Picosecond Dynamics of Proton Transfer of a 7-Hydroxyflavylium Salt in Aqueous-Organic Solvent Mixtures

The intermediacy of the geminate base proton pair (A*center dot center dot center dot H(+)) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base proton pair A*center dot center dot center dot H(+). The geminate pair becomes detectable because of the increase in the recombination rate constant, k(rec), of (A*center dot center dot center dot H(+)) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH(+)*, k(d), and recombination, k(rec) of A*center dot center dot center dot H(+)) from the diffusion controlled rates (dissociation, k(diss) and formation, k(diff)[H(+)], of A*center dot center dot center dot H+), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k(d) and k(rec)) as well as on proton diffusion/migration (k(diss) and k(diff)).

Unusual 1,6-diphenyl-1,3,5-hexatriene (DPH) spectrophotometric behavior in water/ethanol and water/DMSO mixtures

The absorption spectra of DPH at fixed concentration do not change with water content in organic solvents. It exhibits monomer bands, such as those obtained in ethanol. The absorption did not change for solutions up to 54 and 46% of water in ethanol and DMSO, respectively, for [DPH] = 5.0 × 10-6 mol L-1 at 30 °C. However, at the same experimental conditions, a gradual sharp decay of the DPH fluorescence is observed. It is proposed that water molecules below these water concentration limits act as quenchers of the excited states of DPH. Stern-Volmer quenching constants by intensities measurements are 7.4 × 10-2 (water/ethanol) and 2.6 × 10-2 L mol-1 (water/DMSO). DPH lifetime measurements in the absence and presence of water resulted in 7.1 × 10-2 L mol-1 in water/ethanol, which pointed out that the process is a dynamic quenching by water molecules. For experiments using DPH as probe, this process can affect data, leading to misunderstanding interpretation.

Localized modes of binary mixtures of Bose-Einstein condensates in nonlinear optical lattices

The properties of the localized states of a two-component Bose-Einstein condensate confined in a nonlinear periodic potential (nonlinear optical lattice) are investigated. We discuss the existence of different types of solitons and study their stability by means of analytical and numerical approaches. The symmetry properties of the localized states with respect to nonlinear optical lattices are also investigated. We show that nonlinear optical lattices allow the existence of bright soliton modes with equal symmetry in both components and bright localized modes of mixed symmetry type, as well as dark-bright bound states and bright modes on periodic backgrounds. In spite of the quasi-one-dimensional nature of the problem, the fundamental symmetric localized modes undergo a delocalizing transition when the strength of the nonlinear optical lattice is varied. This transition is associated with the existence of an unstable solution, which exhibits a shrinking (decaying) behavior for slightly overcritical (undercritical) variations in the number of atoms.

Statistical models of mixtures with a biaxial nematic phase

We consider a simple Maier-Saupe statistical model with the inclusion of disorder degrees of freedom to mimic the phase diagram of a mixture of rodlike and disklike molecules. A quenched distribution of shapes leads to a phase diagram with two uniaxial and a biaxial nematic structure. A thermalized distribution, however, which is more adequate to liquid mixtures, precludes the stability of this biaxial phase. We then use a two-temperature formalism, and assume a separation of relaxation times, to show that a partial degree of annealing is already sufficient to stabilize a biaxial nematic structure.

Thermo-solvatochromism in binary mixtures of water and ionic liquids: on the relative importance of solvophobic interactions

The thermo-solvatochromism of 2,6-dibromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr(2), has been studied in mixtures of water, W, with ionic liquids, ILs, in the temperature range of 10 to 60 degrees C, where feasible. The objectives of the study were to test the applicability of a recently introduced solvation model, and to assess the relative importance of solute-solvent solvophobic interactions. The ILs were 1-allyl-3-alkylimidazolium chlorides, where the alkyl groups are methyl, 1-butyl, and 1-hexyl, respectively. The equilibrium constants for the interaction of W and the ILs were calculated from density data; they were found to be linearly dependent on N(C), the number of carbon atoms of the alkyl group; van't Hoff equation (log K versus 1/T) applied satisfactorily. Plots of the empirical solvent polarities, E(T) (MePMBr(2)) in kcal mol(-1), versus the mole fraction of water in the binary mixture, chi(w), showed non-linear, i.e., non-ideal behavior. The dependence of E(T) (MePMBr(2)) on chi(w), has been conveniently quantified in terms of solvation by W, IL, and the ""complex"" solvent IL-W. The non-ideal behavior is due to preferential solvation by the IL and, more efficiently, by IL-W. The deviation from linearity increases as a function of increasing N(C) of the IL, and is stronger than that observed for solvation of MePMBr(2) by aqueous 1-propanol, a solvent whose lipophilicity is 12.8 to 52.1 times larger than those of the ILs investigated. The dependence on N(C) is attributed to solute-solvent solvophobic interactions, whose relative contribution to solvation are presumably greater than that in mixtures of water and 1-propanol.

The Effect of the Mixtures of Photoinitiators in Polymerization Efficiencies

The photoinitiated polymerization of methyl methacrylate using the mixtures of camphorquinone (CQ) and acylphosphine oxides (monoacylphosphine oxide, MAPO or bisacylphosphine oxide,BAPO) was studied to determine the possible synergistic effects. The addition of the acylphosphines to CQ resulted in an increase of the polymerization rate compared with CQ alone. On the other hand, a significant decrease of the polymerization quantum yield is observed for the mixtures compared with the pure acylphosphines. Therefore, the increase in the polymerization efficiency of the two rnixtures studied, MAPO/CQ and BAPO/CQ (compared with CQ) can be traced to the larger light absorption range, rather than to the onset of new mechanisms. The presence of the coinitiator ethyl 4-dimethylaminobenzoate, EDB, always present in CQ formulations, has no effect at all on the rates of polymerization photoinitiated by the acylphosphine oxides. From the point of view of photopolymerization quantum yields, an antagonistic effect is observed because Of the energy transfer of the M more efficient initiator (MAPO or BAPO) to the less efficient one (CQ). (C) 2008 Wiley Periodicals, Inc. j Appl Polym Sci 112: 129-134, 2009

Experimental Measurement of Water Diffusion through Fine Aggregate Mixtures

The water diffusion attributable to concentration gradients is among the main mechanisms of water transport into the asphalt mixture. The transport of small molecules through polymeric materials is a very complex process, and no single model provides a complete explanation because of the small molecule`s complex internal structure. The objective of this study was to experimentally determine the diffusion of water in different fine aggregate mixtures (FAM) using simple gravimetric sorption measurements. For the purposes of measuring the diffusivity of water, FAMs were regarded as a representative homogenous volume of the hot-mix asphalt (HMA). Fick`s second law is generally used to model diffusion driven by concentration gradients in different materials. The concept of the dual mode diffusion was investigated for FAM cylindrical samples. Although FAM samples have three components (asphalt binder, aggregates, and air voids), the dual mode was an attempt to represent the diffusion process by only two stages that occur simultaneously: (1) the water molecules are completely mobile, and (2) the water molecules are partially mobile. The combination of three asphalt binders and two aggregates selected from the Strategic Highway Research Program`s (SHRP) Materials Reference Library (MRL) were evaluated at room temperature [23.9 degrees C (75 degrees F)] and at 37.8 degrees C (100 degrees F). The results show that moisture uptake and diffusivity of water through FAM is dependent on the type of aggregate and asphalt binder. At room temperature, the rank order of diffusivity and moisture uptake for the three binders was the same regardless of the type of aggregate. However, this rank order changed at higher temperatures, suggesting that at elevated temperatures different binders may be undergoing a different level of change in the free volume. DOI: 10.1061/(ASCE)MT.1943-5533.0000190. (C) 2011 American Society of Civil Engineers.

Rheological, mechanical and mucoadhesive properties of thermoresponsive, bioadhesive binary mixtures composed of poloxamer 407 and carbopol 974P designed as platforms for implantable drug delivery systems for use in the oral cavity

This study described the formulation and characterisation of the viscoelastic, mechanical and mucoadhesive properties of thermoresponsive, binary polymeric systems composed of poloxamer (P407) and poly(acrylic acid, C974P) that were designed for use as a drug delivery platform within the oral cavity. Monopolymeric and binary polymeric formulations were prepared containing 10, 15 and 20% (w/w) poloxamer (407) and 0.10-0.25% (w/w) poly(acrylic acid, 934P). The flow theological and viscoelastic properties of the formulations were determined using controlled stress and oscillatory rheometry, respectively, the latter as a function of temperature. The mechanical and mucoadhesive properties (namely the force required to break the bond between the formulation and a pre-hydrated mucin disc) were determined using compression and tensile analysis, respectively. Binary systems composed of 10% (w/w) P407 and C934P were elastoviscous, were easily deformed under stress and did not exhibit mucoadhesion. Formulations containing 15 or 20% (w/w) Pluronic P407 and C934P exhibited a sol-gel temperature T(sol/gel), were viscoelastic and offered high elasticity and resistance to deformation at 37 degrees C. Conversely these formulations were elastoviscous and easily deformed at temperatures below the sol-gel transition temperature. The sol-gel transition temperatures of systems containing 15% (w/w) P407 were unaffected by the presence of C934P; however, increasing the concentration of C934P decreased the T(sol/gel) in formulations containing 20%(w/w) P407. Rheological synergy between P407 and C934P at 37 degrees C was observed and was accredited to secondary interactions between these polymers, in addition to hydrophobic interactions between P407 micelles. Importantly, formulations composed of 20% (w/w) P407 and C934P exhibited pronounced mucoadhesive properties. The ease of administration (below the T(sol/gel)) in conjunction with the viscoelastic (notably high elasticity) and mucoadhesive properties (at body temperature) render the formulations composed of 20% (w/w) P407 and C934P as potentially useful platforms for mucoadhesive, controlled topical drug delivery within the oral cavity. (c) 2009 Published by Elsevier B.V.

Effect of extrusion on the emulsifying properties of soybean proteins and pectin mixtures modelled by response surface methodology

The objective of this study was to apply response surface methodology to estimate the emulsifying capacity and stability of mixtures containing isolated and textured soybean proteins combined with pectin and to evaluate if the extrusion process affects these interfacial properties. A simplex-centroid design was applied to the model emulsifying activity index (EAI), average droplet size (D-[4.3]) and creaming inhibition (Cl%) of the mixtures. All models were significant and able to explain more than 86% of the variation. The high predictive capacity of the models was also confirmed. The mean values for EAI, D-[4.3] and Cl% observed in all assays were 0.173 +/- 0.015 mn, 19.2 +/- 1.0 mu m and 53.3 +/- 2.6%, respectively. No synergism was observed between the three compounds. This result can be attributed to the low soybean protein solubility at pH 6.2 (<35%). Pectin was the most important variable for improving all responses. The emulsifying capacity of the mixture increased 41% after extrusion. Our results showed that pectin could substitute or improve the emulsifying properties of the soybean proteins and that the extrusion brings additional advantage to interfacial properties of this combination. (C) 2008 Elsevier Ltd. All rights reserved.

Bayesian nonlinear regression models with scale mixtures of skew-normal distributions: Estimation and case influence diagnostics

The purpose of this paper is to develop a Bayesian analysis for nonlinear regression models under scale mixtures of skew-normal distributions. This novel class of models provides a useful generalization of the symmetrical nonlinear regression models since the error distributions cover both skewness and heavy-tailed distributions such as the skew-t, skew-slash and the skew-contaminated normal distributions. The main advantage of these class of distributions is that they have a nice hierarchical representation that allows the implementation of Markov chain Monte Carlo (MCMC) methods to simulate samples from the joint posterior distribution. In order to examine the robust aspects of this flexible class, against outlying and influential observations, we present a Bayesian case deletion influence diagnostics based on the Kullback-Leibler divergence. Further, some discussions on the model selection criteria are given. The newly developed procedures are illustrated considering two simulations study, and a real data previously analyzed under normal and skew-normal nonlinear regression models. (C) 2010 Elsevier B.V. All rights reserved.

Multivariate measurement error models based on scale mixtures of the skew-normal distribution

Scale mixtures of the skew-normal (SMSN) distribution is a class of asymmetric thick-tailed distributions that includes the skew-normal (SN) distribution as a special case. The main advantage of these classes of distributions is that they are easy to simulate and have a nice hierarchical representation facilitating easy implementation of the expectation-maximization algorithm for the maximum-likelihood estimation. In this paper, we assume an SMSN distribution for the unobserved value of the covariates and a symmetric scale mixtures of the normal distribution for the error term of the model. This provides a robust alternative to parameter estimation in multivariate measurement error models. Specific distributions examined include univariate and multivariate versions of the SN, skew-t, skew-slash and skew-contaminated normal distributions. The results and methods are applied to a real data set.

Bayesian density estimation using Skew Student-t-Normal mixtures

We present a Bayesian approach for modeling heterogeneous data and estimate multimodal densities using mixtures of Skew Student-t-Normal distributions [Gomez, H.W., Venegas, O., Bolfarine, H., 2007. Skew-symmetric distributions generated by the distribution function of the normal distribution. Environmetrics 18, 395-407]. A stochastic representation that is useful for implementing a MCMC-type algorithm and results about existence of posterior moments are obtained. Marginal likelihood approximations are obtained, in order to compare mixture models with different number of component densities. Data sets concerning the Gross Domestic Product per capita (Human Development Report) and body mass index (National Health and Nutrition Examination Survey), previously studied in the related literature, are analyzed. (c) 2008 Elsevier B.V. All rights reserved.

Room Temperature Ionic Liquid-Lithium Salt Mixtures: Optical Kerr Effect Dynamical Measurements

The addition of lithium salts to ionic liquids causes an increase in viscosity and a decrease in ionic mobility that hinders their possible application as an alternative solvent in lithium ion batteries. Optically heterodyne-detected optical Kerr effect spectroscopy was used to study the change in dynamics, principally orientational relaxation, caused by the addition of lithium bis(trifluoromethylsulfonyl)imide to the ionic liquid 1-buty1-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Over the time scales studied (1 ps-16 ns) for the pure ionic liquid, two temperature-independent power laws were observed: the intermediate power law (1 ps to similar to 1 ns), followed by the von Schweidler power law. The von Schweidler power law is followed by the final complete exponential relaxation, which is highly sensitive to temperature. The lithium salt concentration, however, was found to affect both power laws, and a discontinuity could be found in the trend observed for the intermediate power law when the concentration (mole fraction) of lithium salt is close to chi(LiTf(2)N) = 0.2. A mode coupling theory (MCT) schematic model was also used to fit the data for both the pure ionic liquid and the different salt concentration mixtures. It was found that dynamics in both types of liquids are described very well by MCT.

Thin films of carbohydrate based surfactants and carboxymethylcellulose acetate butyrate mixtures: Morphology and thermal behavior

Thin films of mixtures containing carboxymethylcellulose acetate butyrate (CMCAB) and carbohydrate based surfactant, namely, sorbitan monopalmitate (Span 40) or poly(oxyethylene) sorbitan monopalmitate (Tween 40) were spin-coated onto silicon wafers. The effect of surfactant concentration on resulting film morphology and surface toughness Was Studied by atomic force microscopy (AFM). Upon increasing the concentration of Span 40 in the mixture, films became rougher and more heterogeneous, indicating surface enrichment by Span 40 molecules. In the case of mixtures composed by CMCAB and Tween 40, the increase of Tween 40 in the mixture led to smoother and more homogeneous films, indicating compatibility between both components. Differential scanning calorimetry (DSC) revealed that Span 40 and Tween 40 act as plasticizers for CMCAB, leading to dramatic reduction of glass transition temperature of CMCAB, namely, Delta T(g) = -158 degrees C and Delta T(g)=-179 degrees C. respectively. (C) 2008 Elsevier B.V. All rights reserved.

Solvatochromism in Binary Mixtures: First Report on a Solvation Free Energy Relationship between Solvent Exchange Equilibrium Constants and the Properties of the Medium

We have employed UV-vis spectroscopy in order to investigate details of the solvation of six solvatochromic indicators, hereafter designated as ""probes"", namely, 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (RB); 4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePM; 1-methylquinolinium-8-olate, QB; 2-bromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr, 2,6-dichloro-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (WB); and 2,6-dibromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr,, respectively. These can be divided into three pairs, each includes two probes of similar pK(a) in water and different lipophilicity. Solvation has been studied in binary mixtures, BMs, of water, W, with 12 protic organic solvents, S, including mono- and bifunctional alcohols (2-alkoxyethanoles, unsaturated and chlorinated alcohols). Each medium was treated as a mixture of S, W, and a complex solvent, S-W, formed by hydrogen bonding. Values of lambda(max) (of the probe intramolecular charge transfer) were converted into empirical polarity scales, E(T)(probe) in kcal/mol, whose values were correlated with the effective mole fraction of water in the medium, chi w(effective). This correlation furnished three equilibrium constants for the exchange of solvents in the probe solvation shell; phi(W/S) (W substitutes S): phi(S-W/W) (S-W substitutes W), and phi(S-W/S) (S-W substitutes S), respectively. The values of these constants depend on the physicochemical properties of the probe and the medium. We tested, for the first time, the applicability of a new solvation free energy relationship: phi = constant + a alpha(BM) + b beta(BM) + s(pi*(BM) + d delta) + p log P(BM), where a, b, s, and p are regression coefficients alpha(BM), beta(BM), and pi*(BM) are solvatochromic parameters of the BM, delta is a correction term for pi*, and log P is an empirical scale of lipophilicity. Correlations were carried out with two-, three-, and four-medium descriptors. In all cases, three descriptors gave satisfactory correlations; use of four parameters gave only a marginal increase of the goodness of fit. For phi(W/S), the most important descriptor was found to be the lipophilicity of the medium; for phi(S-W/W) and phi(S-W/S), solvent basicity is either statistically relevant or is the most important descriptor. These responses are different from those of E(T)(probe) of many solvatochromic indicators in pure solvents, where the importance of solvent basicity is usually marginal, and can be neglected.