High-resolution birefringence investigation of octylcyanobiphenyl (8CB): an upper bound on the discontinuity at the smectic-A to nematic phase transition.

We report optical birefringence data by two different methods with high temperature resolution for octylcyanobiphenyl (8CB) near the smectic-A to nematic (Sm-A-N) phase transition temperature T(AN). Within the resolution of our experiments, we find that the Sm-A-N phase transition is continuous. For a possible discontinuity in the orientational order parameter S(T) at T(AN), we arrive at an upper limit of 0.0002, which is substantially smaller than other estimates in literature, but consistent with the value of 0.00008 derived from the upper limit of the latent heat from high-resolution adiabatic scanning calorimetry (ASC), which is itself consistent with the Halperin-Lubensky-Ma theory. The temperature derivative of the order parameter exhibits a power law divergence with a critical exponent that is consistent with the value α = 0.31 ± 0.03 for the specific heat capacity obtained by ASC.

Evidence from adiabatic scanning calorimetry for the Halperin-Lubensky-Ma effect at the N-SmA phase transitions in mixtures of 7OCB+heptane with an injected SmA phase.

The high-resolution adiabatic scanning calorimetric technique has been used to investigate the nematic-smectic A transition (N-SmA in binary mixtures of the non-smectogenic liquid crystal heptyloxycyanobiphenyl (7OCB) and heptane, exhibiting a so-called injected smectic A phase. With the exception of a mixture with the lowest heptane mole fraction for which only an upper limit of 0.2 ± 0.2 J kg(-1) for a possible latent heat could be obtained, for all other mixtures finite latent heats were obtained. The mole fraction dependence of the latent heat could be well fitted with a crossover function consistent with a mean-field free energy expression with a non-zero cubic term arising from the Halperin-Lubensky-Ma (HLM) coupling between the SmA order parameter and the orientational director fluctuations. The mole fraction dependence of the heat capacity effective critical exponents is similar to that observed in mixtures of the two liquid crystals octyloxycyanobiphenyl (8OCB) and nonylcyanobiphenyl (9OCB). The thermal behavior observed along the N-SmA phase transition line yields further strong evidence for the HLM coupling effect.

Weakly first-order character of the nematic-isotropic phase transition in liquid crystals.

The classification of phase transitions in first-order and second-order (or continuous) ones is widely used. The nematic-to-isotropic (NI) transition in liquid crystals is a weakly first-order transition, with only small discontinuities in enthalpy and specific volume at the transition which are not always easy to measure. On the other hand, fluctuation effects near the transition, typical for a continuous transition, are present because of the only weakly first-order character. In a recent paper [Phys. Rev. E 69, 022701 (2004)], it was concluded from the static dielectric permittivity in the isotropic phase near the NI transition that less polar mesogens (with little or no pretransitional effects) are characteristic for a first-order NI phase transition, whereas in the case of strongly polar ones (with large pretransitional effects) the NI transition is close to second order. In this paper, we address the question whether it is, indeed, possible to use these fluctuation effects in the isotropic phase to quantify the "strength" of a weakly first-order transition, i.e., how far it is from second order. Therefore, we measured the temperature dependence of the enthalpy near the NI transition of seven liquid crystals with adiabatic scanning calorimetry and compared the measured values of the latent heat with pretransitional effects in the dielectric constant and the specific heat capacity. The compounds used in the comparison are MBBA, 5CB, 8CB, 5NCS, 5CN, 8CHBT, and D7AB. From our analysis we find, contrary to the assertion in the above reference, no correlation between the strength of the NI transition of a given compound and the pretransitional effects observed, neither dielectrically, nor thermally.

Dielectric spectroscopy of aerosil-dispersed liquid crystal embedded in Anopore membranes.

The complex dielectric permittivity epsilon* values are presented for aerosil-dispersed 4-pentyl-4-cyanobiphenyl (5CB) confined in Anopore membranes. The dielectric permittivities are measured in the frequency range from 10(-2) Hz to 1 GHz at temperatures ranging from 50 degrees C down to -20 degrees C. In bulk 5CB, which has only a nematic phase, there exist two main relaxation processes: one due to the rotation of molecules around their short axes for parallel orientation of the director to the probing field and another fast relaxation process due to the librational motion of molecules for perpendicular orientation. Inside Anopore membranes both these main relaxation processes can be observed, but with subtle differences. The relaxation process due to the rotation of molecules around the short axis is faster in Anopores at all temperatures in comparison with the bulk process. Hydrophilic aerosil particles, when dispersed in the liquid-crystal (LC) phase, attach to each other via hydrogen bonds and form a three-dimensional interconnecting aerosil network, thus dividing the LC phase into small domains. Dispersing 5CB with different concentrations of hydrophilic aerosils leads to a decrease in relaxation time with aerosil concentration. In these dispersed systems a slow additional relaxation process emerges. This slow process becomes stronger with higher concentrations of aerosil. From our experiments we conclude that this process is the relaxation of 5CB molecules homeotropically aligned to the surface of the aerosil particles. In the case of 5CB-aerosil dispersions enclosed in Anopore membranes this slow process still exists and increases also with aerosil concentration. The relaxation time of the rotation of the 5CB molecules around their short axis systematically increases in these 5CB-aerosil samples in Anopore membranes with aerosil concentration from the 5CB-Anopore behavior towards the behavior observed for 5CB-aerosil dispersions. We explain the evolution as resulting from opposing tendencies from size effects (in the Anopore membranes) and disorder effects (in the aerosil dispersions).

Influence of nanosized confinements on 4-n-decyl-4'-cyanobiphenyl (10CB): a broadband dielectric study.

Real (epsilon') and imaginary (epsilon") parts of the complex dielectric permittivity (epsilon*) of the liquid crystal (LC) 4-n-decyl-4'-cyanobiphenyl (10CB) embedded in Anopore membranes and Vycor porous glass, as well as dispersed with hydrophilic aerosils, have been studied by means of broadband dielectric spectroscopy in the frequency range from 10(-2) Hz to 1 GHz . In bulk 10CB, which has a direct transition from an isotropic to a smectic- A phase, there exists one main relaxation process for the parallel orientation of the director with respect to the probing field and a faster one for the perpendicular orientation. All molecular relaxation processes in 10CB are of Debye type and have Arrhenius like temperature dependence. For 10CB embedded in untreated and surface treated cylindrical pores of Anopore membranes the dielectric spectra are similar to the bulk with the exception that both the rotation around the short axis and the libration motion are faster in the pores. In the case of 10CB dispersed with two different concentrations of hydrophilic aerosils an emergence of a slow relaxation process, which is stronger for the higher concentration, is additionally observed along with the bulklike processes. The slow process in the LC-hydrophilic aerosil system is attributed to the relaxation of the molecules that are homeotropically aligned close to the surfaces of the aerosil particles. This process also has an Arrhenius type of temperature dependence. For 10CB embedded in narrow channels of Vycor porous glass three relaxation processes are observed. Two of these processes are bulklike and are due to the librational motion of molecules and the rotation of molecules around their short axes. The slowest process seems to be a surface process, similar in origin to that observed for 10CB dispersed with hydrophilic aerosils, and is prominent amongst all. The material in the Vycor porous glass could be supercooled by at least 185 degrees below bulk crystallization temperature. The slow process has a Vogel-Fulcher-Tamman (VFT) type of temperature dependence typical for glass formers in this wide temperature range. In addition, the bulklike processes are found to be strongly modified and also have a VFT like temperature dependence from measurements done in a wide temperature range. This behavior is in contrast to other reports of glassy behavior in confined LC, where the glassy behavior as concluded from a slow relaxation process observed in a narrow temperature range.

Laparoscopic transabdominal pericardial window: new standard in the treatment of recurrent pericardial effusion complicated by cardiac tamponade.

Creation of a transabdominal transdiaphragmatic pericardial window for life-threatening recurrent pericardial effusion has proved to be a safe minimally invasive technique. By inducing adequate pericardial sac decompression while avoiding single-lung ventilation and thoracic drainage in severely ill patients, it provides anatomopathologic diagnosis and can direct further therapeutic measures. The transabdominal approach improves postoperative recovery dramatically by limiting postoperative pain and prevents sometimes invalidating intercostal neuralgia. Transabdominal pericardial sac fenestration should be part of the armamentarium used by every minimally invasive surgeon.

Endosulfan transport: I. Integrative assessment of airborne and waterborne pathways.

To reduce endosulfan (C9H6O3Cl6S; 6,7,8,9,10,10-hexachloro-1,5, 5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin 3-oxide) contamination in rivers and waterways, it is important to know the relative significances of airborne transport pathways (including spray drift, vapor transport, and dust transport) and waterborne transport pathways (including overland and stream runoff). This work uses an integrated modeling approach to assess the absolute and relative contributions of these pathways to riverine endosulfan concentrations. The modeling framework involves two parts: a set of simple models for each transport pathway, and a model for the physical and chemical processes acting on endosulfan in river water. An averaging process is used to calculate the effects of transport pathways at the regional scale. The results show that spray drift, vapor transport, and runoff are all significant pathways. Dust transport is found to be insignificant. Spray drift and vapor transport both contribute low-level but nearly continuous inputs to the riverine endosulfan load during spraying season in a large cotton (Gossypium hirsutum L.)-growing area, whereas runoff provides occasional but higher inputs. These findings are supported by broad agreement between model predictions and observed typical riverine endosulfan concentrations in two rivers.

Effect of stabilizing versus destabilizing interactions on plasminogen activator inhibitor-1.

Plasminogen activator inhibitor-1 (PAI-1) is a unique member of the serpin family, as it spontaneously converts into a latent conformation. However, the exact mechanism of this conversion is not known. Previous studies reported that neutralizing monoclonal antibodies as well as reversal or removal of charges on the s3C-s4C turn results in a destabilization of PAI-1 leading to an accelerated conversion to its latent form. In this study the effect of the reversal or removal of charges in this "gate region" (R186E/R187E, H190E/K191E, H190L/K191L and R356E) on a stable PAI-1-variant (PAI-1-stab) was investigated. Whereas PAI-1-stab has a half-life of 150+/-66 h, PAI-1-stab-R186E-R187E, PAI-1-stab-H190E-K191E, PAI-1-stab-H190L-K191L and PAI-1-stab-R356E have a strongly decreased half-life (p< 0.005 versus PAI-1-stab) of 175+/-48 min, 75+/-34 min, 68+/-38 min and 79+/-16 min, respectively. Wild-type PAI-1 (wtPAI-1) had a half-life of 55+/-19 min. These data indicate that the stabilization induced by the mutated residues in PAI-1-stab is counteracted by the additional mutations, resulting in half-lives similar to that of wtPAI-1, thereby suggesting that the stabilizing and destabilizing forces act mainly independently in these mutants. Extrapolation of these data to other (stable) serpins leads to the hypothesis that the s3C-s4C turn and the distal hinge region of the reactive site loop plays a role for the stability of serpins in general.

Effect of stabilizing versus destabilizing interactions on plasminogen activator inhibitor-1.

Plasminogen activator inhibitor-1 (PAI-1) is a unique member of the serpin family, as it spontaneously converts into a latent conformation. However, the exact mechanism of this conversion is not known. Previous studies reported that neutralizing monoclonal antibodies (MAs) as well as reversal or removal of charges on the s3C-s4C turn ('gate-region') result in a destabilization of PAI-1 leading to an accelerated conversion to its latent form. In this study the effect of the reversal or removal of charges in this 'gate region' (R186E/R187E, H190E/K191E, H190L/K191L and R356E) on a stable PAI-1-variant (PAI-1-stab) was investigated. PAI-1-stab-R186E-R187E, PAI-1-stab-H190E-K191E, PAI-1-stab-H190L-K191L and PAI-1-stab-R356E have a strongly decreased half-life (p < 0.005 versus PAI-1-stab) resulting in half-lives similar or slightly increased to that of wild-type PAI-1 (wtPAI-1). Reversal of the positively charged residues at position 186/187 or at position 356 and removal at position 190/191 do not only have a destabilizing effect on the active conformation, but also on the substrate conformation. However, reversal of the charge at position 190/191 does not affect the stability of the substrate conformation. Moreover this study is to the best of our knowledge the first to report that the distal hinge region of the reactive site loop (Arg 356) may be involved in the substrate behaviour of PAI-1. In addition our results suggest an important role for the gate-region for the inactivation of PAI-1 through the conversion to latency both for the destabilizing mutations and for the neutralizing MAs.