An atomic force microscope was used to measure kinetic friction forces between a colloidal cellulose
sphere and different planar surfaces in air. The planar surfaces were bare silica, silica modified with a
monolayer of alkyl ketene dimer (AKD), a common additive used for paper sizing, and silica with surface
grafted terminal methyl groups. Friction was measured as a function of the effective load resulting from
adhesive interactions between the cellulose sphere and the planar surface. Friction in the regime of low
effective loads was shown to be a linear function of the load, and the friction coefficients determined from
the slope of these curves were 0.26, 0.07, and 0.021 for the cellulose−silica, cellulose−AKD, and cellulose−methylated silica systems, respectively. The ratio between friction coefficients for the two first systems
agrees well with that reported for paper in the presence and absence of AKD sizing agents, respectively.
In the high effective load regime, deviations from linearity between measured friction and load were
observed. This is an indication that we have single asperity contact, at least at high loads. These results
were analyzed in terms of a contact mechanics transition equation. By making assumptions on the moduli
of the materials, the interfacial shear strength τ and the contact radius a
0 were calculated. The results
obtained through this analysis provide an important insight into the relationship between friction and
chemical surface properties.
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Keywords: CϪH···π interactions / Chelates / Noncovalent interactions / Density functional calculations / Weak hydrogen bondsStructural and computational evidence is given for a special type of C−H···π interaction where the C−H group interacts with the π-system of a six-membered chelate ring. An investigation of crystal structures shows that these interactions take place in quite a number of metal complexes, including organometallic compounds; in the CSD we found over 1200
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