Surfactant Control of Gas Uptake into Supercooled Sulfuric Acid

Background

            Surfactant molecules on sulfuric acid droplets may alter the rates of heterogeneous reactions in the upper troposphere and lower stratosphere.  These droplets (~40–80 wt % H2SO4 at ~200–240 K) provide an aqueous medium for reactions of halogen-containing molecules, such as HOCl + HCl -> Cl2 + H2O.  Upon Cl2 photolysis, Cl atoms catalytically destroy ozone.

             When alcohols are dissolved in sulfuric acid, they preferentially segregate to the surface and form -loosely packed monolayers.  Our surface tension measurements, for example, show that n-butanol at concentrations above 0.2 M forms ~80% of a monolayer at the surface of 72 wt % H2SO4 at 208 K.  These surfactant molecules may block gas molecules from entering the acid.  Our objective is to determine the ways in which organic surfactants control gas uptake and modify the interfacial reactivity of the acid.

Experiments with Bare Sulfuric Acid

             We direct a beam of protic molecules HX at a continuously renewed film of supercooled D2SO4/D2O in vacuum.  By using a mass spectrometer, we measure the fraction of HX molecules that thermalize on the surface and undergo H at D exchange with the acid before evaporating as DX.  Our experiments show that molecules that undergo exchange are also those that enter the acid and become available to react with other dissolved species.  Molecules that do not undergo exchange desorb immediately from the surface and do not enter the acid.  

             We find that the strongly hydrogen bonding molecule HNO3 enters 70 wt % D2SO4 at 213 K on nearly every collision, indicating that it forms strong enough bonds to be pulled into solution each time.  However, HCl and HBr, which are more weakly hydrogen bonding, enter only 10% and 20% of the time, respectively, for the same conditions.  The remaining molecules desorb immediately from the surface.  The entry probability increases from 10% to 70% (HCl) and 20% to 90% (HBr) as the acid concentration is decreased from 70 to 54 wt % D2SO4 and more water molecules become available in the interfacial region for hydrogen bonding and subsequent protonation.

New Results with Surfactant-Coated Sulfuric Acid

             Our results thus far contradict the notion that surfactants impede gas transport.  The presence of surface butanol does not alter the rate of D2O evaporation from the liquid surface.  Our most striking result is that surface butanol actually increases the HCl entry probability, implying that HCl dissociates more readily at the interface when butanol is present.  This enhancement may be caused by the dilution of the acid near the surface by segregated butanol molecules, which provide additional OH groups for protonation by HCl.

Future Directions

             We are now exploring n-hexanol on supercooled sulfuric acid.  We plan to investigate longer-chain and branched alcohols and other surfactants such as amines and sulfates, and extend our studies of HCl to other atmospheric gases such as HBr and HOCl.

Please click here to view a PowerPoint presentation of this project.