Silica In Aqueous Solutions Research Articles

In Ukrainian

Hypercholesterolemia significantly increases the risk of myocardial infarction associated with COVID-19. Along with pharmacological treatment, the possibility of the excretion of excess cholesterol from an organism by adsorption is also of great interest. The interaction of cholesterol with the surface of partially hydrophobized silica in aqueous solutions of bile acids was investigated by the PM7 method using the COSMO (COnductor-like Screening MOdel) solvation model. The distribution of electrostatic and hydrophobic potentials of molecules and complexes was calculated. The values of free Gibbs energy adsorption of bile acids on the surface of silica correlate with the distribution coefficients in the n-octanol-water system. The energy of interaction of cholesterol with bile acids affects its adsorption on silica. The stronger the bond of cholesterol with the molecules of bile acids, the less it is released from the primary micelles in solution and adsorbed on the surface.

Size-dependent dissociation of surface hydroxyl groups of silica in aqueous solution

The size-dependence of surface hydroxyl dissociation of solid nanoparticles dispersed in aqueous solutions is a basic colloidal interface phenomenon, but there is no model to describe this phenomenon. In this work, nine amorphous silica samples with different radii (r, 6–196 nm) were chosen as model particles, and their effective surface hydroxyl density (Nseff) and dissociation equilibrium constants (Ka) were determined using acid-base titration, showing an obvious size-dependence. With the decrease of r, the Nseff (mol/m2) and pKa all decrease. The decrease of pKa indicates an enhanced dissociation tendency of surface hydroxyls. Two size-dependent models relating Nseff and Ka separately with r were developed, predicting that both lnNseff and pKa decrease linearly with an increase in 1/r. The model equations can well describe the size-dependent data of Nseff and Ka of silica particles. The characteristic parameters Nseff, apparent pKa, and intrinsic pKa corresponding to flat surfaces (r → ∞) for silica particles are estimated to be 9.12 μmol/m2 (or 5.49 sites/nm2), 9.85, and 8.99, respectively. The size-effect on Nseff and Ka becomes obvious when r < 40 nm. This work deepens the understanding of the size-dependent solid/liquid interface phenomena.

Insights on Alanine and Arginine Binding to Silica with Atomic Resolution.

Interactions of biomolecules with inorganic oxide surfaces such as silica in aqueous solutions are of profound interest in various research fields, including chemistry, biotechnology, and medicine. While there is a general understanding of the dominating electrostatic interactions, the binding mechanism is still not fully understood. Here, chromatographic zonal elution and flow microcalorimetry experiments were combined with molecular dynamic simulations to describe the interaction of different capped amino acids with the silica surface. We demonstrate that ion pairing is the dominant electrostatic interaction. Surprisingly, the interaction strength is more dependent on the repulsive carboxy group than on the attracting amino group. These findings are essential for conducting experimental and simulative studies on amino acids when transferring the results to biomolecule-surface interactions.

Thermodynamic Modeling of the Solubility of Quartz in Water up to High Temperatures and Pressures

Available data on speciation of silica in aqueous solutions show that the main forms of silica are the monomer Si(OH)4 and the dimer Si2O(OH)6, with shares of higher polymers growing with an increase of temperature and the total concentration of silica in solution. Thermodynamic properties of the monomer and dimer in the ideal gas state are determined from the quantum-chemical calculations. The fugacity coefficients of these species in water up to water densities of 1200 kg m–3 and temperatures of up to 1350 K are estimated from an empirical correlation based on the stoichiometry of hydroxide species. The fine-tuning of the model is made by comparing calculations with the experimental solubility and speciation data. It was found that to match the speciation results reported as the share of the monomer, it is necessary to consider polymerization of silica beyond the dimer, which can be achieved without increasing the number of fitting parameters. The resulting model reproduces well the experimental results on the solubility of quartz in water, on the distribution of silica between the vapor and liquid phases of water at subcritical temperatures, and on the speciation of silica in water in equilibrium with quartz.

Size measurement of silica nanoparticles by Asymmetric Flow Field-Flow Fractionation coupled to Multi-Angle Light Scattering: A comparison exercise between two metrological institutes

In this work we present a comparison exercise between two metrological institutes for size measurement of silica nanoparticles by Asymmetrical Flow Field-Flow Fractionation (AF4) coupled to static light scattering. The work has been performed in the frame of a French inter-laboratory comparison (ILC) exercise organized by the nanoMetrology Club (CnM). The general aim of this multi-technique comparison was to improve the measurement process for each technique, after establishing a well-defined measurement procedure. The results obtained by two national metrological institutes (NMIs), the LNE (France) and the SMD (Belgium) by AF4-UV-DRI-MALS will be presented and discussed. Three different samples were characterized: the reference material ERM®-FD304, which is a suspension of colloidal silica in aqueous solution and two silica bimodal samples consisting of two populations of SiO2 nanoparticles of unknown size in aqueous solution, with different populations’ ratios. The procedure for the preparation of the sample before the analysis, and main separation parameters have been previously defined between the two institutes and will be described. The principals measured parameters were the weight-average (dge_w), number-average (dge_n) and z-average (dge_z) geometric diameter; the average hydrodynamic diameter (dh); and the diameter obtained by external calibration using polystyrene latex standards (dcal). Results between the two NMIs were comparable and coherent with the expected size values of those obtained by other techniques like Scanning Mobility Particle Sizer (SMPS) and Scanning Electron Microscopy (SEM) also involved in this ILC exercise. Where discrepancies are observed, they leave the results compatible within their uncertainties and underpin the challenges in analysing data and reporting results, making AF4 a powerful tool to compare to other measurement techniques.

Coffee‐ring‐effect‐induced water scale formation of silicic acid‐containing droplet

AbstractThe drying behavior of silicic acid‐containing water droplets on a glass substrate was investigated by using as‐prepared aqueous silica solutions of varying concentrations to simulate water scale‐like formation. The droplets of aqueous silica solution were found to form circumferential solid silica deposits (water scale). Additionally, the drying behavior of the silica droplets was observed under varying temperature and humidity. Optical microscopy images of the deposited material showed thicker deposits in the inner part of the circumference, indicating the so‐called rush‐hour effect and induced by the coffee‐ring effect. Thus, the formation of water scale observed in daily life can be attributed to the coffee‐ring effect. Scanning electron microscopy confirmed that the as‐prepared water scale consisted of polymerized silica spheres. The hardness of the water scale depended on the amount of silica deposited and the initial concentration of the silica solution. Thus, the formation of water scale in daily life was concluded to occur by two mechanisms: formation of silica spheres by silica polymerization and agglomeration‐deposition by the coffee‐ring effect.

Adsorption of organic pollutants by amine functionalized mesoporous silica in aqueous solution. Effects of pH, ionic strength and some consequences of APTES stability

A mesoporous silica with MCM-41 type pore array was synthesized by hydrothermal method and then was functionalized with known amounts of 3-aminopropyltriethoxysilane (APTES) in order to study the selective adsorption towards several species/pollutants. Antibiotics doxycycline hydrochloride (DC) and norfloxacin (NFX), the dye methyl orange(MO) and the pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) were used as adsorbates. The adsorption has been studied under different batch experimental conditions such as contact time, pH and ionic strength. The modification with APTES produced a decrease in the surface area of the solid and reversed the surface charge although did not change appreciably the mosoporosity and shape of the particles. This change in the charge strongly increased the adsorption of species that behave as anions in aqueous solution such as 2,4-D and MO. On the contrary, for species with several polar groups and charge-development the effect was very low and only detectable at specific pH. Electrostatic attractions between the protonated amine-end groups of APTES-SiO2 and the negative functional groups of 2,4-D and MO were the main responsible of the adsorption process. Partial hydrolysis of the supported coupling agent to regenerate silanol groups played an important role in decreasing the adsorption capacity of polar species and in the reuse capacity for adsorbing anionic species, i.e., the performance decreased up to 75% after four cycles of successive adsorption-desorption. Finding new methodology or functionalization agents to improve the silica surface reactivity should be the main goal of the researchers in the future. This work perhaps contributes with such perspectives.

Synthetic and Catalytic Potential of Amorphous Mesoporous Aluminosilicates Prepared by Postsynthetic Aluminations of Silica in Aqueous Media

AbstractAmorphous aluminosilicate catalysts have been used industrially on a large scale for almost a century. However, the influence of the pH on the alumination of silica in aqueous solutions has remained largely unclear. Herein, room temperature aluminations of different mesoporous amorphous silicas (fumed silica, dried silica gel, SBA‐15, MCM‐41, and COK‐12) with aqueous solutions of various pH (3–13) are explored. The aqueous solutions are prepared using different aluminum sources (Al(NO3)3 or NaAlO2) and alkaline additives (NaOH or NH4OH). The decoupling of pH and Al source using alkaline additives results in a vast experimental potential to prepare unique aluminosilicates, whereby an important role is played by the pH development during the treatment. The bulk and surface composition, acidity, aluminum coordination, morphology, hydrothermal stability, and porosity of the obtained materials are characterized. Optimal samples possess large surface areas and superior acidities (up to 50 % higher) and outstanding stabilities compared to aluminosilicates prepared with state‐of‐the‐art methods. The obtained materials are evaluated in a series of acid‐catalyzed model reactions involving substrates of various chemical reactivity and size, enabling insight in the catalytic functionality of the introduced Brønsted and Lewis sites. The potential of the obtained materials is emphasized by the similar or superior acidity and catalytic performance compared to several benchmark industrial silica–alumina‐based catalysts.

Adsorption of Uranium over NH2-Functionalized Ordered Silica in Aqueous Solutions.

The aim of this work was to obtain an in-depth understanding of the U(VI) adsorption mechanism over amino-functionalized mesoporous silica SBA-15 and highlights its high efficiency in aqueous media for U(VI) removal and preconcentration. The samples were synthesized and functionalized by both grafting and co-condensation methods, using different alkyl-substituted amine groups and were characterized using X-ray diffraction, N2 physisorption, Fourier transform infrared spectroscopy, and elemental C-H-N-S analyses. The properties for U(VI) adsorption were evaluated under discontinuous conditions, with the determination of the effect of several parameters (initial pH, contact time, initial U(VI) concentration, functionalization method, and organic moiety composition). U(VI) adsorption over grafted materials reached equilibrium at around 30 min, with a maximum adsorption capacity of 573 mgU·gads-1 for the most efficient material at its optimal adsorption pH (equal to 6) at 20 °C. Functionalized materials by grafting exhibit better adsorption capacities than co-condensed samples because of higher function surface density and function availability. U(VI) adsorption mechanisms were also studied by measuring the electrophoretic mobilities of the particles, aqueous U(VI) speciation, in situ attenuated total reflection infrared and Raman spectroscopies, and transmission electron microscopy analysis. U(VI) adsorption occurred through the formation of an inner sphere complex. The localization of adsorbed U(VI) has also been determined inside of the mesopores, with the formation of several particles on the nanometer scale, in the size of U-hydroxy phases. Besides, the study of the reusability of amino-functionalized SBA-15 by applying adsorption-desorption cycles was also conducted. The adsorption capacity of the material remains stable for at least four adsorption-desorption cycles without any noticeable capacity decrease.

On the Mechanism of the Dissolution of Quartz and Silica in Aqueous Solutions.

Quartz and silica are common materials, and their dissolution is of significant interest to a wide range of scientists. The kinetics of the dissolution of quartz and silica have been measured extensively, yet no clear theory of dissolution is available. A novel theory of dissolution and crystallization has recently been proposed that envisages the removal of material from the surface to form ions in solution leaving behind a charged surface vacancy. These vacancies create a potential difference across the Stern layer that accelerates or retards the removal of ions. In this way, the surface potential difference is caused by and influences the rate of the removal of ions. From this theory, a model of quartz dissolution is derived that predicts the observed orders of reaction. This prediction of the orders of reaction fits a data set consisting of 285 experiments. The model also describes the effect of Na+, K+, and Li+ ions, as well as the effect of heavy water. A significant component of the model is its ability to describe the zeta potential of the quartz–water interface. The model successfully predicts a transient period at the beginning of the reaction when the rate could either increase or decrease.

Aerobic Oxidation of Cyclohexane Effectively Catalyzed by Simply Synthesized Silica-Supported Cobalt Ferrite Magnetic Nanocrystal

Silica-supported magnetic cobalt ferrite complex oxides, CoFe2O4/SiO2, with different loading of 5, 10, 20, and 50% were simply prepared by a sol–gel autocombustion method using colloidal aqueous silica solution as a cheap silica source. The as-prepared samples were well characterized by X-ray diffractometry (XRD), Fourier transform infrared spectrophotometry (FT-IR), transmission electron microscopy (TEM), and N2 physisorption. Metals contents of the samples were also determined by atomic absorption spectrophotometry. Their catalytic performances were evaluated on cyclohexane oxidation using oxygen as oxidant in the absence of solvents and reductants. The supported catalysts have shown high catalytic activities for cyclohexane oxidation. Especially, when 5% loading of CoFe2O4/SiO2 was employed, 4181 turnover number and 95.4% selectivity for cyclohexanone and cyclohexanol were obtained under 1.6 MPa of initial oxygen pressure at 418 K after 6.0 h of reaction. The sample of 50% of CoFe2O4/SiO2 has strong m...

Critical Salt Effects in the Swelling Behavior of a Weak Polybasic Brush

The swelling behavior of poly(2-(diethylamino)ethyl methacrylate) (PDEA) brushes in response to changes in solution pH and ionic strength has been investigated. The brushes were synthesized by ARGET ATRP methodology at the silica-aqueous solution interface via two different surface-bound initiator approaches: electrostatically adsorbed cationic macroinitiator and covalently anchored silane-based ATRP initiator moieties. The pH-response of these brushes is studied as a function of the solvated brush thickness in a constant flow regime that elucidates the intrinsic behavior of polymer brushes. In situ ellipsometry equilibrium measurements show the pH-induced brush swelling and collapse transitions are hysteretic in nature. Furthermore, high temporal resolution kinetic studies demonstrate that protonation and solvent ingress during swelling occur much faster than the brush charge neutralization and solvent expulsion during collapse. This hysteresis is attributed to the formation of a dense outer region or skin during collapse that retards solvent egress. Moreover, at a constant pH below its pKa, the PDEA brush exhibited a critical conformational change in the range 0.5-1 mM electrolyte, a range much narrower than predicted by the theory of the osmotic brush regime. This behavior is attributed to the hydrophobicity of the collapsed brush. The swelling and collapse kinetics for this salt-induced transition are nearly identical. This is in contrast to the asymmetry in the rate of the pH-induced response, suggesting an alternative mechanism for the two processes dependent on the nature of the environmental trigger.

Coupled reactions and silica diffusion during serpentinization

Silica activity is one of the key factors in controlling reaction paths of serpentinization. We conducted hydrothermal experiments in the olivine (Ol)–orthopyroxene (Opx)–H2O system at 250°C and at a vapor-saturated pressure of 3.98MPa to explore the role of silica diffusion in aqueous fluids during serpentinization. Olivine (Fo91), orthopyroxene (En91), or their composite powders (with Ol/Opx/Ol zones) were set in tube-in-tube vessels, and solution chemistry and the extent of serpentinization were analyzed in detail.In the Ol–H2O experiments, the product changed from serpentine+magnetite to serpentine+brucite+magnetite, accompanied by a Si-drop in the solutions. Serpentinization proceeded uniformly throughout the reaction tube, indicating that the supply of water was not the rate-determining process. In the Opx–H2O experiments, orthopyroxenes were dissolved along the cleavages, and the amount of newly formed serpentine was very small. The silica activity of the solutions in the Opx–H2O experiments was 1–3 orders higher than in the Ol–H2O experiments. In the Ol–Opx–H2O experiments, serpentinization proceeded in both the Ol and Opx zones. In the Opx zone, the extent of serpentinization was constant, whereas in the Ol zone, serpentinization was most extensive along the boundary between the Ol and Opx zones, and it decreased gradually away from the boundary.Serpentinization in the Ol–Opx–H2O experiments was modeled simply by coupled processes involving silica diffusion and two serpentinization reactions: a silica-consuming reaction after olivine and a silica-releasing reaction after orthopyroxene. The spatial pattern of the extent of serpentinization was controlled by the diffusion coefficient of silica in aqueous solution, DSiO2,aq, and the apparent reaction rate constants k′Ol in the olivine zone, and k′Opx in the orthopyroxene zone. Assuming DSiO2,aq=2.0×10−4cm2/s, the observed variation in the extent of serpentinization after a run of 1512h is best fitted by simulation with k′Ol=4.4×10−4 and k′Opx=3.2×10−5s−1, indicating a reaction rate constant ∼14 times higher in the Ol zone than in the Opx zone. Our experimental results represent an analogue of serpentinization in natural hydrothermal systems with a high porosity, and we suggest that the spatial variation of serpentine as a function of the distance from a source of silica could be a useful indicator of the relative magnitudes of reaction, mass transport, fluid flow as well as temperature during hydrothermal alteration of oceanic lithosphere.

Solid-state properties and colloidal stability of thorium(IV)–silica nanoparticles

Thorium(IV) is able to form meta-stable colloids with silica in aqueous solution at pH⩾7 which results in concentrations of colloid-borne Th(IV) of 10−3M. The colloid structure can be regarded as amorphous because it shows no long-range order, indicated by the absence of distinct structural periodicity >4Å. The internal structure of the colloid particles consists of [Th(O(H))n] polyhedra, n=8–9, coordinated partly by [SiO4] polyhedra with Th–Si distances of 3.25±0.02Å, and partly by [Th(O(H))n] polyhedra with Th–Th distances of 3.98±0.02Å. The near-order coordination shows similarity with that of the orthosilicates thorite, α-ThSiO4, and huttonite, β-ThSiO4. XPS analysis of the oxygen bonds revealed the presence of O2−, OH− and H2O. The colloids can be classified as oxyhydroxo colloids [(Th,Si)On(OH)4−n·xH2O]4−2n−(4−n). Silica occurs in the colloid structure either in mononuclear or oligomeric units, depending on the Si/Th ratio and the silica precursor formed in the initial solution. Silica is enriched at the colloid surface if the concentration of the initial solutions is increased. The solution behavior of the particles was analyzed by in situ methods. With rising silica content, the particles gradually change from metal oxide type colloids to silica type colloids which can dramatically increase their colloidal stability.

Adsorption of Branched-Linear Polyethyleneimine–Ethylene Oxide Conjugate on Hydrophilic Silica Investigated by Ellipsometry and Monte Carlo Simulations

The adsorption of and conformation adopted by a branched-linear polymer conjugate to the hydrophilic silica-aqueous solution interface have been studied by in situ null ellipsometry and Monte Carlo simulations. The conjugate is a highly branched polyethyleneimine structure with ethyleneoxide chains grafted to its primary and secondary amino groups. In situ null ellipsometry demonstrated that the polymer conjugate adsorbs to the silica surface from water and aqueous solution of 1 mM asymmetric divalent salt (calcium and magnesium chloride to emulate hard water) over a large pH range. The adsorbed amount is hardly affected by pH and large charge reversal on the negatively charged silica surface occurred at pH = 4.0, due to the adsorption of the cationic polyelectrolyte. The Monte Carlo simulations using an appropriate coarse-grained model of the polymer in solution predicted a core-shell structure with no sharp boundary between the ethyleneimine and ethyleneoxide moieties. The structure at the interface is similar to that in solution when the polymer degree of protonation is low or moderate while at high degree of protonation the strong electrostatic attraction between the ethyleneimine core and oppositely charged silica surface distorts the ethyleneoxide shell so that an "anemone"-like configuration is adopted. The adsorption of alkyl benzene sulfonic acid (LAS) to a preadsorbed polymer layer was also investigated by null ellipsometry. The adsorption data brought additional support for the existence of a strong polymer adsorption and showed the presence of a binding which was further enhanced by the decreased solvency of the surfactant in the salt solution and confirmed the surface charge reversal by the polymer adsorption at pH = 4.0.

Synthesis of Composite Polystyrene/Silica Nanoparticles via Precipitation and Emulsion Polymerization Methods

Polystyrene/silica composite nanoparticles were synthesized via precipitation and emulsion polymerization methods, in the presence of a basic co-monomer (e.g., 4-VP and 1-VID), and a colloidal aqueous silica solution. The effects of key process parameters, that is, solvent type, monomer/co-monomer volume ratio and total monomers concentration for precipitation polymerization, and reaction temperature, pH value, initial silica-sol concentration and initial monomer/co-monomer molar ratio for emulsifier-free emulsion polymerization on the particle morphology, silica content, and particle size distribution of the composite nanoparticles were experimentally investigated. Stable, spherical, and uniform in size composite nanoparticles were synthesized by both techniques. The average particle diameter varied from 108 to 182 nm for the emulsifier-free emulsion polymerization and from 400 to 800 nm for the precipitation polymerization, while the silica content was as high as 38.3 wt.-% for the former method and up to 15.5 wt.-% for the later. The synthesized composite polymer/silica particles were then electrolytically co-deposited with zinc on steel plates to improve the corrosion resistance of the metal's surface.

Factors Affecting Copper(II) Binding to Multiarmed Cyclam-Grafted Mesoporous Silica in Aqueous Solution

Single- as well as multi-anchored cyclam-functionalized silica samples have been prepared by grafting amorphous silica gel (K60) and mesostructured silica (SBA-15) with silylated cyclam precursors bearing one, two, or four triethoxysilyl groups, respectively ascribed to cyclam-mono, cyclam-di, and cyclam-tetra. Their reactivity toward copper(II) has been thoroughly investigated in aqueous solution and discussed with respect to the number of arms tethering the ligand to the silica surface and the structural ordering of the adsorbent in terms of capacity, long-term stability, and speed of access to the binding sites. Less-than-complete metal ion uptake was always observed, even in excess of cyclam groups with respect to solution-phase Cu(II), suggesting lower stability of immobilized complexes relative to those in solution. Therefore, the number of arms attaching cyclam moieties to the silica walls (one, two, or four) was found to dramatically affect the binding properties of these hybrids toward copper(II), revealing significantly larger capacities when reducing the number of arms (less rigidity constraints in the macrocycle). In parallel, multiarm tethering resulted in better chemical resistance toward degradation as evidenced by UV-visible monitoring of Cu-cyclam complexes in solution (i.e., more ligand leaching from the adsorbent for singly tethered cyclam). On the other hand, electron spin resonance (ESR) experiments did not evidence significant differences between complexes bearing one, two, or four alkyl arms, since all Cu(II)-cyclam surface complexes were found to be hexacoordinated with a strong equatorial ligand field. Comparison of amorphous gels and mesostructured materials indicates that the binding properties of the adsorbents were hardly influenced by their level of ordering, suggesting that accessibility to the binding sites was not the limiting factor. Some advantage belonging to mesostructured adsorbents was however observed with respect to the rate of access to the active centers at pH values close to neutrality (due to faster mass transport), but this was no more the case when operating at lower pH values where the formation of the Cu-cyclam complex became the rate-determining step, as pointed out by electrochemistry.

Hydrogen-bond enhanced thermal energy transport at functionalized, hydrophobic and hydrophilic silica–water interfaces

In this Letter, we investigate the nanoscale heat transfer across hydrophilic silanol and hydrophobic silane interfaces. We calculate the thermal conductance at the interface via the vibrational relaxation of silica in aqueous solutions. Additionally, we directly determine the heat flux across the interface by non-equilibrium molecular dynamics. The results indicate a temperature and time dependence of the thermal conductance across the hydrophilic interface, whereas the conductance at the hydrophobic one stays constant, emphasizing the importance of hydrogen-bonded networks. Most importantly, we observe a rectifying effect of the hydrophilic silanol to the bulk substrate depending on the heat current direction.

Experimental Study on Viscosity of Colloidal Silica in Aqueous Electrolytic Solutions

The relative viscosity of colloidal silica dispersion in aqueous electrolytic solutions as the function of volume fraction of dry particles in the solutions has been experimentally determined in this work, in order to study the effects of pH and electrolytes (Na2SO4 and AlCl3) on the hydration of the silica surfaces in the solutions. The results have shown that the maximum relative viscosity of the silica dispersion and the strongest hydration of the silica in aqueous solutions appeared at neutral pH, while the stronger the acidity and the alkalinity of the aqueous solution, the weaker the hydration. In the presence of the electrolytes (Na2SO4 and AlCl3), the relative viscosity of the silica dispersion reduced and the hydration of the silica in aqueous solutions became weak. The higher the concentration of the electrolytes, the weaker the hydration, indicating that the destabilization of the colloidal silica dispersion in aqueous solutions might be realized through adding the high-valence electrolytes to weaken the hydration of the particle surfaces (hydration forces between the particles). Also, it has been shown that the negative zeta potentials of the colloidal silica in aqueous solutions greatly reduced in the presence of the electrolytes. Therefore, the high-valence electrolytes (Na2SO4 and AlCl3) as the coagulant of colloidal silica in aqueous solutions might be originated from that the presence of the electrolytes simultaneously reduces the electrical double layer repulsive force and the hydration repulsive forces between the particles in aqueous solutions.

Thermodynamic evaluation of a partial charge model assumptionfor the dissolved silica system

The Partial Charge Model was developed to predict the hydroxylation, polymerization, and precipitation of ions. The purpose of this study is to evaluate the Partial Charge Model for describing the polymerization of silica in aqueous solutions. The Partial Charge Model predicts the stability of ions and complexes based on the assumption that the stable species will have the same electronegativity as the mean electronegativity of the solution. The silica system was chosen for model validation because of the rare availability of self-consistent thermodynamic data on many dissolved but polymerized silicate anions, including both linear and cyclical species. The electronegativity of each species was calculated using the Partial Charge Model and the results were plotted against the stability constants for the ions. The silicate anions segregated into groups on the plot based on the number of charges per silicon atom in the polymer. Plots of the log of the stability constant versus the change in electronegativity produced a linear relationship for the silica polymers containing one negative charge per silicon atom, which resulted in an r2 of 0.9978. Thus, the Partial Charge Model successfully describes the thermodynamics of silica polymerization in aqueous solution for species that are sufficiently alike, but was not accurate for all silica species.