Interactions between cellulose surfaces: effect of solution pH

The forces acting between cellulose surfaces have been studied using the interferometric surface force apparatus. The cellulose surfaces were prepared by Langmuir-Blodgett deposition of trimethylsilyl cellulose (TMSC) onto hydrophobized mica. Prior to measurements, the surfaces were desilylated to obtain pure cellulose. The degree of silylation and the molecular weight of the TMSC both affect the structure of the deposited layer. This was observed from the surface pressure-area isotherm, force versus distance curves, and atomic force microscopy images. The forces between the cellulose surfaces were found to depend on the pH of the solution. In dilute electrolyte solutions, the cellulose film was uncharged and rather compact when the pH of the solution was 6.0. However, when the pH was increased to 7.3, the cellulose film swelled considerably and a long-range steric force was measured. The swelling of the film is interpreted as being due to the dissociation of a few carboxylic acid groups present along the cellulose chain. The forces measured were, however, dominated by steric interactions. The repulsion does not increase substantially when the pH is increased from 7.3 to 9.5. Our results suggest that the pKa of the acid groups present within the cellulose film is larger than it would be in the bulk aqueous solution.