Use of atomic force microscope for the measurements of hydrophobic forces (original) (raw)
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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2007
Surface forces were measured using an AFM with silica surfaces immersed in C n TACl (n = 12-18) solutions in the absence of added salt. The results showed long-range attractive forces that cannot be explained by the DLVO theory. The long-range attractions increased with increasing surfactant concentration, reaching a maximum at the point of charge neutralization (p.c.n.) and then decreased. The long-range forces decayed exponentially, with the decay lengths increasing from 3 to 32 nm as the chain length of the surfactants increased from C-12 to C-18. The measured forces can be fitted to the charged-patch model of Miklavic et al. [
AFM measurements of forces between silica surfaces
Journal of Sol-Gel Science and Technology, 2012
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/m 2 according to the Derjaguin-Muller-Toporov (DMT) model. Adhesion energy was much less at pH = 2 (0.3 mJ/m 2) due to persistive short range repulsion.
Nanoscale adhesive forces between silica surfaces in aqueous solutions
Journal of Colloid and Interface Science, 2014
Nanoscale adhesive forces between a colloidal silica probe and a flat silica substrate were measured with an atomic force microscope (AFM) in a range of aqueous NaCl, CaCl 2 , and AlCl 3 solutions, with concentrations ranging from 10 À6 to 10 À2 M at pH $5.1. Notably, the measured force curves reveal large pull-off forces in water which increase in electrolyte solutions, with jump-off-contact occurring as a gradual detachment of the probe from the flat substrate rather than as a sharp discontinuous jump. The measured force curves also show that the number and size of the steps increase with concentration and notably with electrolyte valence. For the higher concentration and valence the steps become jumps. We propose that these nanoscale adhesive forces between mineral surfaces in aqueous solutions may arise from newly born cavities or persistent subnanometer bubbles. Formation of cavities or nanobubbles cannot be observed directly in our experiments; however, we cannot disregard them as responsible for the discontinuities in the measured force data. A simple model based on several cavities bridging the two surfaces we show that is able to capture all the features in the measured force curves. The silica surfaces used are clean but not intentionally hydroxylated, as contact angle measurements show, and as such may be responsible for the cavities.
Hofmeister Solute Effects on Hydrophobic Adhesion Forces in SFM Experiments
Langmuir, 2001
One of the chief problems currently facing surface scientists is determining the relationship between mechanical forces, such as those measured by surface forces apparatus (SFA) or scanning force microscopes (SFM), and chemical driving forces, such as those governing interactions between molecules. Hydrophobic forces are important at both levels, but there is no clear relationship between their manifestation at molecular and supramolecular scales. To help bridge this gap, we have used an SFM to measure detachment forces between untreated silicon nitride SFM tips, which are moderately hydrophobic, and the highly hydrophobic surfaces paraffin and octadecylsilane-mica in the presence of various 2.0 and 3.0 M salt solutions. The salts were chosen for their strong abilities to promote or inhibit hydrophobically driven phenomena, according to their positions in the Hofmeister series. Thus, this is the first systematic assessment of Hofmeister salt effects on supramolecular hydrophobic adhesion in solution-phase chemical terms. NaCl has no effect on hydrophobic adhesion force relative to pure water, which agrees with previous SFM and SFA work but lies in contrast with the effects of NaCl on solution-phase behavior such as aqueous nonpolar compound solubility. Chaotropes such SCNand guanidinium + , which promote exposure of molecular nonpolar surfaces to water, decrease adhesion force, whereas kosmotropes such as NH4 + and SO4 2-, which promote sequestration of nonpolar molecular surfaces from water, enhance adhesion force. Our results suggest that solvent structures near molecular and supramolecular hydrophobic surfaces are fundamentally different but that the nonideal effects embodied by the Hofmeister series have similar mechanisms in both length scales. Preferential interactions of solutes with hydrophobic surfaces and potential sources of hydrophobic adhesion, such as cavitation, van der Waals interactions, and solvent ordering, are discussed.
Measurements of Hydrophobic and DLVO Forces in Bubble-Surface Interactions in Aqueous Solutions
Langmuir, 1994
The forces between hydrophilic and hydrophobic silica particles and an air bubble were measured in pure water and in NaCl solutions using an atomic force microscope. In addition to the expected doublelayer and van der Waals forces, strong long-range attractive forces were also observed. A long-range attraction was also measured between a hydrophilic silica particle and a hydrophobic silica plate. A gas bubble thus behaves like a hydrophobic surface. The long-ranged attractive component of the force disappeared when the anionic surfactant sodium dodecylsulfate (SDS) was added to the solution. This effect is explicable in terms of surfactant adsorption at the hydrophobic interfaces which renders them hydrophilic. A "thermodynamic" model is proposed that appears to be consistent with these and previous force and wetting experiments on hydrophobic surfaces. It is also demonstrated that a nonzero water contact angle on clean hydrophilic silica and similar hydrophilic surfaces can arise from DLVO forces alone and is not necessarily an indication of surface contamination or some hydrophobic component in the force. A. Bubble-particle aggregate B. Hydrophobic surface (0 zz 90) C. Hydrophilic surface (0 = 0) D. Relevant properties
Langmuir : the ACS journal of surfaces and colloids, 2005
Forces have been measured between silica surfaces with adsorbed surfactants by means of a bimorph surface force apparatus. The surfactants used are the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and the nonionic surfactant hexakis(ethylene glycol) mono-n-tetradecyl ether (C(14)E(6)) as well as mixtures of these two surfactants. The measurements were made at elevated pH, and the effect of salt was studied. At high pH the glass surface is highly charged, which increases the adsorption of TTAB. Despite the low adsorption generally seen for nonionic surfactants on silica at high pH, addition of C(14)E(6) has a considerable effect on the surface forces between two glass surfaces in a TTAB solution. The barrier force is hardly affected, but the adhesion is reduced remarkably. Also, addition of salt decreases the adhesion, but increases the barrier force. In the presence of salt, addition of C(14)E(6) also increases the thickness of the adsorbed layer. The force barrier...
Langmuir, 2002
Using the atomic force microscope (AFM), the pull-off forces between flat glass or silicon surfaces, and silicon AFM tips or glass microspheres of different sizes, have been extensively studied as a function of relative humidity (RH) in the range 5-90%, as model systems for the behaviour of cohesive powders. The glass and silicon substrates were treated to render them either hydrophobic or hydrophilic. All the hydrophilic surfaces gave simple force curves and pull-off forces increasing uniformly with RH. Small contacts (R~20nm) gave pull-off forces close to values predicted by simple Laplace-Kelvin theory (~20 nN) but the values with microspheres (R~20µm) fell well below predictions for sphere-flat or sphere-sphere geometry, due to roughness and asperity contacts. The hydrophobic silicon surfaces also exhibited simple behaviour, with no significant RH dependence. The pull-off force again fell well below predicted values (JKRS contact mechanics theory) for the larger contacts. Hydrophobic glass gave similar adhesion to silicon over most of the RH range, but against both silicon tips and glass microspheres, there was an anomalously large adhesion in the RH range 20-40%, accompanied by a long range non-contact force. The adhesion on fully hydrophilic surfaces and its RH dependence can be mostly explained by current theories of capillary bridges, but the interpretation is complicated by the sensitivity of theoretical predictions to contact geometry (and hence to roughness effects) and by uncertainties in the thickness of adsorbed water layers. The anomalous behaviour on hydrophobic glass surfaces at intermediate values of RH is not fully understood, but possible causes are (1) dipole layers in the partially-formed water film, giving rise to patch charges and long range forces, or (2) fixed charges at a reactive glass surface, involving specific bonding reactions. The results may be useful in explaining the behaviour of cohesive powders with different coatings, or which show a large humidity dependence (e.g. zeolites) or show electrostatic charging effects (e.g. silica aerogels).
Journal of Colloid and Interface Science, 2013
Surface force measurements conducted with thiolated gold surfaces showed previously that hydrophobic interaction entails a decrease in excess film entropy, suggesting that hydrophobic force originates from changes in the structure of the medium (water) confined between hydrophobic surfaces [1]. As a follow-up work, surface force measurements have been conducted in the present work using an atomic force microscope (AFM) with silica surfaces coated with octadecyltrichlorosilane (OTS) at temperatures in the range of 10-40°C. A thermodynamic analysis of the results show that both the excess film entropy (DS f) and excess film enthalpy (DH f) decrease with decreasing thickness of the water films between the hydrophobic surfaces. It has been found also that |DH f | > |TDS f |, which represents a necessary condition for the excess free energy change (DG f) to be negative and hence the hydrophobic interaction be attractive. Thus, the results obtained with both the thiolated and silylated surfaces show that hydrophobic forces originate from the structural changes in the medium. It is believed that the water molecules in the thin liquid films (TLFs) of water form clusters as a means to reduce the free energy when they cannot form H-bonds to neighboring hydrophobic surfaces.
Chemical Physics Letters, 2003
We have measured the force acting on neutral tips as function of distance to hydrophobic surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to an electrostatic response of the aqueous fluid structure (hydration layer). The exchange of a volume of this region with a dielectric permittivity int by the tip with a dielectric constant tip is responsible for the tip attraction when it is immersed in the hydration layer. Hydrophobic hydration layers, characterized by a variable dielectric permittivity profile, have measured widths of 4and4 and 4and8 nm for hydrophobic silicon and CTAB monolayer covering mica surfaces, respectively.