Absorption of water in the active layer of reverse osmosis membranes

The absorption of water in commercially available reverse osmosis membrane active layers (FT30 by Filmtec and LF10 by Nitto Denko) is investigated by a combination of measurements of water mass uptake and biaxial stress as a function of relative humidity. The experiments are enabled by bonding the active layers of the membranes to either a quartz crystal microbalance or a glass cover-slip using polyimide as an adhesion layer and subsequently removing the porous polysulfone support layer with an organic solvent. Membranes from these two suppliers behave similarly. Water mass uptakes of the active layers are approximately 12% of the mass of the active layer when the relative humidity is changed from 0% to 95%. This degree of water absorption produces a compressive biaxial stress of ≈ -20 MPa. The changes in water mass uptake with relative humidity resemble a Langmuir adsorption isotherm when the humidity is low and the Flory-Huggins theory when the humidity is high. By combining the measurements of water mass uptake and biaxial stress, we estimate that the specific volume of water in the active layers is ≈ 0.28 cm³ g^-1. The self-diffusion coefficient of water in the polyamide active layers of FT30 (D_S = 0.8 × 10^-9 m² s^-1) and LF10 (D_S = 1.2 × 10^-9 m² s^-1) were calculated from the water mass uptake measurements, water permeability values reported in the literature and assuming the solution-diffusion mechanism for water transport. These values for D_S are a factor of 2-3 smaller than the self-diffusion coefficient of liquid water (D_S = 2 × 10^-9 m² s^-1) and comparable to theoretical estimates of D_S in the active layer of FT30 membrane using the free-volume model.