pH-dependent phase transitions in ferrofluids: A Monte Carlo simulation study using an extended DLVO model

The present survey focuses on the pH-dependent phase behavior of acidic ferrofluids using Monte Carlo simulations. Samples based on CoFe2O4@γ-Fe2O3 core@shell nanoparticles with five different mean sizes were investigated. The interparticle interactions were modeled employing an extended DLVO theory, which includes the anisotropic magnetic dipolar interaction. The electrostatic repulsive component considers the dependence of the surface charge density with pH and nanoparticle mean size. The simulations highlight the effect of pH on the phase behavior of the samples, where the pH increase towards neutral induces an increase in the number of agglomerates, while also reducing first-neighbor distances, especially for samples based on larger nanoparticles. The results obtained in these simulations agree with experimental observations carried out on acidic ferrofluid samples and allow a reliable characterization of the stability domains of these systems. Ultimately, the general pH-dependent phase behavior is confirmed through a detailed analysis of the results of the simulated samples.