Alexander T. Dembo

Size distributions of metal nanoparticles in polyelectrolyte gels

Small-angle x-ray scattering is used to study size distributions of noble metal nanoparticles embedded in polyelectrolyte hydrogels with oppositely charged surfactants. A procedure is proposed to subtract matrix scattering and to extract pure scattering due to the nanoparticles allowing to evaluate their size distribution functions by means of a regularization technique. Two kinds of collapsed gel-surfactant complexes were studied: a complex of a cationic gel of poly(diallyldimethylammonium chloride) with an anionic surfactant sodium dodecyl sulfate (PDADMACl/SDS), and that of an anionic gel of poly(methacrylic acid) with a cationic surfactant cetylpyridinium chloride (PMA/CPC). Addition of a gold compound (HAuCl4⋅3H2O) to the PDADMACl/SDS system forms the metal compound clusters and leads to a partial distortion of the gel structure. After subsequent reduction of the gold compound with sodium borohydride (NaBH4) ordering in the gel disappears and gold nanoparticles are formed. Their size distribution inc...

Small-angle X-ray scattering reveals hollow nanostructures in iota- and kappa-carrageenan/surfactant complexes

Complexes of polyelectrolyte gels with oppositely charged surfactants form ordered polymer matrices with unique structure properties, which can be used in a wide range of medical, chemical and physical applications. Ordered matrices made of natural components are of special interest for medical and biological purposes. In the present study, self-organised complexes of native - and -carrageenans formed with oppositely charged cetylpyridinium chloride (CPC) are prepared and structurally characterised using small-angle X-ray scattering (SAXS). Starting from the molar ratio CPC/(charge of carrrageenan) of 0.2, the carrageenan gel shrinks and ordered motifs in its internal structure are formed. The internal order in the sample is reflected by the Bragg peaks in the scattering patterns which permit to compute periodicity and characteristic sizes of the ordered regions. Moreover, strong central scattering appears and the calculated fractal dimensions indicate that these regions are organised in well defined clusters. The periodicity of the ordered motifs computed from the Bragg peaks in the scattering patterns corresponds to the width of the surfactant bilayer (about 4.0 nm) for all the complexes. The crystallite size ranges from 25 nm to 40 nm depending on the type of carrageenan and on the amount of CPC. A model of the ordered fragments is proposed whereby the carrageenan/surfactant bilayers are regularly packed at the walls of hollow cylindric clusters with the outer radius of about 8 nm and height 40 nm. More detailed ab initio models indicate that these particles are formed by bent worm-like substructures with the cross-section coinciding to the thickness of the carrageenan/CPC bilayer. Thanks to a higher charge density per monomer, -carrageenans form more regular structures than -carrageenans.

Metal colloid formation in the complexes of polyelectrolyte gels with oppositely charged surfactants

Abstract The interaction of Pt, Au and Ni ions with the complexes of cationic gel of polydiallyldimethylammonium chloride (PDADMACl) with two anionic surfactants: sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulphonate (SDBS) was investigated. After reduction of metal compounds by hydrazine-hydrate or sodium borohydride, colloidal particles of Pt, Au and Ni embedded in the body of the hydrogel were formed. It was found by small-angle X-ray scattering (SAXS) that when the PDADMACl/SDS complexes interact with metal ions, the nanostructures are disturbed and the degree of disordering depends on the type of metal compound. PDADMACl/SDBS complexes seem to be more stable towards metal ion incorporation. By SEM and AFM data it was shown that gold colloid formation is controlled by the ordering in the PDADMACl/SDS complex which prevents the aggregation of metal colloids.

Small-Angle X-ray Scattering Study of the Structure of Self-Organized Polymer Matrices and Formation of Imbedded Metal Nanoparticles

The structures of self-organizing polymer matrices based on collapsed gels and polyoctadecylsiloxanes have been studied by the methods of conventional and anomalous X-ray small-angle scattering with the use of the laboratory and synchrotron sources of X-ray radiation. The process of formation of metal nanoparticles in such matrices is also studied, their size distributions are calculated, and the models of localization of these particles in the polymer matrices are suggested. It is shown that growth of metal particles is controlled by the structure of the polymer network and is limited by the dimensions of its cells. The electron-density profiles of polycondensed octadecylsiloxane matrices are also calculated.

A new hydrogel system undergoing a volume phase transition upon heating

A new hydrogel system capable of a thermotropic collapse at about 47 °C is the complex poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate (PDADMA-Cl/SDS). It was found that the stoichiometric complex of PDADMA-Cl/SDS being highly ordered in the collapsed state is capable of swelling strongly in the presence of excess SDS. Upon swelling the gel loses its ordered structure and becomes transparent. The hydrogel shrinks upon heating approaching its initial volume. However, in the newly achieved collapsed state the gel has no ordered structure and the volume phase transition is associated with drastic changes in the heat capacity typical of co-operative conformational and phase transitions. It is reasonable to suggest that this hydrogel system is only an example of a wide group of thermosensitive polymer systems which may undergo conformational and phase transitions.