Creating new membranes to separate liquids

Researchers at the Adolphe Merkle Institute are experimenting with new types of finely tuned composite membranes inspired by leaves, designed to allow certain types of liquids to filter through, as part of a European training project.   

Take any leaf and you may discover that it is covered by a thin waxy layer known as the cuticle, whose main function is to protect the leaf against water loss. Leaf cuticles are thin composite films covering the epidermal cells of leaves, consisting of a hydrophobic cutin matrix, polysaccharides, and waxes. This insulating layer protects leaves from dehydration. Take the waxes away, however, and the water permeability of the leaf increases by two to three orders of magnitude. Two parallel pathways for transport of water are present in cuticles: one through the apolar cutin and one using a polar pathway attributed to polysaccharides (pectin and cellulose fibrils) extending from the epidermal cell wall.

The Polymer Chemistry & Materials group at AMI is seeking to translate the composite “hydrophobic matrix – hydrophilic filler” structure of wax-free leaf cuticles to non-porous artificial membranes using a commercially available hydrophobic rubbery copolymer and hydrophilic cellulose-based fillers, such as cellulose nanocrystals (CNCs) and nanofibrillated cellulose (NFC). The water permeability of these composite membranes is being investigated and correlated with the membrane's architecture, notably the distribution of the fillers in the matrix.

The idea is not to mimic a leaf exactly, but to adopt a similar heterogeneous architecture. “If you look at cuticles in nature for example, they are excellent at fulfilling their role as water permeation barriers, but they have limited functionalities and are very fragile,” explains PhD student Aris Kamtsikakis.

Permeable separation membranes are not a new idea. Indeed, pervaporation – a separation process of liquid mixtures – was first described over a century ago. In this process, the separation is based on exploiting the affinity differences between the permeating substance and the membrane. Yet, it was not until the 1980s that pervaporation attracted significant industrial interest for dehydration of solvents and other liquid-liquid separations. The membranes used are extremely dense and act as a selective mass transfer barrier.

Thus, mimicking nature, the AMI researchers seek to develop thin composite membranes that are mechanically robust and selective towards specific permeating species, privileging, for example, alcohols over water. Initial results are promising: “Our first tests show that our idea works much better than we expect-ed,” says Kamtsikakis. “Not only can the permeability be tuned easily by altering the surface chemistry of the materials, but simply flipping the direction in which liquids are directed through the membrane makes a major difference.” The next stage will be to test which liquids pass through the membrane selectively and how performance is influenced by architecture and surface chemistry. According to Kamtsikakis, potential applications could interest the food and petroleum industries.

The project is part of the Innovative Training Net-work PlaMatSu (Plant-Inspired Materials and Surfaces), funded by the European Commission, that allows nine PhD students to work at three institutions in the field of bio-inspired materials: AMI, the University of Freiburg (Germany), and the University of Cambridge (UK). PlaMatSu brings together plant biologists, polymer chemists, and soft matter physicists to study the development, structure, and properties of multifunctional plant cuticles on a fundamental level and to create novel materials and surfaces based on the working principles of cuticles.

These training networks provide students with the opportunity to pursue their academic training with-in an international multidisciplinary framework along with temporary industrial internships. The aim of the program is to boost scientific excellence and business innovation, as well as to enhance researchers’ career prospects through developing their skills in entrepreneurship, creativity, and innovation.