Hydration forces between mica surfaces in electrolyte solutions

The forces between two molecularly smooth mica surfaces were measured over a range of concentrations in aqueous Li⁺, Na⁺, K⁺ and Cs⁺ chloride solutions. Deviations from DLVO forces in the form of additional short-range repulsive “Hydration” forces were observed only above some critical bulk concentration, which was different for each electrolyte. These observations are interpreted in terms of the corresponding ion exchange properties at the mica surface. “hydration” forces apparently arise when hydrated cations adsorbed on mica are prevented from desorbing as two interacting surfaces approach. dehydration of the cations leads to a repulsive hydration force. A simple site-binding model was successfully applied to describe the charging behavior of interacting mica surfaces . By subtraction of the DLVO-regulation theory from the total measured force the net hydration force was obtained for mica surfaces apparently fully covered with adsorbed cations. The magnitude of this extra force followed the series Na⁺ > Li⁺ > K⁺ > Cs⁺ and, in each case, could be described by a double-exponential decay.     

Short-range forces between glass surfaces in aqueous solutions

We found that the force between glass surfaces measured with an atomic force microscope (AFM) has universal character in the short range, less than approximately 1 nm or about 3-4 water molecules, independent of solution conditions, that is, electrolyte ion size, charge and concentration and pH. Our results suggest that the excess DLVO force, obtained by subtracting the DLVO theory with a charge regulation model from the AFM force data, essentially does not change with the electrolytes Na, Ca, and Al, in the range of concentration from 10(-6) to 10(-2) M and the range of pH from 3.1 to 7.9. Single force curves for a glass-silica system in a 10-4 M aqueous NaCl solution at pH approximately 5.1 show oscillations with a period of about 0.25 nm, roughly the diameter of a water molecule. We postulate that the excess force between glass surfaces arises from a surface-induced solvent effect, from the creation of a hydrogen-bonding network at the surface level, rather than from a solvent-induced surface steric hindrance.     

pH dependence of friction forces between silica surfaces in solutions

The pH dependence of the friction between a silica particle and a silica wafer was investigated using lateral force microscopy. Measurements were done in the range of 3.6 < or = pH < or = 10.6 and the effect of high loading force was also examined. It is found that the friction is independent of the pH of solutions and increases linearly with the applied load, when the pH is between 3.6 and 8.6. On the other hand, once the pH is above 9.0, the friction becomes extremely small and the dependence on the applied load becomes nonlinear. It is postulated that this transition is due to the development of a gel layer composed of polymer-like segments of silicilic acid anchored on the surface; at the lower applied load, this layer acts as a boundary lubricant between the surfaces, but, at the higher applied load, the entanglements of these segments and more direct contact between two solid surfaces leads to the increase of the friction. The effects found here are expected to play an important role in elucidating the basic mechanism of the planarization process of silica wafers.     

Interaction forces between silica surfaces in aqueous solutions of cationic polymeric flocculants: effect of polymer charge

Three cationic polymers with molecular weights and charge density of 3.0 x 10(5) g/mol and 10% (D 6010), 1.1 x 10(5) g/mol and 40% (D6040), and 1.2 x 10(5) g/mol and 100% (D6099) were investigated in aqueous NaCl solutions in the presence of silica. The atomic force microscope (AFM) colloidal probe technique was used to determine silica interparticle interaction forces, which were compared to macroscopic information on the strength of interactions such as compressive yield stress measurements. It was found that in 30 mM NaCl solution the 10% charged polymer produced steric repulsion upon approach and long-range adhesion with multiple pull off events upon retraction at the optimum flocculation concentration. This suggests that the polymer was adsorbed in a conformation where segments extend from the surface, resulting in bridging flocculation. The 40 and 100% charged polymers produced attraction upon approach and strong adhesion with snap out from contact upon separation at optimum polymer dosages. This suggests that these polymers are adsorbed with flat conformations and is typical of charge neutralization or patch attraction. The attractions for 40 and 100% charged polymers measured with the AFM are significantly larger than for the 10% charged polymer. The polymer dose that produced the optimum flocculation and the maximum compressive yield stress typically corresponded to the polymer concentration that produced the maximum adhesion for each polymer. It was found that the magnitude of the adhesive force was more significant in determining the compressive yield stresses of the silica particle sediments than the aggregate size and structure.     

Colloidal forces between bitumen surfaces in aqueous solutions measured with atomic force microscope

Colloidal forces between bitumen surfaces in aqueous solutions were measured with an atomic force microscope (AFM). The results showed a significant impact of solution pH, salinity, calcium and montmorillonite clay addition on both long-range (non-contact) and adhesion (pull-off) forces. Weaker long-range repulsive forces were observed under conditions of lower solution pH, higher salinity and higher calcium concentration. Lower solution pH, salinity and calcium concentration resulted in a stronger adhesion forces. The addition of montmorillonite clays increased long-range repulsive forces and decreased adhesion forces, particularly when co-added with calcium ions. The measured force profiles were fitted with extended DLVO theory to show the repulsive electrostatic double layer and attractive hydrophobic forces being the dominant components in the long-range forces between the bitumen surfaces. At a very short separation distance (less than 4–6 nm), a strong repulsion of steric origin was observed. The findings provide a fundamental understanding of bitumen emulsion stability and a mechanism of bitumen “aeration” in bitumen recovery processes from oil sands.     

AFM measurements of forces between silica surfaces

Interaction forces and adhesion between a silica sphere and a flat silica surface in aqueous electrolyte solutions were investigated by atomic force microscopy. The forces were measured as a function of surface separation, pH and NaCl concentration as the surfaces were approaching each other. The adhesion force was determined upon retraction with respect to pH, NaCl concentration and contact time. The magnitude of the long range repulsive force was decreasing with decreasing pH. A short range repulsive force was observed at pH = 2, but no long range repulsive forces were observed at this pH. Force measurements showed that adhesion of silica surfaces in water was obstructed by short and long range repulsive forces. Adhesion was enhanced when both the long and the short range repulsive force was mitigated. A maximum adhesion force of 7.8 mN/m was measured at pH = 12.5 when the short range force vanished and the long range repulsive force was reduced by increasing the NaCl concentration. At pH = 12.5, the work of adhesion was calculated to be 1.2 mJ/m² according to the Derjaguin–Muller–Toporov (DMT) model. Adhesion energy was much less at pH = 2 (0.3 mJ/m²) due to persistive short range repulsion.     

Hydration force between mica surfaces in aqueous KCl electrolyte solution

Liquid-vapor molecular dynamics simulations are performed to study the interaction forces between two mica surfaces in an aqueous KCl electrolyte solution. Strong repulsive hydration force is obtained within a distance of ~2 nm between the two mica surfaces, which cannot be explained by the continuum theory of double-layer repulsion. We find that this short-range repulsive hydration force is much stronger than the double-layer force between mica surfaces. Whereas the simulation system is much smaller than the surface force measurement system, fundamental mechanisms of repulsive hydration force are revealed. In particular, important features of the step-like force oscillatory behavior during normal compression and force hysteresis during retraction are observed. Detailed analysis of the ionic density distributions shows that the "forced adsorption" of diffusive K(+) ions onto mica surfaces during compression and the subsequent "slow desorption" of the absorbed K(+) ions from mica surfaces upon retraction are responsible for the hysteresis phenomenon. From a mechanics point of view, we attribute the load bearing capacity of the dense electrolyte to the very hard hydration shells of K(+) metal ions under confinement. We find that the hydrated K(+) ions and Cl(-) co-ions remain very diffusive in the aqueous film. Water molecules in the hydration layer are also very fluidic, in the sense that the diffusion constant of water molecules is less than its bulk value by at most 3 orders of magnitude under the extreme confinement.     

Colloidal interactions between asphaltene surfaces in aqueous solutions

Asphaltene at oil/water interfaces plays a dominant role in the recovery of crude oil. In this study, asphaltene monolayer films were deposited on hydrophobic silicon wafers and silica spheres from oil-water interfaces using a Langmuir interfacial trough. The morphology of the deposited asphaltene films was characterized with an atomic force microscope (AFM). The colloidal forces between the prepared asphaltene films in aqueous solutions were measured with AFM to shed light on the stabilization of water or oil droplets coated with asphaltene films. Factors such as solution pH, KCl concentration, calcium addition, and temperature all showed a strong impact on colloidal forces between the prepared asphaltene films. The findings provided a better understanding of asphaltene interfacial films at an oil/water interface in stabilizing bitumen-in-water and water-in-bitumen emulsions.     

Dynamic response of AFM cantilevers to dissimilar functionalized silica surfaces in aqueous electrolyte solutions

The dynamic response of an oscillating microcantilever with a gold-coated tip interacting with dissimilar functionalized silica surfaces was studied in electrolyte solutions with pH ranging from 4 to 9. Silica surfaces were chemically modified, yielding dissimilar surfaces with -Br, -NH(2), and -CH(3) functional group terminations. The relative hydrophobicity of the surfaces was characterized by contact angle measurements. The surface charge of the functionalized surfaces was first probed with commonly used static AFM measurements and serves as a reference to the dynamic response data. The amplitude and phase of the cantilever oscillation were monitored and used to calculate the effective interaction stiffness and damping coefficient, which relate to the electrical double layer interactions and also to distance-dependent hydrodynamic damping at the solid/water interface. The data for the dynamic response of the AFM over silica surfaces as a function of chemical functionalization and electrolyte pH show that the effective stiffness has a distinctive dependence on the surface charge of functionalized silica surfaces. The hydrodynamic damping also correlates strongly with the relative hydrophobicity of the surface. The data reported here indicate that interfacial properties can be strongly affected by changing the chemical composition of surfaces.     

Hydration forces between silica surfaces: experimental data and predictions from different theories

Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.     

Polarization model for poorly-organized interfacial water: hydration forces between silica surfaces

The main goal of this paper is to review the theoretical models which can be used to describe the interactions between silica surfaces and to show that a model proposed earlier by the authors (the polarization model), which accounts concomitantly for double layer and hydration forces, can be adapted to explain recent experiments in this direction. When the water molecules near the interface were considered to have an ice-like structure, a strong coupling between the double layer and hydration forces (described by the correlation length between neighboring dipoles, lambda(m)) generates long range interactions, larger than the experimentally determined interactions between silica surfaces. Arguments are brought that a gel layer is likely to be formed on the surface of silica, which, by generating disorder in the interfacial water layers, can decrease strongly the value of lambda(m). Since the prediction of lambda(m) involves a choice for the microscopic structure of water, which is often unknown, the polarization model is also presented here as a phenomenological theory, in which lambda(m) is used as a fitting parameter. Two extreme cases are considered. In one of them, the water molecules near the interface are considered to have an ice-like structure, whereas in the other they are considered randomly distributed. In the first case, the dipole correlation length lambda(m)=14.9 Angstrom. In the second limiting case, lambda(m) can be of the order of 1 Angstrom. It is shown that, for lambda(m)=4 Angstrom, a more than qualitative agreement with the experiment could be obtained, for reasonable values of the parameters involved (e.g. surface dipole strength and density, dipole location, surface charge).     

Chemical end-grafting of homogeneous polystyrene monolayers on mica and silica surfaces

Homogeneous polystyrene monolayers covalently end-attached on mica and silica surfaces were obtained using a "graft to" methodology. The grafting was achieved via nucleophilic substitution between silanol groups (Si-OH) containing surface and monochlorosilyl terminated polystyrene (PS). Different parameters, such as surface activation, grafting reaction time, polymer concentration, nature of solvent, and presence of catalyst, were investigated to determine the optimal conditions for creating very homogeneous and stable polymer monolayers. Ellipsometry, atomic force microscopy (AFM), surface forces apparatus (SFA), and contact angle measurements were used to characterize the polymer-grafted layers. An efficient plasma activation procedure was established to create a maximum number of silanol groups on mica surfaces without increasing the surface roughness. Surface reactivity was investigated by grafting trimethylchlorosilane (TMS) on OH-activated mica and silica. The maximum TMS surface coverage on activated mica is similar to that observed for silica. The stability of covalently attached TMS and PS layers in toluene and water were investigated. Both grafted layers (TMS and PS) partially detached from the mica and silica surfaces when immersed in water. Hydrolysis of the siloxane bond between the monochlorosilyl groups and the surface is the most probable cause of layer degrafting. The degrafting was much slower with the long PS polymer chains, compared to the small TMS molecules, which may act as a protective layer against hydrolysis.     

Existence of hydration forces in the interaction between apoferritin molecules adsorbed on silica surfaces

The atomic force microscope, together with the colloid probe technique, has become a very useful instrument to measure interaction forces between two surfaces. Its potential has been exploited in this work to study the interaction between protein (apoferritin) layers adsorbed on silica surfaces and to analyze the effect of the medium conditions (pH, salt concentration, salt type) on such interactions. It has been observed that the interaction at low salt concentrations is dominated by electrical double layer (at large distances) and steric forces (at short distances), the latter being due to compression of the protein layers. The DLVO theory fits these experimental data quite well. However, a non-DLVO repulsive interaction, prior to contact of the protein layers, is observed at high salt concentration above the isoelectric point of the protein. This behavior could be explained if the presence of hydration forces in the system is assumed. The inclusion of a hydration term in the DLVO theory (extended DLVO theory) gives rise to a better agreement between the theoretical fits and the experimental results. These results seem to suggest that the hydration forces play a very important role in the stability of the proteins in the physiological media.     

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.     

Freezing of fluids confined between mica surfaces

Using grand ensemble simulations, we show that octamethyl-cyclo-tetra-siloxane (OMCTS) confined between two mica surfaces can form a variety of frozen phases which undergo solid-solid transitions as a function of the separation between the surfaces. For atomically smooth mica surfaces, the following sequence of transitions 1[triangle up] --> 1[triangle up]b --> 2B --> 2 square --> 2[triangle up] are observed in the one- and two-layered regimes, where n[triangle up], n[square], and nB denote triangular, square, and buckled phases, respectively, with the prefix n denoting the number of confined layers. The presence of potassium on mica is seen to have a strong influence on the degree of order induced in the fluid. The sequence of solid-solid transitions that occurs with the smooth mica surface is no longer observed. When equilibrated with a state point near the liquid-solid transition, a counterintuitive freezing scenario is observed in the presence of potassium. Potassium disrupts in-plane ordering in the fluid in contact with the mica surface, and freezing is observed only in the inner confined layers. The largest mica separations at which frozen phases were observed ranged from separations that could accommodate six to seven fluid layers. The extent of freezing and the square-to-triangular lattice transition was found to be sensitive to the presence of potassium as well as the thermodynamic conditions of the bulk fluid. The implications of our results on interpretation of surface force experiments as well as the generic phase behavior of confined soft spheres is discussed.     

Breakdown of hydration repulsion between charged surfaces in aqueous Cs+ solutions

Using a surface force balance, we have measured the normal and shear forces between mica surfaces across aqueous caesium salt solutions (CsNO(3) and CsCl) up to 100 mM concentrations. In contrast to all other alkali metal ions at these concentrations, we find no evidence of hydration repulsion between the mica surfaces on close approach: the surfaces appear to be largely neutralized by condensation of the Cs ions onto the charged lattice sites, and are attracted on approach into adhesive contact. The contact separation at adhesion indicates that the condensed Cs ions protrude by 0.3 +/- 0.2 nm from each surface, an observation supported both by the relatively weak adhesion energies between the surfaces, and the relatively weak frictional yield stress when they are made to slide past each other. These observations show directly that the hydration shells about the Cs(+) ions are removed as the ions condense into the charged surface lattice. This effect is attributed to the low energies-resulting from their large ionic radius-required for dehydration of these ions.     

Adhesion of single polyelectrolyte molecules on silica, mica, and bitumen surfaces

In a recent study (Energy Fuels 2005, 19, 936), a partially hydrolyzed polyacrylamide (HPAM) was used as a process aid to recover bitumen from oil sand ores. It was found that HPAM addition at the bitumen extraction step not only improved bitumen recovery but also enhanced fine solids settling in the tailings stream. To understand the role of HPAM, single-molecule force spectroscopy was employed for the first time to measure the desorption/adhesion forces of single HPAM molecules on silica, mica, and bitumen surfaces using an atomic force microscope (AFM). Silicon wafers with an oxidized surface layer and newly cleaved mica were used, respectively, to represent sand grains and clays in oil sands. The force measurements were carried out in deionized water and in commercial plant process water under equilibrium conditions. The desorption/adhesion forces of HPAM obtained on mica, silica, and bitumen surfaces were approximately 200, 40, and 80 pN in deionized water and approximately 100, 50, and 40 pN in the plant process water, respectively. The measured adhesion forces together with the zeta potential values of these surfaces indicate that the polymer would preferentially adsorb onto clay surfaces rather than onto bitumen surfaces. It is the selective adsorption of HPAM that benefits both bitumen recovery and tailings settling when the polymer was added directly to the bitumen extraction process at an appropriate dosage.     

Hydration structure of water confined between mica surfaces

We report further molecular dynamics simulations on the structure of bound hydration layers under extreme confinement between mica surfaces. We find that the liquid phase of water is maintained down to 2 monolayer (ML) thick, whereas the structure of the K(+) ion hydration shell is close to the bulk structure even under D = 0.92 nm confinement. Unexpectedly, the density of confined water remains approximately the bulk value or less, whereas the diffusion of water molecules decreases dramatically. Further increase in confinement leads to a transition to a bilayer ice, whose density is much less than that of ice Ih due to the formation of a specific hydrogen-bonding network.     

Electrostatic depletion forces between planar surfaces

The interaction between two dielectric plates immersed in an electrolyte solution is examined by using a variational perturbation approximation for the grand partition function. This approach differs from previous treatments in that the screening length between the plates is treated as a variational parameter. A key finding is that adjacent to each plate is a layer of ion depletion with thickness given by about one-half of a Bjerrum length. Consequently, for plate-plate separations less than the Bjerrum length, nearly all the electrolyte is excluded from between the plates, and the interaction is given by the sum of a van der Waals interaction and an attractive osmotic depletion force. In contrast to the predictions of previous theories, the interaction between the plates at short range increases with increasing electrolyte concentration and may provide an important contribution to the salt-induced attraction, commonly referred to as salting out. Because the range of the osmotic depletion force is roughly equal to the Bjerrum length, it increases with the square of the valency of the electrolyte. At larger plate-plate separations, the van der Waals interaction is screened as electrolyte enters the space between the plates, leading to an exponential decay of the interactions, as has been previously observed. However, this interaction is slightly stronger than that previously predicted, due to ion depletion from the surface of the interface, also this effect increases with increasing electrolyte concentration.