Application of Evapoporometry to Characterizing the Pore-Size Distribution of Hollow Fiber Membranes

Application of Evapoporometry to Characterizing the Pore-Size Distribution of Hollow Fiber Membranes

Ebrahim Akhondi^a, Filicia Wicaksana^a,b, William B. Krantz^a,c

^aSingapore Membrane Technology Center, Nanyang Technological University, Singapore

^bDept. of Chemical and Materials Engineering, University of Auckland, New Zealand

^cDept. of Chemical and Biological Engineering, University of Colorado, USA

Evapoporometry (EP) is a novel means for characterizing the pore-size distribution (PSD) 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 that is described by the Kelvin equation.  If a wetting liquid is used to completely fill the pores, the vapor pressure will be depressed, whereas if a non-wetting liquid is used, the vapor pressure will be enhanced.  If the membrane is pre-saturated with a wetting volatile liquid, the evaporation rate will monotonically decrease as a function of time because the liquid will evaporate progressively from the largest to the smallest pores. This is because the gas phase immediately adjacent to the membrane surface will be supersaturated in the vapor of the volatile liquid with respect to any pores having a diameter smaller than that of the pores that are draining.

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 on the pan of a microbalance that permits measuring the 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. The PSD then is then expressed as the mass fraction of pores as a function of the pore diameter; that is, EP gives a mass-based PSD and mass-based average pore diameter. In contrast to other techniques for determining the PSD, EP does not require assuming a pore geometry.

In prior studies EP was applied to characterizing the PSD of flat sheet membranes using only isopropyl alcohol (IPA) as the volatile wetting liquid. In this presentation the adaptation of EP to characterizing the PSD of polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) hollow fiber ultrafiltration (UF) membranes using both IPA and water as the volatile wetting liquids will be discussed. The EP test cell and procedure had to be modified to properly seal the hollow fibers at the base of the test cell and to accommodate the free convection mass transfer associated with using water whose vapor, in contrast to IPA, is less dense than air.

EP characterization of the PSD is compared to that determined by liquid displacement porometry (LDP). This comparison indicates that the high pressures used in the LDP characterization (as high as 3.5 MPa) caused compaction of the UF membranes that resulted in the shift of the PSD to smaller pores. EP offers an attractive alternative to LDP since it does not require using any transmembrane pressure or specialized expensive equipment.