Structure of colloid silica determined by viscosity measurements

The viscosity of nanosized colloid silica suspensions, used as binders in the investment casting, was determined as a function of their weight fraction reaching 52%. A new capillary viscometer was used whose construction eliminated sedimentation effects. The experiments have been carried out at fixed pH 10.0 and controlled ionic strength. It was found that for a low silica concentration range (weight fraction below 5%) the suspension viscosity increased more rapidly than the Einstein theory predicts. This anomalous behavior could not be explained in terms of the primary electroviscous effect predicted to be a few orders of magnitude smaller as observed. This discrepancy was accounted for by postulating a fuzzy, gel-like structure of colloid silicas used in our experiments. Hence, the apparent hydrodynamic radius of silica particles in aqueous suspensions was found to be larger than the primary particle size in accordance with previous observations. Based on this postulate, an apparent density of the silica sols was found to be 1.32-1.37 g/cm(3) instead of 2.2-2.32 g/cm(3) as determined from the suspension dilution method. This behavior was interpreted in terms of the core/shell model with high shell porosity, reaching 85%. Similarly, for higher concentration ranges, silica viscosity increased more rapidly with increased sol concentration than predicted by the Batchelor model derived for hard particles. The deviation was attributed to the secondary electroviscous effect stemming from the electrostatic interactions among silica particles in sheared suspensions. This effect has quantitatively been interpreted in terms of Russel's theory. On the other hand, for the high concentration range the experimental results were well accounted for by the Dougherty-Krieger model. By exploiting our experimental findings a sensitive method of determining the structure and apparent density of silica sols in aqueous media was proposed.     

A spirocyclic system comprising both phosphazane and phosphazene rings

The reaction of hexakis(cyclohexylamino) cyclotriphosphazene [NP(CyNH)(2)](3), 1, with phosphorus trichloride yields [NP(CyN)(2)PCl](3), 2, which contains three four-membered phosphazane rings fused in spirocyclic fashion to a central six-membered phosphazene ring and constitutes the first structurally characterized compound that comprises both phosphazene and phosphazane rings. The peripheral P atoms feature stereoactive lone pairs, and, thus, 2 exists in isomeric C(3h) and C(s) forms. The spirocyclic phosphazene-phosphazane derivative 2 carries three reactive PCl functions in peripheral positions, promising an interesting precursor molecule for the synthesis of extended phosphorus nitrogen structures of high rigidity. Extension of the PN moiety can be achieved by reaction of 2 with a primary amine yielding [NP(CyN)(2)PN(H)(t)Bu](3), 3, which features a central scaffold of 6 phosphorus and 12 nitrogen centers and aggregates via N-H...P hydrogen bonds in the solid state. On the contrary, the reaction of 1 with SbCl(3) undergoes incomplete proton abstraction, resulting in the formation of the tricyclic compound N(3)P(3)(CyNH)(4)(CyNSbCl(2))(2), 4, which contains two four-coordinate Sb centers chelated by N(exo)-N(ring) sites of the phosphazene.     

In situ infrared study of SBA-15 functionalized with carboxylic groups incorporated by a co-condensation route

Two samples of SBA-15 mesoporous silica with a different content of carboxylic groups (-COOH) were prepared by a co-condensation route, using 4-(triethoxysilil)butyronitrile as the organosilane agent, then treating the samples with sulfuric acid, which removes the template and simultaneously hydrolyses the -CN to -COOH groups, as shown by IR spectroscopy. Both incorporation of organosilane agents and subsequent acid treatment do not affect the ordered SBA-15 structure. The proton-donor ability of carboxylic groups, as well their accessibility to reactants, has been studied in the IR by dosing ammonia, which forms reversibly COO- groups and NH4+ ions. The related equilibrium constants have been determined by elaboration of IR data. Outgassing the samples at progressively increasing temperatures destabilizes to an increasing extent the ammonium/carboxylate ion pair, because of the decrease in surface polarity brought about by dehydration: this decreases the related equilibrium constants. The amount of carboxylic groups undergoing reaction appears instead to be constant with dehydration, and to coincide with the whole population of COOH groups. Titration with alkali solutions yielded surface concentrations for the two samples of ca. 1.0 and 0.45 COOH/nm2. Proportionality between the surface concentrations and the intensities of the C=O IR band is observed: this suggests that COOH groups are noninteracting with each other and allows the computation of the related extinction molar coefficient.     

Charge neutralization of ions from benzene

Cross-sections have been measured for the charge neutrilization if ions from benzene in kiloelectron-volt collisions with benezene target molecules. Measured values range from 65 Ų for the symmetric [C₆H₆]⁺? C₆H₆ resonant reactions to 8 Ų for [C₃H₃]⁺? C₆H₆ reactions. Cross-sections computed using a simple resonance charge transfer model compare favourably with experimental data for the symmetric reactions. The cross-sections for asymmetric reactions are smaller that those for they symmetric system and magnitudes of the asymmetric cross-sections are correlated with recombination energies of the respective ions.     

Influence of structural changes on the ion selectivities of magnesium ionophores based on malonic acid diamides

In an investigation of octamethylene bis(malonic acid diamides) and their selectivities for magnesium, it was found that presence of secondary amides within the particular ionophore played a considerable role in the enhancement of magnesium selectivity. Similar effects in other ionophores, i.e. tris(malonic acid diamides), were thus systematically looked at with the help of selectivity measurements with the hope of optimizing the number of secondary and tertiary amides so as to improve the magnesium selectivity. The syntheses of several investigated tris(malonic acid diamide) isologues are equally reported.Deceased in November 1992     

Epitaxial growth of sexiphenyl on Al(111): from monolayer to crystalline films

A combination of in situ surface sensitive-techniques, UV photoemission and low energy electron diffraction, with ex situ bulk sensitive X-ray diffraction reveals the formation of epitaxial thin films of sexiphenyl on Al(111) starting from the first monolayer. For room temperature growth, highly ordered films are formed with a unique alignment of the sexiphenyl molecules with the long axes of all molecules aligned parallel to both the surface and the <10> azimuthal directions of Al(111). This is related to a densely packed highly commensurate first monolayer, which acts as a template for the unique (21) crystallite orientation observed.     

Position dependent heavy atom effect in physical triplet quenching by electron donors

The radical yields and rate constants in the quenching reaction of thionine triplet with the complete series of monohalogen substituted anilines as electron donors were determined by flash spectroscopy. Whereas the quenching rate constants show little and unsystematic variation, the radical yields decrease with increasing spin—orbit coupling constant of the halogen substituent. This effect is very sensitive to the position of the halogen in the donor. The results are explained in terms of a heavy atom effect on the intersystem crossing rate constant in a triplet exciplex.     

Electron impact and chemical ionization mass spectra of alkyldiphenylphosphine oxides

In the 70 e V electron impact mass spectra of a series of alkyldiphenylphosphine oxides (R?₂PO, R = Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, t-Bu, neopentyl, n-decyl), molecular ions of low abundance are observed and [M + H]⁺ ions are formed to a small extent at high sample pressures. The major ions include [?₂PO]⁺, [?₂POH]⁺; [?₂CH₂PO]⁺ and [?₂CH₂POH]⁺ which are formed by rearrangement and cleavage processes. The chemical ionization mass spectra obtained with methane and isobutane reagents consist of [M + H]⁺ ions. The proton affinity of R?₂PO was found to be 219 ± 2.5 kcal mol^?1.     

Influence of phytohormones on polar and hydrophobic parts of mixed phospholipid monolayers at water/air interface

Surface parameters--the limiting area, collapse pressure, and compressibility modulus for monolayers of phospholipids containing a determined hydrophobic part (16:0) but different polar parts (PPL) and a determined polar part (PC) but different fatty acids (HPL), characteristic of nonembryogenic (NE) and embryogenic (E) winter wheat calli--were evaluated at 15 degrees C. These parameters were dependent on the kind of hydrophilic group and the size of the hydrophobic part of phospholipids. In the case of PPL, higher values of Alim and picoll were noticed for NE phospholipids. In the case of HPL, lower Alim and higher picoll were detected for NE than for E monolayers. All investigated phospholipid systems stimulated the adsorption of phytohormones from the water subphase. The influence of phytohormones of anionic (IAA, 2-4-D), cationic (kinetin, zeatin), and nonionic character (zearalenone) was examined. It appeared that the surface activity of phytohormones depended strongly on the kind of tissue from which the phospholipid mixture was extracted and, in a lesser degree, on their charge. In PPL systems with a determined hydrophobic part (16:0), no great differences in phytohormone influence on NE and E monolayers were observed (except of IAA). The greatest phytohormone influence on NE monolayers in HPL systems was related to the structure of the hydrophobic part of phospholipids. IAA, the most active (with the highest Alim values) among the phytohormones examined, influenced both HPL and PPL monolayers. This indicated the interactions of IAA with polar groups of phospholipids. Phytohormones also changed the monolayer stability against collapse process and the direction of changes depended on the kind of tissue (embryogenic or nonembryogenic).