Comparison of humic substances isolated from thermal water and surface water by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry.

Characterization of humic substances isolated from thermal water and surface water was carried out by elemental analysis and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Atomic ratios derived from elemental analysis represented compositional differences of humic substances. Hydrogen-to-carbon and oxygen-to-carbon atomic ratios were also calculated from molecular formulae determined by ultra high-resolution mass spectrometry. The van Krevelen diagram was used to illustrate the bias between the atomic ratios from elemental analysis and mass spectrometry.

Surfactant double layer stabilized magnetic nanofluids for biomedical application.

Magnetite nanoparticles were coated with surfactant double layers in order to prepare water based magnetic fluids (MFs). The effects of head group (sulfonate, carboxylate) and alkyl chain length (11-17 C atoms) and the combination of surfactants were studied. Adsorption, dynamic light scattering (DLS) and electrophoretic mobility measurements were performed. The quantity of surfactant varied between 0.3 and 0.5 g, i.e. their specific amount ranges over 1.5-2 mmol g(-1) magnetite in MFs. The adsorption isotherm of Na oleate on magnetite proved the double layer formation with 2 mmol g(-1) saturation value in good harmony with the empirical doses. The effect of diluting MFs, pH and salt concentration was studied. The pH-dependent stability and the salt tolerance of MFs were different owing to the dissociation of the outermost hydrophilic groups and the hydrophobic interactions scaling with the alkyl chain length of surfactant. The hydrophobic interactions are favored only for oleic and myristic acid double layers. In these MFs, aggregation cannot be observed even in fairly dilute systems up to the physiological salt concentration around neutral pH 6-8 favored in biomedical application. The stable oleic and myristic acid double layers can hinder effectively the aggregation of magnetite particles due to the combined steric and electrostatic stabilization.

The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles.

The pH-dependent adsorption of humic acid (HA) on magnetite and its effect on the surface charging and the aggregation of oxide particles were investigated. HA was extracted from brown coal. Synthetic magnetite was prepared by alkaline hydrolysis of iron(II) and iron(III) salts. The pH-dependent particle charge and aggregation, and coagulation kinetics at pH approximately 4 were measured by laser Doppler electrophoresis and dynamic light scattering. The charge of pure magnetite reverses from positive to negative at pH approximately 8, which may consider as isoelectric point (IEP). Near this pH, large aggregates form, while stable sols exist further from it. In the presence of increasing HA loading, the IEP shifts to lower pH, then at higher loading, magnetite becomes negatively charged even at low pHs, which indicate the neutralization and gradual recharging positive charges on surface. In acidic region, the trace HA amounts are adsorbed on magnetite surface as oppositely charged patches, systems become highly unstable due to heterocoagulation. Above the adsorption saturation, however, the nanoparticles are stabilized in a way of combined steric and electrostatic effects. The HA coated magnetite particles form stable colloidal dispersion, particle aggregation does not occur in a wide range of pH and salt tolerance is enhanced.

Comparison of sorption domains in molecular weight fractions of a soil humic acid using solid-state 19F NMR.

Humic acid was fractionated into eight different molecular size components using ultrafiltration. Solid-state CPMAS 13C NMR demonstrated that fractions larger than 100,000 Daltons were primarily aliphatic in character, while fractions smaller than 30,000 Daltons were predominantly aromatic in character. Solid-state 19F NMR examination of the sorptive uptake of hexafluorobenzene (HFB) by HA and each of the fractions gave spectroscopic evidence for the existence of at least three sorption sites in the smaller molecular size fractions, while two predominant sorption sites could be established in the larger molecular size fractions. Sorbed HFB displayed higher mobility in the smaller, more aromatic fractions while HFB in the larger, more aliphatic fractions displayed lower mobility. The relative mobilities of HFB in each sorption domain suggest that the rigid domain may be composed of aliphatic carbon rather than aromatic carbon moieties. In larger size fractions, this domain may be the result of rigid, glassy regions composed of aliphatic molecules or side chains.

Migration of electronic energy from chlorophyll b to chlorophyll a in solutions.

Absorption, emission, and fluorescence excitation spectra of pure solutions of chlorophyll a (Chl a) and chlorophyll b (Chl b) in diethyl ether and of equimolecular mixed solutions of the two pigments, were determined at room temperature as functions of concentration (in the range from 5 x 10(-6) M to 4 x 10(-3) M) and of wavelength of the exciting light (in the regions 380-465 and 550-650 nm). The efficiency of energy transfer from Chl b to Chl a, derived from these data, was found to depend on the wavelength of exciting light. Furthermore, the transfer efficiency calculated from sensitization of Chl a fluorescence by Chl b was substantially smaller than that calculated from quenching of Chl b fluorescence by Chl a. Both these effects are tentatively explained as evidence of superposition of a "fast" energy transfer (taking place before the Boltzmann distribution of vibrational energy had been reached) upon the "delayed" transfer, which takes place after vibrational equilibration. The first-named mechanism is made possible by overlapping of the absorption bands of the two pigments; the second, by overlapping of the emission band of Chl b and the absorption band of Chl a. The first mechanism can lead to repeated transfer of excitation energy between pigment molecules, the second only to a one-time transfer from the donor to the acceptor. Both mechanisms could be of the same, second-order type, with the transfer rate proportional to r(-6). An alternative is for the fast mechanism to be of the first order, with the transfer rate proportional to r(-3), but spectroscopic evidence seems to make this alternative less probable.