FT-IR and Porosity Study of Si-B-C-O Materials Obtained from TEOS-TEB-PDMS Derived Gel Precursors

Mixed vitreous oxycarbide glasses containing silicon and boron were prepared by pyrolysis of hybrid precursors in N₂ at 1100°C. Four different precursors were synthesized through the sol-gel process using TEOS, TEB and PDMS. Poly-condensation between the Si(OH) groups, from the hydrolysed TEOS, and the PDMS was observed by FT-IR spectroscopy. Boron seems not to incorporate into the network until pyrolysis of the precursors that form the oxycarbides. The oxycarbides were porous made of spherical interconnected particles. The porosity and mean pore size of the glasses increased with increasing TEB content.     

Study of the formation of dye-induced premicellar aggregates and its application to the determination of quaternary ammonium surfactants

Formation of dye-induced mixed premicellar aggregates from binary surfactant solutions is proposed for the determination of alkyltrimethylammonium surfactants at the muM level. The Coomassie Brilliant Blue G (CBBG) dye, negatively charged, induces the formation of cationic surfactant aggregates at concentrations far below the cmc. The role of CBBG in the formation of premicelles was studied by using pyrene as a fluorimetric probe. Formation of CBBG-cationic surfactant aggregates of well-defined stoichiometries that depend on the total surfactant concentration added is demonstrated. Also, the influence of analytical parameters affecting the concentration at which a given aggregate is formed was studied. Linear calibrations for alkyltrimethylammonium surfactants were obtained by using different cationic surfactants as titrants; therefore, the previously derived measurement parameter for mixed micelles is applicable to premicellar aggregates as well.     

Influence of Boron Concentration on the Surface Properties of TEOS-PDMS Hybrid Materials

The influence of the concentration of triethylborate (TEB) on the surface properties of hybrid materials prepared from tetraethylortosilicate (TEOS) and polydimethylsiloxane (PDMS) has been studied. TEB concentration has been varied between 0 and 30 wt.% respect to the total amount of alkoxyde whereas PDMS concentration has been set at 40 wt.% in all the hybrid samples. Pore size distributions (PSDs), specific surface areas (SSAs) and surface energies have been determined for such materials. PSDs have shown that pore size and pore volume increase with the amount of TEB forming the hybrid, whereas SSAs decrease. Surface energies have been determined by inverse gas chromatography at infinite dilution (IGC-ID) and have been divided into dispersive and specific surface energies. It was also found that TEB affects to both, dispersive and specific surface energies. Dispersive surface energy values increase from 23.23 mJ⋅m⁻² for the pure TEOS-PDMS hybrid sample to 31.07 mJ⋅m⁻² for the 30wt.% TEB-containing material. This increase has been attributed to the enhanced polarizability of the TEOS-PDMS hybrid when TEB is incorporated into the structure, a result which is consistent with the composition and structural characterization of the studied hybrid materials. Acid and base (k_A, k_B) surface constants also evidence a correlation with the TEB concentration: k_A decreases and k_B increases with the increase of TEB in these hybrids, a result which has been assigned to the lower acidity of boranol groups respect to that of silanols. The formation of an hybrid structure of Si–O–Si(PDMS) and B–O–Si(PDMS) bonds has been confirmed by FT-IR spectroscopy by the presence of a band at 850 cm⁻¹ and a shoulder between 930 and 880 cm⁻¹, respectively.     

Influence of the molecular shape on the volumetric properties of mixtures. The excess volumes of hexafluorobenzene or fluorobenzene +an octane isomer

The excess volumes of C₆F₆ and C₆H₅F+an octane isomer (n-octane, cyclooctane, 2,2,4-trimethylpentane) have been measured at 25°C. The equimolar values increase from 2,2,4-trimethylpentane to n-octane to cyclooctane. The present results cannot be explained in terms of the scheme developed by Patterson for other alkane mixtures. Comparing the present results with others in the literature, the effect of the dipolar moment of the aromatic compound seems to be more complex than in previous studies of polar+non-polar components. The results have been analyzed in terms of two generalized van der Waals models, and a semiempirical equation of state. The results range from poor to fair.     

Acid volatile sulfide determination in sediments using elemental analyzer with thermal conductivity detector

A method for the determination of acid volatile sulfides (AVS) in sediments, using a common elemental analyzer with thermal conductivity detector, is proposed. The method uses a mixture of Sn and V(2)O(5) for pyrolysis and combustion to determine total sulfur (TS), and non volatile sulfur (NVS), after an acidic attack. AVS is calculated as the difference between TS and NVS. The method for TS is validated by analyzing a certified reference material. The recovery in the determination of acid volatile sulfide is determined by spiking a river sediment with ZnS. The method is accurate and gives a good reproducibility, recovering 97.7-99.6% of the sulfur in the 0-3% total sulfur content, with SD of approximately 0.015%.     

Polarizable particle aggregation under rotating magnetic fields using scattering dichroism

We used scattering dichroism to study the dynamics of dipolar chains induced in magnetorheological suspensions under rotating magnetic fields. Both the dichroism (proportional to the total number of aggregated particles) and the phase lag show different behavior below and above a cross-over frequency. The cross-over frequency depends linearly on both the square of the magnetization and the inverse of the viscosity. The Mason number (ratio of viscous to magnetic forces) governs the dynamics. Therefore, there is a cross-over Mason number below which the dichroism remains almost constant and above which the rotation of the field prevents the particle aggregation process from taking place. Our experimental results have been compared with particle dynamics simulations showing good agreement.     

Applicability of the Mayo–Lewis equation to high conversion copolymerization of styrene and methylmethacrylate

In contradiction to reports from this and other laboratories, this study reports that the integrated Mayo–Lewis equation, or Meyer–Lowery equation, adequately describes the high-conversion free radical copolymerization of styrene and methylmethacrylate. The copolymerization was monitored by following the changes in the feed composition using NMR, as well as determination of the resulting copolymer compositions by NMR and UV. “Error in all Variables” statistical techniques were used to produce estimates of the reactivity ratios. The reactivity ratios estimated were, from feed composition, NMR, r₁ (styrene) = 0.472, r₂ = 0.454, from copolymer composition, UV, r₁ = 0.497, r₂ = 0.464, and NMR, r₁ = 0.432, r₂ = 0.422.