Bulk Aqueous Solution Research Articles

Measurement of Eu and Yb in aqueous solutions by underwater laser induced breakdown spectroscopy

In this paper, we report the use of laser induced breakdown spectroscopy (LIBS) to detect dissolved Eu and Yb in bulk aqueous solutions. Ten strong emission lines of Eu and one strong emission line of Yb were identified in the underwater LIBS spectra obtained by using Czerny–Turner spectrometer within the wavelength range of 375–515nm. Temporal evolution of plasma and the effect of laser pulse energy on the spectral emission were studied. Calibration curves using the concentration range from 500 to 10,000ppm were developed and limits of detection for Eu and Yb were estimated to be 209 and 156ppm, respectively.

Rationally Turning the Interface Activity of Mesoporous Silicas for Preparing Pickering Foam and "Dry Water".

We develop a novel protocol to prepare smart, gas/water interface-active, mesoporous silica particles. This protocol involves modification of highly mesoporous silicas with a mixture of hydrophobic octyl organosilane and hydrophilic triamine organosilane. Their structure and compositions are characterized by transmission electron microscopy (TEM), N2 sorption, solid state NMR, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TGA), and elemental analysis. It is demonstrated that our protocol enables the interface activity of mesoporous silica particles to be facilely tuned, so that the stable gas-water interfaces ranging from air bubbles dispersed in water (Pickering foam) and water droplets dispersed in air ("dry water") can be achieved, depending on the molar ratio of these two organosilanes. The "dry water" is not otherwise attainable for the analogous nonporous silica particles, indicting the uniqueness of the chosen mesoporous structures. Moreover, these particle-stabilized Pickering foams and "dry waters" can be disassembled in response to pH. Interestingly, it was found that aqueous potassium carbonate droplets stabilized by these interface-active mesoporous silica particles ("dry K2CO3-containing water") could automatically capture CO2 from a simulated flue gas with enhanced adsorption rate and adsorption capacity when compared to the aqueous potassium carbonate bulk solution. This study not only supplies a novel type of efficient, smart, gas/water interface-active mesoporous silica particles but also demonstrates an innovative application of mesoporous materials in gas adsorption.

Prototropism and dynamics of an anticancer drug in reverse micelles: Focus on the variation of pH in reverse micelles having w0 ≥ 10

The modulations of the photophysics and dynamics of an archetypal anticancer β-carboline drug, namely, harmane (HM) within AOT reverse micelles (RMs) have been explored. To this end, particular focus is given on a pH-dependent study commensurate with the effect of increasing water content within the RM interior. Our results reveal markedly different photophysical behavior of the drug encapsulated within the RM compared to that in bulk aqueous medium. For example, our results show the presence (or absence) of the cationic (or neutral) emission of HM in RMs in pH values apparently much higher (or lower) than that of pure aqueous medium. The characteristic zwitterionic emission in strongly alkaline bulk aqueous solutions (e.g., pH12.2 and 13.5) is also found to be lacking within the reverse micellar core, even in RMs with w0≥10. These data are further substantiated from the study of the dynamical aspects of the interaction scenario, that is, modulation of the fluorescence decay and rotational relaxation behaviors of HM entrapped within the RMs. Cumulatively, our results indicate the presence of a proton gradient across the reverse micellar water pool in which the interfacial regime appears to be more acidic in comparison to the central core. The results with alkaline solutions (e.g., pH10.0, 12.2 and 13.5) suggest selective compartmentalization of the hydroxide ions leaving the effective pH of the water pool lower than that of the bulk aqueous solution.

Molecular Recognition Under Interfacial Conditions: Calix[4]pyrrole-Based Cross-linkable Micelles for Ion Pair Extraction.

An anthracene-functionalized, long-tailed calix[4]pyrrole 1, containing both an anion-recognition site and cation-recognition functionality, has been synthesized and fully characterized. Upon ion pair complexation with FeF2, receptor 1 self-assembles into multimicelles in aqueous media. This aggregation process is ascribed to a change in polarity from nonpolar to amphiphilic induced upon concurrent anion and cation complexation and permits molecular recognition-based control over chemical morphology under interfacial conditions. Photoirradiation of the micelles serves to cross-link the anthracene units thus stabilizing the aggregates. The combination of ion pair recognition, micelle formation, and cross-linking can be used to extract FeF2 ion pairs from bulk aqueous solutions. The present work helps illustrate how molecular recognition and self-assembly may be used to control the chemistry of extractants at interfaces.

Photochemical Formation of Nitrite and Nitrous Acid (HONO) upon Irradiation of Nitrophenols in Aqueous Solution and in Viscous Secondary Organic Aerosol Proxy.

Irradiated nitrophenols can produce nitrite and nitrous acid (HONO) in bulk aqueous solutions and in viscous aqueous films, simulating the conditions of a high-solute-strength aqueous aerosol, with comparable quantum yields in solution and viscous films (10-5-10-4 in the case of 4-nitrophenol) and overall reaction yields up to 0.3 in solution. The process is particularly important for the para-nitrophenols, possibly because their less sterically hindered nitro groups can be released more easily as nitrite and HONO. The nitrophenols giving the highest photoproduction rates of nitrite and HONO (most notably, 4-nitrophenol and 2-methyl-4-nitrophenol) could significantly contribute to the occurrence of nitrite in aqueous phases in contact with the atmosphere. Interestingly, dew-water evaporation has shown potential to contribute to the gas-phase HONO levels during the morning, which accounts for the possible importance of the studied process.

Calcium chloride adsorption at liquid-liquid interfaces: A molecular dynamics simulation study

We carried out molecular dynamics simulations (MD) to investigate the adsorption of calcium chloride (CaCl2) at n-hexane-water interfaces. We also measured the interfacial tensions (IFT) of the selected systems making use of the pendant-drop method. The histograms of hexane, water, and the ions indicate an electrical double layer (EDL) near the interface. The trend of the EDL indicates that chloride anions intend to adsorb to the interface more intrinsically than calcium cations. The measured interfacial width of the n-hexane-water interfaces decreases with the salt concentration. The average densities of the interfacial and bulk aqueous solutions demonstrate density heterogeneity in the direction perpendicular to the interface. The calculated density profiles enabled us to measure the adsorption values of each component. It is found that the salt particles are repelled from the liquid-liquid interface, resulting in an enriched bulk solution with salt. We, therefore, report negative adsorptions of CaCl2 and the consequent reproduced increasing IFT with salt concentration. Consistent with the EDL trends, the negative adsorption of calcium cations is higher than that of chloride anions.

Hydrophobicity of carbohydrates and related hydroxy compounds

The hydrophobic interaction of carbohydrates and other hydroxy compounds with a C18-modified silica gel column was measured with pure water as eluent, thereby expanding the range of measurements already published. The interaction is augmented by structure strengthening salts and decreasing temperature. Although the interaction of the solute with the hydrophobic interface is expected to only imperfectly reflect its state in aqueous bulk solution, the retention can be correlated to hydration numbers calculated from molecular mechanics studies given in the literature. No correlation can be established towards published hydration numbers obtained by physical methods (isentropic compressibility, O-17 NMR relaxation, terahertz spectroscopy, and viscosity). The hydrophobicity is discussed with respect to the chemical structure. It increases with the fraction and size of hydrophobic molecular surface regions.

Proton-Transfer Mechanisms at the Water-ZnO Interface: The Role of Presolvation.

The dissociation of water is an important step in many chemical processes at solid surfaces. In particular, water often spontaneously dissociates near metal oxide surfaces, resulting in a mixture of H2O, H+, and OH- at the interface. Ubiquitous proton-transfer (PT) reactions cause these species to dynamically interconvert, but the underlying mechanisms are poorly understood. Here, we develop and use a reactive high-dimensional neural-network potential based on density functional theory data to elucidate the structural and dynamical properties of the interfacial species at the liquid-water-metal-oxide interface, using the nonpolar ZnO(101̅0) surface as a prototypical case. Molecular dynamics simulations reveal that water dissociation and recombination proceed via two types of PT reactions: (i) to and from surface oxide and hydroxide anions ("surface-PT") and (ii) to and from neighboring adsorbed hydroxide ions and water molecules ("adlayer-PT"). We find that the adlayer-PT rate is significantly higher than the surface-PT rate. Water dissociation is, for both types of PT, governed by a predominant presolvation mechanism, i.e., thermal fluctuations that cause the adsorbed water molecules to occasionally accept a hydrogen bond, resulting in a decreased PT barrier and an increased dissociation rate as compared to when no hydrogen bond is present. Consequently, we are able to show that hydrogen bond fluctuations govern PT events at the water-metal-oxide interface in a way similar to that in acidic and basic aqueous bulk solutions.

IKMTSL-PTE, a Phospholipid-Based EPR Probe for Surface Electrostatic Potential of Biological Interfaces at Neutral pH: Effects of Temperature and Effective Dielectric Constant of the Solvent.

The synthesis and characterization of a lipidlike electrostatic spin probe, (S)-2,3-bis(palmitoyloxy)propyl 2-((4-(4-(dimethylamino)-2-ethyl-1-oxyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-2-yl)benzyl)disulfanyl)ethyl phosphate (IKMTSL-PTE), are being reported. The intrinsic pKa0 of IKMTSL-PTE was determined by X-band (9.5 GHz) electron paramagnetic resonance (EPR) titration of a water-soluble model compound, 4-(dimethylamino)-2-ethyl-2-(4-(((2-hydroxyethyl)disulfanyl)methyl)phenyl)-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl (IKMTSL-ME), an adduct of methanethiosulfonate spin label IKMTSL and 2-mercaptoethanol. The pKa0 of IKMTSL-ME in bulk aqueous solutions was found to be significantly higher than that of 4-(((2-hydroxyethyl)disulfanyl)methyl)-2,2,3,5,5-pentamethylimidazolidin-1-oxyl (IMTSL-ME), an adduct of the corresponding methanethiosulfonate spin label IMTSL and 2-mercaptoethanol (17 °C, pKa0 = 6.16 ± 0.03 vs 20 °C, pKa0 = 3.33 ± 0.03, respectively). A series of EPR titration experiments with IKMTSL-ME in aqueous solutions containing 0-60% v/v isopropanol have been carried out at 17 and 48 °C to determine the effects of temperature and bulk dielectric permittivity constant, ε, on the probe pKa. A linear relationship between the probe pKa and ε has been established and found to be essentially the same at 17 and 48 °C. The polarity term contributing to the pKa of IKMTSL-PTE at an uncharged lipidlike interface was determined by incorporating the probe into electrically neutral micelles formed from nonionic detergent Triton X-100, and it was found, similar to IMTSL-PTE, to be negative. In negatively charged DMPG lipid bilayers, IKMTSL-PTE exhibits ionization transitions with significantly higher pKa values than those previously reported for IMTSL-PTE (e.g., at 17 °C, pKai = 7.80 ± 0.03 vs pKa0 = 5.70 ± 0.05). The surface electrostatic potentials of DMPG lipid bilayers calculated using IKMTSL-PTE titration data were found to be somewhat lower than those calculated using IMTSL-PTE. The lower values measured by IKMTSL-PTE are the likely consequences of the structure of the linker that positions the reporter nitroxide further away from the bilayer plane into aqueous phase. Overall, the ionization transitions of IKMTSL-PTE with pKa values close to the neutral pH range make this lipidlike molecule a valuable spectroscopic EPR probe for studying the electrostatic phenomena at biological interfaces, including lipid bilayer/membrane protein systems, that could be unstable in the acidic pH range accessible by the previously available probes.

Synergy of Miniemulsion and Solvothermal Conditions for the Low-Temperature Crystallization of Magnetic Nanostructured Transition-Metal Ferrites

Crystalline first-row transition-metal (Mn, Fe, Co, Ni, Cu, and Zn) ferrites were prepared by an unprecedented synergetic combination of miniemulsion synthesis and solvothermal route, pursuing unconventional conditions in terms of space confinement, temperature, and pressure. This synergy allowed for obtaining six different crystalline ferrites at much lower temperature (i.e., 80 °C) than usually required and without any postsynthesis thermal treatment. X-ray diffraction (XRD) revealed that analogous ferrites synthesized by miniemulsion at ambient pressure or in bulk (i.e., from an aqueous bulk solution and not in the confined space of the miniemulsion droplets) either at ambient pressure or under solvothermal conditions did not result in comparatively highly crystalline products. To follow the structural evolution at local level as a function of reaction time and depending on the synthesis conditions, X-ray absorption spectroscopy (XAS) was used to determine the cation distribution in these structures. W...

Glycine molecules in ionic liquid based reverse micelles: Investigation of structure and dynamics using molecular dynamics simulations

Abstract Amino acids, the building blocks of proteins, are probed in reverse micellar encapsulation processes of biological molecules. To achieve a theoretical picture of the effect of confinement of amino acids on the structure and dynamics of reverse micellar aqueous core, a water/[C 1 C 10 Im][Br] reverse micelles containing variable number of glycine molecules (0 to 8), dispersed in pure nonane has been studied using atomistic molecular dynamics simulations. The size of the central water pool was found to become stable with increase in the number of glycine molecules. Glycine molecules associate together through H-bonding interaction within the aqueous core of the reverse micelles. The radial density profile of glycine molecules from the center of mass of the reverse micellar aqueous core shows the preferential presence of glycine molecules near the center. Confinement in nano-space region stabilizes the H-bonds formed between glycine molecules compared to bulk aqueous solution. Increase in the number of glycine molecules within the reverse micellar aqueous core increases the survival time of H-bonds among glycine molecules as well as among water molecules. Translational and reorientational motion of glycine and water molecules are found to be directly related to the survival probability of intermolecular glycine–glycine and water–water H-bonds suggesting the correlation between the mobility of molecules with the rearrangement of the H-bonding network.

Magnetic iron oxide nanoparticles for the collection and direct measurement of adsorbed alpha-emitting radionuclides from environmental waters by liquid scintillation analysis.

Radioactive contamination, be it from accidental or intentional release, can create an urgent need to assess water and food supplies and the environment, and monitor human health. In the event of such an emergency, rapid and efficient methods may be needed to assess contamination levels in scores of samples within a short time frame. Internalized exposure to radionuclides that decay by alpha (α) emission can be especially hazardous, given the strongly ionizing nature of the α particle. Unfortunately, the determination of α-emitting radionuclides using traditional radioanalytical methods is typically labor and resource intensive and time consuming. In an effort to devise methods that are fast, require little labor and laboratory expendables, and minimize the use of toxic or corrosive reagents, researchers at PNNL have evaluated superparamagnetic nanoparticles as extracting agents for α-emitting radionuclides from chemically unmodified and acidified (pH 2) aqueous systems. It is demonstrated that bare magnetite nanoparticles exhibit strong affinity for two representative α-emitting radionuclides (241Am and 210Po) from two representative aqueous matrices (river and ground water). Furthermore, use of the superparamagnetic properties of these nanomaterials to concentrate the analyte-bearing solids from the bulk aqueous solution has been demonstrated. The nanoparticle concentrate can be either directly dispensed into a scintillation cocktail, or first dissolved and then added to a scintillation cocktail as a solution for an α-emission assay by liquid scintillation analysis. Despite the severe quenching caused by the metal oxide suspensions in the cocktail, the authors have demonstrated that modern liquid scintillation analyzers can report accurate α activity count rates; the upper limits of nanoparticle suspension concentrations in a cocktail are reported for cases wherein normal instrument count mode and a quench correction protocol are used. Discussions are provided on the presented sample processing and analysis method, the improvement (lowering) of minimum detectable activity concentrations using the nanoparticle-based assay method, and the quenching effects of nanoparticle suspensions in a scintillation cocktail.

Stability of Hairpin Structure of (CCG)4 Trinucleotide Repeats inside Amine-functionalized Silica Mesopores

Thermal stabilities of the hairpin structure of (CCG)4 trinucleotide repeats were examined in a bulk aqueous solution and inside amine-functionalized silica mesopores (pore diameters: 3.5 and 5.7 nm). The results confirmed that the denaturation temperatures of (CCG)4 hairpin within TMAP-MPSs were about 10 °C lower than those in bulk water. On the basis of enthalpy–entropy compensation, enthalpy change upon denaturation (ΔH) inside the pore was estimated to be 0.8 kcal mol−1 lower than that for the bulk system.

Nanoconfinement's Dramatic Impact on Proton Exchange between Glucose and Water.

Glucose nanoconfined by solubilization in water-containing AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles has been investigated using 1H NMR. NMR spectra reveal well-defined signals for the glucose hydroxyl groups that suggest slow chemical exchange between them and the water hydroxyl groups. Using the EXSY (ZZ-exchange) method, the chemical exchange rate from water to glucose hydroxyl groups was measured for glucose in reverse micelles as a function of size (water pool diameter of ∼1-5 nm) at 25 °C. The chemical exchange rates observed in the nanoconfined interior are dramatically slower (5-20 times) than those observed for glucose in bulk aqueous solution at the same concentration as the micelle interior. Exchange rate constants are calculated via a mechanism that accounts for these observations, and implications of these results are presented and discussed.

Micro- and Nano-Liquid Phases Coexistent with Ice as Separation and Reaction Media.

Ice has a variety of scientifically interesting features, some of which have not been reasonably interpreted despite substantial efforts by researchers. Most chemical studies of ice have focused on the elucidation of its physicochemical nature and its roles in the natural environment. Ice often contains impurities, such as salts, and in such cases, a liquid phase coexists with solid ice over a wide temperature range. This impure ice also acts as a cryoreactor, governing the circulation of chemical species of environmental importance. Reactions and phenomena occurring in this liquid phase show features different from those seen in normal bulk aqueous solutions. In the present account, we discuss the chemical characteristics of the liquid phase that develops in a frozen aqueous phase and show how novel analytical systems can be designed based on he features of the liquid phase which are predictable in some cases but unpredictable in others.

Asymmetric hydration structure around calcium ion restricted in micropores fabricated in activated carbons

The adsorbed phase and hydration structure of an aqueous solution of Ca(NO3)2 restricted in micropores fabricated in activated carbons (ACs) having different average pore widths (0.63 and 1.1 nm) were investigated with the analysis of adsorption isotherms and x-ray absorption fine structure (XAFS) spectra on Ca K-edge. The adsorbed density of Ca2+ per unit micropore volume in the narrower pore was higher than in the wider pore, while the adsorbed amount per unit mass of carbon with the narrower pore was half of the amount of ACs with the larger pore. On the other hand, variations in the bands assigned to double-electron (KMI) and 1s → 3d excitations in XAFS spectra demonstrate the formation of a distorted hydration cluster around Ca2+ in the micropore, although the structural parameters of hydrated Ca2+ in the micropores were almost consistent with the bulk aqueous solution, as revealed by the analysis of extended XAFS (EXAFS) spectra. In contrast to the hydration structure of monovalent ions such as Rb+, which generally presents a dehydrated structure in smaller than 1 nm micropores in ACs, the present study clearly explains that the non-spherically-symmetric structure of hydrated Ca2+ restricted in carbon micropores whose sizes are around 1 nm is experimentally revealed where any dehydration phenomena from the first hydration shell around Ca2+ could not be observed.

Effect of Nanoscale Confinement on the Crystallization of Potassium Ferrocyanide

Many crystallization processes of great significance in nature and technology occur in small volumes rather than in bulk solution. This article describes an investigation into the effects of nanoscale confinement on the crystallization of the inorganic compound potassium ferrocyanide, K4Fe(CN)6 (KFC). Selected for study due to its high solubility, rich polymorphism, and interesting physical properties, K4Fe(CN)6 was precipitated within controlled pore glasses (CPG) with pore diameters of 8, 48, and 362 nm. Remarkable effects were seen, such that although anhydrous potassium ferrocyanide was never observed on precipitation in bulk aqueous solution, it was the first phase to crystallize within the CPGs and was present for at least 1 day in all three pore sizes. Slow transformation to the metastable tetragonal polymorph of the trihydrate K4Fe(CN)6·3H2O (KFCT) then occurred, where this polymorph was stable for a month in 8 nm pores. Finally, conversion to the thermodynamically stable monoclinic polymorph of K...

Structural Implications on the Properties of Self-Assembling Supramolecular Hosts for Fluorescent Guests

Nine amphiphilic macromolecules with decyl and oligo(ethylene glycol) side chains, randomly distributed along a common poly(methacrylate) backbone, were synthesized from the radical copolymerization of appropriate methacrylate monomers. The resulting amphiphilic constructs differ in (1) the ratio between their hydrophobic and hydrophilic components, (2) the length of their oligo(ethylene glycol) chains, and/or (3) the molecular weight. When the ratio between hydrophobic and hydrophilic segments is comprised between 6:1 and 1:2, the macromolecules assemble spontaneously into particles with nanoscaled dimensions in neutral buffer and capture hydrophobic borondipyrromethene chromophores in their interior. However, the critical concentration required for the assembly of these supramolecular hosts as well as their hydrodynamic diameter, supramolecular weight, and number of constituent macromolecular building blocks all vary monotonically with the ratio between hydrophobic and hydrophilic components. Specifically, the critical concentration decreases and the other three parameters increase as the relative hydrophobic content raises. Furthermore, an increase in the relative hydrophobic content also discourages interchromophoric interactions between entrapped guests in both ground and excited states as well as delays access of potential quenchers. In fact, these observations demonstrate that the hydrophobic components must be in excess over their hydrophilic counterparts for optimal supramolecular hosts to assemble. Indeed, a ratio of 6:1 between the numbers of decyl and oligo(ethylene glycol) side chains appears to be ideal for this particular structural design. Under these conditions, supramolecular hosts assemble spontaneously even at relatively low polymer concentrations and their fluorescent guests do not escape into the bulk aqueous solution, despite the reversibility of the noncovalent interactions holding the supramolecular container together. Thus, these systematic investigations provide invaluable structural guidelines to design self-assembling supramolecular hosts with optimal composition for the effective encapsulation of fluorescent guests and can lead to ideal delivery vehicles for the transport of imaging probes to target locations in biological samples.

Ion-Specific and Thermal Effects in the Stabilization of the Gas Nanobubble Phase in Bulk Aqueous Electrolyte Solutions

Ion-stabilized nanobubbles in bulk aqueous solutions of various electrolytes were investigated. To understand the ion-specific mechanism of nanobubble stabilization, an approach based on the Poisson--Boltzmann equation at the nanobubble interface and in the near-surface layer was developed. It has been shown that the stabilization of nanobubbles is realized by the adsorption of chaotropic anions at the interface, whereas the influence of cosmotropic cations is weak. With increasing temperature, it should be accounted for by blurring the interface due to thermal fluctuations. As a result, the adsorbed state of ions becomes unstable: the nanobubble loses its stability and vanishes. This prediction was proven in our experiments. It turned out that in the case of liquid samples being kept in hermetically sealed ampules, where the phase equilibrium at the liquid-gas interface is fulfilled for any temperature, the volume number density of nanobubbles decreases with increasing temperature and this decrease is irreversible.

Complex Behavior of Aqueous α-Cyclodextrin Solutions. Interfacial Morphologies Resulting from Bulk Aggregation.

The spontaneous aggregation of α-cyclodextrin (α-CD) molecules in the bulk aqueous solution and the interactions of the resulting aggregates at the liquid/air interface have been studied at 283 K using a battery of techniques: transmission electron microscopy, dynamic light scattering, dynamic surface tensiometry, Brewster angle microscopy, neutron reflectometry, and ellipsometry. We show that α-CD molecules spontaneously form aggregates in the bulk that grow in size with time. These aggregates adsorb to the liquid/air interface with their size in the bulk determining the adsorption rate. The material that reaches the interface coalesces laterally to form two-dimensional domains on the micrometer scale with a layer thickness on the nanometer scale. These processes are affected by the ages of both the bulk and the interface. The interfacial layer formed is not in fast dynamic equilibrium with the subphase as the resulting morphology is locked in a kinetically trapped state. These results reveal a surprising complexity of the parallel physical processes taking place in the bulk and at the interface of what might have seemed initially like a simple system.