Phenomena In Aqueous Solutions Research Articles

Weakly Hydrated Solute of Mixed Hydrophobic-Hydrophilic Nature.

Infrared (IR) spectroscopy is a commonly used and invaluable tool in studies of solvation phenomena in aqueous solutions. Concurrently, density functional theory calculations and ab initio molecular dynamics simulations deliver the solvation shell picture at the molecular detail level. The mentioned techniques allowed us to gain insights into the structure and energy of the hydrogen bonding network of water molecules around methylsulfonylmethane (MSM). In the hydration sphere of MSM, there are two types of populations of water molecules: a significant share of water molecules weakly bonded to the sulfone group and a smaller share of water molecules strongly bonded to each other around the methyl groups of MSM. The very weak hydrogen bond of water molecules with the hydrophilic group causes the extended network of water hydrogen bonds to be not "anchored" on the sulfone group, and consequently, the MSM hydration shell is labile.

Small Amphiphile-Based Coacervation.

Coacervation plays an important role in the molecular assembly towards soft materials with a diversity of function (e. g., underwater adhesives of mussels and membraneless organelles). Coacervation is observed when one homogenous solution spontaneously separates into two immiscible liquid phases of low and high solute concentration, also known as liquid-liquid phase separation (LLPS), which enables spatiotemporally local concentration of specific molecules. LLPS is a common physical phenomenon in aqueous solutions of polyelectrolytes, surfactants and biomolecules, which has been extensively explored for applications in the fields of environmental remediation, cosmetic formulation, protein purification, extractive fermentation and pharmaceutical microencapsulation. This review summarizes the development of LLPS with low molecular weight amphiphiles to construct simple and complex coacervates using conventional surfactants and novel amphiphiles such as azobenzene-derivatives and peptides. We also highlight the applications of these amphiphile coacervates in the extraction of biomolecules, construction of protocell models and drug delivery.

Determining the Efficiency of Photoelectrode Materials by Coupling Cavity‐Microelectrode Tips and Scanning Electrochemical Microscopy

AbstractPhotoelectrochemical water splitting (PEC‐WS) is a promising route to obtain hydrogen (and oxygen) from sunlight and water. However, too many semiconductors show poor stability, due to photodegradation phenomena in aqueous solutions, thus loosing efficiency under operative conditions. Aim of this paper is to introduce a simple and fast method for screening different semiconductor materials and identify their efficiency in H2 (or O2) production with respect to photocorrosion. This method could be used with any finely dispersed semiconductor (powder) for a fast, preliminary evaluation of the material's behaviour without interferences from the supporting material (i. e. FTO) or any binder. The method is based on the combination of scanning electrochemical microscopy (SECM) in the tip generation/substrate collection (TG/SC) mode and of cavity microelectrodes as SECM tips. Here, we show results obtained on three powder materials, namely core‐shell CuI/CuO, CuI and TiO2.

Highly Connected Metal-Organic Framework Constructed from Heptanuclear SBUs: Structure, Topology, and Fluorescence

The novel porous 3D metal-organic framework, [Cd9(BTA)6(H2O)5]·5H2O (1) (BTA = benzene-1,3,5-tri-β-acrylate), has been synthesized under hydrothermal condition. The X-ray crystallography revealed that the compound 1 pertains to an orthorhombic crystal system with space group Ibca. The MOF constructed from a centro-symmetric hexagonal wheel-like anionic cluster [Cd7(O2C)18]4‒ and linked by semi-rigid BTA ligands, leading to an exceptionally complicated topology with point symbol of (42⋅6)6 (436⋅6105⋅812). Furthermore, it shows excellent thermo-stability in aqueous surroundings. Fluorescence of the compound was evaluated under ultraviolet light irradiation at room temperature. The compound can perform as a highly selective and sensitive luminescence sensor toward metal ions Al3+ and Fe3+ based on the luminous enhancing or quenching phenomenon in aqueous solutions. Thus the new crystalline solid material can be expected to have broad practical applications in detecting, sensing or other related fields.

Исследование коллоидных явлений в водных растворах красителя свекольного красного

Исследование коллоидных явлений в водных растворах красителя свекольного красного

Immiscibility of Liquid Phases and Liquid–Liquid Critical Phenomena in Aqueous Solutions of Sodium and Potassium Phosphates

Phase equilibria were studied in aqueous solutions of sodium and potassium phosphates at elevated temperatures (to 425°C) and pressures (to 63 MPa). Experimental data were obtained on the transformation of gas–liquid equilibria into heterogeneous equilibria of two liquid phases with increasing pressure, which ends in the critical phenomena of immiscibility (L1 = L2). Based on the experimental and literature data, it was concluded that the binary water–salt systems containing KH2PO4, K2HPO4, K3PO4, NaH2PO4, and Na2HPO4 are characterized by phase diagrams of the first type [1, 2], which have two critical invariant end points: N (G–L1 = L2) and R (G = L1–L2).

Solvent‐Controlled Assembly of Aromatic Glutamic Dendrimers for Efficient Luminescent Color Conversion

AbstractA binary supramolecular system where self‐sorting and coassembly behavior can be switched by changing the solvent polarity is hereby reported. Glutamic dendron is separately conjugated with pyrene and naphthalimide luminophores through an alkyl spacer. The resulting structurally similar building units can self‐assemble into one‐dimensional micro/nanostructures with hexagonal and lamellar packing, respectively. Varying solvents from polar aqueous solution to nonpolar decane is evidenced to profoundly inverse the superchirality and switch self‐sorted assembly to coassembly of the two building blocks. The moisture sensitivity of the naphthalimide moiety is considered the primary driving force for the self‐sorting phenomenon in aqueous solution, resulting in inevitable hydration to repel its stacking with hydrophobic pyrene moiety. On the other hand, the naphthalimide unit can integrate segmentally with the pyrene unit in decane, greatly facilitating the nanofiber growth and supramolecular gel formation along with improved energy transfer efficiency between luminophores. As a result, the coassembly‐based thin films show efficient luminescent color conversion upon the UV light irradiation. This research presents a useful route for the fabrication of controllable solution‐processed light emitting devices from self‐assembled multicomponent systems.

Unusual Influence of Fluorinated Anions on the Stretching Vibrations of Liquid Water.

Infrared (IR) spectroscopy is a commonly used and invaluable tool in the studies of solvation phenomena in aqueous solutions. Concurrently, ab initio molecular dynamics (AIMD) simulations deliver the solvation shell picture at a molecular detail level and allow for a consistent decomposition of the theoretical IR spectrum into underlying spatial correlations. Here, we demonstrate how the novel spectral decomposition techniques can extract important information from the computed IR spectra of aqueous solutions of BF4- and PF6-, interesting weakly coordinating anions that have been known for a long time to alter the IR spectrum of water in an unusual manner. The distance-dependent spectra of both ions are analyzed using the spectral similarity method that provides a quantitative picture of both the spectrum of the solute-affected solvent and the number of solvent molecules thus altered. We find, in accordance with previous experiments, a considerable blue shift of the νOH stretching band of liquid water by 264 cm-1 for BF4- and 306 cm-1 for PF6-, with the affected numbers being 3.7 and 4.2, respectively. Considering also the additional information on solute-solvent dipolar couplings delivered by radially and spatially resolved IR spectra, the computational IR spectroscopy based on AIMD simulations is shown to be a viable predictive tool with strong interpretative power.

Interaction of Black Phosphorus with Oxygen and Water

Black phosphorus (BP) has attracted significant interest as a monolayer or few-layer material with extraordinary electrical and optoelectronic properties. However, degradation in air and other environments is an unresolved issue that may limit future applications. In particular the role of different ambient species has remained controversial. Here, we report systematic experiments combined with ab-initio calculations that address the effects of oxygen and water in the degradation of BP. Our results show that BP rapidly degrades whenever oxygen is present, but is unaffected by deaerated (i.e., O2 depleted) water. This behavior is rationalized by oxidation involving a facile dissociative chemisorption of O2, whereas H2O molecules are weakly physisorbed and do not dissociate on the BP surface. Oxidation (by O2) turns the hydrophobic pristine BP surface progressively hydrophilic. Our results have implications on the development of encapsulation strategies for BP, and open new avenues for exploration of phenomena in aqueous solutions including solution-gating, electrochemistry, and solution-phase approaches for exfoliation, dispersion, and delivery of BP.

Entropy Drives Calcium Carbonate Ion Association.

The understanding of the molecular mechanisms underlying the early stages of crystallisation is still incomplete. In the case of calcium carbonate, experimental and computational evidence suggests that phase separation relies on so-called pre-nucleation clusters (PNCs). A thorough thermodynamic analysis of the enthalpic and entropic contributions to the overall free energy of PNC formation derived from three independent methods demonstrates that solute clustering is driven by entropy. This can be quantitatively rationalised by the release of water molecules from ion hydration layers, explaining why ion association is not limited to simple ion pairing. The key role of water release in this process suggests that PNC formation should be a common phenomenon in aqueous solutions.

Influence of temperature, added electrolyte, and polymer molecular weight on the counterion-condensation phenomenon in aqueous solution of sodium polystyrenesulfonate: a scaling theory approach

Interactions in aqueous sodium polystyrenesulfonate (NaPSS) – NaCl solutions were studied conductometrically using a recent model based on scaling theory of polyions.

A fully atomistic computer simulation study of cold denaturation of a β-hairpin.

Cold denaturation is a fundamental phenomenon in aqueous solutions where the native structure of proteins disrupts on cooling. Understanding this process in molecular details can provide a new insight into the detailed natures of hydrophobic forces governing the stability of proteins in water. We show that the cold-denaturation-like phenomenon can be directly observed at low temperatures using a fully atomistic molecular dynamics simulation method. Using a highly optimized protein force field in conjunction with three different explicit water models, a replica exchange molecular dynamics simulation scheme at constant pressures allows for the computation of the melting profile of an experimentally well-characterized β-hairpin peptide. For all three water models tested, the simulated melting profiles are indicative of possible cold denaturation. From the analysis of simulation ensembles, we find that the most probable cold-denatured structure is structurally compact, with its hydrogen bonds and native hydrophobic packing substantially disrupted.

Molecular dynamics for the charging behavior of nanostructured electric double layer capacitors containing room temperature ionic liquids

The charging kinetics of electric double layers (EDLs) is closely related to the performance of a wide variety of nanostructured devices including supercapacitors, electro-actuators, and electrolyte-gated transistors. While room temperature ionic liquids (RTIL) are often used as the charge carrier in these new applications, the theoretical analyses are mostly based on conventional electrokinetic theories suitable for macroscopic electrochemical phenomena in aqueous solutions. In this work, we study the charging behavior of RTIL-EDLs using a coarse-grained molecular model and constant-potential molecular dynamics (MD) simulations. In stark contrast to the predictions of conventional theories, the MD results show oscillatory variations of ionic distributions and electrochemical properties in response to the separation between electrodes. The rate of EDL charging exhibits non-monotonic behavior revealing strong electrostatic correlations in RTIL under confinement.

Influence of Peroxynitrite in Gliding Arc Discharge Treatment of Alizarin Red S and Postdischarge Effects

The gliding arc discharge is a cheap and efficient nonthermal plasma technique able to degrade organic compounds dispersed in water at atmospheric pressure. Alizarin Red Sulfonate (ARS) is selected as a stable quinonic dye. Exposure of the dye solution to the discharge in a batch reactor induces two successive reaction steps according to the treatment conditions. Direct exposure of the solution to the discharge induces simultaneous bleaching and COD evolution. In postdischarge conditions, that is, after the discharge is switched off, the reactions keep on developing. This study thus underlines two key features: the ability of glidarc discharges to degrade recalcitrant molecules and the low cost of the process which requires short exposure times. A model mechanism involves peroxynitrite as a likely active species formed in the discharge and involved in postdischarge phenomena in aqueous solutions and suggests short exposure times and much longer postdischarge times for optimized pollutant abatement.

Gel Formation Driven by Tunable Hydrophobic Domain: Design of Acrylamide Macromonomer with Oligo Hydrophobic Segment

Nowadays, biomaterials with amphiphilic properties are undergoing remarkable development. Here, we present one such development, in which we prepared amphiphilic graft copolymers, with a main chain composed of hydroxyethyl acrylamide (HEAA), to introduce hydrophilicity, and a side chain composed of poly(trimethylene carbonate) (PTMC) to introduce tunable hydrophobicity. These macromonomers were created with a novel molecular design, which introduced a ring-opening polymerization by the hydroxyl end group of HEAA in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene, and were analyzed by (1)H NMR and gel permeation chromatography. The amphiphilic graft copolymers were shown to form a hydrogel, the swelling ratio of which was greatly influenced by the number of trimethylene carbonate units. These copolymers also exhibited the Tyndall phenomenon in aqueous solution; they aggregated spontaneously due to hydrogen bonding and hydrophobic interactions, and a sodium 8-anilino-1-naphthalenesulfonate (ANS) fluorescence probe was introduced into the hydrophobic domain. The solution property of ANS in the polymer solution was analyzed by fluorescence measurement and (1)H NMR. The maximum fluorescence wavelength of ANS shifted to shorter wavelengths as the degree of polymerization of the hydrophobic PTMC, the composition of the macromonomer, and the concentration of the copolymer increased. The resulting copolymer formed a polymer micelle structure due to the tunable hydrophobic domain formation in selected solvents. Therefore, these amphiphilic graft copolymers containing a PTMC segment are excellent candidates for use as hydrophobic drug delivery carriers.

Phase equilibria in chemical reactive fluid mixtures

Downstream processing is a major part of nearly all processes in the chemical industries. Most separation processes in the chemical (and related) industries for fluid mixtures are based on phase equilibrium phenomena. The majority of separation processes can be modelled assuming that chemical reactions are of no (or very minor) importance, i.e., assuming that the overall speciation remains unchanged during a separation process. However, there are also a large number of industrially important processes where the thermodynamic properties are influenced by chemical reactions. The phase equilibrium of chemical reactive mixtures has been a major research area of the author’s group over nearly 40 years. In this contribution, three examples from that research are discussed. The first example deals with the vapour phase dimerisation of carboxylic acids and its consequences on phase equilibrium phenomena and phase equilibrium predictions. The second example deals with the solubility of sour gases ( e.g., carbon dioxide and sulfur dioxide) in aqueous solutions of ammonia. That topic has been of interest for many years, e.g., in relation with the gasification and liquefaction of coal and, more recently, with the removal of carbon dioxide from flue gas in the “chilled ammonia process”. The third example deals with phase equilibrium phenomena in aqueous solutions of polyelectrolytes. It deals with the phenomenon of “counter ion condensation” and methods to model the Gibbs free energy of such solutions.

Study of the cloud point variation of amitriptyline hydrochloride solutions in presence of amines and amino acids

In this paper clouding phenomenon in aqueous solutions of an amphiphilic drug amitriptyline hydrochloride is reported. The effect of adding ethylamine, butylamine, hexylamine, heptylamine, octylamine, ethylenediamine, diethylenetriamine, tetraethylenepentramine, aniline, and benzylamine on the cloud point of solutions containing a fixed drug concentration (50 mM) was seen. All the amines decrease the cloud point by intercalating between the head groups with protonated amine groups at the micellar surface. The amino acids used (lysine, arginine, histidine) also behave similarly. The results are discussed in the light of a particular additive’s ability to remove water from the head group region of the drug micelles.

Kinetics of Mu addition to acetone in sub- and supercritical water

Abstract The rate constant for the addition of muonium to acetone in aqueous medium has been measured in sub- and supercritical conditions at 250 bar in the temperature range 100–400 °C using the transverse field muon spin rotation (TF-μSR) technique. At temperatures up to ≈200 °C, the reaction shows the expected Arrhenius behaviour. However above ≈250 °C, the rate constant goes through a maximum then drops rapidly between 300 and 400 °C. This adds to a list of similar observations made for muonium reacting with very different substrates, indicating that the decrease of the value of the rate constant with increasing temperature above supercritical conditions may be a general phenomenon in aqueous solution. Allowing muonium to react with both the keto and enol form of acetone, each with a different rate constant ( k O and k E , respectively), and using various constraints on the kinetic parameters of the system, an empirical fit to the present data provides an estimate of K E , the enolization equilibrium constant of acetone. At 350 °C, one finds k O ≈9×10 7 M −1 s −1 , 1.0×10 10 k E 10 M −1 s −1 and 0.02 K E

Aggregation Phenomena in Aqueous Solutions of Uncharged Star Polymers with a Porphyrin Core

Uncharged star polymers having a porphyrin core and four poly(ethylene glycol)methyl ether (PEGME) arms have been synthesized and investigated (through the study of the structural and dynamical properties) as a function of the molecular mass of the arms, in different thermodynamic conditions. Molecules with short PEGME arms are insoluble in water, whereas in the presence of long PEGME chains, the star polymers are water-soluble; for the intermediate length of the arms, the building up of large rigid monodisperse aggregates, driven by the diffusion limit aggregation mechanism, has been observed by means of static and dynamic light scattering. These aggregated systems, whose sizes are modulated by changing the pH and triggered by the chain flexibility, can be well described in terms of the DLVO interparticle potential in the frame of the colloidal theory.

ATR‐FTIR studies on the effect of strong salting‐out salts on the phase separation scenario in aqueous solutions of poly(N‐isopropylacrylamide) [PNIPA

AbstractAttenuated total reflectance (ATR) FTIR spectroscopy was used to investigate the effect of two strong salting‐out salts (Na2SO4 and K2SO4) on the temperature‐induced phase separation process in aqueous solutions of poly(N‐isopropylacrylamide) [PNIPA]. It was found that the addition of these salts not only depresses the phase transition temperature (as was pointed out by other groups in the past) but, in fact, changes the whole scenario below, at, and above the aggregation temperature. Deconvolution of the amide II peak into three sub‐bands reflecting three types of populations that differ by their hydrogen bonding types revealed that the sulfate ions decreased the amount of amide–water hydrogen bonds, while increasing the amount of amide–amide intramolecular H‐bond population. This led to a relatively compact structure even at low temperatures. Based on the behavior of the hydrophobic IR peaks it is suggested that addition of sulfate ions increases the importance of hydrophobic interactions below the phase separation temperature. This study demonstrates the potential of ATR‐FTIR in providing information at a molecular level on phase separation phenomena in aqueous solutions of hydrophobic polymers and, in particular, on the effects of co‐solutes in such systems. Copyright © 2003 John Wiley & Sons, Ltd.