| 1. |
- Sanchez-Vega, M., et al.
(author)
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Studies of quadrupole collectivity in the γ -soft 106Ru
- 2008
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In: European Physical Journal A. - 1434-6001 .- 1434-601X. ; 35:2, s. 159-165
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Journal article (peer-reviewed)abstract
- Various alternative models were used to describe the structure of 106Ru . For example, the General Collective Model (GCM) predicts shape-coexistence for 106Ru with a spherical and a triaxial minimum and strongly mixed structures, while in the IBA-2 calculations, where 106Ru was considered as transitional from vibrational U(5) to γ -soft O(6) , no need was found to include the shape-coexisting configurations. In order to provide additional constraints on the model interpretations, we have applied the Advanced Time-Delayed (ATD) βγγ(t) method to measure the level lifetimes of the excited levels in 106Ru . The new results include the half-lives of T 1/2 = 183(3) ps and 7.5(30)ps for the 2+ 1 and 2+ 2 states, respectively.
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| 2. |
- Eriksson, Urban, 1968-, et al.
(author)
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Limits of ultra-high-precision optical astrometry : stellar surface structures
- 2007
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 476:3, s. 1389-1400
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Journal article (peer-reviewed)abstract
- Aims. To investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry (e.g. in the context of exoplanet searches) and to quantify the expected effects in different regions of the HR-diagram. Methods. Stellar surface structures (spots, plages, granulation, non-radial oscillations) are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter. We use theoretical considerations supported by Monte Carlo simulations (using a starspot model) to derive statistical relations between the corresponding astrometric, photometric, and radial velocity effects. Based on these relations, the more easily observed photometric and radial velocity variations can be used to predict the expected size of the astrometric jitter. Also the third moment of the brightness distribution, interferometrically observable as closure phase, contains information about the astrometric jitter. Results. For most stellar types the astrometric jitter due to stellar surface structures is expected to be of the order of 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (< 0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of the order of 1 micro-AU, sufficient to allow the astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general a negligible impact on the detection of large (Jupiter-size) planets and on the determination of stellar parallax and proper motion. From the starspot model we also conclude that the commonly used spot filling factor is not the most relevant parameter for quantifying the spottiness in terms of the resulting astrometric, photometric and radial velocity variations.
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| 3. |
- Aslund, Ingrid, et al.
(author)
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Diffusion NMR for determining the homogeneous length-scale in lamellar phases.
- 2008
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In: Journal of Physical Chemistry B. - : The American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 112:10, s. 2782-2794
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Journal article (peer-reviewed)abstract
- The size of the anisotropic domains in a lyotropic liquid crystal is estimated using a new protocol for diffusion NMR. Echo attenuation decays are recorded for different durations of the displacement-encoding gradient pulses, while keeping the effective diffusion time and the range of the wave vectors constant. Deviations between the sets of data appear if there are non-Gaussian diffusion processes occurring on the time-scale defined by the gradient pulse duration and the length-scale defined by the wave vector. The homogeneous length-scale is defined as the minimum length-scale for which the diffusion appears to be Gaussian. Simulations are performed to show that spatial variation of the director orientation in an otherwise homogeneous system is sufficient to induce non-Gaussian diffusion. The method is demonstrated by numerical solutions of the Bloch-Torrey equation and experiments on a range of lamellar liquid crystals with different domain sizes.
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| 4. |
- Cabaleiro-Lago, Celia, et al.
(author)
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Characterization of alkane diol-CD complexes. Acid denitrosation of N-methyl-N-nitroso-p-toluenesulphonamide as a chemical probe
- 2006
-
In: JOURNAL OF INCLUSION PHENOMENA AND MACROCYCLIC CHEMISTRY. - Univ Vigo, Dept Quim Fis, Vigo 36200, Spain. Univ Santiago de Compostela, Dept Quim Fis, Santiago De Compostela 15706, Spain. : SPRINGER. - 1388-3127. ; 54:3-4, s. 209-216
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Journal article (peer-reviewed)abstract
- A study was carried out on the acid denitrosation of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) in mixed systems made up of linear (geminal and terminal) alkyl diols and beta-cyclodextrin (CD). The alkyl diols used allowed us to vary the length of the hydrocarbon chain from 2 to 6 carbon atoms. The observed rate constant, k(obs), decreases in the presence of CD. The inhibition profile decreases as the as the number of carbons in the chain is increased. This behaviour can be interpreted as a consequence of a balance between the complexation processes of MNTS and the alkyl diols by the CD. At a constant CD concentration and increase in the diols concentration decreases the concentration of free cyclodextrin available to complex with MNTS molecules and therefore produces an increases in the observed rate constant. The results were interpreted in terms of two different models; trough the presupposition and non-presupposition of a stoichiometry for the CD-diols complex. Both models agreed quite well and allow us to determine the uncomplexed cyclodextrin concentration in each case as well as the stoichiometry of the complexes. The binding constant for both types of alkane diols increase with increasing the number of carbon in the chain. Besides, the binding constant of the alpha,beta-alkane diols is higher than for the analog alpha,beta-alkane diols. One of the main consequences of this study is that the acid denitrosation of MNTS can be use to obtain the stochiometry of the CD-diol complexes and to monitor the free cyclodextrin concentration.
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| 5. |
- Cabaleiro-Lago, Celia, et al.
(author)
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Effects of zwitterionic vesicles on the reactivity of benzoyl chlorides
- 2006
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In: Journal of Physical Chemistry B. - Univ Vigo, Dept Phys Chem, Fac Chem, Vigo 36310, Spain. Univ Santiago de Compostela, Fac Chem, Dept Phys Chem, Santiago De Compostela 15782, Spain. : AMER CHEMICAL SOC. - 1520-6106 .- 1520-5207. ; 110:16, s. 8524-8530
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Journal article (peer-reviewed)abstract
- A systematic study on the solvolysis reaction of substituted benzoyl chlorides in the presence of zwitterionic vesicles of dipalmitoyl phosphatidylcholine (DPPC) has been performed. Size, shape, surface charge, and polarity of the interface of the vesicular aggregates were determined using various techniques. The application of the pseudophase formalism allowed us to obtain the thermodynamic and kinetic coefficients characteristic of the reaction. The effects of vesicular aggregates on the solvolysis of benzoyl chlorides, which are known to be sensitive to the physical properties of the medium, depend on the nature of the substrate. For benzoyl chlorides with electron-donating groups, which react predominantly through a dissociative mechanism which is strongly affected by medium properties, the rate constant decreases as the vesicle concentration increases. On the other hand, for benzoyl chlorides with electron-withdrawing groups, which react mainly via an associative pathway, DPPC vesicles catalyze the solvolysis reaction.
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| 6. |
- Cabaleiro-Lago, Celia, et al.
(author)
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Fully uncomplexed cyclodextrin in mixed systems of vesicle-cyclodextrin : solvolysis of benzoyl chlorides.
- 2009
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In: Journal of Physical Chemistry B. - 1520-6106 .- 1520-5207. ; 113:19, s. 6749-6755
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Journal article (peer-reviewed)abstract
- In this contribution the influence of beta-cyclodextrin (CD) on the behavior of aqueous systems containing vesicles of dipalmitoyl phosphatidyl choline (DPPC) has been studied by determining the kinetics of the solvolysis reaction of substituted benzoyl chlorides whose solvolysis reactivity entails a high sensitivity on media properties. The application of the pseudophase formalism allowed us to obtain the thermodynamic and kinetic coefficients characteristic of the reaction, which are essentially independent of the concentration of CD. We were able to determine the percentages of uncomplexed cyclodextrin in equilibrium with the vesicular system which were in all cases compatible with 100%. The obtained results led us to conclude that the properties of DPPC vesicles are not affected by the presence of CD in the medium and there is no type of interaction between the CD and the vesicular surfactant monomers and, therefore, all cyclodextrin is present in the mixed system as uncomplexed cyclodextrin.
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| 7. |
- Cabaleiro-Lago, Celia, et al.
(author)
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In search of fully uncomplexed cyclodextrin in the presence of micellar aggregates
- 2006
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In: Journal of Physical Chemistry B. - Univ Vigo, Fac Quim, Dept Quim Fis, Vigo 36310, Spain. Univ Santiago, Fac Quim, Dept Quim Fis, Santiago 15782, Spain. : AMER CHEMICAL SOC. - 1520-6106 .- 1520-5207. ; 110:32, s. 15831-15838
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Journal article (peer-reviewed)abstract
- The chemical behavior of beta-cyclodextrin/nonionic surfactant mixed systems has been investigated using the basic hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide as a chemical probe. The experimental results prove that at the cmc, there are significant quantities of uncomplexed beta-CD in equilibrium with the micellar aggregates. In contrast to the expected situation, the percentage of uncomplexed beta-CD in equilibrium with the micellar system increases on increasing the hydrophobicity of the surfactant molecule. This behavior is due to the existence of two simultaneous processes: complexation of surfactant monomers by cyclodextrin and the process of self-assembly to form micellar aggregates. The autoaggregation of surfactant monomers is expected to be more important than the complexation process in this mixed system. Varying the hydrophobicity of the surfactant monomer enabled us to determine that the percentages of uncomplexed cyclodextrin in equilibrium with the micellar system were in the range of 5-95%.
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| 8. |
- Cabaleiro-Lago, Celia, et al.
(author)
-
Reactivity of benzoyl chlorides in nonionic microemulsions : Potential application as indicators of system properties
- 2005
-
In: Journal of Physical Chemistry B. - Univ Vigo, Fac Chem, Dept Phys Chem, Vigo 36310, Spain. Univ Santiago de Compostela, Fac Chem, Dept Phys Chem, Santiago De Compostela 15782, Spain. : AMER CHEMICAL SOC. - 1520-6106 .- 1520-5207. ; 109:47, s. 22614-22622
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Journal article (peer-reviewed)abstract
- The solvolysis reactivity of benzoyl chlorides entails a high sensitivity on medium properties. A systematic study of the reaction of a series of these substrates, varying the electron-withdrawing character of the substituent, has been performed in nonionic microemulsions. The kinetic effects due to variation of microemulsion compositions can be assigned to modifications in system properties, to be precise, to modifications in interface properties. Microemulsion properties that are obtained from kinetic analysis of solvolysis show a good agreement with the characterization of the microemulsion that was made via H-1 NMR and solvatochromic fluorescence probes. Benzoyl chlorides with electron-donating groups react through a dissociative mechanism, whereas electron-withdrawing groups favor an associative mechanism. A comparative analysis of reactivity between the different substrates at the interface shows a variation in the contributions of both reaction pathways, associative and dissociative, to the whole reaction mechanism. The confined media shift the point where the mechanism changes from an associative to a dissociative pathway, far away from the turning point in water. Furthermore, the change in mechanism can be modulated by modification of the microemulsion composition.
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| 9. |
- Cabaleiro-Lago, Celia, et al.
(author)
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Self-diffusion NMR studies of the host-guest interaction between beta-cyclodextrin and alkyltrimethylammonium bromide surfactants
- 2005
-
In: Langmuir. - Lund Univ, Ctr Chem & Engn, SE-22100 Lund, Sweden. : AMER CHEMICAL SOC. - 0743-7463 .- 1520-5827. ; 21:25, s. 11637-11644
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Journal article (peer-reviewed)abstract
- Diffusion measurements by nuclear magnetic resonance (NMR) spectroscopy were used to investigate the host-guest association between beta-cyclodextrin (CD) and alkyltrimethylammonium bromide surfactants with different chain lengths, ranging from 6 up to 16 carbons. The scope and limitations of the method in the study of formation of inclusion complexes are discussed. The influences of the presence of CD in the micellization process have been studied, and the apparent critical micellar concentration and the self-diffusion coefficients of the species present in the systems have been calculated. The stoichiometries of the different complexes have been determined. Evidence for the formation of a 2:1 complex in the case Of C(16)TAB has been found.
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| 10. |
- Cabaleiro-Lago, Celia, et al.
(author)
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Spectrophotometric study of metal-ligand reactions in isooctane/Brij30/water nonionic microemulsions
- 2007
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In: Colloids and Surfaces A. - Univ Vigo, Dept Quim Fis, Fac Quim, Vigo 36310, Spain. Univ Santiago Compostela, Fac Chem, Dept Chem Phys, Santiago 15782, Spain. : ELSEVIER SCIENCE BV. - 0927-7757 .- 1873-4359. ; 295:1-3, s. 49-54
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Journal article (peer-reviewed)abstract
- The complexation between Ni2+ and pyridine-2-azo-p-dimethylaniline (PADA) was studied in isooctane/polyoxyethylenglycol dodecyl ether (Brij30)/water microemulsions at 25 degrees C. The apparent complexation constant depends on microemulsion composition. The proposed model, which takes into account the heterogeneity of the system at the microscopic scale, allows us to determine the distribution constants of both reactants and the complexation constant at the interface. The complex is less stable in microemulsion than in water due to a more efficient hydration of the nickel hexahydrate coordination complex that arises from the interaction between the polar head group of the surfactant and the interfacial water. (c) 2006 Elsevier B.V. All rights reserved.
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