Control of adhesion and surface forces via potential-dependent adsorption of pyridine

The orientation and extent of adsorption of pyridine on a gold electrode is known to depend on applied potential and is well characterized. By use of the electrochemical surface forces apparatus, we measured the potential dependence of the double-layer interactions and adhesive forces between a gold electrode and a mica surface for different pyridine concentrations. We observed that, unlike mica-mica interactions, the gold-mica interactions were strongly affected by the presence of small concentrations of pyridine. We are able to reach high negative surface potentials (as determined by applying Derjaguin-Landau-Verway-Overbeek theory to our force measurements), which is similar to what is observed in the absence of pyridine. This demonstrates the electronic nature of the forces measured and shows that pyridine does not displace potential-determining ions on the surface. At positive potentials, where the interaction between gold and mica is attractive, pull-off measurements are a strong function of applied potential. The major effect of the presence of pyridine is on the observed shift in the potential of zero force (PZF), moving it to more negative potentials. This effect is caused by the strong dipole of the pyridine molecule. When the applied potential is cast as a deviation from the PZF, the effect of pyridine is to reduce adhesion between gold and mica. We modeled the potential-dependent adhesion of this system using an electrocapillary framework developed previously, and in doing so, we establish the relationship between the gold-liquid and gold-mica surface energies. In addition, we show that pyridine adsorption affects the capacitance of the gold-mica interface.     

The Electrochemical Surface Forces Apparatus: The Effect of Surface Roughness, Electrostatic Surface Potentials, and Anodic Oxide Growth on Interaction Forces, and Friction between Dissimilar Surfaces in Aqueous Solutions

We present a newly designed electrochemical surface forces apparatus (EC-SFA) that allows control and measurement of surface potentials and interfacial electrochemical reactions with simultaneous measurement of normal interaction forces (with nN resolution), friction forces (with μN resolution), and distances (with ? resolution) between apposing surfaces. We describe three applications of the developed EC-SFA and discuss the wide-range of potential other applications. In particular, we describe measurements of (1) force-distance profiles between smooth and rough gold surfaces and apposing self-assembled monolayer-covered smooth mica surfaces; (2) the effective changing thickness of anodically growing oxide layers with ?-accuracy on rough and smooth surfaces; and (3) friction forces evolving at a metal-ceramic contact, all as a function of the applied electrochemical potential. Interaction forces between atomically smooth surfaces are well-described using DLVO theory and the Hogg-Healy-Fuerstenau approximation for electric double layer interactions between dissimilar surfaces, which unintuitively predicts the possibility of attractive double layer forces between dissimilar surfaces whose surface potentials have similar sign, and repulsive forces between surfaces whose surface potentials have opposite sign. Surface roughness of the gold electrodes leads to an additional exponentially repulsive force in the force-distance profiles that is qualitatively well described by an extended DLVO model that includes repulsive hydration and steric forces. Comparing the measured thickness of the anodic gold oxide layer and the charge consumed for generating this layer allowed the identification of its chemical structure as a hydrated Au(OH)(3) phase formed at the gold surface at high positive potentials. The EC-SFA allows, for the first time, one to look at complex long-term transient effects of dynamic processes (e.g., relaxation times), which are also reflected in friction forces while tuning electrochemical surface potentials.     

The electrical double layer on gold probed by electrokinetic and surface force measurements

Gold surfaces, obtained by vacuum deposition of 15-nm gold films on glass and silica wafers, were studied in aqueous solutions by streaming potential measurements and colloidal-probe AFM force measurements. In the force measurements both a bare and a gold-coated silica particle (6 microm in diameter) have been used as colloidal probes. From the streaming potential measurements we determined the zeta-potential of the gold surface, while from the force measurements the diffuse double-layer potential psi(d) was obtained by fitting the data to the DLVO theory or to the nonlinear Poisson-Boltzmann equation. Measured interactions were found to be entirely due to overlap of electric double layers with no indication of attractive Van der Waals forces. Results of both types of measurements are in good agreement. The double layer potential strongly depends on the pH, probably as a result of the presence of oxide species on the gold surface. Insight in the double layer potential of polarizable interfaces such as the gold/electrolyte solution interface is the first step for understanding the effect of externally applied potentials on the adsorption behavior of charged species.     

Forces controlling protein interactions: theory and experiment

This work reviews both the theory and experimental measurements of the fundamental forces that control protein solution behavior. In addition to the Derjaguin–Landau–Verwey–Overbeek (DLVO) forces, we also discuss the relative importance of hydrodynamic, solvation, and lock-and-key interactions in controlling protein solution behavior. The more common computational methods used to calculate both electrostatic and van der Waals potentials are described. Particular attention is given to the differences between proteins and ideal colloidal particles, and the computational methods used to address those differences. In addition to theoretical investigations of protein interactions, the results of recent direct measurements of the forces governing protein interactions are reviewed. These experimental results provide not only measurements against which the theories can be tested, but also demonstrate directly the relative importance of both DLVO and non-classical DLVO forces in the control of protein behavior.     

Forces between surfaces in liquids

The results and implications of direct force measurements between molecularly smooth mica surfaces in liquids are reviewed. These discussions include four interactions fundamental to colloid science: van der Waals forces, double layer forces, adhesion forces and structural or solvation forces (e.g. hydration forces). Also considered are the effects of preferential surface adsorption of solute molecules on these interactions, e.g. surfactant adsorptions from aqueous solutions and water condensation from non-aqueous solvents.In aqueous media it is apparent that the DLVO theory is valid at all surface separations down to the “force barrier”, but that under certain conditions hydration forces can become significant at distances below 30 Å.The measured adhesion force between two solid surfaces can be simply related to their surface energies and where meniscus forces are also present due to “capillary condensation” from vapor solvent, their effect on adhesion can be understood in terms of straightforward bulk thermodynamic principles. Here, too, it is concluded that structural forces cannot be ignored.Our results suggest that structural forces may either very monotonically with distance or be oscillatory with a periodicity equal to the molecular size. Their origin, nature, mode of action and importance for particle interactions will no doubt take many years to sort out.     

Direct measurements of long-range surface forces in gas and liquid media

A modified set-up was applied to carry out direct measurements of the forces of molecular attraction of gold spheres and crossed quartz filaments in air within the region of distances from 10 to 100 nm. Some quantitative deviations from Lifshitz's theory for gold may be attributed to an insufficient reliability of the spectral data used in the calculations. The DLVO theory adequately describes the interaction of glass threads in KCl (10^?3 ÷ 10^?5 N) solutions within the region of 5 to 100 nm. At a distance smaller than 5 nm, the deviations from DLVO theory are attributable to the influence of structural forces.When the contact between crossed hydrophobized quartz threads in water is broken, the attraction forces (which exceed the molecular forces by several orders of magnitude) at a distance of up to 300 nm are detected.     

Probing surface charge potentials of clay basal planes and edges by direct force measurements

The dispersion and gelation of clay suspensions have major impact on a number of industries, such as ceramic and composite materials processing, paper making, cement production, and consumer product formulation. To fundamentally understand controlling mechanisms of clay dispersion and gelation, it is necessary to study anisotropic surface charge properties and colloidal interactions of clay particles. In this study, a colloidal probe technique was employed to study the interaction forces between a silica probe and clay basal plane/edge surfaces. A muscovite mica was used as a representative of 2:1 phyllosilicate clay minerals. The muscovite basal plane was prepared by cleavage, while the edge surface was obtained by a microtome cutting technique. Direct force measurements demonstrated the anisotropic surface charge properties of the basal plane and edge surface. For the basal plane, the long-range forces were monotonically repulsive within pH 6-10 and the measured forces were pH-independent, thereby confirming that clay basal planes have permanent surface charge from isomorphic substitution of lattice elements. The measured interaction forces were fitted well with the classical DLVO theory. The surface potentials of muscovite basal plane derived from the measured force profiles were in good agreement with those reported in the literature. In the case of edge surfaces, the measured forces were monotonically repulsive at pH 10, decreasing with pH, and changed to be attractive at pH 5.6, strongly suggesting that the charge on the clay edge surfaces is pH-dependent. The measured force profiles could not be reasonably fitted with the classical DLVO theory, even with very small surface potential values, unless the surface roughness was considered. The surface element integration (SEI) method was used to calculate the DLVO forces to account for the surface roughness. The surface potentials of the muscovite edges were derived by fitting the measured force profiles with the surface element integrated DLVO model. The point of zero charge of the muscovite edge surface was estimated to be pH 7-8.     

Specific and nonspecific interaction forces between Escherichia coli and silicon nitride, determined by poisson statistical analysis

The nature of the physical interactions between Escherichia coli JM109 and a model surface (silicon nitride) was investigated in water via atomic force microscopy (AFM). AFM force measurements on bacteria can represent the combined effects of van der Waals and electrostatic forces, hydrogen bonding, steric interactions, and perhaps ligand-receptor type bonds. It can be difficult to decouple these forces into their individual components since both specific (chemical or short-range forces such as hydrogen bonding) and nonspecific (long-range colloidal) forces may be present in the overall profiles. An analysis is presented based on the application of Poisson statistics to AFM adhesion data, to decouple the specific and nonspecific interactions. Comparisons with classical DLVO theory and a modified form of a van der Waals expression for rough surfaces were made in order to help explain the nature of the interactions. The only specific forces in the system were due to hydrogen bonding, which from the Poisson analysis were found to be -0.125 nN. The nonspecific forces of 0.155 nN represent an overall repulsive interaction. These nonspecific forces are comparable to the forces calculated from DLVO theory, in which electrostatic-double layer interactions are added to van der Waals attractions calculated at the distance of closest approach, as long as the van der Waals model for "rough" spherical surfaces is used. Calculated electrostatic-double layer and van der Waals interactions summed to 0.116 nN. In contrast, if the classic (i.e., smooth) sphere-sphere model was used to predict the van der Waals forces, the sum of electrostatic and van der Waals forces was -7.11 nN, which appears to be a large overprediction. The Poisson statistical analysis of adhesion forces may be very useful in applications of bacterial adhesion, because it represents an easy way to determine the magnitude of hydrogen bonding in a given system and it allows the fundamental forces to be easily broken into their components.     

Interaction forces between α-alumina fibres in aqueous electrolyte measured with an atomic force microscope

The surface charging properties of polycrystalline α-alumina fibres in aqueous electrolyte solutions have been investigated by direct force and streaming potential measurements. The presence of both Al and Si on the surface of the fibres resulted in a chemically heterogeneous surface. The heterogeneous distribution of Si resulted in large attractive forces between the fibres at moderate to low pH values and a pzc/iep at a pH value of approximately 5.5. The origin of this force was electrostatic in nature as the force profiles were well described by the DLVO theory of colloid stability. The agreement between the direct force and streaming potential measurements was good both in terms of the magnitude of the potentials and the position of the pzc/iep. By acid washing the fibres the chemical heterogeneity of the surface was reduced and the attractive force profiles at lower pH values were not observed. Instead repulsive forces were observed which were well described by DLVO theory at all separation distances greater than 8 nm. At smaller separation distances an additional repulsive force was measured which was attributed to the presence of a Al(OH)₃ like layer on the surface of the alumina. The acid washing treatment also resulted in a shift in the pH at which the pzc/iep occurred to a value of 6.5, presumably due to a lower surface silica concentration.     

Direct evaluation of DLVO theory for predicting long-range forces between a yeast cell and a surface

Using a combined gradient optical trap and evanescent wave light-scattering force-measurement technique, long-range colloidal forces were measured between a single Candida albicans yeast cell and a flat, bare glass surface in electrolyte concentrations ranging from 0.1 to 100 mM NaCl. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was compared to experimentally measured equilibrium force curves and found to provide a close approximation to the decay length of the measured forces for electrolyte concentrations up to about 0.23 mM NaCl. At higher electrolyte concentrations (>/=0.5 mM NaCl), decay lengths of force curves in experimental measurements were consistently longer than Debye lengths calculated from the electrolyte concentrations. In electrolyte concentrations of 10 and 100 mM NaCl, most cells attached rapidly, which prevented measurements of long-range forces. The small fraction of cells remaining unattached in these higher electrolyte concentrations displayed purely repulsive forces. These results show that the DLVO theory accurately describes cell-surface interactions when the Debye length is in the range of 20-30 nm but underpredicts the decay length of the interactions at higher electrolyte concentrations.     

The equation of state for an interface during the adsorption of organic substances on an electrode

The state of the interface between a metal and a solution of an electrolyte containing a neutral surfactant was investigated using a method alternative to the traditional thermodynamic approach. The method was based on the concept that there was a stability limit of a surfactant on an electrode, and the corresponding state could be described in terms of the catastrophe theory. The surface pressure was approximated by the Whitney polynomial in powers of the de Donder parameter (completeness of adsorption) with the coefficients depending on the chemical potential and polarization of the interface. The equation of state and the equation for the stability limit were obtained from the condition of zero first and second derivatives. These equations correctly described the results of electrocapillary measurements in the spirit of the law of corresponding states. The correlation between surface pressure maxima and critical stability potentials predicted by the theory was substantiated by the electrocapillary measurements data provided that the inflexions of surface pressure curves calculated from the electrocapillary data were related to the limiting stability at which the competing forces are balanced during the adsorption of surfactants. A simple equation for surface pressure was suggested in the form of a function of the state of thermodynamic parameters and completeness of adsorption. This function described the state of a surfactant at the interface. Equilibrium equations were derived for the state of a surfactant and the spinodal.     

The role of physicochemical interactions and FLO genes expression in the immobilization of industrially important yeasts by adhesion

One of the industrially important qualities of yeast is their ability to provide the cell-cell and cell-support interactions. This feature of yeast is responsible for technologically significant phenomena such as flocculation (brewing) and yeast biofilm formation (immobilization to supports), whereas these phenomena are time, environment, and strain dependent. Therefore, the goal of this work was to verify the possibility to predict and subsequently select yeast strains capable to colonize solid supports by using physicochemical adhesion models. Three different industrial yeast strains (Saccharomyces cerevisiae) were tested for their adhesion onto spent grain particles in the continuous gas-lift reactor. The cell adhesion energies were calculated, based on physicochemical characteristics of surfaces involved, according to three adhesion models (DLVO theory, thermodynamic approach, and extended DLVO theory). The role of physicochemical surface properties in the cell-cell and cell-support interactions was evaluated by comparing the computed predictions with experimental results. The best agreement between forecast and observation of the yeast adhesion to spent grains was achieved with the extended DLVO (XDLVO) theory, the most complex adhesion model applied in this study. Despite its relative comprehensiveness, the XDLVO theory does not take into account specific biochemical interactions. Consequently, additional understanding of the yeast adhesion mechanism was obtained by means of quantifying the expression of selected FLO genes. The presented approach provides tools to select the appropriately adhesive yeast strains and match them with solid supports of convenient surface properties in order to design immobilized biocatalysts exploitable in biotechnological processes.     

AFM force measurements between SAM-modified tip and SAM-modified substrate in alkaline solution

The reversible desorption and adsorption of ethanethiol (ET) and hexadecane thiol (HDT) self-assembled monolayers (SAMs) on gold substrates are addressed with potential-dependent AFM force measurements where both tip and substrate potentials are controlled independently. For HDT-modified tip and substrate, the potential dependence of the force curve corresponds to the observed voltammetric features. The adhesion interaction between HDT-modified tip and substrate exhibits a large adhesion, whereas the adhesion is reduced to one-quarter of its original value after HDT on the substrate is removed. The presence of both attractive features on the approach curve and large adhesion on retraction after thiol desorption are ascribed to micelle formation from the desorbed, insoluble, thiols above the Au surface. For the ET-modified tip and substrate, the force curve evinces time-dependent recovery after the thiol adsorption peak which arises from the finite time of diffusion of the desorbed thiol back to the substrate. However, the force curves exhibit little potential dependence when the ET-desorbed tip is interacted with ET-modified substrate.     

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.     

Computer simulation of colloidal dispersions

This paper discusses recent applications of statistical mechanics to dispersions with particular emphasis on the computer simulation of the dynamic properties.Fundamental to any computation on a colloidal dispersion is the knowledge of the potential of mean force for at least a pair of suspended particles. At low-to-moderate particle concentrations for stable dispersions, statistical mechanical calculations based on the normal DLVO pair potential produce reasonable agreement with experiment for a number of equilibrium properties of simple latex dispersions. This phenomenon indicates that under these conditions the DLVO pair potential is a reasonable effective pair potential. However, recent Monte Carlo simulations and experimental measurements with liquids of spherical molecules suggest that the force between a pair of dispersed particles at very small separation may differ significantly from that predicted by DLVO theory.The computation of dynamic properties of dispersions involves problems not encountered in the above equilibrium calculations. In particular, one must include the effects of indirect hydrodynamic as well as direct interactions among the particles. This computation may be easily accomplished at moderately low particle concentrations and the results of such calculations are able to give a very detailed analysis of the results of Photon Correlation Spectroscopy measurements on ion exchanged polystyrene latex suspensions at low concentration. These computations also, once again, emphasize the usefulness of DLVO pair potentials as effective pair potentials for systems of strongly interacting particles.     

Charge heterogeneity of surfaces: mapping and effects on surface forces

The DLVO theory treats the total interaction force between two surfaces in a liquid medium as an arithmetic sum of two components: Lifshitz–van der Waals and electric double layer forces. Despite the success of the DLVO model developed for homogeneous surfaces, a vast majority of surfaces of particles and materials in technological systems are of a heterogeneous nature with a mosaic structure composed of microscopic and sub-microscopic domains of different surface characteristics. In such systems, the heterogeneity of the surface can be more important than the average surface character. Attractions can be stronger, by orders of magnitude, than would be expected from the classical mean-field DLVO model when area-averaged surface charge or potential is employed. Heterogeneity also introduces anisotropy of interactions into colloidal systems, vastly ignored in the past. To detect surface heterogeneities, analytical tools which provide accurate and spatially resolved information about material surface chemistry and potential — particularly at microscopic and sub-microscopic resolutions — are needed.Atomic force microscopy (AFM) offers the opportunity to locally probe not only changes in material surface characteristic but also charges of heterogeneous surfaces through measurements of force–distance curves in electrolyte solutions. Both diffuse-layer charge densities and potentials can be calculated by fitting the experimental data with a DLVO theoretical model. The surface charge characteristics of the heterogeneous substrate as recorded by AFM allow the charge variation to be mapped. Based on the obtained information, computer modeling and simulation can be performed to study the interactions among an ensemble of heterogeneous particles and their collective motions. In this paper, the diffuse-layer charge mapping by the AFM technique is briefly reviewed, and a new Diffuse Interface Field Approach to colloid modeling and simulation is briefly discussed.     

Numerical study on the adhesion and reentrainment of nondeformable particles on surfaces: the role of surface roughness and electrostatic forces

In this paper, the reentrainment of nanosized and microsized particles from rough walls under various electrostatic conditions and various hydrodynamic conditions (either in air or aqueous media) is numerically investigated. This issue arises in the general context of particulate fouling in industrial applications, which involves (among other phenomena) particle deposition and particle reentrainment. The deposition phenomenon has been studied previously and, in the present work, we focus our attention on resuspension. Once particles are deposited on a surface, the balance between hydrodynamic forces (which tend to move particles away from the surface) and adhesion forces (which maintain particles on the surface) can lead to particle removal. Adhesion forces are generally described using van der Waals attractive forces, but the limit of these models is that any dependence of adhesion forces on electrostatic forces (due to variations in pH or ionic strength) cannot be reproduced numerically. For this purpose, we develop a model of adhesion forces that is based on the DLVO (Derjaguin and Landau, Verwey and Overbeek) theory and which includes also the effect of surface roughness through the use of hemispherical asperities on the surface. We first highlight the effect of the curvature radius on adhesion forces. Then some numerical predictions of adhesion forces or adhesion energies are compared to experimental data. Finally, the overall effects of surface roughness and electrostatic forces are demonstrated with some applications of the complete reentrainment model in some simple test cases.     

Adsorption of benzene on coinage metals: a theoretical analysis using wavefunction-based methods

The interaction of benzene with a Ag(111) surface has been determined using reliable ab initio electronic structure calculations. The results are compared to a recent detailed analysis of the interaction of benzene with copper and gold surfaces, thus making it possible to derive a consistent picture for the electronic structure changes encountered when benzene is brought into contact with the densely packed coinage metal surfaces. To avoid the problems encountered when the presently most frequently employed computational approach, density functional theory (DFT), is applied to adsorbate systems where dispersion (or van der Waals) forces contribute substantially, we use a wavefunction-based approach. In this approach, the weak van der Waals interactions, which are dominated by correlation effects, are described using second-order perturbation theory. The surface dipole moment and the work function changes induced upon adsorption are also discussed.     

Interaction forces in bitumen extraction from oil sands

Water-based extraction process (WBEP) has been successfully applied to bitumen recovery from Athabasca oil sand ore deposits in Alberta. In this process, two essential steps are involved. The bitumen first needs to be "liberated" from sand grains, followed by "aeration" with air bubbles. Bitumen "liberation" from the sand grains is controlled by the interaction between the bitumen and sand grains. Bitumen "aeration" is dependent, among other mechanical and hydrodynamic variables, on the hydrophobicity of the bitumen surface, which is controlled by water chemistry and interactions between bitumen and fine solids. In this paper, the interaction force measured with an atomic force microscope (AFM) between bitumen-bitumen, bitumen-silica, bitumen-clays and bitumen-fines is summarized. The measured interaction force barrier coupled with the contacted adhesion force allows us to predict the coagulative state of colloidal systems. Zeta potential distribution measurements, in terms of heterocoagulation, confirmed the prediction of the measured force profiles using AFM. The results show that solution pH and calcium addition can significantly affect the colloidal interactions of various components in oil sand extraction systems. The strong attachment of fines from a poor processing ore on bitumen is responsible for the corresponding low bitumen flotation recovery. The identification of the dominant non-contact forces by fitting with the classical DLVO or extended DLVO theory provides guidance for controlling the interaction behavior of the oil sand components through monitoring the factors that could affect the non-contact forces. The findings provide insights into megascale industrial operations of oil sand extraction.