We use molecular dynamics simulations to study osmotically driven transport of water molecules through hexagonally packed carbon nanotube membranes. Our simulation setup comprises two such semipermeable membranes separating compartments of pure water and salt solution. The osmotic force drives water flow from the pure-water to the salt-solution compartment. Monitoring the flow at molecular resolution reveals several distinct features of nanoscale flows. In particular, thermal fluctuations become significant at the nanoscopic length scales, and as a result, the flow is stochastic in nature. Further, the flow appears frictionless and is limited primarily by the barriers at the entry and exit of the nanotube pore. The observed flow rates are high (5.8 water molecules per nanosecond and nanotube), comparable to those through the transmembrane protein aquaporin-1, and are practically independent of the length of the nanotube, in contrast to predictions of macroscopic hydrodynamics. All of these distinct characteristics of nanoscopic water flow can be modeled quantitatively by a 1D continuous-time random walk. At long times, the pure-water compartment is drained, and the net flow of water is interrupted by the formation of structured solvation layers of water sandwiched between two nanotube membranes. Structural and thermodynamic aspects of confined water monolayers are studied.
We present results on the thermodynamic and structural aspects of the hydration of hydrophobic solutes in three tetramethylammonium [N(CH 3 ) 4 + ] salt solutions at various concentrations obtained from molecular dynamics simulations. Monovalent counterions of different sizessF -, Cl -, and a relatively large model ion BIsare chosen in order to cover a range of kosmotropic to chaotropic behaviors. Chemical potentials of hard-sphere solutes obtained using test particle insertions display both salting-in and salting-out effects depending on the type of salt. Water and salt-ion densities in the vicinity of hard-sphere solutes are calculated. Small and strongly hydrated Fions (kosmotropes) are excluded from the vicinity of hydrophobic solutes, leading to an increase in local water densities near hydrophobic solutes (i.e., preferential hydration). This increases the excess chemical potential of hydrophobic solutes in solution which leads to salting-out. Opposite behavior is observed for large, less favorably hydrated BIions (chaotropes) which associate strongly with hydrophobic solutes. Compressive forces due to neighboring water molecules, cations, and anions on the surface of the hard sphere solute are calculated. We find that water molecules make the most significant contribution toward the total compressive force. This explains the observed linear correlation between the extent of preferential hydration or dehydration of the solute surface and salting-out or salting-in effects. The trends in the thermodynamics of hydration of hydrophobic solutes upon addition of salts are explained in terms of the structural hydration of individual salt ions.
The well defined shape and size of carbon nanotubes (CNTs) makes them attractive candidates for theoretical
and experimental studies of various nanoscopic phenomena such as protection and confinement of molecular
species as well as transport of molecules through their interior pores. Here we investigate solute partitioning
and transport using molecular dynamics simulations of CNTs in mixtures of hydrophobic solutes and water.
The hydrophobic pores of CNTs provide a favorable environment for partitioning of hydrophobic solutes.
We find that the transfer of a methane molecule from aqueous solution into the CNT interior is favored by
about 16 kJ/mol of free energy. In 50 molecular dynamics simulations, we observe that methane molecules
replace water molecules initially inside the nanotubes, and completely fill their interior channels over a
nanosecond time scale. Once filled with methane molecules, the nanotubes are able to transport methane
from one end to the other through successive methane uptake and release events at the tube ends. We estimate
a net rate of transport of about 11 methane molecules per nanotube and nanosecond for a 1 mol/L methane
concentration gradient. This concentration-corrected rate of methane transport even exceeds that of water
through nanotubes (∼1 per nanosecond at a 1 mol/L osmotic gradient). These results have implications for
the design of molecule-selective CNT devices that may act through mechanisms similar to those of biological
transmembrane channels.
Molecular dynamics simulations are used to determine the vibrational density of states for a model
montmorillonite clay as well as the spectral shifts with applied strain for significant peaks in the 1000−1300
cm-1 range. Under uniaxial deformation with fixed lateral dimensions, the spectral shifts are found to be around
−29 and −40 cm-1/% strain in the clay, with little dependence on direction of applied strain within the plane of
the clay platelet. Using Eshelby's method, a strain transfer efficiency of 5.6% is predicted for the nanocomposite
with 5 wt % exfoliated clay. This results in a predicted spectral shift of −1.6 to −2.2 cm-1/% macrostrain in the
nanocomposite, in reasonable agreement with the experimental results of Loo and Gleason (Macromolecules
2003, 36, 2587).
Oil production is generally a complicated multiphase flow inside pipelines, with possible water-in-oil (W/O) emulsions present with other usual phases such as free water and free oil. The W/O emulsions formed can present significant hurdles in production facilities for pumping fluids and during pipeline transport. It is well known that high shear rates provided by pumps, chokes, or valves result in stable emulsion behavior for a field in primary production. Several field tests are under way to test the potential of surfactant flooding as a tertiary-recovery mechanism. The effect of addition of surfactants on the emulsion rheology of production fluids, as in alkaline/ surfactant/polymer (ASP) flooding, is not very well understood. This understanding of W/O-emulsion rheology in ASP-injection oil recovery is essential for design of pumps and pipelines as well as for handling flow-assurance issues. In this paper, we report results from experiments as well as modeling of W/O-emulsion rheology that can form during ASP injections. We focus here only on the alkaline/surfactant (AS) part of these injections in order to clearly understand the impact of surfactants, removing the uncertainities that come with large rheology changes with polymer addition. The effect of surfactants on the rheology of W/O emulsions was studied by making two different types of emulsions: (1) native-brine W/O emulsions without surfactants to provide a baseline and (2) brine W/O emulsions with surfactants used in ASP injections. This way, the impact of ASP injections on emulsion rheology can easily be quantified. A new correlation is developed, based on in-house historical experimental data, to describe rheology of emulsions without surfactants. This correlation should assist in managing the uncertainties that come from extrapolating emulsion rheology measured in the laboratory to actual field conditions. Further, to understand the effect of ASP injections, new experimental measurements were made by adding surfactants to brine solutions. The addition of surfactants resulted in different rheology as compared with emulsions formed by brine solutions. These differences have been attributed to the W/O interfacial tension (IFT), and IFT was added to modify the original correlation. To our knowledge, this is the first study that explicitly relates emulsion rheology with IFT.
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