Alkali Metals Ion Exchange on Muscovite Mica

A grahame triple-layer model unifies mica monovalent ion exchange, zeta potential, and surface forces

A triple-layer model of the mica/water electrical double layer (EDL) unifies prediction of zeta potential, ion-exchange, and surface-force isotherms. The theory treats cations as partially dehydrated and complexed specifically to the anionic exchange sites of mica. A diffuse layer commencing at the outer Helmholtz plane (OHP) balances the surface charge not neutralized by adsorbed cations in the inner Helmholtz plane (IHP). Ion-binding equilibrium constants are assessed from zeta-potential measurements and used thereafter to predict ion-exchange isotherms and surface forces. Basal-plane mica surface charge is almost completely neutralized by specific binding of cations, including hydronium ions. The charge in the diffuse layer is only a few percent of the mica crystallographic charge density but leads to long-range electrostatic interactions between charged surfaces. The Grahame triple-layer model of the aqueous EDL provides a robust, quantitative, and unified description of the mica/water interface.

Dynamics of major and trace elements during seawater intrusion in a coastal sedimentary aquifer impacted by anthropogenic activities

This study analyzed the dynamics of major ions and trace elements along the groundwater flow path of the coastal sedimentary Todos Santos aquifer in Baja California Sur, Mexico, moderately impacted by anthropogenic activities. The results indicate that the elements Ca²⁺, Mg²⁺, Ba²⁺, Sr²⁺ and Li⁺ are mobilized from the aquifer matrix during seawater intrusion, whereas the alkali-elements Na⁺, K⁺ and Rb⁺ are removed from solution, possibly due to cationic exchange process. The anions HCO^3- and SO₄^2- and the elements I and B are mobilized due to carbonate mineral weathering, whereas dissolved silica and the halides Br^- and F^- behave conservatively during salinization. Groundwater NO₃^- is provided by sewage infiltration. Regarding trace elements behavior, we identify three groups: i) elements that are mobilized during saline intrusion (Fe, Co, V, Se, Re), ii) elements revealing low or no mobilization (Mo, Ni, Cr, Ta, W) and iii) elements that show an undefined tendency (U, As, Ge, Sb, Cu, Mn). The U and NO₃^- levels in groundwater should be considered carefully because several wells have concentrations close to the permissible levels. This study may be useful as reference for knowing the possible effect of salinization in coastal aquifers under sea level rise scenarios driven by climate change.

Self-assembly and adsorption of cetyltrimethylammonium bromide and didodecyldimethylammonium bromide surfactants at the mica-water interface

The self-assembly and adsorption of the surfactants cetyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) at the muscovite mica-water interface are studied using molecular-dynamics simulations. Adsorption takes place by an ion-exchange mechanism, in which K+ ions are replaced by the organic alkylammonium cations from the solution. Simulations are performed with and without the surface K+ ions, with pure water, and with the surfactants in aqueous solution. CTAB and DDAB form micellar structures in bulk solution, and in the absence of the surface K+ ions, they quickly adsorb and form bilayer structures. The bilayer ordering of CTAB is not perfect, and there is a competition with the formation of cylindrical micelles. DDAB, on the other hand, forms a well-ordered bilayer structure, with the innermost layer showing strong orientational ordering, and the outermost layer being more disordered. The simulations with pure water highlight the molecular ordering and strong electrostatic interactions with the mica-surface atoms. Using simulated scattering length density profiles, the results are compared directly and critically with existing neutron reflectivity measurements. The simulation results are generally consistent with experiments, and yield new insights on the molecular-scale ordering at the mica-water interface.

Synthesis of Ultrathin Biotite Nanosheets as an Intelligent Theranostic Platform for Combination Cancer Therapy

Biotite, also called black mica (BM), is a group of sheet silicate minerals with great potential in various fields. However, synthesis of high-quality BM nanosheets (NSs) remains a huge challenge. Here, an exfoliation approach is provided that combines calcination, n-butyllithium exchange and intercalation, and liquid exfoliating processes for the high-yield synthesis of ultrathin BM NSs. Due to the presence of MgO, Fe2O3, and FeO in these NSs, PEGylated BM can be engineered as an intelligent theranostic platform with the following unique features: i) Fe3+ can damage the tumor microenvironment (TME) through glutathione consumption and O2 production; ii) Generated O2 can be further catalyzed by MgO with oxygen vacancy to generate ·O2 -; iii) The Fe2+-catalyzed Fenton reaction can produce ·OH by disproportionation reactions of H2O2 in the TME; iv) Reactions in (i) and (iii) circularly regenerate Fe2+ and Fe3+ for continuous consumption of glutathione and H2O2 and constant production of ·OH and O2; v) The NSs can be triggered by a 650 nm laser to generate ·O2 - from O2 as well as by an 808 nm laser to generate local hyperthermia; and vi) The fluorescent, photoacoustic, and photothermal imaging capabilities of the engineered NSs allow for multimodal imaging-guided breast cancer treatment.

Interfacial Layering and Screening Behavior of Glyme-Based Lithium Electrolytes

Understanding of electrical double layers is essential to all electrochemical devices, particularly at high charge carrier concentrations. Using a combined approach (surface force apparatus, zeta potential, infrared spectroscopy), we propose a model for the interfacial structure of triglyme electrolytes on muscovite mica. In contact with the pure triglyme, a brush-like polymeric structure grows on the mica surface. When lithium triflate is present in the triglyme, this structure is suppressed by anion adsorption and an extended double layer is formed. A surprising result of great fundamental significance is that the effective screening length measured by surface force apparatus at considerable lithium triflate concentrations (above 0.2 M) is substantially higher than expected from the Debye-Hückel theory. This suggests a high degree of complex salt association as a novel characteristic feature of salt-containing electrolytes.

An Anionic Surfactant on an Anionic Substrate: Monovalent Cation Binding

Neutron reflectometry has been used to study the adsorption of the anionic surfactant bis(2-ethylhexyl) sulfosuccinate cesium salt on the anionic surface of mica. Evidence of significant adsorption is reported. The adsorption is reversible and changes little with pH. This unexpected adsorption behavior of an anionic molecule on an anionic surface is discussed in terms of recent models for surfactant adsorption such as cation bridging, where adsorption has been reported with the divalent ion calcium but not previously observed with monovalent ions.

Correlation of natural muscovite exfoliation with interlayer and solvation forces

Natural muscovite exfoliation was correlated with interlayer and solvation forces respectively in this work.

Preparation of monolayer muscovite through exfoliation of natural muscovite

Monolayer nanosheets were exfoliated from natural muscovite.

Dibenzo crown ether layer formation on muscovite mica

Stable layers of crown ethers were grown on muscovite mica using the potassium-crown ether interaction. The multilayers were grown from solution and from the vapor phase and were analyzed with atomic force microscopy (AFM), matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, and surface X-ray diffraction (SXRD). The results show that the first molecular layer of the three investigated dibenzo crown ethers is more rigid than the second because of the strong interaction of the first molecular layer with the potassium ions on the surface of muscovite mica. SXRD measurements revealed that for all of the investigated dibenzo crown ethers the first molecule lies relatively flat whereas the second lies more upright. The SXRD measurements further revealed that the molecules of the first layer of dibenzo-15-crown-5 are on top of a potassium atom, showing that the binding mechanism of this layer is indeed of the coordination complex form. The AFM and SXRD data are in good agreement, and the combination of these techniques is therefore a powerful way to determine the molecular orientation at surfaces.

Salts drive controllable multilayered upright assembly of amyloid-like peptides at mica/water interface

Surface-assisted self-assembly of amyloid-like peptides has received considerable interest in both amyloidosis research and nanotechnology in recent years. Despite extensive studies, some controlling factors, such as salts, are still not well understood, even though it is known that some salts can promote peptide self-assemblies through the so-called "salting-out" effect. However, they are usually noncontrollable, disordered, amorphous aggregates. Here, we show via a combined experimental and theoretical approach that a conserved consensus peptide NH2-VGGAVVAGV-CONH2 (GAV-9) (from representative amyloidogenic proteins) can self-assemble into highly ordered, multilayered nanofilaments, with surprising all-upright conformations, under high-salt concentrations. Our atomic force microscopy images also demonstrate that the vertical stacking of multiple layers is highly controllable by tuning the ionic strength, such as from 0 mM (monolayer) to 100 mM (mainly double layer), and to 250 mM MgCl2 (double, triple, quadruple, and quintuple layers). Our atomistic molecular dynamics simulations then reveal that these individual layers have very different internal nanostructures, with parallel β-sheets in the first monolayer but antiparallel β-sheets in the subsequent upper layers due to their different microenvironment. Further studies show that the growth of multilayered, all-upright nanostructures is a common phenomenon for GAV-9 at the mica/water interface, under a variety of salt types and a wide range of salt concentrations.

Measurement of interaction forces between fibrinogen coated probes and mica surface with the atomic force microscope: The pH and ionic strength effect

The study of protein-surface interactions is of great significance in the design of biomaterials and the evaluation of molecular processes in tissue engineering. The authors have used atomic force microscopy (AFM) to directly measure the force of attraction/adhesion of fibrinogen coated tips to mica surfaces and reveal the effect of the surrounding solution pH and ionic strength on this interaction. Silica colloid spheres were attached to the AFM cantilevers and, after plasma deposition of poly(acrylic acid), fibrinogen molecules were covalently bound on them with the help of the cross-linker 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in the presence of N-hydroxysulfosuccinimide (sulfo-NHS). The measurements suggest that fibrinogen adsorption is controlled by the screening of electrostatic repulsion as the salt concentration increases from 15 to 150 mM, whereas at higher ionic strength (500 mM) the hydration forces and the compact molecular conformation become crucial, restricting adsorption. The protein attraction to the surface increases at the isoelectric point of fibrinogen (pH 5.8), compared with the physiological pH. At pH 3.5, apart from fibrinogen attraction to the surface, evidence of fibrinogen conformational changes is observed, as the pH and the ionic strength are set back and forth, and these changes may account for fibrinogen aggregation in the protein solution at this pH.

Adsorption of alpha-chymotrypsin onto mica in laminar flow conditions. Adsorption kinetic constant as a function of tris buffer concentration at pH 8.6

We examined the adsorption kinetics of alpha-chymotrypsin (pH 8.6, 10(-2) to 0.5 M Tris buffer) on muscovite mica in conditions of laminar flow through a slit. The range of buffer concentrations is between two limits: (i) no adsorption in 1 M Tris and (ii) no desorption in 10(-3) M Tris. Studying the dependence of adsorption kinetics on the wall shear rate leads to the determination of the interfacial adsorption kinetic constant ka and the diffusion coefficient. The obtained value for the diffusion coefficient is close to the one expected from the molecular size of alpha-chymotrypsin. The interfacial adsorption kinetic constant of alpha-chymotrypsin decreases when ionic strength increases, while the initial desorption constant (over a part of all the adsorbed population) shows the contrary. Although alpha-chymotrypsin is almost at its isoelectric point, the effect of ionic strength on the adsorption kinetics suggests the importance of electrostatic interactions between the protein and mica. We observed an increase in the adsorption rate, at a surface coverage near 0.14 microg cm(-2), for adsorption in 10(-2) M Tris and the low wall shear rates (<300 s(-1)). This change in the adsorption rate suggests a structural transition, that we assume again to be due to electrostatic interactions, but between proteins. The large dipole moment of the protein may induce this transition, illustrated here by the ferroelectric/antiferroelectric pattern. The variation of the zeta potential with interfacial concentration seems to be in agreement with such a model.

Structure and phase transitions of alkyl chains on mica

We use molecular dynamics as a tool to understand the structure and phase transitions [Osman, M. A.; et al. J. Phys. Chem. B 2000, 104, 4433-4439. Osman, M. A.; et al. J. Phys. Chem. B 2002, 106, 653-662] in alkylammonium micas. The consistent force field 91 is extended for accurate simulation of mica and related minerals. We investigate mica sheets with 12 octadecyltrimethylammonium (C(18)) ions or 12 dioctadecyldimethylammonium (2C(18)) ions, respectively, as single and layered structures at different temperatures with periodicity in the xy plane by NVT dynamics. The alkylammonium ions reside preferably above the cavities in the mica surface with an aluminum-rich boundary. The nitrogen atoms are 380-390 pm away from the superficial silicon-aluminum plane. With increasing temperature, rearrangements of C(18) ions on the mica surface are found, while 2C(18) ions remain tethered due to geometric restraints. We present basal-plane spacings in the duplicate structures, tilt angles of the alkyl chains, and gauche-trans ratios to analyze the chain conformation. Agreement with experimental data, where available, is quantitative. In C(18)-mica with less than 100% alkali-ion exchange, the disordered C(18) rods in the island structures [Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913-5917] break at 40 degrees C. At 60 degrees C, the headgroups of the C(18) alkyl chains rearrange on the mica surface, and the broken chain backbones assume a coillike structure. The C(18)-mica obtained on fast cooling of this phase is metastable due to slow reverse rearrangements of the headgroups. In 2C(18)-mica with 70-80% ion exchange, the alkali ions are interspersed between the alkyl chains, corresponding to a single phase on the surface. The observed phase transition at approximately 53 degrees C involves an increase of chain disorder (partial melting) of the 2C(18) ions without significant rearrangements on the mica surface. We propose a geometric parameter lambda for the saturation of the surface with alkyl chains, which determines the preferred self-assembly pattern, that is, islands, intermediate, or continuous. lambda allows the calculation of tilt angles in continuous layers on mica or other surfaces. The thermal decomposition seems to be a Hofmann elimination with mica as a base-template.

Electron and scanning force microscopy studies of alterations in supercoiled DNA tertiary structure

The configuration of supercoiled DNA (scDNA) was investigated by electron microscopy and scanning force microscopy. Changes in configuration were induced by varying monovalent/divalent salt concentrations and manifested by variation in the number of nodes (crossings of double helical segments). A decrease in the concentration of monovalent cations from 50 mM to approximately 1 mM resulted in a significant change of apparent configuration of negatively supercoiled DNA from a plectonemic form with virtually approximately 15 nodes (the value expected for molecules of approximately 3000 bp) to one or two nodes. This result was in good agreement with values calculated using an elastic rod model of DNA and salt concentration in the range of 5-50 mM. The effect did not depend on the identity of the monovalent cation (Na(+), K(+)) or the nature of the support used for electron microscopy imaging (glow-discharged carbon film, polylysine film). At very low salt concentrations, a single denatured region several hundred base-pairs in length was often detected. Similarly, at low concentrations of divalent cations (Mg(2+), Ca(2+), Zn(2+)), scDNA was apparently relaxed, although the effect was slightly dependent on the nature of the cation. Positively supercoiled DNA behaved in a manner different from that of its negative counterpart when the ion concentration was varied. As expected for these molecules, an increase in salt concentration resulted in an apparent relaxation; however, a decrease in salt concentration also led to an apparent relaxation manifested by a slight decrease in the number of nodes. Scanning force microscopy imaging of negatively scDNA molecules deposited onto a mica surface under various salt conditions also revealed an apparent relaxation of scDNA molecules. However, due to weak interactions with the mica surface in the presence of a mixture of mono/divalent cations, the effect occurred under conditions differing from those used for electron microscopy. We conclude that the observed changes in scDNA configuration are inherent to the DNA structure and do not reflect artifacts arising from the method(s) of sample preparation.

Determination of the Cation-Exchange Capacity of Muscovite Mica

High cation-exchange capacity (CEC) muscovite mica with a homoionic surface was prepared by replacing the Li(+) surface ions of partially delaminated Li-mica with K(+). The CEC of this K-mica was determined by exchanging its surface cations with Cs(+), NH(+)(4), methylene blue (MB(+)), and copper triethylenetetramine [Cu(trien)(2+)]. The kinetics of these exchange reactions were studied and showed large differences depending on their relative affinities to mica. The NH(4)(+)/K(+) exchange was slow, while the Cs(+) and Cu(trien)(2+)/K(+) exchange was fast. The MB(+)/K(+) exchange was quite slow and was not completed even after 99 h. Insufficient reaction time is one of the main reasons for the contradictory results reported in the literature for the CEC of aluminosilicates obtained by different methods. The CEC of mica can be photometrically measured by exchanging its surface cations with Cu(trien)(2+). Copyright 2000 Academic Press.

Dodecyl Pyridinium/Alkali Metals Ion Exchange on Muscovite Mica

Model muscovite with high cation exchange capacity was prepared, the surface of which was saturated with a single species of alkali metal ions. The kinetics and thermodynamics of the exchange reaction of the 1-dodecylpyridinium ion (NDP+) with alkali metal ions onto that substrate were studied. The exchange rate depended on the type of alkali metal ions present on the surface because of their different affinities to mica. However, in all cases the reaction was fast at the beginning and about 50% of the ions were exchanged within one hour; then the reaction rate decreased and equilibrium was only reached after several hours. This was attributed to a rate-determining rearrangement step in which the alkyl chains rearrange to adopt a dense packing. The reactivity of the alkali metal ions was in the order Li+ > Na+ > K+ > Rb+, Cs+, and in the case of K+, Rb+, and Cs+, equilibrium was only reached after 72 h. The lithium and sodium ions were exchanged almost quantitatively until a saturation value was nearly reached, while the K+, Rb+, and Cs+ exchange isotherms were less steep. The equilibrium constants (K) as well as the ion exchange capacity (S) were calculated by least-squares fits. Since K is infinite for quantitative exchange and decreases asymptotically upon deviation from this ideal behavior, the high K values (>10) of the NDP+/Li+ and Na+ exchange cannot be accurately determined. K ranges between 1 and 3 for the NDP+/K+, Rb+, and Cs+ exchange. The affinity of NDP+ to muscovite was similar or slightly higher than that of K+, Rb+, and Cs+, but was much higher than that of Li+ and Na+. The presence of oxonium ions in water did not strongly influence the exchange reaction on delaminated mica, as in the case of mica sheets, due to its high CEC. Copyright 1999 Academic Press.