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First-Principles Prediction of Amorphous Silica Nanoparticle Surface Charge: Effect of Size, pH, and Ionic Strength
Abstract
The quantification of surface charge properties of silica nanoparticles is essential for several applications. To determine these properties, many experimental and theoretical methods have been introduced, which are time-consuming and/or challenging to use. In this study, a first-principles approach is developed to determine the surface charge properties of amorphous silica nanoparticles against the nanoparticle size, pH, and ionic strength without relying on experimental data. An amorphous silica nanoparticle of 1.34 nm diameter is simulated by using integrated molecular dynamics and Monte Carlo methods. A detailed analysis of the nanoparticle structure is provided by analyzing the types of silanol groups on the surface. Moreover, a model is developed to estimate the probability distribution of the surface silanol groups based on the nearest neighbor distances and the diameter of the nanoparticle to determine the number of surface silanols on larger nanoparticles. Thereafter, a computational chemistry approach is used to calculate the acid dissociation constants of the corresponding deprotonation reactions. The calculated constants and the point of zero charge value are in excellent agreement with experiments. The surface charge properties of the nanoparticle with various diameters are then estimated by using a mean-field model at different pH and ionic strength values. The results of the developed model are compared to the Poisson−Boltzmann equation as a reference model. The developed model predictions agree well with the reference model for low and mid-electrolyte concentrations (1 and 10 mM) and small nanoparticles (smaller than 100 nm). However, the developed model seems to qualitatively predict the surface charge properties more accurately than the Poisson− Boltzmann model for high electrolyte concentrations.
... According to the results from Zhao's group, EDA was analyzed as a binder between the negatively charged SiO 2 and resin nanoparticles, facilitating the formation of connected SiO 2 -resin hybrid nanoparticles through a two-step method [31]. When we used a one-step method to synthesize AISNPs, we speculated that the observed distance-dependent effect on particle morphology arises because, in the early stages of the reaction, a large number of unreacted ≡Si-O − remain on the surface of the smaller primary particles [33]. The two amine groups in diamines with a distance between them of less than approximately 4 Å are more likely to adsorb onto the same primary particle, similar to monoamines. ...
Herein, we synthesized anisotropic silica nanoparticles (AISNPs) with organic amines with different structures. Monoamines and diamines with distance between amine groups shorter than ca. 4 Å have been observed to facilitate the formation of isotropic silica nanoparticles (ISNPs). AISNPs were synthesized with diamines with distance between amine groups longer than ca. 4 Å and linear structures of triamines. Non-linear structures with amine groups positioned in a triangular configuration and the cage-like structure of tetra-amines directed the formation of ISNPs. It has been speculated that the formation of AISNPs would be due to the spherical primary particles connecting by organic amines with two or more amine groups with distances between them longer than ca. 4 Å. On the contrary, the formation of ISNPs would be attributed to the adsorption of two amine groups on the same primary particles, or to steric hindrance that prevent their connection.
Silica has many industrial (i.e., glass formers) and scientific applications. The understanding and prediction of the interesting properties of such materials are dependent on the knowledge of detailed atomic structures. In this work, amorphous silica subjected to an accelerated alkali silica reaction (ASR) was recorded at different time intervals so as to follow the evolution of the structure by means of high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and electron pair distribution function (e-PDF), combined with X-ray powder diffraction (XRPD). An increase in the size of the amorphous silica nanostructures and nanopores was observed by HRTEM, which was accompanied by the possible formation of Si-OH surface species. All of the studied samples were found to be amorphous, as observed by HRTEM, a fact that was also confirmed by XRPD and e-PDF analysis. A broad diffuse peak observed in the XRPD pattern showed a shift toward higher angles following the higher reaction times of the ASR-treated material. A comparison of the EELS spectra revealed varying spectral features in the peak edges with different reaction times due to the interaction evolution between oxygen and the silicon and OH ions. Solid-state nuclear magnetic resonance (NMR) was also used to elucidate the silica nanostructures.
We have developed a molecular mean-field theory—fourth-order Poisson–Nernst–Planck–Bikerman theory—for modeling ionic and water flows in biological ion channels by treating ions and water molecules of any volume and shape with interstitial voids, polarization of water, and ion-ion and ion-water correlations. The theory can also be used to study thermodynamic and electrokinetic properties of electrolyte solutions in batteries, fuel cells, nanopores, porous media including cement, geothermal brines, the oceanic system, etc. The theory can compute electric and steric energies from all atoms in a protein and all ions and water molecules in a channel pore while keeping electrolyte solutions in the extra- and intracellular baths as a continuum dielectric medium with complex properties that mimic experimental data. The theory has been verified with experiments and molecular dynamics data from the gramicidin A channel, L-type calcium channel, potassium channel, and sodium/calcium exchanger with real structures from the Protein Data Bank. It was also verified with the experimental or Monte Carlo data of electric double-layer differential capacitance and ion activities in aqueous electrolyte solutions. We give an in-depth review of the literature about the most novel properties of the theory, namely Fermi distributions of water and ions as classical particles with excluded volumes and dynamic correlations that depend on salt concentration, composition, temperature, pressure, far-field boundary conditions etc. in a complex and complicated way as reported in a wide range of experiments. The dynamic correlations are self-consistent output functions from a fourth-order differential operator that describes ion-ion and ion-water correlations, the dielectric response (permittivity) of ionic solutions, and the polarization of water molecules with a single correlation length parameter.
Preferential ion adsorption in mixed electrolytes plays a crucial role in many practical applications, such as ion sensing and separation and in colloid science. Using all-atom Molecular Dynamics simulations of aqueous NaCl, CaCl2 and NaCl--CaCl2 solutions confined by charged amorphous silica, we show that Na⁺ ions can adsorb preferentially over Ca²⁺ ions, depending on the surface structure. We propose that this occurs when the local surface structure sterically hinders the first hydration shell of the Ca²⁺ ion. Introducing a protrusion metric as a function of protrusion of deprotonated silanols, ion-specificity is successfully predicted on isolated, vicinal and geminal silanols alike, provided that no other deprotonated silanols are found nearby. Furthermore, we introduce a new strategy to analyze the results as a function of distance to the surface. This approach effectively removes surface roughness effects allowing for direct comparison with classical Electric Double Layer theory and distinction of specifically adsorbed ions and electrostatically adsorbed ions.
Surface charge densities of spherical silica nanoparticles of varied size spanning from 4.1 to 495.7 nm in NaCl solution with a concentration from 0.003 to 1.2 mM at a pH value of 8.0 were determined by converting their corresponding measured zeta potential with Poisson-Boltzmann model. The magnitude of the derived surface charge density (negative) at a given NaCl concentration of 0.225 mM decreases monotonically with increasing particle size, and reaching almost a steady value when the size exceeding 30 nm, revealing clearly the effect of the nanoparticle curvature on the surface charge density. The experimental data provide a consensus for the validity of different models describing the relation between the surface charge density and the electric potential, i.e., the surface complexation model and the Poisson-Boltzmann model for the charged nanospheres in electrolyte solution. We found that if the former includes the curvature-dependent deprotonation constant of the surface silanol groups, both the models would give a consistent result.
It has been long recognized that the surface chemistry of silica, and in particular the type and relative amount of surface bound silanol groups, plays a critical role in many of the properties associated with the material, where a typical example is the discrepant adsorption behavior observed depending on the pretreatment history of the surface. However, in spite of its importance, the direct probing of specific surface silanol groups under water has been hampered by instrumental limitations. Here we make use of vibrational sum frequency spectroscopy (VSFS) to first, identify under water, the OH stretch of isolated surface silanols, and second, explore its acid/base behavior and dependence on the surface pretreatment method. The properties of other types of silanol groups (i.e. hydrogen bonded / geminal) are also inferred from the data. The ability to directly probe these functional groups under water represents a crucial step to further improving our understanding of this widely used mineral oxide.
The use of nanotechnology in medicine and more specifically drug delivery is set to spread rapidly. Currently many substances are under investigation for drug delivery and more specifically for cancer therapy. Interestingly pharmaceutical sciences are using nanoparticles to reduce toxicity and side effects of drugs and up to recently did not realize that carrier systems themselves may impose risks to the patient. The kind of hazards that are introduced by using nanoparticles for drug delivery are beyond that posed by conventional hazards imposed by chemicals in classical delivery matrices. For nanoparticles the knowledge on particle toxicity as obtained in inhalation toxicity shows the way how to investigate the potential hazards of nanoparticles. The toxicology of particulate matter differs from toxicology of substances as the composing chemical(s) may or may not be soluble in biological matrices, thus influencing greatly the potential exposure of various internal organs. This may vary from a rather high local exposure in the lungs and a low or neglectable exposure for other organ systems after inhalation. However, absorbed species may also influence the potential toxicity of the inhaled particles. For nanoparticles the situation is different as their size opens the potential for crossing the various biological barriers within the body. From a positive viewpoint, especially the potential to cross the blood brain barrier may open new ways for drug delivery into the brain. In addition, the nanosize also allows for access into the cell and various cellular compartments including the nucleus. A multitude of substances are currently under investigation for the preparation of nanoparticles for drug delivery, varying from biological substances like albumin, gelatine and phospholipids for liposomes, and more substances of a chemical nature like various polymers and solid metal containing nanoparticles. It is obvious that the potential interaction with tissues and cells, and the potential toxicity, greatly depends on the actual composition of the nanoparticle formulation. This paper provides an overview on some of the currently used systems for drug delivery. Besides the potential beneficial use also attention is drawn to the questions how we should proceed with the safety evaluation of the nanoparticle formulations for drug delivery. For such testing the lessons learned from particle toxicity as applied in inhalation toxicology may be of use. Although for pharmaceutical use the current requirements seem to be adequate to detect most of the adverse effects of nanoparticle formulations, it can not be expected that all aspects of nanoparticle toxicology will be detected. So, probably additional more specific testing would be needed.
Electric double-layer capacitors are a family of electrochemical energy storage devices that offer a number of advantages, such as high power density and long cyclability. In recent years, research and development of electric double-layer capacitor technology has been growing rapidly, in response to the increasing demand for energy storage devices from emerging industries, such as hybrid and electric vehicles, renewable energy, and smart grid management. The past few years have witnessed a number of significant research breakthroughs in terms of novel electrodes, new electrolytes, and fabrication of devices, thanks to the discovery of innovative materials (e.g. graphene, carbide-derived carbon, and templated carbon) and the availability of advanced experimental and computational tools. However, some experimental observations could not be clearly understood and interpreted due to limitations of traditional theories, some of which were developed more than one hundred years ago. This has led to significant research efforts in computational simulation and modelling, aimed at developing new theories, or improving the existing ones to help interpret experimental results. This review article provides a summary of research progress in molecular modelling of the physical phenomena taking place in electric double-layer capacitors. An introduction to electric double-layer capacitors and their applications, alongside a brief description of electric double layer theories, is presented first. Second, molecular modelling of ion behaviours of various electrolytes interacting with electrodes under different conditions is reviewed. Finally, key conclusions and outlooks are given. Simulations on comparing electric double-layer structure at planar and porous electrode surfaces under equilibrium conditions have revealed significant structural differences between the two electrode types, and porous electrodes have been shown to store charge more efficiently. Accurate electrolyte and electrode models which account for polarisation effects are critical for future simulations which will consider more complex electrode geometries, particularly for the study of dynamics of electrolyte transport, where the exclusion of electrode polarisation leads to significant artefacts.
Nanoparticle surface charge density plays an important role in many 7 applications, such as drug delivery and cellular uptake. In this study, surface charge 8 properties of silica nanoparticles with different sizes are studied using a multi-ion surface 9 charge-regulation model. In contrast to most previous studies utilizing constant surface 10 charge, protonation and deprotonation surface reactions are used to obtain the local 11 surface charge, which depends on the particle size and electrolyte solution properties, 12 including the salt concentration and pH. For a fixed particle size, the magnitude of the 13 surface charge typically increases with an increase in pH or background salt 14 concentration. For fixed background salt concentration and pH, the magnitude of 15 surface charge decreases with an increase in the particle size and reaches a constant 16 when the particle size exceeds a critical value. Size dependent surface charge is further 17 characterized by the ratio of electrical double layer thickness to the particle diameter, and the surface charge varies significantly 18 when this dimensionless ratio is above 0.2.
A major requirement for the validation of methods assessing the risk associated with engineered
nanoparticles (ENPs) is the use of reference materials (RMs). In the present contribution we review
available RMs, ongoing projects and characterisation trends in the field. The conclusion is that actual
approaches to RMs mostly deal with metrological considerations about single properties of the ENPs,
typically their primary size, which can hardly be representative of nanoparticles characteristics in real
testing media and therefore, not valid for reliable and comparable toxicological studies. As an alternative,
we discussed the convenience and feasibility of establishing multi-parametric RMs for a series of ENPs,
focusing on silica nanoparticles (SNPs). As a future perspective, the need to develop RMs based on hybrid
nanoparticles is also discussed.
We study the variation of the dielectric response of ionic aqueous solutions as function of their ionic strength. The effect of salt on the dielectric constant appears through the coupling between ions and dipolar water molecules. On a mean-field level, we account for any internal charge distribution of particles. The dipolar degrees of freedom are added to the ionic ones and result in a generalization of the Poisson-Boltzmann (PB) equation called the Dipolar PB (DPB). By looking at the DPB equation around a fixed point-like ion, a closed-form formula for the dielectric constant is obtained. We express the dielectric constant using the "hydration length" that characterizes the hydration shell of dipoles around ions, and thus the strength of the dielectric decrement. The DPB equation is then examined for three additional cases: mixture of solvents, polarizable medium, and ions of finite size. Employing field-theoretical methods, we expand the Gibbs free-energy to first order in a loop expansion and calculate self-consistently the dielectric constant. For pure water, the dipolar fluctuations represent an important correction to the mean-field value and good agreement with the water dielectric constant is obtained. For ionic solutions we predict analytically the dielectric decrement that depends on the ionic strength in a nonlinear way. Our prediction fits rather well a large range of concentrations for different salts using only one fit parameter related to the size of ions and dipoles. A linear dependence of the dielectric constant on the salt concentration is observed at low salinity, and a noticeable deviation from linearity can be seen for ionic strength above 1 M, in agreement with experiments.
Contrary to ordinary solids, which are normally known to harden by compression, the compressibility of SiO2 (silica) glass has a maximum at about 2–4 GPa and its mechanical strength shows a minimum around 10 GPa. At this pressure, the compression of silica glass undergoes a change from purely elastic to plastic, and samples recovered from above 10 GPa are found to be permanently densified. Using an improved, ab initio parametrized interatomic potential for SiO2 we provide here a unified picture of the compression mechanisms based on the pressure-induced appearance of unquenchable fivefold defects. By means of molecular-dynamic simulations we find them to be responsible for the reduction of the mechanical strength and for permanent densification. We also find that the compressibility maximum does not require changes of the tetrahedral network topology.
The purpose of this study was to characterize model, outcrop, and reservoir samples under low salinity, different cationic valency, and pH ranges. Potentiometric titrations and zeta potential measurements were performed in order to determine the surface properties at different pH. The extent of ion adsorption on the mineral surfaces was found by the difference between the two approaches. X-ray diffraction and cation exchange capacity measurements were also performed. All samples except calcite showed high, negative zeta potentials in fresh water at pH > 6, followed by NaCl, low salinity solution (LSS), and solutions with divalent cations. The effect of ion valency on the zeta potential at low pH values was not prominent. Above pH 8, H+ and OH− were not potential determining ions for samples that contained calcite and dolomite more than 1.5%. Above pH 8, the presence of carbonates in outcrop and reservoir samples significantly affected the zeta potential by reversing the charge in divalent solutions. The cation exchange capacity of predominant quartz, mixed quartz-clay-carbonate, and quartz-carbonate sandstones was 0.0, 0.2, and 2.2 meqv/100 g, respectively. The compositional differences between the samples were also reflected in the point of zero charge (pzc) values. Samples containing more than 1.5% carbonates had pzc in the range of pH 8–9, while samples with small fractions of carbonates or without carbonates had pzc values in the range of pH 2.9–3.3.
In this paper, an integrated hybrid modeling approach with first-principles is implemented to model a flocculation process. The application of the framework is demonstrated through a laboratory-scale flocculation case of silica particles in water. In this modeling framework, it is demonstrated that the integration of first-principles models and machine-learning approaches accurately predicts the dynamics of the system. The first-principles model used in this study incorporates a population balance and mass balance models combined with the kinetic expressions of the agglomeration and breakage phenomena. The prediction of such modeling framework is compared with a fully first-principles model, and moreover with a hybrid model that was developed in a prior work, which used a population balance model as the first principles model and a deep learning algorithm for the determination of the flocculation kinetic parameters.
Based on the modified Derjaguin approach and the DLVO theory, an analytical expression has been obtained for the potential of electrostatic and molecular interaction of a charged spherical particle with a membrane pore rounded at the entrance. Profiles of the interaction potential of the "membrane pore-particle -electrolyte solution" system have been constructed for real values of the governing parameters. Relationships have been obtained that make it possible to optimize the level of the potential barrier in terms of these parameters and thereby improve the selective properties and productivity of the membranes to be fabricated. In particular, the maximum of the barrier can be displaced towards the bulk solution-the smaller the pore curvature radius, the greater is the displacement.
These instructions indicate how to prepare a manuscript for rapid publication in GASTROENTEHOLOGY and exemplify the exact format to follow.
We have developed a method for predicting solid-liquid interfacial tension based on density functional theory and the implicit solvent model COSMO-RS. Our model takes into account the solvation contributions and can be used to predict wetting behavior for a solid surface in contact with two liquids. We benchmarked our model against measurements of contact angle from water-in-oil on silica wafers and a range of self assembled monolayers (SAMs) with different composition, ranging from oil-wet to water-wet. By explicitly including deprotonation for the silica surfaces and carboxylic acid self-assembled monolayers, very good agreement was obtained with experimental data for all surfaces. Solid-liquid interfacial tension cannot be measured directly, making predictions such as from our model all the more important.
Ionic transport through a charged nanopore at low ion concentration is governed by the surface conductance. Several experiments have reported various power-law relations between the surface conductance and ion concentration, i.e., Gsurf ∝ c0α. However, the physical origin of the varying exponent, α, is not yet clearly understood. By performing extensive coarse-grained Molecular Dynamics (MD) simulations for various pore diameters, lengths, and surface charge densities, we observe varying power-law exponents even with a constant surface charge and show that α depends on how electrically “perfect” the nanopore is. Specifically, when the net charge of the solution in the pore is insufficient to ensure electroneutrality, the pore is electrically “imperfect” and such nanopores can exhibit varying α depending on the degree of “imperfectness”. We present an ionic conductance theory for electrically “imperfect” nanopores that not only explains the various power-law relationships but also describes most of the experimental data available in the literature.
Nanoporous silica is used in a wide variety of applications, ranging from bioanalytical tools and materials for energy storage and conversion as well as separation devices. The surface charge density of nanopores is not easily measured by experiment yet plays a vital role in the performance and functioning of silica nanopores. Herein, we report a theoretical model to describe charge regulation in silica nanopores by combining the surface-reaction model and the classical density functional theory (CDFT). The theoretical predictions provide quantitative insights into the effects of pH, electrolyte concentration, and pore size on the surface charge density and electric double layer structure. With a fixed pore size, the surface charge density increases with both pH and the bulk salt concentration similar to that for an open surface. At fixed pH and salt concentration, the surface charge density rises with the pore size until it reaches the bulk asymptotic value when the surface interactions become negligible. At high pH, the surface charge density is mainly determined by the ratio of the Debye screening length to the pore size (\lambda_D/D).
The structure of water adjacent to silica is sensitive to the degree of deprotonation of surface silanol groups. As a result, close inspection of signals originating from these water molecules can be used to reveal the surface charge density. We have used nonlinear vibrational spectroscopy of the water O–H stretching band over a temperature range of 10–75°C to account for the increase in surface potential from deprotonation. We demonstrate that the behavior at the silica surface is a balance between increasing surface charge, and a decreasing contribution of water molecules aligned by the surface charge. Together with a model that accounts for two different types of silanol sites, we use our data to report the change in enthalpy and entropy for deprotonation at each site. This is the first experimental determination of these thermodynamic parameters for hydrated silanol groups at the silica surface, critical to a wide range of geochemical and technological applications.
Amorphous silica is an intrinsic challenging system to study. In the last decades some particular chemical properties have been discovered and described, but their description and understanding at the molecular level is experimentally difficult. Therefore, theoretical quantum chemical methods and descriptors, combined with experimental input is a very appropriate set up to tackle this topic. In this study the acidity of silanol groups of amorphous silica in hydrated conditions is investigated. Special attention has been drawn to the chemical shift, but also Bader charges, and vibrational frequencies with their intensities. The known bimodal acidity behavior was recovered and rationalized. The findings support that the chemistry of the silica surface is not determined by a single site or a family of sites but is determined by a broad distribution of very different geometries of silanol groups in a H-bond network that can eventually show very acidic and at the same time very basic properties.
MATLAB provides a platform to solve BVPs which consist of two residual control based, adaptive mesh solvers named as bvp4c and bvp5c .
We perform atomistic simulations of nanometer-separated charged surfaces in the presence of monovalent counterions at fixed water chemical potential. The counterion density profiles are well described by a modified Poisson-Boltzmann (MPB) approach that accounts for non-electrostatic ion-surface interactions, while the effects of smeared-out surface-charge distributions and dielectric profiles are found to be relatively unimportant. The simulated surface interactions are for weakly charged surfaces well described by the additive contributions of hydration and MPB repulsions, but already for a moderate surface charge density of σ=- 0.77 e/nm² this additivity breaks down. This we rationalize by a combination of different effects, namely counterion correlations as well as the surface charge-induced reorientation of hydration water, which modifies the effective water dielectric constant as well as the hydration repulsion.
We present a comprehensive study of the electrochemical capacitance between a one-dimensional electronic material and an electrolyte. In contrast to a conventional, planar electrode, the nanoscale dimension of the electrode (with diameter smaller than the Debye length and approaching the size of the ions in solution) qualitatively changes the capacitance, which we measure and model herein. Furthermore, the finite density of states in these low dimensional electronic systems results in a quantum capacitance, which is comparable to the electrochemical capacitance. Using electrochemical impedance spectroscopy (EIS), we measure the ensemble average, complex, frequency dependent impedance (from 0.1 Hz to 1 MHz) between a purified (99.9%) semiconducting nanotube network and an aqueous electrolyte (KCl) at different concentrations between 10 mM and 1 M. The potential dependence of the capacitance is convoluted with the potential dependence of the in-plane conductance of the nanotube network, which we model using a transmission-line model to account for the frequency dependent in-plane impedance as well as the interfacial impedance between the network and the electrolyte. The ionic strength dependence of the capacitance is expected to have a root cause from the double layer capacitance, which we model using a modified Poisson Boltzmann equation. The relative contributions from those two capacitances can be quantitatively decoupled. We find a total capacitance per tube of 0.67–1.13 fF/µm according to liquid gate potential varying from -0.5 to -0.7 V.
Carbon dioxide (CO2) capture and storage (CCS) is an important climate change mitigation option along with improved energy efficiency, renewable energy, and nuclear energy. CO2 geosequestration, that is, to store CO2 under the subsurface of Earth, is feasible because the world’s sedimentary basins have high capacity and are often located in the same region of the world as emission sources. How CO2 interacts with the connate water and minerals is the focus of this Account. There are four trapping mechanisms that keep CO2 in the pores of subsurface rocks: (1) structural trapping, (2) residual trapping, (3) dissolution trapping, and (4) mineral trapping. The first two are dominated by capillary action, where wettability controls CO2 and water two-phase flow in porous media. We review state-of-the-art studies on CO2/water/mineral wettability, which was found to depend on pressure and temperature conditions, salt concentration in aqueous solutions, mineral surface chemistry, and geometry. We then review some recent advances in mineral trapping.
Nanoparticle manufacturing industries use well-established synthesis protocols or methodology as much as possible. The nanomaterials have unique physical and chemical properties at the atomic and molecular levels which often interfere with the standardized methods and compromise the reproducibility of the results. This chapter provides details of the challenges and associated problems which usually arise during the characterization of the nanomaterials and ways to resolve the issues with an emphasis on minimizing the associated interferences.
The MCM-41 material is very commonly used as a support for catalysts. However, theoretical
investigations are significantly limited due to the lack of appropriate models that well and accurately
describe the real material and enable effective computation at the same time. In this work, our aim is to
obtain calculable models at the DFT level of MCM-41 which are as close as possible to the real material.
In particular the hydration degree has been investigated, and we present and characterize here for the
first time a model for the MCM-41 unit cell filled with explicit solvent water molecules. This is
particularly important, because the models developed here are aimed to be further applied in theoretical
ab initio/DFT studies of adsorption or as a support for modelling active sites in catalysts.
The surface charge density and surface potential of oxide surfaces are to a large extent regulated by the co- and counterion distributions in the electrical double layer. Here we study the effect of different anions (Cl−, Br−, I−, HCOO− and NO3−) in sodium electrolytes and different cations (Li+, Na+, K+ and Cs+) in chloride electrolytes on the surface charge density and relative surface potential of colloidal nanometer sized silica (SiO2) as a function of electrolyte concentration and bulk pH using potentiometric titrations, attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) from a liquid microjet. Our results reveal that the identity of the anion has no significant effect on the nanoparticles surface charge density and relative surface potential at bulk pH where silica is negatively charged, consistent with textbook arguments of electrostatics and their exclusion from the region of the electrical double layer that regulates these properties. By contrast, the identity of the cation in solution is significant on both the surface charge density and surface potential. Specific cation effects are rationalized by their respective distance of closest approach to the nanoparticles surface—the hydrated cation diameter sets the outer Helmholtz plane and in turn the capacitance of the Stern layer.
Charge regulation in the electrical double layer has important implications for ion adsorption, interparticle forces, colloidal stability, and deposition phenomena. While charge regulation generally receives little attention, its consequences can be major, especially when considering interactions between unequally charged surfaces. The present article discusses common approaches to quantify such phenomena, especially within classical Poisson-Boltzmann theory, and pinpoints numerous situations where the consideration of charge regulation is essential. For the interpretation of interaction energy profiles, we advocate the use of the constant regulation approximation, which summarizes the surface properties in terms of two quantities, namely the diffuse layer potential and the regulation parameter. This description also captures some pronounced regulation effects observed in the presence of multivalent ions.
Understanding the microscopic origin of the acid-base behavior of mineral surfaces in contact with water is still a challenging task, for both the experimental and the theoretical communities. Even for a relatively simple material, such as silica, the origin of the bimodal acidity behavior is still a debated topic. In this contribution we calculate the acidity of single sites on the humid silica surface represented by a model for the hydroxylated amorphous surface. Using a thermodynamic integration approach based on ab initio molecular dynamics, we identify two different acidity values. In particular, some convex geminals and some type of vicinals are very acidic (pKa = 2.9 and 2.1, respectively) thanks to a special stabilization of their deprotonated forms. This recalls the behavior of the out-of-plane silanols on the crystalline (0001) α-quartz surface, although the acidity here is even stronger. On the contrary, the concave geminals and the isolated groups present a quite high pKa (8.9 and 10.3, respectively), similar to the one of silicic acid in liquid water.
Second harmonic generation (SHG) is commonly employed to monitor processes at mineral oxide/liquid interfaces. Using SHG, we determine how the starting pH affects the acid-base chemistry of the silica/aqueous interface. We observe three different sites with pKa values of approximately 3.8, 5.2, and ∼9 (pKa-I, pKa-II, pKa-III, respectively), but the presence and relative abundance of these sites is very sensitive to the starting pH. For titrations initiated at pH 12, all three sites are observed, whereas only two sites are observed for titrations initiated at pH 2 or pH 7. Moreover, exposure to pH 2 facilitates the formation of pKa-II and pKa-III sites, while exposure to pH 7 results in pKa-I and pKa-III sites. Based on previous computational work, we assign these sites to three different hydrogen bonding environments at the interface including a hydrophobic site for the most acidic silanol corresponding to pKa-I.
With the miniaturization of electronic devices, it is essential to achieve higher carrier density and lower operation voltage in field-effect transistors (FETs). However, this is a great challenge in conventional FETs owing to the low capacitance and electric breakdown of gate dielectrics. Recently, electric double-layer technology with ultra-high charge-carrier accumulation at the semiconductor channel/electrolyte interface has been creatively introduced into transistors to overcome this problem. Some interesting electrical transport characteristics such as superconductivity, metal–insulator transition, and tunable thermoelectric behavior have been modulated both theoretically and experimentally in electric double-layer transistors (EDLTs) with various semiconductor channel layers and electrolyte materials. The present article is a review of the recent progress in the EDLTs and the impacts of EDLT technology on modulating the charge transportation of various electronics.
Oil-in-water emulsions stabilized by graphene oxide (GO) can be flocculated by either an increase or a decrease in pH. At highly acidic pH, fully reversible flocculation of emulsion droplets can be achieved, whereas when adjusted to high pH, the flocculation is irreversible, which we interpret as a permanent chemical change in the GO. We correlate the effectiveness of GO as a stabilizer to its aqueous aggregation state and explore the effects of the GO surface charge in determining emulsion properties. By directly measuring the interaction forces between two emulsion droplets coated in GO using the atomic force microscope, we demonstrate the basis for the pH-dependent flocculation behavior. It is shown that interfacial charge of the GO and oil–water interface is the overriding drive for the exceptional stability of acidic GO emulsions.
Electrostatic interactions between two charged dielectric objects highly depend on their surface charge. Most existing studies assume constant surface charge densities between the two interacting objects regardless of the separation distance between them. The surface charge of a spherical silica nanoparticle interacting with a flat silica plate is investigated numerically as a function of the separation distance normalized with the electrical double layer thickness (κh), pH, and background salt concentration. The numerical model employs Poisson–Nernst–Planck equations for ionic mass transport and considers surface charge regulation in the presence of multiple ionic species. Relatively weak interactions between the nanoparticle and the plate are observed for κh 1, resulting in uniform surface charge densities. Because of curvature, the surface charge density of the nanoparticle is higher than that of the plate. Strong interactions are observed for κh ≤ 1, leading to spatially nonuniform surface charge densities on the nanoparticle and the plate. This effect increases with decreased separation distance (κh). Enhanced proton concentration in the gap between the particle and the plate leads to reduced surface charge densities on the two objects.
This paper presents the results of imbibition tests using a reservoir crude oil and a reservoir brine solution with a high salinity and a suitable nanofluid that displaces crude oil from Berea sandstone (water-wet) and single-glass capillaries. The Illinois Institute of Technology (IIT) nanofluid is specially formulated to survive in a high-salinity environment and is found to result in an efficiency of 50% for Berea sandstone, compared to 17% using the brine alone at a reservoir temperature of 55 °C. We also present a direct visual evidence of the underlying mechanism based on the structural disjoining pressure for the crude oil displacement using IIT nanofluid from the solid substrate in high-salinity brine. These results aid our understanding of the role of the nanofluid in displacing crude oil from the rock, especially in a high-salinity environment containing Ca2+ and Mg2+ ions. Results are also reported using Berea sandstone and a nanofluid containing silica nanoparticles.
In this article, plasmon-mediated fluorescence biosensing is reported to be distance independent through a full-coupling strategy that effectively activates the entire plasmon coupling region. This concept is demonstrated through collecting the directional surface plasmon-coupled emission (SPCE) signal from fluorescent silica nanoparticles with a size that matches the entire coupling region. Based on this design, the spatial distribution of the fluorophores is confined by the dimension of the nanoparticle. Therefore, these encapsulated fluorophores occupy the maximum coupling dominant region and optimally utilize the coupling effect. Being different from the conventional plasmon-mediated fluorescence, the enhanced fluorescence response becomes nearly independent of distance changes on a wide dynamic range from 0 nm to 30 nm between the fluorescent nanoparticles and metal structure. Full-coupling SPCE appropriately enlarges the distribution of fluorophores, ensuring that the coupling dominant region is filled with enough fluorophores at varying distances to create a stable and detectable signal. This scale of distances is well suited for many biorecognition events. Full-coupling SPCE solves signal deviation challenges originating from the susceptible and unpredictable orientation and conformation of biomolecules on the nanoscale. Immunoassays and DNA detection are shown with high and reliable signals, demonstrating the advantages of distance-independent full coupling. Without the need of a complicated and rigorous architecture for precise distance control, full-coupling SPCE offers great promise for a general platform of chip-based biosensing and bioanalysis.
A Comment on the Letter by John P. Perdew, Kieron Burke, and Matthias Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996). The authors of the Letter offer a Reply.
Acid–base chemistry of clay minerals is central to their interfacial properties, but up to now a quantitative understanding on the surface acidity is still lacking. In this study, with first principles molecular dynamics (FPMD) based vertical energy gap technique, we calculate the acidity constants of surface groups on (0 1 0)-type edges of montmorillonite and kaolinite, which are representatives of 2:1 and 1:1-type clay minerals, respectively. It shows that Si–OH and Al–OH2OH groups of kaolinite have pKas of 6.9 and 5.7 and those of montmorillonite have pKas of 7.0 and 8.3, respectively. For each mineral, the calculated pKas are consistent with the experimental ranges derived from fittings of titration curves, indicating that Si–OH and Al–OH2OH groups are the major acidic sites responsible to pH-dependent experimental observations. The effect of Mg substitution in montmorillonite is investigated and it is found that Mg substitution increases the pKas of the neighboring Si–OH and Si–OH2 groups by 2–3 pKa units. Furthermore, our calculation shows that the pKa of edge Mg–(OH2)2 is as high as 13.2, indicating the protonated state dominates under common pH. Together with previous adsorption experiments, our derived acidity constants suggest that Si–O– and Al–(OH)2 groups are the most probable edge sites for complexing heavy metal cations.
A survey is presented of the research results obtained by the author on the properties of amorphous silica. It covers the following topics: physically adsorbed water; dehydration of the surface and the temperature boundary of this process; dehydroxylation of the surface; concentration of hydroxyl groups on the silica surface, depending on the conditions of activation of silica; the energetic non-uniformity of the surface; chemisorption of water (rehydroxylation of the surface); the difference between the hydroxyl groups on the silica surface and structurally bound water inside the silica particles. For each of these processes a probable mechanism is suggested.
Reduction of fluid drag is important in the micro/nanofluidic systems. Surface charge and boundary slip can affect the fluid drag, and surface charge is also believed to affect boundary slip. The quantification of surface charge and boundary slip at a solid-liquid interface has been widely studied, but there is lack of the understanding of effect of surface charge on boundary slip. In this paper, the surface charge density of borosilicate glass and octadecyltrichlorosilane (OTS) surfaces immersed in saline solutions with two ionic concentrations and deionized (DI) water with different pH values and electric field values is quantified by fitting experimental atomic force microscopy (AFM) electrostatic force data using a theoretical model relating the surface charge density and electrostatic force. Results show that pH and electric field can affect the surface charge density of glass and OTS surfaces immersed in saline solutions and DI water. The mechanisms of the effect of pH and electric field on the surface charge density are discussed. The slip length of OTS surface immersed in saline solutions with two ionic concentrations and DI water with different pH values and electric field values is measured, and their effects on the slip length are analyzed from the point of surface charge. Results show that larger absolute value of surface charge density leads to smaller slip length for the OTS surface.
The work presented here includes both theoretical and experimental aspects of some of the most significant areas of colloidal silica science and technology. This book constitutes an update in the field since Ralph K. Iler, the distinguished silica scientist, published the definitive book on silica chemistry in 1979. This new book includes the 11 plenary lectures presented at an international symposium honoring Iler and 22 related research papers. This book offers a clear introduction to the science and technology of colloidal silica and will increase the reader`s understanding of the most important problems in this area of science.
To study charge-dependent interactions of nanoparticles (NPs) with biological media and NP uptake by cells, colloidal gold nanoparticles were modified with amphiphilic polymers to obtain NPs with identical physical properties except for the sign of the charge (negative/positive). This strategy enabled us to solely assess the influence of charge on the interactions of the NPs with proteins and cells, without interference by other effects such as different size and colloidal stability. Our study shows that the number of adsorbed human serum albumin molecules per NP was not influenced by their surface charge. Positively charged NPs were incorporated by cells to a larger extent than negatively charged ones, both in serum-free and serum-containing media. Consequently, with and without protein corona (i.e., in serum-free medium) present, NP internalization depends on the sign of charge. The uptake rate of NPs by cells was higher for positively than for negatively charged NPs. Furthermore, cytotoxicity assays revealed a higher cytotoxicity for positively charged NPs, associated with their enhanced uptake.
We describe a variational mean field study of polyelectrolyte expansion based on the application of the Gibbs–Bogoliubov inequality and a generalized Gaussian trial Hamiltonian. The screened electrostatic interactions among the charged beads on the polyion are approximated by a pairwise additive Yukawa potential while we treat the excluded volume effects in terms of the Dirac δ function in the way usual in studies of neutral polymers. Expressing the Hamiltonian in terms of Fourier components, the variational procedure yields a set of Euler equations that are analyzed by the method of dominant balance to study the scaling regimes in various limiting situations. The method predicts correct scaling laws for weakly screened polyelectrolytes, dominated by long‐ranged Coulombic repulsions. At strong screening or low degrees of ionization, when the polymer resembles a self‐avoiding walk, the calculations overestimate the scaling exponent, the value of ∼4/3 replacing the Flory value, a deficiency known from earlier applications of the theory to nonionic macromolecules. The numerical solution to the Euler equations is used to calculate the mean square distances between monomer pairs in cyclic polyions as functions of the relative distance along the polymer backbone. Effects of the degree of polymerization and electrolyte screening are studied and the difficulties in providing a general numerical solution to the variational problem are discussed.
Reprinted from Proc. Phys.-Math. Soc. Jpn. 20 (1938), 319-340.
Using nonresonant second harmonic generation spectroscopy, we have monitored the change in surface charge density of the silica/water interface over a broad pH range in the presence of different alkali chlorides. Planar silica is known to possess two types of surface sites with pKa values of 4 and 9, which are attributed to different solvation environments of the silanols. We report that varying the alkali chloride electrolyte significantly changes the effective acid dissociation constant (pKaeff) for the less acidic silanol groups, with the silica/NaClaq and silica/CsClaq interfaces exhibiting the lowest and highest pKaeff values of 8.3(1) and 10.8(1), respectively. Additionally, the relative populations of the two silanol groups are also very sensitive to the electrolyte identity. The greatest percentage of acidic silanol groups was 60(2)% for the silica/LiClaq interface in contrast to the lowest value of 20(2)% for the silica/NaClaq interface. We attribute these changes in the bimodal behavior to the influence of alkali ions on the interfacial water structure and its corresponding effect on surface acidity.
The physicochemical reasons behind the delicate equilibrium established between the adsorbed and the mobile phases are ultimately dictated by the specific interactions between silica surface functionalities and the adsorbate, resulting in different adsorption constants. There are many reasons to report on silica interacting with biomolecules. The most obvious one is that on the Earth's crust, oxygen and silicon are the most abundant atomic species, with percentages of 45.5% and 27.2%, respectively, which manifests itself in a large variety of silica and silicate minerals so that the contact between living matter and these materials is ubiquitous. The adsorption of other molecules, which are not rigorously biomolecules, has also been addressed in section 9, because their behaviors mimic that of residues to be found in real proteins. From the methodological point of view, only methods able to properly handle relatively weak intermolecular interactions should be adopted.
Starting from the screening in conductors, an algorithm for the accurate calculation of dielectric screening effects in solvents is presented, which leads to rather simple explicit expressions for the screening energy and its analytic gradient with respect to the solute coordinates. Thus geometry optimization of a solute within a realistic dielectric continuum model becomes practicable for the first time. The algorithm is suited for molecular mechanics as well as for any molecular orbital algorithm. The implementation into MOPAC and some example applications are reported.