Structure of the electrical double layer at the ice-water interface.

The surface of ice in contact with water contains sites that undergo deprotonation and protonation and can act as adsorption sites for aqueous ions. Therefore, an electrical double layer should form at this interface and existing models for describing the electrical double layer at metal oxide-water interfaces should be able to be modified to describe the surface charge, surface potential, and ionic occupancy at the ice-water interface. I used a surface complexation model along with literature measurements of the zeta potential of ice in brines of various strength and pH to constrain equilibrium constants. I then made predictions of ion site occupancy, surface charge density, and partitioning of counterions between the Stern and diffuse layers. The equilibrium constant for cation adsorption is more than 5 orders of magnitude larger than the other constants, indicating that this reaction dominates even at low salinity. Deprotonated OH sites are predicted to be slightly more abundant than dangling O sites, consistent with previous work. Surface charge densities are on the order of ±0.001 C/m2 and are always negative at the moderate pH values of interest to atmospheric and geophysical applications (6-9). In this pH range, over 99% of the counterions are contained in the Stern layer. This suggests that diffuse layer polarization will not occur because the ionic concentrations in the diffuse layer are nearly identical to those in the bulk electrolyte and that electrical conduction and polarization in the Stern layer will be negligible due to reduced ion mobility.

Ultra-stable CO2-in-water foam by generating switchable Janus nanoparticles in-situ.

HYPOTHESIS: The surface of silica nanoparticles (NP) may be covalently grafted with two amino ligands to balance colloidal stability and interfacial activity via formation of in situ Janus particles. The modified NP may be combined with a like-charged diamine surfactant to create ultra-stable CO2 foam at low NP concentrations. EXPERIMENTS: The NP colloidal stability was measured up to 80 °C in 230 g/L TDS brine with dynamic light scattering. The NP surface was characterized using zeta potential, TEM, TGA, conductometric and potentiometric titrations, NMR and interfacial measurement. CO2/brine foam was generated at 60-80 °C and 15 MPa and apparent viscosity was measured vs foam quality. The foam stability was measured in-situ with an optical microscope. FINDINGS: Upon adding only 0.1 wt% NP, ultra-stable CO2 foam was observed at 60 °C with a bubble coarsening rate 3 orders of magnitude lower than with surfactant alone. Foam bubbles were spherical with NP present, but became polyhedral for the much less stable surfactant-only foams. For this novel like-charged surfactant-NP system, the limited surfactant adsorption on the NP resulted in NP stabilized CO2 foam, while maintaining NP colloidal stability at high surfactant concentrations and high salinity, providing a new perspective of NP-surfactant design.

Examining the role of salinity on the dynamic stability of Pickering emulsions.

HYPOTHESIS: The effect of salinity on Pickering emulsion stability to coalescence under dynamic forces present during flow in porous media for applications including enhanced oil recovery is poorly understood. Recent work suggests the absence of significant electrostatic repulsion in brine prompts unattached particles to assemble into inter-droplet networks that increase emulsion stability. We hypothesize that emulsions stabilized by nanoparticles coated with (3-glycidyloxypropyl)trimethoxysilane (GLYMO) will generate particle networks in brine and exhibit greater stability to coalescence than in deionized water (DI). EXPERIMENTS: We stabilized decane-in-water emulsions with GLYMO-coated silica nanoparticles at various particle concentrations using brine and DI as the aqueous phase. We imaged the emulsions to calculate droplet diameters, then centrifuged the emulsions and weighed the volume of decane released to determine the extent of coalescence. We compared these measurements to evaluate the effect of salinity on emulsion stability. FINDINGS: Emulsions demonstrate greater dynamic stability and smaller droplet diameters with increasing nanoparticle concentration and salinity. Controlling for differences in droplet size, we observe that brine reduces the emulsion coalescence rate by a factor of 78 ± 23 relative to DI. This difference supports and quantifies past work suggesting that unattached nanoparticles aggregate in brine and increase overall emulsion stability, whereas nanoparticles in DI remain separated.

Effect of interparticle forces on the stability and droplet diameter of Pickering emulsions stabilized by PEG-coated silica nanoparticles.

HYPOTHESIS: Attractive and repulsive interparticle forces influence the stability and structure of Pickering emulsions. The effect these forces have on emulsion behavior must be better understood to improve Pickering emulsions for subsurface applications, including enhanced oil recovery and aquifer decontamination. Past work demonstrates improved emulsion stability with increasing salinity and reduced electrostatic repulsion, possibly because of interparticle networks. We hypothesize that emulsion stability is similarly improved by reducing interparticle steric repulsion. EXPERIMENTS: We assessed the effect of interparticle forces on emulsion stability by generating decane-in-water emulsions. We used polyethylene glycol (PEG)-coated silica nanoparticles with different diameters, surface modification, and salinities to modify either vdW, steric, or electrostatic interactions. We measured emulsion stability using centrifugation, imaged emulsion droplets with optical microscopy, and analyzed images with ImageJ to calculate droplet diameters. FINDINGS: Mildly aggregated particles with 0.5-1.0 µmol/m2 surface PEG exhibit the highest emulsion stability. This optimal surface concentration maximizes a trade-off between particle repulsion and aggregation. Droplet diameters are well explained by an energy balance limited coalescence model, generated by solving DLVO equations. We find that while emulsion stability is influenced by interparticle forces, droplet size is dominated by particle-droplet interactions. These results demonstrate the potential of surface modification to significantly improve emulsion stability.

Camphene-Assisted Fabrication of Free-Standing Lithium-Ion Battery Electrode Composites.

Free-standing electrode (FSE) architectures hold the potential to dramatically increase the gravimetric and volumetric energy density of lithium-ion batteries (LIBs) by eliminating the parasitic dead weight and volume associated with traditional metal foil current collectors. However, current FSE fabrication methods suffer from insufficient mechanical stability, electrochemical performance, or industrial adoptability. Here, we demonstrate a scalable camphene-assisted fabrication method that allows simultaneous casting and templating of FSEs comprising common LIB materials with a performance superior to their foil-cast counterparts. These porous, lightweight, and robust electrodes simultaneously enable enhanced rate performance by improving the mass and ion transport within the percolating conductive carbon pore network and eliminating current collectors for efficient and stable Li+ storage (>1000 cycles in half-cells) at increased gravimetric and areal energy densities. Compared to conventional foil-cast counterparts, the camphene-derived electrodes exhibit ∼1.5× enhanced gravimetric energy density, increased rate capability, and improved capacity retention in coin-cell configurations. A full cell containing both a free-standing anode and cathode was cycled for over 250 cycles with greater than 80% capacity retention at an areal capacity of 0.73 mA h/cm2. This active-material-agnostic electrode fabrication method holds potential to tailor the morphology of flexible, current-collector-free electrodes, thus enabling LIBs to be optimized for high power or high energy density Li+ storage. Furthermore, this platform provides an electrode fabrication method that is applicable to other electrochemical technologies and advanced manufacturing methods.

Destabilizing Pickering emulsions using fumed silica particles with different wettabilities.

HYPOTHESIS: Whereas hydrophobic colloidal particles are known to destabilize foams and emulsions stabilized with surfactants, their use for destabilizing Pickering emulsions is unexplored. Pickering emulsions differ from surfactant-stabilized emulsions because they are stabilized with colloidal particles that are adsorbed to the oil/water interface, which provide a steric barrier to droplet coalescence. This can make Pickering emulsions very stable, but it can also make their subsequent destabilization difficult. The hypothesis of this work is that destabilizing a Pickering emulsion should be possible with colloidal particles provided they are sufficiently hydrophobic, which will enable the particles to dewet an emulsion film and induce coalescence. EXPERIMENTS: A model oil-in-water Pickering emulsion was stabilized with polyethylene glycol-modified silica nanoparticles and its stability was assessed by centrifugation and by stirring on a stir plate. Three different fumed silica particles, A200 (bare, hydrophilic), R816 (hexadecylsilane modified, intermediate hydrophobicity), and R805 (octadecylsilane modified, hydrophobic), were added to the emulsion and stirred to evaluate their ability to macroscopically induce coalescence. Optical microscopy was used to visualize the interaction between the model Pickering emulsion and the three different fumed silica particles. RESULTS: The results from this work show there is a strong correlation between the wettability of a fumed silica particle and its ability to destabilize a model Pickering emulsion, with more hydrophobic particles showing a greater tendency to coalesce the Pickering emulsion. The hydrophilic and partially hydrophobic particles, at all concentrations tested, were unable coalesce the model Pickering emulsion. This was because the particles were almost immediately wetted by the continuous phase of the emulsion, which prevented any interactions between the emulsified oil drops and the silica particle surface. The hydrophobic fumed silica particles coalesced 60% of the emulsified oil with just 0.01 wt% added fumed silica, which further increased to 85% with 0.05 wt% added silica.

On the shear stability of water-in-water Pickering emulsions stabilized with silica nanoparticles.

HYPOTHESIS: Water-in-water (w/w) emulsions are known for their low interfacial tensions (IFT) which makes their stability to shear questionable. This is because of low particle attachment energies, which can be just a few kT. Therefore, emulsions stabilized with larger particles should display greater stability to shear because of larger attachment energies (10-100 or more kT). This is typically not an issue with traditional oil-in-water Pickering emulsions because particle attachment energies are much larger due to higher interfacial tensions, even when very small particles are used. EXPERIMENTS: Silica nanoparticles were silanized with 2-(methoxy(polyethyleneoxy)6-9propyl)trimethoxysilane (PEG-silane) to aid in emulsion stabilization. The phase behavior of an aqueous, two-phase system consisting of 20,000 g mol-1 polyethylene glycol (PEG) and magnesium sulfate (MgSO4) was characterized. Optical microscopy was used to characterize the static properties of the particle stabilized emulsions and shear rheology was used to study the stability of emulsions stabilized with 6 nm and 50 nm PEG-silane functionalized particles. RESULTS: We demonstrated that silica nanoparticles silanized with PEG-silane can stabilize MgSO4 drops to produce MgSO4-in-PEG emulsions. We found emulsions stabilized with 6 wt% particles, regardless of particle size (6 nm or 50 nm), had similar viscosities, emulsion drop size, and were statically stable for one week. Emulsion drops stabilized with 6 wt% 50 nm particles doubled in size after 80 min of shear at 10 s-1 whereas those stabilized with 6 wt% 6 nm particles required only 25 min to double in size. We attribute these differences in doubling time to the larger particle attachment energies associated with the 50 nm particles.

Manipulation of Pickering emulsion rheology using hydrophilically modified silica nanoparticles in brine.

HYPOTHESIS: Previous work on Pickering emulsions has shown that bromohexadecane-in-water emulsions (50% oil) stabilized with fumed and spherical particles modified with hexadecyl groups develop a noticeable zero shear elastic storage modulus (G'0) of 200Pa and 9Pa, respectively, while in just 50mM NaCl. This high G'0 can be problematic for subsurface applications where brine salinities are higher and on the order of 600mM NaCl. High reservoir salinity coupled with low formation pressure drops could prevent an emulsion with a high G'0 from propagating deep into formation. It is hypothesized that G'0 of an emulsion can be minimized by using sterically stabilized silica nanoparticles modified with the hydrophilic silane (3-glycidyloxypropyl)trimethoxysilane (glymo). EXPERIMENTS: Bromohexadecane-in-water emulsions were stabilized with low and high coverage glymo nanoparticles. Oscillatory rheology was used to monitor G'0 asa function of nanoparticle concentration, oil volume fraction, salinity, and pH. Cryogenic scanning electron microscopy was used to make observations on the emulsion microstructure. FINDINGS: G'0 of bromohexadecane-in-water emulsions were minimized by using particles with a high coverage of glymo on the particle surface, which reduced the Ca2+/silanol site interactions. Emulsions that were stabilized with low surface coverage particles had noticeably higher G'0, however, their G'0 could be reduced by a factor of 3.3 by simply lowering the solution pH to 3. Cryo-SEM images showed that nanoparticle bridging was more pronounced with nanoparticles that had low glymo coverage as opposed to high coverage.