Structure and anomalous solubility for hard spheres in an associating lattice gas model

In this paper we investigate the solubility of a hard-sphere gas in a solvent modeled as an associating lattice gas. The solution phase diagram for solute at 5% is compared with the phase diagram of the original solute free model. Model properties are investigated both through Monte Carlo simulations and a cluster approximation. The model solubility is computed via simulations and is shown to exhibit a minimum as a function of temperature. The line of minimum solubility (TmS) coincides with the line of maximum density (TMD) for different solvent chemical potentials, in accordance with the literature on continuous realistic models and on the "cavity" picture. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4743635]

Stripe-tetragonal phase transition in the two-dimensional Ising model with dipole interactions: Partition function zeros approach

We have performed multicanonical simulations to study the critical behavior of the two-dimensional Ising model with dipole interactions. This study concerns the thermodynamic phase transitions in the range of the interaction delta where the phase characterized by striped configurations of width h = 1 is observed. Controversial results obtained from local update algorithms have been reported for this region, including the claimed existence of a second-order phase transition line that becomes first order above a tricritical point located somewhere between delta = 0.85 and 1. Our analysis relies on the complex partition function zeros obtained with high statistics from multicanonical simulations. Finite size scaling relations for the leading partition function zeros yield critical exponents. that are clearly consistent with a single second-order phase transition line, thus excluding such a tricritical point in that region of the phase diagram. This conclusion is further supported by analysis of the specific heat and susceptibility of the orientational order parameter.

Structure and electronic properties of hydrated mesityl oxide: a sequential quantum mechanics/molecular mechanics approach

The hydration of mesityl oxide (MOx) was investigated through a sequential quantum mechanics/molecular mechanics approach. Emphasis was placed on the analysis of the role played by water in the MOx syn-anti equilibrium and the electronic absorption spectrum. Results for the structure of the MOx-water solution, free energy of solvation and polarization effects are also reported. Our main conclusion was that in gas-phase and in low-polarity solvents, the MOx exists dominantly in syn-form and in aqueous solution in anti-form. This conclusion was supported by Gibbs free energy calculations in gas phase and in-water by quantum mechanical calculations with polarizable continuum model and thermodynamic perturbation theory in Monte Carlo simulations using a polarized MOx model. The consideration of the in-water polarization of the MOx is very important to correctly describe the solute-solvent electrostatic interaction. Our best estimate for the shift of the pi-pi* transition energy of MOx, when it changes from gas-phase to water solvent, shows a red-shift of -2,520 +/- 90 cm(-1), which is only 110 cm(-1) (0.014 eV) below the experimental extrapolation of -2,410 +/- 90 cm(-1). This red-shift of around -2,500 cm(-1) can be divided in two distinct and opposite contributions. One contribution is related to the syn -> anti conformational change leading to a blue-shift of similar to 1,700 cm(-1). Other contribution is the solvent effect on the electronic structure of the MOx leading to a red-shift of around -4,200 cm(-1). Additionally, this red-shift caused by the solvent effect on the electronic structure can by composed by approximately 60 % due to the electrostatic bulk effect, 10 % due to the explicit inclusion of the hydrogen-bonded water molecules and 30 % due to the explicit inclusion of the nearest water molecules.

A LED Based Photometer for Solid Phase Photometry: Zinc Determination in Pharmaceutical Preparation Employing a Multicommuted Flow Analysis Approach

In this work, a LED (light emitting diode) based photometer for solid phase photometry is described. The photometer was designed to permit direct coupling of a light source (LED) and a photodiode to a flow cell with an optical pathlength of 4 mm. The flow cell was filled with adsorbing solid phase material (C-18), which was used to immobilize the chromogenic reagent 1-(2-thiazolylazo)-2-naphthol (TAN). Aiming to allow accuracy assessment, samples were also analyzed employing ICP OES (inductively coupled plasma optical emission spectrometry) methodology. Applying the paired t-test at the 95% confidence level, no significant difference was observed. Other useful features were also achieved: linear response ranging from 0.05 to 0.85 mg L-1 Zn, limit of detection of 9 mu g L-1 Zn (3 sigma criterion), standard deviation of 1.4% (n = 10), sampling throughput of 36 determinations per h, and a waste generation and reagent consumption of 1.7 mL and of 0.03 mu g per determination, respectively.

Parathyroid adenoma apoplexy as a temporary solution of primary hyperparathyroidism: a case report

Abstract Introduction The natural history of patients with spontaneous parathyroid necrosis is unknown. In this case report we describe the clinical course, laboratory, radiographic, bone densitometry tests, parathyroid ultrasonography and scintigraphy examinations of a patient performed over a period of eight years after she first presented with a sudden episode of spontaneous resolution of primary hyperparathyroidism (PHPT). Case presentation A 24-year-old woman with a clinical history and laboratory and radiographic tests compatible with PHPT suffered a sudden episode of cervical pain and presented with clinical evidence of hypocalcemia. Biopsy of a cervical nodule revealed necrotic material compatible with ischemia of the parathyroid. The follow-up of the patient presented four distinct phases: the first, which lasted two years, was compatible with a period of bone hunger during which it was necessary to introduce calcitriol and calcium carbonate. During this period, the patient showed bone mass gain. The second phase was characterized by normalization of calcium and parathyroid hormone levels and its end was difficult to define. During the third phase there was a recurrence of hypercalcemia associated with elevated parathyroid hormone (PTH) levels and loss of bone mass. The last phase corresponded to the interval after parathyroidectomy, which was characterized by normalization of serum levels of calcium and PTH, as well as bone mass gain. Conclusion This case report indicates that spontaneous resolution of PHPT by adenoma necrosis is potentially temporary. Thus, in cases in which a conservative approach is chosen, clinical and laboratory follow-up is indispensable. Bone mass measurement is a useful tool in the follow-up of these cases. However, this option exposes the patient to a potential roller-coaster ride of bone mass gain and loss, whose long term consequences are still unknown.

A LED based photometer for solid phase photometry: zinc determination in pharmaceutical preparation employing a multicommuted flow analysis approach

Neste trabalho é proposto um fotômetro baseado em LED (diodo emissor de luz) para fotometria em fase sólida. O fotômetro foi desenvolvido para permitir o acoplamento da fonte de radiação (LED) e do fotodetector direto na cela de fluxo, tendo um caminho óptico de 4 mm. A cela de fluxo foi preenchida com material sólido (C18), o qual foi utilizado para imobilizar o reagente cromogênico 1-(2-tiazolilazo)-2-naftol (TAN). A exatidão foi avaliada empregando dados obtidos através da técnica ICP OES (espectrometria de emissão por plasma indutivamente acoplado). Aplicando-se o teste-t pareado não foi observada diferença significativa em nível de confiança de 95%. Outros parâmetros importantes encontrados foram faixa de resposta linear de 0,05 a 0,85 mg L-1 Zn, limite de detecção de 9 µg L-1 Zn (n = 3), desvio padrão de 1,4 % (n = 10), frequência de amostragem de 36 determinações por h, e uma geração de efluente e consumo de reagente de 1,7 mL e 0,03 µg por determinação, respectivamente.