Evapoporometry: A novel technique for determining the pore-size distribution of membranes

• Novel technique advanced for determining the pore-size distribution of microporous membranes.
• Based on the Kelvin effect that describes the vapor pressure reduction in small pores.
• Evaporation rate determined via gravimetric measurements using a microbalance.
• Validated using 100 nm Anopore™ and 20 and 50 nm PVDF membranes.
• Detailed error analysis confirms high accuracy.

Synthetic membranes are used in applications such as controlled drug release, artificial organs, smart sensors, and water treatment that require knowing the pore-size distribution (PSD). Current PSD characterization methods can require dedicated and/or expensive equipment as well as special expertise to implement. Evapoporometry (EP) is a novel characterization technique based on vapor pressure depression that can accurately determine the pore size for ultrafiltration membranes using a simple diffusion cell and a microbalance. A straightforward characterization procedure and comprehensive error analysis are described. EP characterization of the PSD of a 100 nm bilayer Anopore™ membrane used as a standard is shown to compare favorably with the nominal pore diameter, FESEM analysis, and analyses reported by prior investigators. In addition, EP is utilized for PSD characterization of commercially fabricated 20 and 50 nm polyvinylidene fluoride (PVDF) membranes for which the results are compared with those obtained via liquid displacement porometry (LDP). The mean pore diameters determined by EP and LDP are in good agreement for the 50 nm PVDF membranes, but the lower values obtained via LDP for the 20 nm PVDF membranes may reflect the effects of high pressures that are not required in EP. The advantages and limitations of EP as well as other potential applications of this technique are discussed.

(A) Test cell used in the evapoporometry technique for determining the pore-size distribution of membranes having diameters ranging between 4 and 150 nm. The flat-sheet membrane is sealed at the bottom of the test cell and overlain with a wetting noninteracting volatile liquid. (B) The test cell is placed on the pan of a microbalance that permits determining the instantaneous mass from which the evaporation rate can be obtained. After the overlying liquid layer has evaporated, the evaporation progresses from the largest to the smallest pores owing to the vapor pressure depression that is described by the Kelvin effect. Evaporation from the overlying liquid layer permits determining the mass-transfer coefficient. This mass-transfer coefficient in combination with the instantaneous evaporation rate during pore draining then permits determining the mole fraction of the vapor at the surface of the membrane. The pore diameter then can be determined from the Kelvin equation. The gravimetric measurements then permit determining the mass-based pore-size distribution and mass-average pore diameter. Figure optionsDownload high-quality image (132 K)Download as PowerPoint slide