G. La Manna

Study of AOT-stabilized microemulsions of formamide and n-methylformamide dispersed in n-heptane

Abstract A wide investigation of some physicochemical properties (density, viscosity, conductance, IR spectra, permittivity) of AOT-stabilized dispersions of formamide and n -methylformamide in n -heptane has been performed. The experimental data are consistent with the hypothesis that these highly hydrophilic substances are encapsulated within AOT reversed micelles and that this structure is maintained for both systems well above the volume fraction of the dispersed phase where a percolative transition occurs. In addition, the observed properties of these microemulsions reveal the pivotal role of intermicellar attractive interactions in driving the percolative transition. A marked modification of the structural and dynamical properties of formamide and n -methylformamide accompanying their solubilization within AOT reversed micelles has also been observed.

Hydrogenation of light hydrocarbons on palladium: theoretical study of the local surface arrangements

Abstract Quantum mechanical calculations at HF, MP2 and DFT levels were used to rationalise the surface effects of the hydrocarbon lateral interactions occurring on single planar metal surfaces or on different adjacent surfaces during the hydrogenation on palladium catalyst. The different values of electronic charge, found in the different atoms of palladium clusters, were suggested as a possible explanation of the non-homogeneous behaviour already inferred for catalyst surface sites having non-isotropic local arrangement.

About entangled networks of worm-like micelles : a rejected hypothesis

We report new results from small-angle neutron scattering ond12-cyclohexane/lecithin/water micellar solutions performed as a function of the water content (w0), temperature (T) and dispersed phase volume fraction (ϕ). The data from dilute samples are interpretable in terms of the existence of giant cylindrical reverse micelles and are well fit with a core-shell model (that provides the micelle structure and dimensions) with values of 28 and 45 Å for the inner core and the outer shell radii, almost independent on temperature and concentration. Such a result could appear consistent with the current idea that worm-like micelles are living polymers. On the contrary, the appearance of a sharp interference maximum at high concentrations (ϕ>0.15) suggests arguments against the current hypothesis of an entangled network of giant flexible cylinders. Further arguments against the current hypothesis are given by the close similarity between the above described results and those from free of water micelles (for sure not cylinders). All the data are well fitted in terms of a unique model taking into account the micellar form factor plus a hard sphere structure factor. The data analysis suggests a micellar size distribution determined by the competition between concentration and interaction effects on which temperature plays not a minor role. Following our results, the current hypothesis of a gel structure in terms of an entangled network can be assumed as wrong and some caution has to be taken in assuming wormlike micelles as living polymers.

Dielectric properties of water/lecithin/cyclohexane gels

Dielectric relaxation measurements in the frequency range 0.1–15 GHz have been carried out on water/lecithin/cyclohexane gels using a time domain reflectometry method (TDR). Dielectric dispersions describable in terms of a power law typical of percolated systems, but with exponents significantly different from those characterizing statically or dynamically percolated systems are observed. The dielectric dispersions, attributed to water and/or lecithin head group rotational motions, indicate that these motions are characterized by a wide spectrum of relaxation times.

Structural and dynamical investigation of gelation containing water-in-oil microemulsions

The gelatin (Bloom 300)/water/AOT/n-heptane system has been investigated at fixed water/AOT molar ratioR (R=31.1) as a function of the gelatin content. Several experimental techniques (densitometry, refractometry, conductometry, rheology, dielectrometry, ultrasonics, hypersonics) have been used to investigate the role played by the gelatin molecule in the observed sol-gel transition above a critical gelatin content. The results appear consistent with the hypothesis of a rigid network of gelatin-water rods coated by surfactant molecules coexisting with gelatin-free AOT reversed micelles at the gelation point.

Population analysis of pair densities. A study on cyclic systems

Abstract Pair population analysis seems a reliable method for evaluating atomic valency states and bond orders from the MO wavefunction of molecular systems. A test on some cyclic organic systems was performed. The results confirm the capability of the method to provide a correct description of the molecules in terms of simple VB approach.

Dielectric properties of solutions of antimony (III) halides in methanol

The complex permittivities obtained by time domain reflectometry (TDR) measurements in the frequency range 0.1–15 GHz are reported for solutions of SbF3, SbCl3 and SbBr3 in methanol at 25°C. By increasing the concentration of the solutes, a marked increase of the main relaxation time and of the static permittivity of methanol is observed. These effects have been attributed to the ability of antimony (III) halides to strengthen the hydrogen bonding in methanol and to promote the shift of the dynamic equilibria among the various populations of hydrogen bonded methanol aggregates toward those characterized by a parallel orientation of neighboring dipoles. A second relaxation, occurring at low frequencies, was attributed to the rotational dynamics of the solvated solute molecules.

Protonation of acetamidoxime: An ab initio molecular orbital study

Abstract Stabilities of acetamidoxime isomers, conformers and tautomers were investigated by ab initio calculations using different basis sets (up to 6–31G*). Protonation energies evaluated for the most stable tautomers indicate that the best protonation site of the amino—oxime structures is the oxime nitrogen whereas the imino—nitrogen is the preferred site for the other tautomeric forms.

Electrostatic contribution to the interaction of α, β poly (N-hydroxyethyl)-DL-aspartamide with sodium dodecylsulfate micelles

The enthalpic effect due to the interaction between α, β poly(N-hydroxyethyl)-DL-aspartamide (PHEA) and sodium dodecylsulfate (SDS) in aqueous solutions as a function of the surfactant concentration was measured by the calorimetric technique at various NaCl concentrations. A marked influence of the added electrolyte on the PHEA-SDS interaction was observed. An analysis of the experimental enthalpies allows to estimate the electrostatic and the hydrophobic contributions to the enthalpy of interaction between PHEA and SDS micelles. The results were rationalized in terms of effects due to the screening of the charges residing on PHEA and SDS micelles.ZusammenfassungMittels einer kalorimetrischen Methode wurde bei verschiedenen NaCl-Konzentrationen der Enthalpieeffekt der Wechselwirkung zwischen α, β Poly(N-Hydroxyethyl)-DL-Aspartamid (PHEA) und Natriumdodecylsulfat (SDS) in wäßriger Lösung als eine Funktion der Konzentration des oberflächenaktiven Stoffes gemessen. Dabei wurde ein deutlicher Einfluß des Elektrolytzusatzes auf die PHEA-SDS-Wechselwirkung festgestellt. Eine Analyse der experimentell ermittelten Enthalpiewerte gestattet eine Schätzung des elektrostatischen und hydrophobischen Beitrages zu den Enthalpien der Wechselwirkung zwischen PHEA- und SDS-Mizellen.

Cationic structures of acetamidoxime: theoretical analysis of H+ and Li+ complexes

Abstract A series of ab initio calculations at the 3-21G and 6-31G ∗ levels was performed to investigate the molecular conformers deriving from the association between H + , Li + and acetamidoxime in its two tautomeric forms; such complexes are useful models for simulating drug-cationic-receptorsite interactions. The results indicate that, particularly for lithiated systems, a certain number of conformers lie within a few kilocalories of the global minimum. Hence, more than one accessible conformation may be available for interaction with a cationic receptor site and, therefore, may be biologically active. Li + -base complexes are also characterized by a quite significant charge transfer to Li + .