(531e) Three-Dimensional Imaging of Polymeric Membranes for Determination of Critical Structural Metrics in Osmotic Membranes

Engineered osmosis (EO) is a platform membrane separations technology  with applications in water desalination (forward osmosis, FO), power production (pressure-retarded osmosis, PRO), and  dewatering (direct osmotic concentration, DOC).  A key obstacle in EO is the lack of an appropriately designed membrane that minimizes mass transfer resistance during osmosis.  The most common resistance has been identified as internal concentration polarization (ICP), which is a film resistance that occurs in an asymmetric membrane’s support structure.  ICP combines the resistance to diffusion of solutes in contact with the membrane with the resistances imparted by the membrane support layer itself.   The support layer properties that most impact ICP are the porosity, the thickness and the tortuosity of the overall structure. These are often combined into a single parameter, S, which represents a combined term (thickness X tortuosity/porosity). However this parameter has so far only been calculated from experimental flux measurements combined with model fitting, leading to erroneous calculations that can vary greatly from test to test because of inaccurate underlying assumptions.  One could directly calculate these parameters if they were easily measured, but there are a lack of suitable characterization techniques for soft materials that enable these types of measurements.  In this study the asymmetric structures of two commercially available thin film composite reverse osmosis (RO) membranes, the BW30 and SW30-XLE from Dow Water and Process Solutions, have been studied using an analytical (mercury intrusion porosimetry) and a 3D imaging (x-ray computed tomography) technique. A multi-length scale approach was used for the imaging technique in order to study both the bulk properties as well as visualize the microporous structure of the support layer. Structural metrics (porosity, tortuosity and thickness) of the membranes were then calculated from which, for the first time, structural parameters were directly measured. Large variations were observed between the “actual” structural parameter, from either technique, and the values obtained from osmotic flux tests. The results of the study indicate a need to revise the traditional approaches of characterizing membrane structures overall for any application.