Locating heavy atoms by integrating direct methods and SIR techniques.

Direct methods have been applied to the SIR (single isomorphous replacement) case to estimate structure-factor moduli from diffraction magnitudes. The joint probability distribution function P(E(H),E(p),E(d)) has been calculated by explicitly considering, as an additional primitive random variable, the cumulative error arising from measurements and from lack of isomorphism. The specific feature of the approach is that it provides estimated values of |E(H)| which depend on the experimental diffraction data as well as on errors. Some test structures have been used to check the efficiency of the new estimates. Patterson techniques, using the estimated |E(H)|(2) values as coefficients, as well as a tangent procedure, have been implemented into a computer program to locate the heavy atoms automatically. All the experimental tests show that the new approach provides results highly competitive with the traditional |E(H)|(2) estimates.

Control of silver-polymer aggregation mechanism by primary particle spatial correlations in dynamic fractal-like geometry.

Silver nanocrystals have been prepared by reacting silver nitrate with ascorbic acid in aqueous solution containing a low concentration of a commercial polynaphthalene sulfonate polymer (Daxad 19). Various crystalline morphologies have been obtained simply by tuning the reaction temperature. We have investigated the nanoparticle formation mechanism at three different temperatures by in situ and time resolved small angle X-ray scattering measurements. By modeling the scattering intensity with interacting spherical particles in a fractal-like polymer-Ag matrix, we found signatures of nucleation, growth and assembly of primary particles of about 15-20 nm. We observed how the time evolution of both spatial correlations between primary particles and the dynamic fractal geometry of the polymer-Ag matrix could influence and determine both the aggregation mechanism and the morphology of forming nanostructures in solution.

Monitoring early stages of silver particle formation in a polymer solution by in situ and time resolved small angle X-ray scattering.

Silver particles have been prepared by reduction of silver nitrate with ascorbic acid in acidic aqueous solution containing a low concentration of a commercial polynaphthalene sulfonate polymer (Daxad 19) as dispersant agent. The reduction has been induced and controlled by the slow addition of ascorbic acid at a fixed rate; in this way, we were able to monitor the formation of a silver crystalline colloidal dispersion by in situ and time resolved Small Angle X-ray Scattering measurements. Modeling the scattering intensity with interacting spherical particles in a polymer-Ag like-fractal template allowed us to distinguish different stages involving liquid-like ordered cluster nucleation, cluster growth up to primary particle formation and particle coalescence. Between primary particle formation and particle coalescence, we observed the occurrence of a transient phase of core-shell type structures having primary particles as stable cores in expanding shells built by the organic polymer. We discuss these results in a twofold perspective pertaining both to technology, relative to controlled fabrication of metal nanoparticles and to basic chemical physics, dealing with non standard stepwise crystallization from solutions.