Investigating the effects of solid surfaces on ice nucleation

Understanding the role played by solid surfaces in ice nucleation is a significant step toward designing anti-icing surfaces. However, the uncontrollable impurities in water and surface heterogeneities remain a great challenge for elucidating the effects of surfaces on ice nucleation. Via a designed process of evaporation, condensation, and subsequent ice formation in a closed cell, we investigate the ice nucleation of ensembles of condensed water microdroplets on flat, solid surfaces with completely different wettabilities. The water microdroplets formed on flat, solid surfaces by an evaporation and condensation process exclude the uncontrollable impurities in water, and the effects of surface heterogeneities can be minimized through studying the freezing of ensembles of separate and independent water microdroplets. It is found that the normalized surface ice nucleation rate on a hydrophilic surface is about 1 order of magnitude lower than that on a hydrophobic surface. This is ascribed to the difference in the viscosity of interfacial water and the surface roughness.     

Integrated band intensities of 1,1,1-trichlorotrifluoroethane,CFC113a,and 1,1,2-trichlorotrifluoroethane,CFC113

The integrated band intensities of 1,1,2-trichlorotrifluoroethane, CFC113, and 1,1,1-trichlorotrifluoroethane, CFC113a, have been measured, the latter for the first time. The former are in reasonable agreement with the reported results of Varanasi and Chudamani [J. Geophys. Res. 93, 1666 (1988)]. The total integrated intensities in the range 1300 to 700 cm⁻¹ are, respectively, 3218 and 3138 atm⁻¹ cm⁻². Attention is drawn to possible significant sources of error deriving from choices of band shape.     

A review of CFC and halon treatment technologies – The nature and role of catalysts

The purpose of this review article is to provide readers with an account of CFC and halon treatment technologies as depicted in the patent and open literature. Destruction technologies, in which halons and CFCs are converted into species such as CO₂, HX or X₂ (X = Br, Cl, F), are treated less extensively. Emphasis has been placed on conversion processes, which aim at transforming (rather than destroying) CFC or halon into environmentally benign and useful products. It has been found that catalytic hydrodehalogenation over transition metal based catalysts, Pd in particular, has great potential for conversion of CFCs and halons to hydrofluorocarbons. In this regard, the focus of this review is on catalytic hydrodehalogenation, including an assessment of reaction mechanisms, catalytic activity, selectivity and durability.     

Dynamics of ice nucleation on water repellent surfaces

Prevention of ice accretion and adhesion on surfaces is relevant to many applications, leading to improved operation safety, increased energy efficiency, and cost reduction. Development of passive nonicing coatings is highly desirable, since current antiicing strategies are energy and cost intensive. Superhydrophobicity has been proposed as a lead passive nonicing strategy, yet the exact mechanism of delayed icing on these surfaces is not clearly understood. In this work, we present an in-depth analysis of ice formation dynamics upon water droplet impact on surfaces with different wettabilities. We experimentally demonstrate that ice nucleation under low-humidity conditions can be delayed through control of surface chemistry and texture. Combining infrared (IR) thermometry and high-speed photography, we observe that the reduction of water-surface contact area on superhydrophobic surfaces plays a dual role in delaying nucleation: first by reducing heat transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. This work also includes an analysis (based on classical nucleation theory) to estimate various homogeneous and heterogeneous nucleation rates in icing situations. The key finding is that ice nucleation delay on superhydrophobic surfaces is more prominent at moderate degrees of supercooling, while closer to the homogeneous nucleation temperature, bulk and air-water interface nucleation effects become equally important. The study presented here offers a comprehensive perspective on the efficacy of textured surfaces for nonicing applications.     

The role of the ice rules in the thermal properties of potassium dihydrogen phosphate

Calorimetric properties of potassium dihydrogen phosphate are examined by analysis of the heat capacity data taken from the literature and from a recent measurement. The analysis is based on an extensive use of harmonic heat capacity functions to separate the effect of the phase transition from the vibrational contribution. The transition enthalpy and entropy derived are 421 J mol^–1 and 3.79 J K^–1 mol^–1, respectively. Characteristic temperatures of the lattice vibrations including the Debye temperature (254±18) K were determined. The transition entropy, exceeding the value compatible with the ice-rules, is consistent with the temperature dependence of the heat capacity. The implication of the result is discussed by comparison with the hydrogen bond networks in copper formate tetrahydrate and thallium dihydrogen phosphate.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthdayContribution No. 113 from Microcalorimetry Research Center.     

Comparative molecular dynamics study of vapor-exposed basal, prismatic, and pyramidal surfaces of ice

We present the results of molecular dynamics simulations in which ice I(h) slabs with free basal, prismatic, 28° pyramidal, and 14° pyramidal facets are exposed to vapor. All simulations were carried out at 250 K using a six-site intermolecular potential. Characteristics common to all facets include spontaneous development of a quasi-liquid layer (QLL) within ～10 ns and QLL stratification into outer (ε(1)) and inner (ε(2)) sublayers having on average two and three hydrogen bonds, respectively. Vapor pressure, based on the rate of escape of molecules from the QLL to the vapor phase, is found to be greatest for the 14° pyramidal and basal facets (～230 Pa), while significantly lower values are obtained for the prismatic and 28° pyramidal facets (～200 Pa). The geometric thickness of the QLL also varies between facets, with the 14° pyramidal having the greatest thickness. The free prismatic and pyramidal facets exhibit significant anisotropic diffusivity, in-plane motion being faster in the trans-prismatic direction than in the basal-to-basal direction. The in-plane diffusion length is greatest for the 28° pyramidal facet and smallest for the prismatic facet. This diversity of facet-specific properties provides a rich set of possibilities for mechanisms of ice crystal growth and ablation.     

The acidity functions of trifluoroethanol and hexafluoroisopropanol,and their mixtures with water

Acidity measurements in trifluoroethanol and hexafluoroisopropanol as solvents, and in their mixtures with water, are reported. The hydrogen electrode and the glass electrode were used for pH measurements. The “experimental” autoprotolysis constants were determined from measurements in strongly acidic (trifluoromethanesulfonic) media and in strongly basic (alcoholate) media: for trifluoroethanol, pK = -log K/mol² l^-2 = 15 and for hexafluoroisopropanol, pK = 14.8. Evaluation of the pH-indicator potential systems with reference to the ferrocene/ferricinium couple gives the acidity function R₀(H). The values obtained are compared to the H₀ values evaluated in the same conditions.     

The role of phosphate in bacterial interaction with iron-coated surfaces

This study investigated the role of phosphate in the adhesion of bacteria (Staphylococcus aureus ATCC 10537) to iron-coated surfaces. Column experiments were performed at phosphate concentrations ranging from 0.0 to 2.0 mM. Bacterial breakthrough curves were obtained by monitoring effluent, and mass recovery and sticking efficiency were quantified from these curves. At phosphate concentrations between 0 and 0.5 mM, bacterial attachment to iron-coated sand decreased with increasing phosphate concentration (mass recovery increased from 14.0 to 86.3%), possibly due to charge modification of the coated sand from positive to negative by adsorbed phosphate ions. Between 0.5 and 2.0 mM, however, bacterial attachment increased with increasing phosphate concentration (mass recovery decreased from 86.3 to 41.3%), possibly due to compression of the electrical double layer between bacteria and phosphate-adsorbed/negatively charged surfaces by free phosphate ions. This study demonstrates that phosphate can play different roles in bacterial interaction with iron-coated surfaces depending on its concentration.     

Evaporation loss of dissolved volatile substances from ice surfaces

Volatile acidic solutes were used to make dilute solutions, which were frozen by various methods. The concentration of solutes and the pH of the samples were measured before and after being frozen. When the sample solution is frozen from the bottom to the top, solutes are concentrated into the unfrozen solution (i.e., the upper part of the sample) due to the freeze concentration effect. Thereafter, concentrated anions combine with protons to form acids, and the amount of acids in the unfrozen solution increase as the ice formation progresses. At the end of freezing, the acid is saturated at the ice surface, and if the formed acid is volatile, then evaporation occurs. Frozen solutions were allowed to stand below 0 degrees C, where evaporation rates were obtained in the following order: formate > acetate > propionate > n-butyrate > chloride > nitrate. Except for nitrate, evaporation rates were enough to take place in frozen water of the natural environment (e.g., ice crystal, graupel, snow crystal, and frozen droplets). The relationship between the evaporation rate of volatile acids and their physical properties demonstrate that the evaporation rates of weak acids are faster than those of strong acids, and the evaporation rates among weak acids are the same as the volatility of weak acids.     

Sticking of CO to crystalline and amorphous ice surfaces

We present results of classical trajectory calculations on the sticking of hyperthermal CO to the basal plane (0001) face of crystalline ice Ih and to the surface of amorphous ice Ia. The calculations were performed for normal incidence at a surface temperature Ts = 90 K for ice Ia, and at Ts = 90 and 150 K for ice Ih. For both surfaces, the sticking probability can be fitted to a simple exponentially decaying function of the incidence energy, Ei: Ps = 1.0e(-Ei(kJ/mol)/90(kJ/mol)) at Ts = 90 K. The energy transfer from the impinging molecule to the crystalline and the amorphous surface is found to be quite efficient, in agreement with the results of molecular beam experiments on the scattering of the similar molecule, N2, from crystalline and amorphous ice. However, the energy transfer is less efficient for amorphous than for crystalline ice. Our calculations predict that the sticking probability decreases with Ts for CO scattering from crystalline ice, as the energy transfer from the impinging molecule to the warmer surfaces becomes less efficient. At high Ei (up to 193 kJ/mol), no surface penetration occurs in the case of crystalline ice. However, for CO colliding with the amorphous surface, a penetrating trajectory was observed to occur into a large water pore. The molecular dynamics calculations predict that the average potential energy of CO adsorbed to ice Ih is -10.1 +/- 0.2 and -8.4 +/- 0.2 kJ/mol for CO adsorbed to ice Ia. These values are in agreement with previous experimental and theoretical data. The distribution of the potential energy of CO adsorbed to ice Ia was found to be wider (with a standard deviation sigma of 2.4 kJ/mol) than that of CO interacting with ice Ih (sigma = 2.0 kJ/mol). In collisions with ice Ia, the CO molecules scatter at larger angles and over a wider distribution of angles than in collisions with ice Ih.     

Viscosity of nonfreezing thin interlayers between the surfaces of ice and quartz

From the shift rate of the ice column in thin quartz cylindrical capillaries the value of h/η has been measured at different temperatures below 0°C. The viscosity values of nonfreezing water and KCl solution interlayer η are calculated on the basis of the known values of the interlayer thickness h. For the electrolyte solution, the h values are much higher than those for pure water.     

CFC and Halon replacements in the environment

Substitute fluorocarbons may have direct environmental impact, for example as greenhouse gases, or indirect impacts through the products of their decomposition in the environment. The mechanisms of that atmospheric decomposition are reviewed here and shown to be well established now. The end products are halogen acids and trifluoroacetic acid, all of which pre-exist in the environment in quantities greater than are expected to arise from fluorocarbon use and emissions. Furthermore, the growth in use of fluorocarbon replacements has been shown to be far less than the fall in CFC and Halon production. Hydrochlorofluorocarbons (HCFCs) have replaced less than one third of CFCs and are, themselves, ozone depleting substances that will be phased out under the Montreal Protocol. The growth in hydrofluorocarbons (HFCs) amounts to about 10% of the fall in CFCs. It is likely that the impact of new fluorocarbons on climate change will be a very small fraction of the total impact, which comes mainly from the accumulation of carbon dioxide in the atmosphere.     

Photolysis of polycyclic aromatic hydrocarbons on water and ice surfaces

Laser-induced fluorescence detection was used to measure photolysis rates of anthracene and naphthalene at the air-ice interface, and the kinetics were compared to those observed in water solution and at the air-water interface. Direct photolysis proceeds much more quickly at the air-ice interface than at the air-water interface, whereas indirect photolysis due to the presence of nitrate or hydrogen peroxide appears to be suppressed at the ice surface with respect to the liquid water surface. Both naphthalene and anthracene self-associate readily on the ice surface, but not on the water surface. The increase in photolysis rates observed on ice surfaces is not due to this self-association, however. The wavelength dependence of the photolysis indicates that it is due to absorption by the PAH. No dependence of the rate on temperature is seen, either at the liquid water surface or at the ice surface. Molecular oxygen appears to play a complex role in the photolytic loss mechanism, increasing or decreasing the photolysis rate depending on its concentration.     

The role of polymer compatibility in the adhesion between surfaces saturated with modified dextrans

Wet and dry adhesion between dextran-coated surfaces were measured aiming to understand the influence of polymer compatibility. The wet adhesion measurements were performed using the atomic force microscope (AFM) colloidal probe technique whereas the dry adhesion measurements were performed using the micro adhesion measurement apparatus (MAMA). Two types of dextrans were used, one cationically modified dextran (DEX) and one that was both cationically and hydrophobically modified (HDEX), leading to three different combinations of polymer-coated surfaces; (1) DEX:DEX, (2) HDEX:DEX, and (3) HDEX:HDEX. DEX increased dry adhesion more than HDEX did, which likely is due to differences in the ability to form specific interactions, especially hydrogen bonding. HDEX gave strong wet adhesion, probably due to its poorer solvency, while DEX contributed to reducing the wet adhesion due to its hydrophilicity. All combinations showed a steric repulsion on approach in aqueous media. Furthermore, when HDEX was adsorbed on either or both surfaces a long range attractive force between the surfaces was detected outside this steric regime.     

UV-induced protonation of molecules adsorbed on ice surfaces at low temperature

UV irradiation of ice films adsorbed with methylamine molecules induces protonation of the adsorbate molecules at low temperature (50-130 K). The observation indicates that long-lived protonic defects are created in the ice film by UV light, and they transfer protons to the adsorbate molecules via tunneling mechanism at low temperature. The methylammonium ion formed by proton transfer remains to be stable at the ice surface. It is suggested that this solid-phase protonation might play a significant role in the production of molecular ions in interstellar clouds.     

The role of molecular rotation in activated dissociative adsorption on metal surfaces

The role of molecular rotation in dissociative adsorption of H2 on the activated NiAl(110) metal surface is systematically investigated by means of classical dynamics calculations performed on ab initio six-dimensional potential energy surfaces. The calculations show that molecules rotate abruptly when they are close to the surface and that this rotation allows the molecules to adopt the orientation that is more convenient for dissociation (i.e., nearly parallel to the surface). Also, in reactive sectors of the NiAl(110) unit cell, there is an "angular threshold" below which molecules cannot dissociate. This angular threshold goes down as the incidence energy increases, which explains the rise of the dissociation probability and the fact that it reaches a value close to 1 at incidence energies of the order of 2 eV. The fact that switching on molecular rotation favors dissociation establishes a competition between dissociation and rotational excitation of reflected molecules above the dissociation threshold. Measurements on rotational excitation might thus bring indirect evidence on the dissociation dynamics. Sample calculations for nonactivated Pd(111) and activated Cu(110) metal surfaces suggest that some of these conclusions may be of general validity.