Water permeability of ultrathin polyamide membranes: a computational study from molecular to macro scale

Many works focused on predicting water permeability in polyamides utilized in osmotic membranes, but few of them provide Multi-Scale (MS) calculations free from tunable parameters. In this paper, the phenomenological water permeability was calculated by a novel MS approach. Two biphasic models, water/FT-30 polyamide and dilute salt solution/polyamide, were simulated. Molecular Dynamics (MD) –based simulations were first performed to obtain the equilibrium water volume fraction and the water mass concentrations in the polymeric phase. Then the analytical solution of Fick's second law, provided by Penetration Theory, was used to obtain the water diffusion coefficient, exploiting the MD water mass concentrations. The computed water uptake was found to be in good agreement with the values available in the literature using both biphasic models. Moreover, the MS method yields water diffusion coefficients comparable with the smallest available theoretical and experimental values. Water permeability as well, is in agreement with the experimental values referring to ultrathin polyamide membranes, while the agreement is lost for membranes with thicker active layers. Therefore, the proposed MS methodology is reliable for predicting water permeability in this kind of promising membranes and for designing new ultrathin polymer membranes since it is based on atomistic-scale simulations. The strength of the method lies in the suitable assembly of advanced nanoscale simulations with the macroscopic Fick's transport equation.