Electrostatic interactions between double layers: influence of surface roughness, regulation, and chemical heterogeneities

Electrostatic interactions between two surfaces as measured by atomic force microscopy (AFM) are usually analyzed in terms of DLVO theory. The discrepancies often observed between the experimental and theoretical behavior are usually ascribed to the occurrence of chemical regulation processes and/or to the presence of surface chemical or morphological heterogeneities (roughness). In this paper, a two-gradient mean-field lattice analysis is elaborated to quantifying double layer interactions between nonplanar surfaces. It allows for the implementation of the aforementioned sources of deviation from DLVO predictions. Two types of ion-surface interaction ensure the adjustment of charges and potentials upon double layer overlap, i.e., specific ionic adsorption at the surfaces and/or the presence of charge-determining ions for the surfaces considered. Upon double layer overlap, charges and potentials are adjusted via reequilibrium of the different ion adsorption processes. Roughness is modeled by grafting asperities on supporting planar surfaces, with their respective positions, shapes, and chemical properties being assigned at will. Local potential and charge distributions are derived by numerically solving the nonlinear Poisson-Boltzmann equation under the boundary conditions imposed by the surface profiles and regulation mechanism chosen. Finite size of the ions is taken into account. A number of characteristic situations are briefly discussed. It is shown how the surface irregularities are reflected in the Gibbs energy of interaction.     

Measurement of long-range forces on a single yeast cell using a gradient optical trap and evanescent wave light scattering

The long-range equilibrium and viscous interaction forces between a single Candida albicans cell and a flat surface have been measured using a gradient optical trap as a force transducer and evanescent wave light scattering (EWLS) to determine the separation distance. In this technique the trapped cell is probed against the surface by moving the focal point of the optical trap, the equilibrium force is determined by the deflection of the most probable cell position from the trap center, and the viscous forces are determined from the relaxation time of the Brownian fluctuations of the cell in the trap. At low electrolyte concentrations (0.5 mM NaCl) where double layer repulsion was anticipated to be the dominant interaction, equilibrium force–distance profiles for yeast cells and similarly sized polystyrene microspheres on glass surfaces both showed good agreement with predictions of DLVO theory. Also, viscous drag profiles at larger separation distances where interaction forces were small agreed well with Stokes flow predictions. These results appear to validate the technique for use with spherical yeast cells and other bioparticles of similar size. This force measurement methodology therefore provides a complementary alternative to atomic force microscopy for direct force measurement with much greater sensitivity for studying interaction between yeast and surfaces.     

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.     

Fabrication and Visualization of Metal‐Ion Patterns on Glass by Dip‐Pen Nanolithography

Fluorescent self‐assembled monolayers (SAMs) are used as dip‐pen nanolithography (DPN) substrates for the fabrication of patterns of Ca²⁺ and Cu²⁺ ions. The driving force for the transfer of these ions from an atomic force microscopy (AFM) tip to the surface is their complexation to organic ligands on the monolayer. By means of fluorescent surfaces, the patterns can be visualized under a fluorescence microscope. We use a custom‐built atomic force fluorescence microscope (AFFM), a combination of atomic force and confocal fluorescence microscopes, to deposit the metal ions onto the sensing SAMs by DPN and to subsequently visualize modulations of fluorescence intensity in a sequential write–read mode.     

Tip–surface interactions in noncontact atomic force microscopy on reactive surfaces

The imaging process in noncontact atomic force microscopy (AFM) is studied on a number of reactive surfaces, namely, the Takayanagi reconstructed Si(111), InP(110), and GaAs(110). We show that on these surfaces, the short-range dangling-bond type of interaction between the tip and the surface is decisive in achieving atomic resolution. The short-range tip–surface interaction is modeled in the density functional theory within the GGA approximation. We show that we can achieve quantitative agreement with the experimental data in the commonly used frequency modulation technique for AFM surface corrugation with a very simple model for the tip geometry treating the tip–surface interaction in the perturbation theory. The nature of the short-range tip–surface interaction on the three surfaces is considered and the consequences thereof for the experiments is discussed.     

Fragile structured layers on surfaces in highly concentrated solutions of electrolytes of various valencies

In the present study a dynamic mode of atomic force microscopy for force measurements was employed to investigate the hydration repulsion force between charged surfaces in highly concentrated electrolyte solutions of NaCl, MgCl₂ and LaCl₃. A strong dependence of this repulsive force on the approaching rate of surfaces, the prehistory of their contact and the valency of cations was demonstrated. The phenomena were strongly pronounced in the cases of high scan rates, large surfaces and cations of high valency. The results obtained indicate that a fragile structure composed of water molecules, ions and hydrated ions exists outside of the primary layer of water molecules and ions adsorbed firmly on surfaces.     

Adsorption and stability of streptavidin on cluster-assembled nanostructured TiOx films

The study of the adsorption of proteins on nanostructured surfaces is of fundamental importance to understand and control cell-surface interactions and, notably, cell adhesion and proliferation; it can also play a strategic role in the design and fabrication of nanostructured devices for postgenomic and proteomic applications. We have recently demonstrated that cluster-assembled nanostructured TiO x films produced by supersonic cluster beam deposition possess excellent biocompatibility and that these films can be functionalized with streptavidin, allowing the immobilization of biotinylated retroviral particles and the realization of living-cell microarrays for phenotype screening. Here we present a multitechnique investigation of the adsorption mechanisms of streptavidin on cluster-assembled TiO x films. We show that this nanostructured surface provides an optimal balance between adsorption efficacy and protein functionality. By using low-resolution protein arrays, we demonstrate that a layer of adsorbed streptavidin can be stably maintained on a cluster-assembled TiO x surface under cell culture conditions and that streptavidin retains its biological activity in the adsorbed layer. The adsorption mechanisms are investigated by atomic force microscopy in force spectroscopy mode and by valence-band photoemission spectroscopy, highlighting the potential role of the interaction of the exposed carboxyl groups on streptavidin with the titanium atoms of the nanostructured surface.     

Nanoscale analyses of modified polypropylene microtubes internal surface: an approach covering topographical and force spectroscopic parameters

A nanoscale characterization of modified and unmodified polypropylene (PP) microtubes internal surface was performed to investigate their structural, chemical, and physical properties. Nanoroughness, stiffness, elasticity, attraction behavior, adhesion forces, and chemical environment were investigated to test some manufacturer statements regarding Axygen MAXYMum Recovery® products. They announced that this class of material presented special features, originated from a modification to the original PP resin and by using a diamond polished mould, providing lower retention and minor interference on laboratorial tests, such as low roughness and little interaction tendency. Then, in this study, modified and control internal surfaces of PP microtubes were compared by atomic force microscopy, scanning electron microscopy, and Fourier transform infrared spectroscopy. Nanoroughness and force spectroscopy parameters assessed by atomic force microscopy showed out as a sensible and high‐resolution technique, crucial to discriminate differences between the surfaces. This type of investigation can be considered as a promising approach that can be applied to other polymeric systems, considering nanoscale properties, physical/chemical modifications, and as an alternative route for quality control checking concerning polymeric surfaces. Copyright © 2013 John Wiley & Sons, Ltd.     

Modification of polymers by adsorption phenomena

The adsorption of ions and amphiphilic molecules on solid polymers is investigated by direct force measurements using an atomic force microscope (AFM). It is shown that electrolyte ions are changing the surface potentials of the solid polymers as well as their adhesive properties. The experiments show that the interaction with a negatively charged probe is dramatically decreased by the adsorption of anions. The adsorption isotherms are determined by zeta potential measurements (streaming potential of flat plates). In presence of adsorbing anions, the attractive interaction and the adhesion are reduced and can be eliminated completely. So, even solutions of simple electrolyte ions can be applied intentionally in order to modify the interaction of polymer surfaces. A wide variety of technological applications becomes accessible.     

High surface water interaction in superhydrophobic nanostructured silicon surfaces: convergence between nanoscopic and macroscopic scale phenomena

In the present work, we investigate wetting phenomena on freshly prepared nanostructured porous silicon (nPS) with tunable properties. Surface roughness and porosity of nPS can be tailored by controlling fabrication current density in the range 40-120 mA/cm(2). The length scale of the characteristic surface structures that compose nPS allows the application of thermodynamic wettability approaches. The high interaction energy between water and surface is determined by measuring water contact angle (WCA) hysteresis, which reveals Wenzel wetting regime. Moreover, the morphological analysis of the surfaces by atomic force microscopy allows predicting WCA from a semiempiric model adapted to this material.     

Capillary forces: influence of roughness and heterogeneity

A formalism is described to calculate capillary forces between solid surfaces analytically. Assumptions are that the liquid menisci (1) have a much larger extension parallel to the gap than normal and (2) are formed by capillary condensation and are in equilibrium with the vapor. To calculate capillary forces, first the gap between the two surfaces is described by a height distribution function. Roughness is considered with an asperity distribution function. Both distributions can at least in principal be measured by light, electron, or atomic force microscopy or grazing incidence X-ray reflectivity. The total capillary force versus distance or vapor pressure is obtained by a convolution of both distributions and an integration. The formalism is applied to calculate the capillary force between rough spherical particles. In addition, a method to consider surface heterogeneity is suggested.     

Force measurements between Teflon AF and colloidal silica particles in electrolyte solutions

The interaction force between a very hydrophobic polymer surface and colloidal silica particles with a roughness of 10–15 nm has been measured in aqueous solutions of KOH and KCl using an atomic force microscope. The interaction can be described according to the DLVO theory by an electrical double-layer force that is repulsive at long distances and attractive at short distances and an attractive van der Waals force. The electrical double-layer potentials are compared to the zeta potentials of Teflon AF and the silica spheres. The roughness of the silica particles leads to an underestimation of the short-range attraction and the surface potential. Both KCl and KOH solutions affect the potential of the interacting surfaces. OH⁻ ions that adsorb preferentially to the Teflon AF surface create higher potentials than Cl⁻ ions. Range and strength of the attractive interaction are not affected by KCl solutions but reduced by addition of KOH. This can be explained by decreasing potential differences between the silica sphere and Teflon AF with increasing KOH concentration. In addition, the preferential adsorption of OH⁻ ions may lead to a reduction of the van der Waals interaction. The presence of nanobubbles, too, might play a role.     

Cluster properties of peptides on (100) semiconductor surfaces

Peptide adhesion on semiconductor surfaces is quantitatively investigated by atomic force microscopy. The selected peptides are shown to cluster at the surface, with the larger, higher, and softer clusters appearing on the surfaces with lower peptide adhesion. Average cluster diameters vary from 40 nm on GaAs (100) to 300 nm on Si (100). Direct adhesion of the peptides to the surface competes with forming molecular aggregates that offer an overall reduced surface contact.     

Novel methodology to control the adsorption structure of cationic porphyrins on the clay surface using the "size-matching rule"

Saponite-type clays that have different cation exchange capacities were successfully synthesized by hydrothermal synthesis. The structure and properties were analyzed by X-ray diffraction, X-ray fluorescence, (27)Al NMR, FT-IR, thermogravimetric and differential thermal analysis, atomic force microscopy, and cation exchange capacity measurement. The intercharge distances on the synthetic saponite (SS) surfaces were calculated to be 0.8-1.9 nm on the basis of a hexagonal array. The complex formation behavior between SS and cationic porphyrins was examined. It turns out that the average intermolecular distance between porphyrin molecules on the SS surface can be controlled, depending on the charge density of the SS. In the case of tetrakis(1-methylpyridinium-4-yl)porphyrin (H(2)TMPyP(4+)), the average intermolecular distances on the SS surface can be controlled from 2.3 to 3.0 nm on the basis of a hexagonal array. It was also found that absorption maxima of porphyrins depend on the charge density of the SS. The adsorption behavior of porphyrin on the SS surface can be rationally understood by the previously reported "size-matching rule". This methodology using host-guest interaction can realize a unique adsorption structure control of the porphyrin molecule on the SS surface, where the gap distance between guest porphyrin molecules is rather large. These findings will be highly valuable to construct photochemical reaction systems such as energy transfer in the complexes.     

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.     

Friction and adhesion forces of Bacillus thuringiensis spores on planar surfaces in atmospheric systems

The kinetic friction force and the adhesion force of Bacillus thuringiensis spores on planar surfaces in atmospheric systems were studied using atomic force microscopy. The influence of relative humidity (RH) on these forces varied for different surface properties including hydrophobicity, roughness, and surface charge. The friction force of the spore was greater on a rougher surface than on mica, which is atomically flat. As RH increases, the friction force of the spores decreases on mica whereas it increases on rough surfaces. The influence of RH on the interaction forces between hydrophobic surfaces is not as strong as for hydrophilic surfaces. The friction force of the spore is linear to the sum of the adhesion force and normal load on the hydrophobic surface. The poorly defined surface structure of the spore and the adsorption of contaminants from the surrounding atmosphere are believed to cause a discrepancy between the calculated and measured adhesion forces.     

Surface observation of solvent-impregnated Nafion membrane with atomic force microscopy

Surface morphology modification of the Nafion 117 membrane has been investigated by atomic force microscopy. The effects of water and methanol on the topography and structure of the surface were studied using the contact and tapping atomic force microscopy modes. Nafion topography considerably changes when samples absorb water. However, samples stored in methanol are characterized by flat surfaces. Surface modification was linked to an expansion phenomenon during the swelling of Nafion by solvents. Tapping-mode phase images showed that ionic and cluster domains are distinguishable from the surface of samples impregnated either in water or methanol.     

Simultaneous pit generation and visualization of pit topography using combined atomic force–scanning electrochemical microscopy

Combined atomic force microscopy–scanning electrochemical microscopy (AFM–SECM) is for the first time used to generate single corrosion pits on passivating iron surfaces in the micrometer range. The AFM–SECM probe locally generates nitric acid during the oxidation of nitrite ions with the release of protons at selected sites on the surface of the otherwise passive metal. High confinement of passive film breakdown is achieved from the combination of a small probe size and the inhibiting properties of non-reacted nitrite ions on the surrounding passivated surface. Simultaneous visualization of pit nucleation and propagation can be obtained in the same solution without changing the probe by AFM.     

Multiasperity contact adhesion model for universal asperity height and radius of curvature distributions

A new approach to the multiasperities contact interaction between two surfaces is presented. Each asperity is individually considered with its own different height and radius of curvature. Different materials, such as polyvinylchlorine (PVC) and stainless steel, are used as model systems. For each of the model materials, a set of asperities was generated using Monte Carlo method. Both asperity heights and radii were based on their statistical distributions experimentally obtained. Contact forces were determined for each asperity at a given distance between the two surfaces, while the deformation of each asperity was calculated according to the Johnson-Kendall-Roberts (JKR) or the Derjaguin-Muller-Toporov (DMT) model (depending on the material). The contribution of each asperity to the overall surface was summed, and the overall contact force was determined. The developed method was validated against contact force measurements obtained with atomic force microscopy (AFM).     

Boundary lubricant films under shear: effect of roughness and adhesion

The normal interaction and the behavior under shear of mica surfaces covered by two different triblock copolymers of polylysine-polydimethysiloxane-polylysine were studied by combining the capabilities of the surface forces apparatus and the atomic force microscopy. At low pH values these copolymers spontaneously adsorb on the negatively charged mica surfaces from aqueous solutions as a consequence of the positive charge of the polylysine moieties. The morphology of the adsorbed layer is determined by the molecular structure of the particular copolymer investigated. This morphology plays a fundamental role on the behavior of the adsorbed layers under shear and compression. While nonadhesive smooth layers oppose an extremely small resistance to sliding, the presence of asperities even at the nanometric scale originates a frictional resistance to the motion. The behavior of uniform nonadhesive nanorough surfaces under shear can be quantitatively understood in terms of a simple multistable thermally activated junction model. The electric charge of the adsorbed copolymer molecules and hence the adhesion energy between the coated surfaces can be modified by varying the pH of the surrounding media. In the presence of an adhesive interaction between the surfaces the behavior under shear is strongly modified. Time-dependent mechanisms of energy dissipation have to be evoked in order to explain the changes observed.