We use neutral impact ion scattering spectroscopy (NICISS) to measure the depth profiles of ionic surfactants, their counterions and co-ions, and solvent molecules in liquid glycerol. The experiments yield an Angstrom-scale picture of how these species populate the interfacial region. The chosen surfactants are tetrahexylammonium bromide and sodium dodecyl sulfate in the absence and presence of sodium bromide and sodium chloride. NICISS determines the depth profiles of the elements C, O, Na, S, Cl, and Br through the loss in energy of He+ ions that travel into and out of the liquid, which is then converted into depth. In the absence of NaBr, we find that THA+ and its Br- counterion segregate together because of charge attraction, forming a narrow double layer that is 10 Å wide and many times more concentrated than in the bulk. With the addition of NaBr, THA+ is “salted out” to the surface, increasing the interfacial THA+ and Br- concentration and spreading the ions over a ~30 Å depth. Added NaBr similarly increases the interfacial concentration of DS- ions and broadens their positions. However, the dissolved Br- ions are strongly repelled from surface DS- because of charge repulsion. Chloride is less surface active than bromide and generates thinner THA+ segregated regions. The different interfacial Br- propensities correlate with measured gas-liquid reactivities: gaseous Cl2 readily reacts with Br- ions in the presence of THA+ but drops steeply in the presence of DS-, demonstrating that surfactant headgroup charge can control the reactivity of Br- through changes in its depth profile.

Measuring the depth profiles of surfactants, ions, and solvent at the Angstrom scale

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.