1E2
Hydronium at the air-water interface is a superacid
1
( The Hakubi Center for Advanced Research, Kyoto Univ., 2Research Institute for
Sustainable Humanosphere, Kyoto Univ., 3PRESTO, JST) S. ENAMI1,2,3
Ions at aqueous interfaces play vital roles.1 Among
ions, hydronium, H3O+, is exceptinally important since
the proton transfer (PT) through and across aqueous
interfaces is a fundamental process in living systems.
Notwithstanding its importance, it is not generally
realized that interfacial PT is quite different from
conventional PT in bulk water or in gas-phase. Here
the mechanism of PT across air-water boundaries
and the unique properties of interfacial H3O+ are
investigated in experiments in which the protonation of
gaseous base2,3, acid4 and neutral hydrocarbons5 upon
collision with liquid water microjets is monitored by
FIGURE 1 Schematic diagram of the
present experimental setup
interface-specific mass spectrometry6 as a function of
bulk pH (Fig. 1).
We found that H3O+ emerges at the surface of water
pH < 4 by experiments of gaseous trimethylammine
(TMA(g)) uptake on aqueous microjets (Fig. 2),
consistent with previous electrophoretic experiments
on bubbles and droplets in water.2,3 Surprisingly,
although hexanoic acid (PCOOH(aq)) is a very weak
‘base’ (pKBH+ < -3), PCOOH(g) was found to be
converted to PC(OH)2+ on pH < 4 water via a process
that retains some of the exoergicity of its gas-phase
counterpart, PCOOH + H3O+ = PC(OH)2+ + H2O, ΔG ≈
-22 kcal mol-1 (Fig. 3). The large kinetic isotope
effects observed on H2O/D2O microjets4 indicate that
protonation of PCOOH(g) on water involves tunneling
and is faster than H-isotope exchange. We also found
that gaseous unsaturated hydrocarbons (-,-Pinene,
+
FIGURE 2 TMAH signal intensities as
functions of bulk pH on pure water
microjets
or
microjets
with
various
or d-Limonene, C10H16)) are readily protonated (to
dopants exposed to 1 ppmv TMA(g).
C10H17+) and undergo oligomerization (to C20H33+ and
in (A) linear and (B) semilog scales.
C30H49+) upon colliding with the surface of pH < 4 microjets (Fig. 4).5 By considering that the
yields of all products show inflection points at pH ≈ 3.5 and display solvent kinetic hydrogen
isotope effects larger than 2,5 we conclude that the oligomerization process is initiated by
weakly hydrated hydronium ions, H3O+, present at the gas−water interface.
In summary, the very similar titration curves of the products versus bulk pH obtained from
TMA, PCOOH, and unsaturated hydrocarbons are compelling evidence that a barely
hydrated hydronium ion, H3O+, is present on the topmost layers of acidic (pH < 4) water, and
can transfer a proton to colliding gaseous molecules by acting as a superacid (Fig. 5).
FIGURE 3 Enthalpy diagram for the protonation of
+
hexanoic acid PCOOH by H3O under increasing
solvation.
FIGURE 4 (A) Normalized monomer (m/z = 137),
dimer (m/z = 273) and trimer (m/z = 409) mass
spectral signal intensities, (B) ratios of M/(M+1)
FIGURE 5 Hydronium becomes a superacid
mass spectral signal intensities as a function of
once it appears at the water’s topmost layers
bulk pH, in experiments performed on H2O/D2O
【References】
(50:50)
microjets
exposed
to
31
ppmv
(1)
Enami, S.; Mishra, H.; Hoffmann,
-Pinene(g).
M. R.; Colussi, A. J. Hofmeister Effects in Micromolar
Electrolyte Solutions. J. Chem. Phys. 2012, 136, 154707.
(2)
Enami, S.; Hoffmann, M. R.; Colussi, A. J. Proton Availability at the Air/Water
Interface. J. Phys. Chem. Lett. 2010, 1, 1599-1604.
(3)
Enami, S.; Hoffmann, M. R.; Colussi, A. J. Molecular Control of Reactive Gas Uptake
"on Water". J. Phys. Chem. A 2010, 114, 5817-5822.
(4)
Enami, S.; Stewart, L. A.; Hoffmann, M. R.; Colussi, A. J. Superacid Chemistry on
Mildly Acidic Water. J. Phys. Chem. Lett. 2010, 1, 3488-3493.
(5)
Enami, S.; Hoffmann, M. R.; Colussi, A. J. Dry Deposition of Biogenic Terpenes Via
Cationic Oligomerization on Environmental Aqueous Surfaces. J. Phys. Chem. Lett. 2012, 3, 3102-3108.
(6)
Laskin, J.; Laskin, A.; Nizkorodov, S. A. New Mass Spectrometry Techniques for
Studying Physical Chemistry of Atmospheric Heterogeneous Processes. Int. Rev. Phys. Chem. 2013, 32,
128-170.