Ionic liquid imbibition of ceramic nanofiltration membranes

Imbibition kinetics of (ionic) liquids in asymmetrically structured ceramic nanofiltration membranes are investigated experimentally and theoretically. Results of this are employed as a means for a novel preparation method of immobilized liquid membranes (ILM) for gas separations. ILM are composed of liquid domains situated within micro-structured porous supports such as nanofiltration membranes. Experimental investigation of imbibition kinetics is performed by a freezing method: a dyed liquid intrudes into a nanofiltration membrane (support) which afterwards is frozen in liquid nitrogen. The frozen liquid layer thickness in the support is analyzed by light microscopy. Four different support types and two liquids, i.e. the ionic liquid [BMIM][Tf2N] and a silicone oil are employed. The experimental findings are successfully modelled by an adapted Washburn Equation (WE) whose applicability and premises are discussed. Different proposed modelling approaches for representation of the highly asymmetric structure of the employed ceramic nanofiltration membranes are evaluated. A monolayer model, which assumes a uniform pore diameter throughout the whole support is best suited to describe the experimental findings and constitutes a means for the investigated in situ membrane preparation. This semi-empirical Washburn approach is well suited to describe imbibition of both liquids into porous networks where non-continuum physics prevail.