Abstract
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’.
Highlights
Specific ion effects are known since the early work of Hofmeister who has ordered salts according to their ability to salt out proteins [1,2]
The adsorption of dodecyl sulfate surfactants at the air–water interface in the presence of the Li+, and Cs+ cations was studied with a vibrational sum-frequency generation (SFG) and surface tensiometry
lithium dodecyl sulfate (LiDS) and sodium dodecyl sulfate (SDS) showed an excellent agreement in the changes of the SFG amplitude and S-O resonance frequency with a surfactant concentration
Summary
Specific ion effects are known since the early work of Hofmeister who has ordered salts according to their ability to salt out proteins [1,2]. Collins [22,23] proposed a ‘rule of matching water affinities’ which describes the tendency of different oppositely charged ions to associate and to form contact or solvent-shared ion pairs in aqueous solutions. The basis of this rule is linked to the differences in the affinity of ions to bind water molecules, which results in a different ability to strip parts of their solvation shell and to form ion pairs. Ninham et al [24] proposed to extend Collins rule with the consideration of dispersion interactions, while Vlachy et al [19] showed a Hofmeister like ordering for charged headgroups and compared theoretical insights on the Gibbs free energy difference ∆∆G with Collins rule and showed that the tendency of the contact ion pair formation e.g., between sulfate groups and Molecules 2019, 24, 2911; doi:10.3390/molecules24162911 www.mdpi.com/journal/molecules between sulfate groups and alkali ions increases with the size of the cation Li+ < Na+ < K+ < Rb+ < Cs+
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