Water wettability of graphene: interplay between the interfacial water structure and the electronic structure

Wetting phenomena are ubiquitous and impact a wide range of applications. Simulations so far have largely relied on classical potentials. Here, we report the development of an approach that combines density-functional theory (DFT)-based calculations with classical wetting theory that allows practical but sufficiently accurate determination of the water contact angle (WCA). As a benchmark, we apply the approach to the graphene and graphite surfaces that recently received considerable attention. The results agree with and elucidate the experimental data. For metal-supported graphene where electronic interactions play a major role, we demonstrate that doping of graphene by the metal substrate significantly alters the wettability. In addition to theory, we report new experimental measurements of the WCA and the force of adhesion that corroborate the theoretical results. We demonstrate a correlation between the force of adhesion and WCA, and the use of the atomic force microscope (AFM) technique as an alternative measure for wettability at the nanoscale. The present work not only provides a detailed understanding of the wettability of graphene, including the role of electrons, but also sets the stage for studying the wettability alteration mechanism when sufficiently accurate force fields may not be available.

Wettability of graphene is characterized from first principles.

Wetting phenomena are ubiquitous in a variety of practical issues, including adhesion,¹ friction,² interfacial thermal conductance (Kapitza conductance),^3,4 to name just a few. Graphene has emerged as an important material for applications where water wettability plays a major role, e.g., as a lubricant,⁵ small-molecule gas sensor,^6,7 desalination membrane,^8,9 protective coating from electrochemical degradation,¹⁰ promotive coating for dropwise condensation,¹¹etc. Among these applications, to wet or not to wet is the key problem.^12,13 More recently, doping-induced tunable wettability was reported for graphene.^14,15Interactions between the wetting liquid and the solid it rests on are responsible for the wetting properties of a surface. The binding energy of individual molecules on the solid surface, obtained by quantum-mechanical calculations, has at times been used as an indicator of wettability.^14,16 A better indicator, however, is the water contact angle (WCA), an experimentally easily accessible parameter that characterizes macroscopically a surface''s wettability by water.^17,18 Large WCA, >90°, signifies hydrophobic behavior, whereas small WCA, <90°, signifies hydrophilic behavior. The past few years have witnessed increasing efforts to understand the wetting mechanism of graphitic carbon surfaces. Theoretical calculations of WCAs of graphitic carbon surfaces have so far been done primarily using classical potentials, by constructing an analytical interaction potential between water and the solid surface based on interatomic Lennard-Jones potentials^19–21 or classical molecular dynamics (CMD) simulations.^19,22–29 In those pioneering studies with the work-of-adhesion approach, one first computes the work of adhesion of a water slab on a surface and then employs the Young-Dupré equation that relates the work of adhesion to the WCA.^19,25–28 Alternatively, CMD can be sued to measures the shape of a water droplet on a surface and extract the WCA.^22–24Though these approaches have provided significant insights into wetting behavior,^19,22–28 they depend on the availability of reliable classical potentials. The construction of such potentials becomes a difficult task when many atomic species are present. For example, the interplay between ions such as Ca²⁺, Mg²⁺, SO₄²⁻, Na⁺ and Cl⁻ in saline water and calcite (CaCO₃) surfaces is responsible for the wettability alteration for oil recovery.^30,31 There are also cases, e.g., monolayers on metallic substrates, where explicit electron doping effects may play a major role, requiring electronic-structure calculations. While density functional theory (DFT) is the method of choice for predictive, atomic-scale calculations of interactions at the solid–water interface, the major difficulty lies in the limited time and length scales achievable by quantum MD (QMD) simulations.³² So far there exists only one report of QMD simulations of water nanodroplets, on graphene and hexagonal boron nitride monolayers.³³In this paper, we adopt the method based on the work of adhesion and the Young-Dupré equation, and employ an approximation that allows practical but sufficiently accurate DFT-based determination of the WCA. Benchmark calculations confirm that graphitic carbon surfaces are nonpolar and intrinsically hydrophilic, with their wettability determined by the dispersive interaction. The WCA gradually decreases with increasing number of graphene layers N, while a monolayer of adsorbed hydrocarbons is sufficient to render graphene hydrophobic, complementing similar results obtained using classical potentials.^20,21,25,34 In the presence of a metal substrate, electronic structure comes into play and electron doping of the graphene sheet by the metal substrate alters the wettability of graphene. We report new WCA and AFM (atomic force microscopy) measurements for Cu-supported monolayer and multilayer graphene that further corroborate the theoretical results. Finally, we demonstrate a correlation between the force of adhesion and WCA, and the use of the AFM technique as an alternative measure for wettability at the nanoscopic scale.     

Subclinical leaflet thrombosis after transcatheter aortic valve replacement and anticoagulant treatment: lights and shadows

Journal of Thrombosis and Thrombolysis -     

Italian experience with rVIII-single chain: a survey of patients with haemophilia A and their physicians

Journal of Thrombosis and Thrombolysis - rVIII-SingleChain is indicated for treatment and prophylaxis of bleeding in patients with haemophilia A (HA). The safety and efficacy of rVIII-SingleChain...     

Comparison of antithrombotic strategies in patients with cryptogenic stroke and patent foramen ovale: an updated meta-analysis

Purpose

Patients with patent foramen ovale (PFO) and cryptogenic ischemic stroke (CS) are at risk for stroke recurrence. The optimal antithrombotic strategy in patients who undergo medical management is still debated.

Methods

We systematically searched the literature for studies that reported on cerebrovascular event recurrences and/or death in patients with PFO treated with oral anticoagulation (OAC) or antiplatelet therapy (APT) for secondary prevention of CS. The efficacy endpoints were stroke recurrence and the composite of stroke, transient ischemic attack or all-cause death. Major bleedings represented the safety endpoint.

Results

A total of 16 studies with 3953 patients (OAC?=?1527, APT?=?2426) were included. Weighted mean follow-up was 2.9 years. OAC was associated with a significant reduction in the risk of stroke compared with APT (RR 0.65; 95% CI 0.44–0.95; ARR 2%, NNT 49), while no difference was found regarding the composite outcome (RR 0.78; 95% CI 0.57–1.07) and the safety outcome (RR 1.57; 95% CI 0.85–2.90; p?=?0.15).

Conclusions

OAC was more effective than APT in reducing the risk of stroke recurrence in patients with PFO and CS, without a significant increase in the risk of major bleedings. Our findings support the need for further randomized data focused on the comparison of antithrombotic strategies in this setting.

     

Association between vitamin D deficiency and serum Homocysteine levels and its relationship with coronary artery disease

Journal of Thrombosis and Thrombolysis - Homocysteine (Hcy) elevation and vitamin D deficiency have emerged as potential markers of coronary artery disease (CAD). However, even tough...     

Performance of transcatheter pacing system use in relation to patients’ age

Journal of Interventional Cardiac Electrophysiology - Advanced non-fluoroscopic mapping systems for radiofrequency ablation (RFA) have shown to be an effective treatment of atrial fibrillation....     

Mechanical circulatory support in patients with cardiogenic shock not secondary to cardiotomy: a network meta-analysis

Heart Failure Reviews - To compare the efficacy and safety of different mechanical circulatory support (MCS) devices in CS. A total of 24 studies (7 randomized controlled...     

Speckle tracking echocardiography in heart failure development and progression in patients with apneas

Heart Failure Reviews - Obstructive (OA) and central apneas (CA) are highly prevalent breathing disorders that have a negative impact on cardiac structure and function; while OA promote the...