Effects of Hydrophobizing Methods of Surfaces on the Interaction in Aqueous Solutions

Understanding the "Berg limit": the 65° contact angle as the universal adhesion threshold of biomatter

Surface phenomena in aqueous environments such as long-range hydrophobic attraction, macromolecular adhesion, and even biofouling are predominantly influenced by a fundamental parameter-the water contact angle. The minimal contact angle required for these and related phenomena to occur has been repeatedly reported to be around 65° and is commonly referred to as the "Berg limit." However, the universality of this specific threshold across diverse contexts has remained puzzling. In this perspective article, we aim to rationalize the reoccurrence of this enigmatic contact angle. We show that the relevant scenarios can be effectively conceptualized as three-phase problems involving the surface of interest, water, and a generic oil-like material that is representative of the nonpolar constituents within interacting entities. Our analysis reveals that attraction and adhesion emerge when substrates display an underwater oleophilic character, corresponding to a "hydrophobicity under oil", which occurs for contact angles above approximately 65°. This streamlined view provides valuable insights into macromolecular interactions and holds implications for technological applications.

Conditions for the stable adsorption of lipid monolayers to solid surfaces

Lipid monolayers are ubiquitous in biological systems and have multiple roles in biotechnological applications, such as lipid coatings that enhance colloidal stability or prevent surface fouling. Despite the great technological importance of surface-adsorbed lipid monolayers, the connection between their formation and the chemical characteristics of the underlying surfaces has remained poorly understood. Here, we elucidate the conditions required for stable lipid monolayers nonspecifically adsorbed on solid surfaces in aqueous solutions and water/alcohol mixtures. We use a framework that combines the general thermodynamic principles of monolayer adsorption with fully atomistic molecular dynamics simulations. We find that, very universally, the chief descriptor of adsorption free energy is the wetting contact angle of the solvent on the surface. It turns out that monolayers can form and remain thermodynamically stable only on substrates with contact angles above the adsorption contact angle, θads. Our analysis establishes that θads falls into a narrow range of around 60∘-70∘ in aqueous media and is only weakly dependent on the surface chemistry. Moreover, to a good approximation, θads is roughly determined by the ratio between the surface tensions of hydrocarbons and the solvent. Adding small amounts of alcohol to the aqueous medium lowers θads and thereby facilitates monolayer formation on hydrophilic solid surfaces. At the same time, alcohol addition weakens the adsorption strength on hydrophobic surfaces and results in a slowdown of the adsorption kinetics, which can be useful for the preparation of defect-free monolayers.

Tuning Contact Angles of Aqueous Droplets on Hydrophilic and Hydrophobic Surfaces by Surfactants

Adsorption of small amphiphilic molecules occurs in various biological and technological processes, sometimes desired while other times unwanted (e.g., contamination). Surface-active molecules preferentially bind to interfaces and affect their wetting properties. We use molecular dynamics simulations to study the adsorption of short-chained alcohols (simple surfactants) to the water–vapor interface and solid surfaces of various polarities. With a theoretical analysis, we derive an equation for the adsorption coefficient, which scales exponentially with the molecular surface area and the surface wetting coefficient and is in good agreement with the simulation results. We apply the outcomes to aqueous sessile droplets containing surfactants, where the competition of surfactant adsorptions to both interfaces alters the contact angle in a nontrivial way. The influence of surfactants is the strongest on very hydrophilic and hydrophobic surfaces, whereas droplets on moderately hydrophilic surfaces are less affected.

Recent advances for understanding the role of nanobubbles in particles flotation

Traditional froth flotation is the primary method for the separation and upgrading of fine mineral particles. However, it is still difficult for micro-fine and low-quality minerals to effectively separate. It is generally believed that bubble miniaturization is of great significance to improve flotation efficiency. Due to their unique physical and chemical properties, the application of nanobubbles (NBs) in ore flotation and other fields has been widely investigated as an important means to solve the problems of fine particle separation. Therefore, a fundamental understanding of the effect of NBs on flotation is a prerequisite to adapt it for the treatment of fine and low-quality minerals for separation. In this paper, recent advances in the field of nanobubble (NB) formation, preparation and stability are reviewed. In particular, we highlight the latest progress in the role of NBs on particles flotation and focus in particular on the particle-particle and particle-bubble interaction. A discussion of the current knowledge gap and future directions is provided.

Measurement of the Attachment Force between an Air Bubble and a Mineral Surface: Relationship between the Attachment Force and Flotation Kinetics

The interaction forces between air bubbles and mineral surfaces were directly measured during the attachment process using an apparatus developed in our laboratory, and they are defined as the attachment forces. The attachment forces were measured between the air bubble and mineral surfaces modified with surfactants to have different hydrophobicities. Chalcopyrite and galena were used as the mineral surfaces, and their hydrophobicity was controlled by adsorbing xanthates with different hydrocarbon chain lengths. The hydrophobicity is represented by the static contact angle of water on the mineral surface. When the static contact angle was less than 90°, the attachment force increased considerably with increasing static contact angle of the surfaces, irrespective of the mineral type or the hydrocarbon chain length of the adsorbed xanthate. The hydrophobicity of the mineral surface is found to be the dominant factor determining the attachment force. The measured attachment forces agree well with those calculated based on the force balance model derived from the capillary force and Laplace pressure equation. Microflotation experiments to examine the relationship between the attachment force and flotation kinetics were carried out under the same conditions to control surface hydrophobicity. The variation in the flotation kinetic constants and attachment forces with the water contact angle are very similar. As a result, the attachment forces measured by the developed apparatus can provide quantitative information on the interaction between an air bubble and the mineral surface and can be used for predicting the flotation kinetics.

Hydrophobic Attraction Measured between Asymmetric Hydrophobic Surfaces

The interaction forces between silica surfaces modified to different degrees of hydrophobicity were measured using colloidal probe atomic force microscopy (AFM). A highly hydrophobic silica particle was prepared with octadecyltrichlorosilane (OTS), and the interaction forces were measured against silica substrates modified to produce surfaces of varying hydrophobicity. The interaction forces between the highly hydrophobic particle and a completely hydrophilic silicon wafer surface fitted well to the DLVO theory, indicating that no additional (non-DLVO) forces act between the surfaces. When the silicon wafer surface was treated to produce a contact angle of water on surface of 40°, an additional attractive force that is longer ranged than the van der Waals force was observed between the surfaces. The range and magnitude of the attractive force increase with the contact angle of water on the substrate. Beyond the effect on the contact angle, the hydrocarbon chain length and the terminal groups of hydrophobic layer on the substrate only have a minor effect on the magnitude of the force, even when the substrate is terminated with polar carboxyl groups, provided the hydrophobicity of the other surface is high.

Recent experimental advances for understanding bubble-particle attachment in flotation

Bubble-particle interaction is of great theoretical and practical importance in flotation. Significant progress has been achieved over the past years and the process of bubble-particle collision is reasonably well understood. This, however, is not the case for bubble-particle attachment leading to three-phase contact line formation due to the difficulty in both theoretical analysis and experimental verification. For attachment, surface forces play a major role. They control the thinning and rupture of the liquid film between the bubble and the particle. The coupling between force, bubble deformation and film drainage is critical to understand the underlying mechanism responsible for bubble-particle attachment. In this review we first discuss the advances in macroscopic experimental methods for characterizing bubble-particle attachment such as induction timer and high speed visualization. Then we focus on advances in measuring the force and drainage of thin liquid films between an air bubble and a solid surface at a nanometer scale. Advances, limits, challenges, and future research opportunities are discussed. By combining atomic force microscopy and reflection interference contrast microscopy, the force, bubble deformation, and liquid film drainage can be measured simultaneously. The simultaneous measurement of the interaction force and the spatiotemporal evolution of the confined liquid film hold great promise to shed new light on flotation.

Water-Mediated Interactions between Hydrophilic and Hydrophobic Surfaces

All surfaces in water experience at short separations hydration repulsion or hydrophobic attraction, depending on the surface polarity. These interactions dominate the more long-ranged electrostatic and van der Waals interactions and are ubiquitous in biological and colloidal systems. Despite their importance in all scenarios where the surface separation is in the nanometer range, the origin of these hydration interactions is still unclear. Using atomistic solvent-explicit molecular dynamics simulations, we analyze the interaction free energies of charge-neutral model surfaces with different elastic and water-binding properties. The surface polarity is shown to be the most important parameter that not only determines the hydration properties and thereby the water contact angle of a single surface but also the surface-surface interaction and whether two surfaces attract or repel. Elastic properties of the surfaces are less important. On the basis of surface contact angles and surface-surface binding affinities, we construct a universal interaction diagram featuring three different interaction regimes-hydration repulsion, cavitation-induced attraction-and for intermediate surface polarities-dry adhesion. On the basis of scaling arguments and perturbation theory, we establish simple combination rules that predict the interaction behavior for combinations of dissimilar surfaces.

QCM-D study of nanoparticle interactions

Quartz crystal microbalance with dissipation monitoring (QCM-D) has been proven to be a powerful research tool to investigate in situ interactions between nanoparticles and different functionalized surfaces in liquids. QCM-D can also be used to quantitatively determine adsorption kinetics of polymers, DNA and proteins from solutions on various substrate surfaces while providing insights into conformations of adsorbed molecules. This review aims to provide a comprehensive overview on various important applications of QCM-D, focusing on deposition of nanoparticles and attachment-detachment of nanoparticles on model membranes in complex fluid systems. We will first describe the working principle of QCM-D and DLVO theory pertinent to understanding nanoparticle deposition phenomena. The interactions between different nanoparticles and functionalized surfaces for different application areas are then critically reviewed. Finally, the potential applications of QCM-D in other important fields are proposed and knowledge gaps are identified.

From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces

Using all-atom molecular dynamics (MD) simulations at constant water chemical potential in combination with basic theoretical arguments, we study hydration-induced interactions between two overall charge-neutral yet polar planar surfaces with different wetting properties. Whether the water film between the two surfaces becomes unstable below a threshold separation and cavitation gives rise to long-range attraction, depends on the sum of the two individual surface contact angles. Consequently, cavitation-induced attraction also occurs for a mildly hydrophilic surface interacting with a very hydrophobic surface. If both surfaces are very hydrophilic, hydration repulsion dominates at small separations and direct attractive force contribution can-if strong enough-give rise to wet adhesion in this case. In between the regimes of cavitation-induced attraction and hydration repulsion we find a narrow range of contact angle combinations where the surfaces adhere at contact in the absence of cavitation. This dry adhesion regime is driven by direct surface-surface interactions. We derive simple laws for the cavitation transition as well as for the transition between hydration repulsion and dry adhesion, which favorably compare with simulation results in a generic adhesion state diagram as a function of the two surface contact angles.

The hydrophobic force: measurements and methods

The hydrophobic force describes the attraction between water-hating molecules (and surfaces) that draws them together, causing aggregation, phase separation, protein folding and many other inherent physical phenomena.

Measurement of interactions between solid particles, liquid droplets, and/or gas bubbles in a liquid using an integrated thin film drainage apparatus

A novel device was designed to measure drainage dynamics of thin liquid films confined between a solid particle, an immiscible liquid droplet, and/or gas bubble. Equipped with a bimorph force sensor, a computer-interfaced video capture, and a data acquisition system, the newly designed integrated thin film drainage apparatus (ITFDA) allows for the direct and simultaneous measurements of force barrier, true film drainage time, and bubble/droplet deformation under a well-controlled external force, receding and advancing contact angles, capillary force, and adhesion (detachment) force between an air bubble or oil droplet and a solid, a liquid, or an air bubble in an immiscible liquid. Using the diaphragm of a high-frequency speaker as the drive mechanism for the air bubble or oil droplet attached to a capillary tube, this newly designed device is capable of measuring forces over a wide range of hydrodynamic conditions, including bubble approach and retract velocities up to 50 mm/s and displacement range up to 1 mm. The results showed that the ITFDA was capable of measuring hydrodynamic resistance, film drainage time, and other important physical parameters between air bubbles and solid particles in aqueous solutions. As an example of illustrating the versatility, the ITFDA was also applied to other important systems such as interactions between air bubble and oil droplet, two air bubbles, and two oil droplets in an aqueous solution.

Hydrophobic attraction between silanated silica surfaces in the absence of bridging bubbles

The interaction forces between silanated silica surfaces on which there were neither nanobubbles nor a gas phase were measured using colloidal probe atomic force microscopy (AFM). To obtain hydrophobic surfaces without attached nanobubbles, an aqueous solution was introduced between the surfaces after an exchange process involving several solvents. In the approaching force curves obtained, an attractive force was observed from a distance of 10-25 nm, indicating the existence of an additional attractive force stronger than the van der Waals attraction. In the retracting force curves, a strong adhesion force was observed, and the value of this force was comparable to that of the capillary bridging force. The data clearly showed that although the bridging of nanobubbles is responsible for long-range hydrophobic attraction, there also exists an additional attractive force larger than the van der Waals attraction between hydrophobic surfaces without nanobubbles. Both the ionic strength and the temperature of the solution had little influence on the force. The possible origin of the force is discussed on the basis of the obtained results.

SURFACE FORCES AND ADHESION BETWEEN ELECTROLYTIC COPPER CATHODE SURFACES AND MICROSPHERE SURFACES OF GLASS AND POLYSTYRENE IN AQUEOUS ELECTROLYTE SOLUTIONS

Surface forces and adhesion between electrolytic copper cathode surfaces and sphere surfaces of glass and polystyrene were measured with an atomic force microscope (AFM) in water and in aqueous medium containing 104, 10-3 and 10-2 M sodium chloride at ambient temperature. Two types of copper surfaces were considered, oxidized and just-polished. Copper surfaces are submicroscopically rough and so at "contact", a film of intervening fluid separates substrate from probe. At close proximity, the interaction of the copper surfaces with either of the two probes is repulsive and such that extending and retracting force curves are essentially hysteresis-free. At higher separations, extending force curves for any probe-substrate combination were markedly repulsive and DLVO type. The higher the electrolyte concentration, the lower the range of the repulsive force. The system polystyrene-polished substrate displayed contact jumps for the higher electrolyte concentrations considered here, thus suggesting true adhesion between the surfaces. Strong and extremely long-ranged adhesive behavior were measured for the interaction between glass probes and oxidized copper substrates in low salt concentration solutions; at the origin are submicroscopic bubbles or cavities trapped between the surfaces and stabilized by the chemical heterogeneity of the interacting surfaces. A long-ranged, although weak, attractive interaction between polystyrene probes and just-polished copper substrates in aqueous salt solution belongs to the interacting surfaces although the intensity and range seems enhanced by the formation of bubbles.

Non-DLVO silica interaction forces in NMP-water mixtures. II. An asymmetric system

The interaction between energetically asymmetric hydrophilic and hydrophobic surfaces has fundamental and practical importance in both industrial and natural colloidal systems. The interaction forces between a hydrophilic silica sphere and a silanated, hydrophobic glass plate in N-methyl-2-pyrrolidone (NMP)-water binary mixtures were measured using atomic force microscopy (AFM). A strong and long-range attractive force was observed in pure water and was attributed to the formation of capillary bridges associated with nanoscale bubbles initially present on the hydrophobic surface. When NMP was added, the capillary force and corresponding pull-off force became less attractive, which was explained readily in terms of the surface wettability by the binary solvent mixture. Similar to the case of symmetric (two hydrophilic) surfaces, the range of attraction between the asymmetric surfaces was maximized at around 30 vol % NMP, which is consistent with the formation of a thick adsorbed macrocluster layer on the hydrophilic silica surface.

Contact angle and adsorption behavior of carboxylic acids on α-Al2O3 surfaces

The hydrophilic character of aluminum oxide surfaces may be altered through coating such surfaces with carboxylic acids. The initially hydrophilic nature of the solid substrate changes towards a less hydrophilic character as the bulk concentration and the chain length of the acids increases. The acids employed in this work (propionic, valeric and enanthic) show a certain affinity to the liquid-gas, solid-liquid and solid-gas interfaces, being the relative adsorption on them competitive. The adsorption behavior of these carboxylic acids is experimentally investigated combining pendant drop tensiometry, contact angle measurements on α-Al(2)O(3) polycrystalline ceramics and adsorption on particles in aqueous suspensions, as a function of the hydrocarbon chain length of the acids and their bulk concentration, at pH equal to the acids' pKa. The hydrophilic character of the coated alumina decreases with the acids concentration upon a certain concentration beyond that, it increases. The minimum of hydrophilicity is reached right before bi-layer arrangements on the adsorption pattern of the acids on the solid substrates take place.

Drainage, rupture, and lifetime of deionized water films: effect of dissolved gases?

Gas bubbles coalesce in deionized (DI) water because the water (foam) films between the bubbles are not stable. The so-called hydrophobic attraction has been suggested as the cause of the film instability and the bubble coalescence. In this work, microinterferometry experiments show that foam films of ultrapure DI water can last up to 10 s and the contact time between the two gas bubble surfaces at close proximity (approximately 1 microm separation distance) significantly influences the film drainage, rupture, and lifetime. Specifically, when the two bubbles were first brought into contact, the films instantly ruptured at 0.5 microm thickness. However, the film drainage rate and rupture thickness sharply decreased and the film lifetime steeply increased with increasing contact time up to 10 min, but then they leveled off. The constant thickness of film rupture was around 35 nm. Possible contamination was vigorously investigated and ruled out. It is argued that migration of gases inherently dissolved in water might cause the transient behavior of the water films at the short contact time. The film drainage rate and instability at the long contact time were analyzed employing Eriksson et al.'s phenomenological theory of long-range hydrophobic attraction (Eriksson, J. C.; Ljunggren, S.; Claesson, P. M., J. Chem. Soc., Faraday Trans. 2 1989, 85, 163-176) and the hypothesis of water molecular structure modified by dissolved gases, and the extended Stefan-Reynolds theory by incorporating the mobility of the air-DI-water interfaces.

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.

Forces between blank surfaces as measured by the colloidal probe technique and by optical tweezers--a comparison

The well-established atomic force microscopy (AFM)-based colloidal probe technique (CPT) and optical tweezers (OT) are combined to measure the interaction forces between blank SiO(2) surfaces in aqueous ionic solutions (CaCl(2)) of varying concentration at pH 7. Spherical colloids (SiO(2), diameter approximately 4.63 +/- 0.05 microm) taken out of the same batch are used by both methods. In the case of CPT, a single colloid is glued to a cantilever, and the interaction forces with a plain SiO(2) surface are determined in dependence on the concentration of the surrounding medium. For the OT studies, two colloids (one fixed to a micropipet by capillary action, the other held with the optical trap) are approached to each other in nanometer steps, and the resulting forces are measured for the same media as in the CPT experiment. Both techniques fit well to each other and enable one to cover interaction energies ranging from 10(-5) to 1 mN/m. The experimental data are well described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory revealing that the effective surface charge density changes slightly with concentration.

Force trace hysteresis and temperature dependence of bridging nanobubble induced forces between hydrophobic surfaces

An atomic force microscope and the colloidal probe technique are used to probe the interaction between a hydrophobic particle and a hydrophobic surface in water. The characteristics of the observed force curves strongly suggest that a gas bubble is formed when the particle is moved toward the surface and that the bubble ruptures when the particle subsequently is retracted from the surface. We demonstrate that this type of interaction is not unique for hydrophobic surfaces in water since the interaction between hydrophilic surfaces in air provides very similar force curves. However, the interaction between hydrophobic surfaces vanish if water is replaced by an organic solvent with low polarity. The bridging bubble model is employed to explain the hysteresis observed between approach and retraction force traces and experimental conditions where the hysteresis can be almost eliminated are identified. Finally, it is demonstrated that the hydrophobic interaction is strongly temperature dependent and this dependence can be attributed mainly to the decreasing solubility of air in water with increasing temperature.

Interactions between a polystyrene particle and hydrophilic and hydrophobic surfaces in aqueous solutions

The interaction between a colloidal polystyrene particle mounted on an AFM cantilever and a hydrophilic and a hydrophobic surface in aqueous solution is investigated. Despite the apparent simplicity of these two types of systems a variety of different types of interactions are observed. The system containing the polystyrene particle and a hydrophilic surface shows DLVO-like interactions characteristic of forces between charged surfaces. However, when the surface is hydrophobized the interaction changes dramatically and shows evidence of a bridging air bubble being formed between the particle and the surface. For both sets of systems, plateaus of constant force in the force curves are obtained when the particle is retracted from the surface after being in contact. These events are interpreted as a number of individual polystyrene molecules that are bridging the polystyrene particle and the surface. The plateaus of constant force are expected for pulling a hydrophobic polymer in a bad (hydrophilic) solvent. The plateau heights are found to be of uniform spacing and independent of the type of surface, which suggests a model by which collapsed polymers are extended into the aqueous medium. This model is supported by a full stretching curve showing also the backbone elasticity and a stretching curve obtained in pentanol, where the plateau changes to a nonlinear force response, which is typical for a polymer in a good or neutral solvent. We suggest that these polymer bridges are important in particular for the interaction between polystyrene and the hydrophilic surface, where they to some extent counteract the long-range electrostatic repulsion.

Optical observation of gas bridging between hydrophobic surfaces in water

The very strong and long-ranged interaction force between hydrophobic surfaces in water has been a debated issue for a long time. Recent studies suggest that the long-range attraction is attributable to the bridging of nanoscopic bubbles attached on the surfaces. However, it is still unclear at present whether such a bridging is able to exit stably or not. To clarify the existence of the gas bridge, we conducted the optical observation and the force measurement between the hydrophobic glass particle and plate in water simultaneously, using a combined apparatus of an atomic force microscope and an optical inverted microscope. It is found that (i) the image of the dark ring of ca. 1 mum in diameter appears in the region where the existence of the bridge is confirmed by force curves, but disappears when the separation between surfaces becomes shorter than the low limit of the wavelength of visible light, and (ii) the sudden disappearance of the image coincides well with the breakage of the bridge estimated from the separating force curve. The results obtained here are consistent with the above-mentioned mechanism for the long-range attraction between hydrophobic surfaces in water.

Effects of degassing and ionic strength on AFM force measurements in octadecyltrimethylammonium chloride solutions

Sakamoto et al. (Langmuir 2002, 18, 5713) conducted AFM force measurements between silica sphere and fused-silica plate in aqueous octadecyltrimethylammonium chloride (C18TACl) solutions and concluded that long-range attractive force is not observed in carefully degassed solutions. In the present work, AFM force measurements were conducted by following the procedures described by Sakamoto et al. The results showed the presence of an attractive force that was much stronger than the van der Waals force both in air-saturated and degassed solutions. The force was most attractive at 5 x 10(-6) M C18TACl, where contact angle was maximum. At this concentration, which is close to the charge compensation point (ccp) of the glass sphere, the long-range decay lengths (D) were 34 and 38 nm in air-saturated and degassed solutions, respectively. At 10(-5) M, the decay length decreased from 30 to 4 nm upon degassing. This decrease in decay length can be explained by a pH increase (from 5.7 to 6.6), which in turn causes additional surfactant molecules to adsorb on the surface with inverse orientation. The attractive force was screened by an added electrolyte (NaCl), indicating that the attractive force may be of electrostatic origin. Therefore, the very long decay lengths observed in the absence of electrolyte may be ascribed to the fact that the ccp occurs at a very low surfactant concentration.

Water molecule clusters measured at water/air interfaces using atomic force microscopy

During the tip approach to hydrophobic surfaces like the water/air interface, the measured interaction force reveals a strong attraction with a range of approximately 250 nm at some points along the interface. The range of this force is approximately 100 times larger than the measured for gold (approximately 3 nm) and 10 times larger than the one for hydrophobic silicon surfaces (approximately 25 nm). At other points the interface exerts a medium range repulsive force growing stepwise as the tip approaches the interface plane, consequently the hydrophobic force is a strong function of position. To explain these results we propose a model where the force on the tip is associated with the exchange of a small volume of the interface with a dielectric permittivity epsilon(int) by the tip with a dielectric permittivity epsilon(tip). By assuming a oscillatory spatial dependence for the dielectric permittivity it is possible to fit the measured force profiles. This dielectric spatial variation was associated with the orientation of the water molecules arrangement in the interfacial region. Small nanosized hydrogen-bond connected cages of water molecules present in bulk water at the interface are oriented by the interfacial electric field generated by the water molecules broken bonds, one broken hydrogen bond out of every four. This interfacial field orients small clusters formed by approximately 100 water molecules into larger clusters (approximately 100 nm). In the limit of small (less than 5 nm thick) water molecule cages we have modeled the static dielectric permittivity (epsilon) as the average response of those cages. In these regions the dielectric permittivity for water/air interfaces decreases monotonically from the bulk value epsilon approximately 80 to approximately 2 at the interface. For regions filled with medium size cages, the tip senses the structure of each cage and the static dielectric permittivity is matched to the geometrical features of these cages sized approximately 25 to 40 nm. Interfacial electric energy density values were calculated using the electric field intensity and the dielectric permittivity obtained by the fitting of the experimental points. The integration of the electric energy density along the interfacial region gives a value of 0.072 J m(-2) for interfacial energy density for points where the hydrophobic force has a range of approximately 250 nm. Regions formed by various clusters result in lower values of the interfacial energy density.

Tissue-culture surfaces with mixtures of aminated and fluorinated functional groups. Part 1. Synthesis and characterization

Surface chemistry of culture dishes can have profound effects on the phenotype of cultured cells. In the present study, chemisorption from aqueous, binary mixtures of organosilanes onto borosilicate glass created surfaces bearing diamine groups (N2), trifluoropropyl groups (F3) and mixtures of the two. Composition of N2-F3 surfaces was controlled by the ratio of monomers in the silanization bath, as confirmed by electron spectroscopy for chemical analysis and by conjugation of surface amines with fluorescein-5-isothiocyanate. Atomic-force microscopy revealed that silanized surfaces are patchy, though their root-mean-square roughnesses do not differ significantly from that of smooth glass (0.3 nm). Surfaces richest in diamine residues were the most hydrophilic, with advancing water-contact angles < or = 90 degrees. The accompanying paper (the next article in this issue) describes the effects of these surface chemistries on the phenotype of transgenic insulinoma cells in vitro. We conclude that chemisorption from the N2-F3 system provides a simple, one-pot method for tailoring the chemistry of glass culture surfaces.

Measuring forces with the AFM: polymeric surfaces in liquids

This review links together for the first time both the practicalities of force measurement and the work carried out to date on force detection between polymeric surfaces in liquids using the atomic force microscope (AFM). Also included is some of the recent work that has been carried out between surfactant surfaces and biologically coated surfaces with the AFM. The emphasis in this review is on the practical issues involved with force measurement between these types of surfaces, and the similarities and irregularities between the observed types of forces measured. Comparison is made between AFM and surface force apparatus (SFA) measurements, as there is a much longer history of work with the latter. Results indicate that forces between the surfaces reviewed here are a complicated mixture of steric-type repulsion, conformational behaviour on separation and long-range attraction, which is often ascribed to 'hydrophobic' forces. The origin of this latter force remains uncertain, despite its almost ubiquitous appearance in force measurements with these types of surfaces.

Static Method to Evaluate Interaction Forces by AFM

An atomic force microscope (AFM) is a very powerful tool to evaluate interaction forces between surfaces in liquids on the molecular scale, but the apparatus was not designed to measure forces in equilibrium. Hence, data obtained by AFM are not in equilibrium in principle. Here we propose a static method to obtain interaction forces between stationary surfaces in aqueous solutions using AFM. The validity of the proposed method was confirmed by comparing interaction forces measured by this method with those by the normal dynamic method for the system of a mica plate and a silica particle in electrolyte solutions where an equilibrium was nearly achieved because water molecules and ions moved much faster than surfaces. The applicability of this method to the measurement of hydrophobic attraction was then examined, and important information on the attraction was obtained. Copyright 2001 Academic Press.