SPECTROSCOPIC INVESTIGATION OF SODIUM DODECYL-SULFATE ADSORBED TO A TRIMETHYLSILOXANE MONOLAYER ON SILICA

The adsorption of premicellar sodium dodecyl sulfate (SDS) onto methyl ated (C-1) silica surfaces was investigated spectroscopically to explo re the physical origin of premicellar adsorption. Methylation was done to provide a hydrophobic surface without interpenetration of the SDS chains into the monolayer. The premicellar adsorption maxima were foun d to be smaller for the C-1 surface than for the n-octadecylsiloxane ( C-18) surface, which implicates interpenetration as an important facto r in premicellar adsorption maxima. Roughness was found to increase th e premicellar adsorption maximum on the C-1 surface. The rotational di ffusion behavior of acridine orange on the C-1 and C-18 surfaces paint s to a structural transition in the adsorbed SDS layer as the origin o f the large premicellar adsorption maximum of SDS on C-18.