Phosphoric acid can be used to produce cellulose nanocrystals with phosphate half-ester surface groups (P-CNCs), as opposed to the more common acid hydrolysis route that employs sulfuric acid and imparts sulfate half ester group on the surface (S-CNCs). P-CNCs have been shown to have higher thermal stability and have been predicted to interact more favorably with biological systems. Although we have learned worthwhile lessons about weak-acid hydrolysis and the resulting nanoparticle properties by working with P-CNCs, they simply are not as good as expected (or similar properties can be attained by other routes) and generally P-CNCs underperform compared to S-CNCs in the applications we are interested in.<br/><br/>While we can tailor the dimensions of P-CNCs over a wider range than S-CNCs by changing hydrolysis time, temperature and acid concentration, the surface charge density is always too low. The weaker acid hydrolysis gives CNCs with a higher degree of polymerization (DP) and a lower degree of esterification. As such, P-CNCs lack sufficient colloidal stability and for example, when heated in suspension, de-esterification occurs taking the colloidal stability from bad to worse. To summarize where we have failed with P-CNCs: (1) they are not more thermally stable in suspension and are only marginally more thermally stable in a dry state because of their higher DP; (2) because of their colloidal instability they tend to aggregate more in suspension and they do not cross-link when functionalized with orthogonal surface groups to the same extent as S-CNCs which leads to weaker networked structures (gels and aerogels); (3) while phosphate groups should improve compatibility in tissue engineering applications (with bone, cartilage, etc.) the charge density is too low to be of assistance and in bone regeneration tests both S-CNC and P-CNC scaffolds promoted hydroxyapatite growth to the same extent after a calcium chloride rinse. Finally, there are significant environmental implications to using phosphoric acid and audience members at conferences focused on sustainability can get pretty caught up on that!

Ways you can fail with cellulose nanocrystals produced from phosphoric acid hydrolysis

2019 Spring National Meeting

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