Surface solubilization

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Abstract
The surface solubilization of the anionic dye Chrome Azurol S (CAS) into the adsorption layers of three non-ionic surfactants [decylmethylsulphoxide (DEMS), Triton X-100 (TX100) and Triton X-305 (TX305)] on two silica gels of different porosities has been investigated quantitatively. Experimental results showed that the silica gels did not adsorb CAS in the individual aqueous solution, but if a significant amount of non-ionic surfactant is adsorbed on the solid, the silica gel will be dyed to a red or deep red colour (depending on the amount of surfactant adsorbed and the amount of dye solubilized) that is similar to the colour of CAS solubilized in a micellar solution. Linear relationships between the amount of CAS solubilized on the surface and the amount of non-ionic surfactant adsorbed were found for the three systems investigated. The slopes of the straight lines are dependent on the properties of the surfactants, but not on the type of silica gel used. The order of solubilization abilities, TX305 > DEMS > TX100, is the same in both the bulk and surface micelles. In the case of TX305 and TX100 it is suggested that CAS may be solubilized into the adsorbed bilayer of the surfactant oxyethylene chains horizontally. However, in the case of DEMS, CAS may enter the adsorbed bilayer of DEMS perpendicularly. Based on these suggestions the experimental results can be reasonably explained.

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