Dynamic light scattering measurements of particle size development in aqueous humic materials.

Dynamic light scattering (DLS) has been used to monitor changes in aggregate sizes of aqueous humic materials as a function of solution properties. Humic and fulvic acids were dissolved at relatively low concentrations (15-30 mg L(-1)) in solutions of different temperature, cation and ethanol content, and pH. The results could be explained in terms of intramolecular contraction and intermolecular aggregation of humic polymers. The former were prevalent in soil humic acids, and less so in aquatic humic acids and fulvic acids. Increasing the temperature of humic solutions generally led to an increase in particle sizes, which was ascribed to an effect akin to surfactant clouding. The addition of cations led to either contraction or expansion, depending on the charge and concentration of the ion, and the nature of the humic material. Reducing the pH initially caused contraction, followed by growth and precipitation in more highly acidic media.

Time-Resolved Fluorescence Anisotropies in Mixed Surfactant Solutions.

Time-resolved fluorescence anisotropy decays of solutions of Triton X-114 (TX-114) with various amounts of sodium dodecyl sulfate (SDS) were measured using the emission both from the surfactant itself and from added perylene. In the former case, the monomer and aggregate species of the surfactant were spectroscopically isolated and were shown to have significantly different rotational correlation times. The rotational diffusion of perylene in micellar TX-114 with small amounts of added SDS appeared to have a component with a very short correlation time. The anisotropy decay curves showed the existence of limiting anisotropies (r(infinity)), indicating hindered probe rotation in the micellar environment. At higher SDS concentrations, the fast-decaying component slowed down and the limiting anisotropy decreased substantially, suggesting some migration of the probe to the interior of the micelle. Copyright 1999 Academic Press.

The role of selected cations in the formation of pseudomicelles in aqueous humic acid.

The fluorescence intensity enhancement of a pyrene probe in aqueous humic acid solutions was assessed in terms of added lanthanide and thorium cations. Among the trivalent ions it was found that size played a role, with the small Lu(3+) ion producing the greatest increase in pyrene emission. This was attributed to its superior ability to cause pseudomicellization in the humic acid polymer. Slow kinetic effects were observed, leading to substantial fluorescence enhancement over a period of 7 h. This was ascribed to a continuous aggregation process in aqueous humic acid, leading to ever more viscous microenvironments for the probe molecule.

Anisotropy Changes of a Fluorescent Probe during the Micellar Growth and Clouding of a Nonionic Detergent.

The effects of temperature and Triton X-114 (TX-114) concentration on the fluorescence anisotropy of perylene were investigated before and after detergent clouding. The measured anisotropy values were used to estimate the microviscosity of the micellar interior. In the lower detergent concentration range, an anisotropy maximum was observed at the critical micelle concentration (CMC), while the values decreased in the range immediately above the CMC. This was ascribed to the micellar volume increase, which, in the case of TX-114, was not accompanied by a more ordered internal environment. A gradual decrease of anisotropy and microviscosity with increasing temperature below the cloud point was observed. At the cloud point, no abrupt changes were found to occur. Compared to detergents with more flexible hydrophobic moieties, TX-114 micelles have a relatively ordered micellar interior indicated by the microviscosity and calculated fusion energy values. In the separated micellar phase formed after clouding, the probe anisotropy increased as water was eliminated at higher temperatures.

Decontamination of oil-polluted soil by cloud point extraction.

An extraction procedure based on cloud point phase separation of nonionic surfactants was used to remove oil contamination from soils. The detergent employed was Triton X-114, and its clouding behavior was monitored by means of a fluorescence probe. Changes in the I (1)I (3) ratio of pyrene indicated gradual dehydration of the detergent micelles upon heating. The rate of phase separation, and the volume and water content of the micellar phase were determined. In the practical clean-up, 85-98% of the oil present in the soil was found to enter the micellar phase of the separated washing liquid. A 15-min washing time with 3-5% detergent was found to be sufficient for this degree of contaminant removal from soil containing 0.009-0.017% oil, using a liquid:solid ratio of 5:2. The extraction efficiency decreased with increasing carbon content of the soil. The process holds promise for large-scale treatment of oil-polluted soils.

Determination of steroids in urine by micellar HPLC with detection by sensitized terbium fluorescence.

Efficient Förster energy transfer from steroid donors to a terbium ion acceptor occurs in aqueous solution when the two species are sequestered in sodium dodecyl sulfate micelles. The microorganized medium provides both proximity between donor and acceptor and protection from quenching of terbium fluorescence by water. Detection limits down to 100 pg have been achieved for steroids with an alpha,beta-unsaturated carbonyl group in the A-ring. This detection strategy has been used in HPLC separations, using either normal- or reversed-phase chromatography with postcolumn addition of detergent and/or terbium, or micellar chromatography with the detergent solution as the mobile phase. This last method proved to be superior, requiring no sample preparation and allowing direct injection of urine onto the HPLC column.

Determination of sulfur species by cathodic square wave stripping voltammetry; Compounds relevant to natural sulfur mineralization.

The feasibility of rapid analysis of a number of environmentally important sulfur compounds by cathodic square wave stripping voltammetry at a mercury electrode has been investigated. For cysteine/cystine a relatively anodic peak was identified, which is ascribed to the stripping of a mercurous species. The dependence of the peak currents on pH was found to be different for cysteine and cystine. Methionine and thioproline gave similar stripping peaks to those for cysteine. It is proposed that they arise from species deposited by oxidative hydrolysis. A complex ion is proposed to cause the stripping peak of thiosulfate and tetrathionate, while polysulfides give both a HgS stripping peak and a non-adsorptive reduction peak. Limits of detection are in the 10(-8)-10(-9)M range.

A new fluorescent cholesterol derivative: structure and blood plasma analysis.

A fluorescent cholesterol derivative produced by reaction with gaseous HCl and zinc chloride in ethyl acetate is shown to be a chlorine substituted B-ring diene. The species forms relatively rapidly via an i-steroid rearrangement, requiring a temperature around 70 degrees C. A weak, possibly dimeric pi-allyl zinc complex exists in solution, leading to a red shift in the fluorescence emission. The application of the derivative to the determination of cholesterol in bovine plasma provides good sensitivity and precision and requires notably small sample volumes.

Nucleation in premicellar aggregation.

The role of a nucleating agent (cholesterol) in the premicellar aggregation of sodium lauryl sulfate (SLS), cetyltrimethylammonium bromide, Triton X-405 and Brij 35 has been investigated. The I(1)/I(3) ratios of pyrene in the solutions indicate that SLS is strongly nucleated by cholesterol, but the other detergents appear to be so to a lesser extent. However, quenching studies show that premicellar Brij 35 also forms nucleated aggregates that protect the probe from the quencher. Energy transfer data with Triton X-405 suggest that such aggregates have a more open structure than self-assembling ones and are more accessible to water.

Precapacitive processes in single potential-step chronoamperometry.

Extremely short-lived anodic currents were observed in the early parts of the transient response following the application of a cathodic potential step to a mercury working electrode. It is proposed that this phenomenon is due to the existence of a brief precapacitive period, which precedes full development of the double-layer charging current, and which allows momentary reaction (reduction) of species present at the electrode surface. The observed anodic currents are explained in terms of a re-oxidation of such "resident" species that were reduced during this precapacitive period. The subsequent capacitive surge produced by the charging of the electrical double layer leads to an anodic shift of the electrode potential that can be sufficient for the re-oxidation of the precapacitive amalgam. The anodic peak is linearly related to depolarizer concentration and varies with supporting electrolyte concentration, ion mobility and potential step size. Cathodic preconcentration of the depolarizer enhances the effect.

Energy transfer in benzalkonium chloride micelles.

The energy transfer process from the excited phenyl group of micellar benzalkonium chloride (BAC) to solubilized anthracene is described. NMR studies show the solubilization site of the anthracene to be near the phenyl group in the BAC micelles. The system gives an energy transfer efficiency of 0.741 and the donor-acceptor separation is 41.4 A.

Fluorescence in thin liquid films: a simple model.

A model is proposed to account for the effect of various parameters on the fluorescence characteristics of dyestuffs in thin films during drainage of the films.

Corrosion measurements by potential-step chronoamperometry.

A solid-state computer-controlled system has been developed for the measurement of corrosion currents under potentiostatic anodic conditions. The system applies a potential step and measures the resulting current for a variable number of cycles. Data are stored and manipulated by the computer. Tests show that corrosion behaviour of metals in various environments can be evaluated.