Inverse patchy colloids with small patches: fluid structure and dynamical slowing down (original) (raw)
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Molecular dynamics simulations of inverse patchy colloids
The European physical journal. E, Soft matter, 2018
Inverse patchy colloids are patchy particles with differently charged surface regions. In this paper we focus on inverse patchy colloids with two different polar patches and an oppositely charged equatorial belt, and we describe a model and a reliable and efficient numerical algorithm that can be applied to investigate the properties of these particles in molecular dynamics simulations.
Theoretical and numerical investigations of inverse patchy colloids in the fluid phase
The Journal of chemical physics, 2015
We investigate the structural and thermodynamic properties of a new class of patchy colloids, referred to as inverse patchy colloids (IPCs) in their fluid phase via both theoretical methods and simulations. IPCs are nano- or micro- meter sized particles with differently charged surface regions. We extend conventional integral equation schemes to this particular class of systems: our approach is based on the so-called multi-density Ornstein-Zernike equation, supplemented with the associative Percus-Yevick approximation (APY). To validate the accuracy of our framework, we compare the obtained results with data extracted from NpT and NVT Monte Carlo simulations. In addition, other theoretical approaches are used to calculate the properties of the system: the reference hypernetted-chain (RHNC) method and the Barker-Henderson thermodynamic perturbation theory. Both APY and RHNC frameworks provide accurate predictions for the pair distribution functions: APY results are in slightly better...
Inverse patchy colloids: from microscopic description to mesoscopic coarse-graining
Soft Matter, 2011
Typically, patchy systems are characterized by the formation of a small number of directional, possibly selective, bonds due to the presence of attractive regions on the surface of otherwise repulsive particles. Here, we consider a new type of particles with patterned surfaces and we refer to them as inverse patchy colloids because, in this case, the patches on the repulsive particles repel each other instead of attracting. Further, these patches attract the parts of the colloid that are free of patches. Specifically, we consider heterogeneously charged colloids consisting of negatively charged spherical particles carrying a small number of positively charged patches. Making use of the Debye-H€ uckel theory, we derive the effective interaction potential between a pair of inverse patchy colloids with two patches on opposite poles. We then design a simple coarse-grained model via a mapping with the analytical pair potential. The coarsegrained model quantitatively reproduces the features of its microscopic counterpart, while at the same time being characterized by a much higher degree of computational simplicity. Moreover, the mesoscopic model is generalizable to an arbitrary number of patches.
Journal of Molecular Modeling, 2002
We report results of numerical simulations of complex fluids, using a combination of discrete-particle methods. Our molecular modeling repertoire comprises three simulation techniques: molecular dynamics (MD), dissipative particle dynamics (DPD), and the fluid particle model (FPM). This type of model can depict multi-resolution molecular structures (see the found in complex fluids ranging from single micelle, colloidal crystals, large-scale colloidal aggregates up to the mesoscale processes of hydrodynamical instabilities in the bulk of colloidal suspensions. We can simulate different colloidal structures in which the colloidal beds are of comparable size to the solvent particles. This undertaking is accomplished with a two-level discrete particle model consisting of the MD paradigm with a Lennard-Jones (L-J) type potential for defining the colloidal particle system and DPD or FPM for modeling the solvent. We observe the spontaneous emergence of spherical or rod-like micelles and their crystallization in stable hexagonal or worm-like structures, respectively. The ordered arrays obtained by using the particle model are similar to the 2D colloidal crystals observed in laboratory experiments. The micelle shape and its hydrophobic or hydrophilic character depend on the ratio between the scaling factors of the interactions between colloid-colloid to colloid-solvent. Unlike the miscellar arrays, the colloidal aggregates involve the colloid-solvent interactions prescribed by the DPD forces. Different from the assump-tion of equilibrium growth, the two-level particle model can display much more realistic molecular physics, which allows for the simulation of aggregation for various types of colloids and solvent liquids over a very broad range of conditions. We discuss the potential prospects of combining MD, DPD, and FPM techniques in a single three-level model. Finally, we present results from large-scale simulation of the Rayleigh-Taylor instability and dispersion of colloidal slab in 2D and 3D. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.
Phase behaviour of pure and mixed patchy colloids — Theory and simulation
Current Opinion in Colloid & Interface Science, 2017
We review the phase behaviour of pure and mixed patchy colloids, as revealed (mostly) by theory and computer simulation. These experimentally-realisable systems are excellent models for investigating the general problem of the interplay between (equilibrium) phase transitions and self-assembly in soft condensed matter. We focus on how liquid-vapour condensation can be preempted by the formation of different types of aggregates, in particular rings, which we argue is relevant to the criticality of empty fluids and network fluids, and possibly also of dipolar fluids. In this connection we also discuss percolation and gelation in pure and mixed patchy colloids. Finally, we describe the rich phase behaviour of (mostly binary) patchy colloid mixtures.
A numerical study of one-patch colloidal particles: from square-well to Janus
Phys. Chem. Chem. Phys., 2010
We perform numerical simulations of a simple model of one-patch colloidal particles to investigate: (i) the behavior of the gas-liquid phase diagram on moving from a spherical attractive potential to a Janus potential and (ii) the collective structure of a system of Janus particles. We show that, for the case where one of the two hemispheres is attractive and one is repulsive, the system organizes into a dispersion of orientational ordered micelles and vesicles and, at low T , the system can be approximated as a fluid of such clusters, interacting essentially via excluded volume. The stability of this cluster phase generates a very peculiar shape of the gas and liquid coexisting densities, with a gas coexistence density which increases on cooling, approaching the liquid coexistence density at very low T .
Inverse patchy colloids: Synthesis, modeling and self-organization
Current Opinion in Colloid & Interface Science
Inverse patchy colloids are nano-to micro-scale particles with a surface divided into differently charged regions. This class of colloids combines directional, selective bonding with a relatively simple particle design: owing to the competitive interplay between the orientation-dependent attraction and repulsion-induced by the interactions between like/oppositely charged areas-experimentally accessible surface patterns are complex enough to favor the stabilization of specific structures of interest. Most important, the behavior of heterogeneously charged units can be ideally controlled by means of external parameters, such as the pH and the salt concentration. We present a concise review about this class of systems, spanning the range from the synthesis of model inverse patchy particles to their self-assembly, covering their coarse-grained modeling and the related numerical/analytical treatments.
Equilibrium phases of one-patch colloids with short-range attractions
Inspired by experimental studies of short-ranged attractive patchy particles, we study with computer simulations the phase behavior and the crystalline structures of one-patch colloids with an interaction range equal to 5% of the particle diameter. In particular, we study the effects of the patch surface coverage fraction, defined as the ratio between the attractive and the total surface of a particle. Using free-energy calculations and thermodynamic integration schemes, we evaluate the equilibrium phase diagrams for particles with patch coverage fractions of 30%, 50% and 60%. For a 60% surface coverage fraction, we observe stable lamellar crystals consisting of stacked bilayers that directly coexist with a low density fluid. Inside the coexistence region, we observe the formation of lamellar structures also in direct NVT simulations, indicating that the barrier of formation is low and experimental realization is feasible.
The Journal of chemical physics, 2008
We report theoretical and numerical evaluations of the phase diagram for a model of patchy particles. Specifically, we study hard spheres whose surface is decorated by a small number f of identical sites ("sticky spots") interacting via a short-ranged square-well attraction. We theoretically evaluate, solving the Wertheim theory, the location of the critical point and the gas-liquid coexistence line for several values of f and compare them to the results of Gibbs and grand canonical Monte Carlo simulations. We study both ordered and disordered arrangements of the sites on the hard-sphere surface and confirm that patchiness has a strong effect on the phase diagram: the gas-liquid coexistence region in the temperature-density plane is significantly reduced as f decreases. We also theoretically evaluate the locus of specific heat maxima and the percolation line.
Effects of patch size and number within a simple model of patchy colloids
The Journal of Chemical Physics, 2010
We report on a computer simulation and integral equation study of a simple model of patchy spheres, each of whose surfaces is decorated with two opposite attractive caps, as a function of the fraction χ of covered attractive surface. The simple model explored -the two-patch Kern-Frenkel model -interpolates between a square-well and a hard-sphere potential on changing the coverage χ. We show that integral equation theory provides quantitative predictions in the entire explored region of temperatures and densities from the square-well limit χ = 1.0 down to χ ≈ 0.6. For smaller χ, good numerical convergence of the equations is achieved only at temperatures larger than the gasliquid critical point, where however integral equation theory provides a complete description of the angular dependence. These results are contrasted with those for the one-patch case. We investigate the remaining region of coverage via numerical simulation and show how the gas-liquid critical point moves to smaller densities and temperatures on decreasing χ. Below χ ≈ 0.3, crystallization prevents the possibility of observing the evolution of the line of critical points, providing the angular analog of the disappearance of the liquid as an equilibrium phase on decreasing the range for spherical potentials. Finally, we show that the stable ordered phase evolves on decreasing χ from a threedimensional crystal of interconnected planes to a two-dimensional independent-planes structure to a one-dimensional fluid of chains when the one-bond-per-patch limit is eventually reached.