SPECTROSCOPIC INVESTIGATION OF SODIUM DODECYL-SULFATE ADSORPTION ON AHYDROCARBON MONOLAYER

Sodium dodecyl sulfate (SDS), adsorbed from aqueous solution onto a mo nolayer of dimethyloctadecylsiloxane (C18) bonded to silica, was chara cterized spectroscopically. The structural and dynamical properties of the adsorbed SDS were probed by the fluorescence anisotropy of a dilu te hydrophobic fluorophor, 1,4-bis(o-methylstyryl)benzene. Just below the critical micelle concentration (cmc) of 8.3 mM, the orientational distribution of the probe is narrowed and tilted more toward the surfa ce normal, and its rotational diffusion slows, as compared to its beha vior above the cmc. These properties below the cmc were most extreme a t 7 mM in SDS concentration. FTIR spectroscopy was used to measure the amount of adsorbed SDS, and it revealed that there is about 3 times a s much SDS adsorbed below the cmc as there is above it. A distinct max imum in the adsorption isotherm occurs at 7 mM, the same concentration at which the rotational diffusion behavior is most extreme. The FTIR spectral bands are narrowed and shifted below the cmc, showing that th e adsorbed SDS in the premicellar region is organized densely.