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.     

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 repulsion between membranes and polar surfaces: Simulation approaches versus continuum theories

A review of various computer simulation approaches for the study of the hydration repulsion between lipid membranes and polar surfaces is presented. We discuss different methods and compare their advantages and limitations. We consider interaction pressures, interaction thermodynamics, and interaction mechanisms. We take a close look at the influence of the experimental boundary conditions and at repulsion mechanisms due to the unfavorable overlap of interfacial water layers. To this end, we analyze several distinct water order parameters in simulations of interacting polar surfaces and compare the results to the predictions of simple continuum theories.     

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).     

Nanoscale repulsive forces between mica and silica surfaces in aqueous solutions

Nanoscale repulsive forces between mineral surfaces in aqueous solutions were measured for the asymmetric mica-silica system. The force measured with an atomic force microscope (AFM) has universal character in the short range, less than ～1 nm or about 3-4 water molecules, independent of solution conditions, that is, electrolyte ion (Na, Ca, Al), concentration (10(-6)-10(-2)M), and pH (3.9-8.2). Notably, the force is essentially the same as for the glass-silica system. Single force curves for a mica-silica system in a 10(-4)M aqueous NaCl solution at pH ～ 5.1 show oscillations with a period of about 0.25 nm, roughly the diameter of a water molecule, a consequence of a layer-by-layer dehydration of the surfaces when pushed together. This result provides additional support to the idea that nanoscale repulsive forces between mineral surfaces in aqueous solutions arise from a surface-induced water effect; the water between two mineral plates that are pushed together becomes structured and increasingly anchored to the surface of the plates by the creation of a hydrogen-bonding network that prevents dehydration of the surfaces.     

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.     

Hydration forces between surfactant bilayers: An equilibrium binding description

Water vapour pressure measurements at several temperatures are reported for non-ionic surfactant/water liquid crystals. The results are fitted to a simple model involving an equilibrium between head-group-bound and free water. This model, which uses parameters previously derived from NMR data, allows “hydration” forces to be estimated without invoking layers of “structured” water at an interface.     

Hydration force between model hydrophilic surfaces: computer simulations

We performed molecular dynamics simulations to study the interactions between model hydrophilic plates made of carbon atoms distributed on a hexagonal lattice. Although neutral, the plates carry equal amounts of positive and negative charges to represent physical dipoles. Using the thermodynamic perturbation theory we calculated the potential of mean force (PMF) acting between the plates as a function of the distance between these plates. We observed that, at distances when more than one water layer can be found between the plates, the contribution of water into the PMF can be either attractive or repulsive depending on the correlation between the charges situated on the plates.     

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.     

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.     

Wetting on regularly structured surfaces from "core-shell" particles: theoretical predictions and experimental findings

In this paper, we report on a systematic and thorough study of wetting phenomenon on regularly patterned surfaces fabricated from inorganic-organic hybrid "core-shell" particles of different radii (100 nm to 10 microm). Inorganic silica particles were modified through chemical anchoring of polymers and silanes with different hydrophobicities. Modified "core-shell" particles were assembled into regular hexagonally packed structures. The use of regular structured surfaces with specifically designed surface roughness allowed mathematic prediction of the wetting behavior according to existing models and its comparison with experimental observations. It was shown that the character of the wetting behavior varies with the particles size and the chemical nature of the surface immobilized substance. For the regular particle assemblies, an increase in the vertical roughness was achieved with increasing particle radius, but without changing the Wenzel roughness factor.     

Solvation forces between molecularly rough surfaces

Surface heterogeneity affects significantly wetting and adhesion properties. However, most of the theories and simulation methods of calculating solid-fluid interactions assume a standard thermodynamic model of the Gibbs' dividing solid-fluid interface, which is molecularly smooth. This assumption gives rise to a layering of the fluid phase near the surface that is displayed in oscillating density profiles in any theories and simulation models, which account for the hard core intermolecular repulsion. This layering brings about oscillations of the solvation (or disjoining) pressure as a function of the gap distance, which are rarely observed in experiments, except for ideal monocrystal surfaces. We present a detailed study of the effects of surface roughness on the solvation pressure of Lennard-Jones (LJ) fluids confined by LJ walls based on the quenched solid density functional theory (QSDFT). In QSDFT, the surface roughness is quantified by the roughness parameter, which represents the thickness of the surface "corona" - the region of varying solid density. We show that the surface roughness of the amplitude comparable with the fluid molecular diameter effectively damps the oscillations of solvation pressure that would be observed for molecularly smooth surfaces. The calculations were done for the LJ model of nitrogen sorption at 74.4 K in slit-shaped carbon nanopores to provide an opportunity of comparing with standard adsorption experiments. In addition to a better understanding of the fundamentals of fluid adsorption on heterogeneous surfaces and inter-particle interactions, an important practical outcome is envisioned in modeling of adsorption-induced deformation of compliant porous substrates.     

Adhesion forces between wetted solid surfaces

We investigate in this paper the influence of wetting films on the adhesion forces between macroscopic solid surfaces connected by a liquid bridge. We show that the capillary forces are dependent on the interactions governing the wetting layers, and that those interactions may be determined from the measurement of the capillary force in the presence of a condensable vapor. We illustrate those results with a surface force apparatus experiment where the capillary force between high-energy surfaces is measured for different liquid pressures.     

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.     

Friction and forces between cellulose model surfaces: a comparison

Four different cellulose model surfaces, and one silica surface, have been studied by means of atomic force microscopy (AFM). The normal interactions have been found to consist of a longer range double layer force with a short range steric interaction, the nature of which is extensively discussed. Both the surface charge and range of the steric force depend on the type of cellulose substrate used, as does the magnitude of the adhesion. Studies of friction reveal that surface roughness is the determining factor for the friction coefficient, with which it increases monotonically. The absolute value, however, is determined by the surface chemistry. All studied cellulose surfaces show similar behavior in response to xyloglucan addition.     

Interaction between water and defective silica surfaces

We use the density functional theory method to study dry (1 × 1) α-quartz (0001) surfaces that have Frenkel-like defects such as oxygen vacancy and oxygen displacement. These defects have distinctively different effects on the water-silica interface depending on whether the adsorbent is a single water molecule, a cluster, or a thin film. The adsorption energies, bonding energies, and charge transfer or redistributions are analyzed, from which we find that the existence of a defect enhances the water molecule and cluster surface interaction by a large amount, but has little or even negative effect on water thin film-silica surface interaction. The origin of the weakening in film-surface systems is the collective hydrogen bonding that compromises the water-surface interaction in the process of optimizing the total energy. For clusters on surfaces, the lowest total energy states lower both the bonding energy and the adsorption energy.     

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.     

Network development during epoxy curing: experimental ultrasonic data and theoretical predictions

Ultrasonic waves propagation acts as a dynamic mechanical deformation on a material. When, during ultrasonic wave propagation, chemical or physical changes occurs, the evolution of an elastic modulus can be monitored. Therefore, this technique can be considered a powerful tool for non-destructive cure monitoring with a potential for “in situ” applications. In this work, the isothermal cure of a model epoxy resin cured with an amine is studied using propagation of longitudinal ultrasonic wave. The epoxy to amine ratio is optimized in order to reach full conversion of the amine groups during curing. The relative changes in the ultrasonic velocity and attenuation, measured by the transmission technique, have been applied to the calculation of the longitudinal modulus. The ultrasonic modulus has been compared with the degree of reaction measured using Differential Scanning Calorimetry (DSC). Furthermore a correlation between the ultrasonic modulus and the crosslinking density is presented combining DSC data with the stoichiometry of reactants according with the statistical theory of Miller and Macosko. The plot of the ultrasonic modulus as a function of the crosslinking density suggested that the theory of rubber elasticity can not be applied to the ultrasonic bulk longitudinal modulus as a consequence of the small deformation involved in the propagation of the ultrasonic waves.     

Capillary forces between surfaces with nanoscale roughness

The flow and adhesion behavior of fine powders (approx. less than 10 microm) is significantly affected by the magnitude of attractive interparticle forces. Hence, the relative humidity and magnitude of capillary forces are critical parameters in the processing of these materials. In this investigation, approximate theoretical formulae are developed to predict the magnitude and onset of capillary adhesion between a smooth adhering particle and a surface with roughness on the nanometer scale. Experimental adhesion values between a variety of surfaces are measured via atomic force microscopy and are found to validate theoretical predictions.