Influence of wettability and surface charge on the interaction between an aqueous electrolyte solution and a solid surface

The hydrodynamic drainage force of a Newtonian aqueous electrolyte solution squeezed between two surfaces of different wettability was measured using the AFM colloidal probe technique. The surface hydrophobicity, roughness, polarity and approach velocity, and thus the shearing rate of the liquid, were controlled. A direct relationship between the mobility of the aqueous electrolyte solution close to the surfaces and the hydrophobicity of the surfaces was not established. We predict that the mobility of the liquid depends in a more complex fashion on the polarity and charge of the surfaces and on the properties of the electrolyte.     

Effect of solution concentration, surface bias and protonation on the dynamic response of amplitude-modulated atomic force microscopy in water

The dynamic response of amplitude-modulated atomic force microscopy (AM-AFM) is studied at the solid/water interface with respect to changes in ionic concentration, applied surface potential, and surface protonation. Each affects the electric double layer in the solution, charge on the tip and the sample surface, and thus the forces affecting the dynamic response. A theoretical model is developed to relate the effective stiffness and hydrodynamic damping of the AFM cantilever that is due to the tip/surface interaction with the phase and amplitude signals measured in the AM-AFM experiments. The phase and amplitude of an oscillating cantilever are measured as a function of tip-sample distance in three experiments: mica surface in potassium nitrate solutions with different concentrations, biased gold surface in potassium nitrate solution, and carboxylic acid-terminated self-assembled monolayers (SAMs) on gold in potassium nitrate pH buffers. Results show that, over the range where the higher harmonic modes of the oscillation are negligible, the effective stiffness of the AFM cantilever increases to a maximum as the tip approaches the surface before declining again as a result of the repulsive electrical double layer interaction. For attractive electrical double-layer interactions, the effective stiffness declines monotonically as the tip approaches the surface. Similarly, the hydrodynamic damping of the tip increases and then decreases as the tip approaches the solid/water interface, with the magnitude depending on the species present in the solution.     

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.     

Direct measurement of hydrophobic particle–bubble interactions in aqueous solutions by atomic force microscopy: Effect of particle hydrophobicity

Direct measurements of the interaction forces between a spherical silica particle and a small air bubble have been conducted in aqueous electrolyte solutions by using an atomic force microscope (AFM). The silica particle was hydrophobized with a silanating reagent, and the interaction forces were measured by using several particles with different surface hydrophobicities. In the measured force curves, a repulsive force was observed at large separation distances as the particle moved towards the bubble. The origin of the repulsive force was attributed to an electrostatic double-layer force because both the particle and bubble were negatively charged. After the repulsive force, an extremely long-range attractive force acted between the surfaces. These results indicate that the intervening thin water film between the particle and bubble rapidly collapsed, resulting in the particle penetrating the bubble.

The instability of the thin water film between the surfaces suggests the existence of an additional attractive force. By comparing the repulsive forces of the obtained force curves with the DLVO theory, the rupture thickness was estimated. The hydrophobicity of the particle did not significantly change the rupture thickness, whereas the pH of the solution is considered to be a critical factor.     

Structural and adhesion properties of surfaces functionalized with polyelectrolytes and polystyrene particles

Surfaces functionalized with polystyrene particles and polyelectrolytes were used to investigate the morphological and adhesion properties of composite substrates. Atomic force microscopy (AFM) studies showed that surfaces with non-homogeneous topography have non-homogeneous adhesion properties. In addition, the homogeneity of the adhesion properties is dependent upon the chemical species used to functionalize the surface. Force volume (FV) imaging was utilized to map the adhesion of the fabricated substrates with high-resolution. The FV studies revealed that the hydrophobicity of the surface is not uniform despite the fact that the surface was functionalized with the same polyelectrolyte. The analysis methodology we report here opens the possibility to design better surfaces for future tissue engineering applications.     

Protonation of Lysozymes and Its Consequences for the Adsorption onto a Mica Surface

Several physicochemical properties of chicken egg white lysozyme (LSZ) in electrolyte solutions were determined. The hydrodynamic diameter of LSZ at an ionic strength of 0.15 M was found to be 4.0 nm. Using the determined parameters, the number of uncompensated (electrokinetic) charges, N(c), on the molecule surface was calculated from the electrophoretic mobility data. It was found that the N(c) = 2.8 at pH = 3.0 and an ionic strength of I = 0.15 M. At the lower ionic strength, I = 1 × 10(-3) M, this positive charge increased to N(c) = 5.6 at a pH = 3.0 The physicochemical characteristics were supplemented by the dynamic viscosity measurements. The intrinsic viscosity and the hydrodynamic diameter results were compared with theoretical predictions from Brenner's model. Using this approach, it was found that the effective molecule length of LSZ is equal to L(ef) = 5.6 nm. Additional information on the LSZ adsorbed films was obtained by the contact angle measurements. The notably large contact angles were measured on LSZ films formed under the conditions where both the LSZ and the mica were oppositely charged. The higher the positive zeta potential of LSZ, the greater the contact angle measured, which indicates that LSZ affinity for the adsorption on mica increases with its uncompensated charge. The adsorption dependence on the zeta potential of LSZ was explained, assuming a roughly uniform distribution of the net charge on the molecule surface. This assumption is supported by the results of depositing negatively charged, fluorescent latex particles onto the mica surface, which had been modified by LSZ adsorption. The highest latex coverage was formed on mica surfaces that had first been coated with LSZ solutions of lower pH, as a result of the increasing charge of LSZ monolayers in this condition.     

Self-assembled Gemini surfactant film-mediated dispersion stability

The force-distance curves of 12-2-12 and 12-4-12 Gemini quaternary ammonium bromide surfactants on mica and silica surfaces obtained by atomic force microscopy (AFM) were correlated with the structure of the adsorption layer. The critical micelle concentration was measured in the presence or absence of electrolyte. The electrolyte effect (the decrease of CMC) is significantly more pronounced for Gemini than for single-chain surfactants. The maximum compressive force, F(max), of the adsorbed surfactant aggregates was determined. On the mica surface in the presence of 0.1 M NaCl, the Gemini micelles and strong repulsive barrier appear at surfactant concentrations 0.02-0.05 mM, which is significantly lower than that for the single C(12)TAB (5-10 mM). This difference between single and Gemini surfactants can be explained by a stronger adsorption energy of Gemini surfactants. The low concentration of Gemini at which this surfactant forms the strong micellar layer on the solid/solution interface proves that Gemini aggregates (micelles) potentially act as dispersing agent in processes such as chemical mechanical polishing or collector in flotation. The AFM force-distance results obtained for the Gemini surfactants were used along with turbidity measurements to determine how adsorption of Gemini surfactants affects dispersion stability. It has been shown that Gemini (or two-chain) surfactants are more effective dispersing agents, and that in the presence of electrolyte, the silica dispersion stability at pH 4.0 can also be achieved at very low surfactant concentrations ( approximately 0.02 mM).     

溶胶凝胶法制备超疏水二氧化硅涂膜及其表面润湿行为

采用溶胶-凝胶法以正硅酸乙酯(TEOS)和甲基三乙氧基硅烷(MTES)为前驱体制备超疏水SiO2涂层。红外光谱(FTIR)和热重分析(TGA)表征合成SiO2的化学组成,通过透射电镜(TEM)和扫描电镜(TEM)观察制备SiO2的结构形貌,扫描电镜(SEM)和原子力显微镜(AFM)观察SiO2涂膜的表面形貌,通过测试水接触角(WCA)讨论SiO2涂层的表面微观结构与其表面疏水性能的关系。结果表明以TEOS和MTES为共前驱体可以制备得到表面带-CH3基团的SiO2溶胶,SiO2溶胶在老化过程中纳米SiO2粒子由于自组装作用形成草莓状微米-纳米双微观结构,这种结构赋予SiO2涂膜表面不同等级的粗糙度,使得水滴与涂膜表面接触时能够形成高的空气捕捉率和较小的粗糙度因子,与SiO2表面疏水性的-CH3基团共同作用形成类荷叶超疏水结构。     

Thin adsorbed films of a strong cationic polyelectrolyte on silica substrates

The adsorption of poly(diallyldimethyl ammonium chloride) (DADMAC) on planar silica substrates was examined as a function of ionic strength and pH. The study was carried out with reflectometry in an impinging-jet cell and complemented by atomic force microscopy (AFM) and ellipsometry investigations. The adsorption process is initially transport limited, whereby the adsorption rate increases somewhat with increasing ionic strength. This effect is caused by a simultaneous decrease of the hydrodynamic radius of the polymer. After a transient period, the adsorption process saturates and leads to an adsorption plateau. The plateau value increases strongly with increasing ionic strength. This increase can be explained by progressive screening of the electrostatic repulsion between the adsorbing polyelectrolyte chains, as can be rationalized by a random sequential adsorption (RSA) model. The adsorbed amount further increases with increasing pH, and this effect is probably caused by the corresponding increase of the surface charge of the silica substrate.     

Investigation of the effect of dual-size coatings on the hydrophobicity of cotton surface

A series of silica nanoparticles with two different length scales were introduced onto the cotton substrates to study the correlation between the surface structure and observed hydrophobicity. SiO₂ nanoparticles of 7, 12, 20, and 40 nm in size were individually functionalized using 3-aminopropyl triethoxysilane or 3-glycidoxypropyltrimethoxysilane. Amino functionalized silica nanoparticles were durably attached to the cotton surface that was previously treated with epichlorohydrin. By depositing an additional layer of epoxy modified silica nanoparticles, a dual-size hierarchical coating was obtained. It was found that the order of deposition of particles to develop dual-size coatings determines the surface roughness, hydrophobicity and the amount of silica loaded on the cotton substrate. Deposition of the bigger nanoparticles on top of smaller ones resulted in rougher surfaces, higher hydrophobicity and higher amount of silica loading onto the cotton surface. A strong correlation between the size ratio of deposited nanoparticle combinations and the amount of silica loading was observed. It was found that there is also a direct relationship between the surface roughness and the hydrophobicity of the samples generated. Based upon these correlations, it is now possible to tune surface roughness and subsequent wettability by controlling the sizes of the dual-type nanoparticle layers.     

Amination of surfaces via self-assembly of dopamine

Catechols can strongly bind to a variety of substrates so as to functionalize the target surfaces by forming self-assembled monolayer. However, catecholic amine might self-oxidize and polymerize at high pH since the amine is susceptible to nucleophilic addition reaction that results in polymerized oligomers on surfaces. Therefore, the availability of amines for further derivation reaction would be restricted to a large extent. Herein, by controlling pH values to avoid self-oxidative polymerization, dopamine (DA) forms thin and surface-adherent monolayers onto a wide range of inorganic and organic materials, including mica, silica, and Au surface, allowing amination of the surfaces that resemble commercially used aminosilanization. The self-assembly process was traced by surface topography and elemental composition analysis using atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), and electrochemical characterization (electrochemical impedance spectroscopy and cyclic voltammetry measurements). Then, the aminated surfaces were used for secondary derivation reactions to create a variety of ad-layers, including patterned streptavidin through specific binding interaction with biotin and ferrocene surface via amidation reaction. The surface and interface properties of the obtained surfaces were tested by electrochemical measurements.     

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.     

Surface charge microscopy: novel technique for mapping charge-mosaic surfaces in electrolyte solutions

The effective surface potential, called the zeta potential, is commonly determined from electrophoretic mobility measurements for particles moving in a solution in response to an electric field applied between two electrodes. The situation can be reversed, with the solution being forced to flow through a plug of packed particles, and the streaming potential of the particles can be calculated. A significant limitation of these electrokinetic measurements is that only an average value of the zeta potential/streaming potential is measured--regardless of whether the surface charge distribution is homogeneous or otherwise. However, in real-world situations, nearly all solids (and liquids) of technological significance exhibit surface heterogeneities. To detect heterogeneities in surface charge, analytical tools which provide accurate and spatially resolved information about the material surface potential--particularly at microscopic and submicroscopic resolutions--are needed. In this study, atomic force microscopy (AFM) was used to measure the surface interaction forces between a silicon nitride AFM cantilever and a multiphase volcanic rock. The experiments were conducted in electrolyte solutions with different ionic strengths and pH values. The colloidal force measurements were carried out stepwise across the boundary between adjacent phases. At each location, the force-distance curves were recorded. Surface charge densities were then calculated by fitting the experimental data with a DLVO theoretical model. Significant differences between the surface charge densities of the two phases and gradual transitions in the surface charge density at the interface were observed. It is demonstrated that this novel technique can be applied to examine one- and two-dimensional distributions of the surface potential.     

Mucin at solution/air and solid/solution interfaces

In this paper the surface activity of protein mucin at solution/air interface has been studied. The experiments of the adsorbed protein at solution/air interface have been carried out with a range of protein concentrations at a defined pH. The adsorption of the protein to solid surfaces and the degree of hydrophobicity at solid/solution interface of mucin have been evaluated at different pH and in the presence of Hofmeister electrolyte. The results from these studies have been further substantiated by surface potential measurements of mucin covered surface on stainless steel. Quartz crystal microbalance (QCM) has been used to follow the protein adsorption kinetics from solution to solid surface. The results from these measurements show that the adsorption behavior has a remarkable dependence on the degree of maximum coverage and is almost independent of the ionic strength. Other characteristic features such as maximum adsorption values at the protein isoelectric point (IEP4.7) and low-affinity isotherms that showed surface saturation even under unfavorable electrostatic conditions have been observed. The amount of mucin adsorbed in the presence of electrolytes has been estimated using electron spectroscopy for chemical analysis (ESCA). The study clearly shows that there exists an inverse relationship between the hydrophobicity and surface tension of the protein and also on the hydrated radius of Hofmeister electrolyte used.     

AFM study of forces between silica, silicon nitride and polyurethane pads

Interaction of silica and silicon nitride with polyurethane surfaces is rather poorly studied despite being of great interest for modern semiconductor industry, e.g., for chemical-mechanical planarization (CMP) processes. Here we show the results from the application of the atomic force microscopy (AFM) technique to study the forces between silica or silicon nitride (AFM tips) and polyurethane surfaces in aqueous solutions of different acidity. The polyurethane surface potentials are derived from the measured AFM data. The obtained potentials are in rather good agreement with measurements of zeta-potentials using the streaming-potentials method. Another important parameter, adhesion, is also measured. While the surface potentials of silica are well known, there are ambiguous results on the potentials of silicon nitride that is naturally oxidized. Deriving the surface potential of the naturally oxidized silicon nitride from our measurements, we show that it is not oxidized to silica despite some earlier published expectations.     

Particle interactions in kaolinite suspensions and corresponding aggregate structures

The surface charge densities of the silica face surface and the alumina face surface of kaolinite particles, recently determined from surface force measurements using atomic force microscopy, show a distinct dependence on the pH of the system. The silica face was found to be negatively charged at pH>4, whereas the alumina face surface was found to be positively charged at pH<6, and negatively charged at pH>8. The surface charge densities of the silica face and the alumina face were utilized in this study to determine the interaction energies between different surfaces of kaolinite particles. Results indicate that the silica face-alumina face interaction is dominant for kaolinite particle aggregation at low pH. This face-face association increases the stacking of kaolinite layers, and thereby promotes the edge-face (edge-silica face and edge-alumina face) and face-face (silica face-alumina face) associations with increasing pH, and hence the maximum shear-yield stress at pH 5-5.5. With further increase in pH, the face-face and edge-face association decreases due to increasing surface charge density on the silica face and the edge surfaces, and decreasing surface charge density on the alumina face. At high pH, all kaolinite surfaces become negatively charged, kaolinite particles are dispersed, and the suspension is stabilized. The face-face association at low pH has been confirmed from cryo-SEM images of kaolinite aggregates taken from suspension which show that the particles are mostly organized in a face-face and edge-face manner. At higher pH conditions, the cryo-SEM images of the kaolinite aggregates reveal a lower degree of consolidation and the edge-edge association is evident.     

Simultaneous tailoring of surface topography and chemical structure for controlled wettability

Wettability was controlled in a rational manner by individually and simultaneously manipulating surface topography and surface chemical structure. The first stage of this research involved the adsorption of charged submicrometer polystyrene latex particles to oppositely charged poly(ethylene terephthalate) (PET) film samples to form surfaces with different topographies/roughness; adsorption time, solution pH, solution ionic strength, latex particle size, and substrate charge density are external variables that were controlled. The introduction of discrete functional groups to smooth and rough surfaces through organic transformations was carried out in the second stage. Amine groups (-NH(2)) and alcohol groups (-OH) were introduced onto smooth PET surfaces by amidation with poly(allylamine) and adsorption with poly(vinyl alcohol) (PVOH), respectively. On latex particle adsorbed surfaces, a thin layer of gold was evaporated first to prevent particle redistribution before chemical transformation. Reactions with functionalized thiols and adsorption with PVOH on patterned gold surfaces successfully enhanced surface hydrophobicity and hydrophilicity. Particle size and biomodal particle size distribution affect both hydrophobicity and hydrophilicity. A very hydrophobic surface exhibiting water contact angles of 150 degrees /126 degrees (theta(A)/theta(R)) prepared by adsorption of 1-octadecanethiol and a hydrophilic surface with water contact angles of 18 degrees /8 degrees (theta(A)/theta(R)) prepared by adsorption of PVOH were prepared on gold-coated surfaces containing both 0.35 and 0.1 microm latex particles. The combination of surface topography and surface-chemical functionality permits wettability control over a wide range.     

Nonlinear friction characteristics between silica surfaces in high pH solution

Molecular-scale characteristics of friction forces between silica particles and silica wafers in aqueous solutions of the normal (pH 5.6) and high pH (pH 10.6) are investigated, using the lateral force measuring procedure of the atomic force microscope (AFM). Various significant differences of friction characteristics between solutions of normal and high pH's are found. In the case of solutions of normal pH, the friction force increases linearly with increasing loading force, as the Amonton's law for solid bodies indicates. However, in the case of high pH solutions, the increasing rate with the loading force is considerably reduced in the low loading region, but the value increases abruptly above a critical loading force to overcome the magnitude of friction force of normal pH above the region of very high loading. It is very interesting to know that this nonlinear force curve at high pH is independent of the atomic-scale roughness of surfaces, although the magnitude of friction is greatly influenced by the roughness in the case of normal pH. The reason why the friction at high pH is independent of the surface roughness is postulated to be due to the hairy-like layer formed on the silica surface. The existence of hairy-like layers at high pH is proven directly by the dynamic method of normal force measurements with AFM and the thickness is estimated to be at least ca. 1.3 nm.     

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) as a probe of macromolecule adsorption kinetics at functionalized interfaces

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been employed to study the interfacial adsorption kinetics of coumarin-tagged macromolecules onto a range of functionalized planar surfaces. Such studies are valuable in designing polymers for complex systems where the degree of interaction between the polymer and surface needs to be tailored. Three tagged synthetic polymers with different functionalities are examined: poly(acrylic acid) (PAA), poly(3-sulfopropyl methacrylate, potassium salt) (PSPMA), and a mannose-modified glycopolymer. Adsorption transients at the silica/water interface are found to be characteristic for each polymer, and kinetics are deduced from the initial rates. The chemistry of the adsorption interfaces has been varied by, first, manipulation of silica surface chemistry via the bulk pH, followed by surfaces modified by poly(L-glutamic acid) (PGA) and cellulose, giving five chemically different surfaces. Complementary atomic force microscopy (AFM) imaging has been used for additional surface characterization of adsorbed layers and functionalized interfaces to allow adsorption rates to be interpreted more fully. Adsorption rates for PSPMA and the glycopolymer are seen to be highly surface sensitive, with significantly higher rates on cellulose-modified surfaces, whereas PAA shows a much smaller rate dependence on the nature of the adsorption surface.     

Atomic force microscopy measurement of heterogeneity in bacterial surface hydrophobicity

The structure and physicochemical properties of microbial surfaces at the molecular level determine their adhesion to surfaces and interfaces. Here, we report the use of atomic force microscopy (AFM) to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, and to directly correlate the results in the AFM force-distance curves to the macroscopic properties of the microbial surfaces. The surfaces of two bacterial species, Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c, showing different macroscopic surface hydrophobicity were probed with chemically functionalized AFM tips, terminating in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the alternating current mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneous surface morphology was directly correlated with differences in adhesion forces as revealed by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM force curves for both bacterial species showed differences in the interactions of extracellular structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 showed an irregular pattern with multiple adhesion peaks suggesting the presence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c exhibited long-range attraction forces and single rupture events suggesting a more hydrophobic and smoother surface. The adhesion force measurements indicated a patchy surface distribution of interaction forces for both bacterial species, with the highest forces grouped at one pole of the cell for R. erythropolis 20S-E1-c and a random distribution of adhesion forces in the case of A. venetianus RAG-1. The magnitude of the adhesion forces was proportional to the three-phase contact angle between hexadecane and water on the bacterial surfaces.