Скачать или смотреть Drops: Why is capillary condensation dependent on pore geometry and curvature?

Capillary condensation is a process by which multilayer adsorption from the vapour phase into a porous medium and proceeds to the point at which the pore spaces filled with condensed liquid from the vapour phase. Unlike normal condensation, this process takes place in a porous medium (capillary) until the vapour turns to liquid phase (drops), even though the equilibrium vapour pressure is less than saturation vapour pressure, Psat. How does it work? This is because of the tube wall is so narrow, molecules will have a much stronger intermolecular attraction. So, vapour molecules will combine to form small droplets via van der Waals forces. Small droplets will then join together to form larger droplets, eventually condensed.
We demonstrated a small experiment of capillary condensation. Straws were used as the capillary tube, and they were put in the mouth of a hot kettle. Boiling water inside the kettle will cause the hot steam rising. As steam passes through the straws, it quickly condenses into water, even though the steam is still hot. This phenomenon is called capillary condensation.
Capillary condensation will cause the formation of vapour-liquid interface. Thus it can be modelled using Kelvin equation, which provides the relation between equilibrium vapour pressure and saturation vapour pressure.

ln(Pv/Psat)=-(2H γ Vl)/(RT)

Pv = equilibrium vapour pressure, Psat= saturation vapour pressure
H = mean curvature of meniscus, γ = liquid/vapour surface tension
Vl = liquid molar volume T= temperature
R= ideal gas constant

Notice that the terms are all kept constant, except for H and Pv.
Pores geometry means pores shape. Capillary condensation is affected by shapes as different pore shapes give different equilibrium vapour pressure, and thus different curvature of meniscus. In this video we explained the relationship using cone as example. For cylindrical pores, condensation occurs very quick as it has constant radius, so the liquid reaches equilibrium vapour pressure in a short time. In reality pore geometry are not uniform as irregular shape or dust formation in the tube. So Kelvin equation has to be applied separately at each position.
Curvature of meniscus is measured by contact angle where the three interfaces meet. If the angle increased, the curvature of meniscus will decrease. Curvature, H is equals to ½(1/rm+1/rc), where rm = radius of meniscus; rc =radius of capillary tube. If H decreases, the radius of meniscus will increase. Using Kelvin equation, this will result in Pv increasing, which means lesser amount of vapour will be condensing.
In chemical engineering, common application of capillary condensation is the recovery of oil from tiny pores during fractional distillation of crude oil. By injecting mixture of gas and water into the pores, water will condense and occupy the pores spaces. Since oil and water are immiscible, oil is thus displaced out. Capillary condensation also applies in sintering. Sintering is a process of compacting and forming solid from powdered particles using heat, without the needs of melting the solid. Researchers are developing a way of recovering water from diesel exhaust using this concept.
© Copyright 2015