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As the need to augment water supply grows in water-scarce regions of the world, desalination of saline waters such as seawater and brackish groundwater has become crucially important. Reverse osmosis (RO) is the dominant desalination technology, mainly due to its high enegy efficiency and low operating costs compared to other desalination technologies (1, 2). RO is also a central com- ponent in advanced municipal wastewater reuse, a growing technol- ogy for alleviating water scarcity in water-stressed regions (3, 4).
Semipermeable desalination membranes—membranes that allow transport of water and reject salt and other solutes—lie at the heart of the RO technology (1, 5). Because of their outstanding water permeability and water-salt selectivity, thin-film composite (TFC) polyamide membranes have been the gold standard for de- salination since their invention more than four decades ago (6, 7). TFC RO membranes are formed by interfacial polymerization of m- phenylenediamine and trimesoyl chloride on a porous substrate, re- sulting in a thin (100 to 150 nm) cross-linked selective layer (8, 9).
The transport of water and salt through the active layer of RO membranes governs the membrane desalination performance. The widely accepted theory or mechanism to describe water and salt transport in RO membranes is the solution-diffusion (SD) model, which was proposed over half a century ago (10). This model assumes that the membrane active layer is a “dense” polymer phase, where water molecules “dissolve” into the mem- brane and then diffuse through the membrane down their concen- tration gradient (11, 12). Another key assumption in the SD model is that the pressure across the membrane is constant (13, 14). In the absence of a pressure gradient, this assumption implies that the
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