Читать книгу Magnetic Nanoparticles in Human Health and Medicine - Группа авторов - Страница 39

To obtain the nanostructures assembly, it needs, if not fundamental, to understand and engineer the interactions between nanoparticles evolving them into desired structures. The components and the interactions lead to the formation of equilibrium structures, which are reached when the appropriate thermodynamic potential, as the free energies of Gibbs or Helmotz, is low. Moreover, it is not simple to obtain equilibrium conditions (Durbin and Feher 1996; Chayen 2002) and even more difficult if not impossible for the competitive gels or glasses formation (Dawson 2002; Foffi et al. 2002; Sciortino and Tartaglia 2005).

To get the assembling mechanism, it is fundamental that the nanoparticles organize themselves into ordered and macroscopic structures through direct interactions, such as interparticle forces, and indirect interactions, such as an external field.

The assembling mechanism relies on the colloidal interaction between the nanoparticles which organize themselves into ordered and macroscopic structures. The colloidal interaction represented the basis of the balance generated between the individual nanoparticles. These colloidal forces are based on a thermodynamic equilibrium treatment of the nanoparticle interactions. By considering a system where no external forces are present, the repulsive interactions between individual nanoparticles determine their stabilization in suspension. Therefore, it needs a stimulus to obtain an aggregation to create an imbalance and thus reduce repulsive terms of the interaction potential. As reported by Lin and coworkers (1989), the formation of the cluster is influenced by kinetic effects. Therefore, a high interaction potential (>10 kT) leads to a rapid aggregation of the nanoparticles in a disordered assembly due to kinetically trapped structure. Moreover, for moderately attractive potentials (2–3 kT), the formation of more ordered clusters is obtained due to the higher degree of reversibility in the interaction between the nanoparticles through the breakdown of the bonds activated thermally. In this context, the clustering process should take place near equilibrium to obtain a slow and controllable process.

Furthermore, it is demonstrated that long‐range interactions concerning the assembling component sizes were preferred. However, many interaction forces, as van der Waals forces, act only on very low molecular distances. Therefore, to apply them on a larger molecular scale and to allow assembly, it needs to use small components, more precisely in the order of nanometers (2–30 nm).

Numerous factors influencing the strength of the colloidal interaction between nanoparticles are taken into account to obtain a monodisperse suspension of selectable dimensions of clusters with internal order. These factors could be considered as control parameters during the formation of clusters.

For magnetic nanoparticles, van der Waals attraction and the dipole–dipole interaction of magnetic nanoparticles can lead to well‐ordered assembly structures.

Therefore, in this section, we aim to describe the characteristics of various classes of interparticle interactions, as van der Waals, electrostatic, magnetic, and molecular at the nanoscale.

In this regard, we focus on the nanoscale effects that emerge with respect to the smaller molecular systems or, the larger colloids. Furthermore, considerable attention has been posted on theoretical tools to describe interactions at different levels of approximation and finally on their limits.

Each described interaction was accompanied by numerous experimental works to gain a better understanding, by allowing us to evaluate the effects of the assembling process for different interactions. Therefore, our purpose is to describe a general view of the different interparticle potentials to be implemented in the nanoparticle assembly.