Semiaromatic Polyamide-Based Membrane in Forward Osmosis: Molecular Insights

Despite the increased interest in forward osmosis (FO) in recent years, the technology's advancement in commercial and industrial applications has been hampered by the absence of suitable FO membranes and ideal draw solutes, which demands the exploration of new membranes and novel draw solutes targeted for some specific applications. In this context, we considered a semiaromatic polyamide (SAPA) for an application where monovalent salt can be permeated but has high selectivity toward divalent salt and excellent water permeability. In this regard, we constructed an atomistic model for the membrane via a heuristic approach using an equilibrated mixture of hydrolyzed trimesoyl chloride and piperazine monomers and performed nonequilibrium molecular dynamics simulations on the SAPA membrane in the FO process to understand the structural properties and performance of the membrane at the atomistic level. We used pure water as the feed and Na₂SO₄ as the draw solution. It is observed that the SAPA membrane shows excellent water permeability and no reverse draw solute flux. To further test the dynamics of salt ions inside the membranes, we performed two distinct equilibrium simulations on systems consisting of either monovalent salt, such as NaCl, or divalent salt, such as Na₂SO₄. The atomistic details of the interactions between the functional groups of the membrane and salt ions provided in this work can inspire further experiments on SAPA membranes in the context of separation of monovalent and divalent salts, which have applications in the treatment of textile industry wastewater.