Stretching and Solvency of Charged Cellulose Chains

The transition between good and poor solvency conditions was studied for cellulose interacting in an aqueous environment as a function of the pH and ionic strength as well as for cellulose with weakly ionizable grafted groups. For uncharged cellulose, poor solvent conditions were observed for a pH range of 4-10 and an ionic strength between 0.1 and 10 mM, as demonstrated by the characteristic constant force plateau in the single-molecule force spectroscopy data as a function of the polymer extension. The magnitude of the force plateaus was quantized, indicating that on occasion more than a single cellulose chain was stretched into solution. Charged cellulose was also studied with a transition from poor to good solvency observed by an increase in the pH of the solution well above the pK(a) of the carboxyl groups. This transition was characterized by a fundamental change in the form of the force data as a function of extension from the constant force plateau (poor solvent conditions) to the nonlinear increase in adhesion due to elastic stretching (good solvent conditions). Using this single-molecule technique, the Kuhn length of cellulose was determined to be 3.8 A from the fit to a freely jointed chain model. Furthermore, the loop distribution under the good solvent conditions was studied as a function of the pH and ionic strength. The distribution became more compact as the influence of the charged groups was reduced through an increase in the screening or a reduction in the charge density, with chain association dominating because of strong hydrogen bonding.