Molecular switches gate many fundamental processes in natural and artificial systems. Here, we report the development of an electrochemical platform in which a proton carrier switches the activity of a catalyst. By incorporating an alkyl phosphate in the lipid layer of a hybrid bilayer membrane, we regulate proton transport to a Cu-based molecular oxygen reduction reaction catalyst. To construct this hybrid bilayer membrane system, we prepare an example of a synthetic Cu oxygen reduction reaction catalyst that forms a self-assembled monolayer on Au surfaces. We then embed this Cu catalyst inside a hybrid bilayer membrane by depositing a monolayer of lipid on the self-assembled monolayer. We envisage that this electrochemical system can give a unique mechanistic insight not only into the oxygen reduction reaction, but into proton-coupled electron transfer in general.

We explore collisions of hydrogen-bonding molecules with salty water using gas–microjet scattering experiments. Two aqueous solutions, 8 molal (m) LiBr/H2O and ∼4 m K2SO3/H2O at 253 K were exposed to seven organic gases representing different functional groups. These gases comprise weak acids (formic and acetic), weak bases (dimethylamine and piperidine), and an alcohol, ether, and ester (ethanol, dimethyl ether, and methyl formate). The scattering experiments are used to monitor the disappearance of each gas into the aqueous solutions over a ∼100 μs observation time. They demonstrate that formic acid and piperidine disappear into both solutions on almost every collision. Dimers of formic and acetic acid are also captured by the solutions on every collision, despite their pre-existing double hydrogen bonds. The methylene ring of piperidine, (CH2)5NH, also does not interfere with uptake. At the opposite extreme, methyl formate and dimethyl ether are so weakly soluble that they evaporate completely within the observation window, precluding the measurement of their entry probability. Ethanol and dimethylamine represent intermediate cases in which dimethylamine interacts more strongly with dissolved Li+ ions than K+ ions. Collectively, the experiments imply that organic acids and bases reach hydrogen-bonding configurations following nearly every collision, enabling them to be captured by surface water molecules.

Gas−liquid scattering experiments are used to investigate the oxidation−reduction reaction N 2 O 5 (g) + 2Br − (aq) → Br 2 (g) + NO 3 − (aq) + NO 2 − (aq), a model for the nighttime absorption of N 2 O 5 into aerosol droplets containing halide ions. The detection of evaporating Br 2 molecules provides our first observation of a gaseous reaction product generated by a water microjet in vacuum. N 2 O 5 molecules are directed at a 35 μm diameter jet of 6 or 8 m LiBr in water at 263 or 240 K, followed by detection of both unreacted N 2 O 5 and product Br 2 molecules by velocity-resolved mass spectrometry.The N 2 O 5 reaction probability at near-thermal collision energy is too small to be measured and likely lies below 0.2. However, the evaporating Br 2 product can be detected and controlled by the presence of surfactants. The addition of 0.02 m 1-butanol, which creates ∼40% of a compact monolayer, reduces Br 2 production by 35%. Following earlier studies, this reduction may be attributed to surface butanol molecules that block N 2 O 5 entry or alter the near-surface distribution of Br − . Remarkably, addition of the cationic surfactant tetrabutylammonium bromide (TBABr) at 0.005 m (9% of a monolayer) reduces the Br 2 signal by 85%, and a 0.050 m solution (58% of a monolayer) causes the Br 2 signal to disappear entirely. A detailed analysis suggests that TBA + efficiently suppresses Br 2 evaporation because it tightly bonds to the Br 3 − intermediate formed in the highly concentrated Br − solution and thereby hinders the rapid release and evaporation of Br 2 . + 2H + (not shown).Article pubs.acs.org/JPCA

Gas-liquid scattering experiments were employed to measure the entry and dissociation of the acidic gas DCl into salty glycerol coated with dodecyl sulfate ions (DS(-) = CH3(CH2)11OSO3(-)). Five sets of salty solutions were examined: 0.25 and 0.5 M NaCl, 0.25 M MgCl2, 0.25 M CaCl2, and artificial sea salt. DS(-) bulk concentrations were varied from 0 to 11 mM, generating DS(-) surface coverages of up to 34% of a compact monolayer, as determined by surface tension and argon scattering measurements. DS(-) surface segregation is enhanced by the dissolved salts in the order MgCl2 ≈ CaCl2 > sea salt > NaCl. We find that DCl penetration through the dodecyl chains decreases at first gradually and then sharply as more chains segregate to the surface, dropping from 70% entry on bare glycerol to 11% for DS(-) surface concentrations of 1.8 × 10(14) cm(-2). When plotted against DS(-) surface concentration, the DCl entry probabilities fall within a single band for all solutions. These observations imply that the monovalent Na(+) and divalent Ca(2+) and Mg(2+) ions do not bind differently enough to the ROSO3(-) headgroup to significantly alter the diffusive passage of DCl molecules through the dodecyl chains at the same DS(-) chain density. The chief difference among the salts is the greater propensity for the divalent salts to expel the soluble ionic surfactant to the surface.

Liquid microjets provide a powerful means to investigate reactions of gases with salty water in vacuum while minimizing gas-vapor collisions. We use this technique to explore the fate of gaseous HCl and DCl molecules impinging on 8 molal LiCl and LiBr solutions at 238 K. The experiments reveal that HCl or DCl evaporate infrequently if they become thermally accommodated at the surface of either solution. In particular, we observe minimal thermal desorption of HCl following HCl collisions and no distinct evidence for rapid, interfacial DCl→HCl exchange following DCl collisions. These results imply that surface thermal motions are not generally strong enough to propel momentarily trapped HCl or DCl back into the gas phase before they ionize and disappear into solution. Instead, only HCl and DCl molecules that scatter directly from the surface escape entry. These recoiling molecules transfer less energy upon collision to LiBr/H2O than to LiCl/H2O, reflecting the heavier mass of Br(-) than of Cl(-) in the interfacial region.