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.

Intensity of infrared absorptions (as represented by the peak area of νas(CH2)) are correlated with surface excess of SDS at the α-Fe2O3/aqueous solution interface.Figure optionsDownload high-quality image (75 K)Download as PowerPoint slide