Ion-Specific Effects at the Surface of Water
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arXiv
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| Natura: | Preprint |
| Pubblicazione: |
2025
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| _version_ | 1866913991820836864 |
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| author | Sengupta, Sanghamitra Versluis, Jan Bakker, Huib J. |
| author_facet | Sengupta, Sanghamitra Versluis, Jan Bakker, Huib J. |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2508_10811 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Ion-Specific Effects at the Surface of Water Sengupta, Sanghamitra Versluis, Jan Bakker, Huib J. Soft Condensed Matter Chemical Physics 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. |
| title | Ion-Specific Effects at the Surface of Water |
| topic | Soft Condensed Matter Chemical Physics |
| url | https://arxiv.org/abs/2508.10811 |