Acid-base chemistry at the surface of the
hydrogen
bonding, water- like liquid glycerol is investigated by time-of-flight velocity analysis and gas uptake measurements. We monitor how the impact energy and approach angle of an incide
nt gas molecule and its solution phase acidity control gas-liquid energy transfer and trapping and interfacial solvation and dissociation. By colliding deuterated gases with the liquid surface, we determine that some degree of proton exchange occurs in t
he near-interfacial region for all gases with a pKa < 4, although solution phase acidity alone was unable to predict the reaction probability for molecules trapped on the surface. The acidity of DCl is over a million times greater than that
of CF3COOD, but all CF3COOD trapped on the surface dissociate and undergo proton exchange while nearly 25% of trapped DCl molecules desorb before reacting. Surface reactivity appears to be driven not only by solution phase acidity
but also by how strongly a gas molecule is held to the surface. Therefore for this hydrophilic surface, molecules like carboxylic acids, that possess the ability to form very strong hydrogen bonds, have higher reaction probabilities than predicted solely
by their solution phase dissociation constants. However for molecules with similar hydrogen bonding capabilities, the reaction probabilities increase with increasing acidity. DCOOD (pKa ~ 4) undergoes proton exchange to a less degree than CF
3COOD (pKa ~ 0), while over 97% of trapped HBr molecules, which are approximately 100 times more acidic than HCl molecules, undergo irreversible, near-interfacial exchange.
Information about the bulk residence times for gases in solution and the mechanisms associated with the proton exchange reactions discussed above were determined using net gas uptake measurements. These measurements show that CF3COOD underg oes reversible solvation, spending less than 1 ms in solution before desorbing back into vacuum, while dissolved HBr molecules remain in the liquid for long times. HCl molecules that undergo proton exchange dissolve reversibly, resulting in a lower net u ptake than for HBr and a residence time in solution on the order of 1 second. This residence time depends on the incident HCl flux, indicating that desorption must be preceded by recombination of solvated Cl- and a protonated glycerol molecule .