Sofia S. Kantorovich

An iterative, fast, linear-scaling method for computing induced charges on arbitrary dielectric boundaries

Simulating coarse-grained models of charged soft-condensed matter systems in presence of dielectric discontinuities between different media requires an efficient calculation of polarization effects. This is almost always the case if implicit solvent models are used near interfaces or large macromolecules. We present a fast and accurate method (ICC( small star, filled)) that allows to simulate the presence of an arbitrary number of interfaces of arbitrary shape, each characterized by a different dielectric permittivity in one-, two-, and three-dimensional periodic boundary conditions. The scaling behavior and accuracy of the underlying electrostatic algorithms allow to choose the most appropriate scheme for the system under investigation in terms of precision and computational speed. Due to these characteristics the method is particularly suited to include nonplanar dielectric boundaries in coarse-grained molecular dynamics simulations.

The generalized identification of truly interfacial molecules (ITIM) algorithm for nonplanar interfaces

We present a generalized version of the ITIM algorithm for the identification of interfacial molecules, which is able to treat arbitrarily shaped interfaces. The algorithm exploits the similarities between the concept of probe sphere used in ITIM and the circumsphere criterion used in the α-shapes approach, and can be regarded either as a reference-frame independent version of the former, or as an extended version of the latter that includes the atomic excluded volume. The new algorithm is applied to compute the intrinsic orientational order parameters of water around a dodecylphosphocholine and a cholic acid micelle in aqueous environment, and to the identification of solvent-reachable sites in four model structures for soot. The additional algorithm introduced for the calculation of intrinsic density profiles in arbitrary geometries proved to be extremely useful also for planar interfaces, as it allows to solve the paradox of smeared intrinsic profiles far from the interface.

Ferrofluids with shifted dipoles: ground state structures

In the past decades, ferrofluids have become relevant in many applications ranging from engineering to medicine, and have attracted the interest of scientists from many fields. To understand the physical mechanisms serving as a basis for these applications, over the last decades, many of the properties of ferrofluids have been studied and can now be controlled. However, in order to fine-tune various aspects of the interactions in the system and hopefully – in the long run – to build tailored structures, in recent years, magnetic nanoparticles and colloids that deviate from the model of a spherical particle with a dipole moment at its center were examined. Among them are dumbbells, magnetic core-shell particles, elongated ferro-particles, and colloidal particles with a magnetic cap. In this paper, we introduce and examine – using analytical calculations and Monte Carlo simulations – one such model system, namely, magnetic particles in which the dipole moment is shifted from the center of mass towards the particles surface. In this way, an additional anisotropy is introduced to the particles, which results in quite different and surprising microscopic properties of suspensions. Here, we mainly concentrate on ground states of small clusters of shifted-dipole particles, but also take a first glance on suspensions at finite temperature.

Deformation mechanisms in 2D magnetic gels studied by computer simulations

Magnetic gels, so-called ferrogels, consist of a polymer network, into which magnetic nanoparticles are embedded. The interesting properties of ferrogels originate from a complex interplay of the mechanical properties of the polymers with the magnetic interactions of the embedded nanoparticles. The ability to control the system by an external magnetic field may give rise to applications in medicine and engineering. In this paper, we propose and examine two microscopical simulation models for a 2D ferrogel which are suited to explain two distinct mechanisms of deformation in such a system. The first model focusses on deformation of the gel due to the dipole–dipole interaction between the magnetic nanoparticles. In an external magnetic field, a gel of this kind elongates in the direction parallel to the field and shrinks in the perpendicular direction. The second model deals with a distortion of the polymer matrix due to the transmission of torques from the magnetic nanoparticles to the polymer network. In this model, we observe an isotropic shrinking of the gel in an external magnetic field. As the observed deformations are very different in the two models, we conclude that the magnetoelastic behaviour of a magnetic gel strongly depends on the microscopical details of, both, the structure of the network and the coupling between the polymers and the magnetic nanoparticles. This may help to explain seemingly contradicting evidence from different experiments.

Equilibrium properties of a bidisperse ferrofluid with chain aggregates: theory and computer simulations

In this paper we investigate a bidisperse model ferrofluid, where the aggregates are treated as flexible chains, under the influence of an arbitrary valued external magnetic field. An extensive comparison of the theoretical predictions to the results of the computer simulations is provided. Both magnetostatic properties and structural observables are investigated with the help of the newly developed theoretical approach and molecular dynamic simulations. It is shown that the results of the cluster analysis are very sensitive to the cluster definition. Here we use two different criteria for the particles to be bound: an energy criterion which is slightly different in the theory and simulations due to technical problems, and an entropy criterion which is the same for the molecular dynamics and theoretical model. This enables us to compare qualitatively and quantitatively theoretical and numerical microstructural observables, as well as the macro properties of the bidisperse ferrofluids. Finally, an answer to the question which chain criterion should actually be used is provided in this paper.

How to analyse the structure factor in ferrofluids with strong magnetic interactions: a combined analytic and simulation approach

We present a theoretical model for calculating the structure factor for ferrofluids with strong inter-particle magnetic dipole–dipole interactions, where chain aggregates are known to exist. Our analytical model is based on the minimization of a free energy density functional that allows us to explicitly construct the radial distribution functions of the ferroparticles. Both mono- and bi-disperse model systems have been investigated in the absence of an external magnetic field. We perform an extensive comparison of the theoretical model predictions with the results of molecular dynamic computer simulations for a wide range of ferroparticle densities and coupling parameters, and find encouraging agreement between the simulation data and theory. The behaviour of the structure factor in the region of the first peak and in the region of large wave vectors is studied in detail, and related to the observed microstructure. Our results demonstrate that the combined method developed in the present study is suitable for revealing the connection between microstructure and scattering images, and thus can help to interpret experimental results such as small angle neutron scattering images.

Formation of chain aggregates in magnetic fluids: an influence of polydispersity

Abstract The formation of chain-like aggregates in polydisperse ferrofluids is studied theoretically on the basis of the model bidisperse system, consisting of two fractions of small and large ferroparticles. Various topological structures of chains, containing the particles of both fractions, are considered. The equilibrium chain distribution is obtained with the help of the density functional approach. It was found that in real conditions the majority of large particles and the minority of small particles are connected in short chains of 1–3 large particles in the middle and 1–2 small particles at the edges. The chain distribution is greatly dependent on the mole portion of the large particle fraction.

Calculation of the intrinsic solvation free energy profile of an ionic penetrant across a liquid-liquid interface with computer simulations

We introduce the novel concept of an intrinsic free energy profile, allowing one to remove the artificial smearing caused by thermal capillary waves, which renders difficulties for the calculation of free energy profiles across fluid interfaces in computer simulations. We apply this concept to the problem of a chloride ion crossing the interface between water and 1,2-dichloroethane and show that the present approach is able to reveal several important features of the free energy profile which are not detected with the usual, nonintrinsic calculations. Thus, in contrast to the nonintrinsic profile, a free energy barrier is found at the aqueous side of the (intrinsic) interface, which is attributed to the formation of a water “finger” the ion pulls with itself upon approaching the organic phase. Further, by the presence of a nonsampled region, the intrinsic free energy profile clearly indicates the coextraction of the first hydration shell water molecules of the ion when entering the organic phase.

Branching points in the low-temperature dipolar hard sphere fluid

In this contribution, we investigate the low-temperature, low-density behaviour of dipolar hard-sphere (DHS) particles, i.e., hard spheres with dipoles embedded in their centre. We aim at describing the DHS fluid in terms of a network of chains and rings (the fundamental clusters) held together by branching points (defects) of different nature. We first introduce a systematic way of classifying inter-cluster connections according to their topology, and then employ this classification to analyse the geometric and thermodynamic properties of each class of defects, as extracted from state-of-the-art equilibrium Monte Carlo simulations. By computing the average density and energetic cost of each defect class, we find that the relevant contribution to inter-cluster interactions is indeed provided by (rare) three-way junctions and by four-way junctions arising from parallel or anti-parallel locally linear aggregates. All other (numerous) defects are either intra-cluster or associated to low cluster-cluster interaction energies, suggesting that these defects do not play a significant part in the thermodynamic description of the self-assembly processes of dipolar hard spheres.

Communication: Kinetic and pairing contributions in the dielectric spectra of electrolyte solutions

In the late 1970s, Hubbard and Onsager predicted that adding salt to a polar solution would result in a reduced dielectric permittivity that arises from the unexpected tendency of solvent dipoles to align opposite to the applied field. Here we develop a novel non-equilibrium molecular dynamics simulation approach to determine this decrement accurately. Using a thermodynamic consistent all-atom force field we show that for an aqueous solution containing sodium chloride around 4.8 mol/l, this effect accounts for 12% of the total dielectric permittivity. The dielectric decrement can be strikingly different if a less accurate force field for the ions is used. Using the widespread GROMOS parameters, we observe in fact an increment of the dielectric permittivity rather than a decrement, caused by ion pairing and introduced by a too low dispersion force.