Improved design and protocol for evapoporometry determination of the pore-size distribution

• Evapoporometry technique enhanced for characterizing pore-size distribution.
• Test cell modified for reduced mass and to control evaporation rate.
• Analysis protocol incorporated t-layer to boost accuracy for nanometer-scale pores.
• Higher resolution balance detects larger pores and gives lower experimental error.
• No significant difference between replicate measurements and remeasurements.

The capability of evapoporometry (EP) for determining the pore-size distribution (PSD) of flat sheet and hollow fiber membranes, and to characterize internal pore fouling in membranes has been demonstrated in prior studies. A modified test cell, improved data-analysis protocol, and a higher resolution microbalance have been employed in this study to advance the EP technique. The test cell was modified to incorporate a lid with a small hole to control the evaporation rate and to reduce its mass to stay within the tare limits of a higher resolution microbalance. An experimental protocol was developed for determining the effect of the t-layer on the PSD, which is a nanoscale layer of the wetting liquid that adsorbs on both the walls of the test cell and the pores. EP characterization using two microbalances having resolutions of 1 and 10 µg, respectively, was compared by remeasurement of the same sample and replicate runs with different samples. It is shown that the ability of a microbalance to resolve differences in the pore size increases with its mass resolution, but decreases as the pore size increases. For this reason a microbalance with a higher mass resolution can improve the resolution of the larger pores in a membrane that play an important role in determining membrane permeability and selectivity. The PSD was determined for nominal 100 nm Anopore® and polyethersulfone (PES) membranes. A comparison of the average pore diameters based on replicate and remeasurement runs indicated that the variation in determining the PSD is due primarily to experimental variability rather than to differences in the membrane samples. This study indicated that the accuracy of EP is compromised if the evaporation rates are too high. Optimizing the EP procedure involves a trade-off between achieving sufficient accuracy in determining the PSD and the required time for the characterization procedure.