1
Chemistry of Ice Surfaces
KNU Symposium, 2009
Heon Kang
Department of Chemistry, Seoul National University, Korea
Coworkers:
Chang-Woo Lee (PhD, 2008)
Seong-Chan Park (Postdoc, 2008)
Jung-Hwan Kim (PhD, 2008)
Eui-Sung Moon (PhD Student)
Joon-Ki Kim (MSc, 2008)
Funding: KOSEF
2
Structure of Ice Surfaces
The full-bilayer termination of normal hexagonal (Ih) ice is energetically The full-bilayer termination of normal hexagonal (Ih) ice is energetically favored over the half-bilayer termination.
The surface of a crystalline ice film grown on a metal substrate is mostly in a full-bilayer terminated (0001) structure, and there is greatly enhanced vibrational motion of the molecules on the outer surface at 90 K.
Low-energy electron diffraction (LEED), Materer et al., Surf. Sci. (1997), 381, 190 He atom diffraction, Braun et al., Phys. Rev. Lett., (1998), 80, 2638
Self-Diffusion Rate at the Surface and Interior of Ice
-12
-10
surface (Jung et al.) bulk (Brown and Gorge)
b lk (Goto et al ) nter
laye
r (s
)
T (K)100200 150 80250
10-6
Interstellar clouds
StratosphereHot cores
-20
-18
-16
-14
bulk (Goto et al.)
Log
[D (
cm2 s
-1)]
usio
n T
ime
acro
ss O
ne I
ce I
n
103
106
1
10-3
Dsurface / Dbulk > 1 for T < 150 K
If reaction occurs with ice at low temperatures, it will occur predominantly at the ice surface where the molecules may be able to diffuse, rather than in the interior.
0.004 0.006 0.008 0.010 0.012-22
1/T (K-1)
Diff
u106
Goto et al., Jpn. J. Appl. Phys. (1986); Brown and George, J. Phys. Chem. B (1997); K-H. Jung et al., J. Chem. Phys. (2004).
3
Experimental Apparatus for Ice Surface Chemistry
UV light
Ice Film
Ru(0001)
UHV chamberSubstrate
Cs+ ion gun
LHe Cryostat(T ≥ 50 K)
Quadrupole mass spectrometer
Kr resonance lamp (hν = 10.03 & 10.64 eV, Flux ~ 5 × 1015 s–1 sr–1)
MgF2 window
Gas dosing valve
Reactive Ion Scattering of Low Energy Cs+ (RIS)
Cs+ CsX+t = – t = +
XCs+
Xt = 0Cs+
t < 1 ps surfaceinteraction
region
IE of Cs = 3.89 eV
Mass = 133 amu
Ek(Cs+) = 3-100 eV
solid surface
M. C. Yang et al., J. Chem. Phys. 107, 2611 (1997)
H. Kang, Acc. Chem. Res. 38, 893 (2005)
4
Molecular Dynamics Simulation Molecular Dynamics Simulation of Csof Cs++ RIS TrajectoryRIS Trajectory
(i) E-R Abstraction ProcessEk(Cs+) = 10 eVEb = 0.5 eV, ECs+-ads = 0.5 eVAdsorbate mass = 8 amu on Pt(111)
Low Energy Sputtering (LES): Low Energy Sputtering (LES): ImpactImpact--Induced Ion DesorptionInduced Ion Desorption
t = - ∞ t = + ∞
Cs+ Cs+
t = 0
B-
A+
solid surface
matrixB-
A+
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Mass Spectrometric Identification of Surface Species
Cs+ reactive ion scattering (RIS) → neutral species
Low energy sputtering (LES) → ions
LES peaks
RIS products
Hydronium (H3O+) and Hydroxide Ions (OH-)
Important species in aqueous solution chemistry
They exist as ionic defects in the ice lattice
Charge carriers (proton transfer) in ice
How do they affect chemistry of ice surfaces? y y
6
Spatial Distribution of Hydronium Ions near the Ice Surface
The population of hydronium ions produced from the ionization of HCl at the ice surface becomes saturated at high HCl exposure, and the amount of HCl uptake at saturation is almost invariant with the thickness (1-5 BL) of ice film.
hydronium ions do not move from the ice film surface to the
HCl
H2O (1-8 BL)
200
300 All Hydronium ions
HCl on a crystalline ice film (8 BL) at 140 K
dro
niu
m i
on
(c
ps)
600
800
1000
All Hydronium ionydro
niu
m io
n
hydronium ions do not move from the ice film surface to the interior.
Ru(0001)
0 50 100 150 200 250
0
100
Inte
nsi
ty o
f h
yd
HCl exposure time (sec)
1 2 3 4 5 60
200
400
All Hydronium ion
I Max
of
all h
D2O ice thickness (BL)
Park and Kang, JPCB (2005)
Proton transfer from the ice interior to the surface
D2O
H2O (4 BL) H+D+
Ru(0001)
(a) T < 120 K
Protons (H+) migrate from the embedded hydronium ions to the film surface.
(b) T > 120 K
Deuterium-enriched hydronium ionsDeuterium-enriched hydronium ions are produced at the surface, as a result of D+ transfer from the bottom D2O layer via proton hopping and molecule rotation (“hop-and-turn” process).
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Proton transfer from the ice interior to the surface
Hydronium ions do not move from the ice film surface to the interior.
Protons migrate from the hydronium ions in the sandwich layer to the film surface.
→ Thermodynamic propensity for protons to reside at the ice surface
C.-W. Lee et al., Angew. Chem. Int. Ed. (2006); C. W. Lee et al., J. Chem. Phys. (2007).
Distribution of hydroxide ions near the ice surface
1.0x103
1.5x103 a)
y (c
ps) Hydroxide ion H2O
Ru(0001)
Na+
Na OH-
Na + H2O → Na+ + OH- + ½H2
OH- generated from Na hydrolysis tends to float on the surface of ice film, opposite to the migration of Na+ to the film interior.
40
60
0.0
5.0x102
CsNaOD+
b)
sity
(cp
s)
Sodium Ion
Sodium ion (NaF adsorption)
Inte
nsit
y Ru(0001)
→ OH- has thermodynamic tendency to stay at the ice surface.
90 100 110 120 130 140
0
20
Inte
ns
Temperature (K)
J. H. Kim et al., J. Phys. Chem. C (2009)
8
Hydronium and Hydroxide Ions at the Ice Surface
Hydronium and hydroxide ions may critically affect chemistry of ice surfaces: proton transfer, acid-base reaction, proton-catalyzed reaction, charge conduction, etc.
Why hydronium and hydroxide ions prefer to reside at the ice surface ? Why hydronium and hydroxide ions prefer to reside at the ice surface ?
Ice is a very poor solvent in general, because a crystalline ice lattice generates a thermodynamic repulsive force that transfers the trapped foreign species to the surface where the geometry of water molecule can be relaxed. However, the surface segregation phenomena of ions are also determined by chemical specificity of the ions.
When sodium halide salts are ionized on an ice film surface, Na+ and F- migrate , gto the film interior, whereas Cl- and Br- prefer to stay at the surface. [J.-H. Kim, Y.-K. Kim and H. Kang, J. Phys. Chem. C 2007, 111, 8030] Similar chemical specificity appears for both ice and liquid water surfaces.
Why such chemical specificity appears is an interesting theoretical question.
Lifetime of H3O+
In liquid water: Decay time of [H3O+] (bulk conc.) in the temp-jump kinetic measurement, τ1/2 ~ 4 x 10-5 s (k ~ 1 x 1011 L/mol/s near the room temperature)
In crystalline ice: yProton transfer is faster than in water, argued by M. Eigen, 1950’s.Ea (proton transfer) ≈ 0, proton hopping time ~ 1 x 10-13 s
At an ice surface: τ1/2 ~ ?Ea (proton transfer) > 0 according to the studies of the H/D exchange of water at ice surfaces doped with excess protons or hydroxide ions [Park et al., JCP 2004; Kim et al JCP submitted] This suggests the presence of the energetically stabilized statesal., JCP, submitted]. This suggests the presence of the energetically stabilized states of hydronium and hydroxide ions at ice surfaces.
In ice with defects: Decay time of UV-generated [H3O+] in ice, τ1/2 ≥ 1 hr in ice at T ~ 60 K (Moon et al. JCP, 2008)
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Lifetime of UV-generated H3O+ in ice: Protonation of Methylamine at Ice Surfaces by H3O+
1
2
CsMA+
4 cp
s)
Cs+ (x 1/10)
(a) MA(0.35 ML)/H2O(5 BL), before UV irradiation
E(Cs+) = 30 eV
CsH2O+
(x 1/10) UV light
20 30 40 130 140 150 1600
1
2
0
Cs+ (x 1/10)
(b) After UV irradiation (1 x 1016 photons cm-2), T = 53 K
Inte
ns
ity
(10
CsMA+
MAH+
CsH2O+
(x 1/10)
(2x1016 photons)
H2O (5 BL)
Ru(0001)
MA
20 30 40 130 140 150 160m/z (amu/charge)
hv (< 11 eV)CH3NH2 (MA) CH3NH3
+ (MAH+) :
Protonation of MA at the ice surface (T = 50-130 K)
[MA is a weak base in aqueous solution, Kb(MA) = 4.5 x 10-4].
Adsorption of MA after UV Irradiation of Ice Film
2 × 1016 photons/cm2
T = 55 K, E(Cs+) = 30 eV
1
2
3
4
4 c
ps)
(a) UV-irradiated ice
CsH2O+
(x 1/10)
Cs+ (x 1/10)
UV light
(2x1016 photons)
H2O (5 BL)
Ru(0001)
MA+MAH+
0
1
2
3
40
1
Inte
ns
ity
(104
(b) MA adsorption on UV-irradiated ice
CsMA+MAH+
CsH2O+
(x 1/10)
Cs+ (x 1/10)
(0.35 ML)
20 30 40 130 140 150 160m/z (amu/charge)
hvH2O (ice) → H3O+ (ice) : long-lived protonic defects
MA + H3O+ (ice) → MAH+ + H2O (ice) : proton transfer through ice at 55 K
Moon et al. J. Chem. Phys. (2008)
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Plausible Mechanism for UV-induced Protonation of MA
(1) Creation of ionic defects (H3O+ and OH-) by UV radiation
H2O(ice) + hv H3O+ + OH- (ionic defect pair),
Ehv ≥ 6.5 eV [2], ΔG ≈ 1.4 eV
(H O+ + H O + e- H O+ + OH + e- H O+ + OH- )(H2O+ + H2O + e H3O+ + OH + e ··· H3O+ + OH )
(H2O* + H2O ··· H3O+ + OH-)
(2) Trapping of ionic defects at Bjerrum defect sites in ice.Lifetime (τ1/2) of trapped protons in ice ≥ 1 hr at a low temperature (~ 60 K)↔ τ1/2 in liquid water at room temperature ~ 4 x 10-5 s
(3) Proton transfer from H O+ to MA
[Ref. 2] The formation of positive charge carrier in ice has been reported: Petrenko et al., J. Phys. Chem. B 101, 6208 (1997).
(3) Proton transfer from H3O+ to MA
MA + H3O+ MAH+ + H2O
H3O+ + CH3NH2 → H2O + CH3NH3+
τ1/2 ~ 1 x 10-12 s in water at T = 298 Kτ1/2 1 x 10 s in water at T 298 K
> 10 s in amorphous ice at T < 100 K
OHOH-- HH33OO++
CH3NH3+CH3NH2
Ice HH22OOOHOH--
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A Few Examples of Chemistry of Ice Surfaces: Ionization of HCl
?HCl + H2O (ice, T = 50−140 K) → H3O+ + Cl-
H. Kang et al., J. Am. Chem. Soc. (2000)
Ionization of HCl at Ice Surfaces
RIS Spectrum
molecular HCl
no molecular HCl
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Ionization of HCl at Ice Surfaces
hydronium ion due to HCl ionization
H/D exchange:
HD2O+ + D2O ↔ D3O
+ + HDO
Low Energy Sputtering (LES)
HCl ionization
positive ion spectrum negative ion spectrum
HCl exposure = 0.5 L, T = 110K, Cs+ impact energy = 50 eV
H3O+ + NH3 ↔ H2O + NH4+
Acid-Base Reaction at Ice Surfaces
Keq = 1.7 x 109 in water at 298 K
= 1 x 1030 in gas phase
= ? on ice
ClCl--HH33OO++
NHNH33(g)(g)
NHNH44++NHNH33
HCl(g)HCl(g)
HH22OO
Ru(0001)
Ice
HH33OO
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Amines: NH3, (CH3)NH2, and (CH3)2NH
Incomplete Proton-Transfer from Hydronium Ion to Amine at the Ice Surface
ΘHCl = 0.3 L
Q << Keq : the reaction reaches a metastable state by kinetic trapping.
Thermochemical Analysis
Basicity (PA) order of amines
Gas phase: (CH3)3N (918.0 kJ/mol) > (CH3)2NH > CH3NH2 > NH3 (818.8)
Aqueous solution: (CH ) NH (61 5 kJ/mol) > CH NH > (CH ) N > NH (52 8)
Hhydration cluster model for the proton transfer
H3O+(H2O)n + B(H2O)m → H2O(H2O)n + BH+(H2O)m , G*
ice = –RT lnQ
G*ice = −(1 ~ 4) kJ mol-1
Aqueous solution: (CH3)2NH (61.5 kJ/mol) > CH3NH2 > (CH3)3N > NH3 (52.8)
Ice surface: NH3 > (CH3)NH2 ≥ (CH3)2NH (reversed order)
G ice ( ) J o
→ (n – m) = 3 ~ 5 : proton transfer from strongly hydrated hydronium ions to
less hydrated amines at the ice surface.
S-C. Park et al., Angew. Chem. Int. Ed. (2001), ChemPhysChem (2007)
15
Primary alcohol
Reaction of Alcohols with HBr in LiquidReaction of Alcohols with HBr in Liquid
Tertiary alcohol
Ts=100 K, 20 eV Cs+
(a) CH3CH
2OD (3-4 ML)
CsC2H
5DO+
Cs+(1/20)
Reaction of Ethanol with HBr on a Frozen FilmReaction of Ethanol with HBr on a Frozen Film
Cs(C2H
5DO+)
2CsH
2O+
Cs+(1/20)
Inte
nsity
Ts=100 K, 20 eV Cs+
(b) HBr(0.2 L)/CH3CH
2OD (3-4 ML)
C4H
11DO+
C2H
6DO+
Cs(C2H
5DO+)
2
CsC2H
5DO+
CsHDO+
(c) HBr(0 2 L)/CH CH OD (3-4 ML)
protonated alcohol
no ethyl bromide
Cs+(1/200)
m/z(amu/charge)
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280
Ts=100 K, 40 eV Cs+
(c) HBr(0.2 L)/CH3CH
2OD (3-4 ML)
C4H
11DO+
C2H
6DO+
C2H
5
+
H2DO+
CsHBr+
CsC2H
5DO+
CsHDO+
Cs(C2H
5DO+)
2
16
CsC4H
10O+Cs+(x1/100)
(a) (CH3)
3COH (4-5 ML),
20 eV Cs+
Ts = 100 K
Reaction of HBr with a Frozen Reaction of HBr with a Frozen tt--Butyl Alcohol FilmButyl Alcohol Film
CsH2O+
Inte
nsity
CsC4H
10O+
Cs+(x1/100)
Ts = 100 K
20 eV Cs+
C4H
11O+
C4H
9
+ CsH2O+
(b) HBr(0.3 L)/(CH3)
3COH (4-5 ML),
Ts = 100 K (c) HBr(0.3 L)/(CH
3)
3COH (4-5 ML),
protonated alcohol
carbocation
no t-butyl bromide
water intensity is increased
m/z(amu/charge)
0 20 40 60 80 100 120 140 160 180 200 220 240 260
CsC4H
10O+
40 eV Cs+
CsH2O+
Cs+(x1/100)
C3H
5
+
C2H
5
+
C4H
11O+
C4H
9
+
Cs(H2O)
2
+
Reactions on Frozen Alcohol SurfacesReactions on Frozen Alcohol Surfaces
protonatedalcohol
(yield ≥ 99.7%) (yield < 0.3%)Primary alcohol
Tertiary alcoholprotonated
alcohol(yield = 20%) (yield = 2%)
carbocation(yield = 78%)
1) Reaction intermediates can be isolated on the frozen molecular surfaces due to kinetic trapping.
2) Ionic intermediates are preferentially stabilized.
3) Reactivity is well distinguished between primary and tertiary alcohols.
Chemistry Euro. J. (2003)