(695h) Recent Advances in Evapoporometry for Determining the Pore-Size Distribution of Membranes

Evapoporometry (EP) is a novel means for characterizing the pore-size distribution (PSD) of membranes. It is based on the phenomenon described by the Kelvin equation whereby the vapor pressure is reduced owing to the interfacial curvature of a volatile wetting liquid contained in small pores. EP employs a specially designed test cell that permits establishing a partial pressure in the gas phase at the surface of the membrane that corresponds to the vapor pressure of the largest pores that are filled with the volatile wetting liquid. A small hole in the lid of the test cell controls the rate at which evaporation of the volatile wetting liquid occurs from the pores. Owing to the implications of the Kelvin equation, the gas phase adjacent to the membrane is supersaturated with respect to all pores that are smaller than those from which liquid is evaporating. This supersaturation condition causes pores to empty via evaporation progressively from the largest to the smallest. Measuring the instantaneous mass of the test cell containing the membrane sample via a high resolution microbalance permits determining the instantaneous evaporation rate. This can be related to the partial pressure of the volatile wetting liquid at the surface of the membrane via a straightforward analysis of the mass transfer in the test cell. The Kelvin equation then permits determining the instantaneous pore diameter from the partial pressure at the surface of the membrane. The limitations of the Kelvin equation imply that  EP can determine the PSD for flat sheet and hollow fiber ultrafiltration membranes.

EP offers several advantages relative to other methods for determining the PSD. Since the mass can be measured with microgram accuracy, the measurement error in EP is much less than for techniques that require measuring a flow rate, pressure, volume, or heat input. Implementing EP does not require any specialized equipment or expertise. EP also has a relatively small laboratory footprint since the required equipment consists of the test cell, high resolution microbalance, environmental chamber, and an anti-vibration table. Since EP characterization is carried out at the ambient temperature and pressure, it does not alter the morphology of the membrane sample. EP also can be used to determine the PSD of fully hydrated viable biofilms that cannot be done via other PSD characterization methods. A particular advantage of EP is that it can be used to measure the effect of internal pore fouling on the PSD that cannot be done using any method that involves liquid displacement via an inert gas or immiscible liquid.

This presentation will provide an overview of the EP characterization method and discuss recent advances in this technology. The latter include an improved EP test cell design, improving the resolution of EP, extending EP to larger pore sizes, accounting for the effect of the residual adsorbed layer on the pore walls, and adapting EP for characterizing the PSD on both the outer surface and lumen side of hollow fiber membranes.