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.

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.

Homogeneous nucleation rate of methane hydrate formation under experimental conditions from seeding simulations.

In this work, we shall estimate via computer simulations the homogeneous nucleation rate for the methane hydrate at 400 bars for a supercooling of about 35 K. The TIP4P/ICE model and a Lennard-Jones center were used for water and methane, respectively. To estimate the nucleation rate, the seeding technique was employed. Clusters of the methane hydrate of different sizes were inserted into the aqueous phase of a two-phase gas-liquid equilibrium system at 260 K and 400 bars. Using these systems, we determined the size at which the cluster of the hydrate is critical (i.e., it has 50% probability of either growing or melting). Since nucleation rates estimated from the seeding technique are sensitive to the choice of the order parameter used to determine the size of the cluster of the solid, we considered several possibilities. We performed brute force simulations of an aqueous solution of methane in water in which the concentration of methane was several times higher than the equilibrium concentration (i.e., the solution was supersaturated). From brute force runs, we infer the value of the nucleation rate for this system rigorously. Subsequently, seeding runs were carried out for this system, and it was found that only two of the considered order parameters were able to reproduce the value of the nucleation rate obtained from brute force simulations. By using these two order parameters, we estimated the nucleation rate under experimental conditions (400 bars and 260 K) to be of the order of log10 (J/(m3 s)) = -7(5).

Two Indigenous Berberis Species From Spain Were Confirmed as Alternate Hosts of the Yellow Rust Fungus Puccinia striiformis f. sp. tritici.

Puccinia striiformis f. sp. tritici, which causes yellow (or stripe) rust on wheat, is a macrocyclic and heteroecious fungus. In this study, we investigated whether Berberis vulgaris subsp. seroi and B. vulgaris subsp. australis, which are indigenous in Spain, may serve as alternate hosts for P. striiformis f. sp. tritici. Wheat leaves bearing telia of an isolate of P. striiformis f. sp. tritici were harvested and used to inoculate plants of both barberry subspecies. Pycnia were observed on the adaxial side of the leaves from 10 days after inoculation (dai). Following successful fertilization, aecia were observed on the abaxial side of the leaves from 16 dai. At 27 dai, barberry leaves bearing aecia were detached and used to inoculate susceptible wheat seedlings of cultivar Morocco. Uredinia were observed on wheat seedlings from 12 days after aeciospore exposure. Eighty-three single lesions were recovered from individual wheat leaves, of which 43 were genotyped using 19 P. striiformis f. sp. tritici simple sequence repeat markers (SSR). In total, 19 multilocus genotypes (MLGs) were identified among the 43 progeny isolates. The SSR genotyping confirmed that all 43 isolates were derived from the parental isolate. Seven heterozygous SSR markers showed segregation among the progenies, whereas none of the 12 homozygous markers resulted in segregation. These results demonstrated that B. vulgaris subspp. seroi and australis can serve as alternate hosts for P. striiformis f. sp. tritici, which may result in novel virulence combinations that can have a detrimental impact on wheat production. Although P. striiformis f. sp. tritici has not been detected on these barberry species in nature, this study highlights the importance of rust surveillance in barberry areas where suitable conditions for completion of the sexual life cycle may be present.

On interfacial properties of tetrahydrofuran: Atomistic and coarse-grained models from molecular dynamics simulation.

We have determined the interfacial properties of tetrahydrofuran (THF) from direct simulation of the vapor-liquid interface. The molecules are modeled using six different molecular models, three of them based on the united-atom approach and the other three based on a coarse-grained (CG) approach. In the first case, THF is modeled using the transferable parameters potential functions approach proposed by Chandrasekhar and Jorgensen [J. Chem. Phys. 77, 5073 (1982)] and a new parametrization of the TraPPE force fields for cyclic alkanes and ethers [S. J. Keasler et al., J. Phys. Chem. B 115, 11234 (2012)]. In both cases, dispersive and coulombic intermolecular interactions are explicitly taken into account. In the second case, THF is modeled as a single sphere, a diatomic molecule, and a ring formed from three Mie monomers according to the SAFT-γ Mie top-down approach [V. Papaioannou et al., J. Chem. Phys. 140, 054107 (2014)]. Simulations were performed in the molecular dynamics canonical ensemble and the vapor-liquid surface tension is evaluated from the normal and tangential components of the pressure tensor along the simulation box. In addition to the surface tension, we have also obtained density profiles, coexistence densities, critical temperature, density, and pressure, and interfacial thickness as functions of temperature, paying special attention to the comparison between the estimations obtained from different models and literature experimental data. The simulation results obtained from the three CG models as described by the SAFT-γ Mie approach are able to predict accurately the vapor-liquid phase envelope of THF, in excellent agreement with estimations obtained from TraPPE model and experimental data in the whole range of coexistence. However, Chandrasekhar and Jorgensen model presents significant deviations from experimental results. We also compare the predictions for surface tension as obtained from simulation results for all the models with experimental data. The three CG models predict reasonably well (but only qualitatively) the surface tension of THF, as a function of temperature, from the triple point to the critical temperature. On the other hand, only the TraPPE united-atoms models are able to predict accurately the experimental surface tension of the system in the whole temperature range.

Understanding the Phase Behavior of Tetrahydrofuran + Carbon Dioxide, + Methane, and + Water Binary Mixtures from the SAFT-VR Approach.

The high-pressure phase diagrams of the tetrahydrofuran(1) + carbon dioxide(2), + methane(2), and + water(2) mixtures are examined using the SAFT-VR approach. Carbon dioxide molecule is modeled as two spherical segments tangentially bonded, water is modeled as a spherical segment with four associating sites to represent the hydrogen bonding, methane is represented as an isolated sphere, and tetrahydrofuran is represented as a chain of m tangentially bonded spherical segments. Dispersive interactions are modeled using the square-well intermolecular potential. In addition, two different molecular model mixtures are developed to take into account the subtle balance between water-tetrahydrofuran hydrogen-bonding interactions. The polar and quadrupolar interactions present in water, tetrahydrofuran, and carbon dioxide are treated in an effective way via square-well potentials of variable range. The optimized intermolecular parameters are taken from the works of Giner et al. (Fluid Phase Equil. 2007, 255, 200), Galindo and Blas (J. Phys. Chem. B 2002, 106, 4503), Patel et al. (Ind. Eng. Chem. Res. 2003, 42, 3809), and Clark et al. (Mol. Phys. 2006, 104, 3561) for tetrahydrofuran, carbon dioxide, methane, and water, respectively. The phase diagrams of the binary mixtures exhibit different types of phase behavior according to the classification of van Konynenburg and Scott, ranging from types I, III, and VI phase behavior for the tetrahydrofuran(1) + carbon dioxide(2), + methane(2), and + water(2) binary mixtures, respectively. This last type is characterized by the presence of a Bancroft point, positive azeotropy, and the so-called closed-loop curves that represent regions of liquid-liquid immiscibility in the phase diagram. The system exhibits lower critical solution temperatures (LCSTs), which denote the lower limit of immiscibility together with upper critical solution temperatures (UCSTs). This behavior is explained in terms of competition between the incompatibility with the alkyl parts of the tetrahydrofuran ring chain and the hydrogen bonding between water and the ether group. A minimum number of unlike interaction parameters are fitted to give the optimal representation of the most representative features of the binary phase diagrams. In the particular case of tetrahydrofuran(1) + water(2), two sets of intermolecular potential model parameters are proposed to describe accurately either the hypercritical point associated with the closed-loop liquid-liquid immiscibility region or the location of the mixture lower- and upper-critical end-points. The theory is not only able to predict the type of phase behavior of each mixture, but also provides a reasonably good description of the global phase behavior whenever experimental data are available.

Effect of molecular flexibility of Lennard-Jones chains on vapor-liquid interfacial properties.

We have determined the interfacial properties of short fully flexible chains formed from tangentially bonded Lennard-Jones monomeric units from direct simulation of the vapor-liquid interface. The results obtained are compared with those corresponding to rigid-linear chains formed from the same chain length, previously determined in the literature [F. J. Blas, A. I. M.-V. Bravo, J. M. Míguez, M. M. Piñeiro, and L. G. MacDowell, J. Chem. Phys. 137, 084706 (2012)]. The full long-range tails of the potential are accounted for by means of an improved version of the inhomogeneous long-range corrections of Janecek [J. Phys. Chem. B 129, 6264 (2006)] proposed recently by MacDowell and Blas [J. Chem. Phys. 131, 074705 (2008)] valid for spherical as well as for rigid and flexible molecular systems. Three different model systems comprising of 3, 5, and 6 monomers per molecule are considered. The simulations are performed in the canonical ensemble, and the vapor-liquid interfacial tension is evaluated using the test-area method. In addition to the surface tension, we also obtained density profiles, coexistence densities, critical temperature and density, and interfacial thickness as functions of temperature, paying particular attention to the effect of the chain length and rigidity on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the surface tension increases as the molecular chains get longer. Comparison between predictions for fully flexible and rigid-linear chains, formed by the same number of monomeric units, indicates that the main effects of increasing the flexibility, i.e., passing from a rigid-linear to a fully flexible chain, are: (a) to decrease the difference between the liquid and vapor densities; (b) to decrease the critical temperature and to increase the critical density; (c) to smooth the density profiles along the interfacial region; (d) to increase the interfacial thickness; and (e) to decrease the vapor-liquid surface tension.

Influencia del tapiz rodante sobre la variabilidad de la marcha en sujetos jóvenes y sanos / Effects of treadmill on gait variability in healthy young subjects

Objetivo. El objetivo del trabajo fue analizar, a partir de series temporales, las diferencias en la variabilidad de la marcha en dos situaciones distintas (marcha espontánea frente a marcha en tapiz rodante) en sujetos jóvenes, sanos y activos. Método. Las series temporales de diez hombres sanos y físicamente activos fueron evaluadas en dos situaciones experimentales: a) 25 minutos caminando de forma espontánea por un circuito circular abierto; y b) 25 minutos caminando en un tapiz rodante a la misma velocidad que la situación a. Un dispositivo de captura fue diseñado expresamente para la obtención directa y almacenamiento de series de tiempo durante la marcha en ambas situaciones, a partir de un simple pulsador electrónico colocado en la zona del calcáneo de la palmilla del pie dominante introducida en la zapatilla deportiva. Se calcularon la media y desviación estándar (DE), la entropía aproximada (ApEn), la entropía muestral (SampEn) y la entropía multiescala (MSE). Resultados. La DE, la ApEn y la SampEn no presentaron cambios significativos al comparar ambas situaciones experimentales. Todas las MSE disminuyen significativamente al comparar la marcha espontánea con la situación en tapiz. Conclusiones. Existe una pérdida de complejidad de la señal cuando la marcha se realiza en un tapiz rodante y esto debe ser tenido en cuenta para futuras estudios en esta situación(AU)

Objective. The aim of study was to analyze the differences in gait variability (stride interval time series) in two different conditions (spontaneous walking vs. treadmill walking) in healthy and active young subjects. Method. The stride interval time series derived from ten healthy and active males were studied in three experimental conditions: a) walking on level ground at their self-determined usual paces around open circle circuit for 25 minutes; and b) walking on a treadmill for 25 minutes at the same pace as in Situation a. A capture device enabled stride interval time series to be collected directly and stored during walking in both conditions, using a simple electronic push-button mounted in the heel of the dominant foot support of an insole placed in the running shoe. The mean and standard deviation (SD), Approximate Entropy (ApEn), Sample Entropy (SampEn) and Multiscale Entropy (MSE) were calculated. Results. DE, ApEn and SampEn showed no significant changes between both experimental conditions. All MSE significantly decreased from the spontaneous walking to treadmill walking. Conclusions. There was a loss of complexity signal when subjects walked on a treadmill. This should be taken into account for future studies under these conditions(AU)

CO2 laser cordectomy as a day-case procedure.

OBJECTIVE: To evaluate the safety of CO2 laser cordectomy in the treatment of glottic carcinoma as a day-case procedure. STUDY DESIGN: This was a retrospective study of a cohort of 73 patients with glottic carcinoma treated by laser cordectomy as a day-case procedure using a CO2 laser, between January 2000 and June 2004. RESULTS: There were no major complications and no patients required reintubation. More than 90 per cent of the patients were discharged on the day of the procedure. There were seven unplanned admissions to hospital but none of these appeared to be the direct result of the surgery, and the patients were discharged the next day. All the patients stated they would have further surgery in this manner if required. CONCLUSION: Laser cordectomy for glottic carcinoma can be safely performed as an out-patient procedure if patients are carefully selected according to specific criteria.

[Inverted papilloma: treatment and evolution]. / Papiloma invertido: tratamiento y evolución.

Inverted Papilloma is a frequent bening nasosinusal tumour. The high frequency of recurrences and the malignant potential are its clinical characteristics. We present our experience in a retrospective study done in the last 10 years. 10 patients with Inverted Papilloma have been treated during that period. According to the Classification made by krouse who divides the Inverted Papilloma in four stages, we found 3 in stage I, 6 in stage II and 1 in stage IV. The treatment in almost all of the patients in Stage I and II was endonasal surgery. External approach was performed for stage IV. The recurrence rate was 30% and malignization occurred in 10% of the cases. Finally, we have compared other publications, and revised the literature.

Papiloma invertido: tratamiento y evolución / Inverted papilloma: treatment and evolution

El papiloma invertido es una tumoración que afecta principalmente a las cavidades nasosinusales. Clínicamente se caracteriza por su elevado porcentaje de recidivas y la posibilidad de malignización. Presentamos nuestra experiencia de 10 años, a través de un estudio retrospectivo en el cual encontramos 10 pacientes portadores de esta patología. De acuerdo a la clasificación de Krouse, quien divide los Papilomas Invertidos en cuatro estadios, encontramos a tres en Estadio I, seis en Estadio II y uno en Estadio IV. El tratamiento efectuado fue principalmente cirugía endonasal en Estadio I y II y abordaje externo en Estadio IV. La recidiva fue del 30 por ciento y la malignización del 10 por ciento. Finalmente realizamos una comparación con la experiencia acumulada en la literatura revisada. (AU)

Inverted Papilloma is a frequent benign nasosinusal tumour. The high frequency of recurrences and the malignant potential are its clinical characteristics. We present our experience in a retrospective study done in the last 10 years. 10 patients with Inverted Papilloma have been treated during that period. According to the Classification made by krouse who divides the Inverted Papilloma in four stages, we found 3 in stage I, 6 in stage II and 1 in stage IV. The treatment in almost all of the patients in Stage I and II was endonasal surgery. External approach was performed for stage IV. The recurrence rate was 30% and malignization occurred in 10% of the cases. Finally, we have compared other publications, and revised the literature (AU)

Spanish study group on cochlear implants for persons with marginal benefit from acoustic amplification.

Recently the U.S. Food and Drug Administration approved the use of cochlear implants (CI) in patients with severe-to-profound sensorineural hearing loss with marginal benefit (< 30% speech discrimination) from hearing aids (HA). A multicentre trial was developed to determine whether this approval could be applied to the Spanish population. Nine postlingual adults from 3 centres were selected for the study. The mean preoperative unaided PTA threshold (0.25 to 4 KHz) was 108.6 dB HL for the poorer ear and 99.8 dB HL for the better. In all subjects the poorer ear was implanted with a Mini Nucleus 22 device. A single subject design study was used to compare performance with HA preoperatively and with CI postoperatively, or CI with contralateral HA. PTA measures and Spanish open set speech recognition tests were used. Comparing pre-implant levels with results after 6 months of CI use, all subjects improved significantly on all test measures: i) Mean aided soundfield threshold: 73.6 dB vs 39.8 dB ii) Mean bisyllabic recognition: 20% vs 57% iii) Mean consonant discrimination: 20% vs 49% iv) Mean CID sentences: 22% vs 73%. Five subjects discontinued use of the contralateral HA after implantation, three continued occasionally to use the HA, and one continued to use both on a full-time basis. Patients performed significantly better post-implant on speech recognition compared with their best pre-implant HA scores. Therefore the CI appears an appropriate treatment for Spanish speakers with a severe-to-profound hearing loss and marginal benefits with HA.

[Ganglioneuroma of the middle ear. Apropos of a case]. / Ganglioneuroma en oído medio. A propósito de un caso.

We present a clinical case of middle ear Ganglioneuroma correctly diagnosed after its complete exeresis. A previous surgical biopsy of the mass was consistent with an schwannoma of the VIIth cranial nerve. Considering the newness of the case we make this brief note exposing the clinical process as well as the review of the related bibliography of this neoplasm. None previous mention of middle ear ganglioneuroma have been found, according to the reporters perusal.

[Hemangiopericytoma of the maxillary sinus: apropos of a case]. / Hemangiopericitoma del seno maxilar: a propósito de un caso.

Haemangiopericytoma is a rare vascular tumour arising from the proliferation of pericytes which are cells surrounding the capillaries. Since 1949 only 54 cases of haemangiopericytomas of the nose and paranasal sinuses have been reported in the literature. Of the latter, only 8 cases originated from the maxillary sinus. We report a further case originated from the maxillary sinus.

[Chemodectomas of the carotid body. Apropos 2 cases]. / Quemodectomas del corpúsculo carotídeo. A propósito de dos casos.

The chemodectoma a paragangliomas of the carotid body are infrequent tumors in the daily medical practice, but they must be taken into account in the differential diagnosis of any lateral neck mass. We present two new cases, reconsidering the existing literature.