A novel synthesis method of magnetic Janus particles for wastewater applications.

HYPOTHESIS: Magnetic particles are widely used in many adsorption and removal processes. Among the many types of magnetic colloids, magnetic Janus particles offer significant possibilities for the effective removal of several components from aqueous solutions. Nevertheless, the synthesis of structures integrating different types of materials requires scalable fabrication processes to overcome the limitations of the available methodologies. Herein, we hypothesized a fabrication process for dual-surface functionalized magnetic Janus particles. EXPERIMENTS: The primary silica particles with surface-attached amine groups are further asymmetrically modified by iron oxide nanoparticles, exploiting Pickering emulsion and electroless deposition techniques. The dual surface functionality of the particles is designed for its versatility and demonstrated in two wastewater-related applications. FINDINGS: We show that our design can simultaneously remove chromium (VI) and phenol from aqueous solution. The fabricated magnetic-responsive Janus particles are also an effective adsorbent for genomic Deoxyribonucleic acid (DNA) and show superior performance to commercial magnetic beads. Thus, this study provides a novel platform for designing magnetic Janus particles with multifunctional surfaces for wastewater treatment applications.

Unraveling Water-Based Lubrication with Carbon Dots of Asphaltene Origin.

Despite the lower toxicity of water-based lubricants over nonrenewable petroleum-based analogues, they face challenges in achieving widespread adoption due to low stability and inadequate friction-reduction performance. To address this, a cost-effective nanoadditive is synthesized by expansive oxidation of asphaltenes to create biocompatible asphaltene-derived carbon dots [(ACDs); 5 nm]. These ACDs exhibit excellent water redispersibility, promoting long-term friction reduction and marking the first use of an asphaltene-based system for friction reduction in water or oil. Even at low loadings (0.2-4.0 wt %), ACDs significantly reduce friction on steel surfaces (>54%) with tribofilm stability surpassing pristine carbon dots, typical carbon-based graphene quantum dots, and inorganic nanomaterials (commercial 5 and 20 nm silica). The ACDs' attributes include high negative zeta potential, considerable water uptake, varied functional groups, biocompatibility, and a nanodisc shape conducive to stable tribofilm formation through effective particle stacking. The scalable synthesis, high yield, and impressive water redispersibility of ACDs position them favorably for commercial water-based lubrication.

Scale-Dependent Rheology of Synovial Fluid Lubricating Macromolecules.

Synovial fluid (SF) is the complex biofluid that facilitates the exceptional lubrication of articular cartilage in joints. Its primary lubricating macromolecules, the linear polysaccharide hyaluronic acid (HA) and the mucin-like glycoprotein proteoglycan 4 (PRG4 or lubricin), interact synergistically to reduce boundary friction. However, the precise manner in which these molecules influence the rheological properties of SF remains unclear. This study aimed to elucidate this by employing confocal microscopy and multiscale rheometry to examine the microstructure and rheology of solutions containing recombinant human PRG4 (rhPRG4) and HA. Contrary to previous assumptions of an extensive HA-rhPRG4 network, it is discovered that rhPRG4 primarily forms stiff, gel-like aggregates. The properties of these aggregates, including their size and stiffness, are found to be influenced by the viscoelastic characteristics of the surrounding HA matrix. Consequently, the rheology of this system is not governed by a single length scale, but instead responds as a disordered, hierarchical network with solid-like rhPRG4 aggregates distributed throughout the continuous HA phase. These findings provide new insights into the biomechanical function of PRG4 in cartilage lubrication and may have implications in the development of HA-based therapies for joint diseases like osteoarthritis.

The use of UV-C radiation for terminal disinfection of pathogenic Gram-negative rods: a pilot study.

In this controlled study, we found that exposure to ultraviolet-C (UV-C) radiation was able to arrest the growth of selected pathogenic enteric and nonfermenting Gram-negative rods. Further studies are needed to confirm the clinical efficacy and determine optimal implementation strategies for utilizing UV-C terminal disinfection.

Control of interfacial instabilities through variable injection rate in a radial Hele-Shaw cell: A nonlinear approach for late-time analysis.

In this paper, the nonlinear behavior of immiscible viscous fingering in a circular Hele-Shaw cell under the action of different time-dependent injection flow rate schemes is assessed numerically. Unlike previous studies which addressed the infinite viscosity ratio (inviscid-viscous flow), the problem is tackled by paying special attention to flows with finite viscosity ratio (viscous flow) in which the viscosity of the displacing and the displaced fluids can have any arbitrary value. Systematic numerical simulations based on a complex-variable formulation of Cauchy-Green barycentric coordinates are performed at different mobility ratios and capillary numbers with a focus on the late-time fully nonlinear regime. Additionally, numerical optimization is used to obtain the optimal flow rate schedule through a second-order weakly nonlinear stability analysis in contrast to previous studies in which the optimal flow rate was obtained entirely based on linear stability analysis. It is demonstrated that, irrespective of the values of the mobility ratio and/or the capillary number, for patterns whose constant injection counterpart exhibits linear flow regime, the curvature-driven relaxation time is comparable with the operational time of the time-dependent injection flow rate controlling schemes, and most of the controlling schemes work very well and suppress the fingering phenomenon remarkably with the maximum recovery improvement of 15%. As the nonlinearity of the system increases, the schemes may still perform well, but their effectiveness is more pronounced in patterns with less nonlinearity in their constant injection counterpart than those with higher nonlinearity. As the nonlinearity increases, the curvature-driven relaxation time becomes longer than the operational time of the schemes, leading to a reduction in their effectiveness. Additionally, it is shown that employment of the second-order weakly nonlinear stability analysis to formulate the objective function does not result in any remarkable variation in the obtained optimal flow rate schedule.

Dynamic monitoring of dielectric properties during two phase immiscible displacements in sand packs using frequency domain electromagnetic sweeps.

Electromagnetic heating is a promising soil remediation method especially in thin formations. The lack of a wide-spread adoption of this method stems from insufficient knowledge of how the complex dielectric properties, that govern propagation of the electromagnetic waves through porous media, change with changing frequency, water saturation, displacement types and flow regimes. To breach these gaps several sets of spontaneous deionized (DI) water imbibition experiments, followed by the primary drainage floods, that were followed by the secondary DI water imbibition floods in confined uniform sand packs were performed. The frequency domain relative dielectric constant and conductivities were extracted from the two-port complex S-parameter measurements taken with the vector network analyzer during these immiscible displacements at various water saturation levels at ambient conditions. A novel coaxial transmission line core holder was designed and commissioned, and a modified version of a plane-invariant dielectric extraction algorithm was developed for this purpose. Series, parallel and semi-disperse mixing models were applied to fit the water saturation dependent relative dielectric constant and conductivity values sampled at 500 MHz from the extracted frequency domain spectra. The Maxwell-Garnett parallel model was proved to be the most flexible because it could capture the sampled conductivity values in all secondary imbibition floods before and after the breakthroughs, where the inflection points were observed. These inflection points were attributed to silica production and a potential shear-stripping flow. This observation was further confirmed by conducting a single-phase Darcy's law analysis of two DI water imbibition floods.

Multifunctional fully biobased aerogels for water remediation: Applications for dye and heavy metal adsorption and oil/water separation.

Water ecosystem contamination from industrial pollutants is an emerging threat to both humans and native species, making it a point of global concern. In this work, fully biobased aerogels (FBAs) were developed by using low-cost cellulose filament (CF), chitosan (CS), citric acid (CA), and a simple and scalable approach, for water remediation applications. The FBAs displayed superior mechanical properties (up to ∼65 kPa m3 kg-1 specific Young's modulus and ∼111 kJ/m3 energy absorption) due to CA acting as a covalent crosslinker in addition to the natural hydrogen bonding and electrostatic interactions between CF and CS. The addition of CS and CA increased the variety of functional groups (carboxylic acid, hydroxyl and amines) on the materials' surface, resulting in super-high dye and heavy metal adsorption capacities (619 mg/g and 206 mg/g for methylene blue and copper, respectively). Further modification of FBAs with a simple approach using methyltrimethoxysilane endowed aerogel oleophilic and hydrophobic properties. The developed FBAs showed a fast performance in water and oil/organic solvents separation with more than 96% efficiency. Besides, the FBA sorbents could be regenerated and reused for multiple cycles without any significant impact on their performance. Moreover, thanks to the presence of amine groups by addition of CS, FBAs also displayed antibacterial properties by preventing the growth of Escherichia coli on their surface. This work demonstrates the preparation of FBAs from abundant, sustainable, and inexpensive natural resources for applications in wastewater purification.

On the Stability of Pickering and Classical Nanoemulsions: Theory and Experiments.

Emulsification is a crucial technique for mixing immiscible liquids into droplets in various industries, such as food, cosmetics, biomedicine, agrochemistry, and petrochemistry. Quantitative analysis of the stability is pivotal before the utilization of these emulsions. Differences in X-ray attenuation for emulsion components and surface relaxation of the droplets may contribute to X-ray CT imaging and low-field NMR spectroscopy as viable techniques to quantify emulsion stability. In this study, Pickering (stabilized solely by nanoparticles) and Classical (stabilized solely by low molecular weight polymers) nanoemulsions were prepared with a high-energy method. NMR and X-ray CT were employed to constantly monitor the two types of nanoemulsions until phase separation. The creaming rates calculated from NMR match well with the results obtained from X-ray CT. Furthermore, we show that Stokes' law coupled with the classical Lifshitz-Slyozov-Wagner theory underestimates the creaming rate of the nanoemulsions compared to the experimental results from NMR and X-ray CT imaging. A new theory is proposed by fully incorporating the effects of Pickering nanoparticles, hydrocarbon types, volume fraction, size distribution, and flocculation on the droplet coarsening. The theoretical results agree well with the experimentally measured creaming rates. It reveals that the attachment of nanoparticles onto a droplet surface decreases the mass transfer for hydrocarbon molecules to move from the bulk aqueous phase into other droplets, thus slowing the Ostwald ripening. Therefore, Pickering nanoemulsions show a better stability behavior compared to Classical nanoemulsions. The impacts of hydrocarbon and emulsification energy on the stability of nanoemulsions are reported. These findings demonstrate that the stability of the nanoemulsions can be manipulated and optimized for a specific application, setting the stage for subsequent investigations of these nanodroplets.

Formation of biologically influenced palladium microstructures by Desulfovibrio desulfuricans and Desulfovibrio ferrophilus IS5.

A range of Desulfovibrio spp. can reduce metal ions to form metallic nanoparticles that remain attached to their surfaces. The bioreduction of palladium (Pd) has been given considerable attention due to its extensive use in areas of catalysis and electronics and other technological domains. In this study we report, for the first time, evidence for Pd(II) reduction by the highly corrosive Desulfovibrio ferrophilus IS5 strain to form surface attached Pd nanoparticles, as well as rapid formation of Pd(0) coated microbial nanowires. These filaments reached up to 8 µm in length and led to the formation of a tightly bound group of interconnected cells with enhanced ability to attach to a low carbon steel surface. Moreover, when supplied with high concentrations of Pd (≥ 100 mmol Pd(II) g-1 dry cells), both Desulfovibrio desulfuricans and D. ferrophilus IS5 formed bacteria/Pd hybrid porous microstructures comprising millions of cells. These three-dimensional structures reached up to 3 mm in diameter with a dose of 1200 mmol Pd(II) g-1 dry cells. Under suitable hydrodynamic conditions during reduction, two-dimensional nanosheets of Pd metal were formed that were up to several cm in length. Lower dosing of Pd(II) for promoting rapid synthesis of metal coated nanowires and enhanced attachment of cells onto metal surfaces could improve the efficiency of various biotechnological applications such as microbial fuel cells. Formation of biologically stimulated Pd microstructures could lead to a novel way to produce metal scaffolds or nanosheets for a wide variety of applications.

Inhibition of Sulfate Reduction and Cell Division by Desulfovibrio desulfuricans Coated in Palladium Metal.

The growth of sulfate-reducing bacteria (SRB) and associated hydrogen sulfide production can be problematic in a range of industries such that inhibition strategies are needed. A range of SRB can reduce metal ions, a strategy that has been utilized for bioremediation, metal recovery, and synthesis of precious metal catalysts. In some instances, the metal remains bound to the cell surface, and the impact of this coating on bacterial cell division and metabolism has not previously been reported. In this study, Desulfovibrio desulfuricans cells (1g dry weight) enabled the reduction of up to 1500 mmol (157.5 g) palladium (Pd) ions, resulting in cells being coated in approximately 1 µm of metal. Thickly coated cells were no longer able to metabolize or divide, ultimately leading to the death of the population. Increasing Pd coating led to prolonged inhibition of sulfate reduction, which ceased completely after cells had been coated with 1200 mmol Pd g-1 dry cells. Less Pd nanoparticle coating permitted cells to carry out sulfate reduction and divide, allowing the population to recover over time as surface-associated Pd diminished. Overcoming inhibition in this way was more rapid using lactate as the electron donor, compared to formate. When using formate as an electron donor, preferential Pd(II) reduction took place in the presence of 100 mM sulfate. The inhibition of important metabolic pathways using a biologically enabled casing in metal highlights a new mechanism for the development of microbial control strategies. IMPORTANCE Microbial reduction of sulfate to hydrogen sulfide is highly undesirable in several industrial settings. Some sulfate-reducing bacteria are also able to transform metal ions in their environment into metal phases that remain attached to their outer cell surface. This study demonstrates the remarkable extent to which Desulfovibrio desulfuricans can be coated with locally generated metal nanoparticles, with individual cells carrying more than 100 times their mass of palladium metal. Moreover, it reveals the effect of metal coating on metabolism and replication for a wide range of metal loadings, with bacteria unable to reduce sulfate to sulfide beyond a specific threshold. These findings present a foundation for a novel means of modulating the activity of sulfate-reducing bacteria.

The Critical Role of Environmental Synergies in the Creation of Bionanohybrid Microbes.

A wide range of bacteria can synthesize surface-associated nanoparticles (SANs) through exogenous metal ions reacting with sulfide produced via cysteine metabolism, resulting in the emergence of a biological-nanoparticle hybrid (bionanohybrid). The attached nanoparticles may couple to extracellular electron transfer, facilitating de novo photoelectrochemical processes. While SAN-cell coupling in hybrid organisms is opening a range of biotechnological possibilities, observation of bionanohybrids in nature is not commonly reported and their lab-based behavior remains difficult to control. We describe the critical role environmental synergy (microbial growth stage, cell densities, cysteine, and exogenous metal concentrations) plays in controlling the form and occurrence of Escherichia coli and Moorella thermoacetica bionanohybrids. SAN development depends on an appropriate cell density to metal ratio, with too few cells resulting in nanoparticle suppression through cytotoxicity or inhibition of cysteine conversion, and with too many cells diluting the number and size of particles produced. This cell number is governed by the concentration of cysteine present, which acts to protect the cells from metal ion toxicity. Exposing cells to metal and cysteine during the lag phase leads to SAN development, whereas cells in the exponential growth phase predominantly produce dispersed nanoparticles. Applying these principles more broadly, E. coli is shown to biosynthesize composite Bi/Cu sulfide SANs, and Clostridioides difficile can be coaxed into a bionanohybrid lifestyle by fine-tuning the cysteine dosage. Bionanohybrids maintain a remarkable ability for binary fission and sustained growth, opening doors to the production of SANs tailored to specific technological functions. IMPORTANCE Some bacteria can produce nanoscale-sized particles, which remain attached to the surface of the organism. The surface association of these nanoparticles creates a new mode of interaction between the microbe's environment and its internal cellular function, giving rise to a new hybrid lifeform, a biological nanoparticle hybrid (bionanohybrid). These hybrid organisms gain new or enhanced biological functions, and thus their creation opens a wide range of biotechnological possibilities. Despite this potential, the fundamental controls on bionanohybrid formation and occurrence remain poorly constrained. In this study, Escherichia coli K-12, Moorella thermoacetica, and Clostridioides difficile were used to test the combined influences of the growth phase, cell density, cysteine dose, and metal concentration in determining single and composite metal sulfide surface-associated nanoparticle production. The significance of this study is that it defined the critical synergies controlling nanoparticle formation on bacterial cell surfaces, unlocking the potential for bionanohybrid applications in a range of organisms.

Development and Characterization of Field Structured Magnetic Composites.

Polymer composites containing ferromagnetic fillers are promising for applications relating to electrical and electronic devices. In this research, the authors modified an ultraviolet light (UV) curable prepolymer to additionally cure upon heating and validated a permanent magnet-based particle alignment system toward fabricating anisotropic magnetic composites. The developed dual-cure acrylate-based resin, reinforced with ferromagnetic fillers, was first tested for its ability to polymerize through UV and heat. Then, the magnetic alignment setup was used to orient magnetic particles in the dual-cure acrylate-based resin and a heat curable epoxy resin system in a polymer casting approach. The alignment setup was subsequently integrated with a material jetting 3D printer, and the dual-cure resin was dispensed and cured in-situ using UV, followed by thermal post-curing. The resulting magnetic composites were tested for their filler loading, microstructural morphology, alignment of the easy axis of magnetization, and degree of monomer conversion. Magnetic characterization was conducted using a vibrating sample magnetometer along the in-plane and out-of-plane directions to study anisotropic properties. This research establishes a methodology to combine magnetic field induced particle alignment along with a dual-cure resin to create anisotropic magnetic composites through polymer casting and additive manufacturing.

Dispersibility of Poly(vinyl acetate) Modified Silica Nanoparticles in Carbon Dioxide with Several Cosolvents.

The dispersibility and stabilization of silica nanoparticles with surface-capped poly(vinyl acetate) (PVAc) chains are examined in carbon dioxide with four different cosolvents. Three surface coverages of silica-PVAc were formed by using different weight ratios of the silica and PVAc. The dispersibilities of three silica-PVAc nanoparticles in CO2 with the four cosolvents were tested in a rotatable high-pressure variable-volume view cell. The effects of surface coverage, cosolvent type, pressure, and particle concentration on dispersion were investigated. Results show that, in the experimental pressure range (5.5 to 20 MPa), the pressure has no significant effect on the dispersion of nanoparticles, and the cosolvent is the key factor in dispersing silica-PVAc particles in CO2. 1-Butanol is an adequate cosolvent to disperse silica-PVAc in CO2 with any coverage of PVAc on the surface of the particles when the concentration of particles is smaller than 0.31 wt %. Ethanol can only improve the dispersibility of particles with a high surface coverage of PVAc when the concentration of particles is smaller than 0.14 wt %. 1-Hexanol and ethyl acetate cannot disperse the particles in CO2 with any coverage of PVAc. Molecular dynamics simulations were carried out to study the nanoparticle-CO2-cosolvent dispersions. Results suggest that 1-butanol has a good solubility in the CO2 condensed phase and can effectively absorb onto the nanoparticle surface, which help to prevent the formation of nanoparticle aggregation. The precipitation of nanoparticles in the nanoparticle/1-hexanol/CO2 and nanoparticle/ethyl acetate/CO2 systems is attributed to the relatively low solubility of CO2 in 1-hexanol and ethyl acetate. The precipitation of nanoparticles in the nanoparticle/ethanol/CO2 system is the result of less hindrance of ethanol molecules to the aggregation of nanoparticles.

Elucidating the effect of enzymatic polymerized polysaccharide particle morphology on emulsion properties.

Exploiting the shape of Pickering stabilizers offers the ability to unlock the full potential of nanoparticle-stabilized emulsions for applications in enhanced oil recovery, pharmaceuticals, cosmetics, and coatings. In this work, we utilize engineered polysaccharide particles derived from the enzymatic polymerization of glucose from sucrose with controlled shape for the stabilization of dodecane-in-water emulsions. Altering the particle shape (spherical aggregates, fibrids, or platelets), while maintaining a neutral surface charge allows for a systematic examination of the role of particle shape in the stabilization of emulsions. We find that platelet-shaped particles reduce the interfacial tension and result in the smallest droplet size, while emulsions stabilized by aggregates and fibrids are governed by a network of particles in the continuous phase. Exploiting the synergy between these particles allowed for the tuning of their microstructure and rheological signature which allows us to map and tailor these emulsions for a wider variety of applications.

Electro-acoustic excitation of the interface.

Methods that use directed acoustic [14,128] and electric fields [104,115] have shown a promise in oil recovery in the past. These methods are great candidates to not only facilitate oil recovery, but also reduce CO2 emissions to a minimum. However, they have not found big commercial success due to poor understanding of the underlying mechanisms that facilitate the recovery. This paper is focused on breaching this gap by summarizing the main theories and experimental work done in the fields of acoustic and electric wave propagation though liquids, their interfaces and saturated porous media. The main theories of the acoustic and electric wave propagation through liquid and saturated porous media are outlined and discussed here. Key physical mechanisms that occur in the reservoir oil and water phases and at their interface during acoustic and electric wave propagation are identified. The development of the Maxwell stress theory of the liquid-liquid interface under the influence of the electric field by Melcher, Smith, Taylor and Lin et al. is shown to be a useful tool to estimate the critical voltage that disturbs a liquid-liquid interface [78,112,120]. Important aspects in wellbore acoustic transducer design and its selection to fit into the reservoir conditions are covered and scrutinized [66,101]. Characteristics of the magnetostrictive devices are shown to fit reservoir conditions the best.

In situ monitoring of the morphology evolution of interfacially-formed conductive nanocomposite films and their use as strain sensors.

HYPOTHESIS: Understanding and monitoring the film formation of interfacially formed layered films allows for the design of conductive nanocomposite films suitable for strain sensing. EXPERIMENTS: To understand the mechanism of interfacial film formation, the hexane/water interface was monitored during the evaporation process via confocal laser scanning microscopy. Scanning electron microscopy and atomic force microscopy were utilized to investigate final film morphology. Tensile testing was used to determine their mechanical properties under uniaxial strain. FINDINGS: Conductive nanocomposite films were formed at the hexane/water interface. Due to their low colloidal stability in hexane, the Vulcan carbon (VC) nanoparticles settled to the hexane/water interface prior to the onset of paraffin wax precipitation. Consequently, after the evaporation of hexane a two-layer structured film was formed. The bottom (water-facing, VC-rich) layer was conductive due to the existence of a percolated network of nanoparticle aggregates, while the top (hexane facing, paraffin-rich) layer was not conductive. The films showed high sensitivity for strains between 1% and 10%. We propose that the mechanism of strain sensing is similar to that of layer-structured sensors fabricated through embedding conductive nanofillers onto flexible polymeric substrates. The advantage of the films derived by the method proposed here is their ease of fabrication as well as their low cost.

The Effect of Dissimilatory Manganese Reduction on Lactate Fermentation and Microbial Community Assembly.

Fermentation and dissimilatory manganese (Mn) reduction are inter-related metabolic processes that microbes can perform in anoxic environments. Fermentation is less energetically favorable and is often not considered to compete for organic carbon with dissimilatory metal reduction. Therefore, the aim of our study was to investigate the outcome of the competition for lactate between fermentation and Mn oxide (birnessite) reduction in a mixed microbial community. A birnessite reducing enrichment culture was obtained from activated sludge with lactate and birnessite as the substrates. This enrichment was further used to test how various birnessite activities (0, 10, 20, and 40 mM) affected the rates of fermentation and metal reduction, as well as community composition. Increased birnessite activity led to a decrease of lactate consumption rate. Acetate and propionate were the main products. With increasing birnessite activity, the propionate/acetate ratio decreased from 1.4 to 0.47. Significant CO2 production was detected only in the absence of birnessite. In its presence, CO2 concentrations remained close to the background since most of the CO2 produced in these experiments was recovered as MnCO3. The Mn reduction efficiency (Mn(II) produced divided by birnessite added) was the highest at 10 mM birnessite added, where about 50% of added birnessite was reduced to Mn(II), whereas at 20 and 40 mM approximately 21 and 16% was reduced. The decreased birnessite reduction efficiency at higher birnessite activities points to inhibition by terminal electron acceptors and/or its toxicity which was also indicated by retarded lactate oxidation and decreased concentrations of microbial metabolites. Birnessite activity strongly affected microbial community structure. Firmicutes and Bacteroidetes were the most abundant phyla at 0 mM of birnessite. Their abundance was inversely correlated with birnessite concentration. The relative sequence abundance of Proteobacteria correlated with birnessite concentrations. Most of the enriched populations were involved in lactate/acetate or amino acid fermentation and the only previously known metal reducing genus detected was related to Shewanella sp. The sequencing data confirmed that lactate consumption coupled to metal reduction was only one of the processes occurring and did not outcompete fermentation processes.

Insight on Methane Foam Stability and Texture via Adsorption of Surfactants on Oppositely Charged Nanoparticles.

We report the phase behavior of a dispersion of alumina-coated silica nanoparticles in the presence of an anionic surfactant (sodium fatty alcohol polyoxyethylene ether sulfate), and then describe the influence of surfactant/nanoparticle concentration ratio on the stability of methane foam as a potential fluid for enhanced oil recovery application. The surface tension of the methane/aqueous phase interface, surface charge, and size of the particle aggregates and amount of surfactant adsorption were characterized as a function of surfactant/nanoparticle ratio. Five adsorption stages, which are described in terms of the extent and type of the surfactant coverage on the nanoparticle surface, explain the behavior of the solution at different surfactant/nanoparticle ratios. The static foam generation experiments were conducted to monitor the variation of the foam stability and texture over the defined adsorption stages. The surface tension trends illustrate that the affinity of nanoparticles for the gas-liquid interface is strongly affected by the adsorption extent of AES molecules on the particle surface. At high surfactant/nanoparticle ratio, the adsorbed surfactant bilayer causes a high hydrophilicity of the particles that significantly pushed the particles away from the gas-liquid interface. At the most hydrophobic state of the particles which occurred at the ratio of 0.2, the foam structure collapsed quickly. The most stable foam with fine texture was found at surfactant/nanoparticle ratio less than 0.008 at which the particles are partially covered with surfactants and have smaller aggregate size. The findings provide a better understanding of the interaction between oppositely charged nanoparticle/surfactant pairs and how that interaction affects foam stability. It is demonstrated that substitution of absolute concentration by surfactant/nanoparticle ratio can truly govern the foam stability and texture. The results can be beneficial to predict the foam behavior in its numerous applications and whether interactions will be synergistic, antagonistic, or neutral.

The Significance of Graphene Oxide-Polyacrylamide Interactions on the Stability and Microstructure of Oil-in-Water Emulsions.

The emulsification of oil in water by nanoparticles can be facilitated by the addition of costabilizers, such as polymers and surfactants. The enhanced properties of the resulting emulsions are usually attributed to nanoparticle/costabilizer synergy; however, the mechanism of this synergistic effect and its impacts on emulsion stability and microstructure remain unclear. Here, we study the synergistic interaction of graphene oxide (GO) and a high molecular weight anionic polyacrylamide (PAM) in stabilization of paraffin oil/water emulsion systems. We show that the addition of PAM reduces the amount of GO required to stabilize an emulsion significantly. In order to probe the synergistic effect of GO and PAM, we analytically analyze the oil-free GO and GO-PAM dispersions and directly image their morphology via Cryo-TEM and atomic force microscopy (AFM). X-ray diffraction results confirm the adsorption of PAM molecules onto GO sheets resulting in the formation of ultimate GO-PAM complexes. The adsorption phenomenon is a consequence of hydrogen bonding and acid-base interactions, conceivably leading to a resilient electron-donor-acceptor complex. The microstructure of emulsions is captured with two-color fluorescent microscopy and Cryo-TEM. The acquired images display the localization of GO-PAM complexes at the interface while large amount of GO-PAM flocs coexist at the interface and in between oil droplets. Localization of such complexes and flocs at the interface is found to be responsible for their slow creaming rates compared to their GO counterparts. Mechanical properties of both dispersions and emulsions are studied by shear rheology. Rheological measurements confirm that GO-PAM complexes have a higher desorption energy from the interface resulting in higher critical shear strain of GO-PAM emulsions. The results, with insights into both structure and rheology, form a foundational understanding for integration of other polymers and nanoparticles in emulsion systems, which enables efficient design of these systems for an application of interest.