Adsorption of sodium dodecyl sulfate (SDS) at ZnSe and α-Fe₂O₃ surfaces: Combining infrared spectroscopy and batch uptake studies

Adsorption of sodium dodecyl sulfate (SDS) at the solid/aqueous interface was examined as a function of pH and SDS concentration ([SDS]) using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and batch uptake experiments. Two types of sorbent surfaces were compared: (i) a hydrophobic zinc selenide (ZnSe) ATR internal reflection element (IRE) and (ii) the same surface coated with hydrophilic nanoparticulate α-Fe₂O₃ (hematite). The results indicate that adsorption to the ZnSe IRE is affected by both electrostatic attraction and hydrophobic interaction. Batch adsorption and ATR-FTIR spectral results are consistent with SDS forming outer-sphere complexes at the α-Fe₂O₃ surface. There is also no evidence for ligand (SDS)-promoted dissolution of hematite. Adsorption to hematite is dominated by anion exchange and surfactant self-assembly. ATR-FTIR data indicate that adsorption to both surfaces shows a strong pH dependence at low [SDS] and negligible pH dependence when [SDS] exceeds the critical micelle concentration (cmc). Adsorption to ZnSe IRE shows small variation with [SDS], apparently due to a lack of surfactant self-assembly at the interface. Adsorption to α-Fe₂O₃ is a rapid process; equilibrium is reached within a few minutes. Conversely, adsorption to the ZnSe IRE shows strong longer time dependence; evidently, hydrophobic interfacial reactions constitute a much slower process.