Public defence in Bioproduct Technology, MSc Marcel Kröger

Title of the thesis: Insights into the dissociation kinetics of phosphorylated nanocellulose through conductometry

Thesis defender: Marcel Kröger
Opponent: Prof. Yuki Uematsu, Kyushu Institute of Technology, Japan
Custos: Prof. Eero Kontturi, Aalto University School of Chemical Engineering

Cellulose nanocrystals are highly crystalline, rod-shaped particles that can be obtained from native cellulose fibres through acid hydrolysis. Colloidal stability can be achieved by imparting chargeable functional groups to these particles, which induces electrostatic repulsion between the particle surfaces. Consequently, besides the particle morphology, the amount of imparted charge is one of the most important characteristics of cellulose nanocrystals. In this context, nanocelluloses carrying phosphate halfesters are of particular interest, given their potentially high charge and additional affinities for ion capture, biomineralization, or, in dry conditions, increased thermal stability.

In this work, a methodology is presented which conveniently and reliably produces highly substituted phosphorylated cellulose nanocrystals at high yields. However, the determination of the degree of phosphorylation by elemental analysis disagrees with the conclusions drawn from conductometric titrations. This is due to flaws in the contemporary methodology, which ignores the influence of so-called counterion condensation. 

To demonstrate the relevance of this influence and to reconcile the two analytical techniques, a theoretical model for the dissociation of phosphate halfesters on the surface of cellulose nanocrystals was developed. The model is capable of predicting the measured conductivities with reasonable agreement, and it sheds light on the accurate dissociation behavior of the surface groups. The developed model further allows insight into the significant reduction of the achievable surface charge, demonstrating and quantifying the outstanding capacity for ion capture and ion exchange applications. In particular, this work shows how divalent Ca2+ ions, through strong interactions with the phosphate halfesters induce ion exchange processes which, in turn, are visible in conductometry. Again, these effects are reciprocated with reasonable agreement in the modelled datasets. 

The findings in this thesis help to further the current understanding of the surface charge of dispersed nanoparticles in general and to remedy the current methodology of analysing conductometric titration curves, which may specifically reconcile discrepancies and inconsistencies in the reported studies on phosphorylated nanocellulose grades.

Thesis available for public display 7 days prior to the defence at Aaltodoc.