Surface morphology and wetting characteristics of sized cellulose imitations

Wetting characteristics of cellulosic substrates designed by partially imitating sized paper surfaces were investigated with regard to the surface morphology by contact angle measurement, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy. The contact angles of water droplets on cellulose films decreased with increasing surface roughness at the nm level according to Wenzel's theory. Binary alkanethiolate self-assembled monolayer (SAM) prepared as a hydrophobic surface model of size components demonstrated the characteristic water repellency of Cassie's theory. These two surface-design methods were combined to control the surface morphology of sized cellulose imitations. Vaporized Au colloids were deposited on the cellulose film, and subsequently dodecanethiol (DT) self-assembled only at the Au positions. The AFM phase images confirmed that the total Au coverage was less than 20% of the substrate surface, but the water repellency of the model cellulosic surface attained ca. 103°, which is almost as high as that of the DT-SAM surface (ca. 112°). These results strongly suggest that the surface morphology involved both in substrate roughness and size distribution must be an important factor in the sizing enhancement, and a good sizing response appears without complete coverage of the hydrophilic substrates by the hydrophobic size components.