Synthesis and properties of cellulose nanocrystal conjugates with reactive dyes

In this work, we have developed methods of synthesis of cellulose nanocrystal (CNC) conjugates with chlorotriazine reactive dyes (RDs)—reactive violet, reactive-bright red and reactive bright-orange.) The prepared CNC-RD conjugates were characterized by a complex of techniques: UV–Vis, FTIR and solid-state ¹³C NMR spectroscopy, thermogravimetric and elemental analysis, particle size and zeta potential analysis, scanning electron and polarization optical microscopy. An analysis of the solid state ¹³C NMR spectra allowed us to conclude that the crystallinity index of the CNC-RD conjugates, compared to CNCs, did not change, i.e. RDs covalently bonded with the surface of the CNC particles and did not affect the CNC crystalline structure. The amount of the dye covalently bonded to the CNC particle surface was determined based on the elemental analysis data and using a square-cross-section model of the cellulose nanocrystal. The results obtained enabled us to make a conclusion that the RD covalent fixation was selective and occurred on one of the CNC ends (the so-called reducing end of the cellulose polymeric chain). Successful modification of the CNC surface was confirmed by UV–Vis spectroscopy. It was found that aqueous suspensions of the CNC-reactive violet dye conjugates possessed indicator properties. The size and charges of the CNC-RD particles in diluted aqueous suspensions were determined; the CNC-RD aqueous suspensions were found to have high colloidal stability. The stabilizing effect of the CNC-reactive violet dye conjugate for the formation of Pickering emulsions of the oil-in-water type was studied. It was shown that the CNC-reactive violet dye conjugate effectively stabilized n-decane emulsions in water.