Sulfate and molybdate incorporation at the calcite–water interface: insights from ab initio molecular dynamics

Sulfur and molybdenum trace impurities in speleothems (stalagmites and stalactites) can provide long and continuous records of volcanic activity, which are important for past climatic and environmental reconstructions. However, the chemistry governing the incorporation of the trace element-bearing species into the calcium carbonate phases forming speleothems is not well understood. Our previous work has shown that substitution of tetrahedral oxyanions [XO4]2– (X = S and Mo) replacing [CO3]2– in CaCO3 bulk phases (except perhaps for vaterite) is thermodynamically unfavorable with respect to the formation of competing phases, due to the larger size and different shape of the [XO4]2– tetrahedral anions in comparison with the flat [CO3]2– anions, which implied that most of the incorporation would happen at the surface rather than at the bulk of the mineral. Here, we present an ab initio molecular dynamics study, exploring the incorporation of these impurities at the mineral–water interface. We show that the oxyanion substitution at the aqueous calcite (10.4) surface is clearly favored over bulk incorporation, due to the lower structural strain on the calcium carbonate solid. Incorporation at surface step sites is even more favorable for both oxyanions, thanks to the additional interface space afforded by the surface line defect to accommodate the tetrahedral anion. Differences between sulfate and molybdate substitutions can be mostly explained by the size of the anions. The molybdate oxyanion is more difficult to incorporate in the calcite bulk than the smaller sulfate oxyanion. However, when molybdate is substituted at the surface, the elastic cost is avoided because the oxyanion protrudes out of the surface and gains stability via the interaction with water at the interface, which in balance results in more favorable surface substitution for molybdate than for sulfate. The detailed molecular-level insights provided by our calculations will be useful to understand the chemical basis of S- and Mo-based speleothem records

Bandgap engineering in the configurational space of solid solutions via machine learning: (Mg,Zn)O case study

Computer simulations of alloys’ properties often require calculations in a large space of configurations in a supercell of the crystal structure. A common approach is to map density functional theory results into a simplified interaction model using so-called cluster expansions, which are linear on the cluster correlation functions. Alternative descriptors have not been suf-ficiently explored so far. We show here that a simple descriptor based on the Coulomb matrix eigenspectrum clearly outper-forms the cluster expansion both for total energy and bandgap energy predictions in the configurational space of a MgO-ZnO solid solution, a prototypical oxide alloy for bandgap engineering. Bandgap predictions can be further improved by introducing non-linearity via gradient-boosted decision trees or neural networks based on the Coulomb matrix descriptor

Intranasal peptide-induced tolerance and linked suppression: consequences of complement deficiency.

A role for complement, particularly the classical pathway, in the regulation of immune responses is well documented. Deficiencies in C1q or C4 predispose to autoimmunity, while deficiency in C3 affects the suppression of contact sensitization and generation of oral tolerance. Complement components including C3 have been shown to be required for both B-cell and T-cell priming. The mechanisms whereby complement can mediate these diverse regulatory effects are poorly understood. Our previous work, using the mouse minor histocompatibility (HY) model of skin graft rejection, showed that both C1q and C3 were required for the induction of tolerance following intranasal peptide administration. By comparing tolerance induction in wild-type C57BL/6 and C1q-, C3-, C4- and C5-deficient C57BL/6 female mice, we show here that the classical pathway components including C3 are required for tolerance induction, whereas C5 plays no role. C3-deficient mice failed to generate a functional regulatory T (Treg) -dendritic cell (DC) tolerogenic loop required for tolerance induction. This was related to the inability of C3-deficient DC to up-regulate the arginine-consuming enzyme, inducible nitric oxide synthase (Nos-2), in the presence of antigen-specific Treg cells and peptide, leading to reduced Treg cell generation. Our findings demonstrate that the classical pathway and C3 play a critical role in the peptide-mediated induction of tolerance to HY by modulating DC function

Understanding X-ray photoelectron spectra of ionic liquids: experiments and simulations of 1-butyl-3-methylimidazolium thiocyanate

We demonstrate a combined experimental and computational approach to probe the electronic structure and atomic environment of an ionic liquid, based on core level binding energies. The 1-butyl-3-methylimidazolium thiocyanate [C4C1Im][SCN] ionic liquid was studied using ab initio molecular dynamics, and results were compared against previously published and new experimental X-ray photoelectron spectroscopy (XPS) data. The long-held assumption that initial-state effects in XPS dominate the measured binding energies is proven correct, which validates the established premise that the ground state electronic structure of the ionic liquid can be inferred directly from XPS measurements. A regression model based upon site electrostatic potentials and intramolecular bond lengths is shown to account accurately for variations in core-level binding energies within the ionic liquid, demonstrating the important effect of long-range interactions on the core levels and throwing into question the validity of traditional single ion pair ionic liquid calculations for interpreting XPS data

Supramolecular complexation between chain-folding poly(ester-imide)s and polycyclic aromatics: a fractal-based pattern of NMR ring-current shielding

Polycondensation of the diimide-based diols N,N′-bis(2-hydroxyethyl)hexafluoro-isopropylidene-diphthalimide (HFDI), N,N′-bis(2-hydroxyethyl)pyromellitimide (PMDI), or N,N′-bis(2-hydroxyethyl)naphthalene-1,4,5,8-tetracarboxylic-diimide (NDI) with aliphatic diacyl chlorides ClOC(CH2)xCOCl (x = 1 to 8) affords linear poly(ester-imide)s. Homopolymers based on HFDI do not interact with polycyclic aromatics such as pyrene and perylene, as demonstrated by 1H NMR spectroscopy. However, poly(ester-imide)s containing NDI residues show significant upfield complexation shifts of the diimide resonance in the presence of pyrene and perylene, consistent with supramolecular binding of the polycyclic aromatic molecules at the diimide residues. The latter series of poly(ester-imide)s (x = 1 to 8) shows a maximum in complexation shift of the NDI resonance at x = 2. Computational simulations using a density functional based tight-binding (DFTB) method suggest that the maximum at x = 2 is due to the presence of chain folds that are geometrically optimum for a pyrene molecule to bind between pairs of adjacent NDI residues, making near-van-der-Waals contact with both diimide units. As a test of this binding model, 1H NMR studies of pyrene complexation with a copoly(ester-imide) containing both NDI and HFDI units (1 : 1 mole ratio, x = 2) were carried out. The resulting NDI resonance-pattern showed clear evidence of fractal-type character and confirmed tight chain-folding and pairwise binding of pyrene

Novel WS2-based nanofluids for concentrating solar power: performance characterization and molecular-level insights

Nano-colloidal suspensions of nanomaterials in a fluid, nanofluids, are appealing because of their interesting properties related to heat transfer processes. Whilst nanomaterials based on transition metal chalcogenides (TMCs) have been widely studied in catalysis, sensing, and energy storage applications, there are few studies of nanofluids based on TMCs for heat transfer applications. In this study, the preparation and analysis of nanofluids based on 2D-WS2 in a typical heat transfer fluid (HTF) used in concentrating solar power (CSP) plants is reported. Nanofluids prepared using an exfoliation process exhibited well-defined nanosheets and were highly stable. The nanofluids were characterized in terms of properties related to their application in CSP. The presence of WS2 nanosheets did not modify significantly the surface tension, the viscosity, or the isobaric specific heat, but the thermal conductivity was improved by up to 30%. The Ur factor, which characterizes the thermal efficiency of the fluid in the solar collector, shows an enhancement of up to 22% in the nanofluid, demonstrating great promise for CSP applications. The Reynolds number and friction factor of the fluid were not significantly modified by the addition of the nanomaterial to the HTF, which is also positive for practical applications in CSP plants. Ab initio molecular dynamics simulations of the nanoparticle/fluid interface showed an irreversible dissociative adsorption of diphenyl oxide molecules on the WS2 edge, with very low kinetic barrier. The resulting ‘decoration’ of the WS2 edge dramatically affects the nature of the interface interactions and is therefore expected to affect significantly the rheological and transport properties of the nanofluids

Creationism and Intelligent Design

Until recently, little attention has been paid in the school classroom to creationism and almost none to intelligent design. However, creationism and intelligent design appear to be on the increase and there are indications that there are more countries in which schools are becoming battlegrounds over them. I begin by examining whether creationism and intelligent design are controversial issues, drawing on Robert Dearden’s epistemic criterion of the controversial and more recent responses to and defences of this. I then examine whether the notion of ‘worldviews’ in the context of creationism is a useful one by considering the film March of the Penguins. I conclude that the ‘worldviews’ perspective on creationism is useful for two reasons: first, it indicates the difficulty of using the criterion of reason to decide whether an issue is controversial or not; secondly, it suggests that standard ways of addressing the diversity of student views in a science classroom may be inadequate. I close by examining the implications of this view for teaching in science lessons and elsewhere, for example in religious education lessons and at primary level where subject divisions cannot be made in so clear-cut a manner

Quantifying the Emergence of Dengue in Hanoi, Vietnam: 1998–2009

Dengue is the most common vector-borne viral disease of humans, causing an estimated 50 million cases per year. The number of countries affected by dengue has increased dramatically in the last 50 years and dengue is now a major public health problem in large parts of the tropical and subtropical world. It is of considerable importance to understand the factors that determine how dengue becomes newly established in areas where the risk of dengue was previously small. Hanoi in North Vietnam is a large city where dengue appears to be emerging. We analyzed 12 years of dengue surveillance data in order to characterize the temporal and spatial epidemiology of dengue in Hanoi and to establish if dengue incidence has been increasing. After excluding the two major outbreak years of 1998 and 2009 and correcting for changes in population age structure over time, we found there was a significant annual increase in the incidence of notified dengue cases over the period 1999–2008. Dengue cases were concentrated in young adults in the highly urban central areas of Hanoi. This study indicates that dengue transmission is increasing in Hanoi and provides a platform for further studies of the underlying drivers of this emergence

Climate Change and the Future of California's Endemic Flora

The flora of California, a global biodiversity hotspot, includes 2387 endemic plant taxa. With anticipated climate change, we project that up to 66% will experience >80% reductions in range size within a century. These results are comparable with other studies of fewer species or just samples of a region's endemics. Projected reductions depend on the magnitude of future emissions and on the ability of species to disperse from their current locations. California's varied terrain could cause species to move in very different directions, breaking up present-day floras. However, our projections also identify regions where species undergoing severe range reductions may persist. Protecting these potential future refugia and facilitating species dispersal will be essential to maintain biodiversity in the face of climate change

Molybdenum and sulfur incorporation as oxyanion substitutional impurities in calcium carbonate minerals: a computational investigation

Marked increases in sulfur and molybdenum concentration in stalagmites have been proposed as possible evidence of volcanic activity in the past. Thus, speleothems have great potential to deliver long and continuous records of volcanic activity. However, little is known about the chemical nature of these impurities in the calcium carbonate (CaCO3) phases forming stalagmites, which hinders the rationalization of the incorporation mechanisms. While sulfur is known to incorporate as a sulfate anion in CaCO3 polymorphs, the nature and stability of molybdenum incorporation in these minerals has not been investigated yet. Here, we present a computer simulation study, based on density functional theory, comparing the thermodynamics of incorporation of sulfur and molybdenum as tetrahedral oxyanions [XO4]2- (X=S, Mo) in anion sites of CaCO3 polymorphs (calcite, aragonite, vaterite, monohydrocalcite and ikaite). Among the different polymorphs, vaterite incorporates [XO4]2- ions most favourably, which reflects the relatively low density of this carbonate phase. We show that molybdate anions are very unstable (more so than sulfate anions) in the bulk of all three anhydrous carbonate phases, with respect to the formation of naturally occurring competing phases. Most of the Mo impurities found in typical calcite/aragonite stalagmites is therefore likely to concentrate at surface/interface regions such as grain boundaries. Using the calcite (10.4) surface as a model, we show that the energies of substitution are indeed much lower at the surface than at the bulk. Our results suggest that factors affecting the crystallinity of CaCO3 in stalagmites, and therefore the specific surface area, will have a significant effect on the concentration of incorporated molybdenum, which should be a key consideration when interpreting data from Mo-based speleothem archives