Frequency dependence of the dielectric and electro-optic response in suspensions of charged rod-like colloidal particles

We have performed an experimental investigation on the electrokinetic properties of charged rod-like fluorinated latex colloids. Systematic measurements of electrophoretic mobility, dielectric constant and electric birefringence have been performed as a function of the concentration of added nonionic surfactant and salt. In the investigated range of parameters, the zeta potential is a strongly decreasing function of the concentration of nonionic surfactant, while it is basically independent from ionic strength. We have obtained the frequency dependence of dielectric constant and Kerr constant as a function of zeta-potential and ionic strength. We observe the transition from a low frequency behavior, where both the dielectric constant and the Kerr constant are enhanced by the presence of the double layer, to a high frequency behavior, where both quantities take the value expected for unchanged particles in an insulating medium. The shape of the frequency dispersion of the Kerr constant coincides with that of the dielectric constant, but the cut-off frequencies are the same only when the zeta-potential of the particles is low.     

Acoustic variability in vibrato and its perceptual significance

Historically, studies of vocal vibrato have concentrated on pulse rate as being a primary factor in determining whether a given vocal movement is a good or bad vibrato or a tremolo or wobble. More recently, investigators have been studying the extent of frequency variation and amplitude variation around their respective means in order to determine their influence on the perception of vibrato. The present study is an additional attempt to understand the three parameters comprising vibrato, their interrelationship, and their relationship to perception. Samples of sustained sung tones were obtained primarily from recordings. The samples were digitized using a 16-bit A/D converter at a sampling frequency of 10 kHz. Each digitized sample was converted to a useful format for marking purposes in order to derive information on vibrato pulse rate, the mean frequency of the tone, the semitone deviation around the mean, percent frequency deviation and percent amplitude variation around the mean amplitude. Data presentation utilizes representative samples of good vibrato, tremolo and wobble and describes differences in waveforms which may impact on perception.     

Dynamic electrophoretic mobility of spherical colloidal particles in salt-free concentrated suspensions

In this contribution, the dynamic electrophoretic mobility of spherical colloidal particles in a salt-free concentrated suspension subjected to an oscillating electric field is studied theoretically using a cell model approach. Previous calculations focusing the analysis on cases of very low or very high particle surface charge are analyzed and extended to arbitrary conditions regarding particle surface charge, particle radius, volume fraction, counterion properties, and frequency of the applied electric field (sub-GHz range). Because no limit is imposed on the volume fractions of solids considered, the overlap of double layers of adjacent particles is accounted for. Our results display not only the so-called counterion condensation effect for high particle charge, previously described in the literature, but also its relative influence on the dynamic electrophoretic mobility throughout the whole frequency spectrum. Furthermore, we observe a competition between different relaxation processes related to the complex electric dipole moment induced on the particles by the field, as well as the influence of particle inertia at the high-frequency range. In addition, the influences of volume fraction, particle charge, particle radius, and ionic drag coefficient on the dynamic electrophoretic mobility as a function of frequency are extensively analyzed.     

Dielectric response of a concentrated colloidal suspension in a salt-free medium

In this paper the complex dielectric constant of a concentrated colloidal suspension in a salt-free medium is theoretically evaluated using a cell model approximation. To our knowledge this is the first cell model in the literature addressing the dielectric response of a salt-free concentrated suspension. For this reason, we extensively study the influence of all the parameters relevant for such a dielectric response: the particle surface charge, radius, and volume fraction, the counterion properties, and the frequency of the applied electric field (subgigahertz range). Our results display the so-called counterion condensation effect for high particle charge, previously described in the literature for the electrophoretic mobility, and also the relaxation processes occurring in a wide frequency range and their consequences on the complex electric dipole moment induced on the particles by the oscillating electric field. As we already pointed out in a recent paper regarding the dynamic electrophoretic mobility of a colloidal particle in a salt-free concentrated suspension, the competition between these relaxation processes is decisive for the dielectric response throughout the frequency range of interest. Finally, we examine the dielectric response of highly charged particles in more depth, because some singular electrokinetic behaviors of salt-free suspensions have been reported for such cases that have not been predicted for salt-containing suspensions.     

Natural rubber/clay nanocomposites: Influence of poly(ethylene glycol) on the silicate dispersion and local chain order of rubber network

A novel preparation of natural rubber (NR)/Na⁺-montmorillonite (MMT) nanocomposites in only one step by using poly(ethylene glycol) (PEG) has been investigated. PEG behaves as dispersing agent favouring the intercalation of rubber chains into the silicate galleries and providing substantially improved clay dispersion. Intercalated/exfoliated miscible hybrids were observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The influence of PEG on the network structure has also been evaluated by static proton double-quantum nuclear magnetic resonance spectroscopy (¹H DQ NMR) at low-field. Silicate nanoparticles with a high aspect ratio (clay tactoids) and a more crosslinked rubber network have been obtained for an optimum PEG/MMT ratio. Both effects were responsible of the enhancement on mechanical properties.     

Carbon-fluorine bond cleavage in the preparation of Osmium(III) and Osmium(IV) fluorothiolate complexes. Fluorine by fluorine NMR-assignment and fluxional processes

Reactions of OsO4 with HSR (R=C6F5, C6F4H-4,) in refluxing ethanol afford [Os(SC6F5)3(SC6F4(SC6F5)-2)] (1) and [Os(SC6F4H-4)3(SC6F3H-4-(SC6F4H-4)-2)] (2), which involve the rupture of C-F bonds. At room temperature, the compound [Os(SC6F5)3(PMe2Ph)2] or [Os(SC6F5)4(PMe2Ph)] reacts with KOH(aq) in acetone, giving rise to [ Os(SC6F5)(SC6F4(SC6F4O-2)-2)(PMe2Ph)2] (3), through a process involving the rupture of two C-F bonds, while the compound [Os(SC6F4H)4(PPh3)] reacts with KOH(aq) in acetone to afford [Os(SC6F4H-4)2(SC6F3H-4-O-2)(PPh3)] (4), which also implies a C-F bond cleavage. Single-crystal X-ray diffraction studies of 1, 2, and 4 indicate that these compounds include five-coordinated metal ions in essentially trigonal-bipyramidal geometries, whereas these studies on the paramagnetic compound 3 show a six-coordinated osmium center in a distorted octahedral geometry. 19F, 1H, 31P{1H}, and COSY 19F-19F NMR studies for the diamagnetic 1, 2, and 4 compounds, including variable-temperature 19F NMR experiments, showed that these molecules are fluxional. Some of the activation parameters for these dynamic processes have been determined.     

Molecular dynamics simulation of the reaction of hydration of formaldehyde using a potential based on solute-solvent interaction energy components

Molecular dynamics simulations of aqueous solutions at infinite dilution of the reaction of water with formaldehyde, H(2)O + H(2)CO --> H(2)C(OH)(2), were performed using Lennard-Jones 12-6-1 potentials to describe the solute-solvent interactions, and TIP3P to describe the water-water interactions. The Morokuma decomposition scheme of ab initio interaction energies at the SCF level and the dispersion component at MP2 level were used to reproduce the molecular parameters of the solute-water interaction potential. The results show that the functions that use the EX-PL-DIS-ES interaction model to describe the solvation of the reactant and product systems lead to good values of the reaction (DeltaG) and activation (DeltaG(#)) free energy as compared with those from using AMBER-derived parameters, and with the available theoretical and experimental data.     

Molecular simulation of the hydration of ethene to ethanol using ab initio potentials and free energy curves

Molecular dynamics simulations of aqueous solutions at infinite dilution of the reaction of water with ethene: H2O + CH2CH2 --> CH3CH2OH were performed using Lennard-Jones 12-6-1 potentials to describe the solute-solvent interactions, and TIP3P to describe the water-water interactions. The Morokuma decomposition scheme of ab initio interaction energies at the SCF level and the dispersion component at the MP2 level were used to reproduce the molecular parameters of the solute-water interaction potentials. The results show that the functions that use the EX-PL-DIS-ES interaction model to describe the solvation of the reactant, transition state, and product systems lead to good values of the reaction (Delta G) and acceptable values of the activation (Delta G#) free energy as compared with those from using AMBER-derived parameters, using the available theoretical and experimental data as referents.     

Abraham Model Correlations for Triethylene Glycol Solvent Derived from Infinite Dilution Activity Coefficient,Partition Coefficient and Solubility Data Measured at 298.15 K

A gas chromatographic headspace analysis method was used to experimentally determine gas-to-liquid partition coefficients and infinite dilution activity coefficients for 29 liquid organic solutes dissolved in triethylene glycol at 298.15 K. Solubilities were also determined at 298.15 K for 23 crystalline nonelectrolyte organic compounds in triethylene glycol based on spectroscopic absorbance measurements. The experimental results of the headspace chromatographic and spectroscopic solubility measurements were converted to gas-to-triethylene glycol and water-to-triethylene glycol partition coefficients, and molar solubility ratios using standard thermodynamic relationships. Expressions were derived for solute transfer into triethylene glycol by combining our measured experimental values with published literature data. Mathematical correlations based on the Abraham model describe the observed partition coefficient and solubility data to within 0.16 log₁₀ units (or less).     

Characterization of <Emphasis Type="Italic">Anemopsis californica</Emphasis> essential oil–β-cyclodextrin inclusion complex as antioxidant prolonged-release system

Antioxidant activity is a property of Anemopsis californica essential oil (ACO). However, ACO possesses volatile compounds that provide strong flavors and that are susceptible to degradation, complicating its application. An alternative is its encapsulation in a β-cyclodextrin (β-CD) matrix. Therefore, the characterization of β-CD–ACO inclusion complexes as an antioxidant prolonged-release system was the objective of this work. The ACO yield was 8.26 mg/g of dry sample. Elemicin (EL) (68.9%) and methyleugenol (ME) (11.7%) comprised the majority compounds identified by gas chromatography (GC). The β-CD–ACO inclusion complex, with a 90:10 ratio based on the precipitation technique, exhibited best percentage of recovery, greater ACO retention, and major antioxidant capacity determined by DPPH [2,2-diphenyl-1-picrylhydrazyl] and ABTS [2,2′-azino-bis-(3 ethylbenzothiazoline-6-sulfonic acid)] assays. Antioxidant activity was maintained during encapsulation. Free β-CD and β-CD:ACO presented the hysteresis phenomenon according to water sorption isotherms. A prolonged-release system was studied using different relative humidities (0, 33, 66, 96 and 100%). EL demonstrated 93.75% of release in highest relative humidity, while ME released completely. These results suggest application of the β-CD–ACO complex in systems in which it can be passively released and in controlled form, as in active food or drug packaging.