Metastable Solids Research Articles

New Dual Purpose Chemistry for Gas Hydrate and Corrosion Inhibition

Abstract An overlooked benefit of some gas hydrate inhibitors is their potential as corrosion inhibitors. In this paper, a family of molecules was evaluated as gas hydrate inhibitors by an autoclave technique. The chemicals were concurrently evaluated as corrosion inhibitors by linear polarization resistance. The results indicate that there are design criteria based on the chemistry of these systems, which can be used to optimize their performance as both gas hydrate inhibitors and corrosion inhibitors. Introduction and Background Formation of natural gas hydrates is a common problem during oil and gas production, and has been well documented(1). Gas hydrates are molecules of natural gas trapped in crystals of frozen water. They are metastable solids in which hydrogen bonded water molecules (hosts) encase the low boiling hydrocarbon molecules (guests) in a cage-like entity. The cage-like structures can totally enclose or trap a gas molecule. The mechanism of gas hydrate formation is believed to follow a two-step process. First, clusters of hydrogen bonded water molecules form around a non-polar core. This is followed by the joining of these clusters to form gas hydrates. Gas hydrate formation is usually favoured at temperatures closer to the freezing point of water. However, under sufficient pressure, gas hydrates will form at temperatures above the freezing point of water. The resulting solids can form plugs that restrict or block gas flow during oil and gas production. The most common Gas Hydrate Inhibitor (GHI) is methanol. Methanol functions as a thermodynamic inhibitor by shifting the equilibrium requirements for hydrate formation to lower temperatures and higher pressures. However, the high dosage requirements of methanol have created logistic and environmental problems. For this reason, Low Dosage Hydrate Inhibitors (LDHIs) have been developed. LDHIs have been classified as being either kinetic or anti-agglomerant. The Kinetic Inhibitors (KIs) work at low concentrations by delaying the nucleation and growth of crystals. KIs are cost effective, but fail to inhibit agglomeration of the crystals once nucleation occurs(2). KIs also have less sub-cooling potential than anti-aglomerants. For this reason, KIs are less desirable for situations with long shut-in periods. The Anti-Agglomerates (AAs) are believed to function at the gas-water interface preventing the formation of large hydrate crystals. AA mechanisms for gas hydrate inhibition are considerably different than those of kinetic inhibitors. It is believed that AAs have a dual mechanism of action. The first mode of action is believed to affect the structure of the growing hydrate. While the AAs allow hydrates to form, they limit the growth of these hydrates, thus minimizing the risk of pipeline plugging. The growth inhibition is believed to occur as a result of binding of the AA to the surface of the initially formed hydrate, thus altering the structure of the gas cage. AAs second mode of hydrate inhibition is achieved by their behaviour as dispersants. These molecules allow the previously formed hydrates to disperse in the oil phase and allow the transportation of the gas-water mixture through the pipelines(3).

Acid volatile sulfide (AVS)

The sequestration of sulfur within sedimentary pyrite is a major sink in the global biogeochemical sulfur cycle and has consequent impacts on the related cycles of carbon and oxygen. In many sedimentary environments, a zone exists which releases H 2S if treated with acid. The materials that produce this sulfide are described as acid volatile sulfides or AVS and the sulfide which is evolved and collected by this treatment is called acid volatile sulfide (AVS-S). In this research review we show that AVS represents part of a complex and dynamic biogeochemical system which is not defined simply by the analysis of AVS-S. During the 40 years since R.A. Berner first defined the concept of acid volatile sulfide, a number of scientific myths have developed regarding the nature and composition of AVS and its role in the global sulfur cycle. We show that: • AVS is not equivalent to FeS and solid FeS phases have rarely been identified in marine sediments. AVS is a complex and variable component which does not represent any singular or simple group of sediment components. • The commonly used extraction methods do not quantitatively extract AVS minerals (mackinawite and greigite) without also significantly releasing sulfide from pyrite. • AVS varies geographically and AVS is not the same component or set of components wherever it is found. • The solubilities of FeS and Fe 3S 4 in neutral to alkaline systems are presently poorly constrained and pore water saturation state calculations cannot be used to support the presence of specific AVS minerals. • The colored zone in sediments does not reflect the distribution of AVS. • AVS is not always a minor fraction of total reduced sulfide (TRS) which dominantly occurs near the sediment–water interface. AVS minerals are not required precursors for pyrite formation. A better understanding of the sedimentary sulfur system, including its nature, formation and transformations, is a key component in understanding the evolution of the Earth and its future development. However, some basic parameters, such as the solubility and composition of the metastable iron sulfide solids, are still not constrained. Even the presence of these minerals in most sedimentary systems remains equivocal. Aqueous FeS clusters appear to be ubiquitous and appear to have significant geochemical and biological roles. The roles of organisms, especially in modifying rates and products, remain a key area of research. Although AVS may have served its purpose in the earlier exploratory days of the study of sedimentary sulfides, the concept is now probably more of a hindrance than a help in advancing this field. It is reasonable to question if it is not time to simply do away with the AVS concept altogether and get on with the business of trying to deal more directly with the behavior of the more important components known to fall within the group of sulfide species that contribute to sulfide that is volatilized when acid is added to a sediment.

Stability and properties of the cluster assembled solid phases ofX8C12 and YX7C12

We have investigated several of the most stable cluster assembled phases ofX8C12 and YX7C12(X, Y = B, N, Si) using pseudopotential plane-wave density functional theory. We havefound that many of these were stable, conductive and retained the integrity ofthe cluster cage, along with some of its bonding characteristics. A large rangeof metastable molecular solids can be produced in this way. Some of these newcovalent conductors and one semiconductor were considered good candidates for highTc superconductivity and may constitute an improvement over the recently discoveredfcc-C22 superconducting molecular solid.

Deposition and Aggregation of Aspirin Molecules on a Phospholipid Bilayer Pattern

Aspirin and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) are deposited from their alcoholic mixed solution onto highly oriented pyrolytic graphite (HOPG) by spin coating. The film structure and morphology are characterized by atomic force microscopy (AFM). The barely soluble DMPE forms a highly oriented stripe phase as a result of its one-dimensional epitaxy with the HOPG lattice. The bilayer stripe pattern exposes the cross section of the lipid bilayer lamellae and enables the direct visualization of the molecular interactions of drug or biological molecules with either the hydrophobic or the hydrophilic part of the phospholipid bilayer. The bilayer pattern affects the aspirin molecular deposition and aggregation. AFM shows that the aspirin molecules prefer to deposit and aggregate along the aliphatic interior part of the bilayer pattern, giving rise to parallel dimer rods in registry with the underlying pattern. The nonpolar interactions between aspirin and the phospholipid bilayer are consistent with the lipophilic nature of aspirin. The bilayer pattern not only stabilizes the rodlike aggregate structure of aspirin at low aspirin concentration but also inhibits crystallization of aspirin at high aspirin concentration. Molecular models show that the width of the DMPE aliphatic chain interior can accommodate no more than two aspirin dimers. The bilayer confinement may prevent aspirin from reaching its critical nucleus size. This study illustrates a general method to induce a metastable or amorphous form of an active pharmaceutical ingredient (API) by chemical confinement under high undercooling conditions. Metastable and amorphous solids often display better solubility and bioavailability than the stable crystalline form of the API.

Nucleation and phase-selection in undercooled melts

Abstract Electromagnetic levitation technique is applied to undercool bulk melts of metals and alloys. A large undercooling range becomes accessible by avoiding heterogeneous nucleation on container walls and processing the melts under high purity environment. A freely suspended drop is produced with the extra benefit that it becomes accessible for direct observation and diagnostics. Temperature–time profiles (ttp) are measured during the melting and solidification process. The ttp profiles give insight into the thermodynamics of phase-selection processes. The levitation technique is combined with the diagnostic means at European Synchrotron Radiation Facility (ESRF Grenoble) to record time-resolved diffraction spectra during rapid crystallization of deeply undercooled melts. In such a way, the sequence of solidification of different crystallographic phases and (partly) remelting of primarily formed phases during recalescence is unambiguously determined. All these investigations allow for the construction of nucleation phase-selection diagrams examples of which are presented for Fe-based, Ni–V, and Nd–Fe–B alloys with a competition of phase formation of bcc, fcc, and some intermetallic phases, respectively. They give the conditions for the formation of metastable solids of different crystallographic structures as a function of concentration and undercooling achieved prior to solidification.

Threshold Size for Ambient Metastability of Rocksalt CdSe Nanocrystals

We show that metastable rocksalt CdSe nanocrystals can persist at ambient pressure depending on the physical size of the particle. The size-dependence of the hysteresis loop was measured for the solid−solid transition in CdSe nanocrystals, between four- and six-coordinate structures. A systematic shift of the entire hysteresis loop to lower pressure results in a threshold size of ∼11 nm for ambient metastability of the six-coordinate rocksalt structure. Smaller nanocrystals transform back to the four-coordinate structure as occurs in the CdSe bulk solid. Surface energy contributions are used to explain the shift. The results have important implications for the optimum synthesis of metastable nanocrystal solids under ambient conditions.

Solitary waves and supersonic reaction front in metastable solids.

Motivated by an increasing number of remarkable experimental observations on the role of pressure and shear stress in solid reactions, explosions, and detonations, we present a simple one-dimensional model that embodies nonlinear elasticity and dispersion as well as chemical or phase transformation. This generalization of the Toda lattice provides an effective model for the description of the organization during an abrupt transformation in a solid. One of the challenges is to capture both the equilibrium degrees of freedom as well as to quantify the possible role of out-of-equilibrium perturbations. In the Toda lattice, we verify that the particle velocities converge in distribution towards the Maxwell-Boltzmann distribution, thus allowing us to define a bonafide temperature. In addition, the balance between nonlinearity and wave dispersion may create solitary waves that act as energy traps. In the presence of reactive chemistry, we show that the trapping of the released chemical energy in solitary waves that are excited by an initial perturbation provides a positive feedback that enhances the reaction rate and leads to supersonic explosion front propagation. These modes of rupture observed in our model may provide a first-order description of ultrafast reactions of heterogeneous mixtures under mechanical loading.

Computer simulation of cluster assembling

AbstractDensity functional theory is used to investigate the assembling of metallic clusters to yield stable or metastable cluster solids. Motivated by the observed high stability of the Al13H cluster, which has a substantial highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) gap, we have modeled the assembling of those clusters. For a favorable relative orientation of each cluster with respect to all its neighbors, a cluster solid is predicted and its structure appears to be stable at least up to 150 K, which is the highest temperature in our simulations. We have also studied the chemical bonding in the stoichiometric solid alloys PbA, where A is one of the alkali elements Na, K, Rb, or Cs. Those crystals exist in an ordered phase formed by tetrahedral Pb4 clusters surrounded by the alkali atoms. The study of this family of natural cluster compounds reveals the coating role played by the cations, providing further insight into the favorable conditions required for the formation of cluster solids. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Thermally stimulated exoelectron emission from rare gas solids in weak external electric fields

Charge carriers and neutral reactive intermediates were generated by ionizing radiation in model insulators—Ar solids doped with O 2, N 2 and CO. Combining methods of activation spectroscopy—thermally and photon-stimulated exoelectron emission (TSEE and PSEE) with thermally stimulated luminescence we studied electronic and atomic processes of relaxation in these metastable solids. This experimental approach enables us (i) to identify the charge of mobile charge carriers, (ii) to separate reactions of neutral and charged species and (iii) to find the interconnection between thermally driven chemical reactions and charge carrier dynamics in insulators.

Use of metastable, dissociated and charged gas species in synthesis: a low pressure analogue of the high pressure technique

Oxidation of silver using microwave-induced oxygen plasma and oxygen-ozone gas mixture was studied as a function of temperature and partial pressure. The oxide Ag2O was formed at temperatures well above its normal decomposition temperature in oxygen plasma at a pressure of 5 Pa. The higher oxide AgO1−x was formed in O2+O3 gas mixtures at lower temperatures. The oxygen chemical potentials for the oxidation of Ag to Ag2O, Ag2O to AgO1−x and AgO to Ag2O3 were evaluated from thermodynamic data and compared with the experimental results to obtain information on the chemical potential of oxygen in microwave plasma and gases containing ozone. The oxygen potential of the gas phase in microwave plasma operating at a pressure of 5 Pa was found to be in excess of 36 kJ/mol at 750 K. This is equivalent to a pressure of diatomic oxygen gas greater than 3 × 107 Pa. In the O2+O3 mixture at ambient pressure containing 5 mole percent O3, the oxygen potential is ∼112 kJ/mol at 465 K. The equivalent pressure of diatomic oxygen is ∼4 × 1017 Pa. Thus, metastable species such as O3 or charged species such as O− present in plasma can be used as a powerful reagent for the syntheses of metastable oxides. Similar techniques can be used for other metastable inorganic solids such as nitrides for functional applications.

Metastable Eutectic Condensation in a Mg‐Fe‐SiO‐H2‐O2Vapor: Analogs to Circumstellar Dust

Experimental studies of gas-to-solid condensation in a Fe-Mg-SiO-H2-O2 vapor reveal that this process yields only solids with magnesiosilica (MgO.SiO2) and ferrosilica (Fe-oxide.SiO2) compositions that coincide with metastable eutectics in the MgO-SiO2 and (FeO/Fe2O3)-SiO2 binary phase diagrams plus simple metal oxides (MgO, SiO2, and FeO or Fe2O3). No solids form with mixed Mg-Fe-O compositions during condensation nor is there evidence for the formation of ferromagnesiosilica, MgO.FeyOx.SiO2 solids. The experimental evidence demonstrates that condensation of multicomponent vapors yields only a limited number of metastable solids of well-defined composition. These results have interesting consequences for models of grain formation in circumstellar outflows, for predictions concerning the chemical and mineralogical composition of presolar silicates, and for the composition of condensates formed in protostellar systems.

From molecular aggregates to novel organosilicon and phosphorus/arsenic compounds

Abstract

Some recent developments of polysilsesquioxanes chemistry for material science

This review is focused on a new class of materials, the Monophasic Organic–Inorganic Hybrid Materials (MOIHM). These solids correspond to a unique phase prepared from a molecular precursor using the hydrolytic polycondensation which is used in the sol-gel preparation of oxides from alkoxides precursors. In these materials, the oxide matrix is elaborated around the organic moiety, and covalent bonds (mainly SiC bonds) link together the oxide matrix and the organic part. These solids are metastable solids of which their formation is kinetically controlled: all parameters able to modify the kinetics of the hydrolysis polycondensation process are of importance for both the physical and chemical properties of the solids. The organic moieties are also a main factor in both the short range organisation and the texture of the solid: weak interactions are able to control the solid formation and a substructure between the organic units can be evidenced in the case of thienyl units from polymerisation in the solid state. These materials are also a convenient way for the introduction of physical properties in the oxide matrix.

Can there be a second kind of entropy catastrophe in the melting of metastable solids?

We explore here the thermodynamic possibility of the superheating of alloys and the existence of a second order melting transition in metastable alloys subject to concentration fluctuations. Toward the purpose, a novel approach has been adopted which involves simultaneous consideration of the free energy and entropy changes associated with the melting of the fluctuated region. Similar to the well-known Kauzmann paradox in freezing, in principle, there exists an entropy catastrophe in the nucleation of melting in metastable solids, too. However, it would be experimentally difficult to realize the melting of metastable solids because of the tendency to equilibrate at elevated temperatures.

New development in superhard coatings: the superhard nanocrystalline-amorphous composites

The search for superhard materials considered so far mainly metastable solids, such as diamond, c-BN and C 3N 4 and single- and polycrystalline superlattices. More recently, novel superhard nanocrystalline composites have been developed which appear superior to these materials in terms of stability, resistance against oxidation and relatively simple preparation by means of plasma CVD. The present status of the theoretical concept for their design and the experimental results which support it are briefly summarized.

Solution Chemistry for the Elaboration of Solids

This paper is focused on some aspects of the sol-gel process as a solution chemistry route to the preparation of materials. This field is opening new possibilities for molecular chemistry in the synthesis of metastable solids. Three ways emphasizing these possibilities are presented here: (1) the use of precursors containing functional groups for the preparation of solids with specific chemical reactivity leading to metastable phases; (2) the nonhydrolytic sol-gel process for the synthesis of monocomponent oxides with various structures, and of homogeneous bicomponent oxides; (3) organic—inorganic hybrid solids containing covalent bonds. The chemical reactivity of these solids is used as evidence for the presence of substructures which depend on the geometry of the organic moiety.

Infrared Spectroscopic Study of the Crystalline Phases of Sodium Dodecyl Sulfate

Four hydrate forms of sodium dodecyl sulfate (SDS), the 1/8 hydrate, the hemihydrate, the monohydrate, and the dihydrate, and the Rb+ and Mg2+ salts of dodecyl sulfate (RbDS and MgDS) were synthesized under conditions previously used to form crystals for X-ray crystallography. Correlations were made between the IR transmission spectra and crystal structures. IR spectra of 10−75 wt % slurries of DS salts in water were examined at the same concentration and after the same thermal treatments as previously described for “coagel phases” of SDS. The “coagels” of SDS were shown to consist of suspensions of well-characterized crystalline hydrate phases or of metastable solids in the process of conversion to one of the hydrates described; the term “coagel” should be rejected in favor of “crystalline phase”. The crystalline phases of SDS formed under “coagelization” included the hemihydrate, the monohydrate, and the dihydrate. Only the dihydrate was stable (two different forms, interconverted at 19.6 °C) in contact with aqueous solutions at temperatures below 55 °C and compositions below 87.8 wt %, as predicted in the literature. The other hydrates slowly interconverted to the dihydrate when in contact with aqueous solutions, even below 0 °C. The interconversion often required >7 days under the conditions favoring formation of a single hydrate, but did occur in <2 h near 5 °C. The practice of prediction of order in the packing of alkyl chains based on the position of the 2920 cm-1 band was criticized; SDS monohydrate, having one all-trans molecule and one chain with one gauche bond, presented the band at 2915.7 cm-1; the hemihydrate, however, with all-trans bonds presented the band at 2923.2 cm-1. The relative absorbances of the 2850 and 2920 cm-1 peaks in SDS changed drastically with crystallinity and crystal identity. Three of the unit cells (methylene local symmetries, MLS) of the packed all-trans alkyl chains occurring in these phases were novel. The A2/m structures (of SDS mono- and hemihydrate) have been predicted but not previously observed. The MLS of RbDS required four chains to describe the methylene unit cell but is otherwise similar to “hybrid” cells, while the nonintegral chain-to-chain axial offset in the MgDS structure was unique.

The energetics of the GaN MBE reaction: a case study of meta-stable growth

The very large kinetic and thermodynamic barriers present in the GaN system make it ideally suited for studying the fundamental mechanisms involved in synthesizing meta-stable solids. In this paper, the critical factors necessary for the successful growth of GaN thin films under meta-stable conditions are outlined. Experimental results of GaN synthesis by plasma-enhanced Molecular Beam Epitaxy (MBE) are used to illustrate the key steps in the process and to demonstrate the validity of the approach. The issues involved in improving the quality of thin films using meta-stable growth methods are explicitly outlined in order that the method can be successfully applied to other epitaxial systems.

Chemical design of solid inorganic materials

Abstract

Isomeric Differences in the Nucleation of Crystalline Hydrocarbons from Their Melts

Large liquid clusters of the globular hydrocarbon 2,2,3-trimethylbutane were generated in supersonic expansions and frozen, in flight, by evaporative cooling while being monitored by electron diffraction. The nucleation rate rose to 8 × 1028 m-3 s-1 by the time half of the clusters were frozen. Nucleation properties are contrasted with those observed several decades ago by Turnbull and Cormia for three n-alkanes in emulsions. Despite the 15 order of magnitude difference between the rates of nucleation for the globular and linear hydrocarbons, it was possible to make valid comparisons and corroborate Turnbull's inferences that the flexible, linear molecules nucleate by a rather different mechanism. Nucleation of the globular hydrocarbon was similar to that of other simple, rather rigid molecules studied in this laboratory and implied a solid−liquid interfacial free energy of about 4.6 mJ/m2, a value fairly typical for a substance of its (low) heat of fusion. On the other hand, the linear alkanes exhibit interfacial free energies that are twice as great and kinetic prefactors that are considerably greater, presumably because they nucleate in small molecular segments rather than by the association of entire molecules. Unlike several other simple substances which froze to metastable solids when cooled at extremely fast rates in this laboratory, 2,2,3-trimethylbutane froze to its thermodynamically stable form.