The model of hydrophobic attraction in the framework of classical DLVO forces

Monitoring effects of hydrodynamic cavitation pretreatment of sodium oleate on the aggregation of fine diaspore particles through small-angle laser scattering

Hydrodynamic cavitation (HC) enhanced fine particle aggregation could be largely due to the generation of tiny bubbles and their role in bridging particles. However, the lack of adequate characterizations of aggregates severally limits our further understanding of the associated aggregation behaviors. In this study, the aggregation of fine diaspore particles was comparatively investigated in sodium oleate (NaOl) solutions with and without HC pretreatment through the small-angle laser scattering (SALS) technique in a shear-induced aggregation (SIA) system. Results showed that HC pretreatment caused the formation of bulk nanobubbles (BNBs), which significantly modified the particle interactions and thereby modified the size and mass fractal dimension (Df) of aggregates under different SIA conditions. Although HC pretreatment did not noticeably alter the gradual change trend of aggregate size and structure characteristics under specific variables, BNBs bridging facilitated the aggregation process towards the diffusion-limited cluster aggregation model, resulting in the formation of larger but looser aggregates. This effect was more pronounced under relatively high NaOl concentrations. Apart from BNBs, the aggregation was also affected by cavitation bubbles formed during shear cavitation, which was more significant under high stirring intensity conditions (i.e., 1800 rpm) than the low stirring intensity conditions (i.e., 600 rpm).

Biochemical effects of copper nanomaterials in human hepatocellular carcinoma (HepG2) cells

In dose-response and structure-activity studies, human hepatic HepG2 cells were exposed for 3 days to nano Cu, nano CuO or CuCl2 (ions) at doses between 0.1 and 30 ug/ml (approximately the no observable adverse effect level to a high degree of cytotoxicity). Various biochemical parameters were then evaluated to study cytotoxicity, cell growth, hepatic function, and oxidative stress. With nano Cu and nano CuO, few indications of cytotoxicity were observed between 0.1 and 3 ug/ml. In respect to dose, lactate dehydrogenase and aspartate transaminase were the most sensitive cytotoxicity parameters. The next most responsive parameters were alanine aminotransferase, glutathione reductase, glucose 6-phosphate dehydrogenase, and protein concentration. The medium responsive parameters were superoxide dismutase, gamma glutamyltranspeptidase, total bilirubin, and microalbumin. The parameters glutathione peroxidase, glutathione reductase, and protein were all altered by nano Cu and nano CuO but not by CuCl2 exposures. Our chief observations were (1) significant decreases in glucose 6-phosphate dehydrogenase and glutathione reductase was observed at doses below the doses that show high cytotoxicity, (2) even high cytotoxicity did not induce large changes in some study parameters (e.g., alkaline phosphatase, catalase, microalbumin, total bilirubin, thioredoxin reductase, and triglycerides), (3) even though many significant biochemical effects happen only at doses showing varying degrees of cytotoxicity, it was not clear that cytotoxicity alone caused all of the observed significant biochemical effects, and (4) the decreased glucose 6-phosphate dehydrogenase and glutathione reductase support the view that oxidative stress is a main toxicity pathway of CuCl2 and Cu-containing nanomaterials.

Revealing the sorption mechanisms of carbamazepine on pristine and aged microplastics with extended DLVO theory

The co-occurrence of microplastics (MPs) and organic contaminants in aquatic environment can complexify their environmental fate via sorption interactions, especially when the properties of MPs can even vary due to the aging effect. Thus, quantitatively clarifying the sorption mechanisms is required to understand their environmental impacts. This study selected popularly occurring carbamazepine (CBZ) and four types of MPs as model systems, including polyethylene, polyvinyl chloride, polyethylene terephthalate and polystyrene in their pristine and aged forms, to investigate the sorption isotherms, kinetics, and desorption. The variation of MPs during the aging process were analyzed with scanning electron microscopy, contact angle, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. It was found that the aging process elevated the sorption capacity and intensified the desorption hysteresis of CBZ on MPs via increasing the surface roughness, decreasing the particle size, and altering the surficial chemistry of all MPs. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was innovatively applied hereby to calculate the interfacial free energies and revealed that the hydrophobic interaction was significantly lessened after aging for all MPs with the slightly enhanced van der Waals interaction. Then the total interfacial free energies were dropped down for all MPs, which resulted in their declined specific sorption capacity. This work reveals the sorption mechanisms of CBZ on pristine and aged MPs with XDLVO and provides a useful reference to study the sorption of other neutral organics onto MPs.

On the size-regulation of RNA-loaded lipid nanoparticles synthesized by microfluidic device

The use of lipid nanoparticles (LNPs) for nucleic acid delivery is now becoming a promising strategy with a number of clinical trials as vaccines or as novel therapies against a variety of genetic and infectious diseases. The use of microfluidics for the synthesis of the LNPs has attracted interest because of its considerable advantages over other conventional synthetic methods including scalability, reproducibility, and speed. However, despite the potential usefulness of large particles for nucleic acid delivery to dendritic cells (DCs) as a vaccine, the particle size of the LNPs prepared using microfluidics is typically limited to approximately from 30 to 100 nm. In this study, focusing on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the effect of some synthetic parameters, including total flow rate, flow rate ratio, buffer pH, lipid concentration, molar ratio of PEG-lipid as well as salt concentration, on particle size was systematically examined by means of the design of experiment approaches. The findings indicated that the simple addition of salt (e.g. NaCl) to a buffer containing nucleic acids contributed greatly to the synthesis of large LNPs over 200 nm and this effect was concentration-dependent with respect to the salt. The effect of salt on particle size was consistent with a Hofmeister series. The systemic injection of larger mRNA-loaded LNPs resulted in a higher transgene expression in mouse splenic DCs, a higher activation of various splenic immune cells, and had a superior effect as a therapeutic cancer vaccine in a syngeneic mouse model compared to the smaller-sized counterpart with constant lipid composition prepared with lower NaCl concentration. Collectively, size-regulation by the simple addition of salt is a promising strategy for developing potent LNPs.

Application of a Superplasticizer in Scheelite Selective Flotation from Calcite

The superplasticizer sulfonated acetone formaldehyde condensate (SAF) was first used as a depressant in the selective flotation of scheelite and calcite in this paper. First, single mineral flotation tests were performed to compare the depressing performance of SAF and the traditional depressant water glass. Results showed that both −0.074 + 0.037 mm and −0.037 mm particles could be well treated by SAF with more satisfactory results than that of water glass. Contact angle test results showed that SAF could amplify the wettability difference between scheelite and calcite surfaces. Then, the effect of SAF on the surface electrical properties of scheelite and calcite was studied by zeta-potential tests. SAF could negatively charge the calcite surface before adding the collector and hinder the subsequent adsorption of the collector while having little effect on the scheelite. Moreover, microscopic analysis of slurry suspensions showed that SAF could prevent calcite particles from forming hydrophobic agglomerates to achieve the dispersion of gangue minerals. Finally, the selective adsorption effect of the SAF on the calcite and scheelite surfaces was studied by surface analysis using FTIR (Fourier transform infrared) and XPS (X-ray photoelectron spectroscopy). Results showed that Ca2+ on the calcite surface was the main adsorption site for the chemisorption of sulfonic acid groups in SAF.

Electrokinetic potential reduction of fine particles induced by gas nucleation

Electrokinetic potential of particles has been extensively studied in colloidal systems over the past century, while up to date, the influence of gas on electrokinetic behaviors of particles has not been fully understood yet. In this study, the electrokinetic response of particles to gas nucleation was systematically investigated with coal as the object. The results showed that the nucleation of gas (both on particle surfaces and in water) significantly changed the particle' electrokinetic behaviors. Higher gas content and particle's surface hydrophobicity normally trigger more intensive gas nucleation, thus inducing more significant reduction of particle zeta potential. After gas nucleation, numerous nanobubbles (NBs) appear in the suspensions mainly in two forms: NBs adhering onto solid surfaces (ANBs) and NBs stagnating in bulk solutions (BNBs). ANBs not only enhance the surface heterogeneity, but also cause the "steric hindrance" effect, and electric double layer (EDL) overlapping and associated ions shielding towards charged particles, which significantly decrease their electrokinetic potentials. Although BNBs can also reduce the zeta potential of particles by EDL compressing, their functions are rather limited.

Biochemical effects of some CeO2, SiO2, and TiO2 nanomaterials in HepG2 cells

The potential mammalian hepatotoxicity of nanomaterials was explored in dose-response and structure-activity studies in human hepatic HepG2 cells exposed to between 10 and 1000 μg/ml of five different CeO₂, three SiO₂, and one TiO₂-based particles for 3 days. Various biochemical parameters were then evaluated to study cytotoxicity, cell growth, hepatic function, and oxidative stress. Few indications of cytotoxicity were observed between 10 and 30 μg/ml. In the 100 to 300 μg/ml exposure range, a moderate degree of cytotoxicity was often observed. At 1000 μg/ml exposures, all but TiO₂ showed a high degree of cytotoxicity. Cytotoxicity per se did not seem to fully explain the observed patterns of biochemical parameters. Four nanomaterials (all three SiO₂) decreased glucose 6-phosphate dehydrogenase activity with some significant decreases observed at 30 μg/ml. In the range of 100 to 1000 μg/ml, the activities of glutathione reductase (by all three SiO₂) and glutathione peroxidase were decreased by some nanomaterials. Decreased glutathione concentration was also found after exposure to four nanomaterials (all three nano SiO₂ particles). In this study, the more responsive and informative assays were glucose 6-phosphate dehydrogenase, glutathione reductase, superoxide dismutase, lactate dehydrogenase, and aspartate transaminase. In this study, there were six factors that contribute to oxidative stress observed in nanomaterials exposed to hepatocytes (decreased glutathione content, reduced glucose 6-phosphate dehydrogenase, glutathione reductase, glutathione peroxidase, superoxide dismutase, and increased catalase activities). With respect to structure-activity, nanomaterials of SiO₂ were more effective than CeO₂ in reducing glutathione content, glucose 6-phosphate dehydrogenase, glutathione reductase, and superoxide dismutase activities.

Interpretation of Hydrophobization Behavior of Dodecylamine on Muscovite and Talc Surface through Dynamic Wettability and AFM Analysis

In this study, a new approach, “dynamic wettability”, and atomic force microscopy (AFM) imaging analysis techniques were successfully used to characterize the hydrophobization mechanism of the collector dodecylamine (DDA) on muscovite and talc surfaces. The attachment of bubbles to the minerals was studied through the dynamic contact angle to gain a detailed understanding of the hydrophobization mechanism of DDA on a muscovite and talc surface. AFM imaging and interaction forces were performed to explain the DDA adsorption mechanism on both minerals. Finally, flotation tests were performed to verify the effectiveness of these techniques. After treatments with DDA, the contact angles became much larger compared to initial angles, particularly for muscovite, and the attachment of bubbles on the talc surface was much easier than muscovite due to its natural hydrophobicity. From AFM imaging, both the muscovite and talc showed a similar tendency; the higher the DDA concentration, the more the adsorbed amount. However, the adsorbed amount of DDA on talc surface was obviously more than that on muscovite. As far as interaction forces are concerned, the maximum attractions occurred at certain different concentrations respectively for muscovite and talc and agreed well with the AFM-imaging results. Moreover, results obtained from flotation tests were promising and quite in agreement with the phenomenon of these techniques.

The hydrophobic force for bubble–particle attachment in flotation – a brief review

Both exponential decay and power decay laws could be employed to quantitatively describe the hydrophobic force between bubble and particle.

The filamentous fungal pellet and forces driving its formation

Filamentous fungi play an important role not only in the bio-manufacturing of value-added products, but also in bioenergy and environmental research. The bioprocess manipulation of filamentous fungi is more difficult than that of other microbial species because of their different pellet morphologies and the presence of tangled mycelia under different cultivation conditions. Fungal pellets, which have the advantages of harvest ease, low fermentation broth viscosity and high yield of some proteins, have been used for a long time. Many attempts have been made to establish the relationship between pellet and product yield using quantitative approaches. Fungal pellet formation is attributed to the combination of electrostatic interactions, hydrophobicity and specific interactions from spore wall components. Electrostatic interactions result from van der Waals forces and negative charge repulsion from carboxyl groups in the spore wall structure. Electrostatic interactions are also affected by counter-ions (cations) and the physiologic conditions of spores that modify the carboxyl groups. Fungal aggregates are promoted by the hydrophobicity generated by hydrophobins, which form a hydrophobic coat that covers the spore. The specific interactions of spore wall components contribute to spore aggregation through salt bridging. A model of spore aggregation was proposed based on these forces. Additionally, some challenges were addressed, including the limitations of research techniques, the quantitative determination of forces and the complex information of biological systems, to clarify the mechanism of fungal pellet formation.

Thickness-Dependent Hydrophobicity of Epitaxial Graphene

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.

Progress on the Surface Nanobubble Story: What is in the bubble? Why does it exist?

Interfaces between aqueous solutions and hydrophobic solid surfaces are important in various areas of science and technology. Many researchers have found that forces between hydrophobic surfaces in aqueous solution are significantly different from the classical DLVO theory. Long-range attractive forces (non-DLVO forces) are thought to be affected by nanoscopic gaseous domains at the interfaces. This is a review of the latest research on nanobubbles at hydrophobic surfaces from experimental and simulation studies. The review focusses on non-intrusive optical view of surface nanobubbles and gas enrichment on solid surfaces by imaging and force mapping. By use of these recent experimental data in conjunction with molecular simulation work, all major theories on surface nanobubble formation and stability are critically reviewed. Even though the current body of research cannot comprehensively explain all properties of surface nanobubbles observed, the fundamental understanding has been significantly improved. Line tension has been shown to be incapable of explaining the contact angle of nanobubbles. Dense gas layer theory provides a new explanation on both large contact angle and long-time stability. The high density of gas in these domains may significantly affect the gas-water interface which is in line with some observation made on bulk nanobubbles. Along this line of inquiry, experimental and simulation effort should be focussed on measuring the density within surface nanobubbles and the properties of the gas water interface which may be the key to explaining the stability of these nanobubbles.

Interaction mechanism between hydrophobic and hydrophilic surfaces: using polystyrene and mica as a model system

The interactions between hydrophobic and hydrophilic molecules, particles, or surfaces occur in many biological phenomena and industrial processes. In this work, polystyrene (PS) and mica were chosen as a model system to investigate the interaction mechanism between hydrophilic and hydrophobic surfaces. Using a surface forces apparatus (SFA) coupled with a top-view optical microscope, interaction forces between PS and mica surfaces were directly probed in five different electrolyte solutions (i.e., NaCl, CaCl2, NaOH, HCl, and CH3COOH) of various concentrations. Long-range repulsion was observed in low electrolyte concentration (e.g., 0.001 M) which was mainly due to the presence of microscopic and submicroscopic bubbles on PS surface. A modified Derjaguin-Landau-Verwey-Overbeek (DLVO) theory well fits the interaction forces by taking into account the effect of bubbles on PS surface. The range of the repulsion was dramatically reduced in 1.0 M solutions of NaCl, CaCl2, and NaOH but did not significantly change in 1.0 M HCl and CH3COOH, which was due to ion specificity effect on the formation and stability of bubbles on PS surface. The range of repulsion was also significantly reduced to <20 nm in degassed electrolyte solutions. UV-ozone treatment changed the hydrophobic attraction of the untreated PS-PS system to pure repulsion between untreated PS and treated PS, demonstrating the important role of surface hydrophobicity on the formation and stability of bubbles on substrates. Our results indicate that DLVO forces dominate the interaction between hydrophilic surface (i.e., mica) and hydrophobic polymer (i.e., PS), while the types of electrolytes (ion specificity), electrolyte concentration, degassing, and surface hydrophobicity can significantly affect the formation and stability of bubbles on the interacting surfaces, thus affecting the range and magnitude of the interaction forces.

Influence of hydrophobic effects on streaming potential

We study the influence of hydrophobic effects on streaming potential mediated flow through a narrow confinement. In a clear departure from the approach used in prior works, we use a phase-field model to capture the hydrophobicity-induced depletion in the near wall region, and express the variation of viscosity and permittivity across the interfacial layer in terms of the phase-field variable. We then use these in the determination of the flow velocity, and highlight the sensitive interplay between the intrinsic length scale of the electrical double layer and that of the depletion in terms of the variations of an effective normalized viscosity that captures the electroviscous effect. We expect that this work will be an important step forward in the realistic continuum modeling of interfacial physics in the particular context of streaming potential mediated flows.

Electrophoretic mobility of latex nanospheres in electrolytes: Experimental challenges

The electrophoretic mobility of sulfate latex nanospheres (radius 300 ± 10 nm) was measured as a function of ionic strength for different salts. The results were obtained from two similar instruments (Malvern ZetaSizer 3000 HSa and Malvern ZetaSizer Nano) using the same dispersions, in the same conditions. The values predicted from the standard electrokinetic model for constant surface charge were in good agreement with the data over a large range of ionic strength. The influence of the protocol used to fill the cells appears to be of importance between 1–10 mM of added monovalent salt. There, the capillary wall properties seem to influence the electrophoretic measurements, even at fast field reversal (FFR), where electroosmosis should be absent. We found that during a series of measurements with monovalent salts, it was best to fill the cell starting from high ionic strength and decreasing the ionic strength during the series. The measurements with divalent salts were not sensitive to the filling procedure.

Rupture of thin liquid films induced by impinging air-jets

Thin liquid films on partially wetting substrates are subjected to laminar axisymmetric air-jets impinging at normal incidence. We measured the time at which film rupture occurs and dewetting commences as a function of diameter and Reynolds number of the air-jet. We developed numerical models for the air flow as well as the height evolution of the thin liquid film. The experimental results were compared with numerical simulations based on the lubrication approximation and a phenomenological expression for the disjoining pressure. We achieved quantitative agreement for the rupture times. We found that the film thickness profiles were highly sensitive to the presence of minute quantities of surface-active contaminants.

Consistent description of electrohydrodynamics in narrow fluidic confinements in the presence of hydrophobic interactions

Electrohydrodynamics in the presence of hydrophobic interactions in narrow confinements is traditionally represented from a continuum viewpoint by a Navier slip-based conceptual paradigm, in which the slip length carries the sole burden of incorporating the effects of substrate wettability on interfacial electromechanics, precluding any explicit dependence of the interfacial potential distribution on the substrate wettability. Here we show that this traditional way of treating electrokinetics-wettability coupling may lead to serious discrepancies in predicting the resultant transport characteristics as manifested through an effective zeta potential. We suggest that an alternative consistent description of the underlying physics through a free-energy-based formalism, in conjunction with considerations of hydrodynamic and electrical property variations consistent with the pertinent phase-field description, may represent the underlying consequences in a more rational manner, as compared to the traditional slip-based model coupled with a two-layer description. Our studies further reveal that the above discrepancies may not occur solely due to the slip-based route of representing the interfacial wettability, but may be additionally attributed to the act of "discretizing" the interfacial phase fraction distribution through an artificial two-layer route.