Silica surface energy

London Component of the Surface Energy of Heated Treated Silicas. Surface energy is usually considered as the sum of two components the London component (y ), steming from London forces, and the specific component (y p), originating from all other types of forces (polar, H-bonding, metallic, etc). Two methods are commonly used for the measurement of surface energies wettability and adsorption techniques. 

M.J. Wang, S. Wolff, J.B. Donnet. Filler-elastomer interactions. Part I silica surface energies and interactions with model compounds. Rubb. Chem. TechnoL, 64, 559-576,1991. 

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

L. Mascia and T. Tang, Ceramers based on crosslinked epoxy resins-silica hybrids low surface energy systems, J. Sol-Gel Sci. Technol., 1998, 13, 405. 

In some instances, a thin metal film, freely hanging or deposited on a solid support, gives rise to holes on a heat treatment. This effect, noticed long ago, also is supposed to be influenced by surface energy. Silver films, about 400 to 2000 angstroms thick, on silica were heated120 in the presence of oxygen and kept at 266 °C... 

Following the original measurements of the surface energy of sodium chloride by heats of solution by Lipsett et al. 18), much more accurate determinations have been carried out recently by Benson and his co-workers 19-22). Other substances which have been investigated include magnesium oxide 23), calcium oxide and hydroxide 24), and silica, unhydrated and hydrated 25). 

Hydrophobicity - Before plasma treatment, silica powder is highly hydrophilic and immediately sinks in water. After plasma film deposition, the material floats on water for several hours. A significant reduction in polarity and in surface energy compared to untreated silica is found, down to the range of 28.4-47.7 mJ/m2. The water penetration into powder beds of untreated and plasma-treated silica is shown in Fig. 7. The untreated silica absorbs water very fast, whereas the plasma-treated silicas show a significantly decreased water penetration rate. The lowest rate is found for the polythiophene-coated silica (PTh-silica). 

Porous silica is the most widely used sorbent in HPLC and has many advantages 23). Its surface silanols are capable of adsorbing solutes according to their structure and polarity. In solution, the molecules of the solute and solvent compete for the active sites on the silica surface. The adsorption energy of solvents can be characterized by their e° values24>. The figures are usually tabulated for alumina adsorbents. For silica as an adsorbent, a plain proportionality is assumed25 ... 

---