Effects of Ionic Liquid Nanoconfinement on the CO 2 /CH 4 Separation in Poly(vinylidene fluoride)/1-Ethyl-3-methylimidazolium Thiocyanate Membranes.

A combined experimental and molecular dynamics (MD) simulation approach was used to investigate the effects of the nanoconfinement of a highly CO ₂ /CH ₄ -selective ionic liquid (IL), 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]), in porous poly(vinylidene fluoride) (PVDF) matrices on the gas separation performance of the resulting membranes. The observed experimental CO ₂ /CH ₄ permselectivity increased by about 46% when the nominal pore diameter in PVDF, which is a measure of nanoconfinement, decreased from 450 to 100 nm, thus demonstrating nanoconfinement improvements of gas separation. MD simulations corroborated these experimental observations and indicated a suppression in the sorption of CH ₄ by [EMIM][SCN] when the IL nanoconfinement length decreased within the nonpolar PVDF surfaces. This is consistent with the experimental observation that the CH ₄ permeance through the IL confined in nonpolar PVDF is significantly less than the CH ₄ permeance through the IL confined in a water-wetting polar formulation of PVDF. The potential of mean force calculations further indicated that CO ₂ has more affinity to the nonpolar PVDF surface than CH ₄ . Also, a charge/density distribution analysis of the IL in the PVDF-confined region revealed a layering of the IL into [EMIM]- and [SCN]-rich regions, where CH ₄ was preferentially distributed in the former and CO ₂ in the latter. These molecular insights into the nanoconfinement-driven mechanisms in polymer/IL membranes provide a framework for a better molecular design of such membranes for critical gas separation and CO ₂ capture applications.