Paper AS-WeM3
In Situ Monitoring of SDS Adsorption on Positively Charged Surfaces

Wednesday, October 31, 2012, 8:40 am, Room 20

Surfactants are important compounds used in many industrial applications, with sodium dodecyl sulfate (SDS) being one of the most widely used surfactant. This study uses vibrational sum-frequency-generation (SFG) spectroscopy and surface plasmon resonance (SPR) analysis to investigate molecular ordering and orientation within SDS films formed on positively charged surfaces. Substrates with different charge density and polarity include CaF₂ at different pH values and chemically modified CaF₂ and Au surfaces prepared by RF glow discharge plasma deposition of allylamine (AAm) films and heptylamine (HApp), respectively. SFG spectra were recorded in various spectral regionsfor SDS concentrations ranging from µM to mM. At 0.2 mM SDS concentration, the intensity of CH and OH peaks decreased to background levels independently of the substrate. Previous studies have suggested that the SFG intensity minimum at 0.2 mM is due to neutralization effects of the positively charged (CaF₂) surface by the anionic charged head group of SDS.¹ In our studies, we found out that (i) the loss of SFG signal occurring at 0.2 mM is independent of surface charge density and (ii) SFG spectral intensities of lower concentrations vary significantly, whereas above 0.2 mM signals become reproducible. Therefore, in analogy to the behavior observed for alkane thiols on gold,² we interpret the loss of signal to a loss in order induced by a transition from a striped phase to a stand-up phase. As the number density of adsorbed SDS molecules increases above 0.2 mM, a second minimum in SFG intensity can be observed for all substrates, but at concentrations that are substrate dependent. Here we propose a model in which a monolayer is built up, but with opposing head group orientations (towards the substrate and the solution phase). This is supported by (i) SPR data showing a saturating number density towards SDS monolayer coverage at concentrations around the critical micelle concentration and above, (ii) a minimum for methyl vibrations related to an equal number in downward and upward orientations in the monolayer, and (iii) SFG spectral analysis for the polar SO₃^- band revealed a band structure with two contributions of positive and negative phases. This can be associated to spectral shifts in close proximity to the substrate and opposing headgroup orientations (towards the substrate and the solution).

1. Becraft, K. A.; Moore, F. G.; Richmond, G. L., Journal of Physical Chemistry B 2003, 107, (16), 3675-3678.

2. Schreiber, F., Progress in Surface Science 2000, 65, (5-8), 151-256.