Highly Altered State of Proton Transport in Acid Pools in Charged Reverse Micelles

Transport mechanisms of solvated protons of 1 M HCl acid pools,
confined within reverse micelles (RMs) containing the negatively charged surfactant
sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) or the positively charged
cetyltrimethylammonium bromide (CTABr), are analyzed with reactive force field
simulations to interpret dynamical signatures from TeraHertz absorption and dielectric
relaxation spectroscopy. We find that the forward proton hopping events for NaAOT are
further suppressed compared to a nonionic RM, while the Grotthuss mechanism ceases
altogether for CTABr. We attribute the sluggish proton dynamics for both charged RMs
as due to headgroup and counterion charges that expel hydronium and chloride ions
from the interface and into the bulk interior, thereby increasing the pH of the acid pools
relative to the nonionic RM. For charged NaAOT and CTABr RMs, the localization of
hydronium near a counterion or conjugate base reduces the Eigen and Zundel
configurations that enable forward hopping. Thus, localized oscillatory hopping
dominates, an effect that is most extreme for CTABr in which the proton residence time increases dramatically such that even
oscillatory hopping is slow.