Charge-adhered microfibrillar cellulose multilayer as a flow-modulating membrane coating: Solute separation by shape

Abstract

Functional microfibrillar cellulose coatings for ultrafiltration membranes were prepared and characterized. For this purpose, a strategy for converting cotton cellulose into microfibres by incubation in a deep eutectic solvent of choline chloride and malonic acid, with no mechanical disintegration step, was created. The microfibres were furthermore modified to possess either cationic or anionic surface charges to allow charge-assisted adhesion of the fibres on ultrafiltration membrane surface. The adhesion was significantly improved by the electrostatic attraction, and it allowed building durable multilayer coatings by Layer-by-Layer methodology for membrane filtration purposes. The polyelectrolyte-inspired Layer-by-Layer strategy for depositing microfibrillar celluloses onto a membrane makes it possible to create coatings out of pure, only slightly derived cellulose, with no need for incorporating hazardous cross-linkers or synthetic polymers into the matrix. The deposited multilayer introduces a shear-force modulating functionality to the membrane which furthermore allows the permeation of molecules that have larger Stokes-Einstein radii than the membrane pore radius. This permeation is selective towards a linear polymer (poly(ethylene glycol)), while a more globular model substance with a similar molar mass (lignin) is highly rejected. The resulting filtration properties can furthermore be easily tuned by controlling the coating thickness.