SEIRAS Studies of Water Structure in a Sodium Dodecyl Sulfate Film Adsorbed at a Gold Electrode Surface

Surface-enhanced infrared reflection–absorption spectroscopy (SEIRAS) was used to investigate the structure of water that is incorporated within a film of sodium dodecyl sulfate (SDS) adsorbed at a thin gold nanoparticle film deposited onto a silicon substrate. Previous studies on a Au(111) electrode surface showed that SDS molecules form long-range ordered hemicylindrical hemimicelles (phase I) for potentials −0.2 ≤ E ≤ 0.45 V vs Ag/AgCl and a disordered bilayer (phase II) for potentials E ≥ 0.5 V vs Ag/AgCl. The SEIRA spectra demonstrated that the hemimicellar film is water-rich and contains both a network of hydrogen-bonded water and a disturbed network of hydrogen bonds consisting of monomeric and dimeric water in the hydrophobic region of the film. No network water was observed in phase II of the film. However, SEIRAS data showed that sulfate groups in the disordered bilayer are hydrated. The SEIRAS spectra of the film of SDS were compared to the previously measured spectra obtained using subtractively normalized interfacial Fourier transform IR spectroscopy (SNIFTIRS). The complementarity of the spectroscopic information obtained by these two techniques was demonstrated.