Chemical substances moving between phases

Chemical substances can move between different phases in a system that is not at equilibrium. Modelers sometimes refer to the movement of chemicals between environmental "compartments" via such mechanisms, that is, between soil, water, sediment, and air. 

Evidently, in the course of layer formation the plane of inert markers cannot coincide with the initial interface between substances A and B. It would mean that compound layers could grow at the expense of one component. Chemically, this is impossible since any binary compound consists of two components. Position of the layers relative to the initial interface is mainly dependent upon the stoichiometry of chemical compounds, if both ends of a couple are equally free to move. Coincidence of initial and marker planes provides evidence for the lack of contact between reacting phases at that place. 

HPLC is performed on analytical columns packed with a commercially available solid phase containing long hydrocarbon chains (e.g. Cg, Cig) chemically bound onto silica. Chemicals injected onto such a column move along at different rates because of the different degrees of partitioning between the mobile aqueous phase and the stationary hydrocarbon phase. The HPLC method is not applicable to strong acids and bases, metals complexes, surface-active materials, or substances that react with the eluent. The HPLC method is applicable when the log Kow value falls within the range 0 to 6 (OECD 117, 1989). The HPLC method is less sensitive to the presence of impurities in the test compound compared to the shake-flask method. 

Mass transfer processes are governed by the driving force difference in the chemical potentials, the physical proportions (mass transfer coefficient) of the substance, and the surface area (the interface between the phases to be separated). This is known from the basic transport equations of heat and mass transfer. A large surface area, therefore, favors separation processes. A suspension with a distribution of mainly small particles would feature a high interface area. There is, however, a limitation to the size of the disperse solid phase. This is due to the necessary liquid-solid separation at the end of the process, on the one hand, and the necessity of the disperse phase to move in different directions as the main flow direction of the continuous phase so that a maximum of the driving potential between the two phases can be maintained, on the other hand. 

The fundamental driving force that prompts a molecule to diffuse within a polymer or transfer between a polymer and a surrounding phase is its chemical potential. Like the electrical potential of a battery causes electrons to flow through wires, chemical potential is the driving force in physical chemical phenomena. Substances will naturally tend to move from a higher chemical potential to a lower one. The equation for chemical potential is ... 

Chromatographic separation can be defined as the process that involves the distribution of different substances between two phases, a stationary phase and a mobile phase. Solutes distributed preferentially in the mobile phase will move more rapidly through the column than those distributed preferentially in the stationary phase. As a consequence of this, the solutes will elute in order of their increasing partition coefficients towards the stationary phase. During the separation process, it can be assumed that in each point of the column a chemical equilibrium involving the solute molecule will be established between the stationary and the mobile phase. This equilibrium is called partition equilibrium , and it is governed by noncovalent forces... 

When a fluid substance such as carbon dioxide or water is heated under pressure, the density of the liquid phase will decrease as the temperature is increased, whereas the density of the gas phase will increase with increased pressure. If a temperature-pressure phase diagram is constructed, and if we move upward along the curve separating the liquid phase from the gas phase, a temperature and pressure will be reached where the densities of the liquid and gas phases become identical and there will be no distinction between the gas and liquid phases. This point on this phase diagram is known as the critical point, and a supercritical fluid is any fluid that is at a temperature and pressure greater than those at the critical point. The critical temperature and pressure will vary according to the chemical structure of the fluid. For example, carbon dioxide (CO ) has a critical temperature of 304 K and a critical pressure of 74 bar, whereas the critical temperature and pressure of water are 647 K and 221 bar, respectively (Williams et al, 2002). Above the critical temperature, a gas cannot be liquefied by pressure. 

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