Abstract:The stability of charged interfaces varies as a function of the nature of their neutralizing counterions. Changing the counterions from monovalent to divalent cations enhances the interionic correlations and drastically reduces the strong interfacial repulsion, leading, in some cases, to a net attraction. We have used Monte Carlo simulations in the canonical ensemble in order to obtain a direct derivation of the free energy of two charged lamellae, including the contribution of the interionic correlations in t… Show more
“…Above that separation, the local order parameter is zero, corresponding to fully decorrelated orientation of charged disks, as shown by 23 Na NMR relaxometry measurements. 39 Finally, one also notes in Figure 6 a significant reduction of the spatial extend of the locally ordered microdomains whose maximum size reduce from 200 to 160 Å when the electric charge of the disks reaches 400 e. This result is important since the occurrence of some microstructure within the clay dispersion, corresponding to a relative perpendicular orientation of the neighboring colloids, is not only compatible with the minimization of the electrostatic contribution to the free energy of these charged anisotropic colloids 49 but also it may lead to some local locks responsible for the occurrence of the sol/gel transition 42,43 of the dispersion. Let us simply remember that a fully nematic phase does not exhibit any yield stress such as that reported by the rheological behavior of Laponite clay dispersions.…”
We have performed Monte Carlo simulations of the self-organization of large collections of charged disks of various electric charges to probe the influence of electrostatic coupling on the structure and the mechanical stability of aqueous dispersions of charged anisotropic nanoparticles in the presence of salt. A hierarchical approach of the long-range Coulombic potential was used to perform such Monte Carlo simulations of a large number of charged species. By analyzing the influence of the net electric charge of the disks on their self-organization, we detected a negative contribution to the mean force potential resulting from their electrostatic coupling. In addition, it has been shown that the same electrostatic coupling restricts the spatial extend of locally ordered microdomains within dilute dispersions of charged anisotropic colloids.
“…Above that separation, the local order parameter is zero, corresponding to fully decorrelated orientation of charged disks, as shown by 23 Na NMR relaxometry measurements. 39 Finally, one also notes in Figure 6 a significant reduction of the spatial extend of the locally ordered microdomains whose maximum size reduce from 200 to 160 Å when the electric charge of the disks reaches 400 e. This result is important since the occurrence of some microstructure within the clay dispersion, corresponding to a relative perpendicular orientation of the neighboring colloids, is not only compatible with the minimization of the electrostatic contribution to the free energy of these charged anisotropic colloids 49 but also it may lead to some local locks responsible for the occurrence of the sol/gel transition 42,43 of the dispersion. Let us simply remember that a fully nematic phase does not exhibit any yield stress such as that reported by the rheological behavior of Laponite clay dispersions.…”
We have performed Monte Carlo simulations of the self-organization of large collections of charged disks of various electric charges to probe the influence of electrostatic coupling on the structure and the mechanical stability of aqueous dispersions of charged anisotropic nanoparticles in the presence of salt. A hierarchical approach of the long-range Coulombic potential was used to perform such Monte Carlo simulations of a large number of charged species. By analyzing the influence of the net electric charge of the disks on their self-organization, we detected a negative contribution to the mean force potential resulting from their electrostatic coupling. In addition, it has been shown that the same electrostatic coupling restricts the spatial extend of locally ordered microdomains within dilute dispersions of charged anisotropic colloids.
“…The contact densities used in eq 4 are thus extrapolated by fitting the ionic densities on a quadratic law by using a set of five separations ranging between 0.1 and 0.5 Å. Since these separations are smaller than the ionic radius, this procedure was shown to lead to reliable results 2 Concentration profiles of the neutralizing monovalent counterions condensed between two lamellae with a period of 25 Å without (continuous curve) or with (discontinuous curve) ionic exchange between the inner and outer domains. 3 Variation of the interfacial free energy and its energetic and entropic contributions for lamellae neutralized by monovalent counterions without (a) or with (b) ionic exchange between the inner and outer domains. …”
Section: (Ii) Methodsmentioning
confidence: 99%
“…To settle these questions, we have performed Monte Carlo simulations of the distribution of mono- and divalent counterions between two infinite charged lamellae immersed within a large simulation cell. The stability of the charged interface is determined by two independent procedures either by a calculation of the net force acting on the lamellae or by a direct derivation of the entropy of the interface including the first-order contribution from the interionic correlations . A first set of simulations is performed by using the classical canonical Monte Carlo procedure 26 and maintaining the electrical neutrality of each subsystem (i.e., the confined diffuse layer and the remote surfaces).…”
We have applied a restricted grand canonical Monte Carlo procedure to describe, in the framework of the primitive model, the counterion exchange mechanism between diffuse layers of counterions surrounding segregated charged lamellae. The net charge transfer between the dense and dilute domains is shown to vary as a function of the valence of the neutralizing counterions: undercharging of the dense interlayer is detected in the presence of monovalent counterions and overcharging with divalent counterions. Furthermore, no net reduction of the swelling pressure is detected for monovalent counterions, while a large enhancement of the net interlamellar attraction is found for charged lamellae neutralized by divalent counterions.
“…Because of imperfections of the force field, this lowest energy basin usually does not correspond to the native state in most cases, so the rank of native structure in those decoys produced by the force field itself is poor. Therefore, the rank of native structure could be relatively better when ranked by the second force field [20]. In order to compute the average properties from a microscopic description of a real system, one should evaluate integrals over phase space.…”
Section: Molecular Mechanics (Monte Carlo Simulation)mentioning
The nanocrystalline silicon-germanium films (Si/Ge) and Si/Ge nanotubes have low band gaps and high carrier mobility, thus offering appealing potential for absorbing gas molecules. Interaction between hydrogen molecules and bare as well as functionalized Si/Ge nanofilm and nanotube was investigated using Monte Carlo (MC) and Langevin dynamic (LD) simulation methods. It was found that the binding energy of the H2 on the Si/Ge surface is weak, and be enhanced by increasing curvature of surface to tube form and increasing temperature. The structural, total energy and energy band gaps of H2 absorbed nanocrystalline silicon germanium film (Si/Ge) and as it passes through Si/Ge nanotube was also studied. They are computed with MC and LD simulation the methods at different temperatures. All the calculations were carried out using HyperChem 7.0 program package.
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