Capillary condensation is
the "process by which multilayer adsorption from
the vapor [phase] into a porous medium proceeds
to the point at which pore spaces become filled with condensed liquid from the
vapor [phase]." The unique
aspect of capillary condensation is that vapor condensation occurs below the saturation
vapor pressure, Psat, of the pure
liquid. This result is due to an
increased number of van der Waals interactions
between vapor phase molecules inside the confined space of a capillary. Once
condensation has occurred, a meniscus immediately
forms at the liquid-vapor interface which allows for equilibrium below
the saturation
vapor pressure. Meniscus formation is dependent
on the surface tension of
the liquid and the shape of the capillary, as shown by the Young-Laplace equation.
As with any liquid-vapor interface involving a menisci, the Kelvin equation provides
a relation for the difference between the equilibrium vapor pressure and the
saturation vapor pressure. A
capillary does not necessarily have to be a tubular, closed shape, but can be
any confined space with respect to its surroundings.
Capillary condensation is
an important factor in both naturally occurring and synthetic porous
structures. In these structures, scientists use the concept of capillary
condensation to determine pore size distribution and surface area though
adsorption isotherms. Synthetic
applications such as sinterin of
materials are also highly dependent on bridging effects resulting from
capillary condensation. In contrast to the advantages of capillary
condensation, it can also cause many problems in materials science applications
such as Atomic Force Microscopy and Microelectromechanical Systems. See below
figure 1.
Figure 1: An example of a porous structure
exhibiting capillary condensation.
Pores that are not of the same size will fill at
different values of pressure, with the smaller ones filling first. This difference in filling rate can be
a beneficial application of capillary condensation. Many materials have
different pore sizes with ceramics being one of the most commonly encountered.
In materials with different pore sizes, curves can be constructed similar to
Figure 2. A detailed analysis of the shape of these isotherms is done using the Kelvin equation. This enables the pore size distribution to be determined. While this is a relatively simple
method of analyzing the isotherms, a more in depth analysis of the isotherms is
done using the BET method. Another
method of determining the pore size distribution is by using a procedure known
as Mercury Injection Porosimetry. This uses the volume of mercury taken up by
the solid as the pressure increases to create the same isotherms mentioned
above. An application where pore size is beneficial is in regards to oil
recovery. When recovering oil from tiny pores, it is useful to inject gas and
water into the pore. The gas will then occupy the space where the oil once was,
mobilizing the oil, and then the water will displace some of the oil forcing it
to leave the pore.
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Figure 2: Capillary condensation profile
showing a sudden increase in adsorbed volume due to a uniform capillary radius
(dashed path) among a distribution of pores and that of a normal distribution
of capillary radii (solid path)
