The Complemented System Approach: A Novel Method for Calculating the X-ray Scattering from Computer Simulations

In this paper, we review the main problem concerning the calculation of X-ray scattering of simulated model systems, namely their finite size. A novel method based on the Rayleigh-Debye-Gans approximation was derived, which allows sidestepping this issue by complementing the missing surroundings of each particle with an average image of the system. The method was designed to operate directly on particle configurations without an intermediate step (e.g., calculation of pair distribution functions): in this way, all information contained in the configurations was preserved. A comparison of the results against those of other known methods showed that the new method combined several favourable properties: an arbitrary q-scale, scattering curves free of truncation artifacts and good behaviour down to the theoretical lower limit of the q-scale. A test of computational efficiency was also performed to establish a relative scale between the speeds of all known methods: the reciprocal lattice approach, the brute force method, the Fourier transform approach and the newly presented complemented system approach.Comment: 6 pages, 5 figures. Copyright 2010 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in The Journal of Chemical Physics and may be found at http://link.aip.org/link/?jcp/133/17412

Soft-Core Attractive Model Fluid: Structure, Thermodynamics and Inter-colloidal Solvation Force †

Abstract Canonical and grand canonical Monte Carlo simulations are used to study a system of spherical particles interacting via a discontinuous potential combining a repulsive square soft core and an attractive square well. This, so-called coresoftened (CS) potential fluid is known to have both a gas-liquid critical point and a liquid-liquid critical point separating high density liquid (HDL) and low density liquid (LDL) phases. First, the spatial correlations and thermodynamic properties of homogeneous and inhomogeneous CS fluid are investigated. Using open ensemble simulation we study an equilibrium distribution of the CS fluid between the homogeneous phase and the planar pores mimicking the real porous material. The bulk radial distribution function displays discontinuities at the distances coinciding with the ranges of the successive repulsive and attractive parts in the CS potential function. The density profiles of confined CS fluid show the shapes arising from the interplay among the steric effects and the competition between the repulsive and attractive parts of the CS potential. Then, the effective force between a pair of big colloidal spheres immersed in a sea of small spheres interacting via CS potential is explored. The big-small interactions are modeled as hard core pair potentials with attractive or repulsive Yukawa tail leading to the accumulation repulsion and depletion attraction between the two colloids, respectively. For this purpose we apply a special simulation technique based on a separate sampling of the contributions arising from the Yukawa tail and hard-core (collision) parts of the big-small interaction potential to the total force between the colloidal particles

Solvation of Nonionic Poly(Ethylene Oxide) Surfactant Brij 35 in Organic and Aqueous-Organic Solvents

Hypothesis: By combining the experimental small- and wide-angle x-ray scattering (SWAXS) method with molecular dynamics simulations and the theoretical 'complemented-system approach' it is possible to obtain detailed information about the intra- and inter-molecular structure and dynamics of the solvation and hydration of the surfactant in organic and mixed solvents, e.g., of the nonionic surfactant Brij 35 (C12E23) in alcohols and aqueous alcohol-rich ternary systems. This first application of the complemented-system approach to the surfactant system will promote the use of this powerful methodology that is based on experimental and calculated SWAXS data in studies of colloidal systems. By applying high-performance computing systems, such an approach is readily available for studies in the colloidal domain. Experiments: SWAXS experiments and MD simulations were performed for binary Brij 35/alcohol and ternary Brij 35/water/alcohol systems with ethanol, n-butanol and n-hexanol as the organic solvent component at 25 {\deg}C. Findings: We confirmed the presence of solvated Brij 35 monomers in the studied organic media, revealed their preferential hydration and discussed their structural and dynamic features at the intra- and inter-molecular levels. Anisotropic effective surfactant molecular conformations were found. The influence of the hydrophobicity of the organic solvent on the hydration phenomena of surfactant molecules was explained.Comment: 31 pages, 6 figures, 1 tabl

Structural, Rheological and Dynamic Aspects of Hydrogen-Bonding Molecular Liquids: Aqueous Solutions of Hydrotropic tert-Butyl Alcohol

Hypothesis: The structural details, viscosity trends and dynamic phenomena in t-butanol/water solutions are closely related on the molecular scales across the entire composition range. Utilizing the experimental small- and wide-angle x-ray scattering (SWAXS) method, molecular dynamics (MD) simulations and the ‘complemented-system approach’ method developed in our group it is possible to comprehensively describe the structure-viscosity-dynamics relationship in such structurally versatile hydrogen-bonded molecular liquids, as well as in similar, self-assembling systems with pronounced molecular and supramolecular structures at the intra-, inter-, and supra-molecular scales. Experiments: The SWAXS and x-ray diffraction experiments and MD simulations were performed for aqueous t-butanol solutions at 25 °C. Literature viscosity and self-diffusion data were also used. Findings: The interpretive power of the proposed scheme was demonstrated by the extensive and diverse results obtained for aqueous t-butanol solutions across the whole concentration range. Four composition ranges with qualitatively different structures and viscosity trends were revealed. The experimental and calculated zero-shear viscosities and molecular self-diffusion coefficients were successfully related to the corresponding structural details. The hydrogen bonds 2 that were, along with hydrophobic effects, recognized as the most important driving force for the formation of t-butanol aggregates, show intriguing lifetime trends and thermodynamic properties of their formation

Delamination of Layered Double Hydroxide in Ionic Liquids under Ambient Conditions

Liquid phase delamination of layered materials into single- or few-layer nanosheets leads to stable nanoscale dispersions of 2D materials. The delamination of layered double hydroxide (LDH) to double hydroxide nanosheets was studied in two ionic liquids (ILs): ethylammonium nitrate (EAN) and 1-butyl-3-methylimidazolium thiocyanate (BMIMSCN). The as-prepared lamellar structure of LDH disappeared upon dispersing it in ILs due to delamination into 2D nanosheets confirmed by X-ray scattering and diffraction techniques and further evaluated by height profile assessment of the nanoparticles by atomic force microscopy. The results showed that both the thickness and lateral size of the dispersed particles decreased in the IL-based samples, indicating that cleavage of the LDH materials can be observed in addition to delamination. The findings prove the concept of delamination of layered materials by ILs under ambient conditions─an excellent way to prepare 2D double hydroxide nanosheet dispersions in one step using nonvolatile green solvents

Ion Specific Effects on the Stability of Halloysite Nanotube Colloids - Inorganic Salts versus Ionic Liquids

Charging and aggregation processes were studied in aqueous dispersions of halloysite nanotubes (HNTs) in the presence of monovalent inorganic electrolytes and ionic liquid (IL) constituents. The same type of co-ion (same sign of charge as HNT) was used in all systems, while the type of counterions (opposite sign of charge as HNT) was systematically varied. The affinity of the inorganic cations to the HNT surface influenced their destabilizing power leading to an increase in the critical coagulation concentration (CCC) of HNT dispersions in the Cs+ < K+ < Na+ order. This trend agrees with the classical Hofmeister series for negatively charged hydrophobic surfaces. For the IL cations, the CCCs increased in the order BMPY+ < BMPIP+ < BMPYR+ < BMIM+ . An unexpectedly strong adsorption of BMPY+ cations on the HNT surface was observed giving rise to charge neutralization and reversal of the oppositely charged outer surface of HNT. The direct Hofmeister series was extended with these IL cations. The main aggregation mechanism was rationalized within the classical theory developed by Derjaguin, Landau, Verwey, and Overbeek, while ion specific effects resulted in remarkable variation in the CCC values. The results unambiguously proved that the hydration level of the surface and the counterions plays a crucial role in the formation of the ionic composition at the solid−liquid interface and consequently, in the colloidal stability of the HNT particles in both inorganic salt and IL solution

Perturbation Density Functional Theory for Inhomogeneous Fluids †

Abstract A recently developed third order+second order perturbation density functional approximation (DFA) is briefly described. The applicability of this theory is demonstrated in the study of the density profiles of Lennard-Jones (LJ) fluid next to a large hard sphere (mimicking a colloidal particle) of various sizes. The accuracy of DFA predictions is tested against the results of a grand canonical ensemble Monte Carlo simulation. The chosen density and potential parameters for the equilibrium bulk LJ fluid correspond to the conditions situated at &apos;dangerous&apos; regions of the phase diagram, i.e. near the critical temperature or close to the gas-liquid coexistence curve. It is found that the DFA theory performs successfully for both supercritical and subcritical temperatures. It is also shown that the &apos;universality&apos; of the adjustable parameter associated with this theory holds also in the present case of a large spherical particle as a source of external potential. Here the term universality means independence of this parameter on the particular external field responsible for the generation of a non-uniform density profile of the fluid. This DFA results can be used as a useful starting point for further investigation of solvent-induced excess potential of mean force in the similar systems