Ion-Specific Effects at the Surface of Water

TL;DR

This study investigates how different salts influence the surface behavior of surfactants on water, revealing ion-specific effects that alter surfactant aggregation and surface density through spectroscopic measurements.

Contribution

It demonstrates the ion-specific enhancement of surfactant surface density and elucidates the underlying Coulomb screening and hydrophobic interactions using HD-VSFG spectroscopy.

Findings

Surface density of SDS increases dramatically with added salt.

Ion-specific effects determine the salt concentration needed for surfactant collapse.

Hydrophobic interactions trigger surfactant aggregation at critical surface densities.

Abstract

We studied the interaction between salts and surfactants on 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 micromolar bulk concentration in water. The vibrational response of the OH band of near-surface oriented water molecules and the CH bands of the hydrophobic tails of the surfactant are measured. 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 micromolar SDS, the required concentrations of CsCl, KCl, and NaCl for this transition are 2, 5, and 10 mM, respectively.