276425 Recent Developments in Evapoporometry for Characterizing the Pore-Size Distribution of Membranes

Wednesday, October 31, 2012: 4:21 PM
401 (Convention Center )
William B. Krantz, Chemical and Biological Engineering, University of Colorado, Boulder, CO, Alan R. Greenberg, Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO, Elmira Kujundzic, Mechanical Engineering, University of Colorado, Boulder, CO, Adrian Yeo, Singapore Membrane Technology Center, Nanyang Technological University, Singapore, Singapore and Seyed S. Hosseini, Chemical Engineering Department, Tarbiat Modares University, Terhan, Iran

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.

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