(584a) A Novel Technique for Characterizing Membrane Pore Size - Evapoporometry

This paper describes a new technique for characterizing the pore-size distribution in membranes and other microporous materials. This technique is based on the principle that the vapor pressure is affected by the curvature of a volatile liquid contained within the pores of a porous material. This effect is described by the Kelvin equation, which has been shown to be applicable to pores down to the nanometer scale. If a wetting volatile liquid is used to saturate the pores, the vapor pressure will be depressed, whereas if a non-wetting volatile liquid is used, the vapor pressure will be enhanced.

This technique involves saturating the pores with either a wetting or non-wetting volatile liquid, sealing the membrane to be characterized within an appropriately designed test cell, and then measuring the evaporation rate of the volatile liquid as a function of time. If the test cell is appropriately designed, it is possible to determine the effective vapor pressure from the instantaneous evaporation rate. The diameter of the pores draining at any time then can be determined from the vapor pressure via the Kelvin equation.

We refer to this novel technique as ?evapoporometry'. Its advantages are multifold. It can determine the pore-size distribution for a membrane sample over a continuous range of diameters from the micron down to the nanometer scale. This technique can be used to determine the pore-size distribution of both flat sheet as well as hollow fiber membranes. Evapoporometry can also characterize the asymmetry of a membrane. A major advantage is that evapoporometry can be implemented at a very low cost relative to the instrumentation required for any other method for determining pore size. In this paper we test evapoporometry with microporous materials used as standards. These include track-etched polycarbonate (Nuclepore) films with nominal pore diameters of 10, 30, 50, 100 and 200 nm and porous bilayer aluminum oxide samples (Anopore) with nominal pore diameters of 20 and 100 nm. The evapoporometry results are compared with the manufacturer's specifications for these membranes and with image analysis of scanning electron micrographs.