Effect of Counterions on Ordered Phase Formation in Suspensions of Charged Rodlike Cellulose Crystallites

Abstract

Stable colloidal suspensions of cellulose crystallites with negatively charged sulfate groups on their surface were prepared by acid hydrolysis of filter paper. The suspensions, which were free of added electrolyte, formed chiral nematic ordered phases above a critical concentration. A sharp boundary was observed between coexisting chiral nematic and isotropic phases, enabling measurements to be made of the relative amounts of each phase as a function of total cellulose concentration. The isotropic-to-chiral nematic phase equilibrium was sensitive to the nature of the counterions present in the suspension. Samples were prepared with sodium, potassium, cesium, ammonium, tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, trimethylammonium, and triethylammonium counterions. Suspensions with H + counterions formed an ordered phase at the lowest concentrations of crystallites. For inorganic counterions, the critical concentration for ordered phase formation increases in the order H + < Na + < K + < Cs + . For the organic counterions, the critical concentration in general increases with increasing counterion size, suggesting that the equilibrium is governed by a balance between hydrophobic attraction and steric repulsion forces. The nature of the counterions also influences other properties of the suspensions, such as their stability, the temperature dependence of the phase separation and of the chiral nematic pitch, and the redispersability of dried samples made from the suspensions.