Collisions of DCl with a Solution Covered with Hydrophobic and Hydrophilic Ions: Tetrahexylammonium Bromide in Glycerol

Abstract

Gas-liquid scattering experiments are used to investigate interfacial interactions of gaseous DCl with liquid glycerol containing 0.03 M tetrahexylammonium bromide (THABr), an ionic surfactant composed of hydrophobic THA(+) and hydrophilic Br(-) ions. Surface tension and argon scattering measurements indicate that the surface of this solution is dominated by the hexyl chains of the THA(+) ion. The hydrocarbon character of the surface is further explored by comparing Ar and DCl scattering from the THABr solution and liquid squalane. We find that the addition of THABr to glycerol alters the reactivity of DCl in two ways: DCl molecules that land on the surface are more likely to desorb when THA(+) and Br(-) are present and are less likely to dissolve, but they are also more likely to undergo rapid, interfacial DCl --> HCl exchange. Similar trends are observed when THA(+) ions are replaced by Na(+) ions in a 2.7 M NaBr solution, even though THA(+) interacts weakly with glycerol OH groups and Na(+) binds strongly to them. These observations suggest that the THA(+) hexyl chains do not physically block DCl entry and that interfacial cation-OH bonding is not essential for promoting rapid D --> H exchange when Br(-)-OH bonding also occurs. The use of THABr confirms that ions in the top few monolayers, and not those deeper in solution, control DCl entry and rapid D --> H exchange.