March 3, 2026

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

SSSanghamitra SenguptaInstitute for Atomic and Molecular Physics JVJan Versluis HBHuib J. Bakker

Key Points

Observed near-surface water molecules exhibit enhanced orientation due to specific ion effects.
Surface density of sodium dodecyl sulfate increases significantly at specific salt concentrations like 2 mM CsCl.
Utilized heterodyne-detected vibrational sum-frequency generation to assess surfactant interactions.
Findings imply strong influences of ions on molecular behavior at the water surface, highlighting complex interactivity.

Abstract

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.

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