DOI: 10.15125/bath-00222
title: Molecular Weight Cut-Off Determination of Organic Solvent Nanofiltration Membranes Using Poly(propylene) Glycols

description: Dataset relating to Journal Article

A new method for determining the molecular weight cut-off (MWCO) of an organic solvent nanofiltration (OSN) membrane has been developed utilising poly(propylene) glycol (PPG) oligomers. This new MWCO method overcomes the limitations of the currently popular methods: namely the high molecule cost in the popular polystyrene method, the Donnan Exclusion effects when using dye molecules and the solvent compatibility and HPLC separation resolution limitations of the lesser used poly(ethylene) glycol (PEG) method. A new reverse phase high-performance liquid chromatography separation with evaporative light scattering detection (ELSD) allows the concentration of each oligomer of PPG to be accurately determined and from this the MWCO curves are constructed. The method has a high resolution (size increment of 58 g mol-1 corresponding to the OCH(CH3)CH2 structural unit) and can be used in polar, polar aprotic, and non-polar solvents. The accuracy of the method has been demonstrated in three different solvents (methanol, acetone, and toluene) and 5 different OSN membranes (DuraMemTM 150, 200, 500, PuraMemTM 280 and StarMemTM 240). Other advantages include; oligomers of PPG are cheap and widely available, can probe a wide range of MWCO and provide high resolution MWCO curves. Consequently, it is proposed that that this method be adopted as a new standard MWCO test for OSN membranes.
description: Tripropylene glycol (Alfa Aesar), PPG 400 (Alfa Aesar), PPG 725 (Sigma Aldrich), PPG 1000 (Alfa Aesar) of reagent grade were used as the PPG oligomer samples. Organic solvents (HPLC grade) methanol, acetone, acetonitrile and toluene were purchased from VWR and used as received. The membranes used in this study were purchased from Evonik (UK) (DuraMemTM 150, DuraMemTM 200, DuraMemTM 500 and PuraMemTM 280) and Membrane Extraction Technologies (StarMemTM 240). Ultrapure water (18.2 MΩ·cm at 25 °C) and HPLC grade acetonitrile (Alfa Aesar) were used in the gradient elution of the HPLC method. Nitrogen (BOC, 99.998 %) was used as carrier gas for the ELSD.

To determine the MWCO of the commercial OSN membranes, filtration experiments were conducted using a stainless steel dead end filtration cell (HP4750, Sterlitech Corporation USA; active membrane surface area of 14.6 cm2). A magnetic stirrer just above the membrane surface was used for mixing of the feed and minimizing concentration polarization. Typically, solutions of PPG were made up by dissolving 4 g of each PPG sample (tripropylene glycol, 400, 725, 1000) in 1 L of solvent. Before use, membranes were conditioned with the appropriate solvent at the desired applied pressure and temperature for around 2 hours until a consistent flux was achieved. For each experiment a fresh membrane was used to avoid any effects of cross contamination of samples. To perform the MWCO determination, 40 mL of feed was added to the cell of which half was permeated using an applied pressure of 30 bar, except for DuraMemTM 500 where an applied pressure of 10 bar was used. The temperature was maintained at 30 ºC using a water bath.

Determination of the concentration of each PPG oligomer in the feed, permeate and retentate was conducted using reverse phase high-performance liquid chromatography (HPLC) equipped with an evaporative light scattering detector (ELSD). An Agilent Technologies (1260 Infinity) HPLC system consisting of a quaternary pump (G1311B), autosampler (G1329B), column oven (G1316A) and ELSD (GB1530001) was equipped with a Poroshell 120 EC-C18 (4.6 x 50 mm 2.7 μm) column to achieve separation. 

Before analysis, all samples in water miscible solvents were diluted with water by 1/3 (400 μL sample and 800 μL water) to give an improved chromatogram as this reduces fronting of the peaks and gives an improved baseline. Non-polar solvents were evaporated in vacuo and replaced with an equal amount of methanol. This mixture was then diluted with water as above before HPLC analysis.

Further details can be found in (J. Membr. Sci., 2017, 526, 221-228.)
description: Further relevant documentation may be found in the following resources.

   Davey, C. J., Low, Z.-X., Wirawan, R. H., and Patterson, D. A.,  2017.   Molecular weight cut-off determination of organic solvent nanofiltration membranes using poly(propylene glycol).    Journal of Membrane Science, 526, 221-228.  Available from: https://doi.org/10.1016/j.memsci.2016.12.038.