How chains and rings affect the dynamic magnetic susceptibility of a highly clustered ferrofluid

The dynamic magnetic susceptibility, χ(ω), of a model ferrofluid at a 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 the 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∼τB-1, 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. © 2021 American Physical Society.A.O.I. gratefully acknowledges research funding from the Ministry of Science and Higher Education of the Russian Federation (Ural Mathematical Center Project No. 075-02-2021-1387)

Effects of nanoparticle heating on the structure of a concentrated aqueous salt solution

The effects of a rapidly heated nanoparticle on the structure of a concentrated aqueous salt solution are studied using molecular dynamics simulations. A diamond-like nanoparticle of radius 20 Å is immersed in a sodium-chloride solution at 20% above the experimental saturation concentration and equilibrated at T = 293 K and P = 1 atm. The nanoparticle is then rapidly heated to several thousand degrees Kelvin, and the system is held under isobaric-isoenthalpic conditions. It is observed that after 2-3 ns, the salt ions are depleted far more than water molecules from a proximal zone 15-25 Å from the nanoparticle surface. This leads to a transient reduction in molality in the proximal zone and an increase in ion clustering in the distal zone. At longer times, ions begin to diffuse back into the proximal zone. It is speculated that the formation of proximal and distal zones, and the increase in ion clustering, plays a role in the mechanism of nonphotochemical laser-induced nucleation. © 2017 Author(s)

Influence of dipolar interactions on the magnetic susceptibility spectra of ferrofluids

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 χL. The theory is in quantitative agreement with the simulation results as long as χL1. © 2016 American Physical Society

The influence of interparticle correlations and self-assembly on the dynamic initial magnetic susceptibility spectra of ferrofluids

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 low-concentration, 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. © 2016 Elsevier B.V

First-Principles Study on Ligand Binding and Positional Disorder in Pentlandite

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 neighbor metal contacts are more energetically favorable 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 understand the mode of action of industrial xanthate flotation agents better. 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 ethyl xanthate 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, while a Ni-enriched surface will preferentially bind hydroxide anions over xanthate

Disordered Filaments Mediate the Fibrillogenesis of Type I Collagen in Solution

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Thermodynamics of the Stockmayer fluid in an applied field

The thermodynamic properties of the Stockmayer fluid in an applied field are studied using theory and computer simulation. Theoretical expressions for the second and third virial coefficients are obtained in terms of the dipolar coupling constant (, measuring the strength of dipolar interactions as compared to thermal energy) and dipole-field interaction energy (α, being proportional to the applied field strength). These expressions are tested against numerical results obtained by Mayer sampling calculations. The expression for the second virial coefficient contains terms up to λ4, and is found to be accurate over realistic ranges of dipole moment and temperature, and over the entire range of the applied field strength (from zero to infinity). The corresponding expression for the third virial coefficient is truncated at λ3, and is not very accurate: higher order terms are very difficult to calculate. The virial coefficients are incorporated in to a thermodynamic theory based on a logarithmic representation of the Helmholtz free energy. This theory is designed to retain the input virial coefficients, and account for some higher order terms in the sense of a resummation. The compressibility factor is obtained from the theory and compared to results from molecular dynamics simulations with a typical value λ = 1. Despite the mathematical approximations of the virial coefficients, the theory captures the effects of the applied field very well. Finally, the vapour-liquid critical parameters are determined from the theory, and compared to published simulation results; the agreement between the theory and simulations is good. © 2015 Taylor & Francis