The role of small separation interactions in ferrofluid structure

Abstract

Interparticle interactions in colloids are traditionally modeled by means of the DLVO theory, which includes van der Waals and electrical double layer (EDL) interactions However, the validity range limitations become critical in biocompatible magnetic colloids, requiring a more detailed description of the interactions, especially at small intersurface separations. As magnetic colloids, ferrofluids require an extended DLVO (XDLVO) model that includes magnetic interactions. Moreover, the nanoparticles of biocompatible ferrofluids are usually ionic-surfacted, such that their charged surfactants interact both electrically and sterically. In some of such particles, the charge is usually not located at the surface, but at the outer extremities of the surfactant molecules, and this feature restricts the EDL model validity to larger separation distances. We addressed this problem by means of a model proposed by Schnitzer and Morozov, which employs a generalized Derjaguin approximation that makes the EDL repulsion expression valid for all separations. The van der Waals expression of the DLVO theory is also problematic because it shows an unphysical divergence as the intersurface separation tends to zero, a problem that was circumvented by replacing the expression at small separations with another expression based on cohesion energy and the Born-Mayer repulsion. The modifications proposed here are of interest for research on colloids in general and our Monte Carlo simulations show that they acquire even greater importance when it comes to ferrofluids. The influence of magnetic interparticle interactions on the colloid structure is better gauged using these modifications, which prevent magnetic interactions from being obfuscated by artificially large van der Waals and EDL interactions. This conclusion makes the small separation treatment particularly important for the study of magnetic colloids.