New Information on the Hydrophobic Interaction Revealed by Frequency Modulation AFM

Spiers Memorial Lecture: Water at interfaces

In this article we discuss current issues in the context of the four chosen subtopics for the meeting: dynamics and nano-rheology of interfacial water, electrified/charged aqueous interfaces, ice interfaces, and soft matter/water interfaces. We emphasize current advances in both theory and experiment, as well as important practical manifestations and areas of unresolved controversy.

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.

Electroreductive Defluorination of Unsaturated PFAS by a Quaternary Ammonium Surfactant-Modified Cathode via Direct Cathodic Reduction

Remediation of per- and polyfluoroalkyl substances (PFAS) in groundwater remains a technological challenge due to the trace concentrations of PFAS and the strength of their C-F bonds. This study investigated an electroreductive system with a quaternary ammonium surfactant-modified cathode for degrading (E)-perfluoro(4-methylpent-2-enoic acid) (PFMeUPA) at a low cathodic potential. A removal efficiency of 99.81% and defluorination efficiency of 78.67% were achieved under -1.6 V (vs Ag/AgCl) at the cathode modified by octadecyltrimethylammonium bromide (OTAB). The overall degradation procedure started with the adsorption of PFMeUPA onto the modified cathode. This adsorption process was promoted by hydrophobic and electrostatic interactions between the surfactants and PFMeUPA, of which the binding percentage, binding mode, and binding energy were determined via molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The step-wise degradation pathway of PFMeUPA, including reductive defluorination and hydrogenation, was derived. Meanwhile, C-F bond breaking with direct electron transfer only was achieved for the first time in this study, which also showed that the C═C bond structure of PFAS facilitates the C-F cleavage. Overall, this study highlights the crucial role of quaternary ammonium surfactants in electron transfer and electrocatalytic activities in the electroreductive system and provides insights into novel remediation approaches on PFAS-contaminated groundwater.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

Interfacial layering of hydrocarbons on pristine graphite surfaces immersed in water

Interfacial water participates in a wide range of phenomena involving graphite, graphite-like and 2D material interfaces. Recently, several high-spatial resolution experiments have questioned the existence of hydration layers on graphite, graphite-like and 2D material surfaces. Here, 3D AFM was applied to follow in real-time and with atomic-scale depth resolution the evolution of graphite-water interfaces. Pristine graphite surfaces upon immersion in water showed the presence of several hydration layers separated by a distance of 0.3 nm. Those layers were short-lived. After several minutes, the interlayer distance increased to 0.45 nm. At longer immersion times (∼50 min) we observed the formation of a third layer. An interlayer distance of 0.45 nm characterizes the layering of predominantly alkane-like hydrocarbons. Molecular dynamics calculations supported the experimental observations. The replacement of water molecules by hydrocarbons on graphite is spontaneous. It happens whenever the graphite-water volume is exposed to air.

Contribution of the Induced-Dipole Interaction to Methane Aggregation in Water

Apolar molecules in the gas phase have no dipole moments. However, when placed in an aqueous environment, they acquire a dipole moment induced by the electric fields of the surrounding water molecules. Could these induced dipole moments, not present in the gas phase but present in solution, play an important role in the hydrophobic interaction between two apolar molecules? In particular, for two methane molecules, our results show that the interaction between the induced-dipole moments only very weakly plays a role in the aggregation of a pair of methane molecules in water. The induced-dipole-induced-dipole interaction has a magnitude as large as 1 kcal/mol for certain mutual orientations of the induced dipole moments, which is larger than the magnitude of the free energy of aggregation of the methane solutes in water. However, when averaged over all physically occurring conformations for a fixed intersolute separation, this interaction averages to an insignificant value (magnitude less than 0.01 kcal/mol) except, possibly, for some very short intermolecular separation.

Interfacial, Electroviscous, and Nonlinear Dielectric Effects on Electrokinetics at Highly Charged Surfaces

The dielectric constant and the viscosity of water at the interface of hydrophilic surfaces differ from their bulk values, and it has been proposed that the deviation is caused by the strong electric field and the high ion concentration in the interfacial layer. We calculate the dependence of the dielectric constant and the viscosity of bulk electrolytes on the electric field and the salt concentration. Incorporating the concentration and field-dependent dielectric constant and viscosity in the extended Poisson–Boltzmann and Stokes equations, we calculate the electro-osmotic mobility. We compare the results to literature experimental data and explicit molecular dynamics simulations of OH-terminated surfaces and show that it is necessary to additionally include the presence of a subnanometer wide interfacial water layer, the properties of which are drastically transformed by the sheer presence of the interface. We conclude that the origin of the anomalous behavior of aqueous interfacial layers cannot be found in electrostriction or electroviscous effects caused by the interfacial electric field and ion concentration. Instead, it is primarily caused by the intrinsic ordering and orientation of the interfacial water layer.

Comparison of atomic force microscopy force curve and solvation structure studied by integral equation theory

Atomic force microscopy can observe structures of liquids (solvents) on solid surfaces as oscillating force curves. The oscillation originates from the solvation force, which is affected by the interaction between the probe, substrate, and solvents. To investigate the effects of the interactions on the force curve, we calculated the force curves by integral equation theory with various probe and substrate conditions. The probe solvophilicity affected the force curves more than the substrate solvophilicity in our calculation, and its reason is qualitatively explained by the amount of the desolvated solvents. We evaluated the probes and parameters in terms of the qualitative estimation of the number density distribution of the solvent on the wall. The negative of the force curve's derivative with respect to the surface separation reflected the number density distribution better than the force curve. This parameter is based on the method that is proposed previously by Amano et al. [Phys. Chem. Chem. Phys. 18, 15534 (2016)]. The normalized frequency shift can also be used for the qualitative estimation of the number density distribution if the cantilever amplitude is small. Solvophobic probes reflected the number density distribution better than the solvophilic probes. Solvophilic probes resulted in larger oscillation amplitudes than solvophobic probes and are suitable for measurements with a high S/N ratio.

Atomically resolved interfacial water structures on crystalline hydrophilic and hydrophobic surfaces

Hydration layers are formed on hydrophilic crystalline surfaces immersed in water. Their existence has also been predicted for hydrophobic surfaces, yet the experimental evidence is controversial. Using 3D-AFM imaging, we probed the interfacial water structure of hydrophobic and hydrophilic surfaces with atomic-scale spatial resolution. We demonstrate that the atomic-scale structure of interfacial water on crystalline surfaces presents two antagonistic arrangements. On mica, a common hydrophilic crystalline surface, the interface is characterized by the formation of 2 to 3 hydration layers separated by approximately 0.3 nm. On hydrophobic surfaces such as graphite or hexagonal boron nitride (h-BN), the interface is characterized by the formation of 2 to 4 layers separated by about 0.5 nm. The latter interlayer distance indicates that water molecules are expelled from the vicinity of the surface and replaced by hydrocarbon molecules. This creates a new 1.5-2 nm thick interface between the hydrophobic surface and the bulk water. Molecular dynamics simulations reproduced the experimental data and confirmed the above interfacial water structures.

Effect of Strain-Specific Biofilm Properties on the Retention of Colloids in Saturated Porous Media under Conditions of Stormwater Biofiltration

Filter performance can be affected by bacterial colonization of the filtration media, yet little is known about how naturally occurring bacteria modify the surface properties of filtration media to affect colloidal removal. We used sand columns and simulated stormwater conditions to study the retention of model colloidal particles, carboxyl-modified-latex (CML) beads, in porous media colonized by naturally occurring bacterial strains. Colloid retention varied substantially across identical columns colonized by different, in some cases closely related, bacterial strains in a cell density independent manner. Atomic force microscopy was applied to quantify the interaction energy between CML beads and each bacterial strain's biofilm surface. We found interaction energy between CML and each strain was significantly different, with adhesive energies between the biofilm and CML, presumed to be associated with polymer-surface bonding, a better predictor of CML retention than other strain characteristics. Overall, the findings suggest that interactions with biopolymers in naturally occurring bacterial biofilms strongly influence colloid retention in porous media. This work highlights the need for more investigation into the role of biofilm microbial community composition on colloid removal in porous media to improve biofilter design and operation.

Anomalously Slow Conformational Change Dynamics of Polar Groups Anchored to Hydrophobic Surfaces in Aqueous Media

Water molecules within a thin hydration layer, spontaneously generated on hydrophobic protein surfaces, are reported to form a poorly dynamic network structure. However, how such a water network affects the conformational change dynamics of polar groups has never been explored, although such polar groups play a critical role in protein-protein and protein-ligand interactions. In the present work, we utilized as model protein surfaces a series of self-assembled monolayers (SAMs) appended with polar (Fmoc) or ionic (FITC) fluorescent head groups that were tethered via a 1.5-nm-long flexible oligoether chain to a hydrophobic silicon wafer surface, which was densely covered with paraffinic chains. We found that, not only in deionized water but also in aqueous buffer, these oligoether-appended head groups at ambient temperatures both displayed an anomalously slow conformational change, which required ∼10 h to reach a thermodynamically equilibrated state. We suppose that these behaviors reflect the poorly dynamic and low-permittivity natures of the thin hydration layer.

Origin of Ubiquitous Stripes at the Graphite-Water Interface

The investigation of solid-liquid interfaces is pivotal for understanding processes like wetting, corrosion, and mineral dissolution and growth. The graphite-water interface constitutes a prime example for studying the water structure at a seemingly hydrophobic surface. Surprisingly, in a large number of atomic force microscopy (AFM) experiments, well-ordered stripes have been observed at the graphite-water interface. Although many groups have reported on the observation of stripes at this interface, fundamental properties and, in particular, the origin of the stripes are still under debate. Proposed origins include contamination, interplanar stacking of graphene layers, formation of methanol-water nanostructures, and adsorption of nitrogen molecules. Especially, the latter interpretation has received considerable attention because of its potential impact on explaining the long-range nature of the hydrophobic interaction. In this study, we demonstrate that these stripes readily form when using standard plastic syringes to insert the water into the AFM instrument. In contrast, when clean glass syringes are used instead, no such stripes form even though nitrogen was present. We, therefore, conclude that contaminations from the plastic syringe rather than nitrogen constitute the origin of the stripes we observe. We provide high-resolution AFM data that reveal detailed structural insights into the arrangement of the stripes. The rich variability of our data suggests that the stripes might be composed of several different chemical species. Still, we cannot rule out that the stripes observed in the literature might originate from other sources; our study offers a rather straightforward explanation for the origin of the stripes. In the view of these results, we propose to carefully reconsider former assignments.

Gas molecules sandwiched in hydration layers at graphite/water interfaces

Frequency shift-distance curves reveal that each adsorbed gas layer is sandwiched between hydration layers with high water density.

Atomic-scale mapping of hydrophobic layers on graphene and few-layer MoS2 and WSe2 in water

The structure and the role of the interfacial water in mediating the interactions of extended hydrophobic surfaces are not well understood. Two-dimensional materials provide a variety of large and atomically flat hydrophobic surfaces to facilitate our understanding of hydrophobic interactions. The angstrom resolution capabilities of three-dimensional AFM are exploited to image the interfacial water organization on graphene, few-layer MoS2 and few-layer WSe2. Those interfaces are characterized by the existence of a 2 nm thick region above the solid surface where the liquid density oscillates. The distances between adjacent layers for graphene, few-layer MoS2 and WSe2 are ~0.50 nm. This value is larger than the one predicted and measured for water density oscillations (~0.30 nm). The experiments indicate that on extended hydrophobic surfaces water molecules are expelled from the vicinity of the surface and replaced by several molecular-size hydrophobic layers.

Atomic- and Molecular-Resolution Mapping of Solid-Liquid Interfaces by 3D Atomic Force Microscopy

Hydration layers are ubiquitous in life and technology. Hence, interfacial aqueous layers have a central role in a wide range of phenomena from materials science to molecular and cell biology. A complete understanding of those processes requires, among other things, the development of very-sensitive and high-resolution instruments. Three-dimensional atomic force microscopy (3D-AFM) represents the latest and most successful attempt to generate atomically resolved three-dimensional images of solid-liquid interfaces. This review provides an overview of the 3D-AFM operating principles and its underlying physics. We illustrate and explain the capability of the instrument to resolve atomic defects on crystalline surfaces immersed in liquid. We also illustrate some of its applications to imaging the hydration structures on DNA or proteins. In the last section, we discuss some perspectives on emerging applications in materials science and molecular biology.

Molecular features of hydration layers probed by atomic force microscopy

Structurally-ordered layers of water are universally formed on a solid surface in aqueous solution or under ambient conditions. Although such hydration layers are commonly probed via atomic force microscopy (AFM), the current understanding on how the hydration layers manifest themselves in an AFM experiment is far from complete. By using molecular dynamics simulation, we investigate the hydration layers on a hydrophilic or hydrophobic surface probed by a nanoscale tip. We study the density and molecular orientation of water, the free energy, and the force on the tip by varying the tip-surface distance. The force-distance curve oscillates due to the transition between the mono-, bi-, and tri-layers of water confined between the tip and the surface. If both the tip and the surface are hydrophobic, water confined between the tip and the surface evaporates due to the dewetting transition, giving a hydrophobic force without oscillation. The periodicity of oscillation in the force differs from the structural periodicity of water. With a close proximity of the tip, the molecular dipoles align parallel to the surface, regardless of whether the tip and the surface are hydrophilic or hydrophobic.

Solvent-mediated interactions between nanostructures: From water to Lennard-Jones liquid

Solvent-mediated interactions emerge from complex mechanisms that depend on the solute structure, its wetting properties, and the nature of the liquid. While numerous studies have focused on the first two influences, here, we compare the results from water and Lennard-Jones liquid in order to reveal to what extent solvent-mediated interactions are universal with respect to the nature of the liquid. Besides the influence of the liquid, the results were obtained with classical density functional theory and brute-force molecular dynamics simulations which allow us to contrast these two numerical techniques.

Hydration Structure of a Single DNA Molecule Revealed by Frequency-Modulation Atomic Force Microscopy

Hydration interaction shapes biomolecules and is a dominant intermolecular force. Mapping the hydration patterns of biomolecules is therefore essential for understanding molecular processes in biology. Numerous studies have been devoted to this challenge, but current methods cannot map the hydration of single biomolecules, let alone do so under physiological conditions. Here, we show that frequency-modulation atomic force microscopy (FM-AFM) can fill this gap and generate 3D hydration maps of single DNA molecules under near-physiological conditions. Additionally, we present real-space images of DNA in which the double helix is resolved with unprecedented resolution, clearly revealing individual phosphate groups along the DNA backbone. FM-AFM therefore emerges as a powerful enabling tool in the study of individual biomolecules and their hydration under physiological conditions.

Direct comparison between subnanometer hydration structures on hydrophilic and hydrophobic surfaces via three-dimensional scanning force microscopy

Hydration layers on heterogeneous substrates are characterized with subnanometer resolution using three-dimensional scanning force microscopy.

Identification of Single Adsorbed Cations on Mica-Liquid Interfaces by 3D Force Microscopy

Force microscope provides atomically resolved images of surfaces immersed in a liquid. The presence of different chemical species in the interface (cations, anions, water, neutral atoms) complicates the adscription of the observed features to a given species. We develop a 3D atomic force microscopy method to identify the cations adsorbed on a mica surface from a potassium chloride solution. The method is based on measuring the peak value of the attractive force within the Stern layer. The maximum of the attractive force shows site-specific variations. The positions with the highest attractive force values are associated with the presence of adsorbed potassium ions, while the other positions are associated with a local depletion of the hydration layer. This criterion provides a surface coverage of K cations that is consistent with the one reported by other techniques.