Volume 251, 2024

Probing the interfacial structure of aqueous surfactants through helium atom evaporation

Abstract

Dissolved helium atoms evaporate from liquids in super-Maxwellian speed distributions because their interactions are too weak to enforce full thermal equilibration at the surface as they are “squeezed” out of solution. The excess speeds of these He atoms reflect their final interactions with solvent and solute molecules at the surfaces of water and other liquids. We extend this observation by monitoring He atom evaporation from salty water solutions coated with surfactants. These surface-active molecules span neutral, anionic, and cationic amphiphiles: butanol, 3-methyl-1-butanol, pentanol, pentanoic acid, pentanoate, tetrabutylammonium, benzyltrimethylammonium, hexyltrimethylammonium, and dodecyltrimethylammonium, each characterized by surface tension measurements. The helium energy distributions, recorded in vacuum using a salty water microjet, reveal a sharp distinction between neutral and ionic surfactant films. Helium atoms evaporate through neutral surfactant monolayers in speed distributions that are similar to a pure hydrocarbon, reflecting the common alkyl chains of both. In contrast, He atoms appear to evaporate through ionic surfactant layers in distributions that are closer to pure salty water. We speculate that the ionic surfactants distribute themselves more loosely and deeply through the top layers of the aqueous solution than do neutral surfactants, with gaps between the surfactants that may be filled with salty water. This difference is supported by prior molecular dynamics simulations and ion scattering measurements of surfactant solutions.

Graphical abstract: Probing the interfacial structure of aqueous surfactants through helium atom evaporation

Associated articles

Supplementary files

Article information

Article type
Paper
Submitted
20 12 2023
Accepted
05 2 2024
First published
05 2 2024

Faraday Discuss., 2024,251, 342-360

Probing the interfacial structure of aqueous surfactants through helium atom evaporation

X. Gao, D. J. Hood, T. H. Bertram and G. M. Nathanson, Faraday Discuss., 2024, 251, 342 DOI: 10.1039/D3FD00177F

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