the adsorption of polymers favoring both a decrease in the depletion-layer thickness and a spreading of the polymer molecule on the surface. The presence of long side chains favors the adsorption of polymers on the surface, but does not permit the spreading of the polymers. At finite concentration linear polymers and comb polymers with long side chains readily adsorb on the solid surface, while comb polymers with short side chains are unlikely to adsorb. The simple models of comb copolymers with short side chains used here show properties similar to those of associating polymers and of globular proteins in aqueous solutions, and can be used as a first approximation to investigate the mechanism of adsorption of proteins onto hydrophobic surfaces.

Influence of polymer structure upon active-ingredient loading: a Monte Carlo simulation study for design of drug-delivery devices

Abstract Drug-loaded polymers and polymeric microparticles provide an attractive form for controlled drug-delivery systems. Design of new systems requires knowledge of polymer–drug interactions. The effect of polymer architecture and chemistry upon active-ingredient loading is investigated by Monte Carlo simulation. The ensemble-growth method is used to sample conformations of a model polymer comprising polar and nonpolar segments. The polymer is a block copolymer, linear or branched. In our calculations, the polar portion of the polymer contains 21 segments. The polymers are dissolved in either of two types of solvent models. In the first, nonpolar solvent, the polar segments tend to collapse, but the bulky nonpolar groups, easily soluble in the medium, create some cavities in the polymer. These cavities are suitable hosts for the slightly polar active ingredient. In the second solvent, polar, the nonpolar segments contribute to attract the active ingredient within the polymer segments, therefore lowering the burst-release rate. The relative uptake of the active ingredient, proportional to the probability of finding an active ingredient within the radius of gyration of the polymer, is computed as a function of the number of nonpolar segments in the polymer. Simulation results are reported for active ingredients of two different sizes. For given size of the polar portion, short nonpolar tails increase the active-ingredient relative uptake in both solvents considered. Linear block copolymers look promising for obtaining higher entrapment efficiency for the active ingredient and for controlled release.

Thermodynamic modeling of high-pressure equilibria within the McMillan–Mayer framework

Abstract In this work, a new thermodynamic method, based on the McMillan–Mayer solution theory, is proposed to interpret and predict the solubility of low- and high-molecular-weight compounds in compressed CO2. In the thermodynamic approach presented here, the solute is referred to as a pseudo pure component while the compressed CO2 is represented as a continuous medium that affects the interactions among solute molecules. The perturbed-hard-sphere-chain (PHSC) theory is used within the McMillan–Mayer framework to derive an expression for the repulsive and attractive contributions to the Helmholtz free energy of the solute. While easy to handle, the model enlightens the effects of molecular weight and other physical–chemical characteristics on compounds solubility in compressed media. The thermodynamic approach fairly describes the experimental data concerning the solubility of several substances in compressed CO2 at different temperatures. The model also predicts CO2 solubility in PEG polymer and semi-quantitatively reproduces high-molecular-weight component solubility in compressed CO2 containing low amount of ethanol as co-solvent.