We use neutral impact ion scattering spectroscopy (NICISS) to measure the elemental depth profiles of a cationic surfactant, tetrahexylammonium bromide (THABr), dissolved together with NaCl/NaBr salt mixture in liquid glycerol. By analyzing the energy loss of helium atoms backscattered from the surface of the solution, the depth profiles of oxygen, carbon, chloride, and bromide are mapped at the Angstrom scale. These profiles are then used to analyze how solvent, surfactant, and counterions populate the surface region. The experiments encompass a series of solutions with a fixed surfactant concentration (0.01 M THABr) and fixed salt mixture total concentration (0.30 M NaCl + NaBr), while gradually changing the NaCl:NaBr ratio from all NaBr to nearly all NaCl. The interfacial concentrations vary considerably from the bulk as measured over a depth of 0 to 70 Å (7 glycerol layers). Br- is found to be more attracted to THA+ than is Cl- by a factor of 2 to 3, indicating that Br- outcompetes Cl- as a counterion for THA+. The total adsorption of THA+ is also enhanced when the salt mixture contains more Br-. Glycerol solvent molecules are concomitantly repelled from the surface as THA+ replaces glycerol at larger Br- bulk concentrations. As partial proof of the validity of the NICISS study, the adsorption of THA+ and the depletion of glycerol are consistent in terms of a volume trade-off. These structural analyses are in accord with previous reactivity studies of gaseous Cl2 and N2O5 at THA+-coated glycerol containing differing fractions of Cl- and Br- ions.

Measuring the competition between counterions for cationic surfactants at liquid surfaces

ACS Spring Meeting 2021

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