Water sorption capillary equilibrium

The subsequently presented model of water sorption in PEMs reconciles vapor sorption and porosity data. At sufficiently large water contents exceeding the amount of surface water, T > equilibrium water uptake is controlled by capillary forces. Deviations from capillary equilibrium arising at A < can be investigated by explicit ab initio calculations of water at dense interfacial arrays of protogenic surface groups. ° In the presented model, the problem of Schroeder s paradox does not arise and there is no need to invoke vapor in pores or hydrophobicity of internal channels. Here, we will present a general outline... 

Local equilibrium of water in CCLs. How does the loeal water eontent in CCLs depend on materials morphology and operating eonditions By whieh mechanisms does it attain local equilibrium The approaeh pursued in [50, 51] proposes that capillary forces at the liquid-gas interfaces in pores equilibrate the local water content in CCLs. This approach neglects surface film formation or droplet formation in pores of CLs. Ex situ diagnostics, probing porous structures and water sorption characteristics, are needed to relate equilibrated water distributions to structure and conditions in CCLs. 

At sufficient water contents, exceeding the amount of surface water, X > Xg, equilibrium water uptake is established by the action of capillary forces. To support this hypothesis, isopiestic vapor sorption isotherms for Nafion, in Figure 2.17a, are compared with data on pore size distributions in Figure 2.17b, obtained by standard porosimetry. In Figure 2.17a, a simple fit function. 

Membrane structure and external conditions determine water sorption and swelling. The resulting water distribution determines transport properties and operation. Water sorption and swelling are central in rationalizing physical properties and electrochemical performance of the PEM. The key variable that determines the thermodynamic state of the membrane is the water content k. The equilibrium water content depends on the balance of capillary, osmotic, and electrostatic forces. Relevant external conditions include the temperature, relative humidity, and pressure in adjacent reservoirs of liquid water or vapor. The theoretical challenge is to establish the equation of state of the PEM that relates these conditions to A.. A consistent treatment of water sorption phenomena, presented in the section A Model of Water Sorption, revokes many of the contentious issues in understanding PEM structure and function. 

SPME was developed by Pawlisz)m and coworkers in 1987 [161-163]. The reader may find further information on the historical evolution, principles, and commercially available devices of SPME in an excellent review by the pioneer of the technique [164]. SPME is based on a partitioning equilibrium of the solutes between the sorbent phase and the aqueous and/or gas matrix. A small amount of sorbent phase is dispersed on a solid support, which will be exposed to the sample for a predetermined time. Different implementations were developed such as suspended particles, coated-stirrer, vessel walls, disks, stirrers, or membranes, although the fiber and in-tube are explored theoretically and experimentally in depth. The former consists of a thin, fused-silica fiber-coated with sorbent on its surface and mounted in a modified GC syringe, which protects the fiber and allows handling. The latter in-tube implementation consists of an internally coated tube or capillary. The analytes are extracted by sorption when either coated fiber or tube are immersed in the water sample (direct SPME) or in the headspace above the sample (HS-SPME). 

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