To elucidate the nature of processes involved in electrically driven particle aggregation in steady fields, flows near a charged spherical colloidal particle next to an electrode were studied. Electrical body forces in diffuse layers near the electrode and the particle surface drive an axisymmetric flow with two components. One is electroosmotic flow (EOF) driven by the action of the applied field on the equilibrium diffuse charge layer near the particle. The other is electrohydrodynamic (EHD) flow arising from the action of the applied field on charge induced in the electrode polarization layer. The EOF component is proportional to the current density and the particle surface (zeta) potential, whereas our scaling analysis shows that the EHD component scales as the product of the current density and applied potential. Under certain conditions, both flows are directed toward the particle, and a superposition of flows from two nearby particles provides a mechanism for aggregation. Analytical calculations of the two flow fields in the limits of infinitesimal double layers and slowly varying current indicate that the EOF and EHD flow are of comparable magnitude near the particle whereas in the far field the EHD flow along the electrode is predominant. Moreover, the dependence of EHD flow on the applied potential provides a possible explanation for the increased variability in aggregation velocities observed at higher field strengths.
While exhibiting a well-defined nanometer-level structure, surfactant-templated nanoscopic silicas produced via self-assembly do not always possess long-range order. We demonstrate that long-range order can be controlled by guiding the self-assembly of nanostructured silica-surfactant hybrids with low-strength electric fields (E similar to 200 V/m) to produce nanoscopic silica with both the micrometer- and nanometer-level structures oriented parallel to the applied field. Under the influence of the electric field, nanoscopic silica particles migrate, elongate, and merge into fibers with a rate of migration proportional to the applied field strength. The linear dependence with the field strength indicates that the process is governed by electroosmotic flow but not by polarization effects. Realignment of the short-range ordered surfactant nanochannels along the fiber axis accompanies the migration.
Electrohydrodynamic (EHD) flow around a charged spherical colloid near an electrode was studied theoretically and experimentally to understand the nature of long-range particle-particle attraction near electrodes. Numerical computations for finite double-layer thicknesses confirmed the validity of an asymptotic methodology for thin layers. Then the electric potential around the particle was computed analytically in the limit of zero Peclet number and thin double layers for oscillatory electric fields at frequencies where Faradaic reactions are negligible. Streamfunctions for the steady component of the EHD flow were determined with an electro-osmotic slip boundary condition on the electrode surface. Accordingly, it was established how the axisymmetric flow along the electrode is related to the dipole coefficient of the colloidal particle. Under certain conditions, the flow is directed toward the particle and decays as r(-4), in accord with observations of long-range particle aggregation. To test the theory, particle-tracking experiments were performed with fluorescent 300 nm particles around 50 mu m particles over a wide range of electric field strengths and frequencies. Treating the particle surface conductivity as a fitting parameter yields velocities in excellent agreement with the theoretical predictions. The observed frequency dependence, however, differs from the model predictions, suggesting that the effect of convection on the charge distribution is not negligible as assumed in the zero Peclet number limit.
The intercalation reaction of graphite oxide with diaminoalkanes, with the general formula H2N(CH2)(n)NH2 (n = 4-10), was studied as a method for synthesizing pillared graphite with tailored interlayer spacing. Interlayer spacings from 0.8 to 1.0 nm were tailored by varying the size of the intercalant from (CH2)(4) to (CH2)(10). X-ray diffraction and infrared spectroscopy were used to confirm intercalation, and the frequency of the CH2 stretch confirmed that the intercalants are in a disordered state, with an important contribution from the gauche conformer. Sequential intercalation of diaminoalkanes followed by dodecylamine demonstrated the inability of these "stitched" systems to undergo expansion along the c-direction, indicative of cross-linking. Finally, the reaction of graphite oxide with diaminoalkanes under reflux and for extended periods (> 72 h) resulted in the chemical reduction of the graphite oxide to a disordered graphitic structure.
We have synthesized a catalytically active polymer inspired by the naturally occurring protein silicatein alpha and have shown it to catalyze the formation of silica from tetraethoxysilane under near-neutral pH and ambient temperatures. We based the composition of the polymer on the functionalities found in silicatein alpha, specifically those essential components of the catalytically active site for the hydrolysis of silicon alkoxides. Our bioinspired polymer is a block copolymer of poly(2-vinylpyridine-b-1,2-butadiene), functionalized by the addition of hydroxyl groups via hydroboration chemistry. The catalytic action of our polymer on tetraethoxysilane at neutral pH and ambient temperature conditions has been confirmed using a modified molybdic acid assay method, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The structure of the resulting gel is investigated by scanning electron microscopy and solid-state nuclear magnetic resonance. The microscopic features of the material formed resemble that of gels formed by the acid-catalyzed hydrolysis of tetraethoxysilane.
A detailed analysis of the thermal expansion mechanism of graphite oxide to produce functionalized graphene sheets is provided. Exfoliation takes place when the decomposition rate of the epoxy and hydroxyl sites of graphite oxide exceeds the diffusion rate of the evolved gases, thus yielding pressures that exceed the van der Waals forces holding the graphene sheets together. A comparison of the Arrhenius dependence of the reaction rate against the calculated diffusion coefficient based on Knudsen diffusion suggests a critical temperature of 550 degrees C which must be exceeded for exfoliation to occur. As a result of their wrinkled nature, the functionalized and defective graphene sheets do not collapse back to graphite oxide but are highly agglomerated. After dispersion by ultrasonication in appropriate solvents, statistical analysis by atomic force microscopy shows that 80% of the observed flakes are single sheets.
We demonstrate improved atomic force microscopic imaging of surfactant surface aggregates, featuring an increase in the topography contrast by several hundred percent with respect to all previous studies. Surfactant aggregates on rough gold surfaces, which could not be imaged previously because of low resolution, display substantially different morphologies when compared with atomically smooth materials.
A modified graphite oxide material contains a thermally exfoliated graphite oxide with a surface area of from ∼300 m2/g to 2600 m2/g, wherein the thermally exfoliated graphite oxide displays no signature of the original graphite and/or graphite oxide, as detd. by x-ray diffraction. The modified graphite oxide is useful in nanocomposites with, e.g., PMMA. [on SciFinder(R)]