Three-phase equilibria of hydrates from computer simulation. III. Effect of dispersive interactions in the methane and carbon dioxide hydrates.

In this work, the effect of the range of dispersive interactions in determining the three-phase coexistence line of the CO2 and CH4 hydrates has been studied. In particular, the temperature (T3) at which solid hydrate, water, and liquid CO2/gas CH4 coexist has been determined through molecular dynamics simulations using different cutoff values (from 0.9 to 1.6 nm) for dispersive interactions. The T3 of both hydrates has been determined using the direct coexistence simulation technique. Following this method, the three phases in equilibrium are put together in the same simulation box, the pressure is fixed, and simulations are performed at different temperatures T. If the hydrate melts, then T > T3. Conversely, if the hydrate grows, then T < T3. The effect of the cutoff distance on the dissociation temperature has been analyzed at three different pressures for CO2 hydrate: 100, 400, and 1000 bar. Then, we have changed the guest and studied the effect of the cutoff distance on the dissociation temperature of the CH4 hydrate at 400 bar. Moreover, the effect of long-range corrections for dispersive interactions has been analyzed by running simulations with homo- and inhomogeneous corrections and a cutoff value of 0.9 nm. The results obtained in this work highlight that the cutoff distance for the dispersive interactions affects the stability conditions of these hydrates. This effect is enhanced when the pressure is decreased, displacing the T3 about 2-4 K depending on the system and the pressure.

Effect of substrate mismatch, orientation, and flexibility on heterogeneous ice nucleation.

Heterogeneous nucleation is the main path to ice formation on Earth. The ice nucleating ability of a certain substrate is mainly determined by both molecular interactions and the structural mismatch between the ice and the substrate lattices. We focus on the latter factor using molecular simulations of the mW model. Quantifying the effect of structural mismatch alone is challenging due to its coupling with molecular interactions. To disentangle both the factors, we use a substrate composed of water molecules in such a way that any variation on the nucleation temperature can be exclusively ascribed to the structural mismatch. We find that a 1% increase in structural mismatch leads to a decrease of ∼4 K in the nucleation temperature. We also analyze the effect of orientation of the substrate with respect to the liquid. The three main ice orientations (basal, primary prism, and secondary prism) have a similar ice nucleating ability. We finally assess the effect of lattice flexibility by comparing substrates where molecules are immobile to others where a certain freedom to fluctuate around the lattice positions is allowed. Interestingly, we find that the latter type of substrate is more efficient in nucleating ice because it can adapt its structure to that of ice.

Three-phase equilibria of hydrates from computer simulation. I. Finite-size effects in the methane hydrate.

Clathrate hydrates are vital in energy research and environmental applications. Understanding their stability is crucial for harnessing their potential. In this work, we employ direct coexistence simulations to study finite-size effects in the determination of the three-phase equilibrium temperature (T3) for methane hydrates. Two popular water models, TIP4P/Ice and TIP4P/2005, are employed, exploring various system sizes by varying the number of molecules in the hydrate, liquid, and gas phases. The results reveal that finite-size effects play a crucial role in determining T3. The study includes nine configurations with varying system sizes, demonstrating that smaller systems, particularly those leading to stoichiometric conditions and bubble formation, may yield inaccurate T3 values. The emergence of methane bubbles within the liquid phase, observed in smaller configurations, significantly influences the behavior of the system and can lead to erroneous temperature estimations. Our findings reveal finite-size effects on the calculation of T3 by direct coexistence simulations and clarify the system size convergence for both models, shedding light on discrepancies found in the literature. The results contribute to a deeper understanding of the phase equilibrium of gas hydrates and offer valuable information for future research in this field.

Three-phase equilibria of hydrates from computer simulation. II. Finite-size effects in the carbon dioxide hydrate.

In this work, the effects of finite size on the determination of the three-phase coexistence temperature (T3) of the carbon dioxide (CO2) hydrate have been studied by molecular dynamic simulations and using the direct coexistence technique. According to this technique, the three phases involved (hydrate-aqueous solution-liquid CO2) are placed together in the same simulation box. By varying the number of molecules of each phase, it is possible to analyze the effect of simulation size and stoichiometry on the T3 determination. In this work, we have determined the T3 value at 8 different pressures (from 100 to 6000 bar) and using 6 different simulation boxes with different numbers of molecules and sizes. In two of these configurations, the ratio of the number of water and CO2 molecules in the aqueous solution and the liquid CO2 phase is the same as in the hydrate (stoichiometric configuration). In both stoichiometric configurations, the formation of a liquid drop of CO2 in the aqueous phase is observed. This drop, which has a cylindrical geometry, increases the amount of CO2 available in the aqueous solution and can in some cases lead to the crystallization of the hydrate at temperatures above T3, overestimating the T3 value obtained from direct coexistence simulations. The simulation results obtained for the CO2 hydrate confirm the sensitivity of T3 depending on the size and composition of the system, explaining the discrepancies observed in the original work by Míguez et al. [J. Chem Phys. 142, 124505 (2015)]. Non-stoichiometric configurations with larger unit cells show a convergence of T3 values, suggesting that finite-size effects for these system sizes, regardless of drop formation, can be safely neglected. The results obtained in this work highlight that the choice of a correct initial configuration is essential to accurately estimate the three-phase coexistence temperature of hydrates by direct coexistence simulations.

Scaled charges for ions: An improvement but not the final word for modeling electrolytes in water.

In this work, we discuss the use of scaled charges when developing force fields for NaCl in water. We shall develop force fields for Na+ and Cl- using the following values for the scaled charge (in electron units): ±0.75, ±0.80, ±0.85, and ±0.92 along with the TIP4P/2005 model of water (for which previous force fields were proposed for q = ±0.85 and q = ±1). The properties considered in this work are densities, structural properties, transport properties, surface tension, freezing point depression, and maximum in density. All the developed models were able to describe quite well the experimental values of the densities. Structural properties were well described by models with charges equal to or larger than ±0.85, surface tension by the charge ±0.92, maximum in density by the charge ±0.85, and transport properties by the charge ±0.75. The use of a scaled charge of ±0.75 is able to reproduce with high accuracy the viscosities and diffusion coefficients of NaCl solutions for the first time. We have also considered the case of KCl in water, and the results obtained were fully consistent with those of NaCl. There is no value of the scaled charge able to reproduce all the properties considered in this work. Although certainly scaled charges are not the final word in the development of force fields for electrolytes in water, its use may have some practical advantages. Certain values of the scaled charge could be the best option when the interest is to describe certain experimental properties.

Growth rate of CO2 and CH4 hydrates by means of molecular dynamics simulations.

CO2 and CH4 hydrates are of great importance both from an energetic and from an environmental point of view. It is therefore highly relevant to quantify and understand the rate with which they grow. We use molecular dynamics simulations to shed light on the growth rate of these hydrates. We put the solid hydrate phase in contact with a guest aqueous solution in equilibrium with the pure guest phase and study the growth of both hydrates at 400 bars with temperature. We compare our results with previous calculations of the ice growth rate. We find a growth rate maximum as a function of the supercooling in all cases. The incorporation of guest molecules into the solid structure strongly decelerates hydrate growth. Consistently, ice grows faster than either hydrate and the CO2 hydrate grows faster than the CH4 one because of the higher solubility of CO2. We also quantify the molecular motion required to build the solids under study and find that the distance traveled by liquid molecules exceeds by orders of magnitude that advanced by any solid. Less molecular motion is needed in order for ice to grow as compared to the hydrates. Moreover, when temperature increases, more motion is needed for solid growth. Finally, we find a good agreement between our growth rate calculations and experiments of hydrate growth along the guest-solution interface. However, more work is needed to reconcile experiments of hydrate growth toward the solution among each other and with simulations.

The Madrid-2019 force field for electrolytes in water using TIP4P/2005 and scaled charges: Extension to the ions F-, Br-, I-, Rb+, and Cs.

In this work, an extension of the Madrid-2019 force field is presented. We have added the cations Rb+ and Cs+ and the anions F-, Br-, and I-. These ions were the remaining alkaline and halogen ions, not previously considered in the Madrid-2019 force field. The force field, denoted as Madrid-2019-Extended, does not include polarizability and uses the TIP4P/2005 model of water and scaled charges for the ions. A charge of ±0.85e is assigned to monovalent ions. The force field developed provides an accurate description of aqueous solution densities over a wide range of concentrations up to the solubility limit of each salt studied. Good predictions of viscosity and diffusion coefficients are obtained for concentrations below 2 m. Structural properties obtained with this force field are also in reasonable agreement with the experiment. The number of contact ion pairs has been controlled to be low so as to avoid precipitation of the system at concentrations close to the experimental solubility limit. A comprehensive comparison of the performance for aqueous solutions of alkaline halides of force fields of electrolytes using scaled and integer charges is now possible. This comparison will help in the future to learn about the benefits and limitations of the use of scaled charges to describe electrolyte solutions.

Phase diagram of the NaCl-water system from computer simulations.

NaCl aqueous solutions are ubiquitous. They can crystallize into ice, NaCl, or NaCl · 2H2O depending on the temperature-concentration conditions. These crystallization transitions have important implications in geology, cryopreservation, or atmospheric science. Computer simulations can help understand the crystallization of these solids, which requires a detailed knowledge of the equilibrium phase diagram. We use molecular simulations in which we put at contact the solution with the solid of interest to determine points of the solid-solution coexistence lines. We follow two different approaches, one in which we narrow down the melting temperature for a given concentration and the other in which we equilibrate the concentration for a given temperature, obtaining consistent results. The phase diagram thus calculated for the selected model (TIP4P/2005 for water molecules and Joung-Cheatham for the ions) correctly predicts coexistence between the solution and ice. We were only able to determine NaCl · 2H2O-solution coexistence points at higher temperatures and concentrations than in the experiment, so we could not establish a direct comparison in this case. On the other hand, the model underestimates the concentration of the solution in equilibrium with the NaCl solid. Our results, alongside other literature evidence, seem to indicate that ion-ion interactions are too strong in the model. Our work is a good starting point for the improvement of the potential model and for the study of the nucleation kinetics of the solid phases involved in the phase diagram.

Spontaneous NaCl-doped ices Ih, Ic, III, V and VI. Understanding the mechanism of ion inclusion and its dependence on the crystalline structure of ice.

Direct coexistence simulations on a microsecond time scale have been performed for different types of ice (Ih, Ic, III, V, and VI) in contact with a NaCl aqueous solution at different pressures. In line with the previous results obtained for ice Ih [Conde et al., Phys. Chem. Chem. Phys., 2017, 19, 9566-9574], our results reveal the spontaneous growth of a new ice doped phase and the formation of a brine rejection phase in all ices studied. However, both the preferential incorporation of ions into the ice lattice and the inclusion mechanisms depend on the crystalline structure of each ice. This work shows the inclusion of Cl- and Na+ ions in ice from salt using molecular dynamics simulation, in agreement with the experimental evidence found in the literature. The model used for water is TIP4P/2005. For NaCl we employ a set of potential parameters that uses unit charges for the ions.

Homogeneous nucleation of NaCl in supersaturated solutions.

The seeding method is an approximate approach to investigate nucleation that combines molecular dynamics simulations with classical nucleation theory. Recently, this technique has been successfully implemented in a broad range of nucleation studies. However, its accuracy is subject to the arbitrary choice of the order parameter threshold used to distinguish liquid-like from solid-like molecules. We revisit here the crystallization of NaCl from a supersaturated brine solution and show that consistency between seeding and rigorous methods, like Forward Flux Sampling (from previous work) or spontaneous crystallization (from this work), is achieved by following a mislabelling criterion to select such threshold (i.e. equaling the fraction of the mislabelled particles in the bulk parent and nucleating phases). This work supports the use of seeding to obtain fast and reasonably accurate nucleation rate estimates and the mislabelling criterion as one giving the relevant cluster size for classical nucleation theory in crystallization studies.

Single Shot Characterization of High Transformer Ratio Wakefields in Nonlinear Plasma Acceleration.

Plasma wakefields can enable very high accelerating gradients for frontier high energy particle accelerators, in excess of 10 GeV/m. To overcome limits on single stage acceleration, specially shaped drive beams can be used in both linear and nonlinear plasma wakefield accelerators (PWFA), to increase the transformer ratio, implying that the drive beam deceleration is minimized relative to acceleration obtained in the wake. In this Letter, we report the results of a nonlinear PWFA, high transformer ratio experiment using high-charge, longitudinally asymmetric drive beams in a plasma cell. An emittance exchange process is used to generate variable drive current profiles, in conjunction with a long (multiple plasma wavelength) witness beam. The witness beam is energy modulated by the wakefield, yielding a response that contains detailed spectral information in a single-shot measurement. Using these methods, we generate a variety of beam profiles and characterize the wakefields, directly observing transformer ratios up to R=7.8. Furthermore, a spectrally based reconstruction technique, validated by 3D particle-in-cell simulations, is introduced to obtain the drive beam current profile from the decelerating wake data.

Drug interactions with oral antineoplastic drugs: The role of the pharmacist.

The main objective of the study is to determine the pharmacist detection of drug-drug and drug-food interactions in patients receiving oral antineoplastic drugs (OADs). Descriptive, prospective study in a tertiary-care teaching hospital. The study population included patients who received OADs from the Outpatient Pharmacy of the hospital. The study population was attended by a pharmacist who checked potential interactions. The severity of interactions was evaluated using the summary of product characteristics of each drug and three different databases. We included 219 patients with a total of 736 concomitant medications. A total of 34 drug-drug or food-drug interactions were recorded. The most common interaction detected was between erlotinib and ranitidine (major interaction). In 19 of the 34 interactions detected in the experimental group, the pharmacist prevented them from reaching the patient. Interactions were resolved by drug suspensions, drug changes, or changes in schedules always according to the attending physician or the patient. In the remaining 15 interactions, the doctor was not contacted because the interactions were considered to be of little relevance or because they only required surveillance. Hospital pharmacist can improve the patient's safety and the efficiency of oral cytostatic treatment by detecting and preventing drug-drug and drug-food interactions.

Observation of High Transformer Ratio of Shaped Bunch Generated by an Emittance-Exchange Beam Line.

Collinear wakefield acceleration has been long established as a method capable of generating ultrahigh acceleration gradients. Because of the success on this front, recently, more efforts have shifted towards developing methods to raise the transformer ratio (TR). This figure of merit is defined as the ratio of the peak acceleration field behind the drive bunch to the peak deceleration field inside the drive bunch. TR is always less than 2 for temporally symmetric drive bunch distributions and therefore recent efforts have focused on generating asymmetric distributions to overcome this limitation. In this Letter, we report on using the emittance-exchange method to generate a shaped drive bunch to experimentally demonstrate a TR≈5 in a dielectric wakefield accelerator.

Teledermatology-driven topical therapy of actinic keratosis: a comparative study of clinical effectiveness and compliance.

BACKGROUND: Teledermatology (TD) provides efficient care for skin cancer patients. OBJECTIVE: To compare the clinical effectiveness of imiquimod 5% for the treatment of AK with in-person care and through TD. METHODS: Longitudinal prospective controlled study including patients with single AK diagnosed and treated at face-to-face visits (FTF group) or through teledermatology (TD group) with imiquimod 5% cream. The main outcome measures assessed were the complete and global response percentage (CR and GR) under per-protocol (PP) and intention-to-treat (ITT) analysis. RESULTS: A total of 157 patients were enrolled (FTF = 75, TD = 82). PP analysis showed CR in 66.7% of FTF patients and 65.6% in TD patients (P > 0.05). The ITT yielded CR in 64.0% and 51.2% in FTF visits and TD, respectively (P = 0.073). The analysis showed an advantage of FTF care against TD in achieving GR (84.0% vs. 70.7%; P = 0.036). Facial location and local adverse reactions were the only explanatory factors of complete response in the ITT approach. Treatment completion was found in 90.7% and 72.0% in the FTF and TD groups, respectively (P = 0.004). CONCLUSIONS: Improvements in patient counselling at the primary care centre are needed before the implementation of TD as a routine methodology for the management of AK.

Fermi Large Area Telescope as a Galactic Supernovae Axionscope.

In a Galactic core-collapse supernova (SN), axionlike particles (ALPs) could be emitted via the Primakoff process and eventually convert into γ rays in the magnetic field of the Milky Way. From a data-driven sensitivity estimate, we find that, for a SN exploding in our Galaxy, the Fermi Large Area Telescope (LAT) would be able to explore the photon-ALP coupling down to g_{aγ}≃2×10^{-13} GeV^{-1} for an ALP mass m_{a}≲10^{-9} eV. These values are out of reach of next generation laboratory experiments. In this event, the Fermi LAT would probe large regions of the ALP parameter space invoked to explain the anomalous transparency of the Universe to γ rays, stellar cooling anomalies, and cold dark matter. If no γ-ray emission were to be detected, Fermi-LAT observations would improve current bounds derived from SN 1987A by more than 1 order of magnitude.

Precision Control of the Electron Longitudinal Bunch Shape Using an Emittance-Exchange Beam Line.

We report on the experimental generation of relativistic electron bunches with a tunable longitudinal bunch shape. A longitudinal bunch-shaping (LBS) beam line, consisting of a transverse mask followed by a transverse-to-longitudinal emittance exchange (EEX) beam line, is used to tailor the longitudinal bunch shape (or current profile) of the electron bunch. The mask shapes the bunch's horizontal profile, and the EEX beam line converts it to a corresponding longitudinal profile. The Argonne wakefield accelerator rf photoinjector delivers electron bunches into a LBS beam line to generate a variety of longitudinal bunch shapes. The quality of the longitudinal bunch shape is limited by various perturbations in the exchange process. We develop a simple method, based on the incident slope of the bunch, to significantly suppress the perturbations.

Spontaneous NaCl-doped ice at seawater conditions: focus on the mechanisms of ion inclusion.

Molecular dynamics simulations on the microsecond time scale have been performed on an aqueous solution of TIP4P/2005 water and NaCl by using the direct coexistence technique to study the ice growth and the ice/liquid interface water. At ambient pressure, for temperatures above the eutectic point of the salt and at seawater concentrations the brine rejection phenomenon and the spontaneous growth of an ice slab doped by the salt are obtained, as found in natural terrestrial and planetary environments. Experiments indicate that Cl- goes via substitution to ice sites. In line with this evidence we find a new result: the Cl- ion included in the lattice always substitutes not one but two water molecules, leaving the surrounding ice structure not distorted. The Na+ ion shows a lower probability of being included in the ice and it occupies an interstitial site, causing a local distortion of the lattice. No signs of significant ion diffusion are observed in the lattice.

High precision determination of the melting points of water TIP4P/2005 and water TIP4P/Ice models by the direct coexistence technique.

An exhaustive study by molecular dynamics has been performed to analyze the factors that enhance the precision of the technique of direct coexistence for a system of ice and liquid water. The factors analyzed are the stochastic nature of the method, the finite size effects, and the influence of the initial ice configuration used. The results obtained show that the precision of estimates obtained through the technique of direct coexistence is markedly affected by the effects of finite size, requiring systems with a large number of molecules to reduce the error bar of the melting point. This increase in size causes an increase in the simulation time, but the estimate of the melting point with a great accuracy is important, for example, in studies on the ice surface. We also verified that the choice of the initial ice Ih configuration with different proton arrangements does not significantly affect the estimate of the melting point. Importantly this study leads us to estimate the melting point at ambient pressure of two of the most popular models of water, TIP4P/2005 and TIP4P/Ice, with the greatest precision to date.

Structural properties and fragile to strong transition in confined water.

We derive by computer simulation the radial distribution functions of water confined in a silica pore modeled to reproduce MCM-41. We perform the calculations in a range of temperatures from ambient to deep supercooling for the subset of water molecules that reside in the inner shell (free water) by applying the excluded volume corrections. By comparing with bulk water we find that the first shell of the oxygen-oxygen and hydrogen-hydrogen radial distribution functions is less sharp and the first minimum fills in while the oxygen-hydrogen structure does not significantly change, indicating that the free water keeps the hydrogen bond short range order. The two body excess entropy of supercooled water is calculated from the radial distribution functions. We connect the behavior of this function to the relaxation time of the same system already studied in previous simulations. We show that the two body entropy changes its behavior in coincidence with the crossover of the relaxation time from the mode coupling fragile to the strong Arrhenius regime. As for bulk water also in confinement, the two body entropy has a strict connection with the dynamical relaxation.

A scoping review of root canal revascularization: relevant aspects for clinical success and tissue formation.

The aim of this scoping study was to evaluate the survival rate and nature of tissue formed inside root canals of human immature permanent teeth with necrotic pulps (NIPT) under root canal revascularization (RCR). The search was performed in SciVerse Scopus®, PubMed/MEDLINE, Web of Science®, BIREME and in the grey literature up to November 2015. The keywords were selected using MeSH terms and DECs. Two independent reviewers scrutinized the records obtained considering specific inclusion criteria. The included studies were evaluated in accordance with a modified Arksey and O' Malley's framework. From 375 studies that were evaluated, 75 were included. A total of 367 NIPT were submitted to RCR, from which only 21 needed further endodontic treatment. The weighted mean follow-up time was 17.6 months. The data were derived mainly from case reports (69%) or small case series (15%). NaOCl [0.5-6%] was applied as the disinfecting solution in almost all studies. Triple antibiotic paste was as effective as Ca(OH)2 as on intracanal medicament. De novo tissue was cementum and poorly mineralized bone positive to bone sialoprotein (BSP) but negative to dentine sialoprotein (DSP). Failures were associated mainly with reinfection of the root canal. The majority of included studies reported a significant increase in both root length and width. However, as most of these data came from case reports, they must be interpreted with care, as most were focused on treatment successes (not failures). Therefore, well-designed randomized controlled trials comparing RCR with available apexification treatments are needed to address this gap in the literature.