Transport mechanisms thermodynamic factors

When it comes to the equilibration of water concentration gradients, the relevant transport coefficient is the chemical diffusion coefficient, Dwp. This parameter is related to the self-diffusion coefficient by the thermodynamic factor (see above) if the elementary transport mechanism is assumed to be the same. The hydration isotherm (see Figure 8) directly provides the driving force for chemical water diffusion. Under fuel-cell conditions, i.e., high degrees of hydration, the concentration of water in the membrane may change with only a small variation of the chemical potential of water. In the two-phase region (i.e., water contents of >14 water molecules... 

Molecular diffusion remains as the dominant transport mechanism through the best barriers achievable [Daniel 1988]. The rate of transport is governed by several factors, including the inherent properties of the materials, thermodynamic factors, and barrier design. We estimate the diffusion through frozen ground, as follows. 

The kinetic analysis of a complicated electrochemical process involves two crucial steps the validation of the proposed mechanism and the extraction of the kinetic parameter values from experimental data. In cyclic voltammetry, the variable factor, which determines the mass transfer rate, is the potential sweep rate v. Therefore, the kinetic analysis relies on investigation of the dependences of some characteristic features of experimental voltammograms (e.g., peak potentials and currents) on v. Because of the large number of factors affecting the overall process rate (concentrations, diffusion coefficients, rate constants, etc.), such an analysis may be overwhelming unless those factors are combined to form a few dimensionless kinetic parameters. The set of such parameters is specific for every mechanism. Also, the expression of the potential and current as normalized (dimensionless) quantities allows one to generalize the theory in the form of dimensionless working curves valid for different values of kinetic, thermodynamic, and mass transport parameters. 

Fiber coagulation is much more complicated, with respect to flat-sheet membrane, since it involves two surfaces. The thermodynamics and kinetics of PI are affected by both the coagulants. As discussed in Section 1.4, the main factors are the nonsolvent power, its mutual affinity with the dope solvent, the solubility parameter differences between P-NS and S-NS, the solvent and nonsolvent diffusivities connected to their molecular size, and, obviously, temperature. Tasselli and Drioli [43] showed that hollow-fiber morphology, transport, and mechanical properties can be tailored... 

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