Specific Ion Effects in the Near-Surface Molecular Orientation and Surface Coverage of Water

We studied the interaction between salts and surfactants at the water surface using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. We used sodium dodecyl sulfate (SDS) as a prototype surfactant system at 75 μM bulk concentration in water, and measured the vibrational response of the OH band of near-surface oriented water molecules and the CH bands of the hydrophobic tails of the surfactant. We observed a dramatic enhancement of the surface density of the negatively charged SDS (DS^-) within a narrow range of added salt concentrations. We demonstrated this increase is strongly ion-specific and induced by the screening of the lateral Coulomb repulsion of the sulfate headgroups by the added cations, followed by strong hydrophobic interactions (hydrophobic collapse) when the DS^- surface density reaches a critical value. For a solution of 75 μM SDS, the required concentrations of CsCl, KCl, and NaCl for this transition are 2, 5, and 10 mM, respectively.