Suppression of aqueous surface hydrolysis by monolayers of short chain organic amphiphiles
Daniel Clifford, Thorsten Bartels-Rausch and D.J. Donaldson Phys. Chem. Chem. Phys. , 2007 , 9 , 1362-1369
Aqueous aerosols and other water surfaces in the environment may be coated with organic films, which can give rise to significant effects on gas-solution transport and surface reactivity. We have used a surface active fluorescent pH probe to examine the hydrolysis of nitric acid and ammonia at both the uncoated and the organic-coated air-water interface. For uncoated samples, a transient change in pH is observed at the interface upon introduction of acid or base vapour, followed by a relaxation to a final pH which is different than the initial value, and equal to the final bulk pH. Solutions having monolayer and sub-monolayer films of 1-octanol do not display the transient, but do show the same long-time change in pH. The degree of suppression of the surface pH transient depends directly on the amount of octanol present at the surface, however when monolayers of butanol and of uncompressed stearic acid are present at the surface there is little difference seen from the clean interface. We propose that the surface component of the hydrolysis reaction depends directly on the concentration of available water at the interface.
In the upper half of the figure, the time dependence of surface pH is shown after gas phase nitric acid is introduced above a bare interface (in blue) and one having a monolayer of 1-octanol at the surface (red). Beside these traces are photos of the fluorescence observed with higher (top) and lower (bottom) values of the surface pH. The lower portion of the figure shows a cartoon depicting the inhibition of surface hydrolysis by an organic monolayer.