Ca/Na montmorillonite and natural Wyoming bentonite (MX-80) have been studied experimentally and theoretically. For a clay system in equilibrium with pure water, Monte Carlo simulations predict a ...large swelling when the clay counterions are monovalent, while in presence of divalent counterions a limited swelling is obtained with an aqueous layer between the clay platelets of about 10 Å. This latter result is in excellent agreement with X-ray scattering data, while dialysis experiments give a significantly larger swelling for Ca montmorillonite in pure water. Obviously, there is one “intra-lamellar” and a second “extra-lamellar” swelling. Montmorillonite in contact with a salt reservoir containing both Na+ and Ca2+ counterions will only show a modest swelling unless the Na+ concentration in the bulk is several orders of magnitude larger than the Ca2+ concentration. The limited swelling of clay in presence of divalent counterions is a consequence of ion−ion correlations, which reduce the entropic repulsion as well as give rise to an attractive component in the total osmotic pressure. Ion−ion correlations also favor divalent counterions in a situation with a competition with monovalent ones. A more fundamental result of ion−ion correlations is that the osmotic pressure as a function of clay sheet separation becomes nonmonotonic, which indicates the possibility of a phase separation into a concentrated and a dilute clay phase, which would correspond to the “extra-lamellar” swelling found in dialysis experiments. This idea also finds support in the X-ray scattering spectra, where sometimes two peaks corresponding to different lamellar spacings appear.
Aqueous dispersions of pure sodium and calcium smectite clays with platelet sizes on the order of a few hundred nanometers were characterized using a combination of cryo-transmission electron ...microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). With monovalent sodium counterions the clay is dispersed as individual platelets, as seen by cryo-TEM, that order into a nematic phase. From SAXS a one-dimensional swelling of the clay in water is observed with the characteristic spacing h s = δ/ϕ c , where h s is the separation between the platelets, δ = 1 nm is the effective platelet thickness, and ϕc is the clay volume fraction in the sample. In calcium montmorillonite, on the other hand, cryo-TEM images clearly show the presence of tactoids, where the platelets have aggregated into stacks with a periodic spacing of 2 nm. From imaging a large number of tactoids the distribution function f(N) for the number of platelets per tactoid was estimated, and the average number ⟨N⟩ ≈ 10. The characteristic 2 nm spacing as well as the small number of platelets per tactoid was also confirmed by SAXS. The present study demonstrates that cryo-TEM, with carefully prepared specimen, is a very useful technique to characterize clay dispersions, particularly in aggregated systems.
The interaction of two oppositely charged surfaces has been investigated using Monte Carlo simulations and approximate analytical methods. When immersed in an aqueous electrolyte containing only ...monovalent ions, two such surfaces will generally show an attraction at large and intermediate separations. However, if the electrolyte solution contains divalent or multivalent ions, then a repulsion can appear at intermediate separations. The repulsion increases with increasing concentration of the multivalent salt as well as with the valency of the multivalent ion. The addition of a second salt with only monovalent ions magnifies the effect. The repulsion between oppositely charged surfaces is an effect of ion−ion correlations, and it increases with increasing electrostatic coupling and, for example, a lowering of the dielectric permittivity enhances the effect. An apparent charge reversal of the surface neutralized by the multivalent ion is always observed together with a repulsion at large separation, whereas at intermediate separations a repulsion can appear without charge reversal. The effect is hardly observable for a symmetric multivalent salt (e.g., 2:2 or 3:3).
This document was prepared as part of the briefing material for the Workshop of the CERN Council Strategy Group, held in DESY Zeuthen from 2nd to 6th May 2006. It gives an overview of the physics ...issues and of the technological challenges that will shape the future of the field, and incorporates material presented and discussed during the Symposium on the European Strategy for Particle Physics, held in Orsay from 30th January to 2nd February 2006, reflecting the various opinions of the European community as recorded in written submissions to the Strategy Group and in the discussions at the Symposium.
Monte Carlo simulations and density functional calculations have been performed for charged macromolecules confined to planar slits. The force between the confining walls has been evaluated as a ...function of separation, while keeping the chemical potential of the macromolecules constant. Highly charged spherical particles and flexible polyelectrolyte chains in confinement give rise to depletion and structural oscillatory forces as a function of surface separation. The sign and magnitude of the surface charge of the confining walls have no dramatic effect on the qualitative behavior of the confined liquid. With neutral or oppositely charged surfaces, an accumulation of charged macroions is seen in the slit driven by the repulsive interaction between the macroions, while equally charged surfaces give rise to a pure depletion. The net charge, the range of interaction, and the particle density affect the details of the force curve. For spherical macroions, the period of the oscillations scales approximately as the bulk aggregate concentration, c bulk -1/3. Confined polyelectrolyte chains share some of these properties, but they partly display a different behavior. One clear difference is that the polyelectrolyte net charge, that is, the degree of polymerization, has no effect on the osmotic pressure. This is an indication that polyelectrolyte chains pack not as spheres but rather as cylindrical objects. Another difference is that the effective repulsive interaction between polyelectrolyte chains can be more long ranged and oscillatory forces can appear more readily than for a corresponding solution of equally charged spherical macroions.
Aging studies for the ATLAS Transition Radiation Tracker (TRT) Akesson, T.; Barberio, E.; Bondarenko, V. ...
International Workshop on Aging Phenomena in Gaseous Detectors,Hamburg, Germany,2001-10-02 - 2001-10-05,
12/2003, Letnik:
515, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
A summary of the aging and material validation studies carried out for the ATLAS Transition Radiation Tracker (TRT) is presented. Particular emphasis is put on the different phenomena observed in ...straw tubes operating with the chosen Xe/CF
4/CO
2 mixture. The most serious effects observed are silicon deposition on the anode wire and damage of the anode wire gold plating. Etching phenomena and active radical effects are also discussed. With a careful choice of all materials and components, and with good control of the water contamination in the active gas, the ATLAS TRT will operate reliably for 10 years at the LHC design luminosity. To demonstrate this fully, more work is still needed on the gas system purification elements, in particular to understand their interplay with the active species containing fluorine created in the avalanche process under irradiation.
When a protein molecule approaches a charged surface, its protonation state can undergo dramatic changes due to the imposed electric potential. This has a large impact on adsorption strengths that ...may be enhanced by several kT. Using mesoscopic simulation techniques as well as analytical theories, we have investigated this regulation mechanism and demonstrate how it is influenced by salt concentration and solution pH. Using hisactophilin as a test case, we show how the binding to a lipid membrane is governed by small changes in pH and that this is intimately coupled to the charge regulation mechanism.
We used the combined technique of steroid autoradiography and immunohistochemistry to simultaneously visualize estrogen-concentrating and substance P-immunoreactive (sP-IR) cells. A substantial ...proportion (26.1%) of sP-IR cells that bound estrogen was found in the anterior two thirds of the arcuate nucleus with a rapidly diminishing number of cases (8.7%) in the posterior extent of this nucleus. Highest proportions (42.9%) of sP-IR cells retaining estrogen were found in the ventrolateral part of the ventromedial hypothalamic nucleus. The ventral premammillary nucleus was observed to have both populations but coexistence was comparatively infrequent (2.4%). Occasional cases of coexistence were also found in the lateral hypothalamic area. These results provide anatomical evidence that a subset of sP-IR neurons are directly regulated by estrogen and support the hypothesis that sP-containing cells participate in the transduction of hormonal information into a CNS circuitry that regulates gonadotropin secretion and sexual behavior.
Both natural and synthetic polyelectrolytes form strong complexes with a variety of proteins. One peculiar phenomenon is that association can take place even when the protein and the polyelectrolyte ...carry the same charge. This has been interpreted as if the ion−dipole interaction can overcome the repulsive ion−ion interaction. On the basis of Monte Carlo simulations and perturbation theory, we propose a different explanation for the association, namely, charge regulation. We have investigated three different protein−polymer complexes and found that the induced ionization of amino acid residues due to the polyelectrolyte leads to a surprisingly strong attractive interaction between the protein and the polymer. The extra attraction from this charge-induced charge interaction can be several kT and is for the three cases studied here, lysozyme, α-lactalbumin, and β-lactoglobulin, of the same magnitude or stronger than the ion−dipole interaction. The magnitude of the induced charge is governed by a response function, the protein charge capacitance 〈Z 2〉 − 〈Z〉2. This fluctuation term can easily be calculated in a simulation or measured in a titration experiment.
Male and female rats have approximately equal numbers of estrogen(E)-concentrating cells within the medial preoptic area (MPOA). Several cell groups within this brain region are sexually dimorphic, ...however, and these groups may have sexually different numbers of E-containing cells; this, in turn, may reflect sex differences in neural-regulated functions. In order to study this possibility, the distribution of E-concentrating cells was determined using estrogen autoradiography. Except for the lateral portion of the medial preoptic nucleus (MPNl), the density of E-concentrating cells was 3-5-times higher within the most medially situated cell groups of the female than the male, i.e., within the anteroventral periventricular nucleus (AVPv), periventricular preoptic area (PVPO), medial portion of the medial preoptic nucleus (MPNm), and its central portion (MPNc). In addition, we determined whether E-concentrating cells also express the neuropeptide, galanin. An average of 13% of the E-concentrating cells were galanin positive, which represented 15% of the galanin-immunoreactive population. These results demonstrate a frank and dramatic sex difference in the distribution of E-concentrating cells within sexually dimorphic regions of the MPOA, and also suggest that an interaction between galanin and gonadal steroids may be an important means by which cells within the MPOA regulate reproductive function.
PDF
