Self-assembling liquid crystals as building blocks to design nanoporous membranes suitable for molecular separations

The transition from the current “linear” economy to a “circular” economy with a strong focus on the recovery and reuse of materials and resources undoubtedly necessitates efficient and effective separation technologies. Membrane technology will play an important role in this transition to a circular economy. In that perspective, separation at the molecular level to separate and fractionate e.g. individual ions and small molecules for reuse is especially essential. Unfortunately, conventional membrane materials and their fabrication methods mostly lack design and control over pore size and selectivity at a true molecular level. In view of this challenge, nanostructured polymer membranes based on self-assembled materials are gaining more and more interest. Using the self-assembly properties of polymerizable liquid crystal molecules ensures control at a molecular level and gives rise to narrow pore size distributions, high pore densities and control of pore size and functionality. In this review, the potential of liquid crystal materials and their self-assembly properties to fabricate nanoporous membranes for water purification, desalination and selective recovery is presented. The basic principles of liquid crystals, the self-assembling characteristics and methods to control pore size and functionality are discussed in the perspective of membrane properties and applications. Efforts reported in the literature highlighting advances and pointing out important limitations for different pore morphologies are discussed. The versatility of liquid crystal based membranes is highlighted by exploring approaches for post-modification of the nanopores to further tune the pore size and control the pore functionality after polymerization of the liquid crystals. The work provides readers with a thorough understanding of the design and fabrication of nanoporous liquid crystal membranes combined with a perspective on the potential of liquid crystal membranes. Next to recent advances, future challenges are presented as well, with the most crucial two: 1) The formation of thin, defect-free nanoporous liquid crystal layers supported on a microporous support; 2) Large-scale production combined with alignment control over longer length scales.