Thermodynamic stability of clathrate hydrates relative to their separate chemical components

2003 ◽  
Vol 81 (1-2) ◽  
pp. 175-182 ◽  
Author(s):  
M A White ◽  
D C MacLaren ◽  
R A Marriott ◽  
B -Z Zhan
Keyword(s):  
Melting Point ◽  
Ethylene Oxide ◽  
Small Molecules ◽  
Low Temperatures ◽  
Experimental Information ◽  
Clathrate Hydrate ◽  
Clathrate Hydrates ◽  
Chemical Components ◽  
Melting Points ◽  
Guest Species

The thermodynamic changes (ΔH, ΔS, and ΔG) for the association of several small molecules (tetrahydrofuran (THF), ethylene oxide (EO), acetone) with water to form corresponding clathrate hydrates are calculated as a function of temperature from experimental information. For THF clathrate hydrate and EO clathrate hydrate at low temperatures, the clathrate is enthalpically stabilized with respect to the components. This is also the likely case for acetone clathrate hydrate. In all the three cases, above the melting points of the guest species, the clathrate increases in enthalpic stability, but entropic factors favour the separated components. Similar changes for THF clathrate hydrate and EO clathrate hydrate occur at the melting point of ice, eventually favouring the liquid components over the clathrate. PACS No.: 65.40-b

scholarly journals Investigation of the tensile properties of solid mercury and a comparison with those of other metals at low temperatures

1936 ◽  
Vol 156 (888) ◽  
pp. 411-426 ◽  
Keyword(s):  
Mechanical Properties ◽  
Melting Point ◽  
Tensile Properties ◽  
Low Temperatures ◽  
Atmospheric Temperature ◽  
Metals And Alloys ◽  
Melting Points ◽  
Experimental Procedure ◽  
Tension Tests

The mechanical properties of metals at temperatures approaching their melting points present a field of investigation which has not yet been adequately explored. Experimental procedure will vary according to the melting point of the metal. With the exception of mercury, all metals have a melting point considerably above atmospheric temperature. Mercury, however, freezes at —39⋅7° C and, on this account, it is comparatively easy to carry out tension tests at temperatures close to the melting point. The present investigation deals with the strength of mercury under such conditions and a comparison is made with several other metals and alloys at similar temperatures, namely, from 17° C to —130° C.

Predictions of Current Attachment at Thermionic Cathode for TIG Arcs

2008 ◽  
Vol 580-582 ◽  
pp. 319-322 ◽  
Author(s):  
Manabu Tanaka ◽  
Kentaro Yamamoto ◽  
Tashiro Shinichi ◽  
John J. Lowke
Keyword(s):  
Melting Point ◽  
Work Function ◽  
Atmospheric Pressure ◽  
The Other ◽  
Numerical Calculations ◽  
Maximum Temperature ◽  
Arc Plasma ◽  
Melting Points ◽  
Thermionic Cathode ◽  
Uniform Current

Study of current attachment at thermionic cathode for TIG arc at atmospheric pressure is attempted from numerical calculations of arc-electrodes unified model. The calculations show that the maximum temperature of arc plasma close to the cathode tip for W-2% ThO2 reaches 19,000 K and it is the highest value in comparison with the other temperatures for W-2% La2O3 and W-2% CeO2, because the current attachment at the cathode tip is constricted by a centralized limitation of liquid area of ThO2 due to its higher melting point. The calculations also show that, in cases of W- 2% La2O3 and W-2% CeO2, the liquid areas of La2O3 and Ce2O3 are widely expanded at the cathode tip due to their lower melting points and then produce uniform current attachments at the cathode. It is concluded that the current attachment at thermionic cathode is strongly dependent on work function, melting point and Richardson constant of emitter materials.

scholarly journals Predicting Melting Points of Biofriendly Choline-Based Ionic Liquids with Molecular Dynamics

2019 ◽  
Vol 9 (24) ◽  
pp. 5367 ◽  
Author(s):  
Karl Karu ◽  
Fred Elhi ◽  
Kaija Põhako-Esko ◽  
Vladislav Ivaništšev
Keyword(s):  
Crystal Structure ◽  
Molecular Dynamics ◽  
Ionic Liquids ◽  
Melting Point ◽  
Chemical Space ◽  
Mean Square ◽  
Melting Points ◽  
Simulation Based ◽  
Predicted Values ◽  
Dynamics Simulations

In this work, we introduce a simulation-based method for predicting the melting point of ionic liquids without prior knowledge of their crystal structure. We run molecular dynamics simulations of biofriendly, choline cation-based ionic liquids and apply the method to predict their melting point. The root-mean-square error of the predicted values is below 24 K. We advocate that such precision is sufficient for designing ionic liquids with relatively low melting points. The workflow for simulations is available for everyone and can be adopted for any species from the wide chemical space of ionic liquids.

scholarly journals Effects of Local Heating and Premelting in the Terminal Part of the e+ Track

2012 ◽  
Vol 733 ◽  
pp. 15-18 ◽  
Author(s):  
Dmitry Zvezhinskiy ◽  
Sergey V. Stepanov ◽  
Vsevolod Byakov ◽  
Bożena Zgardzińska
Keyword(s):  
Melting Point ◽  
Temperature Dependence ◽  
Local Heating ◽  
Bulk Temperature ◽  
Melting Points ◽  
Terminal Part ◽  
Slowing Down ◽  
Positronium Lifetime ◽  
Molten Region

The terminal part of the e+ track (the positron blob) is formed during ionization slowing down and subsequent ion-electron recombinations produced by a positron. It releases up to 1 keV of energy, which is converted into heat within few picoseconds. If a bulk temperature of a medium is below, but close enough to its melting point, some region of a substance may melt, yielding a peculiar temperature dependence of the lifetime (LT) spectra. We have estimated properties of the molten region with a help of macroscopic heat con- duction equation and suggested a model describing temperature dependence of the ortho- positronium lifetime in frozen methanol, ethanol, butanol and water close to their melting points.

Cluster–Micelle Transition of a Thermo- and Photoresponsive ABC Triblock Copolymer in an Ionic Liquid

2019 ◽  
Vol 72 (2) ◽  
pp. 155
Author(s):  
Aya Saruwatari ◽  
Kei Hashimoto ◽  
Ryota Tamate ◽  
Ryoji Usui ◽  
Hisashi Kokubo ◽  
...  
Keyword(s):  
Ionic Liquid ◽  
Transition Temperature ◽  
Ethylene Oxide ◽  
Uv Irradiation ◽  
Low Temperatures ◽  
Triblock Copolymer ◽  
Abc Triblock Copolymer ◽  
Poly Ethylene Oxide ◽  
Poly Ethylene ◽  
Azobenzene Groups

We report the photocontrollable micelle–cluster transition of an ABC-type triblock copolymer in an ionic liquid (IL). Polystyrene-b-poly(ethylene oxide)-b-poly(4-phenylazobenzyl acrylamide-r-N-isopropylacrylamide) (PSt-b-PEO-b-P(AzoBnAm-r-NIPAm)) was synthesised, where PSt is IL-phobic, PEO is IL-philic, and P(AzoBnAm-r-NIPAm) is photo- and thermoresponsive in the IL. At high temperatures, the triblock copolymer forms micelles with PSt cores; furthermore, at low temperatures, micelles self-assemble into clusters induced by the aggregation of P(AzoBnAm-r-NIPAm). Under UV irradiation, the micelles form clusters at lower temperatures than that in the dark because of the change in the solubility of P(AzoBnAm-r-NIPAm) induced by photoisomerisation of the azobenzene groups, indicating that this triblock copolymer has a photocontrollable micelle–cluster transition temperature.

The “Solidification” of Grain Boundaries with Increasing Temperature

1994 ◽  
Vol 357 ◽  
Author(s):  
Witold Lojkowski ◽  
Bogdan Palosz
Keyword(s):  
Solid State ◽  
Melting Point ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
High Temperatures ◽  
Low Temperatures ◽  
Melting Behavior ◽  
Wetting Transitions ◽  
Increasing Temperature ◽  
Segregation Of Impurities

AbstractThe aim of the paper is to explain the recently observed de-wetting grain boundary transition with increasing temperature. On the example of a bicrystal from the Fe-6at.%Si alloy, it was found recently that as temperature is increased, the following GB transitions take place: “solid” (or regular) GB-→“premelted” GB →“solid” GB. At the same time the wetting/de-wetting transitions have taken place. Another example of such GB behavior was discovered during sintering of alumina. The inverse melting behavior is explained as follows: low melting point impurities cause GB premelting at low temperatures, However de-segregation of impurities at high temperatures causes return of the GB structure to its regular “solid” state.

scholarly journals Nucleation rate analysis of methane hydrate from molecular dynamics simulations

2015 ◽  
Vol 179 ◽  
pp. 463-474 ◽  
Author(s):  
Daisuke Yuhara ◽  
Brian C. Barnes ◽  
Donguk Suh ◽  
Brandon C. Knott ◽  
Gregg T. Beckham ◽  
...  
Keyword(s):  
Molecular Dynamics ◽  
Nucleation Rate ◽  
Methane Hydrate ◽  
Critical Nucleus ◽  
Md Simulations ◽  
Clathrate Hydrate ◽  
Clathrate Hydrates ◽  
Growth Effect ◽  
Nucleus Size ◽  
Critical Nucleus Size

Clathrate hydrates are solid crystalline structures most commonly formed from solutions that have nucleated to form a mixed solid composed of water and gas. Understanding the mechanism of clathrate hydrate nucleation is essential to grasp the fundamental chemistry of these complex structures and their applications. Molecular dynamics (MD) simulation is an ideal method to study nucleation at the molecular level because the size of the critical nucleus and formation rate occur on the nano scale. Various analysis methods for nucleation have been developed through MD to analyze nucleation. In particular, the mean first-passage time (MFPT) and survival probability (SP) methods have proven to be effective in procuring the nucleation rate and critical nucleus size for monatomic systems. This study assesses the MFPT and SP methods, previously used for monatomic systems, when applied to analyzing clathrate hydrate nucleation. Because clathrate hydrate nucleation is relatively difficult to observe in MD simulations (due to its high free energy barrier), these methods have yet to be applied to clathrate hydrate systems. In this study, we have analyzed the nucleation rate and critical nucleus size of methane hydrate using MFPT and SP methods from data generated by MD simulations at 255 K and 50 MPa. MFPT was modified for clathrate hydrate from the original version by adding the maximum likelihood estimate and growth effect term. The nucleation rates calculated by MFPT and SP methods are within 5%, and the critical nucleus size estimated by the MFPT method was 50% higher, than values obtained through other more rigorous but computationally expensive estimates. These methods can also be extended to the analysis of other clathrate hydrates.

Poly(ethylene) oxide for small-molecule crystal growth in gelled organic solvents

2011 ◽  
Vol 44 (1) ◽  
pp. 172-176 ◽  
Author(s):  
Duane Choquesillo-Lazarte ◽  
Juan Manuel García-Ruiz
Keyword(s):  
Crystal Growth ◽  
Ethylene Oxide ◽  
Small Molecules ◽  
Organic Solvents ◽  
Small Molecule ◽  
Poly Ethylene Oxide ◽  
Poly Ethylene

The use of poly(ethylene) oxide (PEO) as a gelator has been evaluated with a selected list of organic solvents. From the 26 solvents tested, eight formed gel matrices. This number was extended to 19 when poly(ethylene) oxide was used with a mixture of solvents. The procedure for the preparation of PEO organogels is described, and their application for the crystallization of small molecules using different crystallization techniques in the presence of organic solvents is discussed.

Melting Point of cis-1,4-Polybutadiene

1965 ◽  
Vol 38 (4) ◽  
pp. 921-923 ◽  
Author(s):  
J. C. Mitchell
Keyword(s):  
Melting Point ◽  
Thermodynamic Data ◽  
Experimental Techniques ◽  
Melting Point Depression ◽  
Melting Points ◽  
Heating Rates ◽  
Orientation Effects ◽  
Flory’S Theory

Abstract Melting point values for cis-1, 4-polybutadiene thus far reported in the literature have not exceeded +1° C. Varying values have been reported, both because of sample variations (differing amounts and distributions of chain imperfections) and because of variations in experimental techniques used to measure the melting point (in particular, varying heating rates). Some workers have extrapolated their data to predict that the melting point of a perfectly regular (100 per cent cis) cis-1, 4- polybutadiene (Tm°) would be close to +1° C. These extrapolations have been used to arrive at heats of fusion for the polymer from Flory's theory for melting point depression due to copolymeric impurities. One such value has been used in another investigation to estimate extents of crystallization from thermodynamic data. We wish to report observation of melting points several degrees higher than the assumed Tm° value of +1° C. Our experiments show that these higher values are not due to orientation effects.

scholarly journals The rigidity of solid unimolecular films

1927 ◽  
Vol 114 (769) ◽  
pp. 690-697 ◽  
Keyword(s):  
Melting Point ◽  
Stearic Acid ◽  
Low Temperatures ◽  
Solid Acid ◽  
Benzene Solution ◽  
Static Method ◽  
Langmuir Trough ◽  
Solid Film ◽  
Small Torque

On evaporation at low temperatures of a benzene solution of palmitic or stearic acid on the surface of water an apparently solid film is left. The experiments of I. Langmuir and N. K. Adam have shown that these films are unimolecular in character, and inasmuch that they possess a fairly well defined melting point, although this varies with the acidity of the underlying solution, we may regard these films as unimolecular sheets of orientated solid acid. Anyone who examines these films even superficially cannot fail to notice their exceptional strength, all the more extraordinary when we consider their extreme thinness. Whilst the compressibility of such films, a property which can be readily determined by examination of the slope of the force area curve obtained with the well-known Langmuir trough apparatus, does not present any unusual features, being of the order anticipated for a hydrocarbon, yet we may expect that the coefficient of rigidity will be unexpectedly high. It seemed a matter of some importance to attempt to measure the rigidity of such films, by applying a suitable torque and determining the displacements effected, a method suggested to us by Prof. G. I. Taylor. In our preliminary experiments we endeavoured to employ a simple static method of placing a disc at the centre of a large circular film and applying torsion by means of a torsion head and wire on the disc to which a mirror is attached. We have to express our thanks to Prof. G. I. Taylor for the loan of an excellent and finely-calibrated head for this purpose. After numerous attempts with various modifications of the method we were reluctantly compelled to abandon it. The results were invariably the same; on applying a small torque to the disc no motion was visible on the image of the mirror attached to the disc. If the torque be increased the film is ruptured and the disc breaks loose and slips, generally forcing itself entirely from attachment to the film. Even with discs coated with wax or corrugated and milled on the circular edge no better results were obtained. Evidently the grip on the disc being only of one molecule thick is not sufficient to hold the slightest movement on the part of the shearing disc.

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