Pseudo-equilibrium Conditions Research Articles

Modeling and validation of the CO2 degassing effect on CaCO3 precipitation using oilfield data

Calcium carbonate scaling can become a challenge for Brazilian Pre-Salt oil production. The high CO2 content that makes up the gas stream in carbonate reservoirs is remarkable. The relationship between CO2 and CaCO3 is evident in the chemical balance: Ca2+(aq) + 2 HCO3–(aq) ⇆ H2O + CaCO3(s) + CO2(aq). Aqueous CO2 present in carbonate reservoir acidifies the medium and keeps CaCO3 dissolved. The head loss provided by oil production moves CO2(aq) to the gaseous state (CO2(aq) → CO2(g)). It disfavors calcium carbonate to keep dissolved, which implies CaCO3 precipitation. The two mentioned chemical reactions are closely linked and are mutually dependent. A previous thermodynamic study carried out by our group, considering a pseudo-equilibrium state, estimated that CO2 degassing is responsible for 61% to 89% of calcium carbonate scaling. However, it is not possible to carry out laboratory experiments with a pseudo-equilibrium condition. Therefore, a methodology was developed based on experimental feasibility to overcome this issue, making possible its application with computational packages. The software includes not only the aqueous solution thermodynamics but also the interaction with the hydrocarbon phases. An applied review of the role of CO2 on precipitation, considering aqueous, oily, and gaseous phases, was carried out with real-field data of Pre-Salt to support the analysis. Likewise for aqueous CO2, oily CO2 degassing (CO2(o) → CO2(g)) also implies CaCO3 scaling, and the CO2 partitioning between the three phases is a phenomenon that critically governs the precipitation. Regarding the simulation results, the percentages that we previously found were confirmed. In runs with real-field data, the results fell within the theoretical percentage range but indicated a distinct behavior from the influence of CO2 with the change in BSW. These discoveries correspond to some operational problems found in oilfields and may help to explain their still unknown causes.

Catalyst Design by Machine Learning and Multiobjective Optimization

The computer technologies of machine learning and multiobjective optimization were introduced to develop the catalyst for fluid catalytic cracking (FCC). Response surface methodology was applied for a training set consisting of 1000 data points with varied catalyst compositions which consist of a variety of catalysts compositions, feedstock properties, pseudo-equilibrium conditions, cracking performance test conditions as input parameters and the cracking test results as outputs. At first, response surface model (RSM) was obtained with four approximation methods, among which the radial basis function (RBF) method was found to give the highest score accurate RSM with the smallest average error and the highest coefficient of determination among them. Then the virtual experiments were carried out with the RSM applied with multiobjective genetic algorithm (MOGA) to optimize the catalyst design considering the multiobjective; to yield less bottoms, less coke, more gasoline and less gas. After 5000 virtual experiments with RSM were carried out, we found that the pareto front was obtained. Finally, the optimum catalyst design was selected from the designs on the pareto front. As a result, the selected catalyst design showed 2.7 % higher gasoline yield and was confirmed to show the excellent performance over conventional FCC catalyst.

Application domain extension of incremental capacity-based battery SoH indicators

The Incremental Capacity (IC) analysis is used to characterise the capacity and the battery state of health, aged by cycling patterns with randomly selected pulsed current levels and duration. The batteries are periodically characterised at 1C current, which is a high value with respect to the typical IC tests in pseudo-equilibrium condition. The high-current IC curves generation from raw voltage/current data includes two filtering stages, one for the input voltage and one for the incremental capacity curve smoothing, which are optimised for the application on the basis of the data characteristics. The correlations between the IC main peak features and the battery full capacity for 28 Lithium–Cobalt oxide batteries with 18650 packaging were evaluated, finding that the main peak area is a general feature to evaluate the state of health under high current tests and random usage pattern, and, therefore, it can be used as a battery health indicator in practical applications. The effects of the computational parameters on the relationship between the peak area and the battery capacity are also investigated. The results are confirmed by a further analysis performed over an additional set of cells with different technology, aged with a fixed cycling pattern. Additionally, the performance of the peak area as a health indicator was compared with an ohmic resistance-based estimation approach.

Stimuli-Induced Nonequilibrium Phase Transitions in Polyelectrolyte-Surfactant Complex Coacervates.

Polyelectrolyte-surfactant complexes (PESCs) are important soft colloids with applications in the fields of personal care, cosmetics, pharmaceutics, and much more. If their phase diagrams have long been studied under pseudoequilibrium conditions, and often inside the micellar or vesicular regions, understanding the effect of nonequilibrium conditions, applied at phase boundaries, on the structure of PESCs generates an increasing interest. In this work we cross the micelle-vesicle and micelle-fiber phase boundaries in an isocompositional surfactant-polyelectrolyte aqueous system through a continuous and rapid variation of pH. We employ two microbial glycolipid biosurfactants in the presence of polyamines, both systems being characterized by their responsiveness to pH. We show that complex coacervates (Co) are always formed in the micellar region of both glycolipids' phase diagram and that their phase behavior drives the PESC stability and structure. However, for glycolipid forming single-wall vesicles, we observe an isostructural and isodimensional transition between complex coacervates and a multilamellar walls vesicle (MLWV) phase. For the fiber-forming glycolipid, on the contrary, the complex coacervate disassembles into free polyelectrolyte coexisting with the equilibrium fiber phase. Last but not least, this work also demonstrates the use of microbial glycolipid biosurfactants in the development of sustainable PESCs.

Synthesis of multilamellar walls vesicles polyelectrolyte-surfactant complexes from pH-stimulated phase transition using microbial biosurfactants

Multilamellar wall vesicles (MLWV) are an interesting class of polyelectrolyte-surfactant complexes (PESCs) for wide applications ranging from house-care to biomedical products. If MLWV are generally obtained by a polyelectrolyte-driven vesicle agglutination under pseudo-equilibrium conditions, the resulting phase is often a mixture of more than one structure. In this work, we show that MLWV can be massively and reproductively prepared from a recently developed method involving a pH-stimulated phase transition from a complex coacervate phase (Co). We employ a biobased pH-sensitive microbial glucolipid biosurfactant in the presence of a natural, or synthetic, polyamine (chitosan, poly-l-Lysine, polyethylene imine, polyallylamine). In situ small angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) show a systematic isostructural and isodimensional transition from the Co to the MLWV phase, while optical microscopy under polarized light experiments and cryo-TEM reveal a massive, virtually quantitative, presence of MLWV. Finally, the multilamellar wall structure is not perturbed by filtration and sonication, two typical methods employed to control size distribution in vesicles. In summary, this work highlights a new, robust, non-equilibrium phase-change method to develop biobased multilamellar wall vesicles, promising soft colloids with applications in the field of personal care, cosmetics and pharmaceutics among many others.

A Novel Analysis of Transient Isothermal 18O Isotopic Exchange on Commercial CexZr1−xO2-Based OSC Materials

A novel methodology for the analysis of 18O2-transient isothermal isotopic exchange (TIIE) response curves obtained over commercial mixed metal oxides upon the gas switch 16O2/Ar (30 min) → 18O2/Kr/Ar (t) at constant T in a flow CSTR micro-reactor in the absence of external and internal mass transport resistances and grain boundary effects, is presented for the first time. The specific aim of this analysis was to derive quantitative descriptors of the16O/18O exchange under pseudo-equilibrium conditions for the transfer rates of oxygen between the gas and solid phases, enabling the assessment of differences in the kinetics of bulk oxygen diffusion over a series of commercial OSC materials used in various catalytic applications (e.g. TWC), without the need of performing any mathematical and kinetic modelling of the surface/bulk oxygen diffusion processes. Based on these descriptors, commercial OSC mixed metal oxides prepared by the same chemical synthesis route but varying the value of one of the synthesis parameters, and having the same at.% composition, were proved to possess largely different transient kinetic rates and other features of the transient response curves of 16O/18O exchange strongly related to the bulk oxygen diffusion in the solids. This novel 18O2-TIIE analysis is quick and provides a reliable means for proving or disproving likely infringement of patented commercial mixed metal oxides OSC materials.

Thermo-kinetic investigation of heavy metal ions adsorption onto lignin considering coupled adsorption–desorption mechanisms: Modeling and experimental validation

A coupled adsorption–desorption thermo-kinetic model is developed incorporating both adsorption and desorption reactions. A local pseudo-equilibrium condition at the interface of adsorbent and adsorbate bulk phases was used as isotherm equation which can even be applied for multi-pollutants scenarios. The developed model is then validated using collected experimental data of heavy metal ions (Pb, Cu, Cd, Zn, and Ni). Comparisons were made for a number of isotherm and kinetic models to examine the performance of the proposed model. The developed model revealed desirable accuracy and superiority over other models in predicting the adsorption behavior and can be used for other systems of concern. The model correlates the adsorption kinetic with an [Formula: see text] value of 0.9391 and desorption kinetic with an [Formula: see text] value of 0.9383. By application of the proposed model to any available adsorption datasets, the individual characteristics of adsorption and desorption can be determined.

Theory of First Order Chemical Kinetics at the Critical Point of Solution.

Liquid mixtures, which have a phase diagram exhibiting a miscibility gap ending in a critical point of solution, have been used as solvents for chemical reactions. The reaction rate in the forward direction has often been observed to slow down as a function of temperature in the critical region. Theories based upon the Gibbs free energy of reaction as the driving force for chemical change have been invoked to explain this behavior. With the assumption that the reaction is proceeding under relaxation conditions, these theories expand the free energy in a Taylor series about the position of equilibrium. Since the free energy is zero at equilibrium, the leading term in the Taylor series is proportional to the first derivative of the free energy with respect to the extent of reaction. To analyze the critical behavior of this derivative, the theories exploit the principle of critical point isomorphism, which is thought to govern all critical phenomena. They find that the derivative goes to zero in the critical region, which accounts for the slowing down observed in the reaction rate. As has been pointed out, however, most experimental rate investigations have been carried out under irreversible conditions as opposed to relaxation conditions [Shen et al. J. Phys. Chem. A 2015, 119, 8784-8791]. Below, we consider a reaction governed by first order kinetics and invoke transition state theory to take into account the irreversible conditions. We express the apparent activation energy in terms of thermodynamic derivatives evaluated under standard conditions as well as the pseudoequilibrium conditions associated with the reactant and the activated complex. We show that these derivatives approach infinity in the critical region. The apparent activation energy follows this behavior, and its divergence accounts for the slowing down of the reaction rate.

Kinetic control on Zn isotope signatures recorded in marine diatoms

Marine diatoms dominate the oceanic cycle of the essential micronutrient zinc (Zn). The stable isotopes of zinc and other metals are increasingly used to understand trace metal micronutrient cycling in the oceans. One clear feature of the early isotope data is the heavy Zn isotope signature of the average oceanic dissolved pool relative to the inputs, potentially driven by uptake of light isotopes into phytoplankton cells and export to sediments. However, despite the fact that diatoms strip Zn from surface waters across the Antarctic polar front in the Southern Ocean, the local upper ocean is not isotopically heavy. Here we use culturing experiments to quantify the extent of Zn isotope fractionation by diatoms and to elucidate the mechanisms driving it.We have cultured two different open-ocean diatom species (T. oceanica and Chaetoceros sp.) in a series of experiments at constant medium Zn concentration but at bioavailable medium Fe ranging from limiting to replete. We find that T. oceanica can maintain high growth rates and Zn uptake rates over the full range of bioavailable iron (Fe) investigated, and that the Zn taken up has a δ66Zn that is unfractionated relative to that of the bioavailable free Zn in the medium. The studied representative of the genus Chaetoceros, on the other hand, shows more significantly reduced Zn uptake rates at low Fe and records more variable biomass δ66Zn signatures, of up to 0.85‰ heavier than the medium. We interpret the preferential uptake of heavy isotopes at extremely low Zn uptake rates as potentially due to either of the following two mechanisms. First, the release of extracellular polymeric substances (EPS), at low Fe levels, may preferentially scavenge heavy Zn isotopes. Second, the Zn uptake rate may be slow enough to establish pseudo-equilibrium conditions at the transporter site, with heavy Zn isotopes forming more stable surface complexes.Thus we find that, in our experiments, Fe-limitation exerts a key control that not only limits diatom growth, but also affects the Zn uptake physiology of diatoms. Uptake of heavy isotopes occurs under Fe-limiting conditions that drive extremely low Zn uptake rates. On the other hand, more rapid Zn uptake rates result in biomass that is indistinguishable from the external bioavailable free Zn pool. These experimental results can, in principle, explain the range of Zn isotopic compositions found in the real surface ocean, given the geographically variable interplay between Fe-limitation, Zn uptake rates, and the degree of organic complexation of oceanic Zn.

Study of 223Ra uptake mechanism by Fe3O4 nanoparticles: towards new prospective theranostic SPIONs

The use of superparamagnetic iron oxide nanoparticles (SPIONs) and radiolabelled nanoparticles (NPs) has grown considerably over the recent years, and the SPIONs labelled with medicinal radionuclides offer new opportunities in multimodal diagnostics and in the drug-delivery systems for targeted alpha-particle therapy (TAT) driven by magnetic field gradient or by biologically active moieties bound on NPs shell. However, the mechanisms of NPs radiolabelling are not studied substantially and still remain unclear, even though the way of label attachment directly implies the stability of the label-nanoparticle construct. Since the 223Ra was the first clinically approved alpha-emitter, it is a promising nuclide for further development of its targeted carriers. We report here on the study of 223Ra uptake by the Fe3O4 SPIONs, together with an attempt to propose the 223Ra uptake mechanism by the Fe3O4 NPs in the presence of a phosphate buffer a typical formulation medium, under the pseudo-equilibrium conditions. Further, the in vitro stability tests of the prepared [223Ra]Fe3O4 NPs were performed to estimate the 223Ra label stability. The potential use of 223Ra-labelled SPIONs in theranostic applications is also discussed.

Struvite precipitation under changing ionic conditions in synthetic wastewater: Experiment and modeling

A computational model was developed and applied to investigate struvite precipitation under different pH levels and ionic concentrations. Ionic species, including ammonium (NH4+), hydrogen phosphate (HPO42−), magnesium (Mg2+), calcium (Ca2+), and struvite, were incorporated into the proposed model. The unknown kinetic coefficients of struvite were identified from the experimental data. In this study, the kinetics of struvite precipitation, which combined the ionic reactions (NH4+/NH3 and HPO42−/H2PO4−) on the basis of pseudo-equilibrium conditions was parameterized. The experiments for model verification were conducted at a constant initial Mg/P ratio with changing ionic concentrations at pH levels of 8.7 and 9.7. The batch experiments showed high struvite precipitation (>90% for 300mg-NL−1 and 100mg-PL−1). The presence of Ca2+ (Ca2+/Mg2+ >0.5) in the reactors interfered with the formation and growth of struvite. The decrease in the pH level with the struvite precipitation verified the simulation data. The model also confirmed the optimal ionic conditions in order to maximize the struvite precipitation (300mg-PL−1, and N/P molar ratio >7). The model responding to the ionic conditions provided good prediction of the decrease in the pH levels and the positive role of the nitrogen levels for struvite precipitation. High nitrogen concentrations provided high P removal due to pH buffering and crystal purity.

Modeling packed bed sorbent systems with the Pore Surface Diffusion Model: Evidence of facilitated surface diffusion of arsenate in nano-metal (hydr)oxide hybrid ion exchange media

This study explores the possibility of employing the Pore Surface Diffusion Model (PSDM) to predict the arsenic breakthrough curve of a packed bed system operated under continuous flow conditions with realistic groundwater, and consequently minimize the need to conduct pilot scale tests. To provide the nano-metal (hydr)oxide hybrid ion exchange media's performance in realistic water matrices without engaging in taxing pilot scale testing, the multi-point equilibrium batch sorption tests under pseudo-equilibrium conditions were performed; arsenate breakthrough curve of short bed column (SBC) was predicted by the PSDM in the continuous flow experiments; SBC tests were conducted under the same conditions to validate the model. The overlapping Freundlich isotherms suggested that the water matrix and competing ions did not have any denoting effect on sorption capacity of the media when the matrix was changed from arsenic-only model water to real groundwater. As expected, the PSDM provided a relatively good prediction of the breakthrough profile for arsenic-only model water limited by intraparticle mass transports. In contrast, the groundwater breakthrough curve demonstrated significantly faster intraparticle mass transport suggesting to a surface diffusion process, which occurs in parallel to the pore diffusion. A simple selection of DS=1/2 DP appears to be sufficient when describing the facilitated surface diffusion of arsenate inside metal (hydr)oxide nano-enabled hybrid ion-exchange media in presence of sulfate, however, quantification of the factors determining the surface diffusion coefficient's magnitude under different treatment scenarios remained unexplored.

Effect of humic acid on the adsorption/desorption behavior of glyphosate on goethite. Isotherms and kinetics.

The effects of humic acid (HA) on the adsorption/desorption of glyphosate (Gly) on goethite were investigated under pseudo equilibrium conditions by adsorption isotherms and under kinetic conditions by ATR-FTIR spectroscopy. Isotherms reveal that the attachment of Gly is almost completely inhibited by HA molecules. The opposite effect is not observed: HA adsorption is not affected by the presence of Gly. ATR-FTIR allowed the simultaneous detection of adsorbed HA and Gly during kinetic runs, revealing that HA at the surface decreases markedly the adsorption rate of Gly likely as a result of a decreased availability of sites for Gly adsorption and because of electrostatic repulsion. In addition, HA in solution increases the desorption rate of Gly. The rate law for Gly desorption could be determined giving important insights on the desorption mechanism. The herbicide is desorbed by two parallel processes: i) a direct detachment from the surface, which is first order in adsorbed Gly; and ii) a ligand exchange with HA molecules, which is first order in adsorbed Gly and first order in dissolved HA. Rate constants for both processes were quantified, leading to half-lives of 3.7 h for the first process, and 1.4 h for the second process in a 400 mg L(-1) HA solution. These data are important for modeling the dynamics of glyphosate in environmentally relevant systems, such as soils and surface waters.

Time Recovery for a Complex Process Using Accelerated Dynamics.

The hyperdynamics method (HD) developed by Voter (J. Chem. Phys. 1996, 106, 4665) sets the theoretical basis to construct an accelerated simulation scheme that holds the time scale information. Since HD is based on transition state theory, pseudoequilibrium conditions (PEC) must be satisfied before any system in a trapped state may be accelerated. As the system evolves, many trapped states may appear, and the PEC must be assumed in each one to accelerate the escape. However, since the system evolution is a priori unknown, the PEC cannot be permanently assumed to be true. Furthermore, the different parameters of the bias function used may need drastic recalibration during this evolution. To overcome these problems, we present a general scheme to switch between HD and conventional molecular dynamics (MD) in an automatic fashion during the simulation. To decide when HD should start and finish, criteria based on the energetic properties of the system are introduced. On the other hand, a very simple bias function is proposed, leading to a straightforward on-the-fly set up of the required parameters. A way to measure the quality of the simulation is suggested. The efficiency of the present hybrid HD-MD method is tested for a two-dimensional model potential and for the coalescence process of two nanoparticles. In spite of the important complexity of the latter system (165 degrees of freedoms), some relevant mechanistic properties were recovered within the present method.

In vivo evaluation of [123I]MNI-420: A novel single photon emission computed tomography radiotracer for imaging of adenosine 2A receptors in brain

Dysregulation of adenosine 2A (A2A) receptor function in brain has been implicated in multiple psychiatric and neurodegenerative disorders, including schizophrenia and Parkinson's disease, making the development of an imaging agent to study A2A receptors in both healthy brain and disease states desirable. In this study, [(123)I]MNI-420 was evaluated as a potential single photon emission computed tomography (SPECT) radiotracer for imaging A2A receptors in brain. Two adult male monkeys (Macaca fascicularis) and three adult female baboons (Papio anubis) were anesthetized and imaged on Neurofocus SPECT cameras. Baboons underwent baseline and displacement studies using varying doses of caffeine (2.0-20mg/kg). Baseline and pre-blocking experiments with multiple doses of preladenant (0.01-1.2mg/kg), a highly selective A2A antagonist, were performed in cynomolgus monkeys. Following bolus intravenous (i.v.) injection, [(123)I]MNI-420 rapidly entered the non-human primate brain. The regional brain accumulation of [(123)I]MNI-420 matched the known distribution of A2A receptors in brain (highest in the striatum). Striatum to cerebellum ratios and binding potentials of around 3.0-3.5 and 2.0-2.5, respectively, were measured in monkey and baboon brain. A dose-dependent occupancy was observed following i.v. injection of caffeine at pseudo-equilibrium conditions during displacement experiments. Pre-treatment with preladenant blocked specific binding in A2A rich regions in a dose-dependent fashion. The data indicate that [(123)I]MNI-420 holds promise as a SPECT radiotracer for imaging A2A receptors in brain and further evaluation is warranted, in order to determine its utility as a SPECT radiotracer for imaging of A2A in brain.

Osmotic Dehydration: Dynamics of Equilibrium and Pseudo-Equilibrium Kinetics

A true equilibrium process usually takes very long to achieve and is very difficult, therefore, a pseudo-equilibrium process is often employed. Several methods exist for predicting pseudo-equilibrium conditions and their accuracies vary. Dynamic equilibrium and pseudo-equilibrium moisture loss (ML) and solids gain (SG) during osmotic dehydration of apple cylinders at different temperature (40, 50, and 60°C) and concentrations (30, 40, 50, and 60°Brix) were evaluated in this study. Pseudo-equilibrium achieved depended on product and processing conditions. Higher concentrations increased the pseudo-equilibrium ML and decreased SG. Two kinds of pseudo-equilibration appear to exist, one for the liquid, which is reached in about 24 h, depending on sample size, the other for the solid matrix, which takes much longer to achieve. Solute penetration in to the product is a much slower process during osmotic dehydration equilibration.

Adsorption and Desorption Kinetics of Naphthalene, Anthracene, and Pyrene in Soil Matrix

The adsorption and desorption kinetics of naphthalene, anthracene, and pyrene in a soil slurry reactor at ambient conditions have been investigated to ascertain the mechanisms controlling the retention and release rates of these compounds in the soil matrix. A stirred-flow method was employed to perform the experiments. The extent of partitioning for the polycyclic aromatic hydrocarbons (PAHs) tested was found to be dependent on their solubility and diffusivity in the aqueous phase. Apparent adsorption and desorption rate coefficients were determined using the Langmuir and Freundlich isotherm models. Equilibrium adsorption and desorption at the external surface and in the internal pore of the soil particle obeyed the Freundlich isotherm equation. The pseudo-equilibrium condition established at the minimum contact time suggests that equilibrium adsorption attained for the contaminant PAHs was not instantaneous but rather time dependent.

Effect of media type and particle size on dissolved organic carbon release from woody filtration media

Sequential batch leaching tests were used to evaluate the mass of DOC released from composted garden organics (yard waste), pine and hardwood under pseudo-equilibrium conditions. All media showed an initial rapid decline in DOC values followed by a slower rate during later time periods. Greater than 50% of the DOC leached occurred within the initial time period (<24h). The mass of DOC leached varied significantly between the materials and to a lesser degree between different particle size ranges. The pine had the lowest leached DOC fraction (2.8–4.8mg/g), while the hardwood (21–27mg/g) and compost (13.6–32.7mg/g) were significantly greater. The type and processing of the woody material incorporated into these systems can have a significant impact on the treated stormwater.

Binding kinetics of 123I[ADAM] in healthy controls: a selective SERT radioligand

[123I]ADAM (2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine) is a promising radioligand for in-vivo quantification of serotonin transporters (SERT) using single photon emission computed tomography (SPECT) in man. We performed tracer kinetic analysis in various brain regions to determine the optimum equilibrium time for SERT quantification with [123I]ADAM and SPECT. Radiosyntheses of [123I]ADAM were performed at MAP Medical Technologies Oy, Tikkakoski, Finland. Thirty healthy male volunteers (21-41 yr) received between 104 and 163 MBq [123I]ADAM intravenously as a bolus. Consecutively, multiple SPECT scans were performed between 14 and 420 min post-injection (p.i.) using a Siemens Multispect 3 camera. Reconstruction was performed applying filtered back projection with a Butterworth filter (cut-off 0.7, order 7) in 128x128 matrices. Regions of interest (ROI) were drawn manually on the individual T1-weighted magnetic resonance image (MRI) comprising midbrain/hypothalamus for specific binding to SERT, and the cerebellum as reference region. After re-orientation to the MRI, the ROI template was applied to SPECT studies. We generated time-activity curves for the ROI and calculated the ratio countstarget/countscerebellum minus 1 (=V3'') as a measure for specific SERT binding. Counts were corrected for applied activity, acquisition time and body-weight. Peak uptakes were observed between 14 and 50 min after bolus injection. Counts per voxel were highest in the midbrain/hypothalamus, 798 (max. 872, min. 728), whereas 462 counts per voxel (max. 599, min. 412) were measured in the cerebellum at a mean time of 31 min p.i. Stable values for V3'' reached 205-320 min p.i. Mean peak V3'' value was 1.43 (95% CI 171-230) for the midbrain/hypothalamus at 205 min p.i. [123I]ADAM is a useful ligand for in-vivo quantification of human SERT by means of SPECT, with a comparatively better signal-to-noise ratio compared to beta-CIT. Our data suggest that the acquisition time for the SPECT scan is optimally, under pseudo-equilibrium conditions, between 205-320 min post-bolus injection of the tracer.

Experimental Gibbs Free Energy Considerations in the Nucleation and Growth of Single-Walled Carbon Nanotubes

Gas feed composition and reaction temperature were varied to identify the thermodynamic threshold conditions for the nucleation and growth of SWNT from methane on supported Fe/Mo catalyst. These reaction conditions closely approximate the pseudoequilibrium conditions that lead to the nucleation and growth of SWNT. These measurements also serve to determine an upper limit of the Gibbs free energy of formation for SWNT. The Gibbs free energy of formation relative to graphite is in good agreement with literature values predicted from simulations for SWNT nuclei containing approximately 80 atoms, while considerably larger than that predicted for bulk (5,5) SWNT. Our estimate over the range 700 to 1000 degrees C of 16.1 to 13.9 kJ mol(-1) falls between the results of these simulations and literature values for diamond.