Molecular dynamics simulation of diffusion behavior of benzene/water in PDMS-calix[4]arene hybrid pervaporation membranes

Molecular dynamics (MD) simulation was employed to investigate diffusion behavior of small penetrants in rubbery-polymer-based hybrid membranes, using pervaporative removal of benzene from its dilute solution by poly(dimethylsiloxane) (PDMS) membranes filled with calix[4]arene (CA) as the model system. In our previous experimental investigation, the normalized permeation rate of benzene (NPRb) and separation factor (benzene/water) through PDMS-CA hybrid membranes did not follow the usual monotonous or single peak/valley change, but accompanied minimum and maximum values instead. In the present study, nonbonding interaction energy between PDMS and CA, mean-square displacement (MSD), free volume characteristics, and diffusion coefficients of benzene and water in pure PDMS and hybrid membranes were analyzed by molecular dynamics simulation. The simulation results revealed that MSD and fractional free volume (FFV) values were closely dependent on interaction energy. Diffusion coefficients of benzene and water at infinite dilution and saturated condition displayed the same changing tendency, although the values at saturated condition were a bit larger. Moreover, it was observed that diffusion coefficients were not only related to FFV but also affected by the interaction between CA and the penetrants. Overall, the MID results agreed well with the experimental results.