| AVS 58th Annual International Symposium and Exhibition | |
| Surface Science Division | Monday Sessions |
| Session SS1-MoA |
| Session: | Selectivity and Reactivity of Chemisorbed Species |
| Presenter: | Sang-Hun Song, National ESCA and Surface Analysis Center for Biomedical Problems |
| Authors: | S.-H. Song, National ESCA and Surface Analysis Center for Biomedical Problems T. Weidner, National ESCA and Surface Analysis Center for Biomedical Problems M.S. Wagner, The Procter & Gamble Company D.G. Castner, National ESCA and Surface Analysis Center for Biomedical Problems |
| Correspondent: | Click to Email |
Surfactants are important compounds used in many industrial applications, with sodium dodecyl sulfate (SDS) being one of the most widely used surfactants. This study uses sum frequency generation (SFG) vibrational spectroscopy and surface plasmon resonance (SPR) sensing to investigate the structure of SDS films formed from the adsorption of SDS onto positively charged and hydrophilic surfaces. The surfaces studied included CaF2 as well as RF glow discharge deposited films of allylamine. The SDS films were prepared by adsorption of SDS from water solutions ranging in concentration from 0.067 to 20 mM. Since the water molecules above the SDS layer interact with the films, peaks from both the SDS molecules and water molecules were studied. SFG spectra of SDS adsorbed onto the positively charged CaF2 surface exhibits two well resolved CH3 peaks at 2877 and 2942 cm-1, and two OH peaks at ~3200 and ~3400 cm-1. At the 0.2 mM SDS concentration on the CaF2 surface the intensity of both the CH3 and OH peaks decrease to close to background levels and then increase as the SDS concentration is raised. As the SDS solution concentration continues to increase the CH3 and OH go through a second intensity minimum. This second intensity minimum occurs between 3-6 mM for the CH3 peaks and near 8 mM for the OH peaks. Previous studies have suggested these SFG intensity minima are due to the neutralization of positively charged CaF2 surfaces by the anionic charged head group of SDS (1). Since the shape and, thus, the phase of the SFG peaks are affected by the molecular environment, fits of the SFG data were used to quantify the orientation and alignment of the SDS layers across the wide range of SDS solution concentration. Since SFG is sensitive to both orientational order and the amount of material adsorbed we used SPR to determine the SDS coverage for the different solution concentrations in order to separate the two contributions. Combining SFG and SPR results provides a more detailed understanding of the structure and interactions of adsorbed SDS films.
(1) Becraft, K. A.; Moore, F. G.; Richmond, G. L. Journal of Physical Chemistry B 2003, 107, 3675.