Gas transport through hexanol films on supercooled sulfuric acid

Films of surfactant molecules potentially alter the growth of sulfuric acid droplets and their ability to catalyze heterogeneous reactions in the upper troposphere. This thesis describes measurements of water evaporation and gas uptake through hexanol films at the surface of supercooled sulfuric acid solutions. The surface segregation of 1-hexanol in 56--68 wt % D 2SO4 at 213 K is characterized using hyperthermal argon atom scattering in conjunction with surface tension measurements. The effect of hexanol films on the evaporation of D2O is investigated by measuring evaporation rates from bare acid and film-coated acid. Molecular beam techniques are employed to measure the fractions of HCl, HBr, and CF3CH 2OH that undergo H→D exchange with the deuterated acid. The exchange fraction is equivalent to the probability of the gas molecule entering the acid.; We find that the water evaporation rates from bare and film-coated 68 wt % D2SO4 are equal, implying that the hexanol film is porous. However, on 56 and 60 wt % D2SO4 the film reduces the water evaporation rate by 20--25% relative to the bare acid. Gas uptake measurements further delineate this change in permeability with acid concentration. The addition of hexanol increases the exchange fractions of HCl and HBr in 68 wt % D2SO4 in a manner that tracks the surface coverage of the film. This enhancement likely arises from the hydroxyl groups of the segregated hexanol molecules, which provide additional interfacial sites for HCl and HBr hydrogen bonding and dissociation. In 60 wt % D2SO4 the HCl exchange fraction rises by ∼10% at low hexanol coverage but drops back to the bare acid value at saturation coverage. Finally, in 56 wt % D2SO4 the HCl exchange fraction is reduced by ∼10%. The experiments suggest that a hexanol film on sulfuric acid droplets can reduce the rates of water evaporation and condensation, but likely will not limit heterogeneous reactions of HCl or HBr with dissolved species because of the small changes in entry probability imposed by the film. To our knowledge, hexanol is the smallest molecule yet observed to form a surface film that can impede gas transport.