Heterogeneity, chemical molecular

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

G. Pacchioni, J.R. Lomas, and F. Illas, Electric field effects in heterogeneous catalysis, Molecular Catalysis A Chemical 119, 263-273 (1997). 

The species boundary condition at the stagnation surface follows from the fact that the diffusive mass flux in the fluid is balanced by a heterogeneous chemical reaction rate on the surface. In general, this can involve multiple and complex surface reactions and complex descriptions of the molecular diffusion. Here, however, we restrict attention to a single species that is dilute in a carrier gas and a single first-order surface reaction. Under these circumstances the surface reaction rate (mass of Y consumed per unit surface area) is given... 

As has been demonstrated, the combination of selective separation techniques with powerful spectroscopic detectors enables complex polymers to be analyzed with respect to all possible types of molecular heterogeneity. Chemical composition distribution can be monitored across the molar mass distribution. Steric and functional peculiarities can be detected over the entire molar mass range. 

Understanding of sediment diagenesis and benthic fluxes has evolved with advances in both experimental methods and modeling. Measurements of chemical concentrations in sediments, their associated pore waters and fluxes at the sediment-water interface have been used to identify the most important reactions. Because transport in pore waters is usually by molecular diffusion, this medium is conducive to interpretation by models of heterogeneous chemical equilibrium and kinetics. Large chemical changes... 

One important aspect of heterogeneous chemical reactions at solid surfaces is related to the presence of adsorbed species that play a key role in determining the rate and efficiency of these processes. Therefore, the knowledge of molecular arrangements on solid catalysts of reactants, reaction intermediates, or products is of outstanding importance in dealing with fundamental aspects of heterogeneous catalysis. 

There is also a need for a more detailed chemical understanding of these material systems. Inevitably, they involve chemical processes occurring at a dynamic heterogeneous interface. The well-defined chemical rules that apply in homogeneous solutions no longer apply. Instead, we must deal with chemical (molecular) interactions in which spatial distribution of active sites occurs at the nanodimensional level, and the fact that the nature of these sites varies as a function of time and environmental stimuli is important. 

In amorphous polymers, discontinuities are caused by the heterogeneous chemical structure of molecular chains or by massive cross-link points in thermoset plastics. Their influence on thermal conductivity depends on their mean cross-link distance D relative to the dominant phonon wavelength ... 

For a long time, the use of chromatography was mainly limited to the determination of molecular masses and to the qualitative estimation of the heterogeneity of the samples. At present, it is a highly efficient technique giving very accurate values [82-89]. Copolymers are complex macromolecular systems, characterized by two distributions in molecular mass and chemical composition liquid chromatography used at the critical point of adsorption allows the determination of the molecular heterogeneities (chemical and molecular mass distributions) [83]. 

Studies of surfaces and surface properties can be traced to the early 1800s [1]. Processes that involved surfaces and surface chemistry, such as heterogeneous catalysis and Daguerre photography, were first discovered at that time. Since then, there has been a continual interest in catalysis, corrosion and other chemical reactions that involve surfaces. The modem era of surface science began in the late 1950s, when instmmentation that could be used to investigate surface processes on the molecular level started to become available. 

Physical and Chemical Properties of Lignosulfonates. Even unmodified lignosulfonates have complex chemical and physical properties. Their molecular polydispersiti.es and stmctures are heterogeneous. They are soluble ia water at any pH but iasoluble ia most common organic solvents. 

---