Influence of chemical environment

These various broad research observations generated questions about the influence of chemical environments in aquatic systems upon plutonium and what chemical species might be present. The oxidation states of plutonium, its associations with DOC, and its complexation by inorganic ions all seemed interrelated and important to the understanding of environmental transport. 

Larson, T. V. (1989). The influence of chemical and physical forms of ambient air acids on airway doses. Environ. Health Perspect. 79, 7-13. 

Fortin, C. and Campbell, P. G. C. (2000). Silver uptake by the green alga Chlamy-domonas reinhardtii in relation to chemical speciation influence of chloride, Environ. Toxicol. Chem., 19, 2769-2778. 

One of the main themes of this volume is the influence of the environment on chemical reactivity. Such a question is of special interest for chemical systems in far from equilibrium conditions. It is today well known that, far from equilibrium, chemical systems involving catalytic mechanisms may lead to dissipative structures.1-2 It has also been shown—and this is one of the main themes of this discussion—that dissipative structures are very sensitive to global features characterizing the environment of chemical systems, such as their size and form, the boundary conditions imposed on their surface, and so on. All these features influence in a decisive way the type of instabilities, called bifurcations, that lead to dissipative structures. 

The probabilities, or molar fractions, are equal to the fraction of time a nucleus will spend in a particular environment when observed for a suitably long period of time. They do not reveal how long the nucleus will stay, on average, in each particular position in other words, the average lifetime of the nucleus in each environment cannot be calculated from the molar fractions. As will be seen, the chemical lifetime or its reciprocal, the chemical dissociation rate, are crucial parameters governing the influence of chemical exchange on NMR parameters. 

Gulens, J Champ, D.R. and Jackson, R.E. (1979) Influence of redox environments on the mobility of arsenic in groundwater, in Chemical Modeling in Aqueous Systems (ed. E.A. Jenne), American Chemical Society. 

The only remaining problem is calculation of the electron-density function, which cannot be done classically. However, for molecules in condensed phases the influence of the environment introduces another simplification. It has been shown that valence-state wave functions of compressed atoms are simpler, than hydrogenic free-atom functions. Core levels are largely unaffected and a nodeless valence-state wave function, which allows chemical distortion of electron density, can be defined. We return to this topic at a later stage. 

The method of visible and UV absorption spectroscopy is at its best when the absorption spectra of the free ions and the associated ions are quite different and known. When the associated ions cannot be chemically isolated and their spectra studied, the type of absorption by the associated ions has to be attributed to electronic transitions known from other well-studied systems. For example, there can be an electron transfer to the ion from its immediate environment (charge-transfer spectra), i.e., from the entities associated with the ion or transitions between new electronic levels produced in the ion under the influence of the electrostatic field of the species associated with the ion (crystal-field splitting). Thus, there is an influence of the environment on the absorption characteristics of a species, and this influence reduces the clarity with which spectra are characteristic of species rather than of their environment. Herein lies what may be considered a disadvantage of visible and UV absorption spectroscopy. 

The periodic approach is not the only one available for atomistic simulations of these materials and we should first mention that much progress has been made in the application of molecular quantum chemical methods using cluster representations of the local structure of oxide materials [1, 2], More recently, this has given way to mixed quantum mechanics/molecular mechanics (QM/MM) calculations. In QM/MM simulations the important region, the active site for catalysis, is represented at a quantum chemical level while the influence of its environment, the extended solid, is represented using the computationally less-demanding atomistic force field approach. This allows complex structures such as metal particles supported on oxides to be tackled [3]. 

These thermodynamic arguments can also consider the hydroxylahon of surfaces by including the chemical potenhal of hydrogen and/or water [108]. This type of analysis shows that AI2O3 surfaces of the a-phase are either A1 or OH terminated under all accessible condihons. However, the influence of the environment is different for each Miller index owing to variation in the calculated surface energies. 

In metal alloys the combination of stress and environment can also lead to premature failures, indicated as Stress Corrosion Cracking, SCC [1]. The influence of the environment on SCC is generally of a chemical nature a chemical reaction occurs between the metal and the environment. Most of the research published on the ESC of polymers focuses on ESC in which the environment influences the material only physically [2-8]. In such cases the mechanism of ESC is studied and models are established for ESC prediction [9]. These models for physical ESC are based predominantly on the solubility parameters of the considered polymer/environment combination. In other words, ESC is mainly a consequence of polymer softening, i.e. it is a reduction of the interaction between the polymer chains that lowers the yield stress. 

The aim of this paper is to consider ESC under not only the physical but also the chemical influence of the environment. In this case the chemical reactions (for example resulting in chain scission) are important and models based on solubility parameters are not valid. Hardly any literature is available on this subject. Moskala [10] reported chemical ESC of poly(ethylene terephthalate) (PET) in an aqueous sodium hydroxide (NaOH) solution. A discontinuous crack growth with an increased crack growth rate was found upon increasing NaOH concentration. However, one major drawback is that the effect of hydrolysis without external stress was not considered. Hydrolysis on its own can potentially lead to premature failures, whether or not the material is in a stressed or unstressed condition. 

Gulens, J., Champ, D. R., and Jackson, R. E., 1979, Influence of Redox Environments on the Mobility of Arsenic in Ground Water in Jenne, E. A., ed.. Chemical modeling in aqueous systems, p. 81-95. 

The influence of chemical equilibrium and/or kinetics on the progress of chemical reactions often determines the abundance, distribution, and fate of substances in the environment. An understanding of the basic concepts of chemical equilibrium and chemical kinetics, therefore, may help us to explain and predict the environmental concentrations of inorganic and organic species in aqueous systems, whether these species are present naturally or have been introduced by humans. In this chapter we will examine chemical equilibrium. The following chapter considers chemical kinetics or the study of rates of chemical reactions. 

The influences of hostile environment on polymers -heat, irradiation, mechanical and electrical abuse, the action of aggressive chemicals - may be generally classified in a) changes of the chemical character and microstructure of the individual macromolecules, and b) changes in the supermolecu-lar structure and texture of the system. 

For practical applications, the strategy was initially exploited by Gerhardt and Adams who determined the diffusion coefficients of several chemical species of neuro-biological interest [72]. They reported the influence of the environment on the diffusion of [Fe(CN)g]3 and [Fe(CN)e]4 into 1.0—2.0 mol L 1 KC1 and 1.0 mol L 1 NaOH solutions Dm values within the 4.7 x 10 6 — 7.6 x 10 6cm2s 1 range were determined. Moreover, they determined the Dm value for the thallium(I) ion (Tl+) in 1.0 mol L-1 KC1 as 1.49 x 10 5 cm2 s, which is in agreement with literature data. Other applications are summarised in Table 5.1. This... 

Williams, III, D. P., Pao, P. S., and Wei, R. P., The Combined Influence of Chemical, Metallurgical and Mechanical Factors on Environment Assisted Cracking, in Environment Sensitive Fracture of Engineering Materials, Z. A. Foroulis, ed., The Minerals, Metals, and Masterials Society-American Institute of Mining, Metallurgical, and Petroleum Engineers (TMS-AIME) (1979), 3-15. 

Shih, T. T., and Wei, R. P., Influences of Chemical and Thermal Environments on Delay in a Ti-6A1-4V Alloy, in Fatigue Crack Growth Under Spectrum Loads, ASTM STP 595, American Soc. of Testing and Materials, Philadelphia, PA (1976), 113-124. 

Azcue JM, Nriagu JO (1994) Arsenic historical perspectives. In Nriagu JO (ed) Arsenic in the Environment. Part 1. Cycling and Characterization. John Wiley Sons Inc, New York, p 1-15 Babich H, Stotzky G (1983) Influence of chemical speciation on the toxicity of heavy metals to the microbiota. Adv Environ Sci Technol 13 1-46... 

This table gives information on the geometric structure of selected molecules in the gas phase, including the overall geometry, interatomic distances, and bond angles. The molecules have been chosen to provide data on a wide variety of chemical bonds and to illustrate the influence of molecular environment on bond distances and angles. The table is restricted to molecules with conventional covalent or ionic bonds, but it should be pointed out that structure data on many loosely bonded complexes of the van der Waals type have recently become available. The references below contain data on many molecules that are not included here and give additional information such as uncertainties and isotopic variations. 

Question B. J. Briscoe (Imperial College of Science and Technology) Early work suggests that the wear rate in these systems is quite sensitive to counterface temperature. Did you control counterface temperature in your experiments Have you any data on wear in different environments which might reveal the influence of chemical processes ... 

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