Molecular dynamics study on the elucidation of polyamide membrane fouling by nonionic surfactants and disaccharides.

Reverse osmosis (RO) is a widely used energy-efficient separation technology for water treatment. Polyamide (PA) membranes are the conventional choice for this process. Fouling is a serious problem for RO separation. This issue leads to significant decreases in the water permeability of PA membranes, and it has yet to be fully elucidated. In particular, the fouling behavior of a nonionic substance on the negatively charged surface of a PA membrane in an aqueous environment has not been previously studied. In this work, the mechanisms of nonionic substances such as polyoxyethylene octyl ether (PE5) and maltose (Mal) were investigated using molecular dynamics (MD) simulations. In a PA membrane in which the carboxyl group was not dissociated, the hydrophobic portion of the membrane was exposed due to the localization of water molecules around the carboxyl groups in the PA membrane. This caused hydrophobic interaction with the hydrophobic groups of PE5. In the case of an amine-modified PA membrane containing no carboxyl groups, water was not localized around the functional group, and the water orientation of the polyamide surface was also low. Due to this membrane property, the presence of stabilized water around PE5 reduced the number of hydrophobic interactions. In similar manner, a PA membrane with a slightly dissociated carboxyl group was hydrophilic, which reduced the PE5 adsorption. The presence of many dissociated carboxyl groups, however, enhanced the adsorption of PE5 due to the increase in interactions between the dissociated carboxyl groups and the hydrophilic groups of PE5. Therefore, PE5 exhibited an amphipathic adsorption wherein both hydrophilic and hydrophobic groups contributed to adsorption onto the PA membrane. Mal, on the other hand, was highly stable in every aqueous environment independent of the state of the functional groups of the PA membrane, and was not easily affected by the properties of the PA membrane.

Molecular Dynamics Simulation Study of Polyamide Membrane Structures and RO/FO Water Permeation Properties.

Polyamide (PA) membranes possess properties that allow for selective water permeation and salt rejection, and these are widely used for reverse osmotic (RO) desalination of sea water to produce drinking water. In order to design high-performance RO membranes with high levels of water permeability and salt rejection, an understanding of microscopic PA membrane structures is indispensable, and this includes water transport and ion rejection mechanisms on a molecular scale. In this study, two types of virtual PA membranes with different structures and densities were constructed on a computer, and water molecular transport properties through PA membranes were examined on a molecular level via direct reverse/forward osmosis (RO/FO) filtration molecular dynamics (MD) simulations. A quasi-non-equilibrium MD simulation technique that uses applied (RO mode) or osmotic (FO mode) pressure differences of several MPa was conducted to estimate water permeability through PA membranes. A simple NVT (Number, Volume, and Temperature constant ensemble)-RO MD simulation method was presented and verified. The simulations of RO and FO water permeability for a dense PA membrane model without a support layer agreed with the experimental value in the RO mode. This PA membrane completely rejected Na⁺ and Cl- ions during a simulation time of several nano-seconds. The naturally dense PA structure showed excellent ion rejection. The effect that the void size of PA structure exerted on water permeability was also examined.