The influence of Li+, Na+ and Cs+ cations on the surface excess and structure of dodecyl sulfate (DS-) anions at the air-water interface was investigated with the vibrational sum-frequency generation (SFG) and surface tensiometry. Particularly, we have addressed the change in amplitude and frequency of the symmetric S-O stretching vibrations as a function of electrolyte and DS- concentration in the presence of Li+, Na+ and Cs+ cations. For the Li+ and Na+ ions, we show that the resonance frequency is shifted noticeably from 1055 cm-1 to 1063 cm-1 as a function of the surfactants' surfaces excess, which we attribute to the vibrational Stark effect within the static electric field at the air-water interface. For Cs+ ions the resonance frequency is independent of the surfactant concentration with the S-O stretching band centered at 1063 cm-1. This frequency is identical to the frequency at the maximum surface excess when Li+ and Na+ ions are present and points to the ion pair formation between the sulfate headgroup and Cs+ counterions, which reduces the local electric field. In addition, SFG experiments of the O-H stretching bands of interfacial H2O molecules are used in order to calculate the apparent double layer potential and the degree of dissociation between the surfactant head group and the investigated cations. The latter was found to be 12.0%, 10.4% and 7.7% for lithium dodecyl sulfate (LiDS), sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CsDS) surfactants, which is in agreement with Collins 'rule of matching water affinities'.
Keywords: Hofmeister series; dodecyl sulfate; specific ion effects; stark shift; sum frequency generation; surface excess.