Vapor pressure | EPFL Graph Search

Vapour pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's thermodynamic tendency to evaporate. It relates to the balance of particles escaping from the liquid (or solid) in equilibrium with those in a coexisting vapor phase. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the attractive interactions between liquid molecules become less significant in comparison to the entropy of those molecules in the gas phase, increasing the vapor pressure. Thus, liquids with strong intermolecular interactions are likely to have smaller vapor pressures, with the reverse true for weaker interactions. The vapor pressure o

A laboratory study of heterogeneous reactions relevant to the atmospheric boundary layer

Dominik Stadler

The present work deals with two subjects. The interaction of NO2 and HONO with different types of soot are examined in the first part whereas in the second part an experimental set-up is presented which has been built in order to measure the kinetics of the degradation of organic compounds by OH radicals. Both soot particles as well as NO2 are mainly produced by fossil fuel and biomass burning. The two species are therefore ubiquitous in the atmospheric boundary layer where they may react with each other. It has been shown that one possible product of this heterogeneous interaction is nitrous acid (HONO). HONO is an important trace gas in atmospheric chemistry because it is easily photolysed resulting in OH radical and NO. Thus, the photolysis of HONO significantly enhances photooxidation processes early in the morning. In the present study two different types of decane soot have been produced. It has been shown that the air/fuel ratio of the diffusion flame is a key parameter influencing the reactivity of soot towards NO2. Whereas soot from a rich flame ("grey" decane soot) leads to HONO with yields up to 100% upon interaction with NO2, only small amounts of HONO, but significant amounts of NO are formed in the presence of soot originating from a lean flame ("black" decane soot). A reaction mechanism has been developed showing that NO2 is initially converted to HONO by a redox reaction. HONO produced in this way is subsequently either quantitatively desorbed into the gas phase on "grey" decane soot or it decomposes to a large extent in a disproportionation reaction on "black" decane soot. The products of disproportionation are NO, which is instantaneously desorbed into the gas phase and NO2, which undergoes secondary reactions. In addition to the HONO producing pathway there is a reaction channel that leads to adsorption of NO2 for both types of soot which is irreversible on the time scale of our standard experiments. Saturation effects that are occurring on the time scale of our experiments are primarily caused by saturation of adsorption sites and not by the depletion of the reducing agent. It seems that the fraction of NO2 that is irreversibly adsorbed is blocking part of the surface sites where NO2 is initially adsorbed. Uptake coefficients γ take initial values of up to 0.1 for both types of soot. However, with increasing uptake of NO2 γ is decreasing. After a NO2 consumption of approximately 8×1013 molecule cm-2, which corresponds to 13% of a formal monolayer, γ drops to values of 3×10-7 for "grey" decane soot and 6×10-7 for "black" decane soot, respectively. Furthermore, our results show that the rate of initial uptake is the rate limiting step of the NO2/soot interaction mechanism. Experiments where "black" decane soot has been exposed to a flow of HONO revealed that it readily decomposes into NO and NO2. For HONO concentrations above 3.3×1011 molecule cm-3 a total product yield of 50%, whereof 40% NO and 10% NO2, has been observed. The initial uptake coefficient ( γ0) for HONO on "black" decane soot did not show clear saturation effects but varied randomly in the concentration range from 1.2×1012 to 5.9×1012 molecule cm-3. The average value measured at these concentrations was equal to 0.027. The strong interaction of HONO with "black" decane soot shows that soot is not only a potential source, but also a potential sink of HONO. On the other hand, no significant uptake could be observed when "grey" decane soot was exposed to HONO. This is consistent with the observation that NO2 is converted to HONO at yields of up to 100%. Acetylene soot is an additional type of soot that has been examined. The results for the initial uptake coefficients were basically the same as for "grey" and "black" decane soot, that is maximum values of γ0 of 0.1 for NO2 uptake decreasing with increasing concentrations of NO2. This similarity holds only for the first few seconds of interaction because acetylene soot saturates much faster than "grey" and "black" decane soot with continuous exposure. The acetylene samples are generally almost completely saturated after only three minutes of exposure to NO2. Soxhlet extractions of "grey" decane soot which have been performed using tetrahydrofuran as a solvent also produced HONO upon interaction with NO2. This shows that the active reactants are not part of the graphitic soot backbone but are extractable, rather polar compounds of the organic fraction of soot. This assumption is supported by the observation that the corresponding benzene extraction is not reactive in relation to NO2 uptake and HONO formation. In the second part of the present work an experimental set-up is described which has been built in order to measure the rate constants of the reaction of organic compounds with the OH radical in the gas phase using a relative rate technique at atmospheric pressure. The system may be called a "mini smog chamber" as the reaction cell has a volume of only 480 cm3. The system was validated by measuring the relative rate constants of the reactant pair cyclohexane/toluene. These measurements have been performed for temperatures ranging from 300 to 360 K and led to results which are in excellent agreement with literature data. However, the experimental set-up also had some important limitations. Biphenyl which has a boiling point of 529 K was the compound with the lowest volatility that could be examined in our reaction cell. Compounds with lower vapor pressures or compounds with polar groups adsorbed to a large extent onto the glass walls of the reaction cell even at temperatures of up to 373 K. Additional problems occurred in the closed ion source of the mass spectrometer leading to non linear and/or unstable MS signals. These phenomena were most probably caused by secondary ion-molecule reactions in the closed ion source.

EPFL2000

Designing interactions across interfaces in ionic liquids

Valentin Michael Manzanares

Ionic liquids (ILs) have attracted much attention because of their unusual charac- teristics (vapor pressure, condictivity, etc.). Furthermore, they can be designed to display desired properties specific to an application (task-specific ionic liquid, TSILs). This makes the production and investigation of ionic liquids derivatized with functional group attractive, especially in the view of green chemistry. The first chapter reviews the recent evolution of ionic liquids, and in particular ionic liquids bearing one or several functional groups. In the second chapter, ILs functionalized with two different functional groups were synthesized. The chosen functional groups (alcohol, ether, nitrile, amine) may influence transition metal-catalyzed reaction such as cross-coupling reactions. The ILs synthesized in this chapter were evaluated as solvents in the palladium-catalyzed Heck and Suzuki cross-coupling reactions. Chapter 3 describes our investigation on ionic liquid-catalyzed carbon dioxide and epoxides cyclization (CEC). The synthesis of several IL catalysts, along with their use in the cyclization of styrene oxide with carbon dioxide is detailed. In chapter 4, we discuss the synthesis of ILs functionalized with perfluoroalkyl chains; the synthesis of imidazole precursors is also considered. The last chapter details our efforts towards the formation of self-assembled IL monolayer onto a silica surface. Additionally, we establish the first steps in the synthesis of uncommon IL surface modifiers.

EPFL2016

Study of the kinetics of condensation and evaporation of water vapor over atmospherically relevant pure and doped ice films

Christophe Eric Ludovic Delval

This thesis presents the different steps in the development of a multiple diagnostic experimental setup and the results concerning the kinetics of condensation and evaporation of water over pure ice, as well as ice films doped with a known amount of atmospheric trace gases. The goal of this project is to obtain reliable data concerning the evaporative flux, Jev [molec cm-2 s-1], and the concomitant rate of condensation, kcond [s-1], of H2O molecules in the presence of an ice film. A further aim of this work is the detailed investigation of the impact on the aforementioned kinetic processes of the deposition of atmospherically relevant trace gases such as hydrogen chloride (HCl), hydrogen bromide and nitric acid (HNO3) under well defined conditions including those that are representative of the atmosphere. The new experimental apparatus consisting of a stainless steel reactor may be used under three different regimes, namely static, stirred flow and dynamic condition which correspond to equilibrium, low rate of pumping and molecular flow, respectively. The separation of the kinetics into individual processes is principally afforded by the application of these 3 regimes. First, using a combination of the first two regimes, we have measured Jev and γ of H2O in the case of pure as well as HCl- or HBr-doped ice in the range 190-240 K. The resulting Arrhenius expressions for pure ice are: in the presence of an average mole fraction of HCl, ΧHCl, in the range 3.2·10-5 to 6.4·10-3 we have found: and for ΧHBr at a HBr mole fraction smaller than 6.4·10-3 was found, where R=1.987 cal mol-1 K-1. Owing to the rate separation, γ applies to an individual rate process and should be called an accommodation rather than an uptake coefficient. Of utmost importance is the fact that during most of the ice film evaporation the vapor pressure remained that of pure ice even in the presence of small amounts of HCl or HBr on the order of a fraction to several formal monolayers. This is consistent with the enthalpies of sublimation that may be derived following the relation ΔHsubl0 = Econd - Eev. It leads to different thermochemical parameters A (pre-exponential factor) and E values (energy of activation) for the three cases that are identical within experimental uncertainty. In addition, FTIR spectroscopy of the ice films involved in these experiments in transmission have revealed that different species such as amorphous HCl·H2O mixtures or the crystalline HCl·6H2O hexahydrate are formed depending on the doping protocol. These two species significantly affect the temporal change of Jev each in a different manner. For the sake of precision and accuracy of the kinetic results, a quartz crystal microbalance (QCM) was subsequently added to the experimental apparatus and used as a substrate for ice film growth. The development of a QCM-based device for the investigation of the kinetic processes of ice films at atmospherically relevant temperatures was a challenge we have successfully surmounted whereas previously published attempt had given barely convincing results. The Arrhenius representation of the zero order evaporative flux Jev (molec cm-2 s-1) of H2O from pure ice displays a discontinuity at 193±2 K that could be observed owing to the accuracy and precision of the QCM measurements. It leads to: In addition, γ of H2O onto ice was found to be lower than 0.5 for temperatures below 240 K in contrast to the value of 1.0 which is often encountered in the literature. Subsequently, the deposited ice films have been doped with HNO3 using different protocols depending on parameters such as temperature, rate of deposition of HNO3 onto ice as well as number ofHNO3 molecules deposited. The reproducible and quantitative dosing of the doping gas was key to the quality of the results. This study has shed light on a complex interplay between several of these parameters that affect the continuous decrease of Jev during ice evaporation. Using atmospherically relevant conditions in the doping of ice by HNO3 we have observed crystalline α-NAT being most likely the first species that forms in/on ice films. During most of the evaporation of the ice film doped with known amounts of HNO3 we have checked that the vapor pressure over the ice film remains that of pure ice. Finally, HCl-doped ice films have been investigated in experiments that are complementary to the first set in order to verify that HCl doping has a similar impact on the change of Jev of H2O in the presence of small albeit known amounts of HCl. The doping protocol is a fundamental parameter determining the decrease of Jev with H2O evaporation time of doped ice films. In summary, we have developed from scratch a new experimental apparatus that allows the separation of the kinetic processes of condensation and evaporation of pure as well as doped ice films using multiple diagnostics under the same experimental conditions which was heretofore not attainable. The main result for pure ice films is that the uptake coefficient, γ of H2O on ice is always lower than approximately 0.5 for atmospherically relevant temperatures. Its negative temperature dependence has been confirmed. An accurate control of the doping protocol has allowed us to observe that for a fraction to a few monolayers of the doping gases, HCl, HBr and HNO3, the nature of the aforementioned protocol is the predominant factor influencing the continuous temporal decrease of Jev and γ during doped ice film evaporation. Of utmost importance is the fact that the vapor pressure over the doped ice films remains that of pure water ice during almost all of the evaporation process. These results may be of importance in atmospheric modeling for determining the lifetime of ice particles involved in clouds in regions that are undersaturated in the vapor pressure of H2O over ice. In this way the physical process of evaporation affects the importance of heterogeneous chemistry of atmospheric trace gases on ice substrates representative of Cirrus clouds, aviation induced Cirrus clouds and contrails as well as type II PSC's.

EPFL2004