Fernando J. A. L. Cruz

Thermodynamics of adsorption of light alkanes and alkenes in single-walled carbon nanotube bundles

The thermodynamics of adsorption of light alkanes and alkenes (CH4, C2H6, C2H4, C3H8, and C3H6) in single-walled carbon nanotube bundles is studied by configurational-bias grand canonical Monte Carlo simulation. The bundles consist of uniform nanotubes with diameters in the range 11.0 < D (A) < 18.1, arranged in the usual close-packed hexagonal lattice. The phase space is systematically analyzed with calculations for adsorption at room temperature and reduced pressure range of 8.7 x 10-9 < (p/p0) < 0.9. The simulation results are interpreted in terms of the molecular nature of the adsorbate and the corresponding solid-fluid interactions. It is shown that confinement in the internal volume of the bundle (interstitial and intratubular) is energetically more favorable than physisorption on the external surface (grooves and exposed surfaces of peripheral tubes), as indicated by the curves of isosteric heat as a function of reduced pressure. However, the zero-loading properties suggest a crossover point to this behavior for D = 18 - 19 A. When interstitial confinement is not inhibited by geometrical considerations, it is possible to establish the following ordering of the zero-loading isosteric heat by type of adsorption site.

Free energy landscapes of the encapsulation mechanism of DNA nucleobases onto carbon nanotubes

The confinement mechanism of DNA nucleobases onto single-walled carbon nanotubes is probed at physiological temperature (310 K) using classical molecular dynamics and fully atomistic molecular potentials. The corresponding free energy landscapes are constructed in terms of two order parameters, ξ1 and ξ2, for both purine (adenine) and pyrimidine (thymine) moieties, by a metadynamics scheme. Two zig-zag topologies, (16, 0) and (23, 0), are employed as solid models with limiting skeletal diameters, D = 1.25 nm and D = 1.8 nm, respectively, considering hydrophobic and electrically charged nanopores, q = +0.05 e−/C. The uncharged nanotubes always exhibit a single free energy minimum located close to the pore center, regardless of individual diameter, whereas the charged nanotubes either inhibit encapsulation or exhibit two energetically distinct confining regions, at the center and close to the pore termini. Very interestingly, the purely hydrophobic solids exhibit a quasi-isotropic free-energy landscape around the nanopore center, F (ξ1, ξ2). Independent umbrella sampling calculations show that for the (23, 0) electrically charged nanotubes, the probability maximum at a distance of Ω = 0.54 nm from the pore center is the global one for adenine. These subtle differences are quantified by decomposing the interaction energy between a nucleobase and the solid into its dispersive and electrostatic contributions, and a critical comparison with DFT and experimental data for graphite is discussed. Purine and pyrimidine show similar interaction energies within the hydrophobic nanopores, however, adenine confinement results in slightly more stable hybrids by 0.9–1.4 kJ mol−1; binding affinity increases with the nanotube curvature as a result of enhanced dispersive energy. The detailed thermodynamical analysis reported herein is of paramount importance to study the sequence specific properties of DNA/carbon nanotube hybrids.

The role of the intermolecular potential on the dynamics of ethylene confined in cylindrical nanopores

The influence of the molecular force field upon the transport properties of ethylene is studied by molecular dynamics simulations, addressing both the bulk fluid and the confined phases within pristine single-walled carbon nanotubes. Five different molecular models with different degrees of coarse-graining were selected, spanning from a simple isotropic Lennard–Jones sphere to a fully detailed all-atom description. The fluid was probed for its self-diffusion coefficient under isochoric (0.026 ≤ ρ (mol L−1) ≤ 15.751) and isothermal (220 ≤ T (K) ≤ 340) conditions, both in the sub- and supercritical regions (Tbulkc = 282.4 K). Although the particular details of each potential model are seen to be nearly irrelevant to the bulk fluid dynamics, they are crucial to correctly describe the inhomogeneous system. The most important aspects affecting fluid transport are the existence/absence of explicit electrostatic contributions and the molecular shape. The effect of temperature on the confined fluid self-diffusion is described by the Arrhenius law, D = Aexp(−Ea/RT), and the nonlinear density dependencies of the activation energy (Ea/R) and pre-exponential factor (A) have been fitted here to empirical equations. In spite of the quasi one-dimensional confining nature of SWCNTs, the isothermal results (T = 300 K) obtained for the bulk and confined systems collapse onto a unique master curve, D = D0ρλ, suggesting that the self-diffusion coefficient of a confined fluid can be estimated from its molecular density, an easily accessible property.