Long-range attraction between surfaces: existence and amplitude?

The possible occurrence of a long-range attraction between hydrophobic surfaces is fundamental for understanding the kinetics of protein folding or self-assembling structures, such as biological membranes, the stability of emulsions and inorganic dispersions. Direct force measurements have revealed the presence in water of a long-range attraction between macroscopic and hydrophobic surfaces. Nevertheless, the existence of this so-called ‘hydrophobic force’ for smaller objects is still under discussion. For macroscopic surfaces, it appears that electrostatic contributions due to surface heterogeneities and gas effects and/or bubble bridging have to be taken into account, but do they define an intrinsic ‘hydrophobic force’? For charged colloidal particles, theoretical predictions of electrostatic attraction and a phase separation when the counterions are multivalent, are partially confirmed by experiments, and recent experimental evidences of an attraction in a confined geometry put an exciting challenge for theoreticians.     

Bridging-cluster model for hydrophobic attraction

A new model is proposed to account for the long-range hydrophobic attraction repeatedly observed for thin water films between two stable (solid) hydrophobic surfaces. The model is based on the notion of structurally organized, elongated water clusters that span the gap between the hydrophobic surfaces. Two features are noted: (i) Mixing entropy due to the mixing of the clusters and the remainder of the water in the thin film is explicitly taken into account. (ii) A term is invoked that depends inversely on the film thickness, which accounts for the free-energy change associated with reorganizing the film as the film thickness varies. Fitting to experimental surface force data resulted in parameter values of reasonable magnitudes. The model developed covers film thicknesses from about 2 nm and above. On this basis, the amazingly long range of the hydrophobic attraction can be attributed to the formation of bridging, quasi-cylindrical clusters having a radius on the order of 1 to 2 nm.     

The role of hydrophobic surfaces in altering water-mediated peptide-peptide interactions in an aqueous environment

Using Born-Oppenheimer molecular dynamics within the density functional framework, we calculated the effective force acting on water-mediated peptide-peptide interaction between antiparallel β-sheets in an aqueous environment and also in the vicinity of a hydrophobic surface. From the magnitude of the effective force (corresponding to the slope of the free energy as a function of the interpeptide distance) and its sign (a negative value indicates an effective attraction, whereas a positive value indicates an effective repulsion) we can elucidate the fundamental differences of the water-mediated peptide-peptide interactions in those two environments. The computed effective forces indicate that the water-mediated interaction between peptides in an aqueous environment is attractive in the range of interpeptide distance d = 7-8 ? when hydrophobic surfaces are not nearby. Due to the stabilization of the water molecules bridging between the two β-sheets, a free energy barrier exists between the direct and indirect (water-mediated) interpeptide interactions. However, when the peptides are in the proximity of hydrophobic surfaces, this free energy barrier decreases because the hydrophobic surfaces enhance the interpeptide attraction by the destabilization and ease-to-libration of the bridging water molecules between them.     

AFM forces measured between gold surfaces coated with self-assembled monolayers of 1-hexadecanethiol

An atomic force microscope (AFM) was used to measure the forces between gold surfaces with and without hydrophobizing them by the self-assembly of 1-hexadecanethiol. The forces measured between bare gold surfaces were fitted to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with a Hamaker constant of 1.2 x 10 (-20) J, which was close to the value determined using the methylene iodide contact angle method but was lower than that calculated using the Lifshitz theory. When the surfaces were hydrophobized in a 0.01 mM thiol-in-ethanol solution for 10 min, the measured forces exhibited a long-range force with a decay length of 35 nm. Despite its high water contact angle (105 degrees ), the force curve was smooth and exhibited no steps. When the surfaces were hydrophobized in a 1 mM thiol solution for longer than 6 h, however, the force curves exhibited steps, indicating that the long-range attractions were caused by bridging bubbles. When the measurements were conducted after washing the substrates with organic solvents, the steps disappeared and long-range attractive forces appeared. In the presence of ethanol, the water contact angle decreased to below 90 degrees , the attraction became weaker, and the force curves became smooth. On the basis of the results obtained in the present work, possible mechanisms for the long-range attractions are discussed.     

Force Measurements between Semifluorinated Thiolate Self-Assembled Monolayers: Long-Range Hydrophobic Interactions and Surface Charge

Long-range interactions between self-assembled monolayers (SAMs) of semifluorinated alkanethiols have been studied by direct force measurements in water and aqueous NaCl solutions. SAMs prepared from three different thiols, with identical fluorinated head groups but varying hydrocarbon spacer lengths, were investigated: CF(3)(CF(2))(9)(CH(2))(x)SH, where x=2, 11, or 17. Force measurements show that the interactions in water and electrolyte solutions are composed of both double-layer interactions emerging from what appears to be charges adsorbed onto the surfaces and long-range "hydrophobic" attractions, in excess of the expected van der Waals forces. The three investigated thiols produce similar results in force measurements, though the contact angles with water are slightly different. The "hydrophobic" attraction has the form of step-like attractive discontinuities in the force profiles at separations ranging from 20 to 40 nm, caused by bridging of microscopic bubbles residing at the surfaces. The shape or range of these discontinuities are not significantly affected by replacement of the water with either 1 mM or 1 M NaCl solutions. The origin of the charges causing the electrostatic double-layer interaction is unclear, but some possible causes are discussed. Copyright 2001 Academic Press.     

Influence of DNA adsorption and DNA/cationic surfactant coadsorption on the interaction forces between hydrophobic surfaces

The forces between hydrophobic surfaces with physisorbed DNA are markedly and irreversibly altered by exposure to DNA/cetyltrimethylammonium bromide (CTAB) mixtures. In this colloidal probe atomic force microscopy study of the interactions between a hydrophobic polystyrene particle and an octadecyltrimethylethoxysilane-modified mica surface in sodium bromide solutions, we measure distinct changes in colloidal forces depending on the existence and state of an adsorbed layer of DNA or CTAB-DNA complexes. For bare hydrophobic surfaces, a monotonically attractive approach curve and very large adhesion are observed. When DNA is adsorbed at low bulk concentrations, a long-range repulsive force dominates the approach, but on retraction some adhesion persists and DNA bridging is clearly observed. When the DNA solution is replaced with a CTAB-DNA mixture at relative low CTAB concentration, the length scale of the repulsive force decreases, the adhesion due to hydrophobic interactions greatly decreases, and bridging events disappear. Finally, when the surface is rinsed with NaBr solution, the length scale of the repulsive interaction increases modestly, and only a very tiny adhesion remains. These pronounced changes in the force behavior are consistent with CTAB-induced DNA compaction accompanied by increased DNA adsorption, both of which are partially irreversible.     

Effects of degassing on the long-range attractive force between hydrophobic surfaces in water

The long-ranged attractions between hydrophobic amorphous fluoropolymer surfaces are measured in water with and without dissolved air. An atomic force microscope is used to obtain more than 500 measured jump-in distances, which yields statistically reliable results. It is found that the range of the attraction and its variability is generally significantly decreased in deaerated water as compared to normal, aerated water. However, the range and strength of the attraction in deaerated water remain significantly greater than the van der Waals attraction for this system. The experimental observations are consistent with (1) nanobubbles being primarily responsible for the long-ranged attraction in normal water, (2) nanobubbles not being present in deaerated water when the surfaces are not in contact, and (3) the attraction in the absence of nanobubbles being most probably due to the approach to the separation-induced spinodal cavitation of the type identified by Bérard et al. [J. Chem. Phys. 1993, 98, 7236]. It is argued that the measurements in deaerated water reveal the bare or pristine hydrophobic attraction unobscured by nanobubbles.     

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.     

Interactions between solid surfaces with adsorbed polyelectrolytes of opposite charge

Adsorption of polyelectrolytes to surfaces of opposite charge typically leads to charge neutralization and subsequent charge reversal. As can be shown by direct force measurements and stability studies, the interaction forces are dominated by repulsive forces originating from diffuse layer overlap and attractive van der Waals forces, in line with the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). Recently, the existence of an additional attractive non-DLVO force was demonstrated, and its likely origin is the attraction between patch-charge heterogeneities. With novel single molecule pulling experiments with the atomic force microscope (AFM) polymer bridging forces could be shown to represent the most important contribution to the adhesion of surfaces coated by polyelectrolytes.     

Non-DLVO silica interaction forces in NMP-water mixtures. I. A symmetric system

Despite the success of DLVO theory, there exist numerous examples of interactions that do not follow its predictions. One prominent example is the interaction between hydrophilic surfaces in mixtures of water with another polar, associating solvent. Interactions of such surfaces are still poorly understood yet play a key role in a wide variety of processes in nature, biology, and industry. The interaction forces between a silica sphere and a glass plate in N-methyl-2-pyrrolidone (NMP)-water binary mixtures were measured using the AFM technique. The interactions in pure NMP and pure water agreed qualitatively with DLVO theory. In contrast, the addition of NMP to water drastically altered the interactions, which no longer followed DLVO predictions. An unusually strong, long-range (50-80 nm), multistepped attractive force was observed on the approach of hydrophilic surfaces in the NMP concentration range of 30-50 vol %, where the adhesive pull-off force was also maximized. The maximum attractive force was observed at an NMP concentration near 30 vol %, consistent with the formation of a strong hydrogen-bonded complex between NMP and water near the solid surface. The analysis of force profiles, zeta potentials, solution viscosity, and contact angles suggests that attraction arises from the bridging of surface-adsorbed macrocluster layers known to form on hydrophilic surfaces in mixtures of associating liquids.     

Excess thermodynamic properties of thin water films confined between hydrophobized gold surfaces

Surface forces between gold surfaces were measured in pure water at temperatures in the range of 10-40 °C using an atomic force microscope (AFM). The surfaces were hydrophobized by self-assembly of alkanethiols (C(n)SH) with n=2 and 16 in ethanol solutions. The data were used to determine the changes in excess free energies (ΔG(f)) of the thin water films per unit area by using the Derjaguin approximation [1]. The free energy data were then used to determine the changes in excess film entropy (ΔS(f)) and the excess film enthalpy (ΔH(f)) per unit area. The results show that both ΔS(f) and ΔH(f) decrease with decreasing film thickness, suggesting that the macroscopic hydrophobic interaction involves building some kind of structures in the intervening thin films of water. It was found that |ΔH(f)|>|TΔS(f)|, which is a necessary condition for an attractive force to appear when the enthalpy and entropy changes are both negative. That macroscopic hydrophobic interaction is enthalpically driven is contrary to the hydrophobic interactions at molecular scale. The results obtained in the present work are used to discuss possible origins for the long-range attractions observed between hydrophobic surfaces.     

Strong attraction among the fully hydrophilic {Mo72Fe30} macroanions

We report the study on the unique driving forces of the self-assembly of fully hydrophilic, soluble {Mo72Fe30} macroanions into single-layer, vesicle-like "blackberry" structures in water and mixed solvents. The hydrophobic interaction that is responsible for the vesicle formation of amphiphilic surfactants does not contribute to the current blackberry formation because of the absence of hydrophobic moiety. The hydrogen bond, van der Waals force, and chemical interaction only play minor roles. Laser light scattering and conductance measurements on a series of {Mo72Fe30}/ethanol/H2O solutions show that a certain amount of negative charges are necessary for the self-assembly, clearly indicating the existence of long-range attraction between macroanions, presumably due to the small counterions in between. The experimental results suggest that the charges on macroanions play a dual effect: short-range electrostatic repulsion and long-range "like-charge attraction", which is the major source of attractive force between hydrophilic macroanions, while van der Waals force, hydrogen bonds, and temporary inter-{Mo72Fe30} Fe-O-Fe chemical linking may also have minor contributions.     

Adsorption of Polyelectrolyte-Nanoparticle Systems on Silica: Influence on Interaction Forces

In this work, we have studied the interfacial properties of cationic polyelectrolyte (PE) and silica nanoparticle (NP) systems at macroscopic silica surfaces by means of ellipsometry. The influence of adsorbed layers on the interactions between silica surfaces was also investigated using the bimorph surface force apparatus. Added nanoparticles were observed to strongly swell the interfacial polyelectrolyte layers, an effect partly related to neutralization of charged polyelectrolyte groups. The effect was more pronounced for low charged than for highly charged polyelectrolytes. Overall, the presence of nanoparticles seemed to increase the repulsive interaction measured between silica surfaces. The force measured on approach was long range and quite strongly repulsive. On separation, an attractive bridging interaction was measured for polyelectrolyte-covered surfaces. For the low charged polyelectrolyte used in the study, the force turned repulsive on addition of nanoparticles. For the highly charged polyelectrolyte used, a change from a very strong attraction (involving a jump of the surfaces out of contact) to a very long-range elastic attractive force was observed on adding nanoparticles. The long-range elastic force indicates that polymer chains and nanoparticles form a transient network in the gap between the surfaces. The observed difference in the outward force curves may explain why the addition of nanoparticles appears to improve, e.g., shear-resistance and reflocculation characteristics of polymeric flocculants. Copyright 2000 Academic Press.     

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.     

Hydrophobic forces and hydrogen bonds in the adhesion between retinoid-coated surfaces

Interactions between hydrophobic chains of lipid monolayers and interactions between hydrophilic headgroups of lipid bilayers (with or without a molecular recognition step) are now well documented, especially for commonly used lipids. Here, we report force measurements between a new class of fluorinated lipid layers whose headgroups (synthetic ligands of retinoid receptors) display a very unusual polar/apolar character and can interact via a combination of hydrophobic forces and hydrogen bonds. Although these two interactions produce adhesion and are therefore not easily distinguishable, we show that it is possible to extract both contributions unambiguously. Experiments are performed both in pure water, where the adhesion is a combination of hydrophobic forces and hydrogen bonds, and in Tris buffer, where the hydrophobic effect is the dominant short-range attractive force. The contribution of hydrophobic forces scaled down to molecular interactions is deduced from force versus distance profiles, and the same value is found independently in pure water and Tris buffer, about 1 kBT. We also show that retinoid lipid layers attract each other through a very long-range (100 nm) exponential force, which is insensitive to the pH and the salinity. The origin of this long-range attraction is discussed on the basis of previously proposed mechanisms.     

Physical properties of nanobubbles on hydrophobic surfaces in water and aqueous solutions

In recent years there has been an accumulation of evidence for the existence of nanobubbles on hydrophobic surfaces in water, despite predictions that such small bubbles should rapidly dissolve because of the high internal pressure associated with the interfacial curvature and the resulting increase in gas solubility. Nanobubbles are of interest among surface scientists because of their potential importance in the long-range hydrophobic attraction, microfluidics, and adsorption at hydrophobic surfaces. Here we employ recently developed techniques designed to induce nanobubbles, coupled with high-resolution tapping-mode atomic force microscopy (TM-AFM) to measure some of the physical properties of nanobubbles in a reliable and repeatable manner. We have reproduced the earlier findings reported by Hu and co-workers. We have also studied the effect of a wide range of solutes on the stability and morphology of these deliberately formed nanobubbles, including monovalent and multivalent salts, cationic, anionic, and nonionic surfactants, as well as solution pH. The measured physical properties of these nanobubbles are in broad agreement with those of macroscopic bubbles, with one notable exception: the contact angle. The nanobubble contact angle (measured through the denser aqueous phase) was found to be much larger than the macroscopic contact angle on the same substrate. The larger contact angle results in a larger radius of curvature and a commensurate decrease in the Laplace pressure. These findings provide further evidence that nanobubbles can be formed in water under some conditions. Once formed, these nanobubbles remain on hydrophobic surfaces for hours, and this apparent stability still remains a well-recognized mystery. The implications for sample preparation in surface science and in surface chemistry are discussed.     

Detachment of Particles from Surfaces: An AFM Study

We have measured interactions between hydrophilic and hydrophobic surfaces in an aqueous medium at various pH and ionic strengths as well as in some organic solvents using atomic force microscopy and analyzed them in terms of particle adhesion and detachment from surfaces. In hydrophilic systems the forces observed were found to be well described by DLVO theory at large separation distances. Very long range hydrophobic forces were not observed in hydrophilic-hydrophobic systems. Nevertheless, the jump into contact was found to occur at distances greater that those predicted by just van der Waals attraction. The interaction between two hydrophobic surfaces was dominated by the long-range attraction due to hydrophobic forces. This interaction was found to be sensitive to the type of substrate as well as to the pH and electrolyte concentration. Measured pull-off forces showed poor reproducibility. However, average values showed clear trends and were used to estimate interfacial energies or work of adhesion for all systems studied by means of the Derjaguin approximation. These values were compared to those calculated by the surface tension component theory using the acid-base approach. Good qualitative agreement was obtained, giving support for the usefulness of this approach in estimating interfacial energies between surfaces in liquid media. A comparison of the measured adhesion force with hydrodynamic detachment experiments showed good qualitative agreement. Copyright 2001 Academic Press.     

Force measurements on hydrophobized silica surfaces by using AFM

Force measurements between SiO₂ surfaces with and without adsorbed phenyl groups in aqueous media using the atomic force microscope (AFM) are compared. An oxidized silicon tip and an oxidized silicon wafer were hydrophobized with phenyl groups, and the long-range attraction induced by hydrophobation is shown in force vs. distance curves. The observed differences prove that the silanol groups of the unmodified SiO₂ surface are replaced by the phenyl groups. Received: 4 May 1998 / Revised: 1 July 1998 / Accepted: 5 July 1998     

Force measurements on hydrophobized silica surfaces by using AFM

Force measurements between SiO₂ surfaces with and without adsorbed phenyl groups in aqueous media using the atomic force microscope (AFM) are compared. An oxidized silicon tip and an oxidized silicon wafer were hydrophobized with phenyl groups, and the long-range attraction induced by hydrophobation is shown in force vs. distance curves. The observed differences prove that the silanol groups of the unmodified SiO₂ surface are replaced by the phenyl groups. Received: 4 May 1998 / Revised: 1 July 1998 / Accepted: 5 July 1998