Tuning electrostatic interactions for controlled structure and rejection of cellulose nanocrystal membranes

Abstract

Precise control over membrane performance by tuning fabrication parameters is highly desirable. We developed ultrafiltration membranes by depositing cellulose nanocrystals (CNCs) on a porous support via tangential flow filtration, followed by irreversibly coagulating the deposited layer by permeating highly concentrated AlCl3 solution. By varying electrostatic interactions between nanocrystals during deposition, via varying ionic strength and pH of CNC suspensions, membrane performance was tuned. Increasing NaCl concentration of CNC suspensions from 0 to 50 mM increased Blue Dextran (5 kDa) rejection from 93% to 98.5% at high tangential flowrate and from 82% to 98% at low tangential flowrate. For membranes fabricated at high tangential flowrate, β-lactoglobulin (18 kDa) rejections increased from 94% to 98% when NaCl concentration increased from 0 to 25 mM. Increasing pH of CNC suspensions also increased probe molecule rejections, implying smaller interparticle distance between CNCs in the deposit layer. Pure water permeance of membranes were comparable to commercial membranes of similar separation properties. Polarized optical microscopy analyses showed that alignment of CNCs and their aggregates, or tactoids, affect the morphology of CNC deposits. In all cases, decreasing electrostatic repulsion between nanocyrstals increased rejection regardless of CNC alignment, which provides a simple parameter for tuning CNC membrane performance.