Atomic force microscopy characterization of ultrafiltration membranes: correspondence between surface pore dimensions and molecular weight cut-off

Abstract

Atomic force microscopy (AFM) has been used to quantify the mean surface pore diameters of a family of five ultrafiltration membranes with molecular weight cut-off (MWCO) in the range 1000-10 000. High aspect ratio tips were used and the images were treated with fast Fourier transform filtering to remove distortions. The mean pore diameters were found to be in the range 1.93-3.14 nm, increasing systematically with the MWCO. Mean pore diameters have also been evaluated from the MWCO values using three published expressions relating MWCO to solute radius. Depending on the expression used, the ratio of the mean pore diameters obtained from MWCO data and AFM measurements (R r ) was in the range 1.04-2.42. Agreement between the diameters obtained in the two ways was best for the membranes with the lowest MWCO values. The overall agreement was substantially better than has been reported previously. The good correspondence between mean surface pore dimensions and MWCO demonstrates the usefulness of AFM in quantifying such a key membrane property.

A wide range and high resolution one-filtration molecular weight cut-off method for aqueous based nanofiltration and ultrafiltration membranes

“…Table 1 gives the MWs and suppliers for the commercial PEG and purer PEG standard used in the method. The properties of commercial membranes used as well as the filtration pressures are provided in 6 GE Osmonics™ GE 1000 [34,35] Composite Polyamide NF…”

“…1.88±0.088 1.08±0.061 1.12 1294±20 1000 [34,35] GE Osmonics™ GH 3.22±0.17 1.71±0.060 3.33 1866±373 1000-2500 [30,34,36,37] GE Osmonics™ GK 9.28±0.99 5.64±0.53 5.66 3956±716 2000-3500 [30,34,36,38] TriSep UA60 9.35±0.73 5.07±0.38 7.94 <678 1000-3500 [39] Millipore Ultracel PLAC04310 2.45±0.062 1.98±0.019 ---1030±62 1000 [40] Millipore Ultracel PLBC04310 6.98±0.70 4.64±0.35 ---1404±31 1000-3000* [41,42] * Manufacturer states a MWCO of 3000 but notes that the MWCO could be 2-3 times smaller than the molecular weight of the solute to be retained when using regenerated cellulose. ) indicates that it may be that the MWCO of the supplied membranes is in fact incorrect.…”

Applications of AFM in Membrane Characterization and Fouling Analysis

“…The results indicated that the average pore diameter of the five membranes fell within the range of 1.93–3.14 nm, increasing with the rise in MWCO. They also used MWCO to estimate the aperture, and the results showed good consistency, which also confirmed the role of AFM in the study of MWCO …”

“…They also used MWCO to estimate the aperture, and the results showed good consistency, which also confirmed the role of AFM in the study of MWCO. 302 The primary limitation affecting the resolution of AFM measurements is the size of the tip. When the scan tip's size is comparable to the membrane pore's aperture, it becomes impractical for the tip to enter the pore and accurately determine its exact size.…”