Hydration, porosity and water dynamics in the polyamide layer of reverse osmosis membranes: A molecular dynamics study
Vesselin Kolev, Viatcheslav Freger*
Technion ? Israel Institute of Technology, Wolfson Department of Chemical Engineering, Haifa 32000, Israel
This paper presents the protocol of synthesizing and the hydration of a polyamide membrane using molecular dynamics simulation. The density of dry polymer was found to be less than previous studies but close to the experimental studies. Volume expansion and mass gain on hydration showed variations due to the void spaces in the polyamide membrane. Strong next-neighbor correlation and random long-distance water ? water correlation was obtained using the radial distribution function.
In this study GROMACS was used for interfacial polymerization of m-phenylene diamine (MPD) and trimesoyl chloride (TMC). The partial atomic charges were found by using Gaussian09. The termination criterion was the atomic fractions of carbon, nitrogen, oxygen and the residual carboxyl groups. Degree of cross linking and branching play major role in physical properties of polymer and not the shape of the simulation box. So, a prismatic box was used instead of the cubic simulation box. A number of ad hoc selections during the course of polymerization were used to avoid unrealistic strains, twists, topology, and asymmetric functionalization of the monomers. A simulation box of 20 x 20 x 30 nm3 with 300 monomers of MPD, 200 monomers of TMC (MPD:TMC::3:2), and 10 dimers of MPD-TMC as the initial clusters was used.
The monomers reacted with the dimers and monomer of the type which reacted was added to the simulation. NVT ensemble at 298.15K was used in this simulation. The polymer formed was rich in COCl groups making the polymer acidic and it caused the reaction to slow down. The routine was corrected with an ad hoc routine given by the following equation
Where A’s represent the atomic fraction of the subscripted atoms and exp represent the target value. Later, COCl groups were hydrolyzed to COOH. The calculated volume (V) and mass density (ρ) are given in the following table.
Reviewer: Aamir Alaud Din