Graphene Oxide and Its Derivatives as Adsorbents for PFOA Molecules.

Effective, low-cost adsorbents are needed to remove perfluoroalkyl and polyfluoroalkyl substances (PFAS) from water sources. Carbon-based materials are promising PFAS adsorbents. Here, we explore the potential of graphite oxide (GO) and its derivatives as PFAS adsorbents by studying the adsorption of perfluorooctanoic acid (PFOA), a model PFAS molecule, on GO surfaces with O/C ratios up to 16.7% using molecular dynamics simulations. An adsorption free energy of approximately -30 kJ/mol (or -310 meV) is obtained for pristine graphene in pure water, and adsorbed PFOA molecules diffuse rapidly. As the O/C ratio increases, hydrophobic interactions' contribution to PFOA adsorption diminishes, but that by electrostatic interactions becomes important. Overall, adsorption is weakened, but favorable adsorption still occurs at an O/C ratio of 16.7%. The in-plane diffusion coefficient of adsorbed PFOA molecules decreases by more than 45 times as the O/C ratio increases to 8.3% but increases significantly when the O/C ratio increases further to 16.7%. Adding salt improves the adsorption owing to the salting-out and screening effects but slows the diffusion of adsorbed PFOA molecules, and these effects are more pronounced at low O/C ratios. These results show that GOs are effective PFOA adsorbents. Such effectiveness, along with GO's potentially low cost and the possibility of regenerating spent GO by removing adsorbed PFOA molecules through a mild electrical potential, makes GO a promising adsorbent for PFOA and similar molecules. The insights from the present study can help the rational design of GOs to realize their full potential.

Transport of Heptane Molecules across Water-Vapor Interfaces Laden with Surfactants.

Molecular transport across liquid-vapor interfaces covered by surfactant monolayers plays a key role in applications such as fire suppression by foams. The molecular understanding of such transport, however, remains incomplete. This work uses molecular dynamics simulations to investigate the heptane transport across water-vapor interfaces populated with sodium dodecyl sulfate (SDS) surfactants. Heptane molecules' potential of mean force (PMF) and local diffusivity profiles across SDS monolayers with different SDS densities are calculated to obtain heptane's transport resistance. We show that a heptane molecule experiences a finite resistance as it crosses water-vapor interfaces covered by SDS. Such interfacial transport resistance is contributed significantly by heptane molecules' high PMF in the SDS headgroup region and their slow diffusion there. This resistance increases linearly as the SDS density rises from zero but jumps as the density approaches saturation when its value is equivalent to that afforded by a 5 nm thick layer of bulk water. These results are understood by analyzing the micro-environment experienced by a heptane molecule crossing SDS monolayers and the local perturbation it brings to the monolayers. The implications of these findings for the design of surfactants to suppress heptane transport through water-vapor interfaces are discussed.

Adsorption of rhodamine 6G and choline on gold electrodes: a molecular dynamics study.

The adsorption of analyte molecules on nano-optoelectrodes (e.g. a combined nanoantenna and nanoelectrode device) significantly affects the signal characteristics in surface-enhanced Raman scattering (SERS) measurements. Understanding how different molecules adsorb on electrodes and their electrical potential modulation helps interpret SERS measurements better. We use molecular dynamics simulations to investigate the adsorption of prototypical analyte molecules (rhodamine 6G and choline) on gold electrodes with negative, neutral, and positive surface charges. We show that both molecules can readily adsorb on gold surfaces at all surface charge densities studied. Nevertheless, the configurations of the adsorbed molecules can differ for different surface charge densities, and adsorption can also change a molecule's conformation. Rhodamine 6G molecules adsorb more strongly than choline molecules, and the adsorption of both molecules is affected by electrode charge in 0.25 M NaCl solutions. The mechanisms of these observations are elucidated, and their implications for voltage-modulated SERS measurements are discussed.

Particle actuation by rotating magnetic fields in microchannels: a numerical study.

Magnetic particles confined in microchannels can be actuated to perform translation motion using a rotating magnetic field, but their actuation in such a situation is not yet well understood. Here, the actuation of a ferromagnetic particle confined in square microchannels is studied using immersed-boundary lattice Boltzmann simulations. In wide channels, when a sphere is positioned close to a side wall but away from channel corners, it experiences a modest hydrodynamic actuation force parallel to the channel walls. This force decreases as the sphere is shifted toward the bottom wall but the opposite trend is found when the channel is narrow. When the sphere is positioned midway between the top and bottom channel walls, the actuation force decreases as the channel width decreases and can reverse its direction. These phenomena are elucidated by studying the flow and pressure fields in the channel-particle system and by analyzing the viscous and pressure components of the hydrodynamic force acting on different parts of the sphere.

Magnetic Actuation of Surface Walkers: The Effects of Confinement and Inertia.

Driven by a magnetic field, the rotation of a particle near a wall can be rectified into a net translation. The particles thus actuated, or surface walkers, are a kind of active colloid that finds application in biology and microfluidics. Here, we investigate the motion of spherical surface walkers confined between two walls using simulations based on the immersed-boundary lattice Boltzmann method. The degree of confinement and the nature of the confining walls (slip vs no-slip) significantly affect a particle's translational speed and can even reverse its translational direction. When the rotational Reynolds number Reω is larger than 1, inertia effects reduce the critical frequency of the magnetic field, beyond which the sphere can no longer follow the external rotating field. The reduction of the critical frequency is especially pronounced when the sphere is confined near a no-slip wall. As Reω increases beyond 1, even when the sphere can still rotate in the synchronous regime, its translational Reynolds number ReT no longer increases linearly with Reω and even decreases when Reω exceeds ∼10.