Internal and free energy in a pair of like-charged colloids: Monte Carlo simulations

The effective interaction between two colloidal particles in a bath of monovalent co- and counterions is studied by means of lattice Monte Carlo simulations with the primitive model. The internal electrostatic energy as a function of the colloid distance is studied fixing the position of the colloids. The free energy of the whole system is obtained introducing a bias parabolic potential, that allows us to sample efficiently small separations between the colloidal particles. For small charges, both the internal and free energy increase when the colloids approach each other, resulting in an effective repulsion driven by the electrostatic repulsion. When the colloidal charge is large enough, on the other hand, the colloid-ion coupling is strong enough to form double layers. The internal energy in this case decreases upon approaching the colloids because more ions enter the double layer. This attractive contribution to the interaction between the colloids is stronger for larger charges and larger ionic concentrations. However, the total free energy increases due to the loss of ionic entropy, and resulting finally in a repulsive interaction potential driven by the entropic contributions. The loss of ionic entropy can be almost quantitatively reproduced with the ideal contribution, the same level of approximation as the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The overall behavior is captured by the DLVO theory qualitatively, and a comparison is made with the functional form predicted by the theory, showing moderate agreement.     

SPME determination of volatile aldehydes for evaluation of in-vitro antioxidant activity

The in-vitro antioxidant activity of natural (essential oils, vitamin E) or synthetic substances ( tert-butyl hydroxy anisole (BHA), Trolox) has been evaluated by monitoring volatile carbonyl compounds released in model lipid systems subjected to peroxidation. The procedure employed methodology previously developed for the determination of carbonyl compounds as their pentafluorophenylhydrazine derivatives which were quantified, with high sensitivity, by means of capillary gas chromatography with electron-capture detection. Linoleic acid and sunflower oil were used as model lipid systems. Lipid peroxidation was induced in linoleic acid by the Fe2+ ion (1 mmol L-1, 37 degrees C, 12 h) and in sunflower oil by heating in the presence of O2 (220 degrees C, 2 h). The change in hexanal (the main lipoxidation product) concentration found in the lipid matrix subjected to oxidation with and without the substance being tested was used to calculate the antioxidant protection effect. These procedures were employed to evaluate the antioxidant activity of the essential oils of cilantro ( Coriander sativum L.), fennel ( Foeniculum vulgare Mill.), rosemary ( Rosmarinus officinalis L.), "salvia negra" ( Lepechinia schiedeana), and oregano ( Origanum vulgare L.), and the well-known antioxidants BHA, vitamin E, and Trolox, its water-soluble analog. In the sunflower oil system, the essential oils had a stronger protective effect against lipid peroxidation than BHA, vitamin E, and Trolox within the range of concentrations examined (1-20 g L-1). The highest protecting effect, corresponding to a 90% drop in hexanal release, was observed for cilantro oil at 10 g L-1.     

Symmetric Products and Social Choice

We apply topological technics to a problem of social choice.     

Idealization of Some Weak Separation Axioms

An ideal is a nonempty collection of subsets closed under heredity and finite additivity. The aim of this paper is to unify some weak separation properties via topological ideals. We concentrate our attention on the separation axioms between T ₀ and T _1/2. We prove that if (X,,I) is a semi-Alexandroff T _I -space and I is a -boundary, then I is completely codense.     

Comparative simulation study of colloidal gels and glasses

Using computer simulations, we identify the mechanisms causing aggregation and structural arrest of colloidal suspensions interacting with a short-ranged attraction at moderate and high densities. Two different nonergodicity transitions are observed. As the density is increased, a glass transition takes place, driven by excluded volume effects. In contrast, at moderate densities, gelation is approached as the strength of the attraction increases. At high density and interaction strength, both transitions merge, and a logarithmic decay in the correlation function is observed. All of these features are correctly predicted by mode coupling theory.     

Core-Modified Coelenterazine Luciferin Analogues: Synthesis and Chemiluminescence Properties

In this work on the design and studies of luciferins related to the blue-hued coelenterazine, the synthesis of heterocyclic analogues susceptible to produce a photon, possibly at a different wavelength, is undertaken. Here, the synthesis of O-acetylated derivatives of imidazo[1,2-b]pyridazin-3(5 H)-one, imidazo[2,1-f][1,2,4]triazin-7(1 H)-one, imidazo[1,2-a]pyridin-3-ol, imidazo[1,2-a]quinoxalin-1(5 H)-one, benzo[f]imidazo[1,2-a]quinoxalin-3(11 H)-one, imidazo[1′,2′:1,6]pyrazino[2,3-c]quinolin-3(11 H)-one, and 5,11-dihydro-3 H-chromeno[4,3-e]imidazo[1,2-a]pyrazin-3-one is described thanks to extensive use of the Buchwald–Hartwig N-arylation reaction. The acidic hydrolysis of these derivatives then gave solutions of the corresponding luciferin analogues, which were studied. Not too unexpectedly, even if these were “dressed” with substituents found in actual substrates of the nanoKAZ/NanoLuc luciferase, no bioluminescence was observed with these compounds. However, in a phosphate buffer, all produced a light signal, by chemiluminescence, with extensive variations in their respective intensity and this could be increased by adding a quaternary ammonium salt in the buffer. This aspect was actually instrumental to determine the emission spectra of many of these luciferin analogues.     

Photoprotective Potential of Baccharis antioquensis (Asteraceae) as Natural Sunscreen

In the quest for new natural agents of photoprotection, we evaluated the photoprotective and antioxidant activity of B. antioquensis leaf extracts as well as its phenolic composition. The methanolic extract treated with activated carbon showed the highest absorption coefficients for UVA‐UVB radiation, as well as an antioxidant capacity comparable to butylated hydroxy toluene. Furthermore, the formulation containing this extract showed suitable sensorial and photostable characteristics for topical use, and significant values of UVAPF, critical wavelength (λ_c), UVA/UVB ratio and sun protection factor (5.3, 378 nm, 0.78 and 9.1 ± 0.1, respectively). In addition, three glycoside derivatives of quercetin, a kaempferol glycoside and a derivative of caffeic acid were the main polyphenolic compounds identified. These results demonstrate the potential of B. antioquensis extracts to be used as active components of novel, natural sunscreens.     

An experimental investigation of change detection in uncertain chain-like systems

Promising ongoing research on “smart” sensing technologies is offering low-cost alternatives and new opportunities for large-scale SHM. Networks of sensors with wireless communication and computational capabilities can be used to increase the spatial resolution of data collection while providing a distributed computing framework for implementing structural health monitoring algorithms. Robust and practical SHM methodologies being able to rapidly and accurately detect and assess changes in the monitored system are required to be at the core of these “smart” structures. A data-driven non-parametric identification technique is used to implement a robust change detection methodology for uncertain MDOF chain-like systems that can be implemented in densely distributed smart-sensor networks. Experimental data from a test-bed structure tested at Los Alamos National Laboratory are used to evaluate the effectiveness and reliability of the proposed SHM methodology. The results of this study showed that the proposed approach was able, in a rigorous statistical framework, to confidently detect the presence of structural changes, accurately locate the structural section where the change occurred, and provide an accurate estimate of the actual level of “change”. Additionally, a full-order finite element model of the test structure, as well as the results from the experimental modal identification using the ERA algorithm were employed to validate the results obtained in this change-detection study.