Effect of polyelectrolyte morphology and adsorption on the mechanism of nanocellulose flocculation

Praveena Raj; Warren Batchelor; Angeles Blanco; Elena de la Fuente; Carlos Negro; Gil Garnier

doi:10.1016/j.jcis.2016.07.048

Effect of polyelectrolyte morphology and adsorption on the mechanism of nanocellulose flocculation

J Colloid Interface Sci. 2016 Nov 1:481:158-67. doi: 10.1016/j.jcis.2016.07.048. Epub 2016 Jul 21.

Authors

Praveena Raj¹, Warren Batchelor¹, Angeles Blanco², Elena de la Fuente¹, Carlos Negro¹, Gil Garnier³

Affiliations

¹ BioResource Processing Research Institute of Australia (BioPRIA), Department of Chemical Engineering, Monash University, Clayton 3800, VIC, Australia; Department of Chemical Engineering, Complutense University of Madrid, Madrid 28040, Spain.
² BioResource Processing Research Institute of Australia (BioPRIA), Department of Chemical Engineering, Monash University, Clayton 3800, VIC, Australia; Department of Chemical Engineering, Complutense University of Madrid, Madrid 28040, Spain. Electronic address: ablanco@ucm.es.
³ BioResource Processing Research Institute of Australia (BioPRIA), Department of Chemical Engineering, Monash University, Clayton 3800, VIC, Australia; Department of Chemical Engineering, Complutense University of Madrid, Madrid 28040, Spain. Electronic address: gil.garnier@monash.edu.

PMID: 27474816
DOI: 10.1016/j.jcis.2016.07.048

Abstract

The effect of polyelectrolyte morphology, charge density, molecular weight and concentration on the adsorption and flocculation of Microfibrillated Cellulose (MFC) were investigated. Linear Cationic Polyacrylamide (CPAM) and Branched Polyethylenimine (PEI) of varying charge density and molecular weight were added at different dosages to MFC suspensions. The flocculation mechanisms were quantified by measuring gel point by sedimentation, and floc size, strength and reflocculation ability through Focussed Beam Reflectance Measurements. Polymer adsorption was quantified through zeta potential and adsorption measurements using polyelectrolyte titration. The flocculation mechanism of MFC is shown to be dependent on polyelectrolyte morphology. The high molecular weight branched polymer, HPEI formed rigid bridges between the MFC fibres. HPEI had low coverage and negative zeta potential at the optimum flocculation dosage, forming flocs of high strength. After breaking of flocs, total reflocculation was achieved because the high rigidity of polymer did not allow reconformation or flattening of the polyelectrolyte adsorbed on MFC surface. The lower molecular weight branched polymer, LPEI (2kDa) showed rapid total deflocculation, complete reflocculation and had maximum flocculation occurring at the point of zero charge. These characteristics correspond to a charge neutralisation mechanism. However, if the flocculation mechanism was purely charge neutralisation mechanism, the minimum gel point would be at the point of zero charge. Since this is not the case, this difference was attributed to the high polydispersity of the commercial LPEI used, allowing some bridges to be formed by the largest molecules, changing the minimum gel point. With the linear 80% charged 4MDa CPAM, bridging mechanism dominates since maximum flocculation occurred at the minimum gel point, negative zeta potential and low coverage required for maximum flocculation. Reflocculation was not possible as the long linear polymer reconformed on the MFC surface under a flat conformation. Flocculation with the linear 50% charged 13MDa CPAM happened by bridging with the minimum gel point and maximum flocculation corresponding to roughly half polyelectrolyte surface coverage on cellulose.

Keywords: Adsorption; Cationic Polyacrylamide (CPAM); Focussed Beam Reflectance Measurements (FBRM); Microfibrillated Cellulose (MFC); Polyethylenimine (PEI); Zeta potential.