Surface morphology and water repellency of sized celkilosic substrates

Factors influencing water repellency of cellulosic substrates designed by partially imitating the 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 droplet on cellulose films decreased with increasing the surface roughness at nm level under the control of Wenzel's theory. Binary alkanethiolate self-assembled monolayer (SAM) prepared as model of hydrophobic surfaces of size components demonstrated the characteristic water-repellency dependent on Cassie's theory. These two technical methods were applied to control the surface morphology of sized paper sheets; the vaporized Au colloids were deposited on the cellulose film, subsequently self-assembling the dodecanethiol (DT) only at the Au positions. The AFM phase images confirmed that the total Au coverage was less than 20% of the substrate surface, although the water repellency of the model cellulosic surface attained ca. 103° as high as that of the DT-SAM surface. These results strongly suggest that the surface morphology involved both in substrate roughness and size distribution state must serve as important factors in the sizing enhancement, and good sizing response appears without any overall coverage of hydrophilic substrates by the hydrophobic size components.