Molecular dynamics simulations of polymeric structure and alcohol-membrane surface affinity of aromatic polyamide membranes

A molecular dynamics (MD) technique was adopted to investigate the free volume, operational temperature effects, feed solution–membrane surface affinity, and water/alcohol diffusion mechanism of aromatic polyamide (PA) membranes for pervaporation (PV) applications. An isothermal–isobaric ensemble (i.e. NPT ensemble) was adopted to analyze the fractional free volume, the fractional accessible volume, and the cavity size distribution of PA membranes at different operational temperatures. The free volume and cavity size analyses indicated that bulkier side groups enhanced the formation of larger free volumes, which increased permeation rates. Increased temperature induced chain mobility and enlarged the cavity size of the membrane. During feed solution–membrane affinity analysis, the alcohols showed a higher interaction with the membrane surface as compared to the water molecules. The diffusion mechanism of water/methanol in the membrane matrix suggested that methanol had a shorter displacement distance relative to the water due to its larger size and stronger interaction with the membrane. Results from the MD simulation agreed well with those from experimental studies reported in the literatures, which demonstrated that this theoretical method is a promising tool for characterizing membrane structures and analyzing feed solution transport at a molecular scale during PV processes.