Molecular beam experiments demonstrate the diverse ways in which
propane, propene, acetaldehyde, formic
acid, dimethyl ether, and ethanol scatter from and react with 98.8 wt
% H2SO4. The events following
contact
between gas and liquid include direct inelastic scattering from the
acid's surface, trapping in the interfacial
region followed by immediate desorption or desorption delayed by
reversible protonation, and trapping followed
by nearly irreversible protonation or further reaction. Molecules
that scatter directly from the surface at
incident and exit angles of 45° transfer from 70 to 80% of their
impact energy of 90−110 kJ/mol. The fate
of a molecule trapped at the interface depends on solute basicity and
reactivity: the more basic molecules
preferentially dissolve and undergo protonation rather than immediately
desorb. Propane and propene desorb
promptly, acetaldehyde and formic acid dissolve reversibly, and
dimethyl ether and ethanol enter the acid for
long times with little or no desorption following trapping.
Substitution of 1-butanol for ethanol increases the
size of the hydrophobic group but is insufficient to protect the OH
group from protonation following thermal
accommodation at the interface.
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