Ab initio molecular-dynamics study of supercritical carbon dioxide

Car-Parrinello molecular-dynamics simulations of supercritical carbon dioxide (scCO2) have been performed at the temperature of 318.15 K and at the density of 0.703 g/cc in order to understand its microscopic structure and dynamics. Atomic pair correlation functions and structure factors have been obtained and good agreement has been found with experiments. In the supercritical state the CO2 molecule is marginally nonlinear, and thus possesses a dipole moment. Analyses of angle distributions between near neighbor molecules reveal the existence of configurations with pairs of molecules in the distorted T-shaped geometry. The reorientational dynamics of carbon dioxide molecules, investigated through first- and second-order time correlation functions, exhibit time constants of 620 and 268 fs, respectively, in good agreement with nuclear magnetic resonance experiments. The intramolecular vibrations of CO2 have been examined through an analysis of the velocity autocorrelation function of the atoms. These reveal a red shift in the frequency spectrum relative to that of an isolated molecule, consistent with experiments on scCO2. The results have also been compared to classical molecular-dynamics calculations employing an empirical potential

Persistence in an antiferromagnetic Ising model with conserved magnetisation

We obtain the persistence exponents for an antiferromagnetic Ising system in which the magnetisation is kept constant. This system belongs to Model C (system with non-conserved order parameter with a conserved density) and is expected to have persistence exponents different from that of Model A (system with no conservation) but independent of the conserved density. Our numerical results for both local persistence at zero temperature and global persistence at the critical temperature however indicate that the exponents are dependent on the conserved magnetisation in both two and three dimensions. This nonuniversal feature is attributed to the presence of the conserved field and is special to the persistence phenomena.Comment: 8 pages, to be published in Physica A (Proceedings of the Statphys-Kolkata IV, 2002

An alternative order parameter for the 4-state Potts model

We have investigated the dynamic critical behavior of the two-dimensional 4-state Potts model using an alternative order parameter first used by Vanderzande [J. Phys. A: Math. Gen. \textbf{20}, L549 (1987)] in the study of the Z(5) model. We have estimated the global persistence exponent $\theta_g$ by following the time evolution of the probability $P(t)$ that the considered order parameter does not change its sign up to time $t$. We have also obtained the critical exponents $\theta$, $z$, $\nu$, and $\beta$ using this alternative definition of the order parameter and our results are in complete agreement with available values found in literature.Comment: 6 pages, 6 figure

A comparative study of the dynamic critical behavior of the four-state Potts like models

We investigate the short-time critical dynamics of the Baxter-Wu (BW) and $n=3$ Turban (3TU) models to estimate their global persistence exponent $\theta _{g}$. We conclude that this new dynamical exponent can be useful in detecting differences between the critical behavior of these models which are very difficult to obtain in usual simulations. In addition, we estimate again the dynamical exponents of the four-state Potts (FSP) model in order to compare them with results previously obtained for the BW and 3TU models and to decide between two sets of estimates presented in the current literature. We also revisit the short-time dynamics of the 3TU model in order to check if, as already found for the FSP model, the anomalous dimension of the initial magnetization $x_{0}$ could be equal to zero

SN1 Reactions in Supercritical Carbon Dioxide in the Presence of Alcohols: the Role of Preferential Solvation

Ethanol (3b) inhibits SN1 reactions of alkyl halides 1 in supercritical carbon dioxide (scCO2) and gives no ethers as products. The unexpected behaviour of alcohols 3 in the reaction of alkyl halides 1 with 1,3-dimethoxybenzene (2) in scCO2 under different conditions is rationalised in terms of Bronsted and Lewis acid-base equilibria of reagents, intermediates, additives and products in a singular solvent characterised by: i) the strong quadrupole and Lewis acid character of carbon dioxide, which hinders SN2 paths by strongly solvating basic solutes; ii) the weak Lewis base character of carbon dioxide, which prevents it from behaving as a proton sink; iii) the compressible nature of scCO2, which enhances the impact of preferential solvation on carbon dioxide availability for the solvent-demanding rate determining step

Effectiveness of acupuncture, special dressings and simple, low-adherence dressings for healing venous leg ulcers in primary healthcare: study protocol for a cluster-randomized open-labeled trial

<p>Abstract</p> <p>Background</p> <p>Venous leg ulcers constitute a chronic recurring complaint that affects 1.0–1.3% of the adult population at some time in life, and which corresponds to approximately 75% of all chronic ulcers of the leg. Multilayer compression bandaging is, at present, the only treatment that has been proved to be effective in treating this type of ulcer. There is no consensus, however, about the dressings that may be applied, beneath the compression, to promote the healing of this type of ulcer, as there does not seem to be any added benefit from using special dressings rather than simple, low-adherence ones. As well as analgesia, acupuncture provokes peripheral vasodilation, in skin and muscles – which has been demonstrated both experimentally and in clinical practice – probably due to the axon reflex, among other mechanisms. The aim of the present study is to measure the effectiveness and cost of compression treatment for venous leg ulcers combined with special dressings, in comparison with low-adherence ones and acupuncture.</p> <p>Methods/design</p> <p>Cluster-randomized open-labeled trial, at 15 primary healthcare clinics in the Sevilla-Sur Healthcare District, with a control group treated with compression bandaging and low-adherence dressings; the experiment will consist, on the one hand, of the compression treatment applied in combination with special dressings (Treatment 1), and on the other, the compression treatment applied in association with low-adherence dressings, together with acupuncture (Treatment 2).</p> <p>Discussion</p> <p>The results will be measured and recorded in terms of the median time elapsed until complete healing of the ulcer, and the rate of complete healing at 3 months after beginning the treatment. An economic analysis will also be made.</p> <p>This study, carried out in the context of real clinical practice, will provide information for decision-taking concerning the effectiveness of special dressings. Moreover, for the first time a high-quality study will evaluate the effectiveness of acupuncture in the process of healing venous leg ulcers.</p> <p>Trial registration</p> <p>Current Controlled Trials ISRCTN26438275.</p

The role of leucocytes in the pathogenesis of skin damage due to chronic venous disease

Leucocyte trapping in the microcirculation has been suggested as the cause of skin damage seen in chronic venous disease. In this thesis the effect of experimental venous hypertension on circulating leucocytes and the vascular endothelium was investigated. Normal volunteers and patients with chronic venous disease both with and without skin damage were studied. Activation markers for neutrophils and monocytes were measured, before and after exposure to experimental venous hypertension. Markers of endothelial activation were also investigated. Finally, leucocyte migration from the circulation into the tissues, and the role of cytokines were explored. Neutrophil and monocyte activation was investigated by measuring the cell surface expression of the integrin CD11b/CD18 and the selectin CD62L by a whole blood assay using fluorescent-labelled monoclonal antibodies. CD62L, which is shed by leucocytes upon activation, was also measured in plasma by a commercially available ELISA. Endothelial adhesion molecules VCAM-1, ICAM-1, ELAM-1 and vWf which act as counter ligands for adhesion molecules expressed by leucocytes and are shed into the plasma were measured by commercially available ELISAs. Neutrophil and monocyte activation occurred in control subjects and patients following experimental venous hypertension. Leucocyte-endothelial adhesion was greater in patients than controls. In-vivo leucocyte migration in volunteers subjected to short term venous hypertension was investigated using the 'Skin Window Technique'. Superficial dermal skin abrasions were made and emigrating leucocytes collected on micropore membranes placed over the abrasions. These were stained and examined by light microscopy to determine the cell types which had emigrated and the distance travelled by the cells within the membrane. Leucocyte migration in the tissue decreased following experimental venous hypertension. The cytokines IL-6 and TNF-α were measured by an ELISA and found to be higher following venous hypertension. These studies show that short-term, experimental, venous hypertension causes leucocyte and endothelial activation and adhesion, which is known to precede extra-vascular migration. Following emigration leucocyte locomotion decreases which may lead to localised accumulation of activated leucocytes releasing toxic metabolites and free radicals resulting in tissue damage