Humic substance is a ubiquitous class of natural organic matter (NOM) in soil and aquatic ecosystems, which severely affects the terrestrial and aquatic environments as well as water-based ...engineering systems by adsorption on solids (e.g., soil minerals, nanoparticles, membranes) via different interaction mechanisms. Herein, the chemical force microscopy (CFM) technique was employed to quantitatively probe the intermolecular forces of humic acid (HA, a representative humic substance) interacting with self-assembled monolayers (SAMs, i.e., OH-SAMs, CH3-SAMs, NH2-SAMs and COOH-SAMs) in various aqueous environments at the nanoscale. The interaction forces measured during approach could be well fitted by the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory by incorporating the hydrophobic interaction. The average adhesion energy followed the trend as: NH2-SAMs (∼3.11 mJ/m2) > CH3-SAMs (∼2.03 mJ/m2) > OH-SAMs (∼1.38 mJ/m2) > COOH-SAMs (∼0.52 mJ/m2) in 100 mM NaCl at pH 5.8, indicating the significant role of electrostatic attraction in contributing to the HA adhesion, followed by hydrophobic interaction and hydrogen bonding. The adhesion energy was found to be dependent on NaCl concentration, Ca2+ addition and pH. For the interaction between NH2-SAMs and HA, their electrostatic attraction at pH 5.8 turned to repulsion under alkaline condition which led to the sudden drop of adhesion energy. Such results promised the adsorption and release of HA using the recyclable magnetic Fe3O4 nanoparticles coated with (3-aminopropyl)tiethoxysilane (APTES). This work provides quantitative information on the molecular interaction mechanism underlying the adsorption of HA on solids of varying surface chemistry at the nanoscale, with useful implications for developing effective chemical additives to remove HA in water treatment and many other engineering processes.
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•Intermolecular forces of humic acid (HA) with self-assembled monolayers (SAMs) are quantified.•HA-SAMs adhesion energy is dependent on ionic strength, ion type and pH.•Electrostatic attraction plays the dominant role, followed by hydrophobic force and H-bonding.•PH responsive adhesion energy of NH2-SAMs facilitates the adsorption and release of HA.•Recyclable APTES/Fe3O4 nanoparticles with –NH2 groups promise excellent HA removal.
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•Interaction between undecane and graphite surface was studied by experiment and simulation.•DLVO and extend DLVO theories were used to describe the forces.•Hydrophobic force was ...observed on hydrophobic surface by chemical force microscopy.•Adhesion force increases with increasing surface hydrophobicity.•Hydration film formed on hydrophilic surface, while a water-depletion layer exists on the hydrophobic surface.
Collectors are often used to increase the hydrophobicity of valuable minerals during flotation. Hence, it is necessary to know the forces between them, especially the hydrophobic force, which closely associated with hydrophobic minerals floating, such as graphite, coal, and molybdenite. In the present study, graphite sheets were used as the hydrophobic mineral, and the force characteristics and interfacial adsorption structures of undecane (a model collector) on graphite surface with different hydrophobicities were investigated by chemical force microscopy and molecular dynamics simulations. Undecane experiences repulsive interactions as it approaches hydrophilic graphite; however, an obvious jump-in phenomenon driven by hydrophobic force was observed for hydrophobic graphite, which triggers their adhesion. Derjaguin–Landau–Verwey–Overbeek (DLVO) and extended DLVO fitting reveal that the hydrophobic force decays at 1.35 nm in a single-exponential manner. The adhesion force during retraction increases with increasing surface hydrophobicity. The hydrophilic surface adsorbs a large amount of water to form a dense and ordered hydration film that interferes with the adsorption of undecane, while a water-depletion layer exists on the hydrophobic surface with closely adsorbed undecane molecules. This study improves our understanding of the action mechanism of flotation collectors for hydrophobic minerals.
This article addresses the much debated question whether the degree of hydrophobicity of single-layer graphene (1LG) is different from that of double-layer graphene (2LG). Knowledge of the water ...affinity of graphene and its spatial variations is critically important as it can affect the graphene properties as well as the performance of graphene devices exposed to humidity. By employing chemical force microscopy with a probe rendered hydrophobic by functionalization with octadecyltrichlorosilane (OTS), the adhesion force between the probe and epitaxial graphene on SiC has been measured in deionized water. Owing to the hydrophobic attraction, a larger adhesion force was measured on 2LG Bernal-stacked domains of graphene surfaces, thus showing that 2LG is more hydrophobic than 1LG. Identification of 1LG and 2LG domains was achieved through Kelvin probe force microscopy and Raman spectral mapping. Approximate values of the adhesion force per OTS molecule have been calculated through contact area analysis. Furthermore, the contrast of friction force images measured in contact mode was reversed to the 1LG/2LG adhesion contrast, and its origin was discussed in terms of the likely water depletion over hydrophobic domains as well as deformation in the contact area between the atomic force microscope tip and 1LG.
A variety of bacterial pathogens use nanoscale protein fibers called type IV pili to mediate cell adhesion, a primary step leading to infection. Currently, how these nanofibers respond to mechanical ...stimuli and how this response is used to control adhesion is poorly understood. Here, we use atomic force microscopy techniques to quantify the forces guiding the adhesion of Pseudomonas aeruginosa type IV pili to surfaces. Using chemical force microscopy and single-cell force spectroscopy, we show that pili strongly bind to hydrophobic surfaces in a time-dependent manner, while they weakly bind to hydrophilic surfaces. Individual nanofibers are capable of withstanding forces up to 250 pN, thereby explaining how they can resist mechanical stress. Pulling on individual pili yields constant force plateaus, presumably reflecting conformational changes, as well as nanospring properties that may help bacteria to withstand physiological shear forces. Analysis of mutant strains demonstrates that these mechanical responses originate solely from type IV pili, while flagella and the cell surface localized and proposed pili-associated adhesin PilY1 play no direct role. We also demonstrate that bacterial–host interactions involve constant force plateaus, the extension of bacterial pili, and the formation of membrane tethers from host cells. We postulate that the unique mechanical responses of type IV pili unravelled here enable the bacteria to firmly attach to biotic and abiotic surfaces and thus maintain attachment when subjected to high shear forces under physiological conditions, helping to explain why pili play a critical role in colonization of the host.
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•Interaction mechanism of hydrocarbon oil was measured by CFM and MD simulations.•RCH3, ROH, RCOOH, and RNH4+ exhibited repulsion when approaching hydrophilic mineral surface.•Polar ...micro-oil droplets are easier to spread on the hydrophilic mineral surface than nonpolar micro-oil droplets.•Polar hydrocarbon oil was effective collector for hydrophilic minerals flotation.
Hydrocarbon oil collectors are typically used in the mineral flotation process to increase the hydrophobicity of the mineral surface. However, the mechanism of action of hydrocarbon oil on hydrophilic mineral surfaces remains unclear. This study innovatively applied chemical force microscopy (CFM) and molecular dynamics (MD) simulations to study the interactions between hydrocarbon oil and hydrophilic mineral surfaces. The CFM results showed that the interactions between RCH3, ROH, RCOOH, and RNH4+ and the hydrophilic mineral surface are always repulsive, and can be well described by Derjaguin–Landau–Verwey–Overbeek theory, which considers both van der Waals and electrostatic forces. The type of functional group significantly influenced the adhesion force between the hydrocarbon oil collector and hydrophilic mineral surface, and the adhesion forces followed the order RNH4+ > ROH > RCOOH > RCH3, indicating that polar hydrocarbon oil molecules adhered strongly to the hydrophilic mineral surface, whereas nonpolar hydrocarbon oil molecules did not adhere to this surface. MD simulation results showed that polar micro-oil droplets easily spread on the hydrophilic surface, while nonpolar micro-oil droplets were difficult to spread. Hydrophilic mineral surfaces preferentially attract water molecules and repel hydrophobic hydrocarbon oils, which reduces the adsorption energy of nonpolar hydrocarbon oils. Polar hydrocarbon oil is an effective collector for increasing the surface hydrophobicity of hydrophilic minerals. The combination of MD and CFM illustrated the mechanism of action of hydrocarbon oil collector molecules, which provides a theoretical reference for mineral flotation separation.
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•The interaction between collector and graphite surface were measured by CFM and MD simulations.•Hydrophobic force dominated the interaction between collector and graphite ...surface.•Nonpolar collector had larger adhesion force and EDLVO force with the graphite surface.•Stronger adsorption was found between nonpolar collector and hydrophobic surface by MD simulation.
Collectors are widely used in flotation to improve the interfacial hydrophobicity of valuable minerals. Clarifying the mechanism of action is therefore a prerequisite for developing new collectors for flotation intensification. In this study, chemical force microscopy was applied to directly characterize the interaction forces between a mineral surface (hydrophobic graphite) and two collector molecules (undecane and polar undecanoic acid). An undecane/undecanoic acid self-assembled monolayer was formed on the Au-coated tip of an atomic force microscopy (AFM) cantilever through chemical deposition. Jump-in phenomena were observed between the collectors and the hydrophobic surface, indicating the existence of an attractive force, which conformed to the EDLVO theories. Various experiments indicated that the interactions between undecane and the hydrophobic surface were stronger than those involving undecanoic acid, indicating that hydrophobic forces dominated the interactions. Using molecular dynamics simulations, the adsorption energies of undecane and undecanoic acid on the graphite surface were determined to be − 17.1 ± 0.3 and − 14.1 ± 0.5 kcal/mol, respectively. The consistency between the force and the adsorption energy results demonstrated that AFM is a credible technique to investigate the interaction mechanism between a mineral and a collector at a molecular level, and so will provide critical guidance for future collector screening and design.
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•A patterning strategy has been developed based on the microcontact printing of ODT and electrografting of aryldiazonium salts.•Well-defined patterned gold surfaces terminated with ...either nitrophenyl (C6H5–NO2) or aminophenyl (C6H5–NH2) functional groups were fabricated.•Nano-FTIR spectroscopy could be applied for the analysis of the patterned surfaces.•pH dependent adhesion forces were studied by chemical force microscopy using COOH-modified AFM tips.•Titration curves revealed the contributions of electrostatic forces.
The aim of this work is the analysis of pH-dependent adhesion on micropatterned electrografted aryldiazonium salts on gold substrates. A PDMS stamp inked with 1-octadecanethiol (ODT) solution was used for microcontact printing. Electrografting of nitrophenyl (C6H5–NO2) occurred in the ODT-free regions. An additional electrochemical reduction step led to aminophenyl (C6H5–NH2) covered surfaces. The corresponding surface chemical states of the pattern were investigated by means of PM-IRRAS and XPS. The local surface structure was microscopically analyzed by means of FE-SEM and AFM as well as nano-FTIR spectroscopy. The ODT covered areas of the patterned substrates allowed for clear distinction between the pH-dependent forces of the nitrophenyl (C6H5–NO2)- and aminophenyl (C6H5–NH2)-terminated layers as measured by chemical force microscopy. COOH-functionalized AFM tips in aqueous electrolytes ranging from pH 5–10 were employed. It could be shown that the aminophenyl (C6H5–NH2) terminated surface led to higher adhesion forces in comparison to the nitrophenyl (C6H5–NO2) group.
This work demonstrates the production of a well-controlled, chemical gradient on the surface of graphene. By inducing a gradient of oxygen functional groups, drops of water and ...dimethyl-methylphosphonate (a nerve agent simulant) are “pulled” in the direction of increasing oxygen content, while fluorine gradients “push” the droplet motion in the direction of decreasing fluorine content. The direction of motion is broadly attributed to increasing/decreasing hydrophilicity, which is correlated to high/low adhesion and binding energy. Such tunability in surface chemistry provides additional capabilities in device design for applications ranging from microfluidics to chemical sensing.
Abstract
Determination of the effect of water stress on the surface properties of bacteria is crucial to study bacterial induced soil water repellency. Changes in the environmental conditions may ...affect several properties of bacteria such as the cell hydrophobicity and morphology. Here, we study the influence of adaptation to hypertonic stress on cell wettability, shape, adhesion, and surface chemical composition of Pseudomonas fluorescens. From this we aim to discover possible relations between the changes in wettability of bacterial films studied by contact angle and single cells studied by atomic and chemical force microscopy (AFM, CFM), which is still lacking. We show that by stress the adhesion forces of the cell surfaces towards hydrophobic functionalized probes increase while they decrease towards hydrophilic functionalized tips. This is consistent with the contact angle results. Further, cell size shrunk and protein content increased upon stress. The results suggest two possible mechanisms: Cell shrinkage is accompanied by the release of outer membrane vesicles by which the protein to lipid ratio increases. The higher protein content increases the rigidity and the number of hydrophobic nano-domains per surface area.
Changes in chemical composition and nanomechanical properties of cell surfaces due to adaptation to hypertonic NaCl stress explain the increase in hydrophobicity measured at nano- and microscale.
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