Dilution effects on combined magnetic and electric dipole interactions: A study of ferromagnetic cobalt nanoparticles with tuneable interactions

Abstract

Improved understanding of complex interactions between nanoparticles will facilitate the control over the ensuing self-assembled structures. In this work, we consider the dynamic changes occurring upon dilution in the self-assembly of a system of ferromagnetic cobalt nanoparticles that combine magnetic, electric, and steric interactions. The systems examined here vary in the strength of the magnetic dipole interactions and the amount of point charges per particle. Scattering techniques are employed for the characterization of the self-assembly aggregates, and zeta-potential measurements are employed for the estimation of surface charges. Our experiments show that for particles with relatively small initial number of surface electric dipoles, an increase in particle concentration results in an increase in diffusion coefficients; whereas for particles with relatively high number of surface dipoles, no effect is observed upon concentration changes. We attribute these changes to a shift in the adsorption/desorption equilibrium of the tri-n-octylphosphine oxide (TOPO) molecules on the particle surface. We put forward an explanation, based on the combination of two theoretical models. One predicts that the growing concentration of electric dipoles, stemming from the addition of tri-n-octylphosphine oxide (TOPO) as co-surfactant during particle synthesis, on the surface of the particles results in the overall repulsive interaction. Secondly, using density functional theory, we explain that the observed behaviour of the diffusion coefficient can be treated as a result of the concentration dependent nanoparticle self-assembly: additional repulsion leads to the reduction in self-assembled aggregate size despite the shorter average interparticle distances, and as such provides the growth of the diffusion coefficient.

Polymer architecture of magnetic gels: a review

“…For single domain particles, typical values for λ range from below one to eight. When λ is significantly larger than one, the magnetic particles will form clusters such as chains, rings and branched structures [33][34][35][36][37][38]. Their orientation is isotropically distributed, as long as no external magnetic field is applied.…”

Ослабление Диполь-Дипольного Взаимодействия В Ансамблях Микросфер Кобальта С Немагнитным Ядром

“…Многочисленные применения магнитных нано-и микрочастиц как функциональных магнитных элементов в биомедицине, электронике, катализе и т. д. порождают ряд проблем и требований к дизайну их формы и свойств [1][2][3][4][5]. В системах с большими массивами магнитных частиц (феррожидкости, феррогели, магнитные эластомеры, гранулированные магнитные композиты) важно влияние магнитного диполь-дипольного взаимодействия [6]. Поскольку диполь-дипольное взаимодействие убывает с расстоянием достаточно медленно (∝ r −3 ), это приводит к его нелокальности для трехмерных систем.…”

Dipole-dipole interaction weakening in ensembles of cobalt microspheres with a nonmagnetic core

“…give rise to a number of problems and requirements for the design of their shape and properties [1][2][3][4][5]. In systems with large arrays of magnetic particles (ferrofluids, ferrogels, magnetic elastomers, granular magnetic composites), the effect of magnetic dipoledipole interaction is important [6]. Since the dipoledipole interaction decreases rather slowly with distance (∝ r −3 ), this leads to its non-locality for three-dimensional systems.…”