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Based on classical density functional theory (DFT), we investigate the demixing phase transition of a two-dimensional, binary Heisenberg fluid mixture. The particles in the mixture are modeled as Gaussian soft spheres, where one component is characterized by an additional classical spin-spin interaction of Heisenberg type. Within the DFT we treat the particle interactions using a mean-field approximation. For certain magnetic coupling strengths, we calculate phase diagrams in the density-concentration plane. For sufficiently large coupling strengths and densities, we find a demixing phase transition driven by the ferromagnetic interactions of the magnetic species. We also provide a microscopic description (i.e., density profiles) of the resulting non-magnetic/magnetic fluid-fluid interface. Finally, we investigate the phase separation using dynamical density functional theory, considering both nucleation processes and spinodal demixing. [ABSTRACT FROM AUTHOR]

We present Monte Carlo simulation results for strongly coupled dipolar fluids, such as ferrofluids, confined to a narrow slit pore accommodating only a few layers of particles. Our results show that the ferromagnetic ordering observed in dense bulk systems and in thick fluid films persists down to nanoscopic wall separations where the system consists of only 3 monolayers. The ferromagnetic transition density in these systems approaches experimentally accessible values. For even smaller wall separations, we observe stripelike defects and finally the breakdown of orientational ordering for systems close to the two-dimensional limit. Our results for the liquid phase are supported by simulations starting from quasicrystalline soft-sphere configurations. [ABSTRACT FROM AUTHOR]

Using replica integral equations in the reference hypernetted-chain (RHNC) approximation we calculate vapor-liquid spinodals, chemical potentials, and compressibilities of fluids with angle-averaged dipolar interactions adsorbed to various disordered porous media. Comparison with previous RHNC results for systems with true angle-dependent Stockmayer (dipolar plus Lennard-Jones) interactions [C. Spöler and S. H. L. Klapp, J. Chem. Phys. 118, 3628 (2003); ibid.120, 6734 (2004)] indicate that, for a dilute hard sphere matrix, the angle-averaged fluid-fluid (ff) potential is a reasonable alternative for reduced fluid dipole moments m*2=μ2/(ε0σ3)≤2.0. This range is comparable to that estimated in bulk fluids, for which RHNC results are presented as well. Finally, results for weakly polar matrices suggest that angle-averaged fluid-matrix (fm) interactions can reproduce main features observed for true dipolar (fm) interactions such as the shift of the vapor-liquid spinodals towards lower temperatures and higher densities. However, the effective attraction induced by dipolar (fm) interaction is underestimated rather than overestimated as in the case of angle-averaged ff interactions.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Based on replica integral equations in the (reference) hypernetted chain approximation we investigate the structural features and phase properties of a dipolar Stockmayer fluid confined to a disordered dipolar matrix. The integral equations are applied to the homogeneous high-temperature phase where the system is globally isotropic. At low densities we find the influence of dipolar interactions between fluid (f) and matrix (m) particles to be surprisingly similar to the previously investigated effect of attractive isotropic (fm) interactions: the critical temperature of the vapor–liquid transition decreases with increasing (fm) coupling, while the critical density increases. The anisotropic nature of the dipolar (fm) interactions turns out to play a more dominant role at high fluid densities where we observe a pronounced sensitivity in the dielectric constant and a strong degree of local orientational ordering of the fluid particles along the local fields generated by the matrix. Moreover, an instability of the dielectric constant, which is a precursor of ferroelectric ordering occurring both in bulk Stockmayer fluids and in fluids in nonpolar matrices, is observed only for very small dipolar (fm) couplings. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The phase behavior of hard core Stockmayer fluids (i.e., dipolar hard spheres with additional Lennard-Jones interactions) adsorbed into a disordered porous hard sphere matrix is studied by means of replica-integral equations in the reference hypernetted chain approximation. The integral equations are solved for the homogeneous isotropic phase and the low-temperature phase behavior is inferred from those thermal fluctuations which strongly increase when the stability limit of the isotropic phase is approached. Analyzing first number density fluctuations and adsorption isotherms we find that the vapor-liquid transition occuring in bulk Stockmayer fluids still takes place in matrices of sufficiently high porosity. The corresponding critical point is shifted towards a lower temperature and density, in agreement with previous results on simpler fluids. At high fluid densities a diverging dielectric constant indicates the presence of an isotropic-to-ferroelectric transition for all systems considered, and the corresponding transition temperatures are found to increase with increasing matrix density. Comparing the fluid-matrix systems with fully equilibrated mixtures we argue that this enhanced tendency for ferroelectric ordering is essentially due to excluded volume effects. [ABSTRACT FROM AUTHOR]

We report Monte Carlo simulation results for a strongly coupled dipolar soft-sphere (DSS) fluid confined to a nanoscopic slit pore with structureless, nonconducting walls. The central topic of our investigation are the conditions under which the pore fluid can spontaneously order into a globally polarized (i.e., ferroelectric) state. Polarized states are observed in bulk DSS fluids at sufficiently low temperatures and high densities/pressures. The confined system is simulated in the (N, L[sub z] ,P[sub ], T) ensemble, where N is the particle number, L[sub z] the wall separation, P[sub ] the pressure parallel to the walls, and T the temperature. Fixing T and P[sub ] such that the corresponding bulk system is ferroelectric, and considering confined films with various thicknesses proportional to L[sub z], we first demonstrate that the long-range orientational order persists down to L[sub z]≈6σ. We then specialize to the case L[sub z]=7σ, for which we investigate in detail the spatial and orientational structure as functions of P[sub ]. It turns out that the transition from the globally isotropic to the globally polarized phase occurs at significantly lower pressures/densities than in the bulk, indicating that spatial confinement can support the onset of ferroelectric order. We explain this phenomenon within the framework of a simple mean-field theory based on the assumption that confinement effectively restricts orientational fluctuations, as suggested by the Monte Carlo results. [ABSTRACT FROM AUTHOR]

Understanding and controlling the growth and stability of molecular thin films on solid surfaces is necessary to develop nanomaterials with well-defined physical properties. As a prominent model system in organic electronics, we investigate the post-growth dewetting kinetics of the fullerene C60 on mica with real-time and in situ X-ray scattering. After layer-by-layer growth of C60, we find a thermally-activated post-growth dewetting, where the smooth C60-layer breaks up into islands. This clearly shows that growth is kinetically limited before the system moves over an activation barrier into an energetically favored configuration. From the temperaturedependent dewetting kinetics we find an effective activation barrier of 0.33 eV, which describes both the temperaturedependent macroscopic changes in the surface morphology and the microscopic processes of inter- and intralayer diffusion during dewetting. [ABSTRACT FROM AUTHOR]

Molecular semiconductors are increasingly used in devices, but understanding of elementary nanoscopic processes in molecular film growth is in its infancy. Here we use real-time in situ specular and diffuse X-ray scattering in combination with kinetic Monte Carlo simulations to study C60 nucleation and multilayer growth. We determine a self-consistent set of energy parameters describing both intra- and interlayer diffusion processes in C60 growth. This approach yields an effective Ehrlich-Schwoebel barrier of EES=110 meV, diffusion barrier of ED=540 meV and binding energy of EB=130 meV. Analysing the particle-resolved dynamics, we find that the lateral diffusion is similar to colloids, but characterized by an atom-like Schwoebel barrier. Our results contribute to a fundamental understanding of molecular growth processes in a system, which forms an important intermediate case between atoms and colloids.