WATER AS A DETECTING AGENT IN THIN LAYER CHROMATOGRAPHY OF SURFACTANTS

Abstract

Appearance of gas bubbles coming out from the spots of some surfactants on silica plates was examined. The plates were dipped into water after chromatograms had been developed. Experimental results had shown that only anionic surfactants gave white spots with gas bubbles (rare and slightly some nonionic surfactants, and never cationics). Therefore, this occurrence can be used for detection (and identification) of anionic surfactants, and for determination of polarity of surfactants. On the basis of the experiments done, it was concluded that the bubbles were air bubbles liberated out of silica gel pores, influenced by water. This occurrence was caused, on one hand,by characteristic negative charge of the silica gel surface which provoked weaker binding of anionic surfactants. On the other hand, anionic surfactant molecules are built from negatively charged hydrophilic head and hydrophobic chain entities that enable foaming. At the interface, silica gel/water anionic surfactants are oriented with hydrophilic heads toward water, and with hydrophobic tails which are poorly bonded to silica gel. Air bubbles appear; a layer of anionic surfactant is formed around the air bubble (foaming). At the water/air interface, the monolayer is formed by surfactant molecules, the surface tension of water is decreased, and emergence of air bubbles from silica gel pores is made possible (water is entering and driving air out of the silica gel pores).