Cellulose nanocrystals (CNC), the smallest building blocks of cellulose that carry its physical properties, are produced by controlled acid hydrolysis from various cellulose sources in a sustainable process. In sulfuric acid hydrolysis of softwood pulp, CNC surface is modified with negatively charged acidic sulfate ester (–OSO3−) groups, resulting in electrostatic repulsion and stable aqueous CNC suspensions. The X-ray crystal structure based models of Iα CNC with both pristine surface and sulfate groups of negative charge 0–0.34 e/nm2 feature a highly polarized “crystal-like” charge distribution. Molecular dynamics relaxation of both neutral pristine and negatively charged sulfated CNC of various length in water solvent induces a right-hand twist in CNC and rearranges its initial structure with a macro-dipole of the oppositely charged faces into a mesh of small spots of alternating charge at each face. This rearrangement observed for both neutral and charged CNC strongly affects the distribution of solution ions around CNC. The statistical-mechanical, 3D-RISM-KH molecular theory of solvation readily yields effective interactions between nanoparticles in electrolyte solution of a given nature and concentration,1 and is used to evaluate the solvation structure and thermodynamics of relaxed CNC in aqueous NaCl at concentration 0.0–0.25 mol/kg.2 The solvation free energy of charged sulfated CNC has a minimum at a particular NaCl concentration depending on surface charge density, and for neutral CNC increases linearly with electrolyte concentration. The 3D map of Na+ density distribution around CNC exhibits the behavior similar to charged nanocolloids in aqueous electrolyte solution: increase in electrolyte concentration shrinks the electric interfacial layer and weakens the effective repulsion between charged CNC particles. The CNC structures and modeling procedure enable studies of effective interactions and formation of ordered phases in CNC suspensions.2 The dependence of layer thickness on electrolyte concentration plays an important role in the mechanisms of phase transitions in CNC suspensions, as the weakening of effective repulsion between charged CNC particles with layer compression at higher ionic strength affects their phase equilibrium and chiral ordering. (1) Molecular Theory of Solvation. Hirata,F.(ed) Kluwer, Dordrecht, 2003. J.Chem.Phys. 1999,110,10095. Pure Appl.Chem. 2013,85,159. (2) Nordic Pulp Paper Res.J. 2014,29,144. Langmuir 2015,31,7106.

Effective interactions in cellulose nanocrystals suspensions from 3D-RISM-KH molecular theory of solvation

251st National Meeting (2016)

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