Erated SDS solutions are consistent with SDS present on the surface

Erated SDS solutions are consistent with SDS present on the surface only above SDS solution concentrations of 0.2 mM. Contaminants such as dodecanol from the as received SDS are likely the dominant adsorbed species contributing to the SFG spectra observed at solution concentrations below 0.2 mM. Dodecanol will continue to be present in the SDS films as the solution concentration is increased to the SDS cmc. ssp polarization SFG spectra in the O O3 stretching region While the previous paragraph discusses spectral contributions at higher frequencies, this section discusses the SFG O O3 SDS headgoup stretching vibrations between 1000 and 1150 cm-1.56 Figure 7 shows the ssp polarization SFG spectra recorded at the SDS/CaF2 interface at a fixed pH 3.5 value and different SDS concentrations. The first observation is the absence of SFG intensity at SDS solution concentrations 2 mM. This further confirms that SDS is not adsorbing onto the CaF2 surface in an ordered fashion at sufficient surface density from the low concentration SDS solutions.(S)-Crizotinib However, the presence of strong CH bands for the as received SDS solutions at concentrations below 0.2 mM indicates the existence of an ordered layer at these concentrations, but the absence of SO3 vibrations makes it unlikely that SDS is the source for these CH spectral features. Bain et al. previously showed that low concentrations of dodecanol form well-ordered films, suggesting dodecanol or similar fatty alcohol is responsible for the CH signals observed at solution concentrations below 0.Imatinib 2 mM made with as-received SDS.PMID:23255394 30 At solution concentrations above 0.2 mM the alkyl chains in both dodecanol and SDS will contribute to the CH signals. Furthermore, starting at 3.5 mM a double peak with frequencies of 1074 and 1087 cm-1 appears and continues to grow in intensity at concentrations where the CH signals are vanishing (between 5 and 8 mM, see Figs. 3 and 4). The presence of SO3 vibrations and the absence of the CH signals in this concentration range can, therefore, be related to a molecular SDS arrangement where the methyl groups are in a symmetric environment while the sulfate headgroups are in an ordered, non-symmetric environment. A symmetric methyl group arrangement can be accomplished by an equal number of opposing SDS chains. This can be realized in a bilayer or a monolayer with opposing orientations of SDS molecules. The most likely bilayer configuration would have the SDS headgroups in the inner layer facing the substrate and the SDS headgroups in the outer layer facing the water phase. The most likely monolayer configuration would require interdigitating SDS chains. If an SDS bilayer does form at solution concentrations between 3 and 8 mM, the reappearance of methyl vibrations at higher solution concentrations would suggest the formation of an additional layer on top of the bilayer as shown in Figure 6(A). This seems energetically unfavorable due to the adjacent location of the charged headgroups from the second and third layers as well as the hydrophobic chains extending into the water phase. A more plausible explanation is a single layer with opposing headgroups and interdigitating alkyl chains as shown in Figure 6(B). Other SDS adsorption models such as hemi-micelles have been considered (e.g., see references 11 and 26). However, the arrangement of the SDS molecules in these models is not consistent with the results of these study. So only the planar, layered models are discussed in detail he.