Surface Free Energy Analysis of Cellulose Nanofibers through Contact Angle Measurements

Abstract

Accurate measurements of the surface free energy （SFE） are essential for determining the interfacial properties of materials, including the adsorption characteristics of solid particles at the water/oil interface in emulsions. The SFE of cellulose nanofibers （CNFs）, which are nanomaterials derived from wood resources, has thus far been evaluated by inverse gas chromatography （IGC）, and the emulsifying capacity of CNFs is effectively explained by the desorption energy calculated from the SFE. However, IGC suffers from a long measurement time and complicated analysis. Among the various methods proposed for measuring the SFE, contact angle measurements have the advantage of simplicity in both measurement and analysis. Here, the SFE of CNFs was evaluated by contact angle measurements. Two types of CNFs with different surface structures were prepared through ion exchange of surface-carboxylated CNFs: one with sodium and the other with tetrabutylammonium ions. The contact angle measurements were performed on the CNF films using a combination of three different probe liquids, and the SFE of the CNFs was determined based on the van Oss‒Chaudhury‒Good theory. However, a substantial decrease in the electron‒donor component of SFE was observed through the ion exchange from sodium to tetrabutylammonium; this result was inconsistent with previous reports. Furthermore, the calculated desorption energies of the CNFs could not explain the emulsification behavior in the experimental system. These discrepancies were attributed to the swelling of the CNF films against the probe liquid. This study showed the limitations of contact angle measurements for accurate SFE determination of CNFs.