Computational IR spectroscopy of interfacial water at fluorinated and non-fluorinated hydrophobic surfaces

We report ab-initio simulations of the interface between water and fluorinated and non-fluorinated hydrocarbon self assembled monolayers (SAMs) and compare with the prototypical interfacial system, the vapor-water interface. The thickness of the microscopic depletion layer between SAMs and water is larger for the fluorinated SAM, consistent with the larger contact angle of fluorinated SAMs. We calculate the infrared absorption spectrum of interfacial water, which displays a prominent sharp peak at around 3700 $\mathrm{cm}^{-1}$, signaling the presence of dangling OH bonds. We describe the vibrational properties of dangling OH bonds by a harmonic model and show that spectral line shifts reflect OH-dangling-bond interactions with the surface and line widths report on the rotational lifetimes of dangling OH configurations

Electrokinetic, electrochemical and electronic surface potentials of the pristine water liquid-vapor interface

Although conceptually simple, the air-water interface displays rich behavior. Different definitions of the electrostatic potential, each relevant for distinct experimental scenarios, lead to widely varying surface potential magnitudes and even different signs. Based on quantum-chemical density-functional molecular dynamics simulations, all relevant surface potentials are evaluated and compared. The spatially averaged surface potential, accessible to electron holography, is dominated by the trace of the water molecular quadrupole moment and amounts to more than + 4 V inside the water phase, very different from results obtained with force-field water models. The surface potential inside a cavity is much smaller, less than 200 mV in magnitude, and depends specifically on the cavity radius. This is the electrochemical surface potential relevant for ion transfer reactions and ion surface adsorption. Charge transfer between water molecules leads to pronounced surface potentials as well. However, when probing electrophoresis by explicitly applying a lateral electric field, the zeta potential turns out to be zero. Thus, charge transfer between water molecules does not translate to a non-zero electrophoretic mobility at the pristine vapor-liquid water interface

Electrokinetic, electrochemical, and electrostatic surface potentials of the pristine water liquid–vapor interface

Although conceptually simple, the air–water interface displays rich behavior and is subject to intense experimental and theoretical investigations. Different definitions of the electrostatic surface potential as well as different calculation methods, each relevant for distinct experimental scenarios, lead to widely varying potential magnitudes and sometimes even different signs. Based on quantum-chemical density-functional-theory molecular dynamics (DFT-MD) simulations, different surface potentials are evaluated and compared to force-field (FF) MD simulations. As well explained in the literature, the laterally averaged electrostatic surface potential, accessible to electron holography, is dominated by the trace of the water molecular quadrupole moment, and using DFT-MD amounts to +4.35 V inside the water phase, very different from results obtained with FF water models which yield negative values of the order of −0.4 to −0.6 V. Thus, when predicting potentials within water molecules, as relevant for photoelectron spectroscopy and non-linear interface-specific spectroscopy, DFT simulations should be used. The electrochemical surface potential, relevant for ion transfer reactions and ion surface adsorption, is much smaller, less than 200 mV in magnitude, and depends specifically on the ion radius. Charge transfer between interfacial water molecules leads to a sizable surface potential as well. However, when probing electrokinetics by explicitly applying a lateral electric field in DFT-MD simulations, the electrokinetic ζ-potential turns out to be negligible, in agreement with predictions using continuous hydrodynamic models. Thus, interfacial polarization charges from intermolecular charge transfer do not lead to significant electrokinetic mobility at the pristine vapor–liquid water interface, even assuming these transfer charges are mobile in an external electric field

Modeling Water Interactions with Graphene and Graphite via Force Fields Consistent with Experimental Contact Angles

Accurate simulation models for water interactions with graphene and graphite are important for nanofluidic applications, but existing force fields produce widely varying contact angles. Our extensive review of the experimental literature reveals extreme variation among reported values of graphene–water contact angles and a clustering of graphite–water contact angles into groups of freshly exfoliated (60° ± 13°) and not-freshly exfoliated graphite surfaces. The carbon–oxygen dispersion energy for a classical force field is optimized with respect to this 60° graphite–water contact angle in the infinite-force-cutoff limit, which in turn yields a contact angle for unsupported graphene of 80°, in agreement with the mean of the experimental results. Interaction force fields for finite cutoffs are also derived. A method for calculating contact angles from pressure tensors of planar equilibrium simulations that is ideally suited to graphite and graphene surfaces is introduced. Our methodology is widely applicable to any liquid-surface combination

Enhanced interfacial water dissociation on a hydrated iron porphyrin single-atom catalyst in graphene

Single Atom Catalysis (SAC) is an expanding field of heterogeneous catalysis in which single metallic atoms embedded in different materials catalyze a chemical reaction, but these new catalytic materials still lack fundamental understanding when used in electrochemical environments. Recent characterizations of non-noble metals like Fe deposited on N-doped graphitic materials have evidenced two types of Fe-N4 fourfold coordination, either of pyridine type or of porphyrin type. Here, we study these defects embedded in a graphene sheet and immersed in an explicit aqueous medium at the quantum level. While the Fe-pyridine SAC model is clear cut and widely studied, it is not the case for the Fe-porphyrin SAC that remains ill-defined, because of the necessary embedding of odd-membered rings in graphene. We first propose an atomistic model for the Fe-porphyrin SAC. Using spin-polarized ab initio molecular dynamics, we show that both Fe SACs spontaneously adsorb two interfacial water molecules from the solvent on opposite sides. Interestingly, we unveil a different catalytic reactivity of the two hydrated SAC motives: while the Fe-porphyrin defect eventually dissociates an adsorbed water molecule under a moderate external electric field, the Fe-pyridine defect does not convey water dissociation

Collective modes and quantum effects in two-dimensional nanofluidic channels

Nanoscale fluid transport is typically pictured in terms of atomic-scale dynamics, as is natural in the real-space framework of molecular simulations. An alternative Fourier-space picture, that involves the collective charge fluctuation modes of both the liquid and the confining wall, has recently been successful at predicting new nanofluidic phenomena such as quantum friction and near-field heat transfer, that rely on the coupling of those fluctuations. Here, we study the charge fluctuation modes of a two-dimensional (planar) nanofluidic channel. Introducing confined response functions that generalize the notion of surface response function, we show that the channel walls exhibit coupled plasmon modes as soon as the confinement is comparable to the plasmon wavelength. Conversely, the water fluctuations remain remarkably bulk-like, with significant confinement effects arising only when the wall spacing is reduced to 7 Å. We apply the confined response formalism to predict the dependence of the solid–water quantum friction and thermal boundary conductance on channel width for model channel wall materials. Our results provide a general framework for Coulomb interactions of fluctuating matter under nanoscale confinement

Dielectric properties of aqueous electrolytes at the nanoscale

Despite the ubiquity of nanoconfined aqueous electrolytes, a theoretical framework that accounts for the nonlinear coupling of water and ion polarization is still missing. We introduce a nonlocal and nonlinear field theory for the nanoscale polarization of ions and water and derive the electrolyte dielectric properties as a function of salt concentration to first order in a loop expansion. Classical molecular dynamics simulations are favorably compared with the calculated dielectric response functions. The theory correctly predicts the dielectric permittivity decrement with rising salt concentration and furthermore shows that salt induces a Debye screening in the longitudinal susceptibility but leaves the short-range water organization remarkably unchanged.Comment: 6 pages, 3 figure

Multiscale Modeling of Aqueous Electric Double Layers

From the stability of colloidal suspensions to the charging of electrodes, electric double layers play a pivotal role in aqueous systems. The interactions between interfaces, water molecules, ions and other solutes making up the electrical double layer span length scales from Ångströms to micrometers and are notoriously complex. Therefore, explaining experimental observations in terms of the double layer’s molecular structure has been a long-standing challenge in physical chemistry, yet recent advances in simulations techniques and computational power have led to tremendous progress. In particular, the past decades have seen the development of a multiscale theoretical framework based on the combination of quantum density functional theory, force-field based simulations and continuum theory. In this Review, we discuss these theoretical developments and make quantitative comparisons to experimental results from, among other techniques, sum-frequency generation, atomic-force microscopy, and electrokinetics. Starting from the vapor/water interface, we treat a range of qualitatively different types of surfaces, varying from soft to solid, from hydrophilic to hydrophobic, and from charged to uncharged

The Impact of Halogenated Phenylalanine Derivatives on NFGAIL Amyloid Formation

The hexapeptide hIAPP(22-27)(NFGAIL) is known as a crucial amyloid core sequence of the human islet amyloid polypeptide (hIAPP) whose aggregates can be used to better understand the wild-type hIAPP ' s toxicity to beta-cell death. In amyloid research, the role of hydrophobic and aromatic-aromatic interactions as potential driving forces during the aggregation process is controversially discussed not only in case of NFGAIL, but also for amyloidogenic peptides in general. We have used halogenation of the aromatic residue as a strategy to modulate hydrophobic and aromatic-aromatic interactions and prepared a library of NFGAIL variants containing fluorinated and iodinated phenylalanine analogues. We used thioflavin T staining, transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) to study the impact of side-chain halogenation on NFGAIL amyloid formation kinetics. Our data revealed a synergy between aggregation behavior and hydrophobicity of the phenylalanine residue. This study introduces systematic fluorination as a toolbox to further investigate the nature of the amyloid self-assembly process

Local control of globally competing patterns in coupled Swift--Hohenberg equations

We present analytical and numerical investigations of two anti-symmetrically coupled 1D Swift--Hohenberg equations (SHEs) with cubic nonlinearities. The SHE provides a generic formulation for pattern formation at a characteristic length scale. A linear stability analysis of the homogeneous state reveals a wave instability in addition to the usual Turing instability of uncoupled SHEs. We performed weakly nonlinear analysis in the vicinity of the codimension-two point of the Turing-wave instability, resulting in a set of coupled amplitude equations for the Turing pattern as well as left and right traveling waves. In particular, these complex Ginzburg--Landau-type equations predict two major things: there exists a parameter regime where multiple different patterns are stable with respect to each other; and that the amplitudes of different patterns interact by local mutual suppression. In consequence, different patterns can coexist in distinct spatial regions, separated by localized interfaces. We identified specific mechanisms for controlling the position of these interfaces, which distinguish what kinds of patterns the interface connects and thus allow for global pattern selection. Extensive simulations of the original SHEs confirm our results