Location of Spectroscopic Probes in Self-Aggregating Assemblies. II. The Location of Pyrene and Other Probes in Sodium Dodecyl Sulfate Micelles

The location of pyrene in sodium dodecyl sulfate (SDS) micelles is determined as a function of the aggregation number, N, by exploiting the fact that spin probes 5- and 16-doxyl stearic acid methyl esters (5DSE and 16DSE, respectively) are effective quenchers of pyrene fluorescence. The locations of the two spin probes are known from Part 1 of this series (J. Phys. Chem. B 2006, 110, 9791) and the distance between the probes and pyrene is determined by using a hydrodynamic theory to predict the quenching rate constant. The hydrodynamic theory requires the microviscosity of the regions through which the probe and pyrene diffuse. The same spin probe that serves as quencher provides a measure of the microviscosity; thus, all the information needed to locate pyrene is available from each spin probe. Employing 5DSE, at N = 53, pyrene is found to diffuse through a zone 67% of which lies within the Stern layer and 33% in the core. As the micelle grows, due to increasing either the surfactant or added-salt concentration, this diffusion zone moves outward such that, at N = 130, near the sphere-rod transition, it lies approximately 75% within the Stern layer and 25% in the core. Employing 16DSE, the location of pyrene is within 0.4 A of that found from 5DSE at low values of N and within 0.8 A at high values. Full information required to locate pyrene by using the currently developed method is not yet available for other spin probes and other commonly employed quenchers; nevertheless, using a variety of strategies and reasonable assumptions leads to the same location of pyrene within the uncertainties of the method. All of the spectroscopic probes employed in this study are largely located within the polar shell of the micelles, the largest departure being about 4% of the diameter of the micelle.

Characterization of the surface interacting ability of carbon black by means of electron paramagnetic resonance analysis of adsorbed Cu2+, supported by surface analysis and atomic absorption

Electron paramagnetic resonance (EPR) has been used to investigate the adsorption capability and the surface interacting ability towards Cu(II) solutions (CuCl2, Cu(NO3)2, CuSO4 in water or ethanol) of various carbon blacks, both graphitized and ungraphitized, selected on the basis of the surface area, namely, Carbograph1 (area = 100 m2/g), Carbograph4 (area = 210 m2/g), and Carbograph5 (area = 560 m2/g), which were indicated as C1g, C4g, C5g (g = graphitized), and C1ng, C4ng, C5ng (ng = ungraphitized). The EPR analysis was supported by surface analysis, for evaluating the surface area, the pore volume and the porosity, and by atomic absorption to obtain the adsorbed Cu(II) amounts. Graphitization provokes a decrease in surface area, but C1g, at low surface area, showed a unexpected increase of the adsorption ability ascribed to the formation of new surface porosity closed by graphite layers. The carbon samples showed a broad unresolved EPR signal due to mobile unpaired electrons in the carbon matrix. Graphitized samples presented a narrower signal than ungraphitized samples, which increases in width with the increase in surface area (with the exception of C5ng due to the high exposition of the wide surface to oxydizing external agents) and upon prolonged thermal treatment. The signal intensity of the carbon paramagnetic centers decreases upon Cu(II) adsorption. Computer aided analysis of the EPR spectra of the solids after Cu(II) adsorption allowed to extract structural information on the Cu-surface site complexes. The Cu2+ ions coordinated with surface polar sites, mainly oxygenated. Adsorption depends on the different Cu(II) salts, caused by the salt solubility and the interacting ability of the counter-ion. In several cases the solutions concentrated in the carbon porosity leading to precipitation of the salt. Ethanol solutions are more adsorbed at the carbon surface than water solutions; Cu(II) partially retains its solvation shell and partially presents electron transfer to the carbon surface. Adsorption is favored to ungraphitized carbons with respect to the graphitized ones due to both the higher surface area, and the higher hydrophilicity of the surface. In summary, these carbon powders, widely used for chromatographic applications, show an adsorption capability towards Cu(II) solutions higher than expected due to both a definite porosity, and the presence of polar groups which are not eliminated with chemical surface treatments.

Physicochemical Studies on the Adsorption Properties of Asbestos

Toxic asbestos fibers are known to be carriers of carcinogenic polyaromatic hydrocarbons (PAH) when ingested by human lungs. To clarify the asbestos-PAH interactions, the adsorption properties of four different asbestos, namely amosite, anthofillite, crocidolite, and chrysotile (termed AMOS, ANTHO, CROX, and CHRYS, respectively), toward pyrene (py) were investigated by electron paramagnetic resonance (EPR) and fluorescence spectroscopy, both steady state and time resolved. Pyrene was labeled with a nitroxide TEMPO radical (py-T) for the EPR study. EPR provided information on the adsorption ability of the surface toward py-T and on the interactions between py-T and surface sites, mainly cooperative in nature. Pyrene molecules organize on AMOS samples in a packed layer; the radical works as a separator and prevents the attaching of the py-T molecules at the surface, but the N-O groups of the few adsorbed molecules directly interact with the surface. The interacting sites at the ANTHO surface are quite far from each other; therefore py-T instead of py may form a monolayer at the asbestos surface. The surface of CROX strongly adsorbs both excimers of py and monomers of py-T, orienting the nitroxide groups toward the polar sites at the surface. The surface of CHRYS is the most adsorbing toward py and py-T, but the nitroxide group does not directly interact with the surface. By increasing concentration, py-T saturates the CROX and the CHRYS surface interacting sites, and then forms a disordered liquid-like multilayer. The close average distance calculated for py-T adsorbed at the CROX surface (12 Å) suggests that the py-T molecules intercalate in an "up-and-down" conformation. The chemical composition of the different asbestos and their different structural properties affect the distribution of the polar sites at the external surface, modifying the adsorption properties. This study confirms the synergetic effect between the toxicity of the asbestos fibers and the adsorption of polyaromatic hydro carbons at their surface, which mainly depends on the surface properties of the asbestos. Copyright 2001 Academic Press.

Use of Nitroxides as Topological Monitors of the Interaction of Silica-Based Particles with Components of the Biological Environment

The interaction of solid particles, such as silica and vitreous fibers, with different surrounding media which well mimic the various environments in a biological medium, such as inhaled in vivo or in a cell culture, has been studied by means of the electron paramagnetic resonance (EPR) spectra of spin labels attached to the solid surface or spin probes inserted in the surrounding medium. Among the solid particles, a MCM-41 type mesoporous silica was found to be very suitable for investigating the binding between the labels and different molecules, due to the high surface area and the availability of interacting sites in the internal channels of the structure. The computer-aided analysis of the spectral lineshape allowed the evaluation of structural and dynamic parameters. A model has been proposed which describes the interactions of the solid surface with: (a) pure solvents at different polarities; (b) molecules present in biological fluids, which mimic the effect of physiological solutions; (c) the components of the cell membrane (phospholypid or proteins in water solution); and (d) a model phospholypid membrane, to mimic the interaction between the solid particles and the cell membrane. The hydration of the surface lets the labels interact preferentially with the water molecules with respect to the surface itself, or the other labels. Apolar molecules decreased the mobility of the labels attached to the surface. Phospholipid bilayers were formed at the solid surface, whose internal structure was more fluid with respect to noninteracting bilayers, whereas the external polar groups trapped probe and label molecules in restricted space at the surface. The labels were partially extracted from the wet surface of the vitreous fibers by the interaction with a protein (albumin) and distributed in two different environments (at different polarities).