Competing Segregation of Br- and Cl- to a Surface Coated with a Cationic Surfactant: Direct Measurements of Ion and Solvent Depth Profiles

Xianyuan Zhao; Gilbert M Nathanson; Gunther G Andersson

doi:10.1021/acs.jpca.0c08859

Competing Segregation of Br^- and Cl^- to a Surface Coated with a Cationic Surfactant: Direct Measurements of Ion and Solvent Depth Profiles

J Phys Chem A. 2020 Dec 31;124(52):11102-11110. doi: 10.1021/acs.jpca.0c08859. Epub 2020 Dec 16.

Authors

Xianyuan Zhao¹, Gilbert M Nathanson¹, Gunther G Andersson²

Affiliations

¹ Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States.
² Centre for Nanoscale Science and Technology, Flinders University, Adelaide, South Australia 5001, Australia.

PMID: 33325710
DOI: 10.1021/acs.jpca.0c08859

Abstract

Ion-surface scattering experiments can be used to measure elemental depth profiles on the angstrom scale in complex liquid mixtures. We employ NICISS (neutral impact collision ion scattering spectroscopy) to measure depth profiles of dissolved ions and solvent in liquid glycerol containing the cationic surfactant tetrahexylammonium bromide (THA⁺/Br^-) at 0.013 M and mixtures of NaBr + NaCl at 0.4 M total concentration. The experiments reveal that Br^- outcompetes Cl^- in its attraction to surface THA⁺, and that THA⁺ segregates more extensively when more Br^- ions are present. Intriguingly, the depths spanned by THA⁺, Br^-, and Cl^- ions generally increase with Br^- bulk concentration, expanding from ∼10 to ∼25 Å for both Br^- and Cl^- depth profiles. This broadening likely occurs because of an increasing pileup of THA⁺ ions in a multilayer region that spreads the halide ions over a wider depth. The experiments indicate that cationic surfactants enhance Br^- and Cl^- concentrations in the surface region far beyond their bulk-phase values, making solutions coated with these surfactants potentially more reactive toward gases that can oxidize the halide ions.