Ag+, ethylene complexes

For both membranes, the temperature eflTect was less pronounced when the partial pressure of ethylene was din shed. When the ethylene partial pressure was reduced to about 100 mm Hg, there was almost no change in the ethylene permeability with a 20 C temperature increase. However, the same was not true for ethane. At constant temperature, the permeability of ethane was almost constant over the same range of partial pressure. The permeabilities of ethane at 5,25 and 35 C were 3.7, 5.8 and 7.5 Barrer, respectively. This suggests that raising the temperature increases the difEusion coefBcients of the gases in the membranes. The mathematical model developed by Way and Noble (27) was used to determine the diffiision coefficients of uncomplexed ethylene (Da) and the Ag -ethylene complex (Dab)- The equiUbrium constants (Keq) for the reversible reaction between ethylene and Ag were also calculated (Tables V and VI). 

Surprisingly, the temperature does not have a great effect on the equilibrium constant for the reaction of Ag and ethylene. All of the equilibrium constants shown in Tables V and VI are of the same magnitude. In contrast, a temperature increase from 5 to 35 "C enhances the ethylene diffusion coefficients by a factor of 3 to 4. Diffusion coefficients for the Ag -ethylene complex also increase by a factor of 2 to 3 over the same temperature range. This implies that the transport mechanism at low partial pressure of gas is controlled by gas solubility. More likely, a low concentration of gas at the gas-membrane interface limits the amount of gas adsorbed into a membrane. 

These calculated diffusion coefficients and equilibrium constant for Nafion 117 are comparable to the values reported by Eriksen et al. (7d) at 25 °C. Eriksen s effective diffusion coefficient for ethylene, as calculated from Pick s law, was 3.3 X 10 cm sec" The effective diffusion coefficient for the Ag" -ethylene complex. Dab, in their work is 2.0 x 10 cm sec The disagreement in these values is attributed to the differences in the Ag concentration in the Nafion 117... 

For the most part, the separation factor decreases as the temperature increases. The transport model shows that the diffusion coefficient for ethylene is more sensitive to temperature variation than the diffusion coefficient of the Ag -ethylene complex. It seems reasonable that the diffusion coefficient of ethane is increased by a larger number than that of the complex. The decreasing separation factor may simply be explained by the fact that temperature favors the transport of ethane over the Ag" - ylene complex. When the ethylene partial pressure becomes small, the transport mechanism is controlled by gas solubility and the concentration of gas at gas-membrane interface. The amount of Ag -ethylene complex is decreased by the snuill amount of ethylene in the membrane and the effect of temperature becomes pronounced. 

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See also in sourсe #XX -- [ ]

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