Three grain sizes of expanded perlite were modified with emulsifiers and their potential usefulness in combating oil spills was studied. The tests in the laboratory show that when this perlite is added to a water-oil mixture, the light perlite particles move on the surface spreading over it very quickly (in fractions of a second). It seems that the emulsifier disperses the oil, but at the same time it disperses the perlite particles. At the end there is an emulsion and also perlite particles saturated with it.The usefulness in combating oil spills at sea depends on the following characteristics:(a) With emulsifier-modified perlite some of the oil can be removed (in the form of emulsion), whereas with emulsifiers only this is not possible. Simultaneously the spill is dispersed quickly, before spreading.(b) The action is quick even with a calm sea. Self-mixing is inherent to the process.(c) The action is quick and limited to the surface, where the perlite particles float. There is little waste of the emulsifier in the bulk of the sea. (C) 2002 Elsevier Ltd. All rights reserved.

Introduction One of the Holy Grails of chemistry is to seek a photo-activated catalyst to split water into H2 and O2 using solar spectra. Significant effort has been undertaken by the scientific community to develop a catalyst that can catalytically split water using visible light with limited success. However, one exception may be the homogeneous catalyst, tris-[1-(4methoxyphenyl)-2-phenyl-1,2ethylenodithiolenic-S,S’]tungsten, reported by researchers at the University of Athens. A unique and highly desirable characteristic of the tris-dithiolene catalyst is its ability to form both hydrogen and oxygen from water without the need to add a consumable (sacrificial) donor. Water is reduced to hydrogen and oxidized to oxygen in a sequential pathway. The energy storage efficiencies of the trisdithiolenes have been reported to range between 7-11%, values approaching the economic break point for solar production of hydrogen. While much work has been reported on the stability and efficiency of the various derivatives and isomers of tris-dithiolenes, little is known about the important steps of the mechanism that leads to hydrogen and oxygen formation. To this end, the understanding of the mechanism for the gas formation will assist in the design of the next generation of PCC’s with the aim in increasing their efficiency.

The reaction of Cu2+ with tris-dithiolenes I and II (vide infra) was studied by electrospray-MS and kinetically. With complex I, there is a multiple attack resulting in the formation of copper mono- and di-dithiolenes at constant ratio and initial kinetics are first-order in the tungsten complex. With complex If, a similar reaction takes place, but the ratio of the copper dithiolenes is not constant and the kinetics are not simple. The ES-MS results indicate that with this complex, there is also attachment of Cu2+ and H+ on the dimethylamine nitrogen. With both I and II, the initial reaction is followed by hydrolysis, condensation and formation of the polynuclear W6O192-.

The separation of Cr(III) from collagen in chromium-tanned leather (wet-blue) is achieved by a combination of the protein (collagen) stabilization (protective cross-linking) and a subsequent labilization of the Cr(III) species bound to the protein. During the separation collagen retains the tertiary triple helical and higher (fibril) structures. The model proposed for the protective cross-linking involves inter- rather than intramolecular bridging, that is, a supramolecular bridging. The process of chromium removal takes place in a restricted aqueous environment, and it can be characterized as semihomogeneous or semiheterogeneous.

Excitation of four unsymmetrically substituted tungsten tris(dithiolenes), in various solvents with 355 nm laser pulses produce weak but easily measurable transient absorptions. The four complexes used are characterized as photocatalyst-catalysts (PC-C). for the photodecom-position of water and their names are tris-[ 1-(4-dimethylaminophenyl)-2-phenyl-l,2-ethylenodithiolenic-S,S′]tungsten,tris-[ 1-(4-methox-yphenyl)-2-phenyl-1, 2-ethylenodithiolenic-S,S′] tungsten, tris-[1 -(4-methoxy-phenyl)-1,2-ethylenodithiolenic-.S,S′] tungsten and tris-[ 1 -(phenyl)-1,2-ethylenodithiolenic-S,S′]tungsten. The lifetimes of the transient absorptions observed are long, persisting into the millisecond time domain. In acetone-water, and in the presence of MV2+ there are at least two transients, one corresponding closely to the known spectrum of MV , assigned to the hydrogen precursor, the other living much longer and assigned to the O2 precursor. The results indicate that the electron is quickly removed from the excited state of the photocatalyst-catalyst and is temporarily stored in the electron acceptor, in a way that is unfavorable to recombination. The first stages of the process can be schematically represented as: In-MV2+-(W-L)--OH2→MV2+ --(W−-L+)--OH2→ MV --(W-L)- +OH2→H2 + (W-L) +O2.The dotted lines connecting the reagents with the PC-C (W-L) were put in this scheme in order to emphasize the supramolecular nature of the interactions. The opposite charges created by light absorption are quickly transformed into repulsive positive charges due to the presence of the electron acceptor MV2+ and two centers are created, two molecular diameters apart, one the precursor of the H2, the other the precursor of O2. The combination of a small space separation charges and of the electrostatic factor (transformation of these charges into repulsive ones) are perhaps crucial factors in the separation of the reduction from the oxidation. However, there is also a kinetic factor. The reduction is quick, irreversibly leading to H2 not to recombination. The splitting of water is time resolved. The reactions at the reduction end of the supramolecular complex (presumably because the involve fast proton and electron transfers) lead fast and irreversibly to H2 formation.

The intermolecular interactions of the catalyst tris(1,2-ethylenedithiolate-S,S')tungsten, W(S2C2H2)(3), with the molecules of a 25 : 75 water-acetone mixed solvent, are examined by statistical mechanical methods, and specifically by a molecular dynamics (MD) technique, using charge distributions obtained by extended Huckel calculations. The results are presented in the form of pair correlation functions (PCF), and show that an average of up to three water molecules come close to the sites of the catalyst, whereas the acetone molecules form an open cage at a somewhat longer distance. The supramolecular structure around the catalyst is asymmetric, and is examined in the two characteristic geometries corresponding to the D-3h and C-2v symmetries which the 'tris(dithiolene)' molecule transiently assumes during its fluxional metamorphoses in solution. At large distance (large values of the correlation parameter r) the system is homogeneous. Yet, at nanometer distances the symmetry breaks down, and the system becomes highly asymmetric with differentiation (selectivity) in space. The mobility of the water and acetone molecules, close to the catalyst, was also studied by estimating the translational self-diffusion coefficients D-wat and D-acet from the center of mass linear velocity correlation functions. The results show that the water molecules are more mobile than the acetone molecules, which is the opposite of what happens without the catalyst.

The electronic spectra of aqueous CrVI solutions in the pH range 3–11 have four isosbestic points, located at 246, 293, 338 and 441 nm. These positions are independent of the total concentration of CrVI. The existence of the isosbestic points indicates that only two species are present, namely CrO42− and Cr2O72−, and that HCrO4− is spectrophotometrically undetectable. This conclusion is further supported by factor analysis, which gives the number of linearly independent species in solution to be two, and also by the method of spectral displacement. At the isosbestic points absorbance in linear in total CrVI concentration. In this sense Beer's law is valid, and the results can be used for analytical purposes.

Of the 20 dithiolene complexes investigated, only three were effective in the photo-oxidation of water to O-2 in the visible region and at the same time sufficiently stable: tris-[1-(4-dimethylaminophenyl)-2-phenyl-1,2-ethylenodithiolenic-S,S']tungsten, tris-[1-(4-methoxy-phenyl)-2-phenyl-1,2-ethylenodithiolenic-S, S']tungsten and tris-[1-(4-methoxyphenyl)-1,2-ethylenodithiolenic-S,S']tungsten. Four more produced oxygen, but were not sufficiently stable for the reaction to be truly catalytic.The three successful catalysts are non-symmetrical and have phenyl rings with para electron-donating substituents. The metals used were W, Mo, Re and Ni. The dithiolene complexes of the first three are trigonal prismatic and of nickel are square planar. The best results in terms of yield and stability were obtained with tungsten, and specifically with tris-[1-(4-dimethylaminophenyl)-2-phenyl-1,2-ethylenodithiolenic-S,S']tungsten; the light storage efficiency for this catalyst was estimated to be better than 9%.As indicated by the oxidation potentials, the excited states of the dithiolenes used were powerful oxidizing agents, sufficient to oxidize water. (C) 1997 Elsevier Science S.A.

Tungsten tris(1,2-dithiolene) complexes [W(S(2)C(2)R(1)R(6))(S(2)C(2)R(2)R(5))(S(2)C(2)R(3)R(4))] (R(4,5,6) = H or Ph; R(1,2,3) = para-substituted Ph), have been found to exhibit properties (H-1 NMR, UV/VIS, IR spectral) closely related to each other through Hammett constant sigma(p)(+), indicating extensive delocalization and conjugation. A comparative study of complexes containing differently substituted dithiolenic rings showed that the electron delocalization is hot confined within each dithiolenic ring separately, but involves all of them, i.e. it is three dimensional. This is further supported by a NMR and/or UV/VIS study of the effect of water and acid on the dimethylamine derivative. The oxidation number was assigned by recording the NMR spectra of the metal nuclei; the metals have effectively been stripped of their valence electrons; which are placed on the ligands. The non-planar 'aromaticity' of the dithiolenes differs from the familiar flat organic aromaticity since it involves mixing of sigma and pi bonding.

The isomerization of nine asymmetric tris(dithiolenes) of tungsten and molybdenum, of the general formula (R(1)R(2)C(2)S(2))(3)M is studied with NMR methods. In the complexes investigated, R(1) = H, R(2) = p-CH(3)OPh, p-CH(3)Ph, or Ph, and M = W or Mo, or R(1) = H, R(2) = p-ClPh or p-BrPh, and M = W, or R(1) = Ph, R(2) = p-CH(3)OPh, and M = W, as shown in formula I. The complexes are proved to be trigonal prismatic in solution and stereochemically nonrigid at room temperature. An equilibrium favoring the trans isomer (formula III) is established, with the concentration of this isomer being three times that of the cis due to entropy reasons. The kinetics and mechanism of the isomerization is investigated and a scheme is proposed involving the rotation of only one ligand around an axis lying on the dithiolenic ring, passing from the metal to the center of the carbon-carbon bond. This mechanism satisfies energy criteria and is allowed by symmetry selection rules, as theoretical EHMO calculations indicate.

Ten new nonsymmetric dithiolenes of the general formula [RR'(C2S2)](3)M were synthesized, from the corresponding 1,3-dithiol-2-ones, or from the corresponding mixed benzoins and tetraphosphorus decasulfide. The redox properties of these ten complexes together with those of the symmetric ones with R = R' = Ph and M = Mo or W, were investigated by cyclic voltammetry. Two processes were only observed, assigned to C --> C1- and C1- --> C2- (C the complex), for which the quasi-reversible E(1/2) potentials are highly dependent on the nature of the central metal ion, and for the same metal on the nature of the ligand and on the substituents on the phenyl rings. The ease by which the complexes are reduced is most likely the reason they give ESMS (negative mode) spectra, in spite of the fact they are neutral in solution. The process of this reduction and the parameters affecting it are investigated and discussed.

The yield of the photocatalytic splitting of water using tris-[1-(4-methoxyphenyl)-2-phenyl-1,2-ethylenodithiolenic-S,S′] tungsten as a photocatalyst-catalyst increases by more than threefold on going from 20 to 70°C, and there is no indication that the effect levels off at this temperature. The intensity of light (within the error limits of our experiments) does not have appreciable effect. The nature of the reversible electron acceptor also influences the energy storage efficiency, e.g. 1,1-dibenzyl-4,4′-bipyridiniumdichloride gives an energy storage efficiency approximately 10% higher than methylviologen. The energy storage efficiency also depends on the presence of electron donors; if Ph3N is added, the energy storage efficiency increases by 20%. With ethylenediaminetetraacetic acid (EDTA) the results are even more spectacular; there is a twofold increase, but only initially. At longer times the system is unstable. Overall light energy storage efficiencies can be as high as 7%, and the expectations for further improvement are very good.

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The monoanion of the title compound acts as a catalyst in the production of H2 from water. The reaction was studied using the free radical derived from methyl viologen as the source of the required electrons, in mixed water-acetone solutions. The kinetics are first order in the concentration of the free radical, and in the concentration of the catalyst. The dependence on hydrogen ion concentration, and on the water content of the solvent is complex. This, and the activation parameters indicate parallel paths. The proposed catalytic cycle involves the sequence: (1) Electron transfer, (2) Proton transfer, (3) Electron transfer, (4) Proton transfer. It is postulated that the rate-determining step is the transfer of the second electron or a concerted combination of this and of the transfer of the second proton.

Physical and chemical measurements of Greek coals lead to a classification by rank into five categories, ranging from subbituminous C to peat. This classification is further documented by ESR spectroscopy and powder X-ray diffraction.

The reduction of cystine by aqueous vanadium(II) was investigated in the p..H range from 7.5 to 12. The product ratio [VIV] /[VIII] reaches a maximum at pH ca. 9 and depends linearly on the excess concentration of cystine. It is also affected by cysteine, but not by initially added vanadium(III). The rate of the oxidation is first order in total vanadium(II) and also depends on cystine and on added cysteine or mercaptoacetic acid. The data are consistent with a mechanism involving two parallel paths leading to vanadium(III) and vanadium(IV), with precursors differing by one cystine ligand. In either case, the net result is scission of the SS bond.

The kinetics of the oxidative addition of hydrochloric acid (6–12 M) to the quadruply bonded Mo2Cl84− ion, 1, to produce the triply bonded hydride Mo2(μ-H) (μ-Cl)2(Cl)63−, 2, are first order in the concentration of the acid, and first order in the Mo24+ reactant. The rate is strongly affected by axial coordination. At lower acid concentrations (< 6 M) the reaction is complicated by hydrolysis. The hydride 2 and the analogous nonachlorodimolybdate Mo2(μ-Cl)3Cl63−, 3, undergo a two-electron reduction by chromous chloride in HCl (6M) to give 1 ; the Mo26+/Mo24+ couple catalyses the anaerobic oxidation of Cr(II) to Cr(III).