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Topic Review
Material Reactions, Degradation and Applications of Polyvinylidene Fluoride
Polyvinylidene fluoride (PVDF), the chemical formula is (C2H2F2)n. Its basic building blocks are therefore carbon, hydrogen, and fluorine. These three elements can form several crystalline chain conformations. Conformations are defined by polar and nonpolar phases. Four phases are most commonly found in the literature: α-, β-, γ-, and δ-. 
  • 594
  • 17 Oct 2022
Topic Review
WDFW Toxics in Biota Fish Monitoring
Fish are an important component of the Washington Department of Fish and Wildlife's (WDFW) Toxics in Biota Program, part of the Puget Sound Ecosystem Monitoring Program (PSEMP, recently renamed from PSAMP). Since 1989 fish indicator species have been used to monitor contamination of Puget Sound by chemicals such as poly-chlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), pesticides, and metals. English Sole, the first species studied for the Toxics in Biota Program, have been used for monitoring by WDFW since 1989. As of 2015 the program also monitors copper rockfish, quillback rockfish, brown rockfish, coho salmon, chinook salmon, Pacific herring, and Pacific staghorn sculpin. Exposure and effects of each contaminant are monitored for each species, and more in-depth studies of specific regions are performed if monitoring indicates a contamination problem. Geographical characteristics making Puget Sound especially susceptible to contaminants include funneling of fish through narrow inlets, restricted exchange of water, shallow sea floor sills, and freshwater inputs from urban sources. One of the largest sources of contaminants is stormwater runoff. During heavy rain events metals, persistent organic pollutants (POPs), and pesticides flow untreated into local waterways, streams, and lakes. Impervious surfaces such as roads provide a route for deposition of metals and POPs by cars and industrial emissions, before subsequent discharge into nearby water bodies. The highly urbanized coastline of southern Puget Sound results in stormwater runoff containing high concentrations of contaminants.
  • 260
  • 17 Oct 2022
Topic Review
Basis Set
A basis set in theoretical and computational chemistry is a set of functions (called basis functions) that is used to represent the electronic wave function in the Hartree–Fock method or density-functional theory in order to turn the partial differential equations of the model into algebraic equations suitable for efficient implementation on a computer. The use of basis sets is equivalent to the use of an approximate resolution of the identity: the orbitals [math]\displaystyle{ |\psi_i\rangle }[/math] are expanded within the basis set as a linear combination of the basis functions [math]\displaystyle{ |\psi_i\rangle \approx \sum_\mu c_{\mu i} |\mu\rangle }[/math], where the expansion coefficients [math]\displaystyle{ c_{\mu i} }[/math] are given by [math]\displaystyle{ c_{\mu i} = \sum_{\nu} \langle \mu|\nu \rangle^{-1} \langle \nu |\psi_i \rangle }[/math]. The basis set can either be composed of atomic orbitals (yielding the linear combination of atomic orbitals approach), which is the usual choice within the quantum chemistry community; plane waves which are typically used within the solid state community, or real-space approaches. Several types of atomic orbitals can be used: Gaussian-type orbitals, Slater-type orbitals, or numerical atomic orbitals. Out of the three, Gaussian-type orbitals are by far the most often used, as they allow efficient implementations of Post-Hartree–Fock methods.
  • 930
  • 17 Oct 2022
Topic Review
1,3-Butadiene
1,3-Butadiene is the organic compound with the formula (CH2=CH)2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a monomer in the production of synthetic rubber. The molecule can be viewed as the union of two vinyl groups. It is the simplest conjugated diene. Although butadiene breaks down quickly in the atmosphere, it is nevertheless found in ambient air in urban and suburban areas as a consequence of its constant emission from motor vehicles. The name butadiene can also refer to the isomer, 1,2-butadiene, which is a cumulated diene with structure H2C=C=CH−CH3. This allene has no industrial significance.
  • 1.2K
  • 17 Oct 2022
Topic Review
Applications of Cu-Based Nanomaterials from Plant-Mediated Synthesis
Plants have been used for multiple purposes over thousands of years in various applications such as traditional Chinese medicine and Ayurveda. The special properties of phytochemicals within plant extracts have spurred researchers to pursue interdisciplinary studies uniting nanotechnology and biotechnology. Plant-mediated green synthesis of nanomaterials utilises the phytochemicals in plant extracts to produce nanomaterials. Principles of plant-mediated Cu-based nanomaterials in biomedical and environmental applications are discussed.
  • 448
  • 17 Oct 2022
Topic Review
Preparation of MXene/Elastomer Nanocomposites
Two-dimensional (2D) transition-metal carbon/nitrogen/carbon nitride (MXene) has extremely high conductivity and easily modifiable surface functional groups. Compared with graphene, another 2D layered material, MXene is easily dispersed in water owing to its hydrophilic groups. Its unique characteristics make MXene a valuable material. Nanocomposites can be endowed with functionality when MXene is compounded with an elastomer. Particularly in electromagnetic interference shielding and sensing, MXene exhibits extraordinary properties. 
  • 384
  • 14 Oct 2022
Topic Review
Steady State
In chemistry, a steady state is a situation in which all state variables are constant in spite of ongoing processes that strive to change them. For an entire system to be at steady state, i.e. for all state variables of a system to be constant, there must be a flow through the system (compare mass balance). A simple example of such a system is the case of a bathtub with the tap running but with the drain unplugged: after a certain time, the water flows in and out at the same rate, so the water level (the state variable Volume) stabilizes and the system is in a steady state. The steady state concept is different from chemical equilibrium. Although both may create a situation where a concentration does not change, in a system at chemical equilibrium, the net reaction rate is zero (products transform into reactants at the same rate as reactants transform into products), while no such limitation exists in the steady state concept. Indeed, there does not have to be a reaction at all for a steady state to develop. The term steady state is also used to describe a situation where some, but not all, of the state variables of a system are constant. For such a steady state to develop, the system does not have to be a flow system. Therefore, such a steady state can develop in a closed system where a series of chemical reactions take place. Literature in chemical kinetics usually refers to this case, calling it steady state approximation. In simple systems the steady state is approached by state variables gradually decreasing or increasing until they reach their steady state value. In more complex systems state variable might fluctuate around the theoretical steady state either forever (a limit cycle) or gradually coming closer and closer. It theoretically takes an infinite time to reach steady state, just as it takes an infinite time to reach chemical equilibrium. Both concepts are, however, frequently used approximations because of the substantial mathematical simplifications these concepts offer. Whether or not these concepts can be used depends on the error the underlying assumptions introduce. So, even though a steady state, from a theoretical point of view, requires constant drivers (e.g. constant inflow rate and constant concentrations in the inflow), the error introduced by assuming steady state for a system with non-constant drivers may be negligible if the steady state is approached fast enough (relatively speaking).
  • 1.1K
  • 14 Oct 2022
Biography
Austin James Barnes
Austin James Barnes was born in Blackpool on 2 June 1945. He went to Gonville and Caius College, Cambridge, in 1963 and obtained his BA degree in 1966. He then moved to the University College of Swansea, where he was awarded his PhD degree in 1969 for research on the infrared spectra of matrix-isolated species under the supervision of the late Harry Hallam. After a period of postdoctoral researc
  • 641
  • 14 Oct 2022
Topic Review
Recrystallization
In chemistry, recrystallization is a technique used to purify chemicals. By dissolving both impurities and a compound in an appropriate solvent, either the desired compound or impurities can be removed from the solution, leaving the other behind. It is named for the crystals often formed when the compound precipitates out. Alternatively, recrystallization can refer to the natural growth of larger ice crystals at the expense of smaller ones.
  • 1.3K
  • 14 Oct 2022
Topic Review
Nonaqueous Titration
Nonaqueous titration is the titration of substances dissolved in solvents other than water. It is the most common titrimetric procedure used in pharmacopoeial assays and serves a double purpose: it is suitable for the titration of very weak acids and very weak bases, and it provides a solvent in which organic compounds are soluble. The most commonly used procedure is the titration of organic bases with perchloric acid in anhydrous acetic acid. These assays sometimes take some perfecting in terms of being able to judge the endpoint precisely. The Karl Fischer titration for water content is another nonaqueous titration, usually done in methanol or sometimes in ethanol. Since water is the analyte in this method, it cannot also be used as the solvent.
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  • 14 Oct 2022
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