Modulating the chiral nematic structure of cellulose nanocrystal suspensions with electrolytes

The iridescent optical properties of films made of cellulose nanocrystals (CNC) are controlled by the pitch and range of the chiral nematic structures. These are further tuned with the addition of electrolyte.

Electrolyte type, valency and concentration were varied. The bulk CNC suspension properties were investigated by combining rheology, polarised optical photography and microscopy, while the spacing between crystals was determined using SAXS.

The addition of electrolyte to a CNC suspension containing chiral nematic structures first causes the nematic pitch to increase indicating the suspension has a weaker structure. Further increases in electrolyte concentration cause aggregation and complete breakdown of the chiral nematic structures. The univalent species cause larger changes to the chiral nematic structure with the onset and magnitude of structure breakdown occurring at lower ionic strengths compared with the divalent species. Cation size influences the chiral nematic structure with the order of influence being K⁺ > Na⁺ ≈ Ca²⁺ > Mg²⁺, which corresponds from the largest to smallest cation. This work demonstrates that both ion valency, concentration and species play a significant role in controlling the chiral nematic structures of CNC suspensions and will be a vital step in the development of CNC liquid crystals, optical materials and sensors.