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+

5

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).

7

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)

10

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--

11

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

12

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

13

HCl and NH3 Adsorbed on D2O-ice

ionized HCl

molecular NH3

NH3 - H3O+ Titration

mo

re NH

3add

ed

14

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)