The frequency-dependent magnetic susceptibility of a ferrofluid is calculated under the assumption that the constituent particles undergo Brownian relaxation only. Brownian-dynamics simulations are carried out in order to test the predictions of a recent theory [A. O. Ivanov, V. S. Zverev, and S. S. Kantorovich, Soft Matter 12, 3507 (2016)1744-683X10.1039/C5SM02679B] that includes the effects of interparticle dipole-dipole interactions. The theory is based on the so-called modified mean-field approach and possesses the following important characteristics: in the low-concentration, noninteracting regime, it gives the correct single-particle Debye-theory results; it yields the exact leading-order results in the zero-frequency limit; it includes particle polydispersity correctly from the outset; and it is based on firm theoretical foundations allowing, in principle, systematic extensions to treat stronger interactions and/or higher concentrations. The theory and simulations are compared in the case of a model monodisperse ferrofluid, where the effects of interactions are predicted to be more pronounced than in a polydisperse ferrofluid. The susceptibility spectra are analyzed in detail in terms of the low-frequency behavior, the position of the peak in the imaginary (out-of-phase) part, and the characteristic decay time of the magnetization autocorrelation function. It is demonstrated that the theory correctly predicts the trends in all of these properties with increasing concentration and dipolar coupling constant, the product of which is proportional to the Langevin susceptibility χ_{L}. The theory is in quantitative agreement with the simulation results as long as χ_{L}≲1.

The structure and dynamics of potassium chloride in aqueous solution over a wide range of concentrations-and in particular beyond saturation-are studied using molecular dynamics simulations to help shed light on recent experimental studies of nonphotochemical laser-induced nucleation (NPLIN). In NPLIN experiments, the duration, t, of the laser pulse (with wavelength 1064 nm) is found to influence the occurrence of crystal nucleation in supersaturated KCl(aq): if t is less than about 5 ps, no crystal nucleation is observed; if t is greater than about 100 ps, crystal nucleation is observed, and with a known dependence on laser power. Assuming that the laser acts on spontaneously formed solute clusters, these observations suggest that there are transient structures in supersaturated solutions with relaxation times on the scale of 5-100 ps. Ion-cluster formation and ion-cluster lifetimes are calculated according to various criteria, and it is found that, in the supersaturated regime, there are indeed structures with relaxation times of up to 100 ps. In addition, the ion dynamics in this regime is found to show signs of collective behavior, as evidenced by stretched exponential decay of the self-intermediate scattering function. Although these results do not explain the phenomenon of NPLIN, they do provide insights into possible relevant dynamical factors in supersaturated aqueous solutions of potassium chloride.

Density functional theory, in conjunction with a cluster expansion model, has been used to study the structure and stability of the positionally disordered iron-nickel sulfide mineral pentlandite (Pn), (Fe, Ni)9S8, with results indicating heterogeneous nearest neighbour metal contacts are more energetically favourable than homogeneous contacts. The virtual crystal approximation was also explored as a means to address positional disorder, but while reliable results could be obtained for the bulk model, the same was not true for the surface, as local distortions which affected the surface model energies could not be reproduced. We also address the binding of ethyl xanthate (CH3CH2OCS2-), water and hydroxide to the [111] Pn surface to better understand the mode of action of industrial xanthate flotation agents. In order to model anionic ligands bound to a periodic boundary condition surface we propose applying a correction derived from the surface work function to remove the additional charge introduced by the ligand. The results obtained from the ligand binding studies indicate that while an ethyl xanthate collector could readily displace up to a full monolayer of water per unit cell, it is likely that Fe-enriched surfaces will bind xanthate in competition with the hydroxide anion, whilst a Nienriched surface will preferentially bind hydroxide anions over xanthate.

Using computer simulations and a mean-field theoretical approach, we study how the growth in dipolar interparticle correlations manifests itself in the frequency-dependent initial magnetic susceptibility of a ferrofluid. Our recently developed theory gives the correct single-particle Debye-theory results in the lowconcentration, non-interacting regime; and it yields the exact leading-order contributions from interparticle correlations. The susceptibility spectra are analysed in terms of the low-frequency behaviours of the real and imaginary parts, and the position of the peak in the imaginary part. By comparing the theoretical predictions to the results from Brownian dynamics simulations, it is possible to identify the conditions where correlations are important, but where self-assembly has not developed. We also provide a qualitative explanation for the behaviour of spectra beyond the mean-field limit. of a randomly chosen magnetic nanoparticle 1.

The dynamic magnetic susceptibility, χ(ω), of a model ferrofluid at very low concentration (volume fraction approximately 0.05%), and with a range of dipolar coupling constants (1 ≤ λ ≤ 8), is examined using Brownian dynamics simulations. With increasing λ, the structural motifs in the system change from unclustered particles, through chains, to rings. This gives rise to a nonmonotonic dependence of the static susceptibility χ(0) on λ, and qualitative changes to the frequency spectrum. The behavior of χ(0) is already understood, and the simulation results are compared to an existing theory. The single-particle rotational dynamics are characterized by the Brownian time, τB, which depends on particle size, carrier-liquid viscosity, and temperature. With λ ≤ 5.5, the imaginary part of the spectrum, χ (ω), shows a single peak near ω ∼ τ −1 B , characteristic of single particles. With λ ≥ 5.75, the spectrum is dominated by the low-frequency response of chains. With λ ≥ 7, new features appear at high frequency, which correspond to intracluster motions of dipoles within chains and rings. The peak frequency corresponding to these intracluster motions can be computed accurately using a simple theory.