Wetting of doped carbon nanotubes by water droplets

Test-area surface tension calculation of the graphene-methane interface: Fluctuations and commensurability

The surface tension (γ) of methane on a graphene monolayer is calculated by using the test-area approach. By using a united atom model to describe methane molecules, strong fluctuations of surface tension as a function of the surface area of the graphene are evidenced. In contrast with the liquid-vapor interfaces, the use of a larger cutoff does not fully erase the fluctuations in the surface tension. Counterintuitively, the description of methane and graphene from the Optimized Potentials for Liquid Simulations all-atom model and a flexible model, respectively, led to a lessening in the surface tension fluctuations. This result suggests that the origin of fluctuations in γ is due to a model-effect rather than size-effects. We show that the molecular origin of these fluctuations is the result of a commensurable organization between both graphene and methane. This commensurable structure can be avoided by describing methane and graphene from a flexible force field. Although differences in γ with respect to the model have been often reported, it is the first time that the model drastically affects the physics of a system.

Strong Coupling between Nanofluidic Transport and Interfacial Chemistry: How Defect Reactivity Controls Liquid-Solid Friction through Hydrogen Bonding

Defects are inevitably present in nanofluidic systems, yet the role they play in nanofluidic transport remains poorly understood. Here, we report ab initio molecular dynamics (AIMD) simulations of the friction of liquid water on defective graphene and boron nitride sheets. We show that water dissociates at certain defects and that these "reactive" defects lead to much larger friction than the "nonreactive" defects at which water molecules remain intact. Furthermore, we find that friction is extremely sensitive to the chemical structure of reactive defects and to the number of hydrogen bonds they can partake in with the liquid. Finally, we discuss how the insight obtained from AIMD can be used to quantify the influence of defects on friction in nanofluidic devices for water treatment and sustainable energy harvesting. Overall, we provide new insight into the role of interfacial chemistry on nanofluidic transport in real, defective systems.

Evidencing the mask effect of graphene oxide: a comparative study on primary human and murine phagocytic cells

Graphene oxide (GO) is attracting an ever-growing interest in different fields and applications. Not much is known about the possible impact of GO sheet lateral dimensions on their effects in vitro, especially on human primary cells. In an attempt to address this issue, we present a study to evaluate, how highly soluble 2-dimensional GO constituted of large or small flakes affects human monocyte derived macrophages (hMDM). For this purpose, the lateral size of GO was tuned using sonication and three samples were obtained. The non sonicated one presented large flakes (~1.32 μm) while sonication for 2 and 26 hours generated small (~0.27 μm) and very small (~0.13 μm) sheets of GO, respectively. Cell studies were then conducted to evaluate the cytotoxicity, the oxidative stress induction, the activation potential and the pro-inflammatory effects of these different types of GO at increasing concentrations. In comparison, the same experiments were run on murine intraperitoneal macrophages (mIPM). The interaction between GO and cells was further examined by TEM and Raman spectroscopy. Our data revealed that the GO sheet size had a significant impact on different cellular parameters (i.e. cellular viability, ROS generation, and cellular activation). Indeed, the more the lateral dimensions of GO were reduced, the higher were the cellular internalization and the effects on cellular functionality. Our data also revealed a particular interaction of GO flakes with the cellular membrane. In fact, a GO mask due to the parallel arrangement of the graphene sheets on the cellular surface was observed. Considering the mask effect, we have hypothesized that this particular contact between GO sheets and the cell membrane could either promote their internalization or isolate cells from their environment, thus possibly accounting for the following impact on cellular parameters.

Biological interactions and safety of graphene materials

As graphene technologies progress to commercialization and large-scale manufacturing, issues of material and processing safety will need to be more seriously considered. The single word "graphene" actually represents a family of related materials with large variations in number of layers, surface area, lateral dimensions, stiffness, and surface chemistry. Many of these materials have aerodynamic diameters below 5 μm and can potentially be inhaled into the human lung. Graphene materials show several unique modes of interaction with biological molecules, tissues, and cells. The limited literature suggests that graphene materials can be either benign or harmful and that the biological response varies according to a material's physicochemical properties and biologically effective dose. The present article reviews the current literature on the graphene-biological interface with an emphasis on the mechanisms and fundamental biological responses relevant to material safety and also to potential biomedical applications.

Wetting and contact-line effects for spherical and cylindrical droplets on graphene layers: a comparative molecular-dynamics investigation

In molecular dynamics (MD) simulations, interactions between water molecules and graphitic surfaces are often modeled as a simple Lennard-Jones potential between oxygen and carbon atoms. A possible method for tuning this parameter consists of simulating a water nanodroplet on a flat graphitic surface, measuring the equilibrium contact angle, extrapolating it to the limit of a macroscopic droplet, and finally matching this quantity to experimental results. Considering recent evidence demonstrating that the contact angle of water on a graphitic plane is much higher than what was previously reported, we estimate the oxygen-carbon interaction for the recent SPC/Fw water model. Results indicate a value of about 0.2 kJ/mol, much lower than previous estimations. We then perform simulations of cylindrical water filaments on graphitic surfaces, in order to compare and correlate contact angles resulting from these two different systems. Results suggest that a modified Young's equation does not describe the relation between contact angle and drop size in the case of extremely small systems and that contributions different from the one deriving from contact line tension should be taken into account.

Direct measurement of the wetting behavior of individual carbon nanotubes by polymer melts: the key to carbon nanotube-polymer composites

Carbon nanotube wetting is the critical parameter for the development of nanocomposites yet, due to the lack of suitable methods of qualifying and, more importantly, quantifying nanoscale wetting and adhesion phenomena, it is often overlooked. Here, we discuss a qualitative approach that provides wetting/nonwetting information and present a microfluidics method to produce "nanodroplets" of polymers on individual carbon nanotubes which enable the direct quantification of contact angles.

The environmental effect on the radial breathing mode of carbon nanotubes. II. Shell model approximation for internally and externally adsorbed fluids

We have previously shown that the upshift in the radial breathing mode (RBM) of closed (or infinite) carbon nanotubes in solution is almost entirely due to coupling of the RBM with an adsorbed layer of fluid on the nanotube surface. The upshift can be modeled analytically by considering the adsorbed fluid as an infinitesimally thin shell, which interacts with the nanotube via a continuum Lennard-Jones potential. Here we extend the model to include internally as well as externally adsorbed waterlike molecules, and find that filling the nanotubes leads to an additional upshift of two to six wave numbers. We show that using molecular dynamics, the RBM can be accurately reproduced by replacing the fluid molecules with a mean field harmonic shell potential, greatly reducing simulation times.

Simulated water adsorption in chemically heterogeneous carbon nanotubes

Grand canonical Monte Carlo simulations are used to study the adsorption of water in single-walled (10:10), (12:12), and (20:20) carbon nanotubes at 298 K. Water is represented by the extended simple point charge model and the carbon atoms as Lennard-Jones spheres. The nanotubes are decorated with different amounts of oxygenated sites, represented as carbonyl groups. In the absence of carbonyl groups the simulated isotherms are characterized by negligible amounts of water uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption-desorption hysteresis loops. In the presence of a few carbonyl groups the simulated adsorption isotherms are characterized by pore filling at lower pressures and by narrower adsorption-desorption hysteresis loops compared to the results obtained in the absence of carbonyl groups. Our results show that the distribution of the carbonyl groups has a strong effect on the adsorption isotherms. For carbonyl groups localized in a narrow section the adsorption of water may be gradual because a cluster of adsorbed water forms at low pressures and grows as the pressure increases. For carbonyl groups distributed along the nanotube the adsorption isotherm is of type V.