Migration Control factors

The concept of a characteristic reaction temperature must, therefore, be accepted with considerable reservation and as being of doubtful value since the reactivity of a crystalline material cannot readily be related to other properties of the solid. Such behaviour may at best point towards the possible occurrence of common controlling factors in the reaction, perhaps related to the onset of mobility, e.g. melting of one component or eutectic formation, onset of surface migration or commencement of bulk migration in a barrier phase. These possibilities should be investigated in detail before a mechanism can be formulated for any particular chemical change. 

The mode of action of the antifouling polymers thus conforms to the bulk abiotic bond cleavage model. All the controlling factors, viz., diffusion of water into the polymer matrix, hydrolysis of the tributyltin carboxylate, diffusion of tributyltin species from the matrix to the surface, phase transfer of the organotin species, and its migration across the boundary layer, are analyzed. It is found that the transport of the mobile tributyltin species in the matrix is the rate limiting factor. 

The liquid acts as a barrier to free migration of gas. The rate of diffusion of gas is brought to a steady state when the rate of diffusion of the gas into the liquid on the upstream side is just equal to the rate at which the gas is diffusing out of the membrane on the downstream side. Various steady-state conditions may exist according to pressure (the solubility of gases in liquids increases with pressure), temperature (the solubility of gases in liquids decreases with temperature but the rate of diffusion increases), and other controllable factors. The inieirelaiionships of these factors can be predicted from Ficl s laws of diffusion and the gas laws. 

As the distribution of samples of a certain succession is more or less even over the export and import fields we may assume that silica has migrated locally from the exporters , i.e. fine-grained sandstones, towards importers , i.e. coarser-grained sandstones, and in this way on the local scale an approximate mass balance was maintained. Nevertheless, the overall data show that an important transfer of silica has taken place on a much wider scale and that this is closely related to temperature as the controlling factor for intergranular pressure solution. In zones of higher thermal maturity pressure solution probably was so efficient that it could act simultaneously as the source and driving force for the mass transfer of silica. In zones of low thermal maturity pressure solution was not particularly effective for the mass transfer of silica and the reduction of the primary porosity. 

With very slow reactions (such as between carbon dioxide and water) the dissolved molecules migrate well into the body of the liquid before reaction occurs so that the overall absorption rate is not appreciably increased by the occurrence of the chemical reaction. In this case, the liquid film resistance is the controlling factor, the liquid at the interface can be assumed to be in equilibrium with the gas, and the rate of mass transfer is governed by the molecular CO2 concentration-gradient between the interface and the body of the liquid. At the other extreme are very rapid reactions (such as those of ammonia with strong acids) where the dissolved molecules migrate only a very short distance before reaction occurs. The... 

For many practically relevant material/environment combinations, thennodynamic stability is not provided, since E > E. Hence, a key consideration is how fast the corrosion reaction proceeds. As for other electrochemical reactions, a variety of factors can influence the rate detennining step. In the most straightforward case the reaction is activation energy controlled i.e. the ion transfer tlrrough the surface Helmholtz double layer involving migration and the adjustment of the hydration sphere to electron uptake or donation is rate detennining. The transition state is... 

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