Water sorption osmotic pressure

The data for the crystal structure, NMR structure, powder x-ray diffraction pattern, water vapor sorption isotherms, glass transition temperature as a function of water, heat capacity, heat of solution properties, vapor pressure, and osmotic pressure are described in the literature. 

It is assumed that cement paste plays a role of semi-permeable membrane which is impermeable for the silicate anions formed as a result of reaction. Some authors postulate that this semi-permeable membrane is composed of sodium and potassium sUicates gel formed during reaction. A concentration gradient appears and linked with him the osmotic pressure. Recently, the difference in discussions of swelling mechanism, attributed to the sorption of water or to the osmotic pressure, became insignificant. There is a view that sodium and potassium ions can diffuse through the semi-permeable membrane more easily than the calcium ions. 

Future work should scrutinize these structural aspects and refine corresponding approaches. The effective permittivity of water in pores increases with pore size, resulting in lower osmotic pressures, compared to the constant permittivity case due to a modified proton distribution. Consequently, the water uptake should be less steep at large RH. The inclusion of a Stern layer would further affect the proton distribution and, hence, water sorption. In fact, a modified Poisson-Boltzmann approach might be better suited to describe the electrostatic phenomena in such PEM pores. 

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. 

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