Experimental determination of the NH4NO3/(NH4)2SO4/H2O phase diagram

We have studied the thermodynamic properties of the ammonium nitrate/ammonium sulfate/water system using differential scanning calorimetry and infrared spectroscopy of thin films at low temperatures. This is the first study focused on low temperatures, as previous experimental work on this system has been at 273 K and above. We have combined our experimental results with melting point data from the literature at high temperatures to create a solid/liquid phase diagram of the ammonium nitrate/ammonium sulfate/water system for temperatures below 343 K. Using phase diagram theory and Alkemade lines, we predict which solids are stable at equilibrium for all concentrations within the studied region. We also observed the decomposition of a solid at low temperatures which has not previously been reported. Finally, we have compared our predicted solids and final melting temperatures to the Aerosol Inorganics Model (AIM).     

Dynamical reweighting: improved estimates of dynamical properties from simulations at multiple temperatures

Dynamical averages based on functionals of dynamical trajectories, such as time-correlation functions, play an important role in determining kinetic or transport properties of matter. At temperatures of interest, the expectations of these quantities are often dominated by contributions from rare events, making the precise calculation of these quantities by molecular dynamics simulation difficult. Here, we present a reweighting method for combining simulations from multiple temperatures (or from simulated or parallel tempering simulations) to compute an optimal estimate of the dynamical properties at the temperature of interest without the need to invoke an approximate kinetic model (such as the Arrhenius law). Continuous and differentiable estimates of these expectations at any temperature in the sampled range can also be computed, along with an assessment of the associated statistical uncertainty. For rare events, aggregating data from multiple temperatures can produce an estimate with the desired precision at greatly reduced computational cost compared with simulations conducted at a single temperature. Here, we describe use of the method for the canonical (NVT) ensemble using four common models of dynamics (canonical distribution of Hamiltonian trajectories, Andersen thermostatting, Langevin, and overdamped Langevin or Brownian dynamics), but it can be applied to any thermodynamic ensemble provided the ratio of path probabilities at different temperatures can be computed. To illustrate the method, we compute a time-correlation function for solvated terminally-blocked alanine peptide across a range of temperatures using trajectories harvested using a modified parallel tempering protocol.     

Computer simulation study of the phase behavior and structural relaxation in a gel-former modeled by three-body interactions

We report a computer simulation study of a model gel-former obtained by modifying the three-body interactions of the Stillinger-Weber potential for silicon. This modification reduces the average coordination number and consequently shifts the liquid-gas phase coexistence curve to low densities, thus facilitating the formation of gels without phase separation. At low temperatures and densities, the structure of the system is characterized by the presence of long linear chains interconnected by a small number of three coordinated junctions at random locations. At small wave vectors the static structure factor shows a nonmonotonic dependence on temperature, a behavior which is due to the competition between the percolation transition of the particles and the stiffening of the formed chains. We compare in detail the relaxation dynamics of the system as obtained from molecular dynamics with the one obtained from Monte Carlo dynamics. We find that the bond correlation function displays stretched exponential behavior at moderately low temperatures and densities, but exponential relaxation at low temperatures. The bond lifetime shows an Arrhenius behavior, independent of the microscopic dynamics. For the molecular dynamics at low temperatures, the mean squared displacement and the (coherent and incoherent) intermediate scattering function display at intermediate times a dynamics with ballistic character and we show that this leads to compressed exponential relaxation. For the Monte Carlo dynamics we always find an exponential or stretched exponential relaxation. Thus we conclude that the compressed exponential relaxation observed in experiments is due to the out-of-equilibrium dynamics.     

Temperature-dependent shift from labile to recalcitrant carbon sources of arctic heterotrophs

Soils of high latitudes store approximately one-third of the global soil carbon pool. Decomposition of soil organic matter (SOM) is expected to increase in response to global warming, which is most pronounced in northern latitudes. It is, however, unclear if microorganisms are able to utilize more stable, recalcitrant C pools, when labile soil carbon pools will be depleted due to increasing temperatures. Here we report on an incubation experiment with intact soil cores of a frost-boil tundra ecosystem at three different temperatures (2 degrees C, 12 degrees C and 24 degrees C). In order to assess which fractions of the SOM are available for decomposition at various temperatures, we analyzed the isotopic signature of respired CO2 and of different SOM fractions. The delta13C values of CO2 respired were negatively correlated with temperature, indicating the utilization of SOM fractions that were depleted in 13C at higher temperatures. Chemical fractionation of SOM showed that the water-soluble fraction (presumably the most easily available substrates for microbial respiration) was most enriched in 13C, while the acid-insoluble pool (recalcitrant substrates) was most depleted in 13C. Our results therefore suggest that, at higher temperatures, recalcitrant compounds are preferentially respired by arctic microbes. When the isotopic signatures of respired CO2 of soils which had been incubated at 24 degrees C were measured at 12 degrees C, the delta13C values shifted to values found in soils incubated at 12 degrees C, indicating the reversible use of more easily available substrates. Analysis of phospholipid fatty acid profiles showed significant differences in microbial community structure at various incubation temperatures indicating that microorganisms with preference for more recalcitrant compounds establish as temperatures increase. In summary our results demonstrate that a large portion of tundra SOM is potentially mineralizable.     

How far are luminescence properties predictable?

Our knowledge of the luminescence of isolators has increased considerably during the past decade. As a consequence it has become possible to understand the luminescence of technically important phosphors and even to predict efficient luminescent materials. We first illustrate how emission spectra of a given activator can be varied by changing the host lattice. Second we consider the factors influencing the luminescent efficiency. Using a single-configurational coordinate model, we have performed calculations on a model system. These illustrate which factors are important for efficiency. Some clear results are reported. Finally, we discuss the mobility of excitation energy. In some phosphors the excitation is highly mobile even at low temperatures. As a consequence emission originates from centers which trap the excitation energy. In other phosphors this mobility is low, and at low temperatures emission occurs from the regular luminescence centers. At higher temperatures the excitation energy can become mobile because of thermal activation.     

Spectroscopic studies on the effect of temperature on pH-induced folded states of human serum albumin

Human serum albumin (HSA) is a very important multi-domain transporter protein in the circulatory system responsible for carriage of various kinds of ligands within the physiological system. HSA is also known to undergo conformational transformation at different pH(s) and temperatures. In this report we have studied the binding interactions of a photosensitizing drug, protoporphyrin IX (PPIX) with various conformers of HSA at different temperatures using picosecond time-resolved fluorescence spectroscopy. Also, using dynamic light scattering (DLS) and circular dichroism (CD) spectroscopy we have followed the structural transition of various conformers of HSA at different temperatures. Ensuring the intact binding of PPIX to various conformers of HSA at different temperatures as revealed through time-resolved fluorescence anisotropy decay and significant spectral overlap of emission of Trp214 residue (donor) in domain-IIA and absorption of PPIX (acceptor) bound to domain-IB of HSA, we have applied F?rster's resonance energy transfer (FRET) technique to determine the interdomain separation under various environmental conditions. The alkali-induced conformer of HSA shows almost no change in donor-acceptor distance in contrast to the native and acid-induced conformers of HSA, which show a decrease in distance with increase in temperature. Through this study the non-covalently bound PPIX is shown to be an efficient FRET probe in reporting the different temperature-induced folded states of HSA in buffer solutions of widely differing pH values.     

A unified theory for charge-carrier transport in organic crystals

To characterize the crossover from bandlike transport to hopping transport in molecular crystals, we study a microscopic model that treats electron-phonon interactions explicitly. A finite-temperature variational method combining Merrifield's transformation with Bogoliubov's theorem is developed to obtain the optimal basis for an interacting electron-phonon system, which is then used to calculate the bandlike and hopping mobilities for charge carriers. Our calculations on the one dimensional (1D) Holstein model at T=0 K and finite temperatures show that the variational basis gives results that compared favorably to other analytical methods. We also study the structures of polaron states at a broad range of parameters including different temperatures. Furthermore, we calculate the bandlike and hopping mobilities of the 1D Holstein model in different parameters and show that our theory predicts universal power-law decay at low temperatures and an almost temperature independent behavior at higher temperatures, in agreement with experimental observations. In addition, we show that as the temperature increases, hopping transport can become dominant even before the polaron state changes its character. Thus, our result indicates that the self-trapping transition studied in conventional polaron theories does not necessarily correspond to the bandlike to hopping transition in the transport properties in organic molecular crystals. Finally, a comparison of our 1D results with experiments on ultrapure naphthalene crystals suggests that the theory can describe the charge-carrier mobilities quantitatively across the whole experimental temperature range.     

Melting kinetics of it-polypropylene crystals over wide heating rates

Melting kinetics of it-polypropylene crystals has been examined over wide heating rates of 0.6 K min^?1?10⁴ K s^?1 using a standard DSC and a fast-scan DSC. With fast-scan DSC, we have an access to the melting of crystals obtained at low temperatures, which are susceptible to re-organization at the heating rates applicable with standard DSC. It is clearly discernible that the appearance and disappearance of multiple melting peaks are strongly influenced by the applied heating rates and dependent on the crystallization temperatures. By examining the heating rate dependence of superheating of melting, we have determined the melting points of as-grown crystals formed under wide crystallization temperatures.     

From sulfur-amine solutions to metal sulfide nanocrystals: peering into the oleylamine-sulfur black box

In this Article, we present our findings on the formation of metal sulfide nanocrystals from sulfur-alkylamine solutions. By pulsed field gradient diffusion NMR along with the standard toolbox of 1D and 2D NMR, we determined that sulfur-amine solutions used as a sulfur precursor exist as alkylammonium polysulfides at low temperatures. Upon heating to temperatures used in nanocrystal synthesis, the polysulfide ions react with excess amine to generate H(2)S, which combines with the metal precursor to form metal sulfide. Four different reaction pathways were found, each of which produced H(2)S and the byproducts identified in this Article. Thioamides were identified as an intermediate and were shown to exhibit much more rapid kinetics than sulfur-alkylamine solutions at low temperatures in the synthesis of metal sulfide nanocrystals.     

Design and synthesis of ordered mesoporous silica materials with high degree of silica condensation at high temperature from a mixture of polymer surfactant with small organic ammonium

Thermal stability on a mixture of triblock polymer (P123) and fluorocarbon surfactant (FC-4) in acidic media for synthesis of ordered mesoporous materials has been carefully investigated by NMR spectroscopy at various treated temperatures (RT-180 degrees C) and the templating mechanism of the mixture on high-temperature synthesis has been proposed. Accordingly, we have designed fluorocarbon-free templates for syntheses of ordered mesoporous silica materials at high temperatures. As expected, ordered mesoporous silica materials with high degree of silica condensation are synthesized at high temperatures from these designed templates.     

Hairdressing shish-kebabs; comprehensive survey and theoretical discussion of overgrowth crystallization

The platelet component of shish-kebabs crystallizes after the core, during cooling or storage below the formation temperature of the core. Three basic platelet morphologies were previously identified which were mutually interconvertible, a process we have termed “hairdressing”. In this paper we show that these three categories are special cases of a continuous range of overgrowth spacings. Crystallization at high temperatures gives widely spaced overgrowths and, as the crystallization temperature is reduced, so the overgrowth spacing decreases gradually. In the extreme case (only obtainable by quenching) the overgrowths become so close as to overlap and appear continuous. We also report a variety of further effects which were caused by exposing the shish-kebabs, while in solvent, to temperatures above their initial formation temperature. A new theoretical approach is described which considers the depletion of material available to form new overgrowths during crystallization. Two versions of this theory are presented (one a computer simulation and one analytic); interpreting our results on the basis of this theory we show that shish-kebabs crystallize at high temperatures even when quenched (90°C and above except in a few exceptional circumstances) and we are able to explain all the features of shish-kebab crystallization that we have observed.     

On the role of inherent structures in glass-forming materials: II. Reconstruction of the mean square displacement by rigorous lifting of the inherent structure dynamics

In a previous paper, we investigated the role of inherent structures in the vitrification process of glass-forming materials, showing that the dynamical transitions between inherent structures (states) can be well predicted by a first-order kinetic scheme based on infrequent-event theory at low temperatures. In this work, we utilize and extend that methodology in order to completely reconstruct the system dynamics in the form of the mean square atomic displacement as a function of time at finite temperatures on the basis of the succession of transitions in a network of states, the vibrational contribution being evaluated on the basis of short molecular dynamics runs artificially trapped within each one of the states. In order to do so, we provide the mathematical formulation for lifting the coarse-grained Poisson process model of transitions between states back to the atomistic level and thereby reproducing the full dynamics of the atomistic system within the Poisson approximation. Our result shows excellent agreement for temperatures around and below the glass transition temperature of our model Lennard-Jones two-component mixtures. Clearly, our approach is able to reproduce the full dynamics of the atomistic system at low temperatures, where the Poisson approximation is valid.     

Helium induced pressure broadening and shifting of HCN hyperfine transitions between 1.3 and 20 K

We have measured the helium induced pressure broadening and shifting of the distinct hyperfine components of the j = 1 <-- 0 and j = 2 <-- 1 transitions of HC14N at temperatures between 1.3 and 20 K. The HCN molecules were cooled to these temperatures using the collisional cooling technique. As a test of this cooling technique we measured the Doppler contribution to the spectral lines, and these measurements confirm that the molecules are at the same temperature as the walls of the spectroscopic cell. We observed that the hyperfine components of the 2 <-- 1 transition have distinct broadening coefficients that differ from one another by as much as 5%. The measured differences are in reasonable agreement with theoretical predictions. We have also performed molecular scattering calculations on three He-HCN potential energy surfaces in order to compare our results with theoretical expectations. At the lowest temperatures these calculations predict broadening coefficients that are considerably larger than the measured coefficients. We have previously found a similar discrepancy for two other molecules at these low temperatures, and we discuss possible experimental and theoretical origins for this persistent discrepancy.     

Kinetics of hydrolysis of 1-benzoyl-1,2,4-triazole in aqueous solution as a function of temperature near the temperature of maximum density,and the isochoric controversy

At temperatures above and below the temperature of maximum density, TMD, for water at ambient pressure, pairs of temperatures exist at which the molar volumes of water are equal. First-order rate constants for the pH-independent hydrolysis of 1-benzoyl-1,2,4-triazole in aqueous solution at pairs of such isochoric temperatures show no unique features. Taken together with previously published kinetic data for the hydrolysis of a range of simple organic solutes in both water and D2O near their respective TMDs, we conclude that special significance in the context of rates of chemical reactions in aqueous solutions should not be attached to the isochoric condition.     

Superficial segregation in nanoparticles: from facets to infinite surfaces

We compare the superficial segregations of the Cu-Ag system for a nanoparticle and for surfaces that are structurally equivalent to each of its facet. Based on a lattice-gas model and within a mean-field formalism, we derive segregation isotherms at various temperatures in the canonical ensemble, i.e., for a given overall solute concentration, and in the semigrand canonical ensemble, i.e., for a given bulk solute concentration. If both processes are very similar for high temperatures, they differ substantially at lower temperatures. Due to the finite-size effect and the indirect coupling between facets and edges, the relative position of the phase transitions of the facets and the corresponding surfaces is inversed when displayed as a function of the solute bulk concentration. Moreover, we show that working in the semigrand canonical ensemble is a much more efficient way to study this phenomenon, although nanoparticles are "canonical" objects in essence.     

Shape memory effect exhibited by smectic-C liquid crystalline elastomers

It was long expected and recently shown that main-chain liquid crystalline elastomers (MC-LCEs) may serve as high performance soft actuators due to a coupling of their intrinsic characteristics of high, yet labile, ordering and network strain. Here, we present the synthesis of new siloxane-based smectic MC-LCEs. These new materials exhibit a unique thermomechanical behavior known as the shape memory effect, which has never been observed before in such LCEs. To achieve targeted transition temperatures required for facile actuation at low temperatures, specifically temperatures ranging from 15 to 65 degrees C, we have designed and prepared such elastomers incorporating two distinct mesogenic groups, termed 5H and 5tB, coupled with hydride-terminated poly(dimethylsiloxane) spacers.     

Effect of rapidly depressurizing and rising temperature on methane hydrate dissociation

Two methods, rapidly depressurizing to 0.1 MPa at a constant temperature and rising temperature under equilibrium P, T conditions, were used to study the dissociation of pure CH4 hydrate formed below the ice point. At a constant temperature with rapidly depressurizing to 0.1 MPa, CH4 hydrate dissociated rapidly at initial dissociation and then the dissociation rate gradually decreased. However, the dissociation of CH4 hydrate at temperatures of 261 to 266 K was much faster than that at temperatures of 269 to 272 K, indicating its anomalous preservation. Under an equilibrium P, T conditions, rising temperature had extensively controlling impact on dissociation of CH4 hydrate at equilibrium pressures of 2.31, 2.16 and 1.96 MPa. In this study, we report the effect of pressure on CH4 hydrate dissociation, especially the effect of equilibrium pressure on dissociation at various melting temperatures. And we find that the ice particles size of CH4 hydrate formed may dominant the CH4 hydrate dissociation. Dissociation of CH4 hydrate formed from ice particles of smaller than 250 μm may not have an anomalous preservation below the ice point, while particles larger than 250 μm may have more extensive anomalous preservation.     

Orientational dynamics for an amphiphilic-solvent solution

In this work, we performed Monte Carlo simulations on a lattice model for spontaneous amphiphilic aggregation, in order to study the orientational and hydrogen-bonding dynamics of water on different regions inside the micellar solution. We employed an associating lattice gas model that mimics the aqueous solvent, which presents a rich phase diagram with first- and second-order transition lines. Even though this is a simplified model, it makes possible to investigate the orientational dynamics of water in an equilibrium solution of amphiphiles, as well as the influence of the different phases of the solvent in the interfacial and bulk water dynamics. By means of extensive simulations, we showed that, at high temperatures, the behavior of the orientational relaxation and hydrogen bonding of water molecules in the bulk, first, and second hydration shells are considerable different. We observe the appearance of a very slow component for water molecules in the first hydration shell of micelles when the system reaches a high-density phase, consistent with previous theoretical and experimental studies concerning biological water. Also, at high temperatures, we find that water molecules in the second hydration shell of micelles have an orientational decay similar to that of bulk water, but with a generally slower dynamics. Otherwise, at low temperatures, we have two components for the orientational relaxation of bulk water in the low density liquid phase, and only a single component in the high density liquid (HDL) phase, which reflect the symmetry properties of the different phases of the solvent model. In the very dense region of water molecules in the first hydration shell of micelles at low temperatures, we find two components for the orientational relaxation on both liquid phases, one of them much slower than that in the single component of bulk water in the HDL phase. This happens even though our model does not present any hindrance to the water rotational freedom caused by the presence of the amphiphiles.     

Role of temperature in the numerical analysis of CaO under high pressure

In this paper we focus on the elastic and thermodynamic properties of the B1 phase of CaO by using the modified TBP model, including the role of temperature. We have successfully obtained the phase transition pressure and volume change at different temperatures. In addition elastic constants and bulk modulus of B1 phase of CaO at different temperatures are discussed. Our results are comparable with the previous ones at high temperatures and pressures. The thermodynamical properties of the B1 phase of CaO are also predicted.