(539d) Recent Developments in Evapoporometry for Characterizing the Pore-Size Distribution of Membranes

Recent
Developments in Evapoporometry for Characterizing the Pore-Size Distribution of
Membranes

William B. Krantz, Alan
R. Greenberg, Elmira
Kujundzic,
Adrian
Yeo, and Seyed S.
Hosseini

Evapoporometry (EP)is
a novel means for characterizing the pore-size distribution in membranes. It is
based on the principle that the vapor pressure is affected by the curvature of
a volatile liquid contained in a porous material, which is described by the
Kelvin equation.  If a wetting liquid is
used to saturate the pores, the vapor pressure will be depressed, whereas if a
non-wetting liquid is used, the vapor pressure will be enhanced.  EP involves first saturating the membrane
with a volatile liquid that is not a solvent or swelling agent for the membrane
material. The sample is then placed in a specially designed test cell that is
placed on a microbalance that permits measuring sample mass as a function of
time.  The slope of the mass versus time
curve provides the evaporation rate, which can then be related to the vapor
pressure at the interface between the liquid in the porous material and the
ambient gas phase.  The vapor pressure in
turn can be related to the pore diameter via the Kelvin equation.  If the porous material is pre-saturated with
a wetting volatile liquid, the evaporation rate will monotonically decrease as
a function of time.  This behavior
results because the liquid will evaporate progressively from the largest to the
smallest pores.

The accuracy and
reproducibility of EP are confirmed by characterizing the pore-size
distribution of 100 nm Anopore membranes. The latter are an alumina-based
inorganic membrane that has very regular non-interconnected columnar pores. The
results of EP characterization are compared with SEM measurements and with SEM and
AFM analyses of other investigators. EP is also used to characterize 20 nm and
50 nm commercial PVDF membranes and compared with an independent
characterization of the pore-size distribution via displacement porometry. This
comparison reveals substantial compaction effects during the displacement
porometry analyses that shift the pore-size distribution towards smaller pores.
This observation is not surprising since pressures as high as 30 bar were
required for the liquid displacement. However, it underscores one of the
advantages of EP in that it permits characterizing the pore-size distribution
of fragile and compressible membrane materials since it can be done at
atmospheric pressure.

Other advantages as
well as the limitations of EP will be discussed. In particular, a detailed
error analysis is presented that shows that EP is a very accurate method for
characterizing small pores, 2-100 nm, but incurs significant error for pores
larger than approximately 200 nm. Although EP is applied to flat sheet membranes
in the work presented here, the use of this technique for other applications
will be outlined.