Oxygen, diffusion coefficient Permeability

The diffusion of oxygen is of particular importance as the corrosion of steel reinforcement in concrete is regarded. Ludwig [157] was among the first authors who initiated the studies focused ou this problem. The linear relation between the permeability and oxygen diffusion rate was found, as presented in Fig. 5.72 [138]. It has been proved in the same experiments that the CO2 diffusion can be evaluated based on the oxygen diffusion coefficient. 

Fig. 5.72 Relation between the permeability of mortar and oxygen diffusion coefficient. (according to[138])...

Auras and co-workers [61] studied the variations in the oxygen diffusion solubility, permeability coefficients and water sorption of polylactide films at different temperatures (5, 23, and 40 °C) and water activities (A = 0-0. 9). The results were compared with the oxygen diffusion, solubility and permeability coefficients obtained for PET films under the same experimental conditions. 

In Figure 3, we have presented the experimentally obtained reciprocal values of (Di )t.ff of oxygen in a sample of the nano-porous hydrophobic material as a function of the total pressure P of gas mixture (02-N2) when the oxygen concentration in the mixture is 21%. From the intercept of the straight line with the ordinate the value of the Knudsen diffusion coefficient can be also determined. It must be underlined that the value of Knudsen diffusion coefficient obtained by these diffusion measurements (2,86.10"2 cm2/s) is in very good coincidence with the value obtained by the gas permeability measurements. 

Table 7.3. Relative Fluorescence Quantum Yield of Crystal Violet,

The effects of equivalent weight (FW = g polymer/mol SO3H) and water content on diffusion coefficient, solubility, and permeability of oxygen for fully hydrated BAM, S-SEBS, ETFE- -PSSA, Nafion 117, and BPSH membranes have been studied. It has been found that the diffusion coefficients of all the studied membranes decrease with increasing EW, while the solubility correspondingly increases. These trends are the same as found in... 

Table I. Oxygen and water vapour permeabilities (P02 PH2O) with mean deviation (m.d.), water solubility (S1J20) and diffusion coefficient (Djj o) at 21°C.

Verhallen P.T.H.M., Oomen L.J.P., v.d.Elsen A.J.J.M., Kruger A.J., and Fortuin J.M.H. (1984) The diffusion coefficients of helium, hydrogen, oxygen and nitrogen in water determined from the permeability of a stagnant liquid layer in the quasi-steady state. Chem. Eng. Sci. 39, 1535-1541. 

Reasonable prediction can be made of the permeabilities of low molecular weight gases such as oxygen, nitrogen, and carbon dioxide in many polymers. The diffusion coefficients arc not complicated by the shape of the penneant, and the solubility coefficients of each of these molecules do not vary much from polymer to polymer. Hence, all that is required is some correlation of the penneant size and the size of holes in the polymer matrix. Reasonable predictions of the pemieabilides of larger molecules such as flavors, aromas, and solvents are not easily made, The... 

From these results, the permeability of oxygen in a Nation film can be obtained as the product of its concentration by its diffusion coefficient in the film DfCf. Figure 1.15 shows the variation of the oxygen permeability with the temperature. Like many perfluorinated materials, the Nation membrane is especially a good solvent of oxygen. 

Example 9-10. Estimate the average value for the diffusion coefficient for oxygen and carbon dioxide in polypropylene with 50% degree of crystallinity at 30 °C. Use the corresponding molecule diameter o and the average Tg value of 267 K for PP. Calculate the corresponding permeability coefficient value using the solubility coefficient value determined in Example 9. 

Gas transport coefficients of PVC and PVC modified with pyridine groups were studied. It was found that there is a strong time dependence of the permeability and diffusivity of oxygen, nitrogen, carbon dioxide and methane in membranes prepared by solvent casting of PVC and pyridine modified PVC. There is a two-fold reduction for PVC of the diffusion coefficients during the first two days,... 

Oxygen transport measurements were conducted at 25°C, 0% and 50% relative humidity RH, 1 atm partial oxygen pressure difference using the commercially manufactured diffusion apparatus OX-TRAN 2/20 (Modem Control Inc.). This apparatus employs a continuous-flow method (ASTM-D 3985-81) to measure oxygen flux, J(t), through polymer films or thin sheets. In order to obtain the diffusion coefficient and to accurately determine the permeability coefficient, the data, flux, J(t), were fitted to the solution of Fick s second law ... 

The second generation of nonporous membranes was silicon based which displayed increased CO2 permeabilities. In 1965, Bramson et al. commercialized the first nonporous membrane BO [18]. Since the diffusion coefficient of oxygen and carbon dioxide in air is about four orders of magnitude higher than in blood, the gas side mass-transfer resistance was negligible. The major resistance to respiratory gas transfer was due to the membrane and the liquid side concentration boundary layer [19]. Though nonporous membrane BOs reduced blood damage, up to 5.5 m membrane surface area was often required to ensure adequate gas transfer rates. 

---