Silica surfaces, topography

Atomic force microscopy (AFM) has been used to characterize dendrimers that have been adsorbed onto a surface such as silica. AFM involves moving a finely tipped stylus across a surface and monitoring the tip movements as it traces the surface topography. In studying adsorbed dendrimers, samples can be scanned repeatedly and in a variety of directions. When this is done, it is found that all the images are the same. True dendrimers form objects of only one size. 

B. Some Considerations of Surface Topography 1. Silica Surfaces. 

In the case of solid supports having crystalline structures, like quartz, zirconium dioxide and o-alumina, the shape of the enthalpic curve is quite different and exhibits a distinct exothermic peak in the vicinity of the cmc (e.g., Figs. 6a and 6b). At first sight, the phenomenon seems strange, especially if all other curves are similar to those for the amorphous silica (see for example Figs. 2b, 3a and 7). An explanation based on the specific model of surface topography is advanced in this paragraph. 

TOPOGRAPHY OF HYDROXYL AND HYDRATE COVER OF SILICA SURFACE... 

Here the stress is directed away from the low interface (silica/copper) towards the stronger silica/palladium interfaee by the topography produced. The surface topography protects weak regions from a high stress field. 

Ramesh, S., Cohen, Y, Aurbach, D., Gedanken, A. Atomic force microscopy investigation of the surface topography and adhesion of nickel nanoparticles to submicrospherical silica. Chem. Phys. Lett. 287,461 67 (1998)... 

It should be noted that the structure of C/silica and C/X/silica surfaces can be complex not only with respect to their topography but also to their chemical composition due to the availability of H, O, etc., atoms in surface groups, new noncarbon phases (oxide or metal as in the case of carbonization of acetylacetonates of Ni and Co), which can affect the adsorption energy of polar and nonpolar adsorbates. 

Silica exists in several crystalline and amorphous forms which greatly differ in their surface characteristics, namely topography (down to a nanometric level), hydrophobicity, silanol distribution and presence of contaminants. 

---