Name: exploring the radical nature of a carbon surface by electron paramagnetic resonance and a calibrated gas flow
Uploaded: 2014-04-24T14:15:00
Description: Watch this Scientific Journal Video about Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow at JoVE.com

SR acknowledges the support of the Israel science foundation, grant no. 280/12.

1. Preparing Carbon Samples
<ol>
	<li>Grind the carbon samples to the desired fraction size (here, coal samples were ground to a fraction size of between 74-250 mm).</li>
	<li>During the grinding process the grinder should be held in a regulated environment (AC cooled to 20 &#176;C). Additionally, purging the grinder chamber with a flow of nitrogen gas prior to grinding minimizes oxidation at this stage.</li>
	<li>Transfer the carbon samples to sealable canisters and replace the air atmosphere with nitrogen. Keep the s...

<ol>
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Surface oxidation of carbon materials is of significant industrial and academic interest. The effects of carbon substrate oxidation have been characterized with a wide range of analytical techniques including EPR. When investigating the interaction of molecular oxygen with carbon substrate such as coal which has a propensity to undergo oxidation (hence its main utilization as an energy resource) sample preparation and storage is extremely important.
Our samples are coal substrates that have be...

Substrates of varying (wt%) ratios of C/H/O atoms present different types and concentrations of stable radicals that are detectable via Electron Paramagnetic Resonance (EPR)8. These radicals depend on the structure of the macromolecules and are highly influenced by their aromatic nature. The EPR spectrum of coal radicals is characterized by a single broad resonance. In such cases, only the g-value, the line width and the spin concentration can be obtained. The g-values of EPR spectra can be used to determine whether a radical is carbon-centered or oxygen-centered. The basic equation for the electron Zeeman interaction <img alt="Equation 1" fo:content-width="0.8in" src="/files/ftp_upload/51548/51548eq1.jpg" /> defines the g-value, where h is the Planck constant, v is the constant mw frequency applied in the experiment, B0 is the resonance magnetic field and &#946;e is the Bohr magneton. For free electrons the g-value is 2.00232. Variations in the g-value from the 2.00232 are related to magnetic interactions involving the orbital angular momentum of the unpaired electron and its chemical environment. Organic radicals usually have g-values close to the free electron g, which depends on the location of the free radical in the organic matrix3,8-10. Carbon-centered radicals have g-values that are close to the free electron g-value 2.0023. Carbon-centered radicals with an adjacent oxygen atom have higher g-values in the range of 2.003-2.004, while oxygen centered radicals have g-values that are &#62;2.004. The g-value of 2.0034-2.0039 is characteristic for carbon-centered radicals in a nearby oxygen heteroatom that results in increased g-values over that of purely carbon-centered radicals11-15. Line-width is governed by the spin-lattice relaxation process. Therefore, an interaction between adjacent radicals or between a radical and paramagnetic oxygen results in a decrease in the spin lattice relaxation time, and hence, an increase in the line-width4-6.
Stopped flow experiments with EPR detection allow the observation of time-dependent changes in the amplitude of an EPR signal at a distinct field value during the interaction of two phases by time sweep acquisition (kinetic display). The result of such a measurement is a rate constant for the formation, decay or conversion of a paramagnetic species. The procedure is analogous to the well-established case of stopped flow operation with optical detection in which a time-dependence of the optical absorption at a distinct wavelength is observed. Typically stopped flow experiments are conducted in a liquid state as radicals that are not EPR detected in liquid state due to short relaxation time T1, as e.g.&#160;hydroxyl (OH&#215;) or superoxide (O2-) cannot be studied directly by EPR-stopped flow techniques. It is, however, possible to study the spin-adducts of these radicals with nitrones, yielding nitroxide-type radicals (spin-traps), as they are EPR-active and their kinetics can be monitored also by stopped flow EPR16-18.
The method of measurement of rates of chemical reactions using fast-flow gaseous techniques with EPR detection has also previously been established19-22. In essence, the method depends on the measurement, by EPR, of the concentration of a reactant as a function of distance (and thus at a constant velocity, the time) over which the reactant has been in contact with a reactive gas in the flow tube. Conditions whereby the concentration of the reactive gas is approximately constant are usually employed so that the measured decay is pseudo first order.
In the current work, a simple gas flow setup was implemented and a constant flow of gas was introduced to the surface of the solid carbon substrate.
With the method detailed in the current work we succeeded in achieving interesting results where this interaction of molecular oxygen with a certain part of the existing stable radical structure can be reversibly affected simply by flowing a diamagnetic gas through the carbon samples at STP. As a result of this method the removal of the interacting paramagnetic gas uncovers a new radical surface with a g value, which is closer to that of a free electron.

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exploring the radical nature of a carbon surface by electron paramagnetic resonance and a calibrated gas flow

While the first Electron Paramagnetic Resonance (EPR) studies regarding the effects of oxidation on the structure and stability of carbon radicals date back to the early 1980s the focus of these early papers primarily characterized the changes to the structures under extremely harsh conditions (pH or temperature)1-3. It is also known that paramagnetic molecular oxygen undergoes a Heisenberg spin exchange interaction with stable radicals that extremely broadens the EPR signal4-6. Recently, we reported interesting results where this interaction of molecular oxygen with a certain part of the existing stable radical structure can be reversibly affected simply by flowing a diamagnetic gas through the carbon samples at STP7. As flows of He, CO2, and N2 had a similar effect these interactions occur at the surface area of the macropore system.
This manuscript highlights the experimental techniques, work-up, and analysis towards affecting the existing stable radical nature in the carbon structures. It is hoped that it will help towards further development and understanding of these interactions in the community at large.

When preforming the EPR experiments on various coal samples, as a function of the exposure time to a diamagnetic gas flow it was noted that during the gas flow, a second species at g~2.0028 appeared. This g-value is close to the value of a free electron and consistent with unsubstituted aliphatic carbon centered radicals. However, the total spin concentration for each sample remained constant within our experimental error (&#177;10%). Figure 3A presents two scans: 0 sec and 1,900 sec after the coal sampl...

Watch this Scientific Journal Video about Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow at JoVE.com

Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow

Stable radicals that are present in carbon substrates interact with paramagnetic oxygen through a Heisenberg spin exchange. This interaction can be significantly reduced under STP conditions by flowing a diamagnetic gas over the carbon system. This manuscript describes a simple method to characterize the nature of those radicals.

While the first Electron Paramagnetic Resonance (EPR) studies regarding the effects of oxidation on the structure and stability of carbon radicals date ...

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Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension.
Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries.
Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( &#820;500 mN/m to near zero). Furthermore, this method requires only a very modest potential (&#60; 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

We present a method to control the interfacial energy of a liquid metal in an electrolyte via electrochemical deposition (or removal) of a surface oxide layer. This simple method can control the capillary behavior of gallium-based liquid metals by tuning the interfacial energy rapidly, significantly, and reversibly using modest voltages.

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where ...

Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The procedure is performed under an inert atmosphere using air- and water-exclusion techniques. The outcome of the polymerization is affected by the ratios of monomer, initiator, and catalyst used as well as the reaction concentration, solvent, and nature of the light source. Temporal control over the polymerization can be exercised by turning the visible light source off and on. Low dispersities of the resultant polymers as well as the ability to chain-extend to form block copolymers suggest control over the polymerization, while chain end-group analysis provides evidence supporting an atom-transfer radical polymerization mechanism.

A method for the atom transfer radical polymerization of functionalized vinyl monomers using perylene as a visible-light photocatalyst is described.

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The ...

In Situ Monitoring of Diffusion of Guest Molecules in Porous Media Using Electron Paramagnetic Resonance Imaging

A method is demonstrated to monitor macroscopic translational diffusion using electron paramagnetic resonance (EPR) imaging. A host-guest system with nitroxide spin probe 3-(2-Iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (IPSL) as a guest inside the periodic mesoporous organosilica (PMO) aerogel UKON1-GEL as a host and ethanol as a solvent is used as an example to describe the protocol. Data is shown from a previous publication, where the protocol has been applied to both IPSL and Tris(8-carboxy-2,2,6,6-perdeutero-tetramethyl-benzo[1,2-d:4,5-d&#8242;]bis(1,3)dithiole) methyl (Trityl) as guest molecules and UKON1-GEL and SILICA-GEL as host systems. 
A method is shown to prepare aerogel samples that cannot be synthesized directly in the sample tube for measurement due to a size change during synthesis. The aerogel is attached to sample tubes using heat shrink tubing and a pressure cooker to reach the necessary temperature without evaporating the solvent in the process. The method does not assume a clearly defined initial distribution of guest molecules at the start of the measurement. Instead, it requires a reservoir on top of the aerogel and experimentally determines the influx rate during data analysis.
The diffusion is monitored continually over a period of 20 hr by recording the 1d spin density profile within the sample. The spectrometer settings for the imaging experiment are described quantitatively. Data analysis software is provided to take the resonator sensitivity profile into account and to numerically solve the diffusion equation. The software determines the macroscopic translational diffusion coefficient by least square minimization of the difference between the experiment and the numerical solution of the diffusion equation.

In Situ Monitoring of Diffusion of Guest Molecules in Porous Media Using Electron Paramagnetic Resonance Imaging

A protocol for the in situ monitoring of the diffusion of guest molecules in porous media using electron paramagnetic resonance (EPR) imaging is presented.

A method is demonstrated to monitor macroscopic translational diffusion using electron paramagnetic resonance (EPR) imaging. A host-guest system with ...

Sediment Core Extrusion Method at Millimeter Resolution Using a Calibrated, Threaded-rod

Aquatic sediment core subsampling is commonly performed at cm or half-cm resolution. Depending on the sedimentation rate and depositional environment, this resolution provides records at the annual to decadal scale, at best. An extrusion method, using a calibrated, threaded-rod is presented here, which allows for millimeter-scale subsampling of aquatic sediment cores of varying diameters. Millimeter scale subsampling allows for sub-annual to monthly analysis of the sedimentary record, an order of magnitude higher than typical sampling schemes. The extruder consists of a 2 m aluminum frame and base, two core tube clamps, a threaded-rod, and a 1 m piston. The sediment core is placed above the piston and clamped to the frame. An acrylic sampling collar is affixed to the upper 5 cm of the core tube and provides a platform from which to extract sub-samples. The piston is rotated around the threaded-rod at calibrated intervals and gently pushes the sediment out the top of the core tube. The sediment is then isolated into the sampling collar and placed into an appropriate sampling vessel (e.g., jar or bag). This method also preserves the unconsolidated samples (i.e., high pore water content) at the surface, providing a consistent sampling volume. This mm scale extrusion method was applied to cores collected in the northern Gulf of Mexico following the Deepwater Horizon submarine oil release. Evidence suggests that it is necessary to sample at the mm scale to fully characterize events that occur on the monthly time-scale for continental slope sediments.

An extrusion method using a calibrated threaded-rod is presented, which allows for mm scale subsampling of aquatic sediment cores. Millimeter-scale sampling is necessary to fully characterize recent event stratigraphy in sediment records.

Aquatic sediment core subsampling is commonly performed at cm or half-cm resolution. Depending on the sedimentation rate and depositional environment, ...

Failure of Cleaning Verification in Pharmaceutical Industry Due to Uncleanliness of Stainless Steel Surface

The aim of this work is to identify the parameters that affect the recovery of pharmaceutical residues from the surface of stainless steel coupons. A series of factors were assessed, including drug product spike levels, spiking procedure, drug-excipient ratios, analyst-to-analyst variability, intraday variability, and cleaning procedure of the coupons. The lack of a well-defined procedure that consistently cleaned the coupon surface was identified as the major contributor to low and variable recoveries. Assessment of cleaning the surface of the coupons with clean-in-place solutions (CIP) gave high recovery (&#62;90%) and reproducible results (Srel&#8804;4%) regardless of the conditions that were assessed previously. The approach was successfully applied for cleaning verification of small molecules (MW &#60;1,000 Da) as well as large biomolecules (MW up to 50,000 Da).

The lack of a well-defined procedure that consistently cleaned coupon surfaces was identified as the major contributor to low and variable recoveries in cleaning verification. This manuscript describes the correct protocol of cleaning stainless steel coupons.

The aim of this work is to identify the parameters that affect the recovery of pharmaceutical residues from the surface of stainless steel coupons. A ...

Grafting Multiwalled Carbon Nanotubes with Polystyrene to Enable Self-Assembly and Anisotropic Patchiness

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a free-radical polymerization strategy to enable the modulation of the nanotube surface properties and produce supramolecular self-assembly of the nanostructures. First, a selective hydroxylation of the pristine nanotubes through a biphasic catalytically mediated oxidation reaction creates superficially distributed reactive sites at the sidewalls. The latter reactive sites are subsequently modified with methacrylic moieties using a silylated methacrylic precursor to create polymerizable sites. Those polymerizable groups can address further polymerization of styrene to produce a hybrid nanomaterial containing PS chains grafted to the nanotube sidewalls. The polymer-graft content, amount of silylated methacrylic moieties introduced and hydroxylation modification of the nanotubes are identified and quantified by Thermogravimetric Analysis (TGA). The presence of reactive functional groups hydroxyl and silylated methacrylate are confirmed by Fourier Transform Infrared Spectroscopy (FT-IR). Polystyrene-grafted carbon nanotube solutions in tetrahydrofuran (THF) provide wall-to-wall collinearly self-assembled nanotubes when cast samples are analyzed by transmission electron microscopy (TEM). Those self-assemblies are not obtained when suitable blanks are similarly cast from analogous solutions containing non-grafted counterparts. Therefore, this method enables the modification of the nanotube anisotropic patchiness at the sidewalls which results into spontaneous auto-organization at the nanoscale.

A procedure for the synthesis of polystyrene-grafted multiwalled carbon nanotubes using successive chemical modification steps to selectively introduce the polymer chains to the sidewalls and their self-assembly via anisotropic patchiness is presented.

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a ...

Electrochemical Impedance Spectroscopy as a Tool for Electrochemical Rate Constant Estimation

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). In the case of fast reversible electrochemical processes, current is predominantly affected by the rate of diffusion, which is the slowest and limiting stage. EIS is a powerful technique that allows separate analysis of stages of charge transfer that have different AC frequency response. The capability of the method was used to extract the value of charge transfer resistance, which characterizes the rate of charge exchange on the electrode-solution interface. The application of this technique is broad, from biochemistry up to organic electronics. In this work, we are presenting the method of analysis of organic compounds for optoelectronic applications.

Electrochemical impedance spectroscopy (EIS) of species that undergo reversible oxidation or reduction in solution was used for determination of rate constants of oxidation or reduction.

Electrochemical impedance spectroscopy (EIS) was used for advanced characterization of organic electroactive compounds along with cyclic voltammetry (CV). ...

Surface Properties of Synthesized Nanoporous Carbon and Silica Matrices

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) with a 4.6 nm pore size, and ordered silica porous matrix, SBA-15, with a 5.3 nm pore size. This work describes the surface properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the differently ordered porous materials with similar pore sizes. For this purpose, OMC and SBA-15 with highly ordered nanoporous structures are synthesized via impregnation of the silica matrix by applying a carbon precursor and by the sol-gel method, respectively. The porous structure of investigated systems is characterized by an N2 adsorption-desorption analysis at 77 K. To determine the electrochemical character of the surface of synthesized materials, potentiometric titration measurements are conducted; the obtained results for OMC shows a significant pHpzc shift toward the higher values of pH, relative to SBA-15. This suggests that investigated OMC has surface properties related to oxygen-based functional groups. To describe the surface properties of the materials, the contact angles of liquids penetrating the studied porous beds are also determined. The capillary rise method has confirmed the increased wettability of the silica walls relative to the carbon walls and an influence of the pore roughness on the fluid/wall interactions, which is much more pronounced for silica than for carbon mesopores. We have also studied the melting behavior of D2O confined in OMC and SBA-15 by applying the dielectric method. The results show that the depression of the melting temperature of D2O in the pores of OMC is about 15 K higher relative to the depression of the melting temperature in SBA-15 pores with a comparable 5 nm size. This is caused by the influence of adsorbate/adsorbent interactions of the studied matrices.

Here we report the synthesis and characterization of ordered nanoporous carbon (with a 4.6 nm pore size) and SBA-15 (with a 5.3 nm pore size). The work describes the surface and textural properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the materials.

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) ...