The hydrophobic effect: molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces

Langmuir and Langmuir-Blodgett Films of Gold and Silver Nanoparticles

Recently the focus of the Langmuir-Blodgett technique as a method of choice to transfer monolayers from the air/water interface onto solid substrates in a controllable fashion has been shifting toward purely hydrophobic gold and silver nanoparticles. The fundamental interactions between particles that become relevant in the absence of polar groups range from dispersive attractions from the metal cores and repulsions between ligand shells to weaker entropic factors. The layer evolution is explored, starting with interfacial self-assembly upon solution spreading and domain and circular island formation, which subsequently merge into a complete monolayer and finally form multilayers or macroscopic wrinkles. Moreover, structural properties such as the core:ligand size ratio are investigated in the context of dispersive forces, whereby the nanoparticles with small cores and long ligands tend not to aggregate sufficiently to produce continuous films, those with large cores and short ligands were found to aggregate irreversibly, and those in between the two extremes were concluded to be able to form highly organized crystalline films. Similarly, the characteristics of the spreading solution such as the concentration and the solvent type crucially influence the film crystallinity, with the deciding factor being the degree of affinity between the capping ligand and the solvent used for spreading. Finally, the most common strategies employed to enhance the mechanical stability of the metal nanoparticle films along with the recent attempts to functionalize the particles in attempts to improve their applicability in the industry are summarized and evaluated in relation to their future prospects. One of the objectives of this feature article is to elucidate the differences between hydrophobic metal nanoparticles and typical amphiphilic molecules that the majority of the literature in the field describes and to familiarize the reader with the knowledge required to design Langmuir-Blodgett nanoparticle systems as well as the strategies to improve existing ones.

A new mechanism of the interfacial water film dominating low ice friction

It is generally accepted that ice is slippery due to an interfacial water film wetting the ice surface. Despite the current progress in research, the mechanism of low ice friction is not clear, and especially little is known about the behavior of this surface water film under shear and how the sheared interfacial water film influences ice friction. In our work, we investigated the ordering and diffusion coefficient of the interfacial water film and the friction of ice sliding on an atomically smooth solid substrate at the atomic level using molecular dynamics simulations. There are two layers of water molecules at the ice-solid interface that exhibit properties very different from bulk ice. The ice-adjacent water layer is ice-like, and the solid-adjacent water layer is liquid-like. This liquid-like layer behaves in the manner of "confined water," with high viscosity while maintaining fluidity, leading to the slipperiness of the ice. Furthermore, we found that the interfacial water exhibits shear thinning behavior, which connects the structure of the interfacial water film to the coefficient of friction of the ice surface. We propose a new ice friction mechanism based on shear thinning that is applicable to this interfacial water film structure.

A van der Waals Integrated Damage-Free Memristor Based on Layered 2D Hexagonal Boron Nitride

2D materials with intriguing properties have been widely used in optoelectronics. However, electronic devices suffered from structural damage due to the ultrathin materials and uncontrolled defects at interfaces upon metallization, which hindered the development of reliable devices. Here, a damage-free Au/h-BN/Au memristor is reported using a clean, water-assisted metal transfer approach by physically assembling Au electrodes onto the layered h-BN which minimized the structural damage and undesired interfacial defects. The memristors demonstrate significantly improved performance with the coexistence of nonpolar and threshold switching as well as tunable current levels by controlling the compliance current, compared with devices with evaporated contacts. The devices integrated into an array show suppressed sneak path current and can work as both logic gates and latches to implement logic operations allowing in-memory computing. Cross-sectional scanning transmission electron microscopy analysis validates the feasibility of this nondestructive metal integration approach, the crucial role of high-quality atomically sharp interface in resistive switching, and a direct observation of percolation path. The underlying mechanism of boron vacancies-assisted transport is further supported experimentally by conductive atomic force microscopy free from process-induced damage, and theoretically by ab initio simulations.

Heterogeneous Microscopic Dynamics of Intruded Water in a Superhydrophobic Nanoconfinement: Neutron Scattering and Molecular Modeling

With their strong confining porosity and versatile surface chemistry, zeolitic imidazolate frameworks-including the prototypical ZIF-8-display exceptional properties for various applications. In particular, the forced intrusion of water at high pressure (∼25 MPa) into ZIF-8 nanopores is of interest for energy storage. Such a system reveals also ideal to study experimentally water dynamics and thermodynamics in an ultrahydrophobic confinement. Here, we report on neutron scattering experiments to probe the molecular dynamics of water within ZIF-8 nanopores under high pressure up to 38 MPa. In addition to an overall confinement-induced slowing down, we provide evidence for strong dynamical heterogeneities with different underlying molecular dynamics. Using complementary molecular simulations, these heterogeneities are found to correspond to different microscopic mechanisms inherent to vicinal molecules located in strongly adsorbing sites (ligands) and other molecules nanoconfined in the cavity center. These findings unveil a complex microscopic dynamics, which results from the combination of surface residence times and exchanges between the cavity surface and center.

Phenolic Hydroxyl Groups in the Lignin Polymer Affect the Formation of Lignin Nanoparticles

Alkaline soda lignin (AL) was sequentially fractionated into six fractions of different molecular size by means of solvent extraction and their phenolic hydroxyl groups were chemoselectively methylated to determine their effect on nanoparticle formation of lignin polymers. The effect of the lignin structure on the physical properties of nanoparticles was also clarified in this study. Nanoparticles were obtained from neat alkaline soda lignin (ALNP), solvent-extracted fractions (FALNPs, i.d. 414–1214 nm), and methylated lignins (MALNPs, i.d. 516–721 nm) via the nanoprecipitation method. Specifically, the size properties of MALNPs showed a high negative correlation (R² = 0.95) with the phenolic hydroxyl group amount. This indicates that the phenolic hydroxyl groups in lignin could be influenced on the nucleation or condensation during the nanoprecipitation process. Lignin nanoparticles exhibited high colloidal stability, and most of them also showed good in vitro cell viability. This study presents a possible way to control nanoparticle size by blocking specific functional groups and decreasing the interaction between hydroxyl groups of lignin.

Kinematics of Photoisomerization Processes of PMMA-BDK-MR Polymer Composite Thin Films

We investigate and report on the kinematics of photoisomerization processes of polymer composite thin films based on azo dye methyl red (MR) hosted in polymethylmethacrylate (PMMA) incorporated with Benzyl dimethyl ketal (BDK) as a photo-initiator. Understanding photoisomerization mechanisms is crucial for several optical applications such as Read/Write/Erase (WRE) optical data storage media, UV light Read/Write heads, and UV light sensors. The as-prepared polymer composite thin films are characterized using UV–Vis spectroscopy. Furthermore, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) are employed to investigate the optical, chemical, and morphological properties of trans- and cis-states of PMMA-BDK-MR polymer composite thin films. The presence of the azo dye MR in the composite is essential for the efficient performance of the cis ↔ trans cycles through illumination ↔ thermal relaxation for Write/Read/Erase optical data storage and UV-light sensors. Moreover, UV–Vis and FTIR results confirm the hysteresis cycle of trans- and cis-states and that PMMA-BDK-MR thin films may be regarded as potential candidates for successful Write/Read/Erase optical data storage and UV-light sensors. In addition, the morphology of the thin film surface is investigated by SEM technique. The SEM images indicate that uncured surfaces of PMMA-BDK-MR thin films are inhomogeneous compared with the corresponding surfaces after curing. The transformation from inhomogeneous surfaces to homogeneous surfaces is attributed to the polymerization of thin films by UV curing.

Simulation of Subnanometer Contrast in Dynamic Atomic Force Microscopy of Hydrophilic Alkanethiol Self-Assembled Monolayers in Water

Atomic resolution imaging of surfaces in liquid environments using atomic force microscopy (AFM) is challenging in terms of both reproducibility and measurement interpretation. To understand the origins of these challenges, we used molecular dynamics simulations of AFM on hydrophilic self-assembled monolayers (SAMs) in water. The force on the model AFM tip was calculated as a function of lateral and vertical position relative to the SAM surface. The contributions of the water and SAMs to the overall force were analyzed, and the former was correlated to the water density distribution. Then, dynamic AFM was modeled by oscillating the tip at a driving amplitude. It was found that the contrast between amplitudes at different lateral positions on the surface was dependent on the vertical position of the tip. Lastly, amplitude maps were produced for two vertical positions at constant height, and the ability to capture atomic resolution was related to the force on the tip. These results offer an explanation for the observed instability in atomic scale imaging using AFM and more generally provide insight into the contrast mechanisms of surface images obtained in liquid environments.

Dependence of water adsorption on the surface structure of silicon wafers aged under different environmental conditions

Most materials exposed to ambient air can adsorb water molecules and the adsorption capability strongly depends on the surface property. The water contact angle has been widely used as a measure for surface wettability; however, a question can still be asked whether the water contact angle can be used as an adequate sole predictor for water adsorption on the surface in humid air. In this paper, HF-etched silicon wafers were aged (oxidized) under different environmental conditions at room temperature to grow surface layers with varying water contact angles from ∼0° (fully hydrophilic) to ∼83° (highly hydrophobic), and water adsorption as a function of relative humidity (RH) was studied on such surfaces. The thickness and structure of the adsorbed water layer were found to depend on not only the surface wettability on each surface, but also the history of surface oxidation conditions. In particular, the silicon wafer surface oxidized in liquid water uptakes significantly more water from humid air than the fully-hydroxylated native oxide surface (SiO_x/OH), even though its water contact angle is higher than that on the SiO_x/OH surface. This could be attributed to the formation of a gel-like structure during oxidation in liquid water.

Effects of Packing Density and Chain Length on the Surface Hydrophobicity of Thin Films Composed of Perfluoroalkyl Acrylate Chains: A Molecular Dynamics Study

A good understanding of the surface hydrophobicity of fluorinated materials is useful for their application as coating materials. The present study investigates the surface hydrophobicity of perfluoroalkyl acrylate (PFA) thin films using molecular dynamics simulations. Surface hydrophobicity is characterized by examining the contact angle of a water droplet on PFA surfaces and the cavity formation free energy in the vicinity of the surface. It is found that the calculated microscopic contact angles are in good agreement with the experimental results and partially capture the difference in the hydrophobicity of the surface arising from the variation of packing density and side chain length of PFA. The variations of cavity formation free energy in the vicinity of the surface elucidate that the surface hydrophobicity is mainly governed by the packing density rather than the chain length of PFA. The hydrophobicity generally increases with decreasing the packing density to some extent and then turns to decrease as further reducing the packing density. At higher packing density, the surface hydrophobicity slightly decreases with increasing the chain length, while at the lower packing density, the surface hydrophobicity is increased when chain length of PFA is longer than six carbons. Furthermore, we found that the influence of packing density on the surface hydrophobicity is directly related to the variation of the surface roughness and chain flexibility, that is, the surface hydrophobicity increases with increase in the surface roughness, while the chain flexibility plays a secondary role in the enhancement by affecting the stability of water staying near the interface. The study provides a significant insight into the local hydrophobicity and microscopic structure of the PFA surfaces, which would be useful for the application of surface modification.

Tunable surface adsorption and wettability of candle soot coated on ferroelectric ceramics

A ferroelectric Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZTO) ceramic was prepared using a solid-state reaction route. A coating of candle soot was provided on poled and unpoled BCZTO samples. X-ray diffraction and Raman spectroscopy confirmed the presence of the graphite form of carbon in the candle soot. Scanning Kelvin probe microscopy determined that the highest surface potentials were ∼34 mV and 1.5 V in the unpoled and poled BCZTO samples, respectively. The candle soot was found to adsorb ∼65%, 80%, and 90% of the methylene blue dye present in acidic, neutral, and basic media, respectively, within 3 h. In both the poled and unpoled cases, the BCZTO samples coated with candle soot showed greater adsorption capacities than the uncoated BCZTO sample. In the cases of poled samples coated with candle soot, the adsorption was found to be greater in the case of candle soot coated on a positively charged surface than that for candle soot coated on a negatively charged BCZTO surface in an acidic medium. In a basic medium, the adsorption was found to be greater in the case of candle soot coated on a negatively charged surface than that for candle soot coated on a positively charged BCZTO surface. The contact angle of the candle soot-coated BCZTO sample was found to be hydrophobic (∼149°). The contact angle decreased (∼149-133°) with an increase in temperature (30-70 °C) in the case of candle soot coated on the positive surface of a poled BCZTO sample. The contact angle increased (∼139-149°) with an increase in temperature (30-70 °C) in the case of candle soot coated on the negative surface of a poled BCZTO sample. Internal electric field-assisted (associated with ferroelectric materials) adsorption could be a potential technique to improve adsorption processes.

Combined Experimental and Simulation Study of Amplitude Modulation Atomic Force Microscopy Measurements of Self-Assembled Monolayers in Water

Atomic force microscopy (AFM) can be used to measure surface properties at the nanoscale. However, interpretation of measurements from amplitude modulation AFM (AM-AFM) in liquid is not straightforward due to the interactions between the AFM tip, the surface being imaged, and the water. In this work, amplitude-distance measurements and molecular dynamics simulations of AM-AFM were employed to study the effect of surface chemistry on the amplitude of tip oscillation in water. The sample surfaces consisted of self-assembled monolayers where the hydrophilicity or hydrophobicity was determined by the terminal group of the alkanethiols. Analysis showed that surface chemical composition influences the hydration structure near the interface which affects the forces experienced by the tip and in turn changes the amplitude profile. This observation could aid our understanding of AM-AFM measurements of interfacial phenomena on various surfaces in water.

Thermodynamic, structural, and dynamical properties of nano-confined water using SPC/E and TIP4P models by molecular dynamics simulations

Different morphologies of water molecules are confined between two parallel graphene surfaces.

Molecular Shape and the Hydrophobic Effect

This review focuses on papers published since 2000 on the topic of the properties of solutes in water. More specifically, it evaluates the state of the art of our understanding of the complex relationship between the shape of a hydrophobe and the hydrophobic effect. To highlight this, we present a selection of references covering both empirical and molecular dynamics studies of small (molecular-scale) solutes. These include empirical studies of small molecules, synthetic hosts, crystalline monolayers, and proteins, as well as in silico investigations of entities such as idealized hard and soft spheres, small solutes, hydrophobic plates, artificial concavity, molecular hosts, carbon nanotubes and spheres, and proteins.

Ice-like water supports hydration forces and eases sliding friction

The nature of interfacial water is critical in several natural processes, including the aggregation of lipids into the bilayer, protein folding, lubrication of synovial joints, and underwater gecko adhesion. The nanometer-thin water layer trapped between two surfaces has been identified to have properties that are very different from those of bulk water, but the molecular cause of such discrepancy is often undetermined. Using surface-sensitive sum frequency generation (SFG) spectroscopy, we discover a strongly coordinated water layer confined between two charged surfaces, formed by the adsorption of a cationic surfactant on the hydrophobic surfaces. By varying the adsorbed surfactant coverage and hence the surface charge density, we observe a progressively evolving water structure that minimizes the sliding friction only beyond the surfactant concentration needed for monolayer formation. At complete surfactant coverage, the strongly coordinated confined water results in hydration forces, sustains confinement and sliding pressures, and reduces dynamic friction. Observing SFG signals requires breakdown in centrosymmetry, and the SFG signal from two oppositely oriented surfactant monolayers cancels out due to symmetry. Surprisingly, we observe the SFG signal for the water confined between the two charged surfactant monolayers, suggesting that this interfacial water layer is noncentrosymmetric. The structure of molecules under confinement and its macroscopic manifestation on adhesion and friction have significance in many complicated interfacial processes prevalent in biology, chemistry, and engineering.

The Combined Effect of Hydrophobic Mismatch and Bilayer Local Bending on the Regulation of Mechanosensitive Ion Channels

The hydrophobic mismatch between the lipid bilayer and integral membrane proteins has well-defined effect on mechanosensitive (MS) ion channels. Also, membrane local bending is suggested to modulate MS channel activity. Although a number of studies have already shown the significance of each individual factor, the combined effect of these physical factors on MS channel activity have not been investigated. Here using finite element simulation, we study the combined effect of hydrophobic mismatch and local bending on the archetypal mechanosensitive channel MscL. First we show how the local curvature direction impacts on MS channel modulation. In the case of MscL, we show inward (cytoplasmic) bending can more effectively gate the channel compared to outward bending. Then we indicate that in response to a specific local curvature, MscL inserted in a bilayer with the same hydrophobic length is more expanded in the constriction pore region compared to when there is a protein-lipid hydrophobic mismatch. Interestingly in the presence of a negative mismatch (thicker lipids), MscL constriction pore is more expanded than in the presence of positive mismatch (thinner lipids) in response to an identical membrane curvature. These results were confirmed by a parametric energetic calculation provided for MscL gating. These findings have several biophysical consequences for understanding the function of MS channels in response to two major physical stimuli in mechanobiology, namely hydrophobic mismatch and local membrane curvature.

Structure and dynamics of TIP3P, TIP4P, and TIP5P water near smooth and atomistic walls of different hydroaffinity

We perform molecular dynamics simulations to observe the structure and dynamics of water using different water models (TIP3P, TIP4P, TIP5P) at ambient conditions, constrained by planar walls, which are either modeled by smooth potentials or regular atomic lattices, imitating the honeycomb-structure of graphene. We implement walls of different hydroaffinity, different lattice constant, and different types of interaction with the water molecules. We find that in the hydrophobic regime the smooth wall generally represents a good abstraction of the atomically rough walls, while in the hydrophilic regime there are noticeable differences in structure and dynamics between all stages of wall roughness. For a small lattice constant however the smooth and the atomically rough wall still share a number of structural and dynamical similarities. Out of the three water models, TIP5P water shows the largest degree of tetrahedral ordering and is often the one that is least perturbed by the presence of the wall.

Interaction of neurotransmitters with a phospholipid bilayer: a molecular dynamics study

We have performed a series of molecular dynamics simulations to study the interactions between the neurotransmitters (NTs) γ-aminobutyrate (GABA), glycine (GLY), acetylcholine (ACH) and glutamate (GLU) as well as the amidated/acetylated γ-aminobutyrate (GABA(neu)) and the osmolyte molecule glycerol (GOL) with a dipalmitoylphosphatidylcholine (DPPC) bilayer. In agreement with previously published experimental data, we found the lowest membrane affinity for the charged molecules and a moderate affinity for zwitterionic and polar molecules. The affinity can be ranked as follows: ACH-GLU<<GABA<GLY<<GABA(neu)<<GOL. The latter three penetrated the bilayer at most with the deepest location being close to the glycerol backbone of the phospholipids. Even at that position, these solutes were noticeably hydrated and carried ∼30-80% of the bulk water along. The mobility of hydration water at the solute is also affected by the penetration into the bilayer. Two time scales of exchanging water molecules could be determined. In the bulk phase, the hydration layer contains ∼20% slow exchanging water molecules which increases 2-3 times as the solutes entered the bilayer. Our results indicate that there is no intermediate exchange of slow moving water molecules from the solutes to the lipid atoms and vice versa. Instead, the exchange relies on the reservoir of unbounded ("free") water molecules in the interfacial bilayer region. Results from the equilibrium simulations are in good agreement with the results from umbrella sampling simulations, which were conducted for the four naturally occurring NTs. Free energy profiles for ACH and GLU show a minimum of ∼2-3 kJ/mol close to the bilayer interface, while for GABA and GLY, a minimum of respectively ∼2 kJ/mol and ∼5 kJ/mol is observed when these NTs are located in the vicinity of the lipid glycerol backbone. The most important interaction of NTs with the bilayer is the charged amino group of NTs with the lipid phosphate group.

Wettability of azobenzene self-assembled monolayers

The wettability properties of azobenzene self-assembled monolayers (SAMs), in the trans and cis forms, are investigated herein by classical Molecular Dynamics simulations of validated assembly structures described with a dedicated force field. The two different methodologies used for the calculation of the contact angle, one based on the Young's equation and the other on geometrical models, have provided a consistent description of the SAMs wettability in line with available experimental results. Furthermore, we provide an atomistic description of the first layers of water molecules at the solvent-SAM interface, which rationalizes the wettability difference between the cis- and trans-SAMs.

Long-range surface-induced water structures and the effect of 1-butanol studied by 1H nuclear magnetic resonance

Thin films of water between glass plates were investigated in this study with regard to water structure and dynamics in the temperature range of 278-313 K. We further investigated how addition of 1-butanol (0.05 and 0.5 M) affects the range and properties of the surface-induced water structures. From the observation of two (1)H nuclear magnetic resonance (NMR) water resonances and two relaxation components, it was found that the interfacial water exists in a two-state mixture in dynamic equilibrium, with the respective structures interpreted as being high-density water (HDW) and low-density water (LDW). In the absence of 1-butanol, the LDW state is more pronounced, with a further shift in equilibrium toward the LDW state with an increase in temperature. However, in water film samples containing 1-butanol, the HDW state dominates at low temperatures while the LDW state becomes more visible at higher temperatures. Furthermore, the addition of 1-butanol significantly increased the extent of the surface-induced water structures. NMR relaxation shows that the dynamics of water in the HDW state is significantly affected by the presence of 1-butanol and further indicates that the distribution of values for the enthalpy of activation associated with translational motion of water molecules in the HDW state is narrower in the 0.05 M 1-butanol sample than in the 0.5 M 1-butanol sample.

Amyloid fibril formation of peptides derived from the C-terminus of CETP modulated by lipids

Cholesteryl-ester transfer protein (CETP) is a plasmatic protein involved in neutral lipid transfer between lipoproteins. Focusing on the last 12 C-terminus residues we have previously shown that mutation D470N promotes a conformational change towards a β-secondary structure. In turn, this modification leads to the formation of oligomers and fibrillar structures, which cause cytotoxic effects similar to the ones provoked by amyloid peptides. In this study, we evaluated the role of specific lipid arrangements on the structure of peptide helix-Z (D470N) through the use of thioflavin T fluorescence, peptide bond absorbance, circular dichroism and electron microscopy. The results indicate that the use of micelles formed with lysophosphatidylcholine and lysophosphatidic acid (LPA) under neutral pH induce a conformational transition of peptide helix-Z containing a β-sheet conformation to a native α-helix structure, therefore avoiding the formation of amyloid fibrils. In contrast, incubation with phosphatidic acid does not change the profile for the β-sheet conformation. When the electrostatic charge at the surface of micelles or vesicles is regulated through the use of lipids such as phospholipid and LPA, minimal changes and the presence of β-structures were recorded. Mixtures with a positive net charge diminished the percentage of β-structure and the amount of amyloid fibrils. Our results suggest that the degree of solvation determined by the presence of a free hydroxyl group on lipids such as LPA is a key condition that can modulate the secondary structure and the consequent formation of amyloid fibrils in the highly flexible C-terminus domain of CETP.

Hydrophobic drying and hysteresis at different length scales by molecular dynamics simulations

We performed molecular dynamics simulations to investigate hydrophobic interactions between two parallel hydrophobic plates immersed in water. The two plates are separated by a distance D ranging from contact to a few nanometers. To mimic the attractive hydrophobic force measurement in a surface force experiment, a driving spring is used to measure the hydrophobic force between two hydrophobic plates. The force-distance curves, in particular the force variations from spontaneous drying to hydrophobic collapse are obtained. These details are usually not accessible in the surface force measurement due to the unstable jump into contact. The length-scale effect on the hydrophobic drying during normal approach and the hydrophobic hysteresis during retraction has been studied. We find that the critical distance at which a spontaneous drying occurs is determined by the shorter characteristic dimension of the plate, whereas the hydrophobic hysteresis is determined by the longer characteristic dimension of the plate. The variations of the potential of mean force versus separation during approach and retraction are also calculated. The results show that water confined between two parallel hydrophobic plates is in a thermodynamic metastable state. This comparably high energy state leads to the spontaneous drying at some critical distance.

Dipolar origin of water etching of amino acid surfaces

The etching induced by water on hydrophobic (001) surfaces of enantiomeric L-, D- and racemic DL-valine crystals has been characterized by means of atomic force microscopy (AFM) at ambient conditions. Well-defined chiral parallelepipedic shallow patterns, one bilayer deep, are observed for the enantiomeric crystals with sides (steps) oriented along low index crystallographic directions. Hence, chirality can be readily identified by visual inspection of an AFM image after etching. The formation of such regular patterns can be rationalized using basic concepts of electrical dipolar interactions. The key factor that determines the relative etching rate for each step and thus defines the shape of the etching patterns is the orientation of the molecular dipoles with respect to the step edge. The simplicity of the approach allows the prediction of the effect of water etching on other amino acid crystals as well as the effect of the interaction of water with amino acid molecules forming part of more complex structures.

Dewetting transitions in protein cavities

In a previous analysis of the solvation of protein active sites, a drying transition was observed in the narrow hydrophobic binding cavity of Cox-2. With the use of a crude metric that often seems able to discriminate those protein cavities that dry from those that do not, we made an extensive search of the PDB, and identified five other proteins that, in molecular dynamics simulations, undergo drying transitions in their active sites. Because such cavities need not desolvate before binding hydrophobic ligands they often exhibit very large binding affinities. This article gives evidence that drying in protein cavities is not unique to Cox-2.

Hydrophobic amino acid adsorption on surfaces of varying wettability

The adsorption behavior of the model nonpolar amino acid leucine at a solid surface of tunable wetting ability has been examined by means of molecular dynamics simulation. When leucine adsorbs at a highly hydrophobic surface it is found to orient itself with its nonpolar side chain toward the surface and its charged amine and carboxyl groups directed toward the bulk. When the surface is less hydrophobic, leucine alternates between two stable orientations, a standing orientation like that observed at nonwetting surfaces, and a laying orientation in which its side chain and charged groups are located nearly the same distance from the surface. These results are rationalized by a water-density-dependent ordering scheme in which interfacial water structure governs the adsorbed structures. We pay particular attention to how the structure of surface water is changed in the presence of adsorbed leucine in both orientations. We propose that a water density model may be applied as a general technique for understanding adsorption from solution at solid hydrophobic surfaces.

Wedging transfer of nanostructures

We report a versatile water-based method for transferring nanostructures onto surfaces of various shapes and compositions. The transfer occurs through the intercalation of a layer of water between a hydrophilic substrate and a hydrophobic nanostructure (for example, graphene flakes, carbon nanotubes, metallic nanostructures, quantum dots, etc.) locked within a hydrophobic polymer thin film. As a result, the film entrapping the nanostructure is lifted off and floats at the air-water interface. The nanostructure can subsequently be deposited onto a target substrate by the removal of the water and the dissolution of the polymeric film. We show examples where graphene flakes and patterned metallic nanostructures are precisely transferred onto a specific location on a variety of patterned substrates, even on top of curved objects such as microspheres. The method is simple to use, fast, and does not require advanced equipment.

Simulations of water at the interface with hydrophilic self-assembled monolayers

Simulations of water at hydrophilic self-assembled monolayer (SAM) surfaces are especially relevant for biological interfaces. Well-defined, atomically smooth surfaces that can be continuously varied are possible with SAMs. These characteristics enable more accurate measurements than many other surfaces with the added advantage of tailoring the surface to treat specific chemical groups. A fundamental question is how solid surfaces affect the structure and dynamics of water. Measurements of the structure and dynamics of water at solid surfaces have improved significantly, but there remain differences among the experiments. In this article, the authors review simulations of water at the interface with hydrophilic SAMs. These simulations find that while the interfacial water molecules are slower than the bulk water molecules, the interfacial dynamics remains that of a liquid. A major biological application of SAMs is for making coatings resistant to protein adsorption. SAMs terminated with ethylene glycol monomers have proven to be excellent at resisting protein adsorption. Understanding the mechanisms behind this resistance remains an unresolved issue. Recent simulations suggest a new perspective of the role of interfacial water and the inseparable interplay between the SAM and the water.

Nanobubbles and the nanobubble bridging capillary force

Interactions between hydrophobic surfaces at nanometer separation distances in aqueous solutions are important in a number of biological and industrial processes. Force spectroscopy studies, most notably with the atomic force microscope and surface-force apparatus, have found the existence of a long range hydrophobic attractive force between hydrophobic surfaces in aqueous conditions that cannot be explained by classical colloidal science theories. Numerous mechanisms have been proposed for the hydrophobic force, but in many cases the force is an artifact due to the accumulation of submicroscopic bubbles at the liquid-hydrophobic solid interface, the so called nanobubbles. The coalescence of nanobubbles as hydrophobic surfaces approach forms a gaseous capillary bridge, and thus a capillary force. The existence of nanobubbles has been highly debated over the last 15 years. To date, experimental evidence is sound but a theoretical understanding is still lacking. It is the purpose of this review to bring together the many experimental results on nanobubbles and the resulting capillary force in order to clarify these phenomena. A review of pertinent nanobubble stability and formation theories is also presented.

Cholic acid micelles--controlling the size of the aqueous cavity by PEGylation

Data show that cholic acid (CA) micelles are less densely packed and much smaller than micelles formed by typical surfactants, suggesting that CA derivatives can be used to synthesize drug nanocarriers. Presumably, the formation of internal cavities is favored by the facial characteristics of the CA molecule, i.e. the convex molecular structure that is hydrophobic on one side and hydrophilic on the other. Here, we present a thermodynamical approach to quantify the effect of facial characteristics on forces governing the self-assembling process of CA molecules. We show that facial characteristics favor the entrapment of water molecules at interfaces upon CA aggregation, which weakens the attraction between CA hydrophobic moieties. Our computer simulations suggest that these effects contribute significantly to the tendency of CA molecules to form small "hollow-core" micelles. The attachment of polyethylene glycol (PEG) molecular chains to CA increases the repulsive forces in the system, reducing even further the micelle size. We use the present molecular model and experimental critical micelle concentration (cmc) data for CA-PEG systems to predict the change of the micelle size and cavity volume with the increase of the PEG chain length (x). Our computations indicate that the CA-PEG micelles are good candidates for drug delivery. The structural stability of CA-PEG micelles was further assessed by molecular dynamics simulations. We also tested the drug loading efficiency of this system and found an average of 0.5 mg paclitaxel load per 20 mg of CA-PEG polymer. The present study helps to identify critical parameters that control structural properties of the CA based nanocarriers and suggests practical means to optimize the ratio between micelle size and volume of the internal cavity.

Interfacial thermodynamics of confined water near molecularly rough surfaces

We study the effects of nanoscopic roughness on the interfacial free energy of water confined between solid surfaces. SPC/E water is simulated in confinement between two infinite planar surfaces that differ in their physical topology: one is smooth and the other one is physically rough on a sub-nanometre length scale. The two thermodynamic ensembles considered, with constant pressure either normal or parallel to the walls, correspond to different experimental conditions. We find that molecular-scale surface roughness significantly increases the solid-liquid interfacial free energy compared to the smooth surface. For our surfaces with a water-wall interaction energy minimum of -1.2 kcal mol(-1), we observe a transition from a hydrophilic surface to a hydrophobic surface at a roughness amplitude of about 3 angstroms and a wavelength of 11.6 angstroms, with the interfacial free energy changing sign from negative to positive. In agreement with previous studies of water near hydrophobic surfaces, we find an increase in the isothermal compressibility of water with increasing surface roughness. Interestingly, average measures of the water density and hydrogen-bond number do not contain distinct signatures of increased hydrophobicity. In contrast, a local analysis indicates transient dewetting of water in the valleys of the rough surface, together with a significant loss of hydrogen bonds, and a change in the dipole orientation toward the surface. These microscopic changes in the density, hydrogen bonding, and water orientation contribute to the large increase in the interfacial free energy, and the change from a hydrophilic to a hydrophobic character of the surface.

Static and dynamic contact angles of water droplet on a solid surface using molecular dynamics simulation

The present study investigates the variation of static contact angle of a water droplet in equilibrium with a solid surface in the absence of a body force and the dynamic contact angles of water droplet moving on a solid surface for different characteristic energies using the molecular dynamics simulation. With increasing characteristic energy, the static contact angle in equilibrium with a solid surface in the absence of a body force decreases because the hydrophobic surface changes its characteristics to the hydrophilic surface. In order to consider the effect of moving water droplet on the dynamic contact angles, we apply the constant acceleration to an individual oxygen and hydrogen atom. In the presence of a body force, the water droplet changes its shape with larger advancing contact angle than the receding angle. The dynamic contact angles are compared with the static contact angle in order to see the effect of the presence of a body force.

Molecular dynamics simulations of water confined between matched pairs of hydrophobic and hydrophilic self-assembled monolayers

We have conducted a molecular dynamics (MD) simulation study of water confined between methyl-terminated and carboxyl-terminated alkylsilane self-assembled monolayers (SAMs) on amorphous silica substrates. In doing so, we have investigated the dynamic and structural behavior of the water molecules when compressed to loads ranging from 20 to 950 MPa for two different amounts of water (27 and 58 water molecules/nm2). Within the studied range of loads, we observe that no water molecules penetrate the hydrophobic region of the carboxyl-terminated SAMs. However, we observe that at loads larger than 150 MPa water molecules penetrate the methyl-terminated SAMs and form hydrogen-bonded chains that connect to the bulk water. The diffusion coefficient of the water molecules decreases as the water film becomes thinner and pressure increases. When compared to bulk diffusion coefficients of water molecules at the various loads, we found that the diffusion coefficients for the systems with 27 water molecules/nm2 are reduced by a factor of 20 at low loads and by a factor of 40 at high loads, while the diffusion coefficients for the systems with 58 water molecules/nm2 are reduced by a factor of 25 at all loads.

Hydrophobe-water interactions: methane as a model

Recent molecular-dynamics simulations have demonstrated that the use of an empirical hydrophobic potential displaying two minima, i.e., one for hydrophobes in close contact and one for hydrophobes separated by a hydration layer, leads to a marked improvement in protein structure prediction. This potential is supported by experimental data and simulations, but its physical origin and mathematical formulation have not been derived as yet. Here we show that water-mediated attraction (the "wetting regime") between two hydrophobic molecules originates in the interaction between the dipoles induced at the surface of the hydrophobes by the surrounding structured water. As an example, we derive the effective hydrophobic potential that describes the interaction between two methane molecules, a classical model of a double-well energy function. We found an excellent agreement with published results from all-atom, explicit solvent molecular-dynamics simulations of this interaction. The approach presented here provides the theoretical basis for implementing an adequate representation of the wetting regime of the hydrophobic interactions in force fields used for structure prediction. The results are useful for modeling both protein folding and binding.