Hydrophobic Attraction Measured between Asymmetric Hydrophobic Surfaces

Separation of Microplastic Particles from Sewage Sludge Extracts Using Magnetic Seeded Filtration

Microplastic particles (MP) are efficiently retained in wastewater treatment plants and enriched in sewage sludge. For monitoring MP contents in wastewater systems, sewage sludge is thus well suited, but also requires an isolation of MP from the sludge matrix, as other sewage sludge components may interfere with the MP identification and quantification. Although organic matter in sludge samples can be removed through acid and enzymatic digestion procedures, cellulose - mainly from toilet paper - remains in the digests, due to its high chemical resistivity and similar density to MP. We apply the separation concept of magnetic seeded filtration to isolate MP through selective hetero-agglomeration with magnetic seed particles. MP and cellulose differ in their hydrophobic properties and we investigate to what extent these differences can be exploited to selectively form MP-magnetite hetero-agglomerates in the presence of cellulose. These hetero-agglomerates are subsequently separated using a magnet. Five MP types (Polyethylene terephthalate (PET), polypropylene (PP), low density polyethylene (LDPE), polyvinyl chloride (PVC) and polystyrene (PS)) and cellulose particles were mixed in different combinations with both hydrophilic and hydrophobic (silanized) magnetite particles. PET, PP, LDPE and PS only poorly agglomerated with pristine (hydrophilic) magnetite, but efficiently formed hetero-agglomerates with hydrophobic magnetite and were successfully removed from suspensions ( 80 - 100 % ). PVC agglomerated more efficiently with pristine than with hydrophobic magnetite and cellulose only agglomerated to a limited extent with either hydrophilic or hydrophobic magnetite, resulting in a high process selectivity. Results from experiments conducted at different ionic strengths and with hydrophilic and hydrophobic magnetite suggests that the agglomeration process was dominated by hydrophobic interactions. Enzymatic and oxidative treatment of the MP only marginally affected the separation efficiencies and (treated) MP spiked to sewage sludge extracts were successfully recovered using magnetic seeded filtration.

Influence of Thin Film Deposition on AFM Cantilever Tips in Adhesion and Young’s Modulus of MEMS Surfaces

Adhesion is a critical factor in microelectromechanical systems (MEMSs) and is influenced by many parameters. In important fields, such as microassembly, an improved understanding of adhesion can result in higher precision. This study examines the influence of deposition of gold and titanium onto the atomic force microscope (AFM) tips in adhesion forces and Young’s modulus, between a few MEMS substrates (silicon, gold, and silver) and the AFM tips. It was found that, except for gold substrate, an AFM tip coated with gold has the highest adhesion force of 42.67 nN for silicon substrates, whereas the titanium-coated AFM tip decreases the force for all the samples. This study suggests that such changes must be taken into account while studying the adhesion force. The final results indicate that utilizing gold substrate with titanium AFM tip led to the lowest adhesion force, which could be useful in adhesion force measurement during microassembly.

Nanoscopic Analyses of Protein Adsorption on Poly(2-methoxyethyl acrylate) Surfaces for Tailoring Cell Adhesiveness

Regulation of protein adsorption on the surface of biomaterials is important for modulating cell adhesion. Two important proteins in this regard are fibrinogen and fibronectin. Poly(2-methoxyethyl acrylate) (PMEA) and its...

Study of High-Density Polyethylene (HDPE) Kinetics Modification Treated by Dielectric Barrier Discharge (DBD) Plasma

In this work, the plasma was used in the dielectric barrier discharge (DBD) technique for modifying the high-density polyethylene (HDPE) surface. The treatments were performed via argon or oxygen, for 10 min, at a frequency of 820 Hz, voltage of 20 kV, 2 mm distance between electrodes, and atmospheric pressure. The efficiency of the plasma was determined through the triple Langmuir probe to check if it had enough energy to promote chemical changes on the material surface. Physicochemical changes were diagnosed through surface characterization techniques such as contact angle, attenuated total reflection to Fourier transform infrared spectroscopy (ATR-FTIR), X-ray excited photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Plasma electronics temperature showed that it has enough energy to break or form chemical bonds on the material surface, impacting its wettability directly. The wettability test was performed before and after treatment through the sessile drop, using distilled water, glycerin, and dimethylformamide, to the profile of surface tensions by the Fowkes method, analyzing the contact angle variation. ATR-FTIR and XPS analyses showed that groups and bonds were altered or generated on the surface when compared with the untreated sample. The AFM showed a change in roughness, and this directly affected the increase of wettability.

Nanoscale Profiling of 2D Surface Hydrophobicity Recognition of Environmental Media via AFM Measurements In Situ

The surface hydrophobicity and its heterogeneity are essential physicochemical properties of functional materials and environmental media, which directly influence many critical processes, such as the adsorption capacity of absorbents, water/fertilizer retention of soil and oil-water separation performances of membranes. The conventional method to characterize the surface hydrophobicity is based on the water/air/oil contact angle, which could only analyze the macroscale local hydrophobicity of the surfaces. Until now, it is impossible to profile two-dimensional surface hydrophobicity recognition in the nanoscale. Here we utilized an atomic force microscopy (AFM)-based chemical force spectroscopy to measure the topography and the local adhesion forces in the nanoscale. A novel approach is established to exploit adhesion forces to extract the hydrophobic attractions, enabling mapping of the surface hydrophobicity of environmental media in the nanoscale, which was validated by studying synthetic self-assembled monolayers of known composition. The new method was then applied to directly measure the hydrophobicity of porous biochar particles, to profile two-dimensional nanoscale hydrophobicity images of graphene oxide, and to observe the in situ variations of the graphite surface hydrophobicity in the adsorption process of benzylamine, which cannot be monitored by the conventional methods. The advantages of direct observations of the surface hydrophobicity recognition from a single AFM image dynamically and quantitatively may provide an in-depth insight into the surface hydrophobicity in the nanoscale.

Forces between solid surfaces in aqueous electrolyte solutions

This review addresses experimental findings obtained with direct force measurements between two similar or dissimilar solid surfaces in aqueous electrolyte solutions. Interpretation of these measurements is mainly put forward in terms of the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). This theory invokes a superposition of attractive van der Waals forces and repulsive double layer forces. DLVO theory is shown to be extremely reliable, even in the case of multivalent ions. However, such a description is only successful, when appropriate surface charge densities, charge regulation characteristics, and ion pairing or complexation equilibria in solution are considered. Deviations from DLVO theory only manifest themselves at distances of typically below few nm. More long-ranged non-DLVO forces can be observed in some situations, particularly, in concentrated electrolyte solutions, in the presence of strongly adsorbed layers, or for hydrophobic surfaces. The latter forces probably originate from patch-charge surface heterogeneities, which can be induced by ion-ion correlation effects, charge fluctuations, or other types of surface heterogeneities.

Forces between zinc sulphide surfaces; amplification of the hydrophobic attraction by surface charge

Smooth Zinc Sulphide (ZnS) surfaces were prepared by magnetron sputtering and the interaction forces were measured between them as a function of pH. At the isoelectric point (iep) of pH 7.1 the attractive force was well described by the van der Waals interaction calculated using Lifshitz theory for a layered system. Away from the iep, the forces were fitted using DLVO theory extended to account for surface roughness. At pH 9.8 the surfaces acquire a negative charge and an electrostatic repulsion is evident. Below the iep the surfaces acquire a positive charge leading to electrostatic repulsion. The forces in the range 3.8 < pH < 4.8 show an additional attraction on approach and much greater adhesion than at other pH values. This is attributed to the hydrophobic attraction being amplified by a small degree of charge on the surface as has previously been reported for adhesion measurements. The range of the measured forces is attributed to the long-range orientational order of water (>5 nm).

Poly(N-isopropylacrylamide) Response to Salt Concentration and Anion Identity: A Brush-on-Brush Study

The stability of poly(N-isopropylacrylamide) (PNIPAM) brush-modified colloidal silica particles was compared to asymmetric and symmetric PNIPAM brush direct force measurements in the presence of 1, 10, and 500 mM aqueous salt solution of KCl, KNO₃, and KSCN between 10 and 45 °C. Dynamic light scattering measurements highlighted subtle variations in the salt-mediated thermoresponse, while atomic force microscopy (AFM) force curves between a bare silica or PNIPAM brush-modified colloid probe and a planar PNIPAM brush elucidated differences in brush interactions. The AFM force curves in the presence of KCl primarily revealed steric interactions between the surfaces, while KNO₃ and KSCN solutions exhibited electrosteric interactions on approach as a function of the chaotropic nature of the ion and the solution concentration. The symmetric PNIPAM brush interaction highlighted significant variations between KCl and KSCN at 1 and 500 mM concentrations, while the approach and retraction force curves were relatively similar at 10 mM concentration. The combination of these techniques enabled the stability of PNIPAM brush-modified colloidal dispersions in the presence of electrolyte to be better understood with specific ion binding and the solution Debye length playing a significant role.

Measurement of the Attachment Force between an Air Bubble and a Mineral Surface: Relationship between the Attachment Force and Flotation Kinetics

The interaction forces between air bubbles and mineral surfaces were directly measured during the attachment process using an apparatus developed in our laboratory, and they are defined as the attachment forces. The attachment forces were measured between the air bubble and mineral surfaces modified with surfactants to have different hydrophobicities. Chalcopyrite and galena were used as the mineral surfaces, and their hydrophobicity was controlled by adsorbing xanthates with different hydrocarbon chain lengths. The hydrophobicity is represented by the static contact angle of water on the mineral surface. When the static contact angle was less than 90°, the attachment force increased considerably with increasing static contact angle of the surfaces, irrespective of the mineral type or the hydrocarbon chain length of the adsorbed xanthate. The hydrophobicity of the mineral surface is found to be the dominant factor determining the attachment force. The measured attachment forces agree well with those calculated based on the force balance model derived from the capillary force and Laplace pressure equation. Microflotation experiments to examine the relationship between the attachment force and flotation kinetics were carried out under the same conditions to control surface hydrophobicity. The variation in the flotation kinetic constants and attachment forces with the water contact angle are very similar. As a result, the attachment forces measured by the developed apparatus can provide quantitative information on the interaction between an air bubble and the mineral surface and can be used for predicting the flotation kinetics.

Direct Observation of Gas Meniscus Formation on a Superhydrophobic Surface

The formation of a bridging gas meniscus via cavitation or nanobubbles is considered the most likely origin of the submicrometer long-range attractive forces measured between hydrophobic surfaces in aqueous solution. However, the dynamics of the formation and evolution of the gas meniscus is still under debate, in particular, in the presence of a thin air layer on a superhydrophobic surface. On superhydrophobic surfaces the range can even exceed 10 μm. Here, we report microscopic images of the formation and growth of a gas meniscus during force measurements between a superhydrophobic surface and a hydrophobic microsphere immersed in water. This is achieved by combining laser scanning confocal microscopy and colloidal probe atomic force microscopy. The configuration allows determination of the volume and shape of the meniscus, together with direct calculation of the Young-Laplace capillary pressure. The long-range attractive interactions acting on separation are due to meniscus formation and volume growth as air is transported from the surface layer.

Water Interactions with Hydrophobic versus Hydrophilic Nanosilica

It is well-known that interaction of hydrophobic powders with water is weak, and upon mixing, they typically form separated phases. Preparation of hydrophobic nanosilica AM1 with a relatively large content of bound water with no formation of separated phases was the aim of this study. Unmodified nanosilica A-300 and initial AM1 (A-300 completely hydrophobized by dimethyldichlorosilane), compacted A-300 (cA-300), and compacted AM1 (cAM1) containing 50-58 wt % of bound water were studied using low-temperature ¹H NMR spectroscopy, thermogravimetry, infrared spectroscopy, microscopy, small-angle X-ray scattering, nitrogen adsorption, and theoretical modeling. After mechanical activation (∼20 atm) upon stirring of AM1/water mixture at the degree of hydration h = 1.0 or 1.4 g of distilled water per gram of dry silica, all water is bound and the blend has the bulk density of 0.7 g/cm³. The temperature and interfacial behaviors of bound water depend strongly on a dispersion media type (air, chloroform, and chloroform with trifluoroacetic acid (4:1)) because the boundary area between immiscible water and chloroform should be minimal. Water and chloroform molecules are of different sizes affecting their distribution in pores (voids between silica nanoparticles in their aggregates) of different sizes. Structural, morphological, and textural characteristics of silicas, and environmental features affect not only the distribution of bound water, but also the amounts of strongly (frozen at T < 260 K) and weakly (frozen at 260 K < T < 273 K) bound and strongly (chemical shift δ_H = 4-6 ppm) and weakly (δ_H = 1-2 ppm) associated waters. Despite the changes in the characteristics of cAM1, it demonstrates a flotation effect. The developed system with cAM1/bound water could be of interest from a practical point of view due to controlled interactions with aqueous surroundings.

Solvent-mediated interactions between nanostructures: From water to Lennard-Jones liquid

Solvent-mediated interactions emerge from complex mechanisms that depend on the solute structure, its wetting properties, and the nature of the liquid. While numerous studies have focused on the first two influences, here, we compare the results from water and Lennard-Jones liquid in order to reveal to what extent solvent-mediated interactions are universal with respect to the nature of the liquid. Besides the influence of the liquid, the results were obtained with classical density functional theory and brute-force molecular dynamics simulations which allow us to contrast these two numerical techniques.