M. V. Avdeev

Contrast variation in small-angle scattering experiments on polydisperse and superparamagnetic systems: basic functions approach

The development of the basic functions approach [Stuhrmann (1995). Modern Aspects of Small-Angle Scattering, edited by H. Brumberger, pp. 221–254. Dordrecht: Kluwer Academic Publishers] for the contrast variation technique in small-angle scattering from systems of polydisperse and superparamagnetic non-interacting particles is presented. For polydisperse systems the modified contrast is introduced as the difference between the effective mean scattering length density (corresponding to the minimum of the scattering intensity as the function of the scattering length density of the solvent) and the density of the solvent. Then, the general expression for the scattering intensity is written in the classical way through the modified basic functions. It is shown that the shape scattering from the particle volume can be reliably obtained. Modifications of classical expressions describing changes in integral parameters of the scattering (intensity at zero angle, radius of gyration, Porod integral) with the contrast are analyzed. In comparison with the monodisperse case, the residual scattering in the minimum of intensity as a function of contrast (effective match point) in polydisperse systems makes it possible to treat the Guinier region of scattering curves around the effective match point quite precisely from the statistical viewpoint. However, limitations of such treatment exist, which are emphasized in the paper. In addition, the effect of magnetic scattering in small-angle neutron scattering from superparamagnetic nanoparticles is considered in the context of the basic functions approach. Conceptually, modifications of the integral parameters of the scattering in this case are similar to those obtained for polydisperse multicomponent particles. Various cases are considered, including monodisperse non-homogeneous and homogeneous magnetic particles, and polydisperse non-homogeneous and homogeneous magnetic particles. The developed approach is verified for two models representing the main types of magnetic fluids – systems of polydisperse superparamagnetic particles located in liquid carriers.

Comparative structure analysis of non-polar organic ferrofluids stabilized by saturated mono-carboxylic acids

The structure of ferrofluids (magnetite in decahydronaphtalene) stabilized with saturated mono-carboxylic acids of different chain lengths (lauric, myristic, palmitic and stearic acids) is studied by means of magnetization analysis and small-angle neutron scattering. It is shown that in case of saturated acid surfactants, magnetite nanoparticles are dispersed in the carrier approximately with the same size distribution whose mean value and width are significantly less as compared to the classical stabilization with non-saturated oleic acid. The found thickness of the surfactant shell around magnetite is analyzed with respect to stabilizing properties of mono-carboxylic acids.

Models of cluster formation in solutions of fullerenes

A review of experimental and theoretical studies of the formation and growth of clusters in solutions of fullerenes is given. General problems of fullerene cluster formation in solutions are considered. The main directions and goals of studies are specified. The experimental data on solutions with various polarities obtained by various methods, including visible and UV spectroscopy, dynamic light scattering, small-angle neutron scattering, mass spectrometry, transmission electron microscopy, etc., are generalized. The conditions of cluster formation and mechanism of cluster stabilization and the role played by clusters in certain effects observed in the corresponding systems are discussed. Cluster growth models are considered on the basis of nucleation theory for solutions of fullerenes with various polarities. It is shown that the description of the observed cluster state requires modification of the kinetic equations of the classic approach using the drop model of clusters. Modified kinetic equations with corrections for the mechanism of cluster stabilization are used to analyze cluster growth and related phenomena.

Analysis of the structure of aqueous ferrofluids by the small-angle neutron scattering method

The structures of several aqueous magnetic fluids stabilized by different combinations of surfactants have been compared using small-angle neutron scattering. The size distribution functions of colloidal particles in water have been determined. The degree of clustering of magnetic nanoparticles has been obtained from comparison with electron microscopy data. The combinations of surfactants that lead to a minimum clustering have been revealed.

Structure of water-based ferrofluids with sodium oleate and polyethylene glycol stabilization by small-angle neutron scattering: contrast-variation experiments

Contrast variation in small-angle neutron scattering (SANS) experiments is used to compare the structures of a water-based ferrofluid, where magnetite nanoparticles are stabilized by sodium oleate, and its mixture with biocompatible polyethylene glycol, PEG. The basic functions approach is applied, which takes into account the effects of polydispersity and magnetic scattering. Different types of stable aggregates of colloidal particles are revealed in both fluids. The addition of PEG results in a reorganization of the structure of the aggregates: the initial comparatively small and compact aggregates (about 40 nm in size) are replaced by large (more than 120 nm in size) fractal-type structures. It is postulated that these large structures are composed of single magnetite particles coated with PEG, which replaces sodium oleate. Micelle formation involving free sodium oleate is observed in both fluids. The structures of the fluids remain unchanged with increasing temperature up to 343 K. New and specific possibilities of SANS contrast variation with respect to multicomponent systems with different aggregates are considered.

Structure and in vitro biological testing of water-based ferrofluids stabilized by monocarboxylic acids.

Water-based ferrofluids (magnetic fluids) with double-layer steric stabilization by short monocarboxylic acids (lauric and myristic acids) are considered to be a potential source of magnetic nanoparticles in brain cancer (glioblastoma) treatment. Structure characterization in the absence of an external magnetic field is performed, including transmission electron microscopy, magnetization analysis, and small-angle neutron scattering with contrast variation. It is shown that despite the good stability of the systems a significant part of the magnetite nanoparticles are in aggregates, whose inner structure depends on the stabilizer used. In particular, an incomplete coating of magnetite particles is concluded in the case of myristic acid stabilization. The ferrofluids keep their structure unchanged when added to the cancer cell medium. The intracellular accumulations of magnetite from the ferrofluids added to cancer cell cultures as well as its cytotoxicity with respect to human brain cells are investigated.

Magnetic Nanofluids: Synthesis and Structure

Recent results are reviewed concerning the synthesis of magnetic nanoparticles (MNP) and various types of magnetic nanofluids (MNF) or ferrofluids, their structural properties and behavior in an external magnetic field, specially tailored to meet the requirements of some specific engineering and biomedical applications. There are described the chemical co-precipitation procedure and the liquid- and gas-phase thermal decomposition methods to obtain magnetic nanoparticles (Fe3O4, γ-Fe2O3, CoFe2O4, Co, Fe and Fe-C) of adequate size distribution to prepare magnetic nanofluids. Sterical stabilization of MNPs in organic and water carrier liquids is discussed in details, related to the diagrams of magnetic nanofluid synthesis procedures. The macroscopic behavior, especially the magnetic and flow properties of magnetic fluids and their compatibility with various media and exploitation conditions in magnetofluidic devices are described related to composition and structural characteristics, such as nature and size of magnetic nanoparticles, nature and chain length of surfactants used for ultrastable dispersion of magnetic nanoparticles in various non-polar and polar carrier liquids, hydrodynamic size of particles, as well as the formation and characteristics of agglomerates induced by an applied magnetic field.

Small-angle neutron scattering analysis of a water-based magnetic fluid with charge stabilization: contrast variation and scattering of polarized neutrons

Structure analysis of a magnetic fluid (nanoparticles of maghemite dispersed in water with charge stabilization and without surfactant) by means of small-angle neutron scattering is presented. A combination of the contrast variation technique and scattering of polarized neutrons was applied. In the first case, the scattering curves obtained for the unmagnetized fluid with variation of the heavy water content in the carrier are treated in terms of the basic functions approach. The almost homogeneous character of the nanoparticles with respect to the nuclear scattering length density makes it possible to separate information about their characteristic nuclear and magnetic radii. Polarized neutrons are then used to separate and analyze independently the nuclear and magnetic scattering contributions for the fully magnetized fluid. Both methods reveal a significant excess of the apparent nuclear size over the magnetic one, which is explained by a difference in the nonmagnetic and magnetic interactions in the system. The results indicate that from the viewpoint of magnetic interaction the studied fluid behaves under a magnetic field as a purely superparamagnetic system of independent particles. The magnetic scattering length density of the maghemite nanoparticles is found to be ∼25% less than the bulk value, which is in agreement with the data of the magnetization analysis.

Effect of the age of the C60/N-methyl-2-pyrrolidone solution on the structure of clusters in the C60/N-methyl-2-pyrrolidone/water system according to the small-angle neutron scattering data

Fullerene clusters in the C60/N-methyl-2-pyrrolidone (NMP)/water system have been investigated by small-angle neutron scattering. It is shown that the scattering cross section corresponding to the size range 10–100 nm depends on the water content in the mixture. Addition of water to a C60/NMP solution in an amount exceeding 40% leads to a sharp increase in the average scattering cross section. This effect depends on the interval between the times of preparation of a C60/NMP solution and its dilution with water: the size of the clusters formed as a result of adding water increases with increasing the age of the initial solution. The reasons for this effect are discussed.

Solvatochromism and Fullerene Cluster Formation in C60/N-methyl-2-pyrrolidone

UV-Vis spectroscopy and small-angle neutron scattering experiments are performed on the cluster solution of fullerene C60 in N-methyl-2-pyrrolidone before and after dilution of the system with pure solvent. Some changes in the UV-Vis spectra showing solvatochromism at dilution are observed, while the neutron scattering signal does not change. The effect is discussed with respect to the relation between solvatochromism and cluster formation for fullerene solutions in nitrogen-containing solvents.