Title: Ions at Water Surfaces
1Ions at Water Surfaces
1.2 M Aqueous Sodium Halide Solutions
Top View
Side View
Jungwirth, P. Tobias, D.J. Chem. Rev. 2006, 106,
1259-1281.
2Salty Glycerol Solutions
What happens when gaseous DCl collides with pure
glycerol?
Near-Interfacial D?H Exchange (3)
Incident Molecule
Inelastic Scattering
DCl
DCl
HCl
DCl
Non-Reactive Desorption (10)
HCl
Desorption After Bulk Solvation
Glycerol Vapor Pressure 10-4 Torr Viscosity
1400 cP Dielectric Constant 44 Surface
Tension 63 dyn cm-1 No surface dangling
hydrogen bonds
H
H / D
Cl-
Bulk Solvation (87)
Bulk residence time of HCl 0.2 s
Cl-
Salts alter non-reactive and reactive pathways
Concentration
Cation Charge
Identity
1.2, 2.6, 3.9 M NaI 0.7, 1.3 M CaI2
Na (1.16 Å) Ca2 (1.14 Å)
NaI, NaBr, KI, LiI, and KFa surprise!
3Molecular Beam Scattering Experiments
Liquid Reservoir
90 kJ mol-1 DCl Beam
Nozzle
Skimmer
Post-Chopper Time-of-Flight Wheel
Scraper
Exiting HCl / DCl Molecules
Rotating Glass Wheel
Mass Spectrometer
Pure or Salty Glycerol Solution
Double-Decker Reservoir System
4Are Ions Present at the Surface?
Inelastic Scattering (IS)
Surface Mass Ratios
Experimental Data Planar Cut Prediction
Ar Signal
Pure 3.9 M NaI
Maxwell-Boltzmann fits at 291 K
ThermalDesorption (TD)
Arrival Time (µs)
800
0
5Ions Enhance Non-Reactive Pathways
Inelastic Scattering (IS)
DCl Scattering Pathways
More DCl scatters from salt solutions
TD
90 kJ mol-1 (36RT)
IS
45?
45?
Pure / Salty Glycerol
DCl Signal
More DCl undergoes non-reactive desorption from
salt solutions
Pure 2.6 M NaI
Thermal Desorption (TD)
Arrival Time (µs)
0
800
6Isolating Near-Interfacial Exchange
DCl?HCl Exchange in Glycerol
HCl Scattering Pathways
2.6 M NaI in Glycerol
90 kJ mol-1
Interfacial Exchange
45?
50 µs pulses
? 10-5 s
? 10-6 s
HCl Signal
Pure / Salty Glycerol
Pure Glycerol
Maxwell-Boltzmann fits convoluted with a
characteristic bulk residence time, ?, at 291 K
0
1200
Arrival Time (µs)
7Changing Cation Anion Identity
1.0
Note All salt solutions are 2.6 M
Bulk Solvation
0.8
Non-Reactive Desorption
0.6
Near-Interfacial Exchange
Fraction of Trapped DCl Molecules
0.4
0.2
Pure KI NaI LiI NaBr
Pure KI NaI LiI NaBr
Pure KI NaI LiI NaBr
The addition of alkali halide salts to pure
glycerol increases the fraction of DCl molecules
that thermalize on the surface and leave as DCl
or undergo an near-interfacial D?H exchange.
Muenter, A.H. DeZwaan, J.L. Nathanson, G.M. J.
Phys. Chem. C 2007.
8Concentration Charge Effects
Non-Reactive DCl Desorption
Near-Interfacial D?H Exchange
CaI2
CaI2
NaI
NaI
Fraction of Trapped DCl
Fraction of Trapped DCl
Salt Concentration (Molar)
Salt Concentration (Molar)
Bulk Solvation
Near linear trends for NaI implies no changes in
interactions in the interface at high
concentrations
NaI
When I- 2.6 M, Ca2 has a larger effect than
Na
Fraction of Trapped DCl
CaI2
Salt Concentration (Molar)
9Interfacial Interactions
Incoming DCl molecules need 3 hydrogen bonding
sites to form a contact ion pair and dissociate
Ions can take up these bonding sites, enhancing
DCl desorption and scattering
Chorny, I. Benjamin, I. J. Phys. Chem. B, 2004,
108, 995.
Ions can interrupt the proton relay, increasing
near-interfacial D?H exchange
Once trapped, DCl molecules can dissociate
10Eliminating Interfacial Reactivity
CaI2, NaI, LiI, KI, NaBr have all been shown to
enhance interfacial reactivity, but how can we
eliminate it?
For a 4 M KF solution
KF eliminates near-interfacial DCl?HCl exchange
Incident Molecule
Near-Interfacial Exchange
Inelastic Scattering
DCl
DCl
HF
DCl
Non-Reactive Desorption (5)
X
HF Desorption
F-
H
D
Cl-
K
Bulk Solvation (95)
F-
Cl-
H F- ? HF
11KF Suppresses DCl Desorption!
Inelastic Scattering (IS)
Non-Reactive DCl Desorption
Fraction of Trapped DCl
DCl Signal
KF Concentration (M)
Pure 4 M KF 1.3 M KF
Thermal Desorption (TD)
0
800
Arrival Time (µs)
12Interfacial DCl Scavenging by F-
A possible mechanism
DCl
HF
Cl?-
D?
Cl-
F-
F-
DF
K
Hydrogen Bond Strengths1
D?H Exchange
Bond kJ mol-1 F-HF 163 F-HOCH3 125 Cl-HCl 96
F-HCl 92 I- HCl 58
HF ? H F-
Ka(H2O) 1 ? 10-3
1Klepeis, N. E. et al. J. Phys. Chem. 1993, 99,
3865.
13Future Work with Cold, Salty Water
Ar Scattering from 7.05 m aqueous LiBr at -60 ?C
Ar
IS
90 kJ mol-1 (36RT)
TD
45?
45?
Ar Signal
Br-
Li
Arrival Time (µs)
0
1400