Publications

The hypothetical four-center nitrogen exchange reaction of N-2 + N-2 is analyzed. We show that the three level crossings accompanying the least-motion nitrogen exchange reaction occur at different points along the reaction coordinate, leading to a mechanism requiring three `'singly forbidden'' reaction steps. Simple MO arguments show that the loss of one electron in N-2 + N-2(+) reduces the energy demand associated with the energetically dominating first and third level crossing,, suggesting that ionization of the reaction system lowers significantly the high activation barrier. This is supported by nonlocal density functional calculations on various N-4 and N-4(+) structures, which, however, also indicate that the barrier still remains at high energy: the tetraazacyclobutadiene intermediate involved in the neutral reaction is 166.7 kcal/mol higher in energy than N-2 + N-2; the corresponding radical cation is only 52.2 kcal/mol above N-2 + N-2(+). The DFT results also indicate that the N-2 + N-2(+) nitrogen exchange reaction, if it occurs at all, may also proceed via a competing mechanism involving a T-shaped transition state at 102.8 kcal/mol above N-2 + N-2(+). Suggestions for further experimental investigations emerge from this analysis.

Formation and surface properties of microcapsules, which are based on interaction of gelatin and sodium dodecyl sulfate (SDS) were investigated. It was found that microcapsules of oil droplets can be obtained at a specific range of SDS/gelatin concn. ratios. Both the amt. of gelatin and SDS which were bound to the oil/water interface were maximal in this range, in agreement with Zeta-potential measurements and fluorescence microscopy performed for FITC labeled gelatin. [on SciFinder(R)]

The formation of microcapsules which contain rosemary oil, is described. The process is based on two steps; (a) formation of oil-in-water emulsions, by using lecithin as emulsifier, thus imparting neg. charges on the oil droplets; (b) addn. of a cationic biopolymer, chitosan, in conditions that favor the formation of an insol. chitosan-lecithin complex. Zeta potential measurements revealed that addn. of very low concns. of chitosan to lecithin stabilized emulsions, led to reversal of charge. At a suitable pH range the chitosan pptd. around the oil droplets forming pos. charged microcapsules. The chitosan-lecithin insol. complex is composed of a 1:1 molar ratio of the chitosan monomeric unit and lecithin, as evaluated by elementary anal. and turbidity measurements. [on SciFinder(R)]

Surface active gelatin was formed by covalent attachment of hydrophobic groups to gelatin mols. The modification was carried out at various degrees of attachment and with various chain lengths. These modified gelatins (MGs) were synthesized in dry DMSO by a simple and rapid method. The new method leads to high yields and allows high degrees of modification. The MGs, which have various hydrophobicities, have better surface activity than the native gelatin, as detd. by surface tension redn. The surface tension redn. is correlated to the hydrophobicity of the modified mol., which was detd. by a fluorescent probe. It appears that both the increase in the no. of the hydrophobic groups and the increase in the chain lengths lead to decreased surface tension. [on SciFinder(R)]

A theoretical study is made on He scattering from three basic classes of disordered adlayers: (a) translationally random adsorbates, (b) disordered compact islands, and (c) fractal submonolayers. The implications of the results to experimental studies of He scattering from disordered surfaces are discussed, and a combined experimental-theoretical study is made for Ag submonolayers on Pt(111). Some of the main theoretical findings are: (1) The scattering intensities from the three disorder classes differ significantly, and can be used to distinguish between them. (2) Structural aspects of the calculated intensities from translationally random clusters were found to be strongly correlated with those of individual clusters. (3) For fractal islands, just as for all surfaces considered here, the off-specular intensity depends on the parameters of the He/Ag interaction, and does not follow a universal power law as previously proposed in the literature. In the experimental-theoretical study of Ag on Pt(111), we use experimental He scattering data from low-coverage (single adsorbate systems to determine an empirical He/Ag-Pt potential of good quality. Then, we carry out He scattering calculations for high coverage and compare with experiments for these systems. The conclusion is that the actual experimental phase corresponds to small compact Ag clusters of narrow size distribution, with partial translational disorder. (C) 1997 American Institute of Physics.

Appeared also in French.

Hydrophobic modification of human IgG by fatty acid esters (C8, C12 and C16) of N-hydroxysuccinimide was carried out. Such a modification leads to a spontaneous formation of micelle-like colloidal clusters with a mean diam. of 19.9-22.7 nm for C8-modified IgG. C12- and C16-modified IgGs form larger clusters in spite of a lower no. of attached alkyl chains. The adsorption of the modified IgGs onto latex particles was studied. It was found that the affinity of modified IgGs to the neg. charged hydrophobic polystyrene surfaces is higher than that of the native protein, although an increase in hydrophobicity is also followed by an increase in the net charge of the protein mol. In all cases, the highest equil. concns. correspond to the nearly satd. layer of adsorbed protein mols., this layer being more compact for hydrophobized IgG. The mol. areas of IgGs on the surface are close to those calcd. from the known structural data for the "leg-on" disposition of the "T"- or "Y"-shaped mols. The modified IgG retains recognition ability in ELISA tests, the activity decreases only at a high degree of modification. [on SciFinder(R)]