Sorption measurements, membrane

J. S. Mackie and P. Meares (86) could largely avoid these, however, by introducing an empirical formula of the activity coefficients. For the purpose they carried out sorption measurements at ion-exchange membranes. 

Nitrogen sorption measurements were performed by use of a Sorptomatic 1900 Turbo apparatus by Carlo Erba Instruments. All samples were degassed at 393 K before measurement for at least 24 hours at 10 mbar. The mercury porosimetry measurements were carried out on a Porosimeter 2000 apparatus by Carlo Erba Instruments. A contact angle of 141.3° for Hg was used. The samples were degassed at 393 K before measurement for 24 h. SEM of the porous glass membranes was carried out on a Phillips ESEM XL 30 PEG microscope. 

Henry constants and saturation concentrations obtained from the gravimetric sorption measurements on silicalite particles and diffusion constants obtained by the membrane method of methane and n-butane. After Vroon et al. [72-74]... 

Further anomalies are seen in the diffusional behavior of cesium ion for the perfluorosulfonate membrane. Figure 10 represents the plot of the logarithm of the diffusion coefficient vs. the function Vp/(l-Vp), where Vp is the volume fraction of polymer in the water-swollen material and 1 - Vp is the volume fraction of water, as calculated from sorption measurements. This plot corresponds to a test of the equation... 

The results of ion and water sorption measurements for the two polymers under these solution conditions help to explain this difference. Table 5 " lists the concentrations of various sorbed species and the mole ratio of water to cation/anion in the polymer phase for NaCl and NaOH solution environments. This ratio decreases both in the polymers and in solution with increasing concentration. In solution, the ratio varies from 10.8 to 4.0 over the concentration range of 5-12.5 M NaOH, so that ions in the polymer phase exist in a significantly less aqueous environment compared to the solution phase. As noted by Mauritz and co-workers for perfluorosulfonate membranes, these water contents are insufficient to provide even primary hydration spheres for sodium ions, sorbed anions, and exchange sites, and the likelihood... 

Mauritz and co-workersand Hopfinger have reported spectroscopic and sorption studies of perfluorosulfonate membranes. Several conclusions drawn in these studies are useful in the interpretation of current efficiencies as a function of caustic solution concentration. Sorption measurements of an 1100-EW film in equilibrium with NaOH solutions from 7.5 to 18 M show that the... 

For the case of equilibrium methods, for mesoporous materials gas porosimetry is complemented by Small Angle Neutron Scattering to obtain information on pore size distribution. For microporous membranes the extraction of structural information from the equilibrium sorption measurements can be based on techniques like Grand Canonical Monte Carlo Simulation. 

Permeabilities measured for pure gases can serve as a rough guide for selection of membrane materials. For design, data must be obtained on gas mixtures, where selectivities are often found to be much lower than those calculated from pure-component measurements. This effect is often due to plasticisation of the membrane by sorption of the most soluble component of the gas. This allows easier penetration by the less-permeable components. The problem of concentration polarisation, which is often encountered in small-scale flow tests, may also be responsible. Concentration polarisation results when the retention time of the gas in contact with the membrane is long. This allows substantial depletion of the most permeable component on the feed side of the membrane. The membrane-surface concentration of that component, and therefore its flux through the membrane, decreases. 

Pulsed field gradient (PFG)-NMR experiments have been employed in the groups of Zawodzinski and Kreuer to measure the self-diffusivity of water in the membrane as a function of the water content. From QENS, the typical time and length scales of the molecular motions can be evaluated. It was observed that water mobility increases with water content up to almost bulk-like values above T 10, where the water content A = nn o/ nsojH is defined as the ratio of the number of moles of water molecules per moles of acid head groups (-SO3H). In Perrin et al., QENS data for hydrated Nation were analyzed with a Gaussian model for localized translational diffusion. Typical sizes of confining domains and diffusion coefficients, as well as characteristic times for the elementary jump processes, were obtained as functions of A the results were discussed with respect to membrane structure and sorption characteristics. ... 

This condition has been recently used in a vaporization-exchange model for water sorption and flux in phase-separated ionomer membranes. The model allows determining interfacial water exchange rates and water permeabilities from measurements involving membranes in contact with flowing gases. It affords a definition of an effective resistance to water flux through the membrane that is proportional to... 

The sorption and the permeation of pure CO2, CH4, N2 and O2 through an ABS membrane (Lustran 246) have been measured (87). 

The quantitative formulation of the site sorption mode, on the other hand, has the virtue of simplicity, but is undoubtedly rather highly idealised. Ideally, Eq. (9) refers to a collection of distinct, permanent and independent sites each accomodating one penetrant molecule Sq measures the concentration of these sites in the membrane and K2 their affinity for the penetrant assuming them to be isoenergetic 35,36). On this basis, the temperature dependence of K2 should yield a constant enthalpy for this sorption mode AH2 35,36). Consistency with the physical picture presented above requires moreover that AH2 be more exothermic than AH, U). 

If the component phases denoted by A and B are of sufficient, but still microscopic, size (cf. introductory section) and do not interact appreciably, their individual sorption and diffusion properties may be deduced from measurements on the pure bulk phases. Then, the overall solubility coefficient is given by an additive relation analogous to Eq. (5), except that the volume fractions vA, vB( = 1 — vA) of the respective components in the membranes must be taken into account ... 

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