Institute of Food Nutrition and Health, ETH Zurich , 8092 Zurich, Switzerland.
Biomacromolecules. 2017 Dec 11;18(12):4060-4066. doi: 10.1021/acs.biomac.7b01119. Epub 2017 Oct 20.
Nanocrystalline cellulose (NCC) is a promising material for formation of hydrogels and nematic liquid crystals. While salt addition is known to facilitate hydrogel formation, it remains unclear whether this originates from cationic bridging or charge screening effects. Herein, we demonstrate the effect of mono- and divalent salts on NCC gelation and nematic ordering. A strong correlation of NCC suspension zeta-potential and rheological behavior was found. Lower concentrations of divalent cations were needed to decrease NCC zeta-potential and form hydrogels. The same zeta-potentials and gel strengths were achieved at higher concentrations of monovalent salts. Salt-induced NCC aggregation is thus caused by intermolecular attractive forces rather than cationic bridging. Against excluded volume argumentation, salt addition was found to promote NCC nematic phase formation. Increased nematic ordering was observed in a transition regime of moderate salt addition before complete aggregation occurs. This regime is governed by an equilibrium of repulsive and attractive forces. Small angle neutron scattering suggests lateral orientation of NCC. Hence, NCC gelation and nematic ordering can be modulated via its zeta-potential by targeted salt addition.
纳米纤维素（NCC）是一种很有前途的水凝胶和向列液晶形成材料。虽然已知添加盐可以促进水凝胶的形成，但尚不清楚这是源于阳离子桥接还是电荷屏蔽效应。本文中，我们展示了单盐和二价盐对 NCC 凝胶化和向列有序性的影响。发现 NCC 悬浮zeta 电位和流变行为之间存在很强的相关性。需要较低浓度的二价阳离子来降低 NCC 的 zeta 电位并形成水凝胶。在较高浓度的单价盐下可以达到相同的 zeta 电位和凝胶强度。因此，盐诱导的 NCC 聚集是由分子间吸引力引起的，而不是阳离子桥接。与排除体积论证相反，发现盐的添加促进了 NCC 向列相的形成。在完全聚集发生之前，在适度添加盐的过渡区观察到增加的向列有序性。该区域受排斥力和吸引力之间平衡的控制。小角中子散射表明 NCC 的横向取向。因此，通过有针对性地添加盐，可以调节 NCC 的 zeta 电位来调节 NCC 的凝胶化和向列有序性。
