Nucleation of bulk phases in the HCl/H2O system

We report experimental results on the low-temperature uptake of HCl on H(2)O ice (ice). HCl was deposited on the surface at greater than monolayer amounts at 85 K, and the ice substrate was heated. The temperature dependence of the HCl vapor pressure from this phase was measured from 110 to 150 K, with the nucleation of a bulk hydrate phase observed at 150 K. Measurements were conducted in a closed system by simultaneous application of gas phase mass spectrometry and surface spectroscopy to characterize vapor/solid equilibrium and the nucleation of bulk hydrate phases. Combining the nucleation data reported here with data we reported previously (180 to 200 K) and data from two other laboratories (165 and 170 K), the thermodynamic boundaries for the nucleation of both the metastable bulk solution and bulk hydrate phases subsequent to monolayer adsorption of HCl have been determined. The nucleation of the metastable bulk solution phase occurs promptly at monolayer coverage at the ice/liquid coexistence boundary on the binary bulk phase diagram. The nucleation of the bulk hexahydrate occurs from this metastable solution along a locus of points defining a state of constant solution free energy. This measured free energy is -51.2 +/- 0.9 kJ/mol. Finally, the temperature dependence of the HCl vapor pressure from the low-temperature phase is reported here for the first time and is consistent with that of the metastable solution predicted by this thermodynamic model of uptake, extending the range of validity of this model of adsorption followed by bulk solution and hydrate nucleation to a lower bound in temperature of 110 K.     

HCl adsorption and ionization on amorphous and crystalline H2O films below 50 K

Molecular beams were used to grow amorphous and crystalline H(2)O films and to dose HCl upon their surface. The adsorption state of HCl on the ice films was probed with infrared spectroscopy. A Zundel continuum is clearly observed for exposures up to the saturation HCl coverage on ice upon which features centered near 2530, 2120, 1760, and 1220 cm(-1) are superimposed. The band centered near 2530 cm(-1) is observed only when the HCl adlayer is in direct contact with amorphous solid water or crystalline ice films at temperatures as low as 20 K. The spectral signature of solid HCl (amorphous or crystalline) was identified only after saturation of the adsorption sites in the first layer or when HCl was deposited onto a rare gas spacer layer between the HCl and ice film. These observations strongly support conclusions from recent electron spectroscopy work that reported ionic dissociation of the first layer HCl adsorbed onto the ice surface is spontaneous.     

Probing the interaction of hydrogen chloride with low-temperature water ice surfaces using thermal and electron-stimulated desorption

The interaction and autoionization of HCl on low-temperature (80-140 K) water ice surfaces has been studied using low-energy (5-250 eV) electron-stimulated desorption (ESD) and temperature programmed desorption (TPD). There is a reduction of H(+) and H(2)(+) and a concomitant increase in H(+)(H(2)O)(n=1-7) ESD yields due to the presence of submonolayer quantities of HCl. These changes are consistent with HCl induced reduction of dangling bonds required for H(+) and H(2)(+) ESD and increased hole localization necessary for H(+)(H(2)O)(n=1-7) ESD. For low coverages, this can involve nonactivated autoionization of HCl, even at temperatures as low as 80 K; well below those typical of polar stratospheric cloud particles. The uptake and autoionization of HCl is supported by TPD studies which show that for HCl doses ≤0.5 ± 0.2 ML (ML = monolayer) at 110 K, desorption of HCl begins at 115 K and peaks at 180 K. The former is associated with adsorption of a small amount of molecular HCl and is strongly dependent on the annealing history of the ice. The latter peak at 180 K is commensurate with desorption of HCl via recombinative desorption of solvated separated ion pairs. The activation energy for second-order desorption of HCl initially in the ionized state is 43 ± 2 kJ/mol. This is close to the zero-order activation energy for ice desorption.     

Ionization and solvation of HCl adsorbed on the D2O-ice surface

The interaction of HCl with the D(2)O-ice surface has been investigated in the temperature range 15-200 K by utilizing time-of-flight secondary ion mass spectroscopy, temperature-programmed desorption, and x-ray photoelectron spectroscopy. The intensities of sputtered H(+)(D(2)O) and Cl(-) ions (the H(+) ions) are increased (decreased) markedly above 40 K due to the hydrogen bond formation between the HCl and D(2)O molecules. The HCl molecules which form ionic hydrates undergo H/D exchange at 110-140 K and a considerable fraction of them dissolves into the bulk above 140 K. The neutral hydrates of HCl should coexist as evidenced by the desorption of HCl above 170 K. They are incorporated completely in the D(2)O layer up to 140 K. The HCl molecules embedded in the thick D(2)O layer dissolve into the bulk, and the ionic hydrate tends to segregate to the surface above 150 K.     

Photoionization and photodissociation of HCl(B 1Sigma+, J=0) near 236 and 239 nm using three-dimensional ion imaging

The electronically excited states HCl(*)(E,upsilon(')=0,J(')=0) and HCl(*)(V,upsilon(')=12,J(')=0) have been prepared by two-photon resonant absorption of ground state HCl via Q(0) transitions at 238.719 and at 236.000 nm, respectively. The consequent one-or two-photon excitation at the same wavelength results in the production of H(+), Cl(+), and HCl(+) ions. The speed distributions and anisotropy parameters beta for these ions have been determined by three-dimensional photo-fragment ion imaging based on a position-sensitive delay-line anode assembly. Several results are presented: first, we measured velocity (speed and angle) distributions for HCl(+) due to the electron recoil in the photoionization of HCl(*). Such distributions give information on the photoionization process and on the vibrational distribution of HCl(+) after the laser pulse. Second, the measured beta parameters for Cl(+) and H(+) distributions give information on the symmetries of the upper states in the one-photon photoexcitation of HCl(*). Third, the measured speed distributions for H(+) help to understand the mechanism of the photodissociation of HCl(+) ions.     

A theoretical and experimental study on translational and internal energies of H2O and OH from the 157 nm irradiation of amorphous solid water at 90 K

The photodesorption of H(2)O in its vibrational ground state, and of OH radicals in their ground and first excited vibrational states, following 157 nm photoexcitation of amorphous solid water has been studied using molecular dynamics simulations and detected experimentally by resonance-enhanced multiphoton ionization techniques. There is good agreement between the simulated and measured energy distributions. In addition, signals of H(+) and OH(+) were detected in the experiments. These are inferred to originate from vibrationally excited H(2)O molecules that are ejected from the surface by two distinct mechanisms: a direct desorption mechanism and desorption induced by secondary recombination of photoproducts at the ice surface. This is the first reported experimental evidence of photodesorption of vibrationally excited H(2)O molecules from water ice.     

Interaction of methanol with amorphous solid water

The interaction of methanol (MeOH) with amorphous solid water (ASW) composed of D2O molecules, prepared at 125 K on a polycrystalline Ag substrate, was studied with metastable-impact-electron spectroscopy, reflection-absorption infrared spectroscopy, and temperature-programmed desorption mass spectroscopy. In connection with the experiments, classical molecular dynamics (MD) simulations have been performed on a single CH3OH molecule adsorbed at the ice surface (T=190 K), providing further insights into the binding and adsorption properties of the molecule at the ice surface. Consistently with the experimental deductions and previous studies, MeOH is found to adsorb with the hydroxyl group pointing toward dangling bonds of the ice surface, the CH3 group being oriented upwards, slightly tilted with respect to the surface normal. It forms the toplayer up to the onset of the simultaneous desorption of D2O and MeOH. At low coverage the adsorption is dominated by the formation of two strong hydrogen bonds as evidenced by the MD results. During the buildup of the first methanol layer on top of an ASW film the MeOH-MeOH interaction via hydrogen-bond formation becomes of importance as well. The interaction of D2O with solid methanol films and the codeposition of MeOH and D2O were also investigated experimentally; these experiments showed that D2O molecules supplied to a solid methanol film become embedded into the film.     

Adsorption, ionization, and migration of hydrogen chloride on ice films at temperatures between 100 and 140 K

Adsorption of hydrogen chloride (HCl) on water ice films is studied in the temperature range of 100-140 K by using Cs+ reactive ion scattering (Cs+ RIS), low energy sputtering (LES), and temperature-programmed-desorption mass spectrometry (TPDMS). At 100 K, HCl on ice partially dissociates to hydronium and chloride ions and the undissociated HCl exists in two distinct molecular states (alpha- and beta-states). Upon heating of the ice films, HCl molecules in the alpha-state desorb at 135-150 K, whereas those in the beta-state first become ionized and then desorb via recombinative reaction of ions at 170 K. An adsorption kinetics study reveals that HCl adsorption into the ionized state is slightly favored over adsorption into the molecular states at 100 K, leading to earlier saturation of the ionized state. Between the two molecular states, the beta-state is formed first, and the alpha-state appears only at high HCl coverage. At 140 K, ionic dissociation of HCl is completed. The resulting hydronium ion can migrate into the underlying sublayer to a depth <4 bilayers, suggesting that the migration is assisted by self-diffusion of water molecules near the surface. When HCl is covered by a water overlayer at 100 K, its ionization efficiency is enhanced, but a substantial portion of HCl remains undissociated as molecules or contact ion pairs. The observation suggests that three-dimensional surrounding by water molecules does not guarantee ionic dissociation of HCl. Complete ionization of HCl requires additional thermal energy to separate the hydronium and chloride ions.     

Segregation of hydroxide ions to an ice surface

Hydroxide ions that are initially buried within an ice film segregate to the ice film surface at elevated temperatures. This process was observed by conducting experiments with an ice film constructed with a bottom H(2)O layer and an upper D(2)O layer, with an excess of hydroxide ions trapped at the H(2)O/D(2)O interface as they were generated by Na hydrolysis. The transport of hydroxide ions from the interfacial layer to the surface was examined as a function of time using a low energy sputtering method. The progress of the H/D exchange reaction in surface water molecules was also monitored with the Cs(+) reactive ion scattering technique. At 90 K, only a small portion of buried hydroxide ions moved to the surface in the form of OD(-) species. This was due to hydroxide transport via proton hopping through a D(2)O layer, 3 BL thick, in the surface region. At 135 K, at which point water self-diffusion is active in the ice film, the majority of the buried hydroxide ions segregated to the surface after ～1 h. Both OH(-) and OD(-) species were produced at the surface, at an OH(-)/OD(-) population ratio ≥1. Based on kinetic measurements for the transport of OH(-) and OD(-) species and the H/D exchange of surface water molecules, we concluded that the major transport channel for hydroxide ions in this regime is the migration of molecular hydroxide species. H/D exchange reactions also occur between surface hydroxide ions and water molecules. No evidence was observed for the occurrence of the hop-and-turn process at 135 K, although it is known as an important mechanism of proton transport in ice.     

Cation exchange at the mineral-water interface: H3O+/K+ competition at the surface of nano-muscovite

This article describes a (39)K nuclear magnetic resonance (NMR) spectroscopic study of K (+) displacement at the muscovite/water interface as a function of aqueous phase pH. (39)K NMR spectra and T 2 relaxation data for nanocrystalline muscovite wet with a solid/solution weight ratio of 1 at pH 1, 3, and 5.5 show substantial liquid-like K (+) only at pH 1. At pH 3 and 5.5, all K (+) appears to be associated with muscovite as inner- or outer-sphere complexes, indicating that H 3O (+) does not displace basal surface K (+) beyond the (39)K detection limit under these conditions. In our pH 1 mixture, only approximately 1/3 of the initial basal surface K (+) population is located more than 3-4 A from the surface. (29)Si and (27)Al MAS NMR spectra and SEM images show no evidence of dissolution during the (39)K experiments, consistent with the liquid-like (39)K fraction originating from displaced basal surface K (+). Assuming no muscovite dissolution or interlayer exchange, the K (+)/H 3O (+) ratio relevant to the solution/surface exchange equilibrium is controlled by the total amount of K (+) on the surface and H 3O (+) in solution (K (+) surf/H 3O (+) aq). These parameters, in turn, depend on the basal surface area, solution pH, and the solid/solution ratio. The results here are consistent with significant displacement of surface K (+) only under conditions where the initial K (+) surf/H 3O (+) aq ratio is less than approximately 1. Computational molecular models of the muscovite/water interface should account for both K (+) and H 3O (+) in the near-surface region.     

A kinetic model for uptake of HNO3 and HCl on ice in a coated wall flow system

A simple model of gas flow and surface exchange with a single site Langmuir mechanism has been developed to describe effects of adsorption and desorption on trace gas concentrations at the outflow from a coated wall flow tube reactor. The model was tested by simulating experimental results for the uptake of HNO3 and HCl on ice films at temperatures and gas concentrations corresponding to the ice stability region in the upper troposphere. The experimental time-dependent uptake profiles were best fitted with an additional process involving diffusion of the adsorbed molecules into the ice film. The model allowed true surface coverages to be distinguished from total uptake including transfer to the bulk, leading to more accurate estimates of the Langmuir constant, Keq, for surface adsorption. A revised expression was obtained for the temperature dependence of the Keq=-(4.43 +/- 0.77)x 10(5)T+(10.72 +/- 1.75)x 10(7) hPa-1. Reasonable fits to the desorption profiles observed following cessation of exposure of the film to HNO3 or HCl were obtained at high surface coverage but at low coverage desorption was too slow. The analysis suggested that the ice surface was characterised by sites of different binding energy, some weakly bound sites from which the acid molecules desorbed rapidly, and some strong-binding sites which led to essentially irreversible uptake. Experiments involving competitive co-adsorption of HNO3 and HCl, conducted at relatively high equilibrium surface coverage, were well simulated by the model, as were those where the same surface was repeatedly exposed to gas phase acids.     

A calorimetric study on the low temperature dynamics of doped ice V and its reversible phase transition to hydrogen ordered ice XIII

Doped ice V samples made from solutions containing 0.01 M HCl (DCl), HF (DF), or KOH (KOD) in H(2)O (D(2)O) were slow-cooled from 250 to 77 K at 0.5 GPa. The effect of the dopant on the hydrogen disorder --> order transition and formation of hydrogen ordered ice XIII was studied by differential scanning calorimetry (DSC) with samples recovered at 77 K. DSC scans of acid-doped samples are consistent with a reversible ice XIII <--> ice V phase transition at ambient pressure, showing an endothermic peak on heating due to the hydrogen ordered ice XIII --> disordered ice V phase transition, and an exothermic peak on subsequent cooling due to the ice V --> ice XIII phase transition. The equilibrium temperature (T(o)) for the ice V <--> ice XIII phase transition is 112 K for both HCl doped H(2)O and DCl doped D(2)O. From the maximal enthalpy change of 250 J mol(-1) on the ice XIII --> ice V phase transition and T(o) of 112 K, the change in configurational entropy for the ice XIII --> ice V transition is calculated as 2.23 J mol(-1) K(-1) which is 66% of the Pauling entropy. For HCl, the most effective dopant, the influence of HCl concentration on the formation of ice XIII was determined: on decreasing the concentration of HCl from 0.01 to 0.001 M, its effectiveness is only slightly lowered. However, further HCl decrease to 0.0001 M drastically lowered its effectiveness. HF (DF) doping is less effective in inducing formation of ice XIII than HCl (DCl) doping. On heating at a rate of 5 K min(-1), kinetic unfreezing starts in pure ice V at approximately 132 K, whereas in acid doped ice XIII it starts at about 105 K due to acceleration of reorientation of water molecules. KOH doping does not lead to formation of hydrogen ordered ice XIII, a result which is consistent with our powder neutron diffraction study (C. G. Salzmann, P. G. Radaelli, A. Hallbrucker, E. Mayer, J. L. Finney, Science, 2006, 311, 1758). We further conjecture whether or not ice XIII has a stable region in the water/ice phase diagram, and on a metastable triple point where ice XIII, ice V and ice II are in equilibrium.     

Temperature dependent thermodynamic model of the system H(+)-NH₄(+)-Na(+)-SO₄²⁻-NO₃⁻-Cl⁻-H₂O

A thermodynamic model of the system H(+)-NH?(+)-Na(+)-SO?2?-NO??-Cl?-H?O is parametrized and used to represent activity coefficients, equilibrium partial pressures of H?O, HNO?, HCl, H?SO?, and NH?, and saturation with respect to 26 solid phases (NaCl(s), NaCl·2H?O(s), Na?SO?(s), Na?SO?·10H?O(s), NaNO?·Na?SO?·H?O(s), Na?H(SO?)?(s), NaHSO?(s), NaHSO?·H?O(s), NaNH?SO?·2H?O(s), NaNO?(s), NH?Cl(s), NH?NO?(s), (NH?)?SO?(s), (NH?)?H(SO?)?(s), NH?HSO?(s), (NH?)?SO?·2NH?NO?(s), (NH?)?SO?·3NH?NO?(s), H?SO?·H?O(s), H?SO?·2H?O(s), H?SO?·3H?O(s), H?SO?·4H?O(s), H?SO?·6.5H?O(s), HNO?·H?O(s), HNO?·2H?O(s), HNO?·3H?O(s), and HCl·3H?O(s)). The enthalpy of formation of the complex salts NaNH?SO?·2H?O(s) and Na?SO?·NaNO?·H?O(s) is calculated. The model is valid for temperatures < or approximately 263.15 up to 330 K and concentrations from infinite dilution to saturation with respect to the solid phases. For H?SO?-H?O solutions the degree of dissociation of the HSO?? ion is represented near the experimental uncertainty over wide temperature and concentration ranges. The parametrization of the model for the subsystems H(+)-NH?(+)-NO??-SO?2?-H?O and H(+)-NO??-SO?2?-Cl?-H?O relies on previous studies (Clegg, S. L. et al. J. Phys. Chem. A 1998, 102, 2137-2154; Carslaw, K. S. et al. J. Phys. Chem. 1995, 99, 11557-11574), which are only partly adjusted to new data. For these systems the model is applicable to temperatures below 200 K, dependent upon liquid-phase composition, and for the former system also to supersaturated solutions. Values for the model parameters are determined from literature data for the vapor pressure, osmotic coefficient, emf, degree of dissociation of HSO??, and the dissociation constant of NH? as well as measurements of calorimetric properties of aqueous solutions like enthalpy of dilution, enthalpy of solution, enthalpy of mixing, and heat capacity. The high accuracy of the model is demonstrated by comparisons with experimentally determined mean activity coefficients of HCl in HCl-Na?SO?-H?O solutions, solubility measurements for the quaternary systems H(+)-Na(+)-Cl?-SO?2?-H?O, Na(+)-NH?(+)-Cl?-SO?2?-H?O, and Na(+)-NH?(+)-NO??-SO?2?-H?O as well as vapor pressure measurements of HNO?, HCl, H?SO?, and NH?.     

Water molecule adsorption on short alanine peptides: how short is the shortest gas-phase alanine-based helix?

Water adsorption measurements have been performed under equilibrium conditions for unsolvated Ac-A(n)K+H(+) and Ac-KA(n)+H(+) peptides with n = 4 - 10. Previous work on larger alanine peptides has shown that two dominant conformations (helices and globules) are present for these peptides and that water adsorbs much more strongly to the globules than to the helices. All the Ac-KA(n)+H(+) peptides studied here (which are expected to be globular) adsorb water strongly, and so do the Ac-A(n)K+H(+) peptides with n < 8. However, for Ac-A(n)K+H(+) with n = 8-10 there is a substantial drop in the propensity to adsorb water. This result suggests that Ac-A(8)K+H(+) is the smallest Ac-A(n)K+H(+) peptide to have a significant helical content in the gas phase. Water adsorption measurements for Ac-V(n)K+H(+) and Ac-L(n)K+H(+) with n = 5-10 suggest that the helix emerges at n = 8 for these peptides as well.     

Secondary ion emission induced by fission fragment impact in CO-NH(3) and CO-NH(3)-H(2)O ices: modification in the CO-NH(3) ice structure

CO-NH(3) and CO-NH(3)-H(2)O ices at 25-130 K were bombarded by (252)Cf fission fragments ( approximately 65 MeV at the target surface) and the emitted secondary ions were analyzed by time-of-flight mass spectrometry (TOF-SIMS). It is observed that the mass spectra obtained from both ices have similar patterns. The production of hybrid ions (formed from CO and NH(3) molecules) emitted from CO-NH(3) ice has already been reported by R. Martinez et al., Int. J. Mass. Spectrom. 262 (2006) 195; here, the secondary ion emission and the modifications of the CO--NH(3) ice structure during the temperature increase of the ice are addressed. These studies are expected to throw light on the sputtering from planetary and interstellar ices and the possible formation of new organic molecules in CO-NH(3)-H(2)O ice by megaelectronvolt ion bombardment. The presence of water in the CO-NH(3) ice mixture generates molecular ion series such as (NH(3))(p-q)(H(2)O)(q)CO(+) and replaces the cluster series (NH(3))(n)NH(4) (+) emission by the hybrid series (NH(3))(I-i)(H(2)O)(i=1, 2...I)H(+). The distribution of NH(3) and H(2)O molecules within the cluster groups indicates that ammonia and water mix homogeneously in the icy condensate at T = 25 K. The desorption yield distribution of the cluster series (NH(3))(n)NH(4) (+) is described by the sum of two exponential functions: one, slow-decreasing, attributed to the fragmentation of the solid target into clusters; and another, fast-decreasing, due to a local sublimation followed by recombination of ammonia molecules. The analysis of the time-temperature dependence of these two yield components gives information on the formation process of molecular ions, the transient composition of the ice target and structural changes of the ice. Data suggest that the amorphous and porous structure of the NH(3) ice, formed by the condensation of the CO--NH(3) gas at T = 25 K, survives CO sublimation until the occurrence of a phase transition around 80 K, which produces a more fragile ice structure.     

Surface abundance change in vacuum ultraviolet photodissociation of CO2 and H2O mixture ices

Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O((3)P(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO(2). Two different adsorption states of CO(2), i.e., physisorbed CO(2) on the surface of amorphous solid water and trapped CO(2) in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H(2)O. Since the absorption cross section of CO(2) is smaller than that of H(2)O at 157 nm, the CO(2) surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.     

Kinetics of ClONO(2) reactive uptake on ice surfaces at temperatures of the upper troposphere

The reactive uptake kinetics of ClONO(2) on pure and doped water-ice surfaces have been studied using a coated wall flow tube reactor coupled to an electron impact mass spectrometer. Experiments have been conducted on frozen film ice surfaces in the temperature range 208-228 K with P((ClONO)(2)) < or = 10(-6) Torr. The uptake coefficient (gamma) of ClONO(2) on pure ice was time dependent with a maximum value of gamma(max) approximately 0.1. On HNO(3)-doped ice at 218 K the gamma(max) was 0.02. HOCl formation was detected in both experiments. On HCl-doped ice, uptake was gas-phase diffusion limited (gamma > 0.1) and gas-phase Cl(2) was formed. The uptake of HCl on ice continuously doped with HNO(3) was reversible such that there was no net uptake of HCl once the equilibrium surface coverage was established. The data were well described by a single site 2-species competitive Langmuir adsorption isotherm. The surface coverage of HCl on HNO(3)-doped ice was an order of magnitude lower than on bare ice for a given temperature and P(HCl). ClONO(2) uptake on this HCl/HNO(3)-doped ice was studied as a function of P(HCl). gamma(max) was no longer gas-phase diffusion limited and was found to be linearly dependent on the surface concentration of HCl. Under conditions of low HCl surface concentration, hydrolysis of ClONO(2) and reaction with HCl were competing such that both Cl(2) and HOCl were formed. A numerical model was used to simulate the experimental results and to aid in the parametrization of ClONO(2) reactivity on cirrus ice clouds in the upper troposphere.     

Mechanistic study of proton transfer and HD exchange in ice films at low temperatures (100-140 K)

We have examined the elementary molecular processes responsible for proton transfer and HD exchange in thin ice films for the temperature range of 100-140 K. The ice films are made to have a structure of a bottom D(2)O layer and an upper H(2)O layer, with excess protons generated from HCl ionization trapped at the D(2)OH(2)O interface. The transport behavior of excess protons from the interfacial layer to the ice film surface and the progress of the HD exchange reaction in water molecules are examined with the techniques of low energy sputtering and Cs(+) reactive ion scattering. Three major processes are identified: the proton hopping relay, the hop-and-turn process, and molecular diffusion. The proton hopping relay can occur even at low temperatures (<120 K), and it transports a specific portion of embedded protons to the surface. The hop-and-turn mechanism, which involves the coupling of proton hopping and molecule reorientation, increases the proton transfer rate and causes the HD exchange of water molecules. The hop-and-turn mechanism is activated at temperatures above 125 K in the surface region. Diffusional mixing of H(2)O and D(2)O molecules additionally contributes to the HD exchange reaction at temperatures above 130 K. The hop-and-turn and molecular diffusion processes are activated at higher temperatures in the deeper region of ice films. The relative speeds of these processes are in the following order: hopping relay>hop and turn>molecule diffusion.     

Competitive Adsorption of Ferricyanide and Ferrocyanide on gamma-Al(2)O(3) Surface

In the past 3 decades, research has proven the significance of competitive adsorption in the equilibrium of pollutants between solid and liquid phases. However, studies on the competitive adsorption of complex ions are very limited in spite of its important role in transporting pollutants in the natural environment. The objective of this study is to derive the thermodynamic parameters of the competitive adsorption between ferricyanide and ferrocyanide from the modified Langmuir isotherm and the triple-layer model (TLM) to determine the location of adsorption. The effects of pH, temperature, and ion concentration on competitive adsorption onto gamma-Al(2)O(3) were investigated. The results demonstrate that ferrocyanide is more competitive than ferricyanide. By comparing the derived K(app) with K(int), we inferred that the adsorption of ferricyanide and ferrocyanide onto gamma-Al(2)O(3) was achieved through outer-sphere complexation. The negative DeltaH degrees indicated that the adsorption was exothermic. The positive entropy (Delta S degrees ) was caused by the replacement and release of a greater number of smaller surface ions by adsorbed ferricyanide and ferrocyanide ions of larger size. Copyright 2000 Academic Press.     

Electron attachment in ice-HCl clusters: an ab initio study

Experimental work has shown that small amounts of HCl strongly enhance electron capture in ice films. The purpose of the present study was to investigate the effect of adsorbed HCl on the interaction of electrons with small clusters of water. Studies were made with clusters of 6 and 12 water molecules with various geometries both with and without one HCl attached. A number of distinct HCl coordination motifs were examined. All of the neutral structures with HCl exhibited zero thresholds for electron attachment and formed dipole bound anionic states (DBS). The relaxation processes for these "initial DBS" depended on the number of H(2)O (n) and on the number and type of H-bonds to the HCl (x). The initial DBS of systems with only O-H...Cl H-binding underwent dissociative electron attachment (DEA), forming H atoms. Relaxation for systems with ClH...OH(2) bonds was more complex. For the two layer n = 12 systems with x = 2 or 3 the HCl proton moved to the nearest oxygen to form H(3)O(+). Then rearrangement of the proton network occurred, and the Cl(-) became solvated by three HO-H...Cl(-) bonds. The presence of Cl(-) and H(3)O(+) increases the dipole moment and the electron binding energy (EBE) of the network. Further stabilization is achieved by decay into deeper DBS electron traps and/or by reaction of the excess electron with H(3)O(+) to form H(*) atoms. The HCl(H(2)O)(6) clusters with a single Cl-H...OH(2) bond behaved differently. They increased their stability by becoming more linear. This raised the dipole moment and the EBE therefore increased, reducing the total energy. None of these species showed any signs of increasing the number of H-bonds to Cl. The implication of these observations for the interpretation of the results of the experiments with 0.2 monolayer of HCl on 5 monolayer of H(2)O at 20 K, and on the possible role of cosmic ray-induced ionization in polar stratospheric clouds in ozone depletion is discussed.