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The structure and thermodynamics of both 2--2 and 1--1 model electrolytes at a charged interface have been determined. The solvent is modeled as a structureless dielectric continuum. The structure is calculated from the `singlet ' version of the Ornstein-Zernike integral equation, using as input the structure of the bulk electrolyte from a recent i...
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We perform structural and thermodynamic calculations in the framework of the modified hypernetted chain (MHNC) integral equation closure to the Ornstein-Zernike equation for binary mixtures of size-different particles interacting with hard-core Yukawa pair potentials. We use the Percus-Yevick (PY) bridge functions of a binary mixture of hard-sphere...
The exact transfer-matrix solution for the longitudinal equilibrium properties of the single-file hard-disk fluid is used to study the limiting low- and high-pressure behaviors analytically as functions of the pore width. In the low-pressure regime, the exact third and fourth virial coefficients are obtained, which involve single and double integra...
The structure and thermodynamics for model 2–2 electrolytes at a charged interface have been determined by the so-called “pair” approximation of integral equation theory. In addition to Coulombic interactions, the potential models for the ion–ion and ion–wall interactions employ “soft” continuous potentials rather than “hard”-sphere or “hard”-wall...
The behavior of a two-dimensional neutral Coulomb fluid in the strong association regime (low density, high ionic charge) is explored by means of computer simulation and the hypernetted chain integral equation. The theory reproduces reasonably well the structure and thermodynamics of the system but presents a no-solution region at temperatures well...
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... In light of the singlet-based OZ integral equation theory, the appearance of a multilayer structure is related to the short-range correlations between ions. Specifically, Booth et al. 85 showed that while a hypernetted chain closure (that accounts for the short-range correlations) can predict the structure oscillations and overscreening, the closure form that ignored the short-ranged correlations between ions led to a Gouy−Chapman type of behavior with no multilayers or overscreening. ...
Capacitive energy storage devices are receiving increasing experimental and theoretical attention due to their enormous potential for energy applications. Current research in this field is focused on the improvement of both the energy and the power density of supercapacitors by optimizing the nanostructure of porous electrodes and the chemical structure/composition of the electrolytes. However, the understanding of the underlying correlations and the mechanisms of electric double layer formation near charged surfaces and inside nanoporous electrodes is complicated by the complex interplay of several molecular scale phenomena. This Perspective presents several aspects regarding the experimental and theoretical research in the field, discusses the current atomistic and molecular scale understanding of the mechanisms of energy and charge storage, and provides a brief outlook to the future developments and applications of these devices.
... performed using a C-S-H covered AFM tip and a C-S-H flat surface support the presence of strong non contact attractive forces, arising from the fluctuations of the electrolyte confined between the walls, whose strength increases with the ion coupling, characterized by valence, surface charge and concentration (and in general also dielectric constant and temperature) [53,54]. The predicted (and measured) interparticle force displays a strong attractive minimum and a secondary maximum, due to the crossover from anionic to cation concentration profiles [47,54,103]. ...
The way materials form in real conditions influences their function and performance. A material such as cement is an example of how the environmental and chemical conditions determine its structure and mechanics. Cement is not only a complex material, challenging to study, but also of huge importance for civil engineering. In this thesis, I have investigated the development of calcium-silicate-hydrate (C-S-H) gels under out-of-equilibrium conditions, which form during cement hydration and are the main responsible for cement mechanical strength. I have proposed a new model and numerical approach, based on soft matter, to follow the gel formation upon precipitation and aggregation of nano-scale hydration products. This enabled me to systematically connect the formation protocol to the material microstructure and mechanics at all stages. In particular, I use Grand Canonical Monte Carlo to mimic precipitation events during Molecular Dynamics simulations, with their rate corresponding to the hydrate production rate set by the chemical environment. The particle effective interactions are consistent with forces measured between the nano-scale hydrates in experiments at fixed lime concentrations. I have explored the influence of the out-of-equilibrium aggregation to the C-S-H densification and mechanicsby analyzing the microstructure morphology and local packing. Moreover, I studied the equilibrium phases and metastable states of the effective interactions without precipitation in order to determine the effect of the underlying thermodynamics into the structure of C-S-H gels. Finally, I achieved a good qualitative agreement with chemical, structural and mechanical experimental data of cement.
... Theories of divalents in bulk solution do not do well in a wide range of concentrations, if at all in physiological concentrations or in mixed solutions. (This literature, which is particularly relevant for CRCs, can be found through the classical papers of bioelectrostatics (McLaughlin et al., 1981;McLaughlin, 1989) and in the recent chemistry literature (Kjellander and Mitchell, 1994;Booth et al., 1995;Ennis et al., 1995;Kjellander, 1995;Hummer et al., 1996;Kalko et al., 1996;Mehler, 1996).) Nonetheless, PNP should be tried even in these cases, to see where and how it fails, to focus theoretical attention on the practical issues that demand resolution. ...
Current voltage (I-V) relations were measured from the calcium release channel (CRC) of the sarcoplasmic reticulum of cardiac muscle in 12 KCl solutions, symmetrical and asymmetrical, from 25 mM to 2 M. I-V curves are nearly linear, in the voltage range +/- 150 mV approximately 12kT/e, even in asymmetrical solutions, e.g., 2 M // 100 mM. It is awkward to describe straight lines as sums of exponentials in a wide range of solutions and potentials, and so traditional barrier models have difficulty fitting this data. Diffusion theories with constant fields predict curvilinear I-V relations, and so they are also unsatisfactory. The Poisson and Nernst-Planck equations (PNP) form a diffusion theory with variable fields. They fit the data by using adjustable parameters for the diffusion constant of each ion and for the effective density of fixed (i.e., permanent) charge P(x) along the channel's "filter" (7-A diameter, 10 A long). If P(x) is described by just one parameter, independent of x (i.e., P(x) = P0 = -4.2 M), the fits are satisfactory (RMS error/RMS current = 6.4/67), and the estimates of diffusion coefficients are reasonable D(K) = 1.3 x 10(-6) cm2/s, D(Cl) = 3.9 x 10(-6) cm2/s. The CRC seems to have a small selectivity filter with a very high density of permanent charge. This may be a design principle of channels specialized for large flux. The Appendix derives barrier models, and their prefactor, from diffusion theories (with variable fields) and argues that barrier models are poor descriptions of CRCs in particular and open channels in general.
Calcium-silicate hydrate (C-S-H) is the main binding agent in cement and concrete. It forms at the beginning of cement hydration, it progressively densifies as cement hardens and is ultimately responsible of concrete performances. This hydration product is a cohesive nano-scale gel, whose structure and mechanics are still poorly understood, in spite of its practical importance. Here we review some of the open questions for this fascinating material and a statistical physics approach recently developed, which allows us to investigate the gel formation under the out-of-equilibrium conditions typical of cement hydration and the role of the nano-scale structure in C-S-H mechanics upon hardening. Our approach unveils how some distinctive features of the kinetics of cement hydration can be related to changes in the morphology of the gels and elucidates the role of nano-scale mechanical heterogeneities in the hardened C-S-H.
We investigate the development of gels under out-of-equilibrium conditions, such as calcium-silicate-hydrate (C-S-H) gels that form during cement hydration and are the major factor responsible for cement mechanical strength. We propose a new model and numerical approach to follow the gel formation upon precipitation and aggregation of nano-scale colloidal hydrates, whose effective interactions are consistent with forces measured in experiments at fixed lime concentrations. We use Grand Canonical Monte Carlo to mimic precipitation events during Molecular Dynamics simulations, with their rate corresponding to the hydrate production rate set by the chemical environment. Our results display hydrate precipitation curves that indeed reproduce the acceleration and deceleration regime typically observed in experiments and we are able to correctly capture the effect of lime concentration on the hydration kinetics and the gel morphology. Our analysis of the evolution of the gel morphology indicates that the acceleration is related to the formation of an optimal local crystalline packing that allows for large, elongated aggregates to grow and that is controlled by the underlying thermodynamics. The defects produced during precipitation favor branching and gelation that end up controlling the deceleration. The effects on the mechanical properties of C-S-H gels are also discussed.
Recently the properties of both charged and un-charged solutes in aqueous solution have been elucidated by computer simulations and approximate integral equation theories. While answering some questions, others are raised. In particular the structure of water next to a charged surface (or a very large charged solute) is a challenge. Approximate integral equation theories can address this challenge.
We have performed molecular dynamics computer simulations of water in homogeneous external electric fields which were varied in a wide range of field strengths. The dielectric response is found to be linear up to fields E0≈0.01 V/Å from where dielectric saturation effects become important. At fields of E0≈3 V/Å a phase transition into an ordered, ice-like structure is observed, which is stabilized through hydrogen-bonds. With an increasing external electric field, the frequency spectrum of the water dynamics shows a remarkable red shift of the intramolecular modes and a blue shift of the librational motions, where the frequency varies quadratically with the field strength. A simple analytical model is discussed which reproduces the observed behavior.
Inhomogeneous correlation functions for model ‘ soft’ 2–2 and 1–1 electrolytes at a charged interface have been determined by the so–called ‘pair’ approximation of integral equation theory. The solvent is modeled as a structureless dielectric continuum at 25°C. The wall–ion–ion structure is calculated using the inhomogeneous Ornstein–Zernike relation, together with the hypernetted chain closure, and one of two choices for the functional relationship between the singlet and pair correlation functions. Both the interfacial density profiles and the inhomogeneous pair correlation functions are calculated. For most cases, the inhomogeneous pair correlation functions near the interface vary significantly from the homogeneous pair correlation functions. This deviation generally becomes stronger as the charge on the surface increases, and the deviation generally extends out further from the interface as the
surface charge increases. The density profiles predicted by the pair approximations generally show less structure than the singlet approximation density profiles, whereas the inhomogeneous pair correlation functions generally predict more structure than would be expected by simply assuming bulk pair correlation functions. The density profiles and inhomogeneous correlation functions are also found to agree qualitatively with previous simulations which used ‘charged hard–sphere/charged hard—wall’ potentials.
Recently the properties of both charged and un-charged solutes in aqueous solution have been elucidated by computer simulations and approximate integral equation theories. While answering some questions, others are raised. In particular the structure of water next to a charged surface (or a very large charged solute) is a challenge. Approximate integral equation theories can address this challenge.
