Controlling Nonpolar Colloidal Asphaltene Aggregation by Electrostatic Repulsion

While aromatic chemicals are applied to petroleum oil systems to thermodynamically prevent asphaltene precipitation, amphiphilic dispersants can truncate the precipitation process and create stable suspensions of asphaltene colloids in the submicrometer size range. Bulk sedimentation and dynamic light scattering have shown that stabilizing dispersants inhibit colloidal asphaltene aggregation at approximately the same concentration as is needed to effectively slow bulk sedimentation. At the same time, these same types of dispersants can alter the electrostatic properties of colloidal asphaltenes in nonpolar suspensions. While electrostatic stabilization has been linked to aggregation dynamics in several types of colloidal systems, both aqueous and nonpolar, the complete linkage between electrostatic interactions and aggregation inhibition has yet to be shown in colloidal asphaltene suspensions. In this work, we present dynamic light scattering and electrophoresis measurements in colloidal asphaltene suspensions, using three different petroleum fluids and a dispersant which truncates asphaltene precipitation and colloidal aggregation by enabling uniform electrostatic charging at the colloidal asphaltene surface

Particle Formation during Oxidation Catalysis with Cp* Iridium Complexes

Real-time monitoring of light scattering and UV–vis profiles of four different Cp*Ir^III precursors under various conditions give insight into nanoparticle formation during oxidation catalysis with NaIO₄ as primary oxidant. Complexes bearing chelate ligands such as 2,2′-bipyridine, 2-phenylpyridine, or 2-(2′-pyridyl)-2-propanolate were found to be highly resistant toward particle formation, and oxidation catalysis with these compounds is thus believed to be molecular in nature under our conditions. Even with the less stable hydroxo/aqua complex [Cp*₂Ir₂(μ-OH)₃]­OH, nanoparticle formation strongly depended on the exact conditions and elapsed time. Test experiments on the isolated particles and comparison of UV–vis data with light scattering profiles revealed that the formation of a deep purple-blue color (∼580 nm) is <i>not</i> indicative of particle formation during oxidation catalysis with molecular iridium precursors as suggested previously

Relating Silica Scaling in Reverse Osmosis to Membrane Surface Properties

We investigated the relationship between membrane surface properties and silica scaling in reverse osmosis (RO). The effects of membrane hydrophilicity, free energy for heterogeneous nucleation, and surface charge on silica scaling were examined by comparing thin-film composite polyamide membranes grafted with a variety of polymers. Results show that the rate of silica scaling was independent of both membrane hydrophilicity and free energy for heterogeneous nucleation. In contrast, membrane surface charge demonstrated a strong correlation with the extent of silica scaling (<i>R</i>² > 0.95, <i>p</i> < 0.001). Positively charged membranes significantly facilitated silica scaling, whereas a more negative membrane surface charge led to reduced scaling. This observation suggests that deposition of negatively charged silica species on the membrane surface plays a critical role in silica scale formation. Our findings provide fundamental insights into the mechanisms governing silica scaling in reverse osmosis and highlight the potential of membrane surface modification as a strategy to reduce silica scaling

Cp* versus Bis-carbonyl Iridium Precursors as CH Oxidation Precatalysts

We previously reported a dimeric Ir^IV-oxo species as the active water oxidation catalyst formed from a Cp*Ir­(pyalc)Cl {pyalc = 2-(2′-pyridyl)-2-propanoate} precursor, where the Cp* is lost to oxidative degradation during catalyst activation; this system can also oxidize unactivated CH bonds. We now show that the same Cp*Ir­(pyalc)­Cl precursor leads to two distinct active catalysts for CH oxidation. In the presence of external CH substrate, the Cp* remains ligated to the Ir center during catalysis; the active specieslikely a high-valent Cp*Ir­(pyalc) specieswill oxidize the substrate instead of its own Cp*. If there is no external CH substrate in the reaction mixture, the Cp* will be oxidized and lost, and the active species is then an iridium-μ-oxo dimer. Additionally, the recently reported Ir­(CO)₂(pyalc) water oxidation precatalyst is now found to be an efficient, stereoretentive CH oxidation precursor. We compare the reactivity of Ir­(CO)₂(pyalc) and Cp*Ir­(pyalc)­Cl precursors and show that both can lose their placeholder ligands, CO or Cp*, to form substantially similar dimeric Ir^IV-oxo catalyst resting states. The more efficient activation of the bis-carbonyl precursor makes it less inhibited by obligatory byproducts formed from Cp* degradation, and therefore the dicarbonyl is our preferred precatalyst for oxidation catalysis

Combined Organic Fouling and Inorganic Scaling in Reverse Osmosis: Role of Protein–Silica Interactions

We investigated the relationship between silica scaling and protein fouling in reverse osmosis (RO). Flux decline caused by combined scaling and fouling was compared with those by individual scaling or fouling. Bovine serum albumin (BSA) and lysozyme (LYZ), two proteins with opposite charges at typical feedwater pH, were used as model protein foulants. Our results demonstrate that water flux decline was synergistically enhanced when silica and protein were both present in the feedwater. For example, flux decline after 500 min was far greater in combined silica scaling and BSA fouling experiments (55 ± 6% decline) than those caused by silica (11 ± 2% decline) or BSA (9 ± 1% decline) alone. Similar behavior was observed with silica and LYZ, suggesting that this synergistic effect was independent of protein charge. Membrane characterization by scanning electron microscopy and Fourier transform infrared spectroscopy revealed distinct foulant layers formed by BSA and LYZ in the presence of silica. A combination of dynamic light scattering, transmission electron microscopy , and energy dispersive X-ray spectroscopy analyses further suggested that BSA and LYZ facilitated the formation of aggregates with varied chemical compositions. As a result, BSA and LYZ were likely to play different roles in enhancing flux decline in combined scaling and fouling. Our study suggests that the coexistence of organic foulants, such as proteins, largely alters scaling behavior of silica, and that accurate prediction of RO performance requires careful consideration of foulant–scalant interactions

Ultrathin Nanotube/Nanowire Electrodes by Spin–Spray Layer-by-Layer Assembly: A Concept for Transparent Energy Storage

Fully integrated transparent devices require versatile architectures for energy storage, yet typical battery electrodes are thick (20–100 μm) and composed of optically absorbent materials. Reducing the length scale of active materials, assembling them with a controllable method and minimizing electrode thickness should bring transparent batteries closer to reality. In this work, the rapid and controllable spin–spray layer-by-layer (SSLbL) method is used to generate high quality networks of 1D nanomaterials: single-walled carbon nanotubes (SWNT) and vanadium pentoxide (V₂O₅) nanowires for anode and cathode electrodes, respectively. These ultrathin films, deposited with ∼2 nm/bilayer precision are transparent when deposited on a transparent substrate (>87% transmittance) and electrochemically active in Li-ion cells. SSLbL-assembled ultrathin SWNT anodes and V₂O₅ cathodes exhibit reversible lithiation capacities of 23 and 7 μAh/cm², respectively at a current density of 5 μA/cm². When these electrodes are combined in a full cell, they retain ∼5 μAh/cm² capacity over 100 cycles, equivalent to the prelithiation capacity of the limiting V₂O₅ cathode. The SSLbL technique employed here to generate functional thin films is uniquely suited to the generation of transparent electrodes and offers a compelling path to realize the potential of fully integrated transparent devices

Electrochemical Activation of Cp* Iridium Complexes for Electrode-Driven Water-Oxidation Catalysis

Organometallic iridium complexes bearing oxidatively stable chelate ligands are precursors for efficient homogeneous water-oxidation catalysts (WOCs), but their activity in oxygen evolution has so far been studied almost exclusively with sacrificial chemical oxidants. In this report, we study the electrochemical activation of Cp*Ir complexes and demonstrate true electrode-driven water oxidation catalyzed by a homogeneous iridium species in solution. Whereas the Cp* precursors exhibit no measurable O₂-evolution activity, the molecular species formed after their oxidative activation are highly active homogeneous WOCs, capable of electrode-driven O₂ evolution with high Faradaic efficiency. We have ruled out the formation of heterogeneous iridium oxides, either as colloids in solution or as deposits on the surface of the electrode, and found indication that the conversion of the precursor to the active molecular species occurs by a similar process whether carried out by chemical or electrochemical methods. This work makes these WOCs more practical for application in photoelectrochemical dyads for light-driven water splitting

Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

A series of Cp*Ir^III dimers have been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed O₂ evolution. The oxidative stability of the precursors toward nanoparticle formation and their oxygen evolution activity have been investigated and compared to suitable monomeric analogues. We found that precursors bearing monodentate NHC ligands degraded to form nanoparticles (NPs), and accordingly their O₂ evolution rates were not significantly influenced by their nuclearity or distance between the two metals in the dimeric precursors. A doubly chelating bis-pyridine–pyrazolide ligand provided an oxidation-resistant ligand framework that allowed a more meaningful comparison of catalytic performance of dimers with their corresponding monomers. With sodium periodate (NaIO₄) as the oxidant, the dimers provided significantly lower O₂ evolution rates per [Ir] than the monomer, suggesting a negative interaction instead of cooperativity in the catalytic cycle. Electrochemical analysis of the dimers further substantiates the notion that no radical oxyl-coupling pathways are accessible. We thus conclude that the alternative path, nucleophilic attack of water on high-valent Ir-oxo species, may be the preferred mechanistic pathway of water oxidation with these catalysts, and bimolecular oxo-coupling is not a valid mechanistic alternative as in the related ruthenium chemistry, at least in the present system

Cp* Iridium Precatalysts for Selective C–H Oxidation with Sodium Periodate As the Terminal Oxidant

Sodium periodate (NaIO₄) is shown to be a milder and more efficient terminal oxidant for C–H oxidation with Cp*Ir (Cp* = C₅Me₅) precatalysts than ceric­(IV) ammonium nitrate. Synthetically useful yields, regioselectivities, and functional group tolerance were found for methylene oxidation of substrates bearing a phenyl, ketone, ester, or sulfonate group. Oxidation of the natural products (−)-ambroxide and sclareolide proceeded selectively, and retention of configuration was seen in <i>cis</i>-decalin hydroxylation. At 60 °C, even primary C–H bonds can be activated: whereas methane was overoxidized to CO₂ in 39% yield without giving partially oxidized products, ethane was transformed into acetic acid in 25% yield based on total NaIO₄. ¹⁸O labeling was demonstrated in <i>cis</i>-decalin hydroxylation with ¹⁸OH₂ and NaIO₄. A kinetic isotope effect of 3.0 ± 0.1 was found in cyclohexane oxidation at 23 °C, suggesting C–H bond cleavage as the rate-limiting step. Competition experiments between C–H and water oxidation show that C–H oxidation of sodium 4-ethylbenzene sulfonate is favored by 4 orders of magnitude. <i>In operando</i> time-resolved dynamic light scattering and kinetic analysis exclude the involvement of metal oxide nanoparticles and support our previously suggested homogeneous pathway

Loss of Phospholipid Membrane Integrity Induced by Two-Dimensional Nanomaterials

The interaction of two-dimensional (2D) nanomaterials with biological membranes has important implications for ecotoxicity and human health. In this study, we use a dye-leakage assay to quantitatively assess the disruption of a model phospholipid bilayer membrane (i.e., lipid vesicles) by five emerging 2D nanomaterials: graphene oxide (GO), reduced graphene oxide (rGO), molybdenum disulfide (MoS₂), copper oxide (CuO), and iron oxide (α-Fe₂O₃). Leakage of dye from the vesicle inner solution, which indicates loss of membrane integrity, was observed for GO, rGO, and MoS₂ nanosheets but not for CuO and α-Fe₂O₃, implying that 2D morphology by itself is not sufficient to cause loss of membrane integrity. Mixing GO and rGO with lipid vesicles induced aggregation, whereas enhanced stability (dispersion) was observed with MoS₂ nanosheets, suggesting different aggregation mechanisms for the 2D nanomaterials upon interaction with lipid bilayers. No loss of membrane integrity was observed under strong oxidative conditions, indicating that nanosheet-driven membrane disruption stemmed from a physical mechanism rather than chemical oxidation. For GO, the most disruptive nanomaterial, we show that the extent of membrane integrity loss was dependent on total surface area, not edge length, which is consistent with a lipid-extraction mechanism and inconsistent with a piercing mechanism