Ion specificity of the zeta potential of alpha-alumina, and of the adsorption of p-hydroxybenzoate at the alpha-alumina-water interface

The Role of Sum-Frequency Generation Spectroscopy in Understanding On-Surface Reactions and Dynamics in Atmospheric Model-Systems

Surfaces, both water/air and solid/water, play an important role in mediating a multitude of processes central to atmospheric chemistry, particularly in the aerosol phase. However, the study of both static and dynamic properties of surfaces is highly challenging from an experimental standpoint, leading to a lack of molecular level information about the processes that take place at these systems and how they differ from bulk. One of the few techniques that has been able to capture ultrafast surface phenomena is time-resolved sum-frequency generation (SFG) spectroscopy. Since it is both surface-specific and chemically sensitive, the extension of this spectroscopic technique to the time domain makes it possible to study dynamic processes on the femtosecond time scale. In this Perspective, we will explore recent advances made in the field both in terms of studying energy dissipation as well as chemical reactions and the role the surface geometry plays in these processes.

Evaporation Driven Hydrovoltaic Generator Based on Nano-Alumina-Coated Polyethylene Terephthalate Film

Collecting energy from the ambient environment through green and sustainable methods is highly expected to alleviate pollution and energy problems worldwide. Here, we report a facile and flexible hydrovoltaic generator capable of utilizing natural water evaporation for sustainable electricity production. The generator was fabricated by coating nano-Al2O3 on a twistable polyethylene terephthalate film. An open circuit voltage of 1.7 V was obtained on a piece of centimeter-sized hydrovoltaic generator under ambient conditions. The supercapacitor charged by the hydrovoltaic device can power a mini-motor efficiently. Moreover, by expanding the size or connecting it in series/parallel, the energy output of the generator can be further improved. Finally, the influence factors and the mechanism for power generation were primarily investigated. Electrical energy is produced by the migration of water through charged capillary channels. The environmental conditions, the properties of the solution and the morphology of the film have important effects on the electrical performance. This study is anticipated to offer enlightenment into designing novel hydrovoltaic devices, providing diverse energy sources for various self-powered devices and systems.

Synthesis and Characterization of α-Al2O3/Ba-β-Al2O3 Spheres for Cadmium Ions Removal from Aqueous Solutions

The search for adsorbent materials with a certain chemical inertness, mechanical resistance, and high adsorption capacity, as is the case with alumina, is carried out with structural or surface modifications with the addition of additives or metallic salts. This research shows the synthesis, characterization, phase evolution and Cd(II) adsorbent capacity of α-Al₂O₃/Ba-β-Al₂O₃ spheres obtained from α-Al₂O₃ nanopowders by the ion encapsulation method. The formation of the Ba-β-Al₂O₃ phase is manifested at 1500 °C according to the infrared spectrum by the appearance of bands corresponding to AlO₄ bonds and the appearance of peaks corresponding to Ba-O bonds in Raman spectroscopy. XRD determined the presence of BaO·Al₂O₃ at 1000 °C and the formation of Ba-β-Al₂O₃ at 1600 °C. Scanning electron microscopy revealed the presence of spherical grains corresponding to α-Al₂O₃ and hexagonal plates corresponding to β-Al₂O₃ in the spheres treated at 1600 °C. The spheres obtained have dimensions of 4.65 ± 0.30 mm in diameter, weight of 43 ± 2 mg and a surface area of 0.66 m²/g. According to the curve of pH vs. zeta potential, the spheres have an acid character and a negative surface charge of -30 mV at pH 5. Through adsorption studies, an adsorbent capacity of Cd(II) of 59.97 mg/g (87 ppm Cd(II)) was determined at pH 5, and the data were fitted to the pseudo first order, pseudo second order and Freundlich models, with correlation factors of 0.993, 0.987 and 0.998, respectively.

Hydropower generation by transpiration from microporous alumina

Traditional hydropower generation is one of the most sustainable energy sources; however, the local environmental impact of hydroelectric dams and reservoirs is serious, and hydroelectric power requires high-cost turbines and generators. Because these installations utilize gravitational potential energy of massive volumes of falling water, this sort of hydropower generation is unsuitable for ubiquitous, small-scale energy production. Here, we report that wetting and evaporation of pure water from a tiny block of porous alumina generates electrical current in the direction of water transpiration. The current induced in microporous alumina is associated with mass transport of water accompanying ions that accumulate near the negatively charged surface of alumina pores. Without any pre-treatment or additives, once evaporation commences, a 3 × 3 cm² piece of alumina can generate an open-circuit voltage as large as 0.27 V. The power generation scheme we propose here is simple, clean, and versatile, and it can be employed anywhere, as it utilizes only spontaneous capillary action of water and Coulombic interaction at the alumina-water interface, without requiring any input of heat or light.

Computational and experimental assessment of pH and specific ions on the solute solvent interactions of clay-biochar composites towards tetracycline adsorption: Implications on wastewater treatment

Experimental and computational investigations have been conducted in this study to assess the influence of municipal waste pyrolyzed biochar impregnated clay composites on antibiotic removal as a material for wastewater treatment and simultaneous value-addition for waste. The surface potential (zeta potential) of the pristine biochar and composite samples are found to be within the range ~10 to ~ -40 mV in the pH range 2-10. The presence of different inorganic salt solutions influences the electrophoretic mobility of the dispersed phase in a suspension, as well as its zeta potential. In addition of Na⁺ salt solutions, the Na⁺ ions undergo electrostatic interaction with the negatively charged biochar samples and form a double layer at the interface of biochar and ionic salt solution. Molecular dynamics simulations have been employed to understand experimental findings, ions adsorption and solute-solvent interactions at the molecular level of two biochar B7 (seven benzene rings, one methoxy, one aldehyde and two hydroxyls groups) and B17 (seventeen benzene rings, one methoxy, two hydroxyls and two carboxylic acid groups) in salts aqueous solutions. The results confirm that hydroxyls and carboxylate groups of biochar are responsible for solute-solvent interactions. Successful removal of tetracycline antibiotics is observed with 26 mg/g maximum adsorption capacity with montmorillonite biochar composite. This study confirms that interactions between amide and hydroxyl groups of tetracycline with hydroxyl and carboxylate groups of biochar play the key role in the adsorption process. The solution pH and presence of different background electrolytes effectively influence the process of solute-solvent interactions as well as adsorption efficacy towards tetracycline adsorption.

Specific Ion Effects on the Colloidal Stability of Layered Double Hydroxide Single-layer Nanosheets

The surface charge properties and aggregation behavior of positively charged Mg-Al-NO₃ layered double hydroxide (LDH) single-layer nanosheets dispersed in water were investigated in the presence of K⁺ salts with different mono-, di-, and trivalent anions, using electrophoresis and dynamic light scattering techniques. An increase in the salt concentration can significantly decrease the effective surface charge density (σ_eff) of LDHs, leading to the aggregation of nanosheets. The critical coagulation concentration (CCC) or ionic strength (CCIS) of salts for nanosheets significantly decreases with an increase in the valence of anions. Specific ion effects, with a partially reverse Hofmeister series, are observed. On the basis of the Stern model and the DLVO theory, the relationship of CCC with σ_eff and the ionic valences of salts (z_i) is theoretically analyzed, which can accurately describe the dependence of CCC on the σ_eff and z_i but cannot explain the origin of specific ion effects. To explore the origin of specific ion effects, a correlation between CCIS and the specific adsorption energy (E_sc) of anions within the Stern layer is developed. Especially, an empirical relationship of E_sc with the characteristic physical parameters of anions is proposed. Our model can accurately predict the CCISs of at least monovalent anions and divalent anions (CO₃^2- and SO₄^2-), demonstrating that the specific ion effects observed can be attributed to the differences in ionic size, polarizability, and hydration free energy (or the formation capacity of anion-cation pairs) of different anions. This work not only deepens the understanding of specific ion effects on the colloidal stability but also provides useful information for the potential applications of LDH single-layer nanosheets.

Analysis of Silica Pulp Viscoelasticity in Saline Media: The Effect of Cation Size

The effect of alkali metal chlorides on the viscoelastic behavior and yielding properties of silica suspensions was studied through creep-recovery and dynamic oscillatory tests with stress control. Then, the viscoelasticity of the pulps was correlated with the silica zeta potential, aggregate size, and the percentage of cations adsorbed on the surface of the ore. The results indicate that larger cations are more prone to adhere to the silica surface, which increases the number of ionic bonds that bind the particles. This generates stronger particle networks and a greater agglomeration of particles, especially those smaller than 10 µm. As the size of the bare cations increases, the rheological response provides higher values of yield stress, complex viscosity, and viscoelastic moduli, but in turn, pulps undergo minor deformations under the application of stress. Dynamic oscillatory tests suggest structural changes, with the phase angle following the inverse relationship with the bare cation size, indicating that the liquid-like character of the pulps increases as the size of the cations increases.

Effects of Ionic Strength, Salt, and pH on Aggregation of Boehmite Nanocrystals: Tumbler Small-Angle Neutron and X-ray Scattering and Imaging Analysis

The US government currently spends significant resources managing the legacies of the Cold War, including 300 million liters of highly radioactive wastes stored in hundreds of tanks at the Hanford (WA) and Savannah River (SC) sites. The materials in these tanks consist of highly radioactive slurries and sludges at very high pH and salt concentrations. The solid particles primarily consist of aluminum hydroxides and oxyhydroxides (gibbsite and boehmite), although many other materials are present. These form complex aggregates that dramatically affect the rheology of the solutions and, therefore, efforts to recover and treat these wastes. In this paper, we have used a combination of transmission and cryo-transmission electron microscopy, dynamic light scattering, and X-ray and neutron small and ultrasmall-angle scattering to study the aggregation of synthetic nanoboehmite particles at pH 9 (approximately the point of zero charge) and 12, and sodium nitrate and calcium nitrate concentrations up to 1 m. Although the initial particles form individual rhombohedral platelets, once placed in solution they quickly form well-bonded stacks, primary aggregates, up to ∼1500 Å long. These are more prevalent at pH = 12. Addition of calcium nitrate or sodium nitrate has a similar effect as lowering pH, but approximately 100 times less calcium than sodium is needed to observe this effect. These aggregates have fractal dimension between 2.5 and 2.6 that are relatively unaffected by salt concentration for calcium nitrate at high pH. Larger aggregates (>∼4000 Å) are also formed, but their size distributions are discrete rather than continuous. The fractal dimensions of these aggregates are strongly pH-dependent, but only become dependent on solute at high concentrations.

Colloid properties of hydrophobic modified alginate: Surface tension, ζ-potential, viscosity and emulsification

Micelle properties of hydrophobic modified alginate (HM-alginate) in various dispersion media have been studied by surface tension, ζ-potential, and viscosity measurements. Effect of salt on micelle properties showed that the presence of counter ion weakened the repulsive interaction between surfactant ions, decreased the critical micelle concentration (CMC) value of the HM-alginate, reduced the effective volume dimensions of HM-alginate and hence viscosity, which coincide with the corresponding ζ-potential values. Soy oil-in-water emulsions, stabilized solely by HM-alginate, were produced in high speed homogenization conditions and their stability properties were studied by visual inspection, optical microscopy and droplet size measurements. The results showed that emulsions (oil-water ratio was 1:7) containing 15mg/mL HM-alginate presented better stability during 15days storage, which stating clearly that HM-alginate is an effective emulsifier to stabilize oil-in-water emulsions. The herein presented homogeneous method for preparation of emulsion has the potential to be used in food industry.

Isoelectric points and points of zero charge of metal (hydr)oxides: 50years after Parks' review

The pH-dependent surface charging of metal (hydr)oxides is reviewed on the occasion of the 50th anniversary of the publication by G.A. Parks: "Isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems" in Chemical Reviews. The point of zero charge (PZC) and isoelectric point (IEP) became standard parameters to characterize metal oxides in aqueous dispersions, and they define adsorption (surface excess) of ions, stability against coagulation, rheological properties of dispersions, etc. They are commonly used in many branches of science including mineral processing, soil science, materials science, geochemistry, environmental engineering, and corrosion science. Parks established standard procedures and experimental conditions which are required to obtain reliable and reproducible values of PZC and IEP. The field is very active, and the number of related papers exceeds 300 a year, and the standards established by Parks remain still valid. Relevant experimental techniques improved over the years, especially the measurements of electrophoretic mobility became easier and more reliable, are the numerical values of PZC and IEP compiled by Parks were confirmed by contemporary publications with a few exceptions. The present paper is an up-to-date compilation of the values of PZC and IEP of metal oxides. Unlike in former reviews by the same author, which were more comprehensive, only limited number of selected results are presented and discussed here. On top of the results obtained by means of classical methods (titration and electrokinetic methods), new methods and correlations found over the recent 50years are presented.

Quantitative characterization of non-classic polarization of cations on clay aggregate stability

Soil particle interactions are strongly influenced by the concentration, valence and ion species and the pH of the bulk solution, which will also affect aggregate stability and particle transport. In this study, we investigated clay aggregate stability in the presence of different alkali ions (Li+, Na+, K+, and Cs+) at concentrations from10-5 to 10-1 mol L-1. Strong specific ion effects on clay aggregate stability were observed, and showed the order Cs+>K+>Na+>Li+. We found that it was not the effects of ion size, hydration, and dispersion forces in the cation-surface interactions but strong non-classic polarization of adsorbed cations that resulted in these specific effects. In this study, the non-classic dipole moments of each cation species resulting from the non-classic polarization were estimated. By comparing non-classic dipole moments with classic values, the observed dipole moments of adsorbed cations were up to 104 times larger than the classic values for the same cation. The observed non-classic dipole moments sharply increased with decreasing electrolyte concentration. We conclude that strong non-classic polarization could significantly suppress the thickness of the diffuse layer, thereby weakening the electric field near the clay surface and resulting in improved clay aggregate stability. Even though we only demonstrated specific ion effects on aggregate stability with several alkali ions, our results indicate that these effects could be universally important in soil aggregate stability.

Noncovalent magnetic control and reversible recovery of graphene oxide using iron oxide and magnetic surfactants

The unique charging properties of graphene oxide (GO) are exploited in the preparation of a range of noncovalent magnetic GO materials, using microparticles, nanoparticles, and magnetic surfactants. Adsorption and desorption are controlled by modification of pH within a narrow window of <2 pH units. The benefit conferred by using charge-based adsorption is that the process is reversible, and the GO can be captured and separated from the magnetic nanomaterial, such that both components can be recycled. Iron oxide (Fe2O3) microparticles form a loosely flocculated gel network with GO, which is demonstrated to undergo magnetic compressional dewatering in the presence of an external magnetic field. For composites formed from GO and Fe2O3 nanoparticles, it is found that low Fe2O3:GO mass ratios (<5:1) favor flocculation of GO, whereas higher ratios (>5:1) cause overcharging of the surfaces resulting in restabilization. The effectiveness of the GO adsorption and magnetic capture process is demonstrated by separating traditionally difficult-to-recover gold nanoparticles (d ≈ 10 nm) from water. The fully recyclable nature of the assembly and capture process, combined with the vast adsorption capacity of GO, presents obvious and appealing advantages for applications in decontamination and water treatment.

Adhesion of alkane as a functional group on muscovite and quartz: dependence on pH and contact time

The interactions between mineral surfaces and organic molecules in water control many processes in nature and in the production of modern materials. To improve the understanding of fluid-surface interactions, we investigated the interface behavior of quartz and muscovite, a model for clay minerals, in aqueous solutions where the pH and composition were controlled. We used atomic force microscopy (AFM) in chemical force mapping (CFM) mode to measure adhesion using tips functionalized with alkyl, -CH3. By combining adhesion forces measured as a function of pH, with data from streaming potential experiments and DLVO calculations, we were able to determine the surface charge density. We observed increased adhesion between the mineral surface and the hydrophobic tips as the contact time increased from 7 ms to ∼2 s. The diffusion of dissolved ions takes time, and density functional theory (DFT) calculations did not indicate a strong hydration of the mineral surfaces. Therefore, we interpret that the loss of ions from the confined space between the tip and sample is a likely explanation of the correlation between the dwell time and adhesion. The maximum adhesion increase with dwell time for muscovite, i.e., 400 ± 77 pN, was considerably larger than for quartz, 84 ± 15 pN, which fits with the different surface structure and composition of the two minerals. We propose two mechanisms to explain these results: (1) cations that are structured in the solution and on the surface remain associated at the tip-sample interface initially but diffuse away during extended contact time and (2) adventitious carbon, the organic material that comes spontaneously from air and solution, can diffuse to the tip-sample interface during contact. This material decreases the surface energy by aggregating near the alkyl tip and increases adhesion between the tip and sample.

In situ non-DLVO stabilization of surfactant-free, plasmonic gold nanoparticles: effect of Hofmeister's anions

Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br(-), SCN(-), or I(-)) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F(-) or SO4(2-)) seem to destabilize the gold colloid, whereas Cl(-) and NO3(-) give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount.

Harmful algae blooms removal from fresh water with modified vermiculite

Vermiculite and vermiculite modified with hydrochloric acid were investigated to evaluate their flocculation efficiencies in freshwater containing harmful algae blooms (HABs) (Microcystis aeruginosa). Scanning electron microscope, Fourier transform infrared spectroscopy, X-ray diffraction, converted fluorescence microscope, plasma-atomic emission spectrometry, and Zetasizer were used to study the flocculation mechanism of modified vermiculite. It was found that the vermiculite modified with hydrochloric acid could coagulate algae cells through charge neutralization, chemical bridging, and netting effect. The experimental results show that the efficiency of flocculation can be notably improved by modified vermiculite. Ninety-eight per cent of algae cells in algae solution could be removed within 10 min after the addition ofmodified vermiculite clay. The method that removal of HABs with modified vermiculite is economical with high efficiency, and more research is needed to assess their ecological impacts before using in practical application.

Dynamic Light Scattering Based Microelectrophoresis: Main Prospects and Limitations

Microelectrophoresis based on the dynamic light scattering (DLS) effect has been a major tool for assessing and controlling the conditions for stability of colloidal systems. However, both the DLS methods for characterization of the hydrodynamic size of dispersed submicron particles and the theory behind the electrokinetic phenomena are associated with fundamental and practical approximations that limit their sensitivity and information output. Some of these fundamental limitations, including the spherical approximation of DLS measurements and an inability of microelectrophoretic analyses of colloidal systems to detect discrete charges and differ between differently charged particle surfaces due to rotational diffusion and particle orientation averaging, are revisited in this work. Along with that, the main prospects of these two analytical methods are mentioned. A detailed review of the role of zeta potential in processes of biochemical nature is given too. It is argued that although zeta potential has been used as one of the main parameters in controlling the stability of colloidal dispersions, its application potentials are much broader. Manipulating surface charges of interacting species in designing complex soft matter morphologies using the concept of zeta potential, intensively investigated recently, is given as one of the examples. Branching out from the field of colloid chemistry, DLS and zeta potential analyses are now increasingly finding application in drug delivery, biotechnologies, physical chemistry of nanoscale phenomena and other research fields that stand on the frontier of the contemporary science. Coupling the DLS-based microelectrophoretic systems with complementary characterization methods is mentioned as one of the prosperous paths for increasing the information output of these two analytical techniques.

Hofmeister challenges: ion binding and charge of the BSA protein as explicit examples

Experiments on bovine serum albumin (BSA) via potentiometric titration (PT) and electrophoretic light scattering (ELS) are used to study specific-ion binding. The effect is appreciable at a physiological concentration of 0.1 M. We found that anions bind to the protein surface at an acidic pH, where the protein carries a positive charge (Z(p) > 0), according to a Hofmeister series (Cl(-) < Br(-) < NO(3)(-) < I(-) < SCN(-)), as well as at the isoionic point (Z(p) = 0). The results obtained require critical interpretation. The measurements performed depend on electrostatic theories that ignore the very specific effects they are supposed to reveal. Notwithstanding this difficulty, we can still infer that different 1:1 sodium salts affect the BSA surface charge/pH curve because anions bind to the BSA surface with an efficiency which follows a Hofmeister series.

Specific ion effects on adsorption at the solid/electrolyte interface: a probe into the concentration limit

Adsorption of organic acid at the mineral oxide-electrolyte interface has been explored. The adsorption of 2,4-dihydroxybenzoic acid onto α-alumina illustrates that specific ion effects show up at very low salt concentration (<0.05 mM). These surprising Hofmeister effects occur at salt concentrations an order of magnitude lower than in a previous study ( J. Colloid Interface Sci. 2010, 344, 482 ). Salts enhance adsorption and specifically at ≤0.05 mM. With increasing concentration of ion, the adsorption density decreases. The results are accounted for by incorporating the ion size and dispersion forces in the theoretical modeling based on ab initio calculations of polarizabilities. The order appears to be governed by ion size, determining the maximum concentration that ions can attain near the surface due to close packing.

Delamination of layered zeolite precursors under mild conditions: synthesis of UCB-1 via fluoride/chloride anion-promoted exfoliation

New material UCB-1 is synthesized via the delamination of zeolite precursor MCM-22 (P) at pH 9 using an aqueous solution of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride at 353 K. Characterization by powder X-ray diffraction, transmission electron microscopy, and nitrogen physisorption at 77 K indicates the same degree of delamination in UCB-1 as previously reported for delaminated zeolite precursors, which require a pH of greater than 13.5 and sonication in order to achieve exfoliation. UCB-1 consists of a high degree of structural integrity via (29)Si MAS NMR and Fourier transform infrared spectroscopies, and no detectable formation of amorphous silica phase via transmission electron microscopy. Porosimetry measurements demonstrate a lack of hysteresis in the N(2) adsorption/desorption isotherms and macroporosity in UCB-1. The new method is generalizable to a variety of Si:Al ratios and leads to delaminated zeolite precursor materials lacking amorphization.

The pH-dependent surface charging and points of zero charge: V. Update

The points of zero charge (PZC) and isoelectric points (IEP) from the recent literature are discussed. This study is an update of the previous compilation [M. Kosmulski, Surface Charging and Points of Zero Charge, CRC, Boca Raton, FL, 2009] and of its previous update [J. Colloid Interface Sci. 337 (2009) 439]. In several recent publications, the terms PZC/IEP have been used outside their usual meaning. Only the PZC/IEP obtained according to the methods recommended by the present author are reported in this paper, and the other results are ignored. PZC/IEP of albite, sepiolite, and sericite, which have not been studied before, became available over the past 2 years.