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.     

Substrate and surfactant effects on the glass-liquid transition of thin water films

Temperature-programmed time-of-flight secondary ion mass spectrometry (TP-TOF-SIMS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption, desorption, and glass-liquid transition of water on the graphite and Ni(111) surfaces in the temperature range 13-200 K. Water wets the graphite surface at 100-120 K, and the hydrogen-bonded network is formed preferentially in the first monolayer to reduce the number of nonbonding hydrogens. The strongly chemisorbed water molecules at the Ni(111) surface do not form such a network and play a role in stabilizing the film morphology up to 160 K, where dewetting occurs abruptly irrespective of the film thickness. The surface structure of the water film formed on graphite is fluctuated considerably, resulting in deweting at 150-160 K depending on the film thickness. The dewetted patches of graphite are molecularly clean, whereas the chemisorbed water remains on the Ni(111) surface even after evaporation of the film. The abrupt drop in the desorption rate of water molecules at 160 K, which has been attributed to crystallization in the previous TPD studies, is found to disappear completely when a monolayer of methanol is present on the surface. This is because the morphology of supercooled liquid water is changed by the surface tension, and it is quenched by termination of the free OH groups on the surface. The surfactant methanol desorbs above 160 K since the hydrogen bonds of the water molecules are reconstructed. The drastic change in the properties of supercooled liquid water at 160 K should be ascribed to the liquid-liquid phase transition.     

Relaxation of bimolecular layer films on water surfaces

Ferric stearate, a three-tailed amphiphile, forms bimolecular layers on water surfaces. Molecules in the lower layer are in an "asymmetric" configuration, Fe-containing heads touching water and three hydrocarbon tails in air, while molecules in the upper layer are in a "symmetric" configuration, in pairs of "Y and inverted Y" disposition of tails about the Fe-bearing head. Pressure relaxation at constant area (pi- t curves) and area relaxation at constant pressure ( A- t curves) of this bimolecular layer can be modeled as a sum of three exponential decay terms with distinct time constants and weight factors. Relating the long-term decay with desorption of the total film thus indicates a remarkable long-term stability of the bimolecular layer film. An X-ray reflectivity study of the bimolecular films deposited horizontally on Si(001) at various conditions of relaxation shows no further growth along the vertical of any other layer. Under pressure relaxation molecules are transferred from the upper layer to the lower layer with a change from symmetric to asymmetric configuration, while under area relaxation the transfer is from the lower layer to the upper layer with a configurational change from symmetric to asymmetric.     

Acetone and water on TiO2(110): competition for sites

The competitive interaction between acetone and water for surface sites on TiO2(110) was examined using temperature programmed desorption (TPD). Two surface pretreatment methods were employed, one involving vacuum reduction of the surface by annealing at 850 K in ultrahigh vacuum (UHV) and another involving surface oxidation with molecular oxygen. In the former case, the surface possessed about 7% oxygen vacancy sites, and in the latter, reactive oxygen species (adatoms and molecules) were deposited on the surface as a result of oxidative filling of vacancy sites. On the 7% oxygen vacancy surface, excess water displaced all but about 20% of a saturated d6-acetone first layer to physisorbed desorption states, whereas about 40% of the first layer d6-acetone was stabilized on the oxidized surface against displacement by water through a reaction between oxygen and d6-acetone. The displacement of acetone on both surfaces is explained in terms of the relative desorption energies of each molecule on the clean surface and the role of intermolecular repulsions in shifting the respective desorption features to lower temperatures with increasing coverage. Although first layer water desorbs from TiO2(110) at slightly lower temperature (275 K) than submonolayer coverages of d6-acetone (340 K), intermolecular repulsions between d6-acetone molecules shift its leading edge for desorption to 170 K as the first layer is saturated. In contrast, the desorption leading edge for first layer water (with or without coadsorbed d6-acetone) shifted to no lower than 210 K as a function of increasing coverage. This small difference in the onsets for d6-acetone and water desorption resulted in the majority of d6-acetone being compressed into islands by water and displaced from the first layer at a lower temperature than that observed in the absence of coadsorbed water. On the oxidized surface, the species resulting from reaction of d6-acetone and oxygen was not influence by increasing water coverages. This species was stable up to 375 K (well past the first layer water TPD feature) where it decomposed mostly back to d6-acetone and atomic oxygen. These results are discussed in terms of the influence of water in inhibiting acetone photo-oxidation on TiO2 surfaces.     

Interaction of formic acid with solid water

The interaction of formic acid (HCOOH) with solid water, deposited on tungsten at 80 K, was investigated. We have prepared and annealed formic acid (FA)/water interfaces (FA layers on thin films of solid water and H(2)O adlayers on thin FA films). Metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy (He I and II) were utilized to study the electron emission from the 10a' to 6a' molecular orbitals (MOs) of FA, and the 1b(1), 3a(1), and 1b(2) MOs of H(2)O. These spectra were compared with results of density-functional theory calculations on FA-H(2)O complexes reported in Ref. 14 [A. Allouche, J. Chem. Phys. 122, 234703(2005), (preceding paper)]. Temperature programmed desorption was applied for information on the desorption kinetics. Initially, FA is adsorbed on top of the water film. The FA spectra are distorted with respect to those from FA monomers; it is concluded that a strong interaction exists between the adsorbates. Even though partial solvation of FA species takes place during annealing, FA remains in the top layer up to the desorption of the water film. When H(2)O molecules are offered to FA films at 80 K, no water network is formed during the initial stage of water exposure; H(2)O molecules interact individually via H bonds with the formic acid network. Experiment and theory agree that no water-induced deprotonation of the formic acid molecules takes place.     

The first layer of water on Rh(111): microscopic structure and desorption kinetics

The adsorption states and growth process of the first water (D2O) layer on Rh(111) were investigated using infrared reflection absorption spectroscopy, temperature programed desorption, and spot-profile-analysis low energy electron diffraction. Water molecules wet the Rh(111) surface intact. At the early stage of first layer growth, a (square root 3 x square root 3)R30 degrees commensurate water layer grows where "up" and "down" species coexist; the up and down species represent water molecules which have free OD, pointing to a vacuum and the substrate, respectively. The up domain was a flatter structure than an icelike bilayer. Water desorption from Rh(111) was a half-order process. The activation energy and the preexponential factor of desorption are estimated to be 60 kJ/mol and 4.8 x 10(16) ML(1/2)/s at submonolayer coverage, respectively. With an increase in water coverage, the flat up domain becomes a zigzag layer, like an ice bilayer. At the saturation coverage, the amount of down species is 1.3 times larger than that of the up species. In addition, the activation energy and the preexponential factor of desorption decrease to 51 kJ/mol and 1.3 x 10(14) ML(1/2)/s, respectively.     

Interaction of acetic acid with solid water

The interaction of acetic acid (AA, CH(3)COOH), with solid water, deposited on metals, tungsten and gold, at 80 K, was investigated. We have prepared acid/water interfaces at 80 K, namely, acid layers on thin films of solid water and H(2)O adlayers on thin acid films; they were annealed between 80 and 200 K. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy UPS(HeII) were utilized to obtain information on the electronic structure of the outermost surface from the study of the electron emission from the weakest bound MOs of the acids, and of the molecular water. Temperature-programmed desorption (TPD) provided information on the desorption kinetics, and Fourier-transformed infrared spectroscopy (FTIR) provided information on the identification of the adsorbed species as well as on the water and acid crystallization. The results are compatible with the finding of ref 1 (preceding paper), made on the basis of DFT calculations, that AA adsorbs on ice as cyclic dimers. Above 120 K, a rearrangement of the AA dimers is suggested by a sharpening of the spectral features in the IR spectra and by spectral changes in MIES and UPS; this is attributed to the glass transition in AA around 130 K. Above 150 K the spectra transform into those characteristic for polycrystalline polymer chains. This structure is stable up to about 180 K; desorption of water takes place from underneath the AA film, and practically all water has desorbed through the AA film before AA desorption starts. There is no indication of water-induced deprotonation of the acid molecules. For the interaction of H(2)O molecules adsorbed on amorphous AA films, the comparison of MIES with the DFT results of ref 1 shows that the initial phase of exposure does not lead to the formation of a top-adsorbed closed water film at 80 K. Rather, the H(2)O molecules become attached to or incorporated into the preexisting AA network by H bonding; no water network is formed in the initial stage of the water adsorption. Also under these conditions no deprotonation of the acid can be detected.     

Collective interactions in the mechanism of adhesion of condensed phase nuclei to a crystal surface. 1. Spatial organization

Self-organization into domains with spontaneous polarization is revealed for an ensemble of water molecules occurring in a contact layer on a defectless polarizable crystal surface. These domains are the sources for specific heteropolarization interactions of condensed phase nuclei with a substrate. The formation of a spatially nonuniform structure is energetically advantageous due to a nonlinear dependence of polarization energy on field strength. Domain sizes equal to several nanometers are governed by the competition between the energy advantage resulting from the coalescence of the domains and the entropy gain caused by their disintegration into smaller fragments. The forces of spontaneous mutual polarization between an adsorbed film divided into domains and a polarized substrate enhance the adhesion to the surface and markedly affect the adsorption mechanism. Computer simulation of the domain formation in a film of water molecules adsorbed on the surface of crystalline silver iodide particles is implemented by the Monte Carlo method with the summation of the long-range electrostatic interactions by means of the Fourier expansion of the field potential. Comparative analysis of the collective behavior of molecules, which underlies the layer-by-layer nucleation, and the indirect signs of domain formation is performed on the basis of experimental data on polarized surfaces with a hexagonal crystalline structure.     

Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. II. Diffusion limited kinetics in amorphous solid water

The adsorption, desorption, and diffusion kinetics of N2 on thick (up to approximately 9 microm) porous films of amorphous solid water (ASW) films were studied using molecular beam techniques and temperature programmed desorption. Porous ASW films were grown on Pt(111) at low temperature (<30 K) from a collimated H2O beam at glancing incident angles. In thin films (<1 microm), the desorption kinetics are well described by a model that assumes rapid and uniform N2 distribution throughout the film. In thicker films (>1 microm), N2 adsorption at 27 K results in a nonuniform distribution, where most of N2 is trapped in the outer region of the film. Redistribution of N2 can be induced by thermal annealing. The apparent activation energy for this process is approximately 7 kJ/mol, which is approximately half of the desorption activation energy at the corresponding coverage. Preadsorption of Kr preferentially adsorbs onto the highest energy binding sites, thereby preventing N2 from trapping in the outer region of the film which facilitates N2 transport deeper into the porous film. Despite the onset of limited diffusion, the adsorption kinetics are efficient, precursor mediated, and independent of film thickness. An adsorption mechanism is proposed, in which a high-coverage N2 front propagates into a pore by the rapid transport of physisorbed second layer N2 species on top of the first surface bound layer.     

Magnetic field‐assisted liquid crystal alignment on a solid substrate: drift of easy orientation axis,anchoring energy and angular distribution of adsorbed liquid crystal molecules

The anchoring properties of a film of anisotropically adsorbed liquid crystal (LC) molecules on a rigid substrate have been studied. The LC film was prepared by cooling it from the isotropic phase in the presence of a magnetic field parallel to the surface of the substrate. Relationship between the anchoring energy, easy axis direction and angular distribution of the adsorbed molecules, and changes in their angular distribution due to adsorption–desorption, were studied. The dependence of the anchoring energy on the duration and the temperature at which the LC film is annealed allowed an estimation of the activation energy of desorption of LC molecules on ITO surface, ΔE≈0.55 eV. The results suggest that hydrogen bonds are responsible for the adsorption of LC molecules on the substrate.     

Water Structure in the Contact Layer on the Surface of Crystalline Silver Iodine

The basal face of a silver iodide crystal in unsaturated water vapor is covered by a continuous molecular layer which serves as an underlying film. The structure of the film demonstrates long-range molecular order and looks like a honeycomb. Thus, macroscopic manifestations of the substrate wetting are due to the structure of the underlying film rather than the substrate crystal surface as such. A quarter of hydrogen bonds of the film molecules participate in bonding with the ions of the second crystallographic layer of the substrate. Three other quarters ensure the integrity of the film. The interactions with the ions of the first crystallographic layer are antibonding in nature. No free molecules serving as hydrogen bond donors are left on the film surface to keep vapor molecules. The shape of the free energy function associated with the adsorption of vapor molecules indicates its markedly layered nature.     

Comparative study of the interaction of pyridine with polycrystalline Ag and amorphous solid water

The interaction of pyridine (C5H5N) with polycrystalline Ag and amorphous solid water (D2O) is compared. Metastable impact electron spectroscopy (MIES) and reflection-absorption infrared spectroscopy (RAIRS) were utilized to obtain information on the structure of the pyridine-Ag and pyridine-water interfaces. On polycrystalline Ag, C5H5N adsorbs with its molecular axis perpendicular to the surface whereby a work function decrease of 1.5 eV takes place during the build up of the first layer. In the second layer the molecular axis is tilted with respect to the surface normal. On amorphous solid water, C5H5N is initially adsorbed on top with its ring plane oriented preferentially near parallel with respect to the surface, reflecting the contribution of two different interactions to the bonding, the formation of a pi-hydrogen bond, and competitive bonding via the nitrogen lone pair. Coverage-driven reorientation takes place during the completion of the first monolayer and increases the average tilt angle. We have followed the growth of pyridine films up to the third layer which, according to RAIRS, shows clear signs of condensation. No embedding of pyridine species into the underlying water film can be noticed when heating up to desorption. The exposure of a pyridine film at 124 K to D2O molecules does not lead to on top adsorption. Instead, D2O becomes initially embedded into the pyridine film, and RAIRS indicates solvation of the pyridine species.     

Melt processing of hexa-peri-hexabenzocoronene on the water surface

A discotic polycyclic aromatic hydrocarbon, hexa-peri-hexabenzocoronene, was oriented by slow cooling from the isotropic phase on a water surface as a film. For melt processing at low temperatures, an HBC derivative with long swallow-tailed alkyl side chains was chosen. The supramolecular organization in the resulting thin layer was investigated by electron microscopy. In high-resolution mode, the structural study showed large domains in which the columnar structures were oriented uniaxially with an edge-on arrangement of the hydrophobic molecules. The length of the stacks exceeded several hundred nanometers without obvious defects. The small-area analysis by TEM allowed the direct visualization of individual packed molecules. Electron diffraction revealed a high in-plane order of the columnar superstructures in which the discs were tilted by ca. 40° with respect to the stacking direction. This is the first example of a discotic system melt processed on the water surface yielding a pronounced order.     

The effect of the incident collision energy on the phase and crystallization kinetics of vapor deposited water films

Molecular beam techniques are used to grow water films on Pt(111) with incident collision energies from 5 to 205 kJ/mole. The effect of the incident collision energy on the phase of vapor deposited water films and their subsequent crystallization kinetics are studied using temperature programmed desorption and infrared spectroscopy. We find that for films deposited at substrate temperatures below 110 K, the incident kinetic energy (up to 205 kJ/mole) has no effect on the initial phase of the deposited film or its crystallization kinetics. Above 110 K, the substrate temperature does affect the phase and crystallization kinetics of the deposited films but this result is also independent of the incident collision energy. The presence of a crystalline ice template (underlayer) does affect the crystallization of amorphous solid water, but this effect is also independent of the incident beam energy. These results suggest that the crystallization of amorphous solid water requires cooperative motion of several water molecules.     

Structure and dynamics of surfaces of thin films and water microdroplets

Systems containing 3456 water molecules in a periodic rectangular cell are studied by molecular dynamics simulation. The cell parameter along the z axis noticeably exceeds parameters along the x and y axes. Thin film with a thickness of about 30 Å is formed in a cell. Some molecules are transferred into the vapor phase; however, due to the periodicity along the z axis, they are poured into periodic images of the simulated layer above or below this layer. The width of the transition surface layer is about 6–7 Å in density upon passage from the liquid to vapor phases is generally related to the roughness of the surface rather than to a decrease in a local density. The self-diffusion coefficient of molecules in the surface layer is greatly larger than inside the film. Noticeable anisotropy in the diffusion motion of molecules in the surface layer is not revealed. As all of the cell parameters increase, the film is transformed into nearly spherical micro-droplet with a strongly roughed surface. The self-diffusion coefficient of surface molecules of microdroplet is also larger than for molecules inside the droplet.     

Domain nucleation in the contact layer at an interface of water and a polarizable substrate

The growth of a molecular water film on the basic plane of a silver iodide monocrystal is studied through computer simulation. Decomposition into domains with spontaneous polarization is observed in the contact layer of the film at the interface with the substrate. The formation of domains is found to be sharply enhanced on a model substrate with the double polarizability of iodine ions; heteropolarization interactions caused by the formation of domain structures increase the film’s coupling with the substrate. It is demonstrated that the vapor pressure needed for molecular film growth is reduced appreciably via heteropolarization interactions.     

Non-Fickian water vapor sorption dynamics by Nafion membranes

Dynamics of water absorption from a saturated vapor and water desorption into dry air for Nafion 1100 EW ionomers have been measured for film thicknesses between 51 and 606 microm and at temperatures ranging from 30 to 90 degrees C. Water absorption and desorption exhibit two distinct non-Fickian characteristics: (1) desorption is 10 times faster than absorption and (2) the normalized mass change does not collapse to a single master curve when plotted against time normalized by membrane thickness squared, t/l2, for either absorption or desorption. Water desorption data were fit well by a model in which diffusion is rapid and interfacial mass transport resistance is the rate-limiting process for water loss. Water absorption is described by a two-stage process. At early times, interfacial mass transport controls water absorption, and at longer times, water absorption is controlled by the dynamics of polymer swelling and relaxation.     

Magnetic field-assisted liquid crystal alignment on a solid substrate: drift of easy orientation axis, anchoring energy and angular distribution of adsorbed liquid crystal molecules

The anchoring properties of a film of anisotropically adsorbed liquid crystal (LC) molecules on a rigid substrate have been studied. The LC film was prepared by cooling it from the isotropic phase in the presence of a magnetic field parallel to the surface of the substrate. Relationship between the anchoring energy, easy axis direction and angular distribution of the adsorbed molecules, and changes in their angular distribution due to adsorption-desorption, were studied. The dependence of the anchoring energy on the duration and the temperature at which the LC film is annealed allowed an estimation of the activation energy of desorption of LC molecules on ITO surface, ΔE≈0.55 eV. The results suggest that hydrogen bonds are responsible for the adsorption of LC molecules on the substrate.     

Hydrophobic hydration of alkanes: its implication for the property of amorphous solid water

We measured the incorporation of adsorbed alkanes in and their desorption from the amorphous solid water (ASW) by means of secondary ion mass spectroscopy and temperature programmed desorption. The heavier alkanes such as butane and hexane are incorporated completely in the bulk of the nonporous ASW layer below 100 K probably due to the preferential formation of ice structures around the solute molecules. The self-diffusion of water molecules occurs above the glass transition temperature (136 K). The liquid water emerges above 165 K, as evidenced by simultaneous occurrence of the dehydration of alkanes and the morphological change of the water layer induced by the surface tension.     

Nonsolvents cause swelling at the interface with poly(methyl methacrylate) films

Density profiles of a perdeuterated poly(methyl methacrylate) (dPMMA) film spin-coated on a substrate in water, hexane, and methanol, which are "nonsolvents" for dPMMA, were examined along the direction normal to the interface by specular neutron reflectivity (NR). The interfaces of dPMMA with the liquids were diffuse in comparison with the pristine interface with air; the interfacial width with water was thicker than that with hexane. Interestingly, in water, the dPMMA film was composed of a swollen layer and the interior region, which also contained water, in addition to the diffused layer. The interface of dPMMA with hexane was sharper than that with water. Although there were slight indications of a swollen layer for the dPMMA in hexane, the solvent molecules did not penetrate significantly into the film. On the other hand, in methanol, the whole region of the dPMMA film was strikingly swollen. To conserve mass, the swelling of the film by the nonsolvents is accompanied by an increase in the film thickness. The change in the film thickness estimated by NR was in excellent accord with the results of direct observations using atomic force microscopy (AFM). The modulus of dPMMA in the vicinity of the interfaces with liquids was also examined on the basis of force-distance curves measured by AFM. The modulus decreased closer to the outermost region of the film. The extent to which the modulus decreased in the interfacial region was consistent with the amount of liquid sorbed into the film.