Surface energy of oxides and silicates

Published data and the author’s own data on the surface energy of hydrophilic oxides, silicates, and hydrophobic adsorbents based on them are reviewed. The prospects of using the combined Gibbs-Helmholtz-Young equation to obtain data on the surface pressure, heat of wetting, and wetting contact angle of hydrophilic and hydrophobic adsorbents are demonstrated. These data are used to estimate the thermodynamic characteristics of the surface and interfacial regions at the boundary between the materials and water. It is shown that the boundary layers of water close to the hydrophobic surfaces are more ordered while those close to the hydrophobic surfaces are less ordered than with liquid water. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 3, pp. 133–149, May–June, 2006.     

Molecular explanation for why talc surfaces can be both hydrophilic and hydrophobic

While individual water molecules adsorb strongly on a talc surface (hydrophilic behavior), a droplet of water beads up on the same surface (hydrophobic behavior). To rationalize this dichotomy, we investigated the influence of the microscopic structure of the surface and the strength of adhesive (surface-water) interactions on surface hydrophobicity. We have shown that at low relative humidity, the competition between adhesion and the favorable entropy of being in the vapor phase determines the surface coverage. However, at saturation, it is the competition between adhesion and cohesion (water-water interactions) that determines the surface hydrophobicity. The adhesive interactions in talc are strong enough to overcome the unfavorable entropy, and water adsorbs strongly on talc surfaces. However, they are too weak to overcome the cohesive interactions, and water thus beads up on talc surfaces. Surprisingly, even talc-like surfaces that are highly adhesive do not fully wet at saturation. Instead, a water droplet forms on top of a strongly adsorbed monolayer of water. Our results imply that the interior of hydrophobic zeolites suspended in water may contain adsorbed water molecules at pressures much lower than the intrusion pressure.     

Surface energy of hydrophobic adsorbents

The adsorption of water vapor and the heat of wetting of hydrophilic hydromica and hydrophobized samples of kaolinite and Silochrom were studied. The contact angles for the wetting of the investigated materials with water were obtained. The thermodynamic characteristics of the surface of the sorbents and the interfacial region at their boundary with water were calculated from the obtained data. It was shown that the boundary water layers close to the hydrophilic surface of the hydromica are more ordered while those close to the hydrophobic surfaces of the modified samples of kaolinite, Silochrom, and the reference sample (extremely hydrophobic Teflon) are less ordered than liquid water. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 2, pp. 87–91, March–April, 2006.     

Current status and prospects for development of research on the hydrophilicity of disperse systems

The current status of research on the hydrophilicity of disperse systems was discussed. The criterial values of the surface pressure of the adsorbed water film, heat and contact angle of wetting with water which separate the hydrophilic and hydrophobic surfaces were determined. An analysis of the thermodynamic characteristics showed that the boundary layers of water are more ordered near a hydrophilic surface (mica, kaolinite) and less ordered near a hydrophobic surface (organokaolinite, graphite) in comparison to liquid water. The biexponential character of the change in the structural component of the disjoining pressure with the thickness of the water film was demonstrated for disperse materials with a hydrophilic surface (kaolinite). The classification of the forms of water bound by hydrophilic disperse materials was discussed. The changes in the thermodynamic functions of vermiculite in adsorption of water were found with complex adsorptioncalorimetric and dilatometric measurements. This adsorbent undergoes a first-order phase transition during adsorption. The prospects for use of the adsorption-calorimetric method for determining the amount and energy characteristics of hydrophilic sites was demonstrated for ZSM-5 hydrophobic zeolite.Presented to the meeting of the Chemistry Section of the Academy of Sciences of the Ukraine on October 27–29, 1992.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 29, No. 2, pp. 100–115, March–April, 1993.     

Wettability of tri-block copolymer coated hydrophobic surfaces Predictions and measurements

Hydrophobic surfaces with adsorbed tri-block copolymers are wetted by oil in spite of the hydrophilic buoy groups of the block copolymer that are present near the surface. The effect of the buoy group length of the adsorbed molecules on the wettability of hydrophobic surfaces is studied by contact angle measurements and by computer modelling.

The computer model predicts an increase in interfacial free energy with increasing buoy group length for equilibrium adsorption of block copolymer from water. Molecules with large buoy groups occupy more lateral space; therefore the “bare” surface gets more exposed and the anchor groups contribute less to the interfacial free energy which thus increases with the buoy group length.

The calculations showed that the variation of the interaction parameter between solvent and buoy group hardly influences the interfacial free energy. In contrast the interaction parameter between solvent and surface influences the interfacial free energy to a large extent because the oil/surface interactions have a lower energetic value as compared to water/surface interactions and therefore the interfacial free energy is lower than in water. The interfacial free energy varies slightly with increasing buoy group length, depending on the value chosen for the solvent/surface interaction parameter.

Advancing and receding contact angles of hexadecane, sunflower oil and hydrolysate (partly hydrolysed sunflower oil) were measured on hydrophobic surfaces. All oil/water contact angles were small, indicating a hydrophobic apolar surface character. It was found that, for oils with a “good” interaction with the surface (hexadecane and sunflower oil), the contact angle has a minimum value at a certain buoy group length. For hydrolysate (less-strong interaction with the surface) the contact angle decreases monotonically with increasing buoy group length. The results for hexadecane, sunflower oil and hydrolysate are in reasonable agreement with the model predictions. The effect of increasing buoy group length is weak; both decreasing and increasing angles are found, depending on the type of oil used.     

Rationalization of the behavior of solid-liquid surface free energy of water in Cassie and Wenzel wetting states on rugged solid surfaces at the nanometer scale

The present work aims to contribute to the understanding at a molecular level of the origin of the hydrophobic nature of surfaces exhibiting roughness at the nanometer scale. Graphite-based smooth and model surfaces whose roughness dimension stretches from a few angstroms to a few nanometers were used in order to generate Cassie and Wenzel wetting states of water. The corresponding solid-liquid surface free energies were computed by means of molecular dynamics simulations. The solid-liquid surface free energy of water-smooth graphite was found to be -12.7 ± 3.3 mJ/m(2), which is in reasonable agreement with a value estimated from experiments and fully consistent with the features of the employed model. All the rugged surfaces yielded higher surface free energy. In both Cassie and Wenzel states, the maximum variation of the surface free energy with respect to the smooth surface was observed to represent up to 50% of the water model surface tension. The solid-liquid surface free energy of Cassie states could be well predicted from the Cassie-Baxter equation where the surface free energies replace contact angles. The origin of the hydrophobic nature of surfaces yielding Cassie states was therefore found to be the reduction of the number of interactions between water and the solid surface where atomic defects were implemented. Wenzel's theory was found to fail to predict even qualitatively the variation of the solid-liquid surface free energy with respect to the roughness pattern. While graphite was found to be slightly hydrophilic, Wenzel states were found to be dominated by an unfavorable effect that overcame the favorable enthalpic effect induced by the implementation of roughness. From the quantitative point of view, the solid-liquid surface free energy of Wenzel states was found to vary linearly with the roughness contour length.     

Liquid–paper interactions during liquid drop impact and recoil on paper surfaces

The spreading and recoiling of water drops on several flat and macroscopically smooth model surfaces and on sized paper surfaces were studied over a range of drop impaction velocities using a high-speed CCD camera. The water drop spreading and recoiling results on several model hydrophobic and hydrophilic surfaces were found to be in agreement with observations reported in the literature. The maximum drop spreading diameter for those model surfaces at impact was found to be dependent upon the initial drop kinetic energy and the degree of hydrophobicity/hydrophilicity of the surface. The extent of the maximum drop recoiling was found to be much weaker for hydrophilic substrates than for hydrophobic substrates. Sized papers, however, showed an interesting switch of behaviour in the process of water drop impaction. They behave like a hydrophobic substrate when a water drop impacts on it, but like a hydrophilic substrate when water drop recoils. Although the contact angle between water and hydrophilic or hydrophobic non-porous surfaces changes from advancing to receding as reported in literature, the change of contact angle during water impact on paper surface is unique in that the level of sizing was found to have a smaller than expected influence on the degree of recoil. Atomic force microscopy (AFM) was used to probe fibres on a sized filter paper surface under water. The AFM data showed that water interacted strongly with the fibre even though the paper was heavily sized. Implications of this phenomenon were discussed in the context of inkjet print quality and of the surface conditions of sized papers. Results of this study are very useful in the understanding of inkjet ink droplet impaction on paper surfaces which sets the initial condition for ink penetration into paper after impaction.     

Surface immobilization of fibronectin-derived PHSRN peptide on functionalized polymer films – Effects on fibroblast spreading

The Pro-His-Ser-Arg-Asn (PHSRN) sequence in fibronectin is a second cell-binding site that synergistically affects Arg-Gly-Asp (RGD). The PHSRN peptide also induces cell invasion and accelerates wound healing. We report on the surface immobilization of PHSRN by spontaneous adsorption on polysiloxane thin films which have different surface free energy characteristics. Low-surface energy (hydrophobic) polysiloxane and the corresponding high-surface energy (hydrophilic) surfaces obtained by UV–ozone treatments were used as adsorbing substrates. The peptide adsorption process was investigated by quartz crystal microbalance with dissipation monitoring and atomic force microscopy. Both adsorption kinetics and peptide rearrangement dynamics at the solid interface were significantly different on the surface-modified films compared to the untreated ones. Fibroblast cells cultures at short times and in a simplified environment, i.e., a medium-free solution, were prepared to distinguish interaction events at the interface between cell membrane and surface-immobilized peptide for the two cases. It turned out that the cell-adhesive effect of immobilized PHSRN was different for hydrophobic compared to hydrophilic ones. Early signatures of cell spreading were only observed on the hydrophilic substrates. These effects are explained in terms of different spatial arrangements of PHSRN molecules immobilized on the two types of surfaces.     

State and structure of water in vicinity of hydrophobic surfaces

Criterial values of the specific heat of water wetting, surface pressure, and contact angle classifying surfaces into hydrophilic and hydrophobic are proposed based on the analysis of own and published data. The most characteristic properties of hydrophobic surfaces, i.e., large surface area per water molecule in the conventional adsorption monolayer and the absence of continuous two-layer water film on the adsorbent surface at vapor pressure close to saturation, are discussed using nonporous carbon-based materials as example. The presence of residual hydrophilic groups that act as sites of the clusterization of polar molecules on the surface of graphitized carbon black is confirmed by gas chromatography and the concentration of these sites is calculated. The amount of water molecules in the surface cluster is determined at different stages of adsorption. Procedures for preparing organically modified layered silicates and silica as basic objects of the study of the interaction between water molecules and hydrophobic surfaces are considered. It is proven that the boundary water layer in the vicinity of hydrophobic surface consists of a thin (∼0.5 nm) depletion layer with a density of 0.4 g/cm³ and a considerable amount (25–30%) of water molecules with free OH groups and thicker (∼35 nm) layer, which is characterized by a more ordered network of hydrogen bonds compared to liquid water. Data obtained by X-ray scattering and neutron and reflection methods, and sum-frequency vibrational spectroscopy are compared with the results of calorimetric study of the interaction between water and hydrophobic surface, as well as with the data of molecular-statistical calculations of the state of water molecules in the surface layer.     

Influence of nanobubbles on the adsorption of nanoparticles

A quartz crystal microbalance was used to study the influence of nanobubbles on the adsorption of polystyrene nanoparticles onto surfaces coated with gold, or coated with dodecanethiol or mercaptoundecanoic acid self-assembled monolayers (SAMs). Adsorption of the nanoparticles onto the surface causes the resonant frequency of the quartz crystal to decrease. We found that particles were adsorbed onto the gold-coated quartz crystal in air-rich water, but not in degassed water. This finding supports the long-standing hypothesis that nanobubbles play a key role in the long-range attractive force between hydrophobic surfaces in aqueous solutions. When the experiments were conducted using quartz crystals coated with a hydrophobic dodecanethiol SAM, the nanoparticles were adsorbed onto the surface even in degassed water due to the short-range hydrophobic interactions between the nanoparticles and the dodecanethiol molecules. In contrast, the nanoparticles were adsorbed to a lesser degree onto the hydrophilic mercaptoundecanoic acid-coated crystals due to electrostatic repulsive forces.     

Cationic electrolyte adsorption layers on hydrophilic and hydrophobic surfaces

The capillary electrokinetics method (measurements of streaming potential and current in original and hydrophobized fused quartz capillaries with radii of 5–7 μm) is employed to study the formation of adsorption layers upon contact with solutions containing a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). It is shown that polyelectrolyte adsorption causes the charge reversal of both hydrophilic and hydrophobic surfaces, with a smaller amount of the substance being adsorbed on the hydrophobic than on the hydrophilic surface. The adsorption on both surfaces increases with the polymer solution concentration. The cationic polyelectrolyte adsorption on the pure quartz surface occurs mainly due to the electrostatic attraction, while, in the case of the hydrophobic surface, the contribution of hydrophobic interactions increases. The study of the layer deformability shows that, on the hydrophilic surfaces, the layer ages and its structure depends on the polymer solution concentration. On the modified surface, the deformation of even freshly formed layers is slight, which suggests that a denser layer is formed on the hydrophobic surface. In contrast to the hydrophilic surface, the polyelectrolyte is partly desorbed from the hydrophobic surface.     

Properties of a water layer on hydrophilic and hydrophobic self-assembled monolayer surfaces: A molecular dynamics study

The microscopic behaviors of a water layer on different hydrophilic and hydrophobic surfaces of well ordered self-assembled monolayers (SAMs) are studied by molecular dynamics simulations. The SAMs consist of 18-carbon alkyl chains bound to a silicon(111) substrate, and the characteristic of its surface is tuned from hydrophobic to hydrophilic by using different terminal functional groups ( CH 3 , COOH). In the simulation, the properties of water membranes adjacent to the surfaces of SAMs were reported by comparing pure water in mobility, structure, and orientational ordering of water molecules. The results suggest that the mobility of water molecules adjacent to hydrophilic surface becomes weaker and the molecules have a better ordering. The distribution of hydrogen bonds indicates that the number of water-water hydrogen bonds per water molecule tends to be lower. However, the mobility of water molecules and distribution of hydrogen bonds of a water membrane in hydropho- bic system are nearly the same as those in pure water system. In addition, hydrogen bonds are mainly formed between the hydroxyl of the COOH group and water molecules in a hydrophilic system, which is helpful in understanding the structure of interfacial water.     

Dynamics of water confined in the interdomain region of a multidomain protein

Molecular dynamics simulations are performed to study the dynamics of interfacial water confined in the interdomain region of a two-domain protein, BphC enzyme. The results show that near the protein surface the water diffusion constant is much smaller and the water-water hydrogen bond lifetime is much longer than that in bulk. The diffusion constant and hydrogen bond lifetime can vary by a factor of as much as 2 in going from the region near the hydrophobic domain surface to the bulk. Water molecules in the first solvation shell persist for a much longer time near local concave sites than near convex sites. Also, the water layer survival correlation time shows that on average water molecules near the extended hydrophilic surfaces have longer residence times than those near hydrophobic surfaces. These results indicate that local surface curvature and hydrophobicity have a significant influence on water dynamics.     

How to make the Cassie wetting state stable?

Wetting of rough hydrophilic and hydrophobic surfaces is discussed. The stability of the Cassie state, with air trapped in relief details under the droplet, is necessary for the design of true superhydrophobic surfaces. The potential barrier separating the Cassie state and the Wenzel state, for which the substrate is completely wetted, is calculated for both hydrophobic and hydrophilic surfaces. When the surface is hydrophobic, the multiscaled roughness of pillars constituting the surface increases the potential barrier separating the Cassie and Wenzel states. When water fills the hydrophilic pore, the energy gain due to the wetting of the pore hydrophilic wall is overcompensated by the energy increase because of the growth of the high-energetic liquid-air interface. The potential barrier separating the Cassie and Wenzel states is calculated for various topographies of surfaces. Structural features of reliefs favoring enhanced hydrophobicity are elucidated.     

Molecular modeling of interactions between L-lysine and functionalized quartz surfaces

Molecular modeling techniques have been used to investigate the interaction of L-lysine in aqueous medium with silanol and methyl sites onto quartz substrates. The substrate effect has been studied for partially hydrophilic surfaces formed by silanol and methyl groups with a ratio of 1:5 and hydrophobic fully methylated surfaces. Molecular dynamics and static calculations indicate that L-lysine does not show any significant interaction with fully methylated surfaces, while its interaction with hydroxylated/methylated surfaces is dominated by electrostatic and H-bond terms. Accordingly, on fully methylated surfaces there is no preferential orientation of L-lysine with respect to the surface, while for hydroxylated/methylated surfaces the L-lysine-surface interaction mainly depends on the molecular orientation, with a preferred geometry involving the ammonium group pointing toward the silanol site. The structure of water shells around L-lysine molecules was shown to be strongly affected by the relative hydrophilic/hydrophobic character of the surfaces. In particular, the order is almost completely lost for partially hydrophilic surfaces, while well-defined hydration shells around L-lysine are obtained for hydrophobic surfaces.     

Adsorption behavior and cross-linking of EHEC and HM-EHEC at hydrophilic and hydrophobic modified surfaces monitored by SPR and QCM-D

The adsorption behavior of ethyl(hydroxyethyl) cellulose EHEC and hydrophobically modified EHEC (HM-EHEC) at hydrophilic and hydrophobic surfaces has been studied using surface plasmon resonance (SPR) and quartz crystal microbalance with dissipation monitoring (QCM-D) methods. The adsorbed amounts measured with the different methods were different due to large amounts of water in the films. The slow adsorption process made it reasonable to assume a continuous polymer reconfiguration process at the surface. This was mostly seen for HM-EHEC at the hydrophobic surface, where a more flexible structure was adopted during the adsorption process. A cross-linking agent was seen to truly interpolymer cross-link EHEC at the hydrophilic surface and HM-EHEC at the hydrophobic surface. For EHEC at a hydrophobic surface and for HM-EHEC at a hydrophilic surface, the polymers adsorbed in an individually phase-separated manner, making an interpolymer cross-linking reaction unsuccessful.     

Electrokinetic properties of methylated quartz capillaries

Electrokinetic (zeta)-potentials of methylated (trimethylchlorosilane) quartz capillaries (5-6 microm in radius) were determined in 10(-4) M KCl solution. Over the course of time, the absolute values of the zeta-potential decrease, as a result of the formation of small bubbles on the rough methylated surface, generated from the flowing, nitrogen gas-saturated solution. This decrease is attributed to screening of a part of the solid surface. After the passage of time, a sharp increase in the zeta-potentials was observed, as the pressure was increased and the initial potential values were recovered. Sometimes, oscillations in the zeta-potentials were observed. This behaviour was explained by detachment of bubbles from the methylated surfaces by the flowing solution. Addition of non-ionic surfactant, which made the methylated surface hydrophilic, decreased the measured zeta-potentials. This was attributed to suppression of water slippage, an effect known to occur for hydrophobic solid surfaces. A mixed mechanism of charge formation is characteristic for these methylated quartz surfaces and is connected with presence of hydrophobic and hydrophilic areas. The ratio between these areas controls both the formation of surface charge as well as the contact angles.     

Assessment of the Surface Heterogeneity of Talc Materials

The hydrophobic and hydrophilic components of the surface of talc materials in aqueous solution were determined using ionic surfactants and their polar headgroup adsorption isotherms. The hydrophilic and hydrophobic surface areas are inferred from the amount of probe molecule adsorbed and the structure of the adsorbed layer. Natural dispersion of talc shows at 298 K a pH of 9.4 and the electrophoretic measurements indicate that the particles are negatively charged. The hydrophilic surface area is estimated from the adsorption of benzyltrimethylammonium ions (BTMA(+)) through electrostatic interactions as supported by the increase of divalent ions in the bulk phase and the decrease in the exothermic displacement enthalpy. It was also observed from the adsorption isotherm of benzene sulfonate anions that the density of positive surface sites is very low and is thus neglected. The adsorption of an anionic surfactant essentially occurs through dispersive interactions between the nonpolar organic tail of the molecule and the hydrophobic surface. Furthermore, some assumptions on the structure of dodecyl sulfate surfactant aggregates at the interface allow the hydrophobic part of the talc particles to be estimated. The cationic surfactant adsorption has been investigated and found to corroborate the hydrophilic and hydrophobic area values first obtained. Copyright 2001 Academic Press.     

Physical Properties of Water Near a Gold Surface: A Nanorheological Analysis

Water at room temperature is not simply a medium for which uniform properties can always be assumed. Water close to solid hydrophobic or hydrophilic surfaces has elasticity, which is measured by monitoring the quartz crystal microbalance (QCM) resonant frequency and resistance. Small additions of salt are shown to modify this elasticity. Furthermore, near the hydrophobic QCM gold electrode, undersaturated aqueous NaCl solutions present a high concentration of ion pairs, which is confirmed by atomic force microscopy through force versus distance measurements.