Infrared Reflection Absorption Spectroscopy (IRRAS) of Aqueous Nonsurfactant Salts, Ionic Surfactants, and Mixed Ionic Surfactants

When a chromophore in solution is not surface active, it can still be detected via IRRAS at the air/water interface, down to the concentration level of 1-10 mM. This is demonstrated with new data on sodium sulfate and sodium methyl sulfate. With surface active solutes, like sodium dodecyl sulfate (SDS) and sodium dodecylsulfonate (SDSn), which adsorb from solution and form monolayers at the air/water interface, IRRAS primarily detects the monolayers up to about 10 mM, with little or no influence of bulk solution absorption. Above about 10mM at 25 °C, the presence of monomers or micelles in solution may be detected, in addition to the monolayer. For both SDS and SDSn, the RA intensities, and thus the adsorbed monolayer densities, increase with their concentration and sodium chloride concentration (10 or 100 mM), as expected from the equilibrium surface tension data. The wavenumbers of the CH2 IR stretch bands indicate that the monolayers are not close-packed, consistent with the commonly reported minimum areas per molecule (ca. 40 Ų) determined from tensiometry. The polar group bands are not well-resolved and suggest little crystallinity. The splitting of the antisymmetric SO₃ band, observed for SDS in the presence of salt, is not observed for SDSn or mixtures of SDS with SDSn. Adsorption from 50/50, by mole, mixtures of SDSn and SDS, or deuterated SDS, in water or 100 mM NaCl was also probed by IRRAS. Data from the polar group bands and the CH₂ or CD₂ bands indicate that the adsorbed monolayer is mixed and enriched in SDS.