Abstract: The reactions of 2-pyridinealdoxime with Zn(O(2)CPh)(2).2H(2)O have led to a mononuclear complex and a dodecanuclear cluster; the Zn(12) compound, whose metallic skeleton describes a tetrahedron encapsulated in a distorted cube, is the biggest Zn(II) oxime cluster discovered to date and displays photoluminescence with a maximum at 354 nm upon maximum excitation at 314 nm.
Abstract: Sahara desert sand (SaDeS) was employed as a mineral sorbent for retaining organic dyes from aqueous solutions. Natural sand has demonstrated a strong affinity for organic dyes but significantly lost its adsorption capacity when it was washed with water. Therefore, characterization of both natural and water washed sand was performed by XRD, BET, SEM and FTIR techniques. It was found that water-soluble kyanite, which is detected in natural sand, is the dominant factor affecting adsorbance of cationic dyes. The sand adsorbs over 75% of cationic dyes but less than 21% for anionic ones. Among the dyes studied, Methylene Blue (MB) demonstrated the strongest affinity for Sahara desert sand (Q(e)=11.98 mg/g, for initial dye solution concentration 3.5 x 10(-5)mol/L). The effects of initial dye concentration, the amount of the adsorbent, the temperature and the pH of the solution on adsorption capacity were tested by using Methylene Blue as model dye. Pseudo-first-order, pseudo-second-order and intraparticle diffusion models were applied. It was concluded that adsorption of Methylene Blue on Sahara desert sand followed pseudo-second order kinetics. Gibbs free energy, enthalpy change and entropy change were calculated and found -6411 J/mol, -30360 J/mol and -76.58 J/mol K, respectively. These values indicate that the adsorption is an exothermic process and has a spontaneous nature at low temperatures.
Abstract: Organogels were obtained by adding small amounts of water to a solution of lecithin in organic solvents. Either isooctane or isopropyl palmitate and isopropyl myristate were used as the continuous organic phase of the gels. EPR spectroscopy using both DSA membrane-sensitive and lipophilic spin probes was applied to define the dynamic structure of the surfactant monolayer and the continuous oil phase of lecithin organogels. It was found that by increasing the water quantity, an increase of the polar head area per lecithin molecule was induced, and as a consequence the total interface expanded. It was found that the use of esters as organic solvents induced a decrease of the size of the dispersed structures. The interconnection of the aqueous microdomains and their dynamics were monitored by both static and time-resolved fluorescence quenching spectroscopy using Ru(bipy)32+ as fluorophore and Fe(CN)63- as quencher. It was found that the rates of inter- and/or intra-micellar exchange of water molecules were very slow because they appeared quite immobilized close to the lecithin polar heads. According to the results of the dynamic studies, appropriate organogels were formulated and used to incorporate model bioactive compounds with medicinal or cosmetic interest such as caffeine and theophylline. When these systems were tested for trans-membrane diffusion, they showed a 24 h permeation of 20% and 35%, respectively.
Abstract: Nanocomposite organic-inorganic gels synthesized through the sol-gel procedure by using Ureasil precursors can be employed as efficient sorbents for retaining metal cations and small anions from aqueous solutions. Sulfate or sulfonate as well as nitrate ions demonstrate a high affinity for Ureasils. High affinity was also demonstrated by trivalent metal cations. This affinity was explained by the presence of urea and of ether groups in Ureasils, which have the ability to complex many different types of ionic species.
Abstract: Nanocrystalline titania films have been deposited on glass slides by the sol-gel technique in the presence of surfactant, which plays the role of template of the nanostructure. Several different dyes, both anionic and cationic, have been adsorbed on these films from aqueous solutions. Some of these dyes were adsorbed at large quantities some at lower quantities. Some of them were adsorbed in monomeric form and others formed aggregates. Aggregates are easily distinguished by absorption spectrophotometry, since absorption of light is observed at a different wavelength than monomer absorption in a dilute solution. In all cases, aggregation demonstrated a hypsochromic shift, indicating repulsive interactions, which are justified in view of the fact that titania surface is charged and that adsorbed molecules are aligned in parallel. The above titania films are hydroxylated. Therefore, cationic dyes were readily adsorbed. Anionic dyes could be adsorbed only from aqueous solutions brought at low pH. Photodegradation rates of adsorbed dyes were generally fast since these films are efficient photocatalysts. Nevertheless, photodegradation of an adsorbed dye was faster when the quantity of the dye was smaller. When the adsorbed dyes formed aggregates, aggregation had adverse effect on photodegradation rates.
Abstract: Gels obtained through the sol-gel procedure by mixing an alkoxide with a carboxylic acid emit intense intrinsic photoluminescence when the alkoxide contains an amine or amide group. However, also simple polyether chains with two end amino groups but no alkoxysilane groups gel by interaction with carboxylic acids and also emit photoluminescence. Luminescence emission in these gels could be justified by a model similar to the emission of Green Fluorescent Protein. According to this model, amide groups, abundant in these gels, go through multiple reactions with other amide or amino groups so that they could yield conjugated species that are capable to emit photoluminescence. This model could open the possibility to synthesize a great variety of photoluminescent species based on amine-carboxylate interactions.
Abstract: Poly(N,N-dimethylacrylamide) hydrogel forms complexes with terpyridine and various trivalent ions, like Eu(3+), Tb(3+), Gd(3+), and In(3+). The hydrogel can be obtained in three different phases: swollen with water, lyophilized (i.e., dried by freeze-drying), where it loses the solvent but preserves the swollen configuration, and dried in the air where it shrinks. The three hydrogel phases affect the type of complex formed between terpyridine and the metal ion. Thus, in the swollen and lyophilized phases, metal-centered emission can be obtained by energy transfer from the excited ligand. In the shrunk phase, an intense green fluorescence is emitted, which is ligand-centered and is independent of the complexed ion. In the absence of any ion, the ligand emits blue luminescence, independently of the hydrogel phase. In the presence of europium(III) ions, blue, green, or red emission can be thus produced at appropriate compositions and hydrogel phases. Analysis of the photophysical behavior of the polymer-ligand-metal ion complex is related with the photophysical behavior of the ligand and its complexes in various pure solvents.
Abstract: The use of di-2-pyridyl ketone, (py)2CO, in zinc(II) nitrate chemistry has yielded a dinuclear complex and a cationic tetranuclear cluster. The 1:1 Zn(NO3)2.4H2O/(py)2CO reaction system in EtOH gives [Zn2(NO3)2{(py)2C(OEt)O}2].0.5H2O (1.0.5H2O), whereas the same reaction system in MeCN yields [Zn4(NO3)3{(py)2C(OH)O}4(H2O)](NO3) (2). The monoanionic derivatives of the hemiacetal and the gem-diol forms of di-2-pyridyl ketone have been derived from the ZnII-mediated addition of solvent (EtOH, H2O involved in MeCN) on the carbonyl group of (py)2CO. Each (py)2C(OEt)O- ion functions as an eta1:eta2:eta1:mu2 ligand in 1.0.5H2O chelating the two ZnII atoms through the 2-pyridyl nitrogen atoms and the common bridging, deprotonated oxygen atom; one asymmetric chelating nitrate completes six coordination at each metal center. The tetranuclear cluster cation of 2 has a cubane topology with the ZnII ions and the deprotonated oxygen atoms from the four eta1:eta3:eta1:mu3 (py)2C(OH)O- ligands occupying alternate vertices. Three monodentate nitrates and one aqua ligand complete the sixth coordination site at the metal ions. The two complexes have been characterized by IR and far-IR spectroscopies. Characteristic bands are discussed in terms of the known structures and the coordination modes of the nitrato ligands. Upon excitation at 371 nm, complex 2 displays blue photoluminescence in the solid state at room temperature with two emission maxima at 430 and 455 nm.
Abstract: CdS nanoparticles have been synthesized and stabilized in poly(N,N-dimethylacrylamide) hydrogels. The properties of the composite material have been characterized by UV-vis spectroscopy, scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and steady-state and time-resolved luminescence spectroscopy. This material can be obtained in three different states: swollen, shrunk, and freeze-dried. The swollen and the freeze-dried states correspond to a nanocomposite organic/inorganic (wet or dry) gel containing CdS nanoparticles of approximately 50 nm diameter while the shrunk state is a two-phase system containing CdS crystals, which precipitate forming interesting geometrical shapes.
Abstract: Water-in-oil microemulsions have been made with lecithin in the presence of some alcohols. The structure of the microemulsions has been studied by steady-state and time-resolved analysis of the luminescence quenching of Ru(bipy)32+ by Fe(CN)63-. We found that well-defined microemulsions can be made only in the presence of short-chain alcohols such as propanol-1 and butanol-1. There exists a threshold for water content in order to obtain typical reverse micelles. Thus in the case of propanol-1, water/surfactant ratio wo should be above 20. By varying water content in the range 20 < wo </= 40, the microemulsion droplets suffer dramatic structural changes and the system passes through a percolation threshold. Copyright 1997 Academic Press. Copyright 1997Academic Press
