Vicinity Of Hydrophobic Surface Research Articles

Rheology of Water Flows Confined between Multilayer Graphene Walls.

Water confined by hydrophilic materials shows unique transport properties compared to bulk water, thereby offering new opportunities for the development of nanofluidic devices. Recent experimental and numerical studies showed that nanoconfined water undergoes liquid- to solid-phase-like transitions depending on the degree of confinement. In the case of water confined by graphene layers, the van der Waals forces are known to deform the graphene layers, whose bending leads to further nonuniform confinement effects. Despite the extensive studies of nanoconfined water under equilibrium conditions, the interplay between the confinement and rheological water properties, such as viscosity, slip length, and normal stress differences under shear flow conditions, is poorly understood. The current investigation uses a validated all-atom nonequilibrium molecular dynamics model to simultaneously analyze the continuum transport and atomistic structural properties of water in a slit between two moving graphene walls under Couette flow conditions. A range of different slit widths and velocity strain rates are considered. It is shown that under subnanometer confinement, water loses the rotational symmetry of a Newtonian fluid. Under such conditions, water transforms into ice, where the atomistic structure is completely insensitive to the applied shear force and behaves like a frozen slab sliding between the graphene walls. This leads to the shear viscosity increase, although it is not as dramatic as the normal force increase that contributes to the increased friction force reported in previous experimental studies. On the other end of the spectrum, for flows at large velocity strain rates in moderate to large slits between the graphene walls, water is in the liquid state and reveals shear thinning behavior. In this case, water exhibits a constant slip length on the wall, which is typical of liquids in the vicinity of hydrophobic surfaces.

Electrokinetics over hydrophobic surfaces.

Segregation of phases in the vicinity of hydrophobic surfaces may turn out to be immensely consequential towards altering the coupling of electrostatics and hydrodynamics over interfacial scales. Here, we review the fundamental advances towards bringing out the various facets of electrokinetic transport over hydrophobic interfaces. We lay significant emphasis on the developments in understanding the slippery electrohydrodynamics over such interfaces, by appealing to the considerations across various spatio-temporal scales as unveiled by molecular dynamics as well as mesoscopic modelling paradigms (such as phase field and lattice Boltzmann). We envisage that despite significant advancements being achieved towards relating the macroscopic slip-length with the underlying molecular or mesoscopic phenomena, future efforts could be directed towards developing more robust statistically based models that may connect rarefied gas dynamics in the segregated phase with bulk electrokinetic transport and possible giant augmentations in the consequent fluid flow.

Curcumin as a tool to assess the surface hydrophobicity of proteins

ABSTRACTCurcumin is non-fluorescent in aqueous media; however, in the vicinity of hydrophobic surface it fluoresces. This property is used to assess the use of curcumin as a surface hydrophobic probe. The surface hydrophobicity of proteins was measured by calculating the binding affinity and surface hydrophobicity index value. Surface hydrophobicity of bovine serum albumin, β-lactoglobulin, soy lipoxygenase-1, ovalbumin, and lysozyme is in the following order: bovine serum albumin > β-lactoglobulin > soy lipoxygenase-1 > ovalbumin > lysozyme. The binding affinities of curcumin decreased with the decrease in surface hydrophobicity of proteins. The orders of surface hydrophobicity index value, determined using curcumin, show similar trend with the reported values of standard probes, viz. cis-parinaric acid and 1-anilinonaphthalene-8-sulfonate. The surface hydrophobicity index value of proteins determined using curcumin decreased in the presence of urea, suggesting the possible use of curcumin as a probe to determine the surface hydrophobicity of proteins.

Raman Spectroscopic Studies of Clathrate Hydrate Formation in the Presence of Hydrophobized Particles.

In the present work, Raman spectroscopy was used to study the structure of water molecules in the vicinity of glass particles with different hydrophobicity, immersed in water and in tetrahydrofuran and cyclopentane hydrates. The glass particle surfaces were clean (hydrophilic), coated with N,N-dimethyl-N-octadecyl-3-aminopropyl trimethoxysilyl chloride (partially hydrophobic), or coated with octadecyltrichlorosilane (hydrophobic). The Raman spectra indicate that, prior to nucleation, water molecules in the vicinity of hydrophobic surfaces are more ice-like ordered than those in the bulk liquid or near either hydrophilic or partially hydrophobic surfaces. Furthermore, the degree of hydrogen-bond ordering of water observed prior to hydrate nucleation, as measured by the ratio of the inter- and intramolecular Raman OH bands, was found to have an inverse relationship with the mean induction time for hydrate formation. Following hydration formation, no significant difference in the water molecule structure was observed in the hydrate phase based on their Raman OH bands, irrespective of surface hydrophobicity. These observations made with Raman spectroscopy provide the foundations for a quantitative link between hydrate nucleation promotion and water-ordering near solid surfaces, which could enable direct comparisons with results from corresponding molecular dynamics simulations.

Lateral Growth of Polypyrrole Electropolymerized along Hydrophobic Insulative Substrates

Effect of hydration property of insulative surfaces on two-dimensional growth of polypyrrole thinfilm along the surfaces was studied. When a platinum rod electrode was simply placed on an insulative hydrophobicsubstrate immersed in an aqueous solution containing pyrrole as monomer, a polypyrrole thin film started to grow two-dimensionally along the surface toward the counter electrode which was a platinum ring electrode placed around the insulative substrate. Such lateral growth of polypyrrole was not observed on a hydrophilic surface. The mechanism of the lateral growth is significantly different from that proposed for metal thin films grown along air/liquid or liquid/liquid interfaces. The lateral growth of polypyrrole seems to be induced by a highly concentrated pyrrole in the vicinity of hydrophobic surfaces. Finally, we have demonstrated that the lateral growth of polypyrrole recognizes the boundary of hydration property on a glass substrate on which hydrophilic and hydrophobic regions coexist.

Wetting Behavior of Water near Nonpolar Surfaces

We use molecular simulation to study the wetting behavior of water near flat nonpolar surfaces. The interface potential approach is used to capture the evolution of various interfacial properties over a broad range of temperature and substrate strength. Three model substrates are considered: an atomistically detailed face-centered cubic (FCC) lattice, a graphite lattice, and a structureless wall. We first examine the evolution of the contact angle with substrate strength for conditions ranging from near drying to complete wetting at select temperatures. A notable characteristic of all systems is the presence of a surface strength at which the contact angle is independent of temperature. We also identify an analogous point in which the quantity γlv cos θ is temperature invariant. We discuss the relationship between this point and the excess entropy of the solid–liquid interface. We next consider the temperature dependence of interfacial properties. We find the contact angle to decrease with temperature at relatively strong surfaces and increase with the temperature at relatively weak surfaces. The work of adhesion is found to be a useful quantity for describing the interfacial properties of water. The enthalpic and the entropic contributions to the work of adhesion are obtained from its temperature dependence. These properties are found to be directly related to the affinity of water for a solid surface and therefore serve as useful measures of the hydrophobicity of a surface. Finally, we examine the effect of surface strength and temperature on the density depletion associated with water at hydrophobic surfaces. We study various metrics that quantify the density and compressibility of water in the vicinity of hydrophobic surfaces. Comparisons are drawn between the behavior of water and simple nonpolar fluids at solvophobic surfaces.

Electrochemical Surface Potential Due to Classical Point Charge Models Drives Anion Adsorption to the Air-Water Interface.

We demonstrate that the driving forces for ion adsorption to the air-water interface for point charge models result from both cavitation and a term that is of the form of a negative electrochemical surface potential. We carefully characterize the role of the free energy due to the electrochemical surface potential computed from simple empirical models and its role in ionic adsorption within the context of dielectric continuum theory. Our research suggests that the electrochemical surface potential due to point charge models provides anions with a significant driving force for adsoprtion to the air-water interface. This is contrary to the results of ab initio simulations that indicate that the average electrostatic surface potential should favor the desorption of anions at the air-water interface. The results have profound implications for the studies of ionic distributions in the vicinity of hydrophobic surfaces and proteins.

Surface forces in thin liquid films of n-alcohols and of water–ethanol mixtures confined between hydrophobic surfaces

An atomic force microscope (AFM) was used to measure the surface forces in thin liquid films (TLFs) confined between gold surfaces hydrophobized by chemisorption of alkylthiols. The measurements were conducted in different H-bonding liquids and in water–ethanol solutions. Attractive forces stronger and longer-ranged than the van der Waals force were observed in water, ethanol, and 1-butanol in a descending order. The attractive force measured in methanol was much weaker.The surface forces measured in ethanol solutions varied with concentration. Initially, the attractive forces decreased rapidly upon ethanol addition, passed through a broad minimum centered around mole fractions in the range of 0.15–0.20, and then increased as the ethanol mole fraction was further increased above ∼0.5. Thermodynamic analysis of the data suggests that clusters of water and ethanol may form in TLFs. Thus, the attractive forces may arise from changes in liquid structure. It appears that structuring is a consequence of H-bonding liquid to minimize its free energy in the vicinity of hydrophobic surfaces and in the presence of foreign species in solution.

Characterizing hydrophobicity of interfaces by using cavity formation, solute binding, and water correlations

Hydrophobicity is often characterized macroscopically by the droplet contact angle. Molecular signatures of hydrophobicity have, however, remained elusive. Successful theories predict a drying transition leading to a vapor-like region near large hard-sphere solutes and interfaces. Adding attractions wets the interface with local density increasing with attractions. Here we present extensive molecular simulation studies of hydration of realistic surfaces with a wide range of chemistries from hydrophobic (-CF(3), -CH(3)) to hydrophilic (-OH, -CONH(2)). We show that the water density near weakly attractive hydrophobic surfaces (e.g., -CF(3)) can be bulk-like or larger, and provides a poor quantification of surface hydrophobicity. In contrast, the probability of cavity formation or the free energy of binding of hydrophobic solutes to interfaces correlates quantitatively with the macroscopic wetting properties and serves as an excellent signature of hydrophobicity. Specifically, the probability of cavity formation is enhanced in the vicinity of hydrophobic surfaces, and water-water correlations correspondingly display characteristics similar to those near a vapor-liquid interface. Hydrophilic surfaces suppress cavity formation and reduce the water-water correlation length. Our results suggest a potentially robust approach for characterizing hydrophobicity of more complex and heterogeneous surfaces of proteins and biomolecules, and other nanoscopic objects.