Kinetics of dissociative congruent evaporation based on the transition state theory.

Non-transition metal oxides, including major minerals of the early Solar System, are known to evaporate decomposing into multiple gas molecules, while maintaining their stoichiometric compositions (dissociative congruent evaporation). Here, we derived the absolute rate of this type of evaporation using the transition state theory. In our modified transition state theory, the activation energy closely corresponds to the average energy of the molecules at the transition state, reflecting the degree of decomposition at the potential energy barrier along the reaction coordinate of evaporation. By comparing the theoretical and experimental evaporation rates for the reaction MgO (s) → Mg (g) + O (g), we found that there is an activation barrier close to the product side (i.e., "late" barrier) where the decomposition is almost achieved. The present theory is advantageous to the Hertz-Knudsen equation, which is essentially formulated as the evaporation rate in equilibrium based on the detailed balance, in that it describes dissociative congruent evaporation as a non-equilibrium process and thus provides the link between the experiments and the reaction dynamics.

Controlling the formation of ionic complex vesicles through double-tailed surfactants.

OBJECTIVE: Liposomes are often used in cosmetics since they are naturally derived and have excellent texture enhancing capabilities. However, when preparing them by using phospholipids with unsaturated acyl groups, they easily suffer from oxidative degradation. Accordingly, hydrogenated phospholipids are preferred, however, it is difficult to prepare stable liposomes due to its high gel-liquid crystalline phase transition temperature. On the other hand, although dialkyl dimethyl ammonium type cationic surfactants are widely known to form vesicles, they have rarely been used for skincare products except for water-in-oil type emulsion creams stabilized by organically modified clay minerals. We decided to overcome all of the problems above through ionic complex vesicles formed by double-tailed cationic and anionic surfactants. METHODS: Distearyl dimethyl ammonium chloride (DSAC) and sodium dilauramidoglutamide lysine (DLGL) were selected as cationic and anionic surfactants, respectively. Differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SWAXS) measurements were performed to confirm the DSAC/DLGL/water ternary phase diagram. Newly developed ionic complex vesicle formation was confirmed by cryogenic transmission electron microscopy (cryo-TEM). The adsorbed cosmetic film structure on the skin in vivo was evaluated through the polarized infrared external reflection (PIR-ER). Finally, a cosmetic lotion formula was developed and the vesicle size was determined by dynamic light scattering (DLS). RESULTS: DSC and SWAXS data indicated that stable vesicles could be obtained at a molar ratio of DLGL to DSAC = 6:4. At this molar ratio, multi lamellar vesicles with diameters less than 100 nm were observed through cryo-TEM. PIR-ER data revealed that the developed vesicles formed a highly perpendicular orientation to the human skin surface. We have succeeded in formulating a cosmetic lotion containing developed vesicles with a mean diameter of 63.2 nm, which was stable over 1 month at 0, 37, and 50°C. CONCLUSIONS: Our newly developed vesicles can be easily obtained through a coagulation process. Also, the adsorbed film structure supported by PIR-ER experiments implies that the developed lotion has an excellent texture that is the same as cosmetic lotions containing liposomes. Therefore, it's possible that this ionic complex vesicle could take the place of liposomes.

OBJECTIF: Les liposomes sont souvent utilisés dans les cosmétiques, car ils sont d'origine naturelle et ont d'excellentes capacités d'amélioration de la texture. Cependant, lorsqu'ils sont préparés en utilisant des phospholipides avec des groupes acyles insaturés, ils souffrent facilement de dégradation oxydative. Par conséquent, les phospholipides hydrogénés sont préférés, mais il est difficile de préparer des liposomes stables en raison de la température élevée pour la transition de gel à liquide cristal. D'autre part, bien que les tensioactifs cationiques de type dialkyle diméthylammonium soient largement connus pour former des vésicules, ils ont rarement été utilisés pour les produits de soins de la peau, à l'exception des crèmes émulsifiées de type eau dans l'huile stabilisées par des minéraux d'argile organiquement modifiés. Nous avons décidé de surmonter tous les problèmes susmentionnés grâce à des vésicules complexes ioniques formées par des tensioactifs cationiques et anioniques à double queue. MÉTHODES: le chlorure de distéaryl­diméthyl­ammonium (DSAC) et la lysine de dilauramidoglutamide (DLGL) ont été sélectionnés comme agents de surface cationiques et anioniques, respectivement. Des mesures de calorimétrie différentielle à balayage (DSC) et de diffraction des rayons X à petit angle et à grand angle (SWAXS) ont été effectuées pour confirmer le diagramme de phase ternaire en DSAC/DLGL/eau. La formation de vésicules complexes ioniques nouvellement développées a été confirmée par microscopie électronique à transmission cryogénique (cryo­TEM). La structure du film cosmétique adsorbé sur la peau in vivo a été évaluée par réflexion externe infrarouge polarisée (PIR­ER). Enfin, une formule de lotion cosmétique a été développée et la taille de la vésicule a été déterminée par diffraction dynamique de la lumière (DLS). RÉSULTATS: les données DSC et SWAXS ont indiqué que des vésicules stables pouvaient être obtenues à un rapport molaire de DLGL sur DSAC = 6:4. À ce rapport molaire, des vésicules pluri­lamellaires de diamètre inférieur à 100 nm ont été observées par cryo­TEM. Les données PIR­ER ont révélé que les vésicules développées forment une orientation très perpendiculaire à la surface de la peau humaine. Nous avons réussi à formuler une lotion cosmétique contenant des vésicules développées d'un diamètre moyen de 63,2 nm, qui était stable pendant un mois à 0°C, 37°C et 50°C. CONCLUSIONS: Nos vésicules nouvellement développées peuvent être facilement obtenues grâce à un processus de coagulation. De plus, la structure du film adsorbé soutenue par les expériences PIR­ER implique que la lotion développée a une excellente texture qui est la même que celles des lotions cosmétiques contenant des liposomes. Il est donc possible que cette vésicule complexe ionique puisse remplacer les liposomes.

Structure of crystalline water ice formed through neon matrix sublimation under cryogenic and vacuum conditions.

Ice I has three forms depending on the stacking arrangements of its layers: hexagonal ice Ih, cubic ice Ic, and stacking disordered ice Isd. Below ∼60 K, amorphous water becomes metastable, and the formation of any form of ice I is often implicitly precluded. Using a newly developed low-temperature reflection high-energy electron diffraction (RHEED) technique, we show that crystalline ice with cubic stacking sequences (i.e., ice Ic) formed through Ne sublimation from a solid H2O/Ne (1:1000 ratio) matrix at 13 K. The extent of stacking disorder (disordered cubic and hexagonal stacking sequences) in the ice formed by Ne matrix sublimation is smaller than that in vapor-deposited ice Isd prepared at 143 K and below the limit of detection of low-temperature RHEED. Dependence of the resulting ice structures on the thickness of the H2O/Ne matrix shows that amorphous water first forms in the early stages of Ne sublimation, and the cubic stacking sequence subsequently takes place. As the cubic ice Ic formed here at a much lower temperature (13 K) than previously observed (typically above 78 K), Ne matrix sublimation represents a novel route to the formation of cubic ice Ic under low-temperature and low-pressure conditions.

Promotion of glyoxylic acid penetration into human hair by glycolic acid.

OBJECTIVE: Glyoxylic acid (GA) is widely used as a straight perming agent for hair care products, however, advanced GA penetration-enhancing agents are desired due to the peculiar odour and hair colour fading caused by the continuous use of GA products. Hence, it is important to develop a penetration-enhancing agent that helps minimize the GA concentration. We have found that the combined use of GA and glycolic acid (GCA) has a strong hair straightening effect. METHODS: Straightening hair test was carried out to the evaluation of the effect of additives. Liquid chromatography-mass spectrometry (LC/MS) was performed to quantify the GA penetration amount into human hair. Attenuated total reflection (ATR) Fourier transform-infrared spectroscopy (FT-IR) and FT-IR microscope were implemented to estimate the localization of GA in the hair. RESULTS: Straightening hair tests indicated that the hair straightening effect by GA was enhanced by the presence of GCA. LC/MS results showed that the addition of GCA enhanced the amount of GA that penetrated human hair by about four times. ATR FT-IR and FT-IR microscope measurements indicated that GA was localized more in the innermost region of hair (medulla) than the cortex and cuticle. The GA accumulated in the medulla disappeared after a hair straightener treatment at 180°C due to the chemical reaction. CONCLUSIONS: The GA penetration-enhancing effect of GCA is worth investigating to reduce the GA concentration in products for more comfortable use.

OBJECTIF: L'acide glyoxylique (AG) est largement utilisé en tant qu'agent de lissage pour les produits de soins capillaires. Cependant, des agents avancés améliorant la pénétration de l'AG sont souhaités en raison de l'odeur particulière et de la décoloration des cheveux causées par l'utilisation continue de produits à base d'AG. Il est donc important de mettre au point un agent améliorant la pénétration qui contribue à minimiser la concentration d'AG. Nous avons constaté que l'utilisation combinée de l'AG et de l'acide glycolique (AGC) a un fort effet lissant sur les cheveux. MÉTHODES: Un test de lissage des cheveux a été effectué pour évaluer l'effet des additifs. Une chromatographie en phase liquide avec spectrométrie de masse (liquid chromatography-mass spectrometry, LC/MS) a été réalisée pour quantifier le volume de pénétration de l'AG dans les cheveux humains. Une spectroscopie infrarouge à transformée de Fourier (Fourier transform-infrared spectroscopy, FT-IR) à réflexion totale atténuée (RTA) et un microscope FT-IR ont été adoptés pour estimer la localisation de l'AG dans les cheveux. RÉSULTATS: Les tests de lissage des cheveux ont indiqué que l'effet de lissage des cheveux de l'AG était renforcé par la présence d'AGC. Les résultats de la LC/MS ont montré que l'ajout d'AGC augmentait d'environ quatre fois la quantité d'AG pénétrant dans les cheveux humains. Les mesures de la FT-IR à RTA et du microscope FT-IR ont indiqué que l'AG était plus localisé dans la région la plus interne du cheveu (médulla) que dans le cortex et la cuticule. L'AG accumulé dans la médulla a disparu après un traitement au lisseur à cheveux à 180 °C en raison de la réaction chimique. CONCLUSIONS: L'effet d'amélioration de la pénétration de l'AG observé avec l'AGC mérite d'être étudié afin de réduire la concentration d'AG dans les produits pour une utilisation plus confortable.

The Surface of Ice under Equilibrium and Nonequilibrium Conditions.

The ice premelt, often called the quasi-liquid layer (QLL), is key for the lubrication of ice, gas uptake by ice, and growth of aerosols. Despite its apparent importance, in-depth understanding of the ice premelt from the microscopic to the macroscopic scale has not been gained. By reviewing data obtained using molecular dynamics (MD) simulations, sum-frequency generation (SFG) spectroscopy, and laser confocal differential interference contrast microscopy (LCM-DIM), we provide a unified view of the experimentally observed variation in quasi-liquid (QL) states. In particular, we disentangle three distinct types of QL states of disordered layers, QL-droplet, and QL-film and discuss their nature. The topmost ice layer is energetically unstable, as the topmost interfacial H2O molecules lose a hydrogen bonding partner, generating a disordered layer at the ice-air interface. This disordered layer is homogeneously distributed over the ice surface. The nature of the disordered layer changes over a wide temperature range from -90 °C to the bulk melting point. Combined MD simulations and SFG measurements reveal that the topmost ice surface starts to be disordered around -90 °C through a process that the topmost water molecules with three hydrogen bonds convert to a doubly hydrogen-bonded species. When the temperature is further increased, the second layer starts to become disordered at around -16 °C. This disordering occurs not in a gradual manner, but in a bilayer-by-bilayer manner. When the temperature reaches -2 °C, more complicated structures, QL-droplet and QL-film, emerge on the top of the ice surface. These QL-droplets and QL-films are inhomogeneously distributed, in contrast to the disordered layer. We show that these QL-droplet and QL-film emerge only under supersaturated/undersaturated vapor pressure conditions, as partial and pseudopartial wetting states, respectively. Experiments with precisely controlled pressure show that, near the water vapor pressure at the vapor-ice equilibrium condition, no QL-droplet and QL-film can be observed, implying that the QL-droplet and QL-film emerge exclusively under nonequilibrium conditions, as opposed to the disordered layers formed under equilibrium conditions. These findings are connected with many phenomena related to the ice surface. For example, we explain how the disordering of the topmost ice surface governs the slipperiness of the ice surface, allowing for ice skating. Further focus is on the gas uptake mechanism on the ice surface. Finally, we note the unresolved questions and future challenges regarding the ice premelt.

Probing the Molecular Structure and Orientation of the Leaf Surface of Brassica oleracea L. by Polarization Modulation-Infrared Reflection-Absorption Spectroscopy.

The surface of most aerial plant organs is covered with the cuticle, a membrane consisting of a variety of organic compounds, including waxes, cutin (a polyester) and polysaccharides. The cuticle serves as the multifunctional interface between the plant and the environment, and plays a major role in protecting plants against various environmental stress factors. Characterization of the molecular arrangements in the intact cuticle is critical for the fundamental understanding of its physicochemical properties; however, this analysis remains technically challenging. Here, we describe the nondestructive characterization of the intact cuticle of Brassica oleracea L. leaves using polarization modulation-infrared (IR) reflection-absorption spectroscopy (PM-IRRAS). PM-IRRAS has a probing depth of less than several hundreds of nanometers, and reveals the crystalline structure of the wax covering the cuticle surface (epicuticular wax) and the nonhydrogen-bonding character of cutin. Combined analysis using attenuated total reflection-IR spectra suggested that hemicelluloses xylan and xyloglucan are present in the outer cuticle region close to the epicuticular wax, whereas pectins are dominant in the inner cuticle region (depth of ≤2 µm). PM-IRRAS can also determine the average orientation of the cuticular molecules, as indicated by the positive and negative spectral peaks. This unique advantage reveals the orientational order in the intact cuticle; the hydrocarbon chains of the epicuticular wax and cutin and the backbones of hemicelluloses are oriented perpendicular to the leaf surface. PM-IRRAS is a versatile, informative and easy-to-use technique for studying plant cuticles because it is nondestructive and does not require sample pretreatment and background measurements.

Chain-propagation, chain-transfer, and hydride-abstraction by cyclic carbocations on water surfaces.

Atmospheric particles contain a wide range of oligomers, but the formation mechanism and the origin of complexity are still unclear. Here, we report the direct detection of carbocationic oligomers generated from the exposure of a series of cyclic unsaturated hydrocarbon gases to acidic water microjets through interface-sensitive mass spectrometry. By changing gas concentrations, H2O (D2O) solvent, bulk pH and comparing results from experiments on acyclic, cyclic, and aromatic compounds, we elucidated three competing reaction mechanisms: chain propagation (CP), chain transfer (CT), and hydride abstraction (HA). We found that conjugative π-electron delocalization in the carbocation is the most important factor for the interfacial oligomerization processes. Our results showed that electrophilic attack on C[double bond, length as m-dash]C double bonds (CP and CT) is limited, and that on C-H single bonds (HA) is enhanced for carbocations lacking conjugation, which is not the case in bulk organic solutions. Carbocationic oligomers generated by the encounter of gaseous unsaturated hydrocarbons and acidic water surfaces potentially contribute to the molecular complexity in atmospheric particles.

Controlling factors of oligomerization at the water surface: why is isoprene such a unique VOC?

Recent studies have shown that atmospheric particles are sufficiently acidic to enhance the uptake of unsaturated volatile organic compounds (VOCs) by triggering acid-catalyzed oligomerization. Controlling factors of oligomerization at the aqueous surfaces, however, remain to be elucidated. Herein, isoprene (2-methyl-1,3-butadiene, ISO), 1,3-butadiene (1,3-b), 1,4-pentadiene (1,4-p), 1-pentene (1-p), and 2-pentene (2-p) vapors are exposed to an acidic water microjet (1 ≤ pH ≤ 5), where cationic products are generated on its surface within ∼10 µs and directly detected using surface-sensitive mass spectrometry. We found that carbocations form at the air-water interface in all the cases, whereas the extent of oligomerization largely depends on the structure in the following order: ISO ≫ 1,3-b > 1,4-p ≫ 1-p ≈ 2-p. Importantly, the cationic oligomerization of ISO yields a protonated decamer ((ISO)10H+, a C50 species of m/z 681.6), while the pentenes 1-p/2-p remain as protonated monomers. We suggest that ISO oligomerization is uniquely facilitated by (1) the resonance stabilization of (ISO)H+ through the formation of a tertiary carbocation with a conjugated C[double bond, length as m-dash]C bond pair, and (2) π-electron enrichment induced by the neighboring methyl group. Experiments in D2O and D2O : H2O mixtures revealed that ISO oligomerization on the acidic water surface proceeds via two competitive mechanisms: chain-propagation and proton-exchange reactions. Furthermore, we found that ISO carbocations undergo addition to relatively inert 1-p, generating hitherto uncharacterized co-oligomers.

Quantum tunneling observed without its characteristic large kinetic isotope effects.

Classical transition-state theory is fundamental to describing chemical kinetics; however, quantum tunneling is also important in explaining the unexpectedly large reaction efficiencies observed in many chemical systems. Tunneling is often indicated by anomalously large kinetic isotope effects (KIEs), because a particle's ability to tunnel decreases significantly with its increasing mass. Here we experimentally demonstrate that cold hydrogen (H) and deuterium (D) atoms can add to solid benzene by tunneling; however, the observed H/D KIE was very small (1-1.5) despite the large intrinsic H/D KIE of tunneling (≳ 100). This strong reduction is due to the chemical kinetics being controlled not by tunneling but by the surface diffusion of the H/D atoms, a process not greatly affected by the isotope type. Because tunneling need not be accompanied by a large KIE in surface and interfacial chemical systems, it might be overlooked in other systems such as aerosols or enzymes. Our results suggest that surface tunneling reactions on interstellar dust may contribute to the deuteration of interstellar aromatic and aliphatic hydrocarbons, which could represent a major source of the deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. These findings could improve our understanding of interstellar physicochemical processes, including those during the formation of the solar system.

Fast crystalline ice formation at extremely low temperature through water/neon matrix sublimation.

Crystalline ice formation requires water molecules to be sufficiently mobile to find and settle on the thermodynamically most stable site. Upon cooling, however, diffusion and rearrangement become increasingly kinetically difficult. Water ice grown by the condensation of water vapor in laboratory is thus generally assumed to be in a metastable amorphous form below 100 K. Here, we demonstrate the possibility of crystalline ice formation at extremely low temperature using a water/neon matrix (1/1000, 30 000 monolayers) prepared at 6 K, which is subsequently warmed to 11-12 K. In situ infrared spectroscopy revealed the assembly of the dispersed water molecules, forming crystalline ice I during the sublimation of the neon matrix for 40-250 seconds. This finding indicates that the high mobility of the water molecules during matrix sublimation can overcome the kinetic barrier to form crystals even at extremely low temperature.

Reactive Uptake of Gaseous Sesquiterpenes on Aqueous Surfaces.

Sesquiterpenes emitted from biogenic sources play important roles in atmospheric HOx cycles and new particle formation. Current atmospheric models, however, fail to account for their fates, possibly due to missing heterogeneous sinks. Here we apply interface-specific mass spectrometry to detect carbocation products of the reactive uptake of gaseous sesquiterpenes C15H24 (ß-caryophyllene (ß-C), α-humulene (α-H), and alloaromadendrene (a-d)) on the surface of aqueous microjets as functions of water acidity and gas concentration. We find that these gases are effectively protonated to C15H25+ upon colliding with the surface of pH < 5 water microjets. We determine inflection points from plots of product yields vs bulk pH: pH1/2 = 4.17 ± 0.05, 4.28 ± 0.06, and 4.36 ± 0.19, and kinetic isotope effects (KIEs) from H2O/D2O (1:1 = vol/vol) experiments: KIE = 2.31 ± 0.08, 1.95 ± 0.05, and 2.71 ± 0.11, for ß-C, α-H, and a-d, respectively. These results are analyzed vis-a-vis previous reports on isoprene and monoterpenes experiments. We estimate 6.2 × 10-5 ≤ γ ≤ 3.1 × 10-4 for the reactive uptake of gaseous sesquiterpenes on acidic (1 < pH < 3) water surfaces. The atmospheric implications of present findings are discussed.

Surface Temperature Dependence of Hydrogen Ortho-Para Conversion on Amorphous Solid Water.

The surface temperature dependence of the ortho-to-para conversion of H_{2} on amorphous solid water is first reported. A combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques allowed us to sensitively probe the conversion on the surface of amorphous solid water at temperatures of 9.2-16 K. Within a narrow temperature window of 8 K, the conversion time steeply varied from ∼4.1×10^{3} to ∼6.4×10^{2} s. The observed temperature dependence is discussed in the context of previously suggested models and the energy dissipation process. The two-phonon process most likely dominates the conversion rate at low temperatures.

Carboxylate Ion Availability at the Air-Water Interface.

Amphiphilic organic compounds at the air-water interface play key roles in the nucleation, growth, and aging process of atmospheric aerosol. Surface-active species are expected to react preferentially with atmospheric oxidants, such as the OH radical, at the air-water interface via specific mechanisms. Establishing the relative availability of the different amphiphilic species to gas-phase oxidants at the air-water interface under atmospherically relevant conditions is, however, challenging. Here we report the interfacial availability of atmospherically relevant carboxylate ions Rn-COO- (n = 1-7) and n-, cyclo-, aromatic-R6-COO- at the air-water interface via a novel application of mass spectrometry of aqueous microjets. The breakup mechanism of microjets lets us determine the relative interfacial affinities of carboxylate ions in equimolar solutions of the corresponding carboxylic acids in the 1 µM to 1 mM range under ambient conditions. We find that the interfacial affinity of Rn-COO- increases exponentially with both chain-length and solvent-accessible surface area (SASA) except in the case of R1-COO-. The relative interfacial affinities for n-heptanoate (n-R6-COO-) > cyclohexanecarboxylate (c-R6-COO-) > benzoate (Ar-R6-COO-) are also determined. We attribute the smallest availability of Ar-R6-COO- at the air-water interface among the three carboxylate ions to a strong π-H bonding between the aromatic ring and water molecule. Molecular mechanisms on the availability of carboxylate ions at the air-water interface and the atmospheric implications are discussed.

Chiral Spinodal-like Ordering of Homoimmiscible Water at Interface between Water and Chiral Ice III.

Diversity in structures of water endowed by a hydrogen-bonding network plays crucial roles in wide varieties of phenomena in nature. Chiral ordering of water molecules is an intriguing phenomenon from the viewpoint of bimolecular functions. However, experimental reports on chiral ordering have been limited to the water molecules interacting with biomolecules on the molecular scale. It remains unclear whether pure liquid water forms long-range chiral ordering without any interaction with biomolecules. Here, we show that chiral anisotropy can be observed in the macro/mesoscopic network pattern of an unknown water layer formed via spinodal phase separation-like dynamics at the interface between water and ice III with a chiral crystal structure. We named this unknown water homoimmiscible water. Our observations infer that the unknown water is a chiral liquid crystal. This possibility opens new avenues for a wide variety of research fields such as liquid polymorphism, biology, earth and planetary science, and so forth from the perspective of chirality.

Impurity contribution to ultraviolet absorption of saturated fatty acids.

Saturated fatty acids are abundant organic compounds in oceans and sea sprays. Their photochemical reactions induced by solar radiation have recently been found as an abiotic source of volatile organic compounds, which serve as precursors of secondary organic aerosols. However, photoabsorption of wavelengths longer than 250 nanometers in liquid saturated fatty acids remains unexplained, despite being first reported in 1931. Here, we demonstrate that the previously reported absorption of wavelengths longer than 250 nanometers by liquid nonanoic acid [CH3(CH2)7COOH)] originates from traces of impurities (0.1% at most) intrinsically contained in nonanoic acid reagents. Absorption cross sections of nonanoic acid newly obtained here indicate that the upper limit of its photolysis rate is three to five orders of magnitude smaller than those for atmospherically relevant carbonyl compounds.

Anisotropy in spinodal-like dynamics of unknown water at ice V-water interface.

Experimentally demonstrating the existence of waters with local structures unlike that of common water is critical for understanding both the origin of the mysterious properties of water and liquid polymorphism in single component liquids. At the interfaces between water and ices Ih, III, and VI grown/melted under pressure, we previously discovered low- and high-density unknown waters, that are immiscible with the surrounding water. Here, we show, by in-situ optical microscopy, that an unknown water appears at the ice V-water interface via spinodal-like dynamics. The dewetting dynamics of the unknown water indicate that its characteristic velocity is ~ 90 m/s. The time evolution of the characteristic length of the spinodal-like undulation suggests that the dynamics may be described by a common model for spinodal decomposition of an immiscible liquid mixture. Spinodal-like dewetting dynamics of the unknown water transiently showed anisotropy, implying the property of a liquid crystal.

Low- and High-Density Unknown Waters at Ice-Water Interfaces.

Experimental confirmation of liquid polymorphs of water, high-density liquid (HDL) and low-density liquid (LDL), is desired for understanding not only the liquid state of matter but also the origin of the mysterious properties of water. However, this remains challenging because the liquid-liquid critical point of water lies in experimentally inaccessible supercooling conditions known as "no man's land". Here, we show by in situ optical microscopy that droplets and layers of low- and high-density unknown waters (LDUW and HDUW) appear macroscopically depending upon ice polymorphs at non-equilibrium interfaces between water and ices under experimentally accessible (de)pressurization conditions. These unknown waters were found to have characteristic velocities (about 20 and 100 m/s for LDUW and HDUW, respectively) different from water (about 40 m/s) and quasi-liquid layers (QLLs) (about 2 and 0.2 m/s for droplet and layer forms of QLLs, respectively). Our discoveries provide insight on liquid polymorphism of water.

Direct Observation and Quantitative Measurement of OH Radical Desorption During the Ultraviolet Photolysis of Liquid Nonanoic Acid.

Ultraviolet (UV) photolysis of fatty acid surfactants─which cover the surfaces of atmospheric liquid aerosols and are found in the oceans─such as nonanoic acid (NA) has recently been suggested as a source of hydroxyl (OH) radicals in the troposphere. We used laser-induced fluorescence to directly observe OH radicals desorbed from the surface of neat liquid NA as a primary photoproduct following 213 nm irradiation. The upper limit of photoreaction cross section for the OH radical desorption was estimated to be 9.0(4.1) × 10-22 cm2, which is only 1.2 ± 0.8% of the photoreaction cross section established for the photolysis of gas-phase acetic acid monomers. Vibrational sum-frequency generation spectroscopy for liquid NA revealed the hydrogen-bonded, cyclic, dimer structure of the NA molecules at the liquid surface. This dimerization can inhibit the formation of OH radicals and lead the present low photochemical reactivity of liquid NA.

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.

Experimental studies of surface reactions among OH radicals that yield H2O and CO2 at 40-60 K.

We investigated the OH-related formation routes of two astrophysically important molecules, H(2)O and CO(2), under relatively warm astrophysical conditions. OH radicals, together with other neutral species such as H, O, H(2), and O(2), were produced in H(2)O microwave-discharge plasma and cooled to 100 K before being deposited on an Al substrate at 40-60 K. H(2)O formed at 40 and 50 K, but not at 60 K. Taking the experimental conditions into account, a possible route of H(2)O formation is via reactions involving OH + OH, which yield H(2)O(2) as the main reaction product. The present study is the first to show experimentally that surface reactions of two OH radicals can yield H(2)O at low temperatures. The products' branching ratio was 0.2 and 0.8 for H(2)O and H(2)O(2), respectively. When CO was co-deposited with neutral species that formed in the H(2)O plasma, CO(2) was formed at 40-60 K. H(2)CO(3) formed at 40 and 50 K. The present results may suggest that chemical reactions related to OH radicals are effective at yielding various molecules in relatively warm astrophysical environments, such as protostars.