Boiling point | EPFL Graph Search

The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding environmental pressure. A liquid in a partial vacuum, i.e., under a lower pressure, has a lower boiling point than when that liquid is at atmospheric pressure. Because of this, water boils at under standard pressure at sea level, but at at altitude. For a given pressure, different liquids will boil at different temperatures. The normal boiling point (also called the atmospheric boiling point or the atmospheric pressure boiling point) of a liquid is the special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level, one atmosphere. At that temperature, the vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and allow bubbles of vapor to form inside the bulk of the liquid.

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

Stability and physical properties of alkylsilane and alkylphosphonate self-assembled monolayer (SAM) films on metal oxide substrates characterized at the micro- and nanoscale

Enamul Hoque

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.

EPFL2007