As a novel chemical engineering unit operation, membrane separation has many advantages such as high efficiency and energy saving. Recently, at nanoscale, the increasingly anomalous phenomena of ultra-fast flux and breaking the tradeoff between flux and selectivity are found in membrane separations (such as pervaporation, reverse osmosis, nano-filtration, etc.), which promote considerable attentions. However, traditional models or mechanisms fail to explain these phenomena or to predict the behavior. The main reason of this is that when the size of systems shrinks to nanoscale, the intermolecular forces between the fluids and membrane pore wall (including primarily steric interactions/hydration, van der Waals interactions and electrostatic interactions) have become the most prominent ones in nanoconfined systems, which make significant contributions to mass-transfer and engender unique performance. The relevant mass-transfer theories especially under nanoconfinement lacks common mass-transfer mechanism and controlling methods for these abnormal phenomena on its mass-transfer, severely restricting the design and development of related membrane materials. To quantitatively describe the nanoconfined membrane process, a universal theoretical framework for nanoconfined mass-transfer is needed and then the contributions of various influencing factors at nanoscale to the flux and selectivity should be illustrated quantitatively. In this review, we first analyzed the challenges of classical mass-transfer models in the confinement conditions, such as solution-diffusion model to dense membrane. The main problem can be attributed to that these models in essence describe an equilibrium state plus a dynamic process, which not includes the interfacial influence. Secondly, we explored the application of non-equilibrium thermodynamics linearization method in establishing the mass-transfer model under nanoconfinement. We simplified the interfacial phenomenon to assume that system located at a distance close to interface can be regarded as a microelement and its deviation from equilibrium remained in the linear region. We established a model to describe nano- and micro-interfacial transfer confined in a complex structure, using CO2 separation by ionic liquids as an example, assuming that the transfer in phase interface comprised two steps (surface reaction and diffusion) in the same phase. The mass-transfer model on the basis of linear non-equilibrium thermodynamics was proven to decouple reaction and transfer more effectively for achieving process intensification by adjusting easy-to-control factors, compared with the classical solution-diffusion model. Thirdly, the factors affecting the behavior of the confined fluid molecules were so complex that it was difficult to do single-factor analysis and to determine controlling factors. Related advances on the analysis of the influencing factor for nanoconfined mass-transfer were further explored. We evaluated the effects of the influencing factors for water molecules transfer in various nanochannels or nanotubes, including pore size, pore wall hydrophilicity, pore geometrical shape and pore mouth modification, based on the numerous results derived from molecular simulation and experimental method. Finally, we had a brief conclusion and summarized the development directions. (1) Linear non-equilibrium thermodynamics method has great potential in the modeling of mass-transfer at nanoscale. (2) Experimentally synthesizing ordered nanochannel material should be encouraged to build a platform for comparison of the factor analysis results from molecular simulation and experiment. (3) The original experimental characterization methods are expected to be developed to probe the intermolecular interactions quantitatively.
- Molecular simulation