Is collision-induced dissociation of low-energy carbonyl sulfide cations adversely affected by asymmetry?

We have measured relative abundances of fragment ions resulting from collision-induced dissociation of OCS(+) ions in collision with xenon neutrals as a function of ion kinetic energy and scattering angle. The lowest energy dissociation product, S(+), dominates at all energies up to 53 eV kinetic energy studied here. Surprisingly, the second most abundant dissociation channel is CS(+) and not CO(+) even though the thermochemical threshold for CO(+) is lower than that for CS(+) and CO(+) is more abundant than CS(+) in the normal mass spectrum of OCS. We do not observe any significant abundance of CO(+) in this energy range, suggesting that collision-induced excitation and dissociation of OCS(+) is significantly different to that of symmetric triatomic ions. A possible role of asymmetry in the molecular ion's collisional activation via neutral collision is suggested for the different behavior.     

Importance of micellar kinetics in relation to technological processes

The association of many classes of surface-active molecules into micellar aggregates is a well-known phenomenon. Micelles are in dynamic equilibrium, constantly disintegrating and reforming. This relaxation process is characterized by the slow micellar relaxation time constant, tau(2), which is directly related to the micellar stability. Theories of the kinetics of micelle formation and disintegration have been discussed to identify the gaps in our complete understanding of this kinetic process. The micellar stability of sodium dodecyl sulfate micelles has been shown to significantly influence technological processes involving a rapid increase in interfacial area, such as foaming, wetting, emulsification, solubilization, and detergency. First, the available monomers adsorb onto the freshly created interface. Then, additional monomers must be provided by the breakup of micelles. Especially when the free monomer concentration is low, which is the case for many nonionic surfactant solutions, the micellar breakup time is a rate-limiting step in the supply of monomers. The Center for Surface Science & Engineering at the University of Florida has developed methods using stopped flow and pressure jump with optical detection to determine the slow relaxation time of micelles of nonionic surfactants. The results showed that the ionic surfactants such as SDS exhibit slow relaxation times in the range from milliseconds to seconds, whereas nonionic surfactants exhibit slow relaxation times in the range from seconds (for Triton X-100) to minutes (for polyoxyethylene alkyl ethers). The slow relaxation times are much longer for nonionic surfactants than for ionic surfactants, because of the absence of ionic repulsion between the head groups. The observed relaxation times showed a direct correlation with dynamic surface tension and foaming experiments. In conclusion, relaxation time data of surfactant solutions correlate with the dynamic properties of the micellar solutions. Moreover, the results suggest that appropriate micelles with specific stability or tau(2) can be designed by controlling the surfactant structure, concentration, and physicochemical conditions (e.g., salt concentration, temperature, and pressure). One can also tailor micelles by mixing anionic/cationic or ionic/nonionic surfactants for a desired stability to control various technological processes.     

Diffusion behavior of pharmaceutical O/W microemulsions studied by dynamic light scattering

Dynamic light scattering experiments have been performed at various concentrations, of pharmaceutical oil-in-water microemulsions consisting of Eutanol G as oil, a blend of a high (Tagat O2) and a low (Poloxamer 331) hydrophilic–lipophilic balance surfactant, and a hydrophilic phase (propylene glycol/water). We probe the dynamics of these microemulsions by dynamic light scattering. In the measured concentration range, two modes of relaxation were observed. The faster decaying mode is ascribed classically to the collective diffusion D _c (total droplet number density fluctuation). We show that the slow mode is also diffusive and suggest that its possible origin is the relaxation of polydispersity fluctuations. The diffusion coefficient associated with this mode is then the self-diffusion D _s of the droplets. It was found that D _c and D _s had opposite volume fractions of oil plus surfactants (ϕ) dependence and a common limiting value D ₀ for ϕ=0. Average hydrodynamic radius (R _h=10.5 nm) of droplets was calculated from D ₀. R _h is supposed to compose the inner core, a surfactant film including possible solvent molecules, which migrate with the droplet. The concentration dependence of diffusion coefficients reflects the effect of hard sphere and the supplementary repulsive interactions which arises due to loss of entropy, when absorbed chains of surfactant intermingle on the close approach of the two droplets. This mechanism could also explain the observed stability of our systems. The estimated extent of polydispersity is 0.22 from the amplitude of slower decaying mode. The polydispersity in microemulsion systems is dynamic in origin. Results indicate that the time scale for local polydispersity fluctuations is at least three orders of magnitude longer than the estimated time between droplet collisions.     

Radical polymerization of styrene initiated by styrene–arsenic sulfide complex

Styrene forms a complex with arsenic sulfide which in DMF at 85°C initiates radical polymerization yielding high molecular weight polymer. The order of reaction, with respect to initiator and monomer, is 0.5 ± 0.02 and unity, respectively. The energy of activation for the system computed as 96 kJ mol^?1 and the polymerization is retarded by hydroquinone. A possible mechanism for reaction has also been proposed.     

Studies on some newer polyhydrazides containing amide linkages. III

Novel polyhydrazides having inherent viscosities in the range of 0.24–0.38 dL g^?1 were prepared by polymerizing a series of hydrazides with different diacid chlorides in polar aprotic solvents. These polymers display a wide range of solubility in a number of solvents. They start decomposing at about 200°C in air. Results of differential thermal and thermogravimetric analysis show that a steep weight loss takes place mainly in the range of 300–440°C.     

Dynamic light-scattering analysis of the electrostatic interaction of hexahistidine-tagged cytochrome P450 enzyme with semiconductor quantum dots.

Currently, there is great interest in the development of methods suitable for determining the stoichiometry of biomolecules attached to nanoparticles. We describe the use of the dynamic light-scattering technique (DLS) to determine the stoichiometry of the protein cytochrome P450(BSbeta) attached to CdS and CdSe quantum dots (QDs). The enzyme-conjugated QDs have different diffusion characteristics compared to the QD and enzyme precursors, expressed in their size, scattering intensity as well as zeta-potential values. The significant enhancement of the scattering intensity of QDs observed upon conjugation with the P450(BSbeta) due to the refractive-index increment and the systematic variation in zeta potential resulting from charge neutralization of the anionic QDs by the cationic histidine-tagged P450(BSbeta) have been used for stoichiometry determination.     

Influence of the nature of organic diluents on the extraction of uranium(VI) by bis(2-ethylhexyl) sulfoxide from nitric acid solutions

The extraction of uranium(VI) from nitric acid solutions by bis(2-ethylhexyl) sulfoxide (BESO) has been examined using sixteen inert organic diluents in order to establish the correlation between its distribution coefficient and some physico-chemical properties of the diluents. The extracted solvated species is shown to be UO₂(NO₃)₂·2BESO, irrespective of their nature. The extraction rate is lower in halogen substituted hydrocarbons as compared to the other diluents used. Among benzene derivatives, extraction is found to decrease with number of substituted methyl groups. Extraction efficiency decreases as the organic solvent is varied in the order: benzene>nitromethane>toluene> >nitrobenzene>cyclohexane>p-xylene>monochlorobenzene>dodecane>o-dichlorobenzene> >hexane>decalin>1,2-dichloroethane>1,1,1-trichloroethane>carbon tetrachloride> >tetrachloroethane>chloroform. Among the properties showing good correlations with distribution coefficient are Hansen's three-dimensional solubility parameters and Dimroth's empirical solvent polarity parameters [E _T(30) ]. Polarizability indices are most satisfactorily applicable to a wide variety of solvents.     

Determination of gallium in tumour-affected tissues by means of spectroscopic techniques

A comparative investigation was carried out on the suitability of atomic absorption spectrometry and of emission spectrography with arc and hollow-cathode excitation sources for determination of gallium (μg g^-1) in biological samples. The three methods give reliable results. Hollow-cathode emission spectrography was found to be influenced to a lesser extent by matrix effects and to be more precise than the other two techniques.