Ink-bottle effect in mercury intrusion porosimetry of cement-based materials

Mercury intrusion porosimetry (MIP) is a widely used method for studying porous materials, in particular, cement-based materials. The usual interpretation of such measurements is based on certain assumptions. One of these is that each pore is connected to the sample surface directly or through larger pores. Pores not meeting this assumption are called ink-bottle pores. The effect that sample size has on the MIP characteristics of concrete samples, like the ink-bottle effect and hysteresis, was studied by measuring additional extrusion and intrusion cycles. In order to characterize the extrusion and ink-bottle behavior, the amount of entrapped mercury chi(p) was estimated. Superimposition of extrusion and second intrusion curves is achieved if the contact angle theta is adjusted from theta(i), the intrusion contact angle, to theta(e), the extrusion contact angle. The threshold radius is often assumed to be a dominant pore radius, yet in this study the entrapped mercury content shows no evidence for the presence of a dominant pore radius. Even if characteristic properties of cement-based materials can be estimated with MIP, comparison of results is rendered difficult by the significant effects of sample preparation techniques and sample size and the ink-bottle effect due to randomly present air bubbles.     

Modeling desorption of fluids from disordered mesoporous materials

The desorption mechanism of fluids in disordered mesoporous glasses is studied by Monte Carlo simulations of a coarse-grained lattice model with realistic matrix configurations representative of Vycor. Two methods of simulation are considered: grand canonical ensemble Monte Carlo simulations and dynamic Monte Carlo simulations which mimic the diffusion of the fluid in and out of the material using Kawasaki dynamics. In the grand canonical simulations, cavitation via nucleation of bubbles inside the pores plays the dominant role in determining the fluid configurations along the desorption isotherm. The Kawasaki dynamics simulations indicate that such configurations are achieved dynamically via the gradual advancement of macroscopic front interfaces toward the interior. This is made possible by the bubble nucleation mechanism operating on a length scale that is determined by both the typical pore size and the strength of the solid-fluid interaction.     

On the cavitation and pore blocking in slit-shaped ink-bottle pores

We present GCMC simulations of argon adsorption in slit pores of different channel geometry. We show that the isotherm for an ink-bottle pore can be reconstructed as a linear combination of the local isotherms of appropriately chosen independent unit cells. Second, depending on the system parameters and operating conditions, the phenomena of cavitation and pore blocking can occur for a given configuration of the ink-bottle pore by varying the geometrical aspect ratio. Although it has been argued in the literature that the geometrical aspects of the system govern the evaporation mechanism (either cavitation or pore blocking), we here put forward an argument that the local compressibility in different parts of the ink-bottle pore is the deciding factor for evaporation. When the fluid in the small neck is strongly bound, cavitation is the governing process, and molecules in the cavity evaporate to the surrounding bulk gas via a mass transfer mechanism through the pore neck. When the pore neck is sufficiently large, the system of neck and cavity evaporates at the same pressure, which is a consequence of the comparable compressibility between the fluid in the neck and that in the cavity. This suggests that local compressibility is the measure of cohesiveness of the fluid prior to evaporation. One consequence that we derive from the analysis of isotherms of a number of connected pores is that by analyzing the adsorption branch or the desorption branch of an experimental isotherm may not lead to the correct pore sizes and the correct pore volume distribution.     

Modeling mercury porosimetry using statistical mechanics

We consider mercury porosimetry from the perspective of the statistical thermodynamics of penetration of a nonwetting liquid into a porous material under an external pressure. We apply density functional theory to a lattice gas model of the system and use this to compute intrusion/extrusion curves. We focus on the specific example of a Vycor glass and show that essential features of mercury porosimetry experiments can be modeled in this way. The lattice model exhibits a symmetry that provides a direct relationship between intrusion/extrusion curves for a nonwetting fluid and adsorption/desorption isotherms for a wetting fluid. This relationship clarifies the status of methods that are used for transforming mercury intrusion/extrusion curves into gas adsorption/desorption isotherms. We also use Monte Carlo simulations to investigate the nature of the intrusion and extrusion processes.     

Mercury porosimetry in mesoporous glasses: a comparison of experiments with results from a molecular model

We present results from experiments and molecular modeling of mercury porosimetry into mesoporous Vycor and controlled pore glass (CPG) solid materials. The experimental intrusion/extrusion curves show a transition from a type H2 hysteresis for the Vycor glass to a type H1 hysteresis for the CPG. Mercury entrapment is observed in both materials, but we find that the amount of entrapped mercury depends on the chosen experimental relaxation time. No additional entrapment is found in a second intrusion/extrusion cycle, but hysteresis is still observed. This indicates that hysteresis and entrapment are of different origin. The experimental observations are qualitatively reproduced in theoretical calculations based on lattice models, which provide significant insights of the molecular mechanisms occurring during mercury porosimetry experiments in these porous glasses.     

On the hysteresis loop and equilibrium transition in slit-shaped ink-bottle pores

Bin grand canonical Monte Carlo simulations have been carried out to study adsorption–desorption of argon at 87.3 K in a model ink-bottle mesoporous solid in order to investigate the interplay between the pore blocking process, controlled by the evaporation through the pore mouth via the meniscus separating the adsorbate and the bulk gas surroundings, and the cavitation process, governed by the instability of the stretched fluid (with a decrease in pressure) in the cavity. The evaporation mechanism switches from pore blocking to cavitation when the size of the pore neck is decreased, and is relatively insensitive to the neck length under conditions where cavitation is the controlling mechanism. We have applied the recently-developed Mid-Density scheme to determine the equilibrium branch of the hysteresis loop, and have found that, unlike ideal simple pores of constant size and infinite length, where the equilibrium transition is vertical, the equilibrium branch of an ink-bottle pore has three distinct sub-branches within the hysteresis loop. The first sub-branch is steep but continuous and is close to the desorption branch (which is typical for a pore with two open ends); this is associated with the equilibrium state in the neck. The third sub-branch is much steeper and is close to the adsorption branch (which is typical for either a pore with one end closed or a closed pore), and is associated with the equilibrium state in the cavity. The second sub-branch, connecting the other two sub-branches, has a more gradual slope. The domains of these three sub-branches depend on the relative sizes of the cavity and the neck, and their respective lengths. Our investigation of the effects of changing neck length clearly demonstrates that cavitation depends, not only on fluid properties, as frequently stated, but also on pore geometry.     

Pores in bilayer membranes of amphiphilic molecules: coarse-grained molecular dynamics simulations compared with simple mesoscopic models

We investigate pores in fluid membranes by molecular dynamics simulations of an amphiphile-solvent mixture, using a molecular coarse-grained model. The amphiphilic membranes self-assemble into a lamellar stack of amphiphilic bilayers separated by solvent layers. We focus on the particular case of tensionless membranes, in which pores spontaneously appear because of thermal fluctuations. Their spatial distribution is similar to that of a random set of repulsive hard disks. The size and shape distribution of individual pores can be described satisfactorily by a simple mesoscopic model, which accounts only for a pore independent core energy and a line tension penalty at the pore edges. In particular, the pores are not circular: their shapes are fractal and have the same characteristics as those of two-dimensional ring polymers. Finally, we study the size-fluctuation dynamics of the pores, and compare the time evolution of their contour length to a random walk in a linear potential.     

Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel micro- and mesoporous silicas

We report results of nitrogen and argon adsorption experiments performed at 77.4 and 87.3 K on novel micro/mesoporous silica materials with morphologically different networks of mesopores embedded into microporous matrixes: SE3030 silica with worm-like cylindrical channels of mode diameter of approximately 95 angstroms, KLE silica with cage-like spheroidal pores of ca. 140 angstroms, KLE/IL silica with spheroidal pores of approximately 140 angstroms connected by cylindrical channels of approximately 26 angstroms, and, also for a comparison, on Vycor glass with a disordered network of pores of mode diameter of approximately 70 angstroms. We show that the type of hysteresis loop formed by adsorption/desorption isotherms is determined by different mechanisms of condensation and evaporation and depends upon the shape and size of pores. We demonstrate that adsorption experiments performed with different adsorptives allow for detecting and separating the effects of pore blocking/percolation and cavitation in the course of evaporation. The results confirm that cavitation-controlled evaporation occurs in ink-bottle pores with the neck size smaller than a certain critical value. In this case, the pressure of evaporation does not depend upon the neck size. In pores with larger necks, percolation-controlled evaporation occurs, as observed for nitrogen (at 77.4 K) and argon (at 87.3 K) on porous Vycor glass. We elaborate a novel hybrid nonlocal density functional theory (NLDFT) method for calculations of pore size distributions from adsorption isotherms in the entire range of micro- and mesopores. The NLDFT method, applied to the adsorption branch of the isotherm, takes into account the effect of delayed capillary condensation in pores of different geometries. The pore size data obtained by the NLDFT method for SE3030, KLE, and KLE/IL silicas agree with the data of SANS/SAXS techniques.     

Multiporous Materials From Thermoreversible Poly(vinyledene fluoride) Gels

Poly(vinylidene fluoride)(PVF₂) produces thermoreversible gels with alkyl diesters of general formula (CH₂)_n (COOEt)₂ as well as with camphor, a naturally occurring ketone. These gels containing polymer-solvent intercalates yield multiporous materials when subjected to controlled solvent removal techniques. The micro and meso pores are attributed to polymer-solvent complexation while the macro pores are formed as a result of removal of the solvent trapped in the fibrillar network. PVF₂ –diethyl azelate (DEAZ, n = 6) and PVF₂ -camphor gels produce porous polymer network when dried by cyclohexane leaching. FESEM images exhibit porous network structures with fibrillar morphology. Mercury intrusion porosimetry (MIP) shows presence of pores having diameter in the range 4 nm–400µm for both the systems. The BJH pore size distribution curves for both systems confirm the presence of mesoporosity. The HK pore size distribution plots indicate that micropores are also created and it also puts evidence of single molecule solvent intercalation between the PVF₂ strands. The hysteresis between the extrusion and the intrusion curves indicates the presence of channel type/ink-bottle type structure in these systems.     

What can positronium tell us about adsorption?

The condensation and evaporation of n-heptane at 298 K in mesopores of silica material obtained by the polymer templating method have been studied by PALS measurements. It is demonstrated that the ortho-positronium lifetimes and intensities provide valuable information on pore filling and emptying which are not accessible from a conventional adsorption experiment. The results confirm the specific adsorption mechanism of n-heptane in pores with narrow openings (ink-bottle shape) which is different from that known for other pore geometries. The results from PALS experiment are compared to those derived from the conventional n-heptane and nitrogen adsorption data.     

BROMOC suite: Monte Carlo/Brownian dynamics suite for studies of ion permeation and DNA transport in biological and artificial pores with effective potentials

The transport of ions and solutes by biological pores is central for cellular processes and has a variety of applications in modern biotechnology. The time scale involved in the polymer transport across a nanopore is beyond the accessibility of conventional MD simulations. Moreover, experimental studies lack sufficient resolution to provide details on the molecular underpinning of the transport mechanisms. BROMOC, the code presented herein, performs Brownian dynamics simulations, both serial and parallel, up to several milliseconds long. BROMOC can be used to model large biological systems. IMC‐MACRO software allows for the development of effective potentials for solute–ion interactions based on radial distribution function from all‐atom MD. BROMOC Suite also provides a versatile set of tools to do a wide variety of preprocessing and postsimulation analysis. We illustrate a potential application with ion and ssDNA transport in MspA nanopore. © 2014 Wiley Periodicals, Inc.     

Mutants only Partially Represent Characteristics of Calcium-Release-Activated Calcium Channel Gating

Calcium-release-activated calcium (CARC) channels are one of the major pathways of calcium entry in non-excitable cells. Despite a decade or two of research, its regulatory mechanism is not yet thoroughly understood. The slow progress is due to the complexity of its pores (i.e., Orai) on one hand and the difficulty in capturing its regulatory complex on the other hand. As a result, possible gating mechanisms have often been speculated by exploring the structure and properties of constitutive open mutants. However, there is much debate about how they can truly reflect the gating of CRAC channels under physiological conditions. In the present study, we combined molecular dynamics simulations with free energy calculations to study three dOrai mutants (G170P, H206A, and P288A), and further calculated their current-voltage curves. Results show that these constructs adopt different approaches to maintain their conductive state. Meanwhile they have unique pore structures and distinctive rectification properties and ion selectivity for cations compared to wild-type pores. We conclude that although the mutants may partially capture the gating motion characteristics of wild-type pores, the information obtained from these mutants is likely not a true reflection of CRAC channel gating under physiological conditions.     

Phase transformation in a lattice system in the presence of spin-exchange dynamics

A joint action of the Glauber single-spin-flip and the Kawasaki spin-exchange mechanisms upon the processes of phase transformation is examined in the framework of the one-dimensional kinetic Ising model. It is shown that the addition of the Kawasaki dynamics to that of Glauber accelerates the process of phase transformation in the initial stage, but slows it down in later stages. For the truncated form of Glauber dynamics, which excludes the processes of splitting and coagulation of clusters, the addition of the Kawasaki dynamics always accelerates the phase transformation process. Acting alone, the Kawasaki mechanism provides a cluster growth proportional to t(1/2) (where t is the time) in the initial stage and proportional to t(1/3) (Lifshitz-Slyozov-Wagner law) in the intermediate stage. In the final stage, a cluster size approaches exponentially its equilibrium value.     

Sol–gel synthesis,characterization and catalytic activity of mesoporous γ-alumina prepared from boehmite sol by different methods

In this study, boehmite sols were used as alumina precursors for preparing mesoporous γ-aluminas by two different methods. In one case polyethylenimine was used as a structure-directing agent, and in another case ultrasound treatment was applied. Nitrogen physisorption showed that aluminas that had been prepared by these methods demonstrated different porous structures. The sample obtained without additional treatment had closely packed spherical particles and pores had ink-bottle neck morphology. Ultrasound treatment led to the transformation of ink-bottle pores into cylindrical form and to the increase in surface area and pore volume. Aluminas prepared using polyethylenimine as a template showed larger cylindrical wormhole-like mesopores with a broader pore size distribution, high surface area and pore volume. Catalytic tests showed that textural properties as well as crystallite size were very important parameters of synthesized samples which affected the catalytic activity in the methanol dehydration reaction. It was found that γ-Al₂O₃ prepared by ultrasound treatment had large crystallite size and demonstrated high catalytic activity.     

Kinetics of phase transformation on a Bethe lattice in the presence of spin exchange

Kinetics of phase transformation on a Bethe lattice governed by single-spin-flip Glauber and spin-exchange Kawasaki dynamics is examined. For a general Glauber dynamics for which all processes (splitting and coagulation, growth and decay of clusters, as well as creation and annihilation of single-spin clusters) take place, the addition of the Kawasaki dynamics accelerates the transformation process without changing the qualitative behavior. In the growth-decay regime of the Glauber dynamics, regime in which the splitting and coagulation, and creation and annihilation processes due to single-spin flips are negligible, the Kawasaki dynamics strongly increases the fraction of transformed phase because of the splitting and coagulation of clusters induced by the spin-exchange processes. Acting alone, the Kawasaki dynamics leads to the growth of the clusters of each of the phases after the quenching of the temperature to a lower value. When the final temperature T(f) is smaller than a certain temperature T(f0), the average cluster radius grows linearly with time during both the initial and intermediate stages of the kinetic process, and diverges as log(2)(t(d)-t)(-1) when the time t approaches the value t(d) at which infinite clusters arise. It is shown that, among the various spin-exchange processes involved in Kawasaki dynamics, the main contribution is provided by those which decrease or increase the number of clusters by unity.     

Conformational transition pathway in the allosteric process of calcium-induced recoverin: molecular dynamics simulations

Recoverin is an important neuronal calcium sensor (NCS) protein, which have been implicated in a wide range of Ca(2+) signaling events in neurons and photoreceptors. To characterize the conformational transition of recoverin from the myristoyl sequestered state to the extrusion state, a series of conventional molecular dynamics (CMD) and targeted molecular dynamics (TMD) simulations were performed. The 36.8 ns long CMD and TMD simulations on recoverin revealed a reliably conformational transition pathway, which can be viewed as a sequential two-stage process. A very important mechanistic conclusion from the present TMD simulations is that the hydrophobic and hydrophilic interactions modulate the allostery cooperatively in the conformational transition pathway. In the first stage, three salt-bridges broken between Lys-84 and Gly-124, between Lys-5 and Glu-103 and between Gly-16 and Lys-97 are major components to destabilize the structure of state T and trigger the swivel of the N- and C-terminal domains. In the second stage, the rupture of H-bond Phe-56-O(...)H(O)-Thr-21 leads to the two helices of EF-1 apart from each other, destabilizing the hydrophobic interactions of the myristoyl group with its environment, whereas the making of H-bond Leu-108-O(...)H(O)-Ser-72 helps the interfacial domain maintain its structural integrity during the course of the myristoyl extrusion. The molecular dynamics simulations results are beneficial to understanding the mechanism of how and why NCS proteins make progress in the photo-signal transduction processes. Further experimental and theoretical studies are still very desirable.     

Long-time correlations and hydrophobe-modified hydrogen-bonding dynamics in hydrophobic hydration

The physical mechanisms behind hydrophobic hydration have been debated for over 65 years. Spectroscopic techniques have the ability to probe the dynamics of water in increasing detail, but many fundamental issues remain controversial. We have performed systematic first-principles ab initio Car-Parrinello molecular dynamics simulations over a broad temperature range and provide a detailed microscopic view on the dynamics of hydration water around a hydrophobic molecule, tetramethylurea. Our simulations provide a unifying view and resolve some of the controversies concerning femtosecond-infrared, THz-GHz dielectric relaxation, and nuclear magnetic resonance experiments and classical molecular dynamics simulations. Our computational results are in good quantitative agreement with experiments, and we provide a physical picture of the long-debated "iceberg" model; we show that the slow, long-time component is present within the hydration shell and that molecular jumps and over-coordination play important roles. We show that the structure and dynamics of hydration water around an organic molecule are non-uniform.     

Model for dynamics of inhomogeneous and bulk fluids

An accurate model for the density of states (DOS) for strongly inhomogeneous and bulk fluids has been proposed based on gamma distributions. The contribution to the density of states from the collective dynamics is modeled as an incomplete gamma distribution and the high frequency region is obtained from the solution of the memory equation using a sech memory kernel. Using only the frequency moments as input, the model parameters for the collective dynamics are obtained by matching moments of the resulting distribution. The model results in an analytical expression for the self-diffusivity of the fluid. We present results for soft sphere fluids confined in slit-shaped pores as well as bulk soft sphere liquids. Comparisons of the DOS, velocity autocorrelation functions, and memory kernels with molecular dynamics simulations reveal that the model predicts features in the DOS over the entire frequency range and is able to capture changes in the DOS as a function of fluid density and temperature. As a result the predicted VACFs, memory kernels, and self-diffusivities are accurately predicted over a wide range of conditions. Since the frequency moments for bulk liquids can be obtained from pair correlation functions, our method provides a direct route from fluid structure to dynamics. For fluids confined in slit-shaped pores, where the frequency moments are obtained from molecular dynamics simulations, the predicted self-diffusivities capture the resulting oscillations due to variations in the solvation pressure, and in the case of smooth walled pores, the predictions are superior to those obtained using kinetic theory.