Alkali hydrolysis of sulfated cellulose nanocrystals: optimization of reaction conditions and tailored surface charge

Authors: Jordan, Jacobs; Easson, Michael; Condon, Brian

Submitted to: Nanomaterials
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2019
Publication Date: 8/30/2019
Citation: Jordan, J.H., Easson, M.W., Condon, B.D. 2019. Alkali hydrolysis of sulfated cellulose nanocrystals: optimization of reaction conditions and tailored surface charge. Nanomaterials. 9(9):1232. https://doi.org/10.3390/nano9091232.
DOI: https://doi.org/10.3390/nano9091232

Interpretive Summary: Cellulose nanocrystals (CNC) are a biorenewable resource that is environmentally-friendly, and possess unique physical properties such as high crystallinity, aspect ratio, and large surface area among others. These physical properties are dependent upon the preparatory method, which imparts chemical changes to the CNC surface, including excessive surface charge. In some instances, high surface charge is desirable, however, it is often the case, that surface charged groups must be removed prior to further functionalization for specific purposes. There are multiple methods available to remove surface functional groups, each with their specific benefits and drawbacks. The two most common are hydrolysis with excess acid or base. This changes the surface chemistry of the CNC and in some instances, changes their bulk properties to undesirable effect, moreover, obtaining specific desired surface charge is elusive. In this work, the removal of surface charged groups using base hydrolysis methods was investigated. A Design of Experiments (DOE) was used to determine optimal conditions that minimizes the use of excess base to achieve a tailored response and retain desirable properties.

Technical Abstract: Cellulose nanocrystals (CNC) are a biorenewable resource, which may be chemically modified to impart specific properties. Modified CNCs have found use in imaging applications, as rheology modifiers, polymer reinforcements, barrier and/or optical films, and nanocomposites. Nanoparticle dimensions of CNCs are typically 5–10 nm in width with lengths of <100–300 nm. However, the physical properties are dependent upon the number and nature of the surface charge groups imparted during preparation. In the case of CNCs produced from sulfuric acid hydrolysis, the sulfated surface groups may be partially removed prior to further functionalization. This gives more available hydroxyls yet renders the CNCs less colloidally stable. Furthermore, conditions vary significantly, and there is no consensus for the optimal conditions for partial removal of sulfate functionality or conditions developed to give specific surface charge. In the following, alkali hydrolysis of sulfate half-esters was quantified by conductometric titration of the strong acid groups, and using a design of experiments (DOE), optimal conditions were determined to produce CNCs with tailored surface charge.