Riccardo Ricceri

THE ADSORPTION OF POLYSTYRENE SULFONATE AND ETHOXYLATED NON-IONIC SURFACTANTS AT CARBON BLACK-WATER INTERFACE

ABSTRACT The adsorption of a series of sodium polystyrene sulfonates with a narrow distribution of molecular weight and ethoxylated non-ionic surfactants ( NP-12, NP-40, NP-100 and C16E20 ) was investigated at carbon black-water interface. Adsorption isotherms of the Langmuir type were obtained for all the samples with negative adsorption free energies. The dependence of maximum of adsorption on molecular weight tor PSSNa was used to obtain information on the macromolecular conformation at the interface; surface geometry effects on adsorption conformation were analyzed using a simplified fractal approach. PSSNa does not adopt a flat conformation at the interface. Non-ionic surfactants ( NP-12, NP-40, NP-100 and C16E20 ) are adsorbed at the interface with the EO chains in a coil conformation.

Information storage based on photochemical effects in mixed Langmuir–Blodgett films

Mixed Langmuir–Blodgett (LB) films of two novel thiophene-spaced N-alkylpyridinium malononitrile zwitterionic dyes have been prepared. The LB films present two sharp, non-overlapping charge-transfer absorption bands which can be selectively photobleached in a irreversible fashion. The former could have potential application in multifrequency optical data storage.

Monolayers and Langmuir–Blodgett films of a palladium(II)-tetraazaporphyrin

Abstract Spread monolayers and Langmuir–Blodgett films were prepared with 2,3-bis( N -(3-thiopropyl)phthalimido)-7,8,12,13,17,18-hexakis(octylthio)-5,10,15,20-tetraazaporphyrinate palladium(II) (PdR 6 R 2 ′TAP). Although monolayers at the water/air interface tend to aggregate, transfer as multilayer films onto quartz slides was possible when the proper experimental conditions were chosen. The properties of the Langmuir–Blodgett films were investigated by UV-Vis and FTIR spectroscopy. Their structures were markedly influenced by transfer temperature. When the monolayer was transferred at T=309 K, the corresponding LB film showed a different packing structure with respect to the films transferred at lower temperatures. This resulted in a better homogeneity, as revealed by optical microscopy. The monolayer thickness and the refractive index were respectively determined by means of ellipsometry and with the aid of dark m -lines spectroscopy for LB films transferred at T=309 K.

Vectorial PET in LB bilayers and in vesicles: nanosecond fluorescence and laser flash-photolysis investigation

Abstract Vectorial photoinduced electron transfer was investigated in Langmuir-Blodgett bilayers and in vesicles with the same phospholipid composition. The donor was covalently bound to one of the Hydrophobie chains of the phospholipid in the mimetic membrane, whereas the acceptor was adsorbed at the polar head group surface. The location of the redox couple was preliminarily determined studying the photophysical properties of the donor in the phospholipid bilayer as well as the process of adsorption of the acceptor at the phospholipid-water interface. The photoinduced electron transfer reaction was studied measuring the quenching of pyrene fluorescence in the presence of increasing concentrations of acceptor, the process was studied both with steady-state and time-resolved fluorescence emission in the nanosecond time scale. The reaction transient species formed upon photoexcitation were identified performing nanosecond laser-flash photolysis experiments in vesicle systems. The dynamics of the charge recombination process following photoinduced electron transfer, was investigated studying the kinetic of decay of the absorption signal of the transient species.

Langmuir-Blodgett films of a new hemicyanine dye

Abstract We report on Langmuir–Blodgett (LB) characterization of 1-( N -ethyl-1-sulphonate-4-pyridinio)-2-[ N –( n –hexadecyl)pyrrol–2–yl]ethene (C 16 H 33 -ESPPE). Surface pressure-area isotherms of spread monolayers were investigated at pure water/air interface; LB films were characterized by means of UV-vis and steady state fluorescence spectroscopies. The compound is photobleachable and undergoes a dimerization reaction in monolayer and in solution, the strong absorbing band due to a π → π * transition in the visible region disappears by exposing the compound to visible light. A blue shift of this band in LB films with respect to the solution is attributed to a deck-of-cards packing of the chromophore in the film. The photobleachable absorption band could have potential applications in optical data storage; the molecule could also be very interesting for two-photon pumped (TPP) frequency-upconversion lasing applications.

LANGMUIR-BLODGETT FILMS OF PYRIDINIUM-DICYANOMETHANIDE DYES MIXTURES WITH PHOTOBLEACHABLE ABSORPTION BANDS

We report on Langmuir-Blodgett (LB) films characterization of 4-[5-dicyanomethanido)thien-2-y1]-N-(n-hexadecyl)pyridinium (C16H33-PDCNT), and 1-(N-(n-hexadecy]-4-pyridinio)-2-[5-(dicyanomethanido)thien-2-yl]ethene (C16H33-PDCNTE); LB films of the pure compounds and of the mixtures of the two compounds were prepared at 291 K: UV-vis investigation revealed the presence of photobleachable absorption bands, the ones at about 530 nm and 640 nm were due to charge transfer transitions of the monomer of C16H33-PDCNT and C16H33-PDCNTE, respectively; the sharp, photobleachable ones shifted to shorter wavelengths were due to H-aggregates of the two compounds. By changing the molar ratios of the two compounds in the mixtures and in other cases by annealing the LB films, the absorption maxima of the sharp, photobleachable bands due to H-aggregates could be tuned in the range 415–467 nm. These LB films are thus very promising in view of optical data storage applications.

THE EFFECT OF Na2SO4 ON THE STABILIZATION OF TiO2 AQUEOUS DISPERSIONS BY POLYSTYRENE SULFONATE

ABSTRACT The adsorption of sodium polystyrene sulfonate (PSSNa) on TiO2 in aqueous dispersion has been investigated in pure water and in the presence of different concentrations of sodium sulphate. The stability of TiO2 dispersions in PSSNa and Na2SO4 solutions has also been studied. Correlation between adsorption isotherms,zeta potential data and stability measurements has shown that the specific adorption of sulphate anions influences the adsorption of PSSNa and then the stability of the dispersions. The adding of proper amounts of Na2SO4 increases the stability of TiO2 dispersions in PSSNa solutions.

ADSORPTION OF POLYSTYRENE SULPHONATE AND CETYLTRIMETHYLAMMONIUM ON TITANIUM DIOXIDE

ABSTRACT Adsorption of sodium polystyrene sulphonate (PSSNa)and cetyltrimethylammonium bromide (CTAB) on TiO2 in aqueous dispersion has been investigated. Correlation between adsorption isotherms, zeta potential and stability measurements has shown that interactions between PSSNa and CTAB influence strongly the stability of the dispersions.

The crystal structure and spectroscopic characterization of 1-(N-ethyl-1-sulphonate-4-pyridinio)-2-[N-methylpyrrol-2-yl]ethene

Abstract The X-ray structure of 1-(N-ethyl-1-sulphonate-4-pyridinio)-2-[N-methylpyrrol-2-yl]ethene · H2O is reported. The molecule belongs to the spatial group P21/n with unit cell constants a = 6.841(5), b = 21.259(5), c=10.195(5), β=110.262(15)°. The compound was characterized by means of UV-vis and steady-state fluorescence spectroscopies. In the visible region the powder shows a blue shift of the main absorption band with respect to the solution. Analogously a marked red shift and decrease of intensity is observed in the fluorescence spectra. Packing effects are most likely at the origin of the difference. The molecule is expected to have promising proprieties for two-photon pumped (TPP) frequency-upconversion lasing experiments.

VECTORIAL PHOTOINDUCED ELECTRON TRANSFER IN PHOSPHOLIPID VESICLES AND LB BILAYERS

SummaryPhotoinduced electron transfer (PET) was studied in phospholipid vesicles and in Langmuir-Blodgett bilayers in the attempt to produce a model for electron transfer processes in biological media. Spatial organization of the reaction centers in lipid membranes needs to be controlled in order to provide high efficiency of light-to-chemical energy conversion. We designed a phospholipid system where the donor is localized in the inner bilayer whereas the acceptor is at the polar groups-water interface. We used dipalmitoylphosphatidic acid vesicles containing low molar fractions of dipalmitoylphosphatidylcholine with pyrene (donor) bound to one of the alkyl chains. Methylviologen (acceptor) was added to the external aqueous phase; upon photoexcitation of the donor we observed the electron transfer to take place in a unidirectional manner from the inside of the bilayer to the interface. Information about the location of the donor was obtained studying the photophysical properties of the pyrene chromophore in vesicles and in LB layers. The photoinduced electron transfer reaction was evidenced by quenching of pyrene fluorescence in the presence of increasing concentrations of acceptor, the process was studied both with steady-state and time-resolved fluorescence emission. Fluorescence intensity was found to decrease with increasing concentration of methylviologen, similar results were obtained for vesicles and LB layers of analog composition immersed in a methylviologen solution. Lifetimes of the excited species were found to be of the same order of magnitude in vesicle and LB-layer systems.