Molecular motion and phase transitions of clathrate hydrates**

Abstract

Heat capacities and complex dielectric permittivities of three clathrate hydrates of type II, encaging tetrahydrofuran (THF), acetone (Ac), and trimethylene oxide (TMO), were measured at low temperatures. The heat capacity measurement was done in the temperature range 13–300 K by using an adiabatic calorimeter with a built-in cryorefrigerator. The permittivities were measured in the temperature range 20–260 K and in the frequency range 20 Hz-1 MHz. For pure samples, with a glass transition due to freezing out of water, reorientational motion of the host lattice was observed calorimetrically at 85 K for THF and at 90 K for Ac hydrates, respectively. Spontaneous temperature drift rates of the calorimetric cell were measured under adiabatic conditions to derive the characteristic time for enthalpy relaxation. The enthalpy relaxation times thus derived were well correlated in an Arrhenius plot with the dielectric relaxation times derived from the dielectric relaxation of orientation polarization. The situation is the same as hexagonal ice which has a similar four co-ordinated hydrogen-bonded network.     

Phase transitions in the hydroquinone hydrogen sulfide clathrate compound

Heat capacities of [C₆H₄(OH)₂]₃·(H₂S) _x were measured between 1 and 15 K. Heat capacity peaks were found at (7.56±0.09) K, (7.61±0.05) K, and (7.65±0.07) K for the compounds withx=0.92, 0.95, and 0.96. A weak anomaly was observed around 6.75 K for the compound withx=0.85. The temperatures of these anomalies are unusually low among the phase transitions of molecular crystals. The decrease of the transition temperature from that of crystalline H₂S (=103.52 K) is a clear indication of the effect of enclathration on the molecular interaction. A comparison of the rotational heat capacity of the trapped hydrogen sulfide molecules with that of crystalline hydrogen sulfide shows that the trapped hydrogen sulfide molecules have a large rotational freedom at low temperatures (13 K). This agrees with the results from far infrared spectroscopic data. The dielectric constant of the clathrate compound obeyed the Curie-Weiss law above 30 K and no significant dielectric loss was found over the whole temperature range. These results showed that the trapped hydrogen sulfide molecules execute free rotation or are orientationally disordered above 20 K.Dedicated to Professor H. M. Powell.     

Structure and phase stability of binary zintl-phase compounds: lithium-group 13 intermetallics and metal-doped group 14 clathrate compounds

The structure/bonding relationship in a series of intermetallic phases of Li with Al, Ga, and In was investigated by density functional theory and complemented by a model based on tight-binding theory and the method of moments. The combination of these two approaches provides a simple scheme which allows for both a comprehensive understanding of structural trends and the ability to predict low-energy structures for a given composition. This analysis gives a straightforward picture of phase stability in terms of local geometric features such as triangular, square, and hexagonal arrangements of atoms. The approach was extended to examine the structural properties of metal-doped clathrate compounds of C, Si, Ge, and Sn. Clathrate-type phases based on the frameworks Si172, Ge172, Si40, and Ge40 are not only likely to be energetically favorable but may also exhibit high thermoelectric efficiency.     

Computational study of methyl group dynamics in the hydroquinone clathrate of acetonitrile

We report molecular dynamics simulations of the acetonitrile clathrate of hydroquinone, with a focus on the dynamics of acetonitrile methyl groups. There are three inequivalent acetonitrile molecules in the unit cell, one with its dipole parallel to the c-axis, and the other two antiparallel. Although these three guest molecules have previously been found to exhibit two slightly different frequencies of rotation over a wide range of temperatures, the frequencies could not be assigned to specific methyl groups. Perhaps counterintuitively, our simulations suggest that the molecule with the lower frequency is one of the two molecules oriented the same way, the different dynamical behaviour being due to subtle differences in the environments of the molecules.     

Molecular orbital calculations on transition element compounds

In an attempt to improve the theoretical treatment of the lower excited states of the permanganate and chromate anions beyond the simple treatment described in Part II, the MCZDO method of Brown and Roby has been investigated. Although this method includes one-centre exchange integrals and is able to discriminate between integrals involving atomic orbitals of different auxiliary quantum number, neither of which features is present in the CNDO calculations of Part II, we have been unsuccessful in devising a better interpretation of the spectra of MnO ₄ ⁺ and CrO ₄ ^–– than that presented in Part II. The predicted energies of excited states have been found to depend strongly on the values used for two-centre nuclear attraction integrals _A¦V _B¦ _A and on other two-centre integral values. It may be necessary to use more elaborate functions than Slater-type orbitals to evaluate these integrals satisfactorily. It would seem that a theoretical technique even more elaborate than the MCZDO method is required to give a consistently good account of the ultraviolet spectra of transition-element compounds. The success of the CNDO calculations reported in Part II for MnO ₄ ^– and CrO ₄ ^–– is probably fortuitous.

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Zusammenfassung Im Rahmen eines Versuches, die theoretische Behandlung der unteren angeregten ZustÄnde des Permanganat- und Chromat-Anions über die einfache Behandlung von Teil II zu verbessern, wurde die MCZDO-Methode von Brown und Roby untersucht. Obwohl diese Methode die Einzentrenwechselwirkungsintegrale mit einschlie\t und zwischen Integralen für AO mit verschiedenen Nebenquantenzahlen unterscheidet, war der Versuch erfolglos, eine verbesserte Interpretation der Spektren von MnO ₄ ^–1 und CrO ₄ ^–– zu entwickeln. Die Energien der angeregten ZustÄnde waren stark abhÄngig von den Werten der Zweizentrenintegrale. Möglicherweise müssen andere Funktionen als solche vom Slater-Typ verwendet werden, um diese Integrale befriedigend auszurechnen. Es scheint überhaupt, da\ nur subtilere Verfahren als die MCZDO-Methode gute Resultate für die Ultraviolettspektren mehrerer übergangselement-Verbindungen gleichzeitig ergeben können und der Erfolg der CNDO-Rechnungen von II für MnO ₄ ^– und CrO ₄ ^–– ist wahrscheinlich zufÄllig.

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Résumé La méthode MCZDO de Brown et Roby a été étudiée lors d'une tentative pour améliorer le traitement théorique des états excités des anions permanganate et chromate au delà du traitement simple décrit dans la partie II. Quoique cette méthode introduise des intégrales d'échange monocentriques et distingue les intégrales entre des orbitales atomiques de nombres quantiques secondaires différents, ce qui n'est pas fait dans les calculs CNDO de la partie II, aucune interprétation plus satisfaisante des spectres de MnO ₄ ^– et CrO ₄ ^–– n'a pu Être obtenue. Les énergies prédites pour les états excités dépendent fortement des valeurs utilisées pour les intégrales d'attraction nucléaire bicentriques _A¦V _B¦ _A et les autres intégrales bicentriques. Il peut s'avérer nécessaire d'utiliser des fonctions plus élaborées que les orbitales de Slater pour évaluer ces intégrales d'une manière satisfaisante. Il semblerait q'une technique théorique éventuellement plus élaborée que la méthode MCZDO soit nécessaire pour rendre compte des spectres électroniques des composés d'éléments de transition. Le succès des calculs CNDO de la partie II pour MnO ₄ ^– et CrO ₄ ^–– est probablement fortuit.

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Commonwealth of Australia, Department of Supply, Scientific Officer.     

Molecular orbital calculations on transition element compounds

A series of SCFMO calculations on the ground states of MnO ₄ ^– and CrO ₄ ^–– , and a corresponding series of configuration interaction calculations based on these ground state wave functions and all singly excited configuration, are reported. The molecular orbital calculations included all 24 valence electrons (ligand 2s electrons being regarded as part of the core) and included CNDO-type approximations. The various calculations illustrate the effect of different parameterizations upon the numerical results obtained and again emphasise the dangers of placing any quantitative reliance upon a single calculation. Predicted energies of excited states are sensitive to the value of the averaged one-centre electron repulsion integral for the metal atom orbitals, but not so sensitive to the value of the average one-centre nuclear attraction integral. The evaluation of the core Hamiltonian in a full overlap basis and transformation of the matrix to an orthogonal basis proves better than the approximate direct evaluation on an orthogonal basis. The use of scaling factors for two-centre integrals to reproduce most nearly the values obtained from Hartree-Fock orbitals does not lead to the optimum calculations. It is emphasized that the usual CNDO approximation of using averaged values integrals for all valence orbitals on a given centre produces over-occupation of 3d orbitals and under-occupation of 4s and 4p orbitals on the metal atom. By assigning the first three transitions in the spectrum of MnO ₄ ^– to 2et ₁, 2e2t ₂ and 3t ₂t ₁ respectively (corresponding to ¹ T ₂¹ A ₁ in each case, the indicated excitations making the main contributions to the configuration-interaction functions) it proves possible to account for the observed magnetic circular dichroism of these transitions and to account for the observed oscillator strengths of the bands. This qualitative order of energies of the lowest excited states is obtained for a rather wide range of parameter values in the CNDO scheme. For CrO ₄ ^–– a similar assignment of the spectrum emerges but it seems particularly from the magnetic circular dichroism data, that the second and third transitions are superimposed.

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Zusammenfassung Eine Reihe von SCFMO-Rechnungen für die Grundzustände von MnO ₄ ^– und CrO ₄ ^–– sowie die Resultate der entsprechenden Konfigurationswechselwirkung von Grundzustand und allen einfach angeregten Konfigurationen werden mitgeteilt. Einbezogen in die CNDO-artigen Näherungsrech-nungen werden alle 24 Valenzelektronen (die 2s-Elektronen der Liganden zum Rumpf gerechnet). Die Rechnungen illustrieren den Effekt der verschiedenen Parameterisierung auf die Resultate und weisen erneut darauf hin, wie problematisch es ist, sich auf eine bestimmte Rechnung allein zu verlassen. Die Energien der angeregten Zustände hängen stark vom Durchschnittswert der Einzentren-Elektronenwechselwirkungsintegrale für die Metall-AO's, dagegen nicht so stark vom Durchschnittswert der Einzentren-Kernwechselwirkungsintegrale ab. Die Berechnung des Rumpf-Hamilton-operators mit voller Einbeziehung der Überlappung und Übergang zu orthogonalierten Funktionen erweist sich als besser als die näherungsweise direkte Berechnung bei angenommener Orthogonalität. Der Verwendung von scaling-Faktoren für Zweizentrenintegrale zur Annäherung an Werte für Hartree-Fock Orbitale führen nicht zu den optimalen Ergebnissen. Es wird betont, daß die üblichen CNDO-Näherungen (Durchschnittswerte für Integrale für alle Valenzorbitale) zu Überbesetzung von 3d-Zuständen und Unterbesetzung von 4s und 4p Zuständen führt. Weist man den ersten drei Banden im Spektrum von MnO ₄ ^– die Übergänge 2et₁, 2e2t ₂ und 3t ₂t ₁ zu (dies entspricht in jedem Fall ¹ T ₂¹ A ₁, die angezeigten Anregungen geben lediglich den Hauptanteil wieder), läßt sich zeigen, daß der beobachtete magnetische zirkulare Dichroismus dieser Übergänge und die beobachteten Oszillatorstärken der Bänder erklärt werden können. Die Reihenfolge der Energien der niedrigsten angeregten Zustände bleibt für einen ziemlich groen Bereich der CNDO-Parameterwerte unverändert. Für CrO ₄ ^–– ergibt sich eine ähnliche Zuweisung des Spektrums; allerdings deuten besonders die Werte für den magnetischen zirkulären Dichroismus darauf hin, daß der zweite und dritte Übergang einander überlagern.

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Résumé Série de calculs SCFMO sur les états fondamentaux de MnO ₄ ^– et CrO ₄ ^–– et de calculs d'interaction de configurations incluant toutes les configurations monoexcitées. Les calculs ont été effectués sur tous les 24 électrons de valence (les électrons 2s du ligand étant considérés comme faisant partie du coeur) en employant des approximations de type CNDO. Les différents calculs illustrent l'effet de la paramétrisation sur les résultats numériques obtenus, soulignant à nouveau le danger de se fier quantitativement à un seul calcul. Les énergies prédites pour les états excités sont sensibles à la valeur moyenne de l'intégrale de répulsion électronique monocentrique pour les orbitales de l'atome métallique, tout en étant moins sensibles à la valeur moyenne de l'intégrale de l'attraction nucléaire monocentrique. L'évaluation de l'hamiltonien de coeur dans une base avec recouvrement et la représentation de cette matrice dans une base orthogonale s'avère meilleure que l'évaluation directe approchée dans une base orthogonale. L'emploi de facteurs de réduction pour les intégrales bicentriques afin de reproduire au mieux les valeurs obtenues à partir des orbitales de Hartree-Fock ne conduit pas aux meilleurs calculs. On souligne que l'approximation CNDO habituelle employant des valeurs moyennes des intégrales pour tous les électrons de valence d'un centre produit une «sur-occupation» des orbitales 3d et une «sous occupation» des orbitales 4s et 4p sur l'atome métallique. Si l'on assigne les trois premières transitions du spectre de MnO ₄ ^– à 2et ₁, 2e2t ₂ et 3t ₂t ₁ respectivement (ce qui correspond dans chaque cas à ¹ T ₂¹ A ₁, les excitations indiquées étant celles qui ont le plus grand poids dans la fonction d'interaction de configuration), on peut rendre compte du dichroïsme circulaire magnétique observé pour ces transitions ainsi que des forces oscillatrices observées pour ces bandes. L'ordre qualitatif des énergies des états excités les plus bas est obtenu pour un grand éventail de valeurs des paramètres en méthode CNDO. Pour CrO ₄ ^–– une assignation similaire du spectre apparaît, mais il semble en particulier à partir des données du dichroïsme circulaire magnétique que la seconde et la troisième transition sont superposées.

     

Molecular orbital calculations on transition element compounds

The Ewald method has been used to calculate the effect of electrostatic environments (crystal lattices) on the molecular orbitals and spectroscopic state levels of the oxyanions, permanganate and chromate. An estimate of factor group splitting based on a crude model suggests that it may be an observable effect in molecular ions in crystals. However the shifts in energy and the splitting of bands due to electrostatic effect of the lattice are predicted to be more important than the factor group splittings. Attention is drawn to the significant effect of the lattice field on the molecular orbital energies, making the uppermost filled orbital of the ground state strongly bonding, in contrast to the situation in the isolated anions.

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Zusammenfassung Nach dem Verfahren von Ewald wurde der Einfluß des elektrostatischen Beitrages des Kristallgitters auf die MO's und spektroskopische Zustände des Permanganat- und Chromat-Anions berechnet. Eine rohe Abschätzung der Faktorgruppen-Aufspaltung legt die Vermutung nahe, daß auch dieser Effekt beobachtbar ist. Jedoch sind die Energieverschiebungen und Aufspaltungen infolge der Umgebung des Kristallgitters wahrscheinlich größer als die Faktorgruppen-Aufspaltung. Es wird besonders auf den bedeutenden Einfluß des Gitterfeldes auf die MO-Energien hingewiesen, die das oberste besetzte Orbital des Grundzustandes stark bindend machen, ganz im Gegensatz zur Situation für die isolierten Anionen.

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Résumé La méthode d'Ewald a été utilisée pour calculer l'effet de l'environnement électrostatique (réseaux cristallins) sur les orbitales moléculaires et les états spectroscopiques des oxyanions permanganate et chromate. L'estimation de la séparation du groupe facteur sur la base d'un modèle grossier suggère que cet effet peut être observé dans les ions moléculaires dans les cristaux. Cependant les déplacements et les séparations des bandes dûs à l'effet électrostatique du réseau sont prédits comme étant plus grands que les séparations du groupe facteur. On attire l'attention sur l'effet significatif du champ du réseau sur les énergies des orbitales moléculaires, la plus haute orbitale occupée devenant fortement liante, contrairement à ce qui a lieu dans les anions isolés.

     

Pressure induced phase transitions in hydroquinone

High pressure behavior of alpha-hydroquinone (1,4-dihydroxybenzene) has been studied using Raman spectroscopy up to pressures of 19 GPa. Evolution of Raman spectra suggests two transitions around 3.3 and 12.0 GPa. The first transition appears to be associated with the lowering of crystal symmetry. Above 12.0 GPa, Raman bands in the internal modes region exhibit continuous broadening suggesting that the system is progressively evolving into a disordered state. This disorder is understood as arising due to distortion of the hydrogen-bonded cage across the second transition around 12 GPa.     

Structural transformation and guest dynamics of methanol-loaded hydroquinone clathrate observed by temperature-dependent Raman spectroscopy

The structural transformations and guest dynamics of methanol-loaded β-form hydroquinone (HQ) clathrate were investigated using temperature-dependent Raman spectroscopy. Methanol-loaded β-form HQ clathrate was obtained by recrystallization and characterized by elemental analysis, synchrotron X-ray diffraction, solid-state (13)C NMR spectroscopy, and Raman spectroscopy. Temperature-dependent Raman spectra of methanol-loaded β-form HQ clathrate were measured in the temperature range 300-412 K at increments of 4 K. Although no significant changes were evident in the temperature range 300-376 K, abrupt changes in the relative intensity and shape of the Raman bands were observed between 380 and 412 K indicating the structural transition from methanol-loaded β-form HQ clathrate to pure α-form HQ. Methanol molecules were gradually released from the β-form HQ clathrate in the range 364-380 K. Upon returning to ambient conditions, the crystal structure of the HQ sample remained identical to that of pure α-form HQ. Therefore, the temperature-induced structural transition of methanol-loaded HQ clathrate is completely irreversible and α-form HQ is more stable at ambient conditions.     

Phenol as a guest molecule in clathrate compounds

Phenol, having favourable physical and chemical properties, can be enclosed as the guest component in the clathrates of tetracyano complexes. Six compounds of Hofmann and similar type clathrates M(NH₃)₂M' (CN)₄.nG and M(en)_m M'(CN)₄.nG were prepared and identified: Ni(NH₃)₂Pt.2C₆H₅OH; Ni(en)₂Pt(CN)₄.O.14C₆H₅OH; Ni(NH₃)₂Pt(CN)₄.C₆H₅OH.H₂O; Zn(NH₃)₂Ni(CN)₄.O.1C₆H₅OH.H₂O; Cu(NH₃)₂Ni(CN)₄. 2C₆H₅OH and Fe(NH₃)₂Ni(CN)₄.2C₆H₅OH. The phenol containing clathrates are more stable than clathrates containing other guest molecules. In the case of Ni(en)₂ Pt(CN)₄.O.14C₆H₅OH thermal loss of the guest molecule leaves the host lattice intact, but further heating results in the rupture of the host lattice. The compounds were capable in the solid state of sorbing other organic molecules once they had been heated to the temperature required for almost complete loss of guest molecule i.e. n→o.     

Structural properties and phase transitions in a silica clathrate

Melanophlogite, a low-pressure silica polymorph, has been extensively studied at different temperatures and pressures by molecular dynamics simulations. While the high-temperature form is confirmed as cubic, the low-temperature phase is found to be slightly distorted, in agreement with experiments. With increasing pressure, the crystalline character is gradually lost. At 8 GPa, the radial distribution function is consistent with an amorphous state. Like pristine glass, the topology changes, plastic behavior, and permanent densification appear above ～12 GPa, triggered by Si coordination number changes. We predict that a partial crystalline and amorphous sample can be obtained by recovering the sample from a pressure of ～12-16 GPa.     

Molecular mechanisms of hydrogen-loaded beta-hydroquinone clathrate

Molecular dynamics simulations are used to investigate the molecular interactions of hydrogen-loaded beta-hydroquinone clathrate. It is found that, at lower temperatures, higher loadings are more stable, whereas at higher temperatures, lower loadings are more stable. Attractive forces between the guest and host molecules lead to a stabilized minimum-energy configuration at low temperatures. At higher temperatures, greater displacements take the system away from the shallow energy minimum, and the trend reverses. The nature of the cavity structure is nearly spherical for a loading of one, leads to preferential occupation near the hydroxyl ring crowns of the cavity with a loading of two, and at higher loadings, leads to occupation of the interstitial sites (the hydroxyl rings) between cages by a single H(2) molecule with the remaining molecules occupying the equatorial plane of the cavity. Occupation of the interstitial positions of the cavities leads to facile diffusion.     

Charge-density-wave to Mott-Hubbard phase transition in quasi-one-dimensional bromo-bridged Pd compounds

A -Pd(III)-Br-Pd(III)-Mott-Hubbard state was observed in a quasi-one-dimensional bromo-bridged Pd compound [Pd(en)2Br](C5-Y)2 x H2O (en = ethylenediamine, C5-Y = dipentylsulfosuccinate) for the first time. The phase transition between Mott-Hubbard and charge-density-wave states occurred at 206 +/- 2 K and was confirmed by using X-ray, ESR, Raman and electronic spectroscopies, electrical resistivity, and heat capacity. From X-ray powder diffraction patterns and Raman spectra of a series of Pd-Br compounds, [Pd(en)2Br](C(n)-Y)2 x H2O (n = 4, 5, 6, 7, 8, 9, and 12), chemical pressure from the alkyl chains of the counterions caused the phase transition.     

Application of clathrate compounds for hydrogen storage

The review considers current works on clathrate hydrogen compounds, aimed at creating hydrogen accumulators suitable for practical application. Analysis of published data showed that clathrate hydrates formed by pure hydrogen are unsuitable for this purpose in view of their fairly low limiting hydrogen content and the necessity for their synthesis of extremely high pressures (>100 MPa) that are still industrially unfeasible. The possibilities for hydrogen storage in double (including auxiliary guest molecules along with hydrogen) clathrate hydrates are considered. It is concluded from published data that sorbents on the basis of the so-called “metal-organic frameworks” (MOFs) with a pore size of 1–2 nm hold a greater promise for hydrogen storage at temperatures of about 100 and moderately (up to 10 MPa) high pressures, but the development of all the considered methods of hydrogen storage has not yet grown out of laboratory experiments.     

Construction of literature databases for clathrate and inclusion compounds

An attempt to construct the literature databases for inclusion complexes since the beginning of this century is illustrated. The original literatures were cited and cross-checked by several secondary sources including Chemical Abstracts. The existing database systems appear to be insufficient to obtain necessary information efficiently in such a newly and rapidly developing field as inclusion chemistry. The trends in research revealed in the course of the database construction are analyzed and discussed for some important categories of inclusion chemistry.     

Unanticipated guest motion during a phase transition in a ferroelastic inclusion compound

Urea inclusion compounds (UICs) have been used as tools to understand ferroelastic domain switching and molecular recognition during crystal growth. Although the vast majority of UICs contain helical arrangements of host H-bonds, those containing guests with the formula X(CH(2))(6)Y (X, Y = Br, Cl, CN, NC) adopt an alternative P2(1)/n packing mode in which the host molecules exist as stacked loops of urea hexamers. Such structures may be further separated into two classes, ones distorted away from hexagonal symmetry along [100] (Br(CH(2))(6)Br, Br(CH(2))(6)Cl, and Cl(CH(2))(6)Cl) and those distorted along [001] (e.g. NC(CH(2))(6)CN). In each of these systems, guests exist as equilibrium mixtures of gauche conformers whose populations control the direction and magnitude of the observed distortion. Such UICs are potentially ferroelastic, but the n-glide requires that domains are not related by a simple rotation-translation mechanism as in the helical systems. Ferroelastic (degenerate) domain reorientation would necessitate a large-scale reorganization of the urea framework and rupture of numerous H-bonds. Coupled with distortions of 2 to 10%, this mechanism-based barrier to domain switching has precluded observation of this phenomenon. To prepare ferroelastic UICs with minimal distortions from hexagonal symmetry, attempts were made to form solid solutions of UICs containing guests from the two classes. This failed, however: solid solution formation of the stacked loop form is usually possible within a series (e.g. with Cl(CH(2))(6)Cl and Br(CH(2))(6)Br), but not between series (e.g. Cl(CH(2))(6)Cl and NC(CH(2))(6)CN). Crystals of Cl(CH(2))(6)CN/urea, in which a single guest contains substituents from each class, are distorted along [001] by only 0.5% from hexagonal symmetry at 298 K and exhibit ferroelastic domain reorientation at high forces. At -66 degrees C, Cl(CH(2))(6)CN/urea undergoes a topotactic phase transition that is unexpectedly nontopochemical. The structure of the low-temperature phase, including the orientation of the methylene chain, closely matches the structures of UICs distorted by 10% along [100] (e.g. Cl(CH(2))(6)Cl/urea). In this transition, small conformation changes of guests give rise to large-scale guest translations of approximately 5.5 A down the channel axis, even though an analogous gauche-to-gauche jump is well established in closely related materials that adopt either high- or low-temperature forms (e.g. NC(CH(2))(6)CN/urea, Cl(CH(2))(6)Cl/urea). The large guest displacement during this transition explains the difficulty in preparing solid solutions of the P2(1)/n form with guests of formula X(CH(2))(6)Y from two different series (e.g. Cl(CH(2))(6)Cl and NC(CH(2))(6)CN). This failure arises not from the different orientations of guest-induced strain, but from preferential occupation of different sites along the channel by the two types of guests. The subtlety of this process and of the interactions involved highlights the difficulty in using simple considerations of isomorphism to design new materials.     

Contact line motion in confined liquid-gas systems: Slip versus phase transition

In two-phase flows, the interface intervening between the two fluid phases intersects the solid wall at the contact line. A classical problem in continuum fluid mechanics is the incompatibility between the moving contact line and the no-slip boundary condition, as the latter leads to a nonintegrable stress singularity. Recently, various diffuse-interface models have been proposed to explain the contact line motion using mechanisms missing from the sharp-interface treatments in fluid mechanics. In one-component two-phase (liquid-gas) systems, the contact line can move through the mass transport across the interface while in two-component (binary) fluids, the contact line can move through diffusive transport across the interface. While these mechanisms alone suffice to remove the stress singularity, the role of fluid slip at solid surface needs to be taken into account as well. In this paper, we apply the diffuse-interface modeling to the study of contact line motion in one-component liquid-gas systems, with the fluid slip fully taken into account. The dynamic van der Waals theory has been presented for one-component fluids, capable of describing the two-phase hydrodynamics involving the liquid-gas transition [A. Onuki, Phys. Rev. E 75, 036304 (2007)]. This theory assumes the local equilibrium condition at the solid surface for density and also the no-slip boundary condition for velocity. We use its hydrodynamic equations to describe the continuum hydrodynamics in the bulk region and derive the more general boundary conditions by introducing additional dissipative processes at the fluid-solid interface. The positive definiteness of entropy production rate is the guiding principle of our derivation. Numerical simulations based on a finite-difference algorithm have been carried out to investigate the dynamic effects of the newly derived boundary conditions, showing that the contact line can move through both phase transition and slip, with their relative contributions determined by a competition between the two coexisting mechanisms in terms of entropy production. At temperatures very close to the critical temperature, the phase transition is the dominant mechanism, for the liquid-gas interface is wide and the density ratio is close to 1. At low temperatures, the slip effect shows up as the slip length is gradually increased. The observed competition can be interpreted by the Onsager principle of minimum entropy production.     

On the thermodynamic stability and structural transition of clathrate hydrates

Gas mixtures of methane and ethane form structure II clathrate hydrates despite the fact that each of pure methane and pure ethane gases forms the structure I hydrate. Optimization of the interaction potential parameters for methane and ethane is attempted so as to reproduce the dissociation pressures of each simple hydrate containing either methane or ethane alone. An account for the structural transitions between type I and type II hydrates upon changing the mole fraction of the gas mixture is given on the basis of the van der Waals and Platteeuw theory with these optimized potentials. Cage occupancies of the two kinds of hydrates are also calculated as functions of the mole fraction at the dissociation pressure and at a fixed pressure well above the dissociation pressure.