Supersaturated Aqueous Solutions Research Articles

Условия зарождения и стабильности объемных нанопузырьков

The thermodynamic analysis of the nucleation of bulk nanobubbles (BNB) in a supersaturated aqueous solution is carried out. In the model of a charged BNB with double electric layer (DEL), the minimum work of nucleation and the radius of the critical the embryo are calculated. Simple analytical expressions are derived, when the BNB will not dissolve in water, i.e. it will be stable. For the first time, the authors successful to explain the mystery of the long life times of the BNB.

Organic contaminants and atmospheric nitrogen at the graphene-water interface: a simulation study.

Ordered nanoscale patterns have been observed by atomic force microscopy at graphene–water and graphite–water interfaces. The two dominant explanations for these patterns are that (i) they consist of self-assembled organic contaminants or (ii) they are dense layers formed from atmospheric gases (especially nitrogen). Here we apply molecular dynamics simulations to study the behavior of dinitrogen and possible organic contaminants at the graphene–water interface. Despite the high concentration of N2 in ambient air, we find that its expected occupancy at the graphene–water interface is quite low. Although dense (disordered) aggregates of dinitrogen have been observed in previous simulations, our results suggest that they are stable only in the presence of supersaturated aqueous N2 solutions and dissipate rapidly when they coexist with nitrogen gas near atmospheric pressure. On the other hand, although heavy alkanes are present at only trace concentrations (micrograms per cubic meter) in typical indoor air, we predict that such concentrations can be sufficient to form ordered monolayers that cover the graphene–water interface. For octadecane, grand canonical Monte Carlo suggests nucleation and growth of monolayers above an ambient concentration near 6 μg m−3, which is less than some literature values for indoor air. The thermodynamics of the formation of these alkane monolayers includes contributions from the hydration free-energy (unfavorable), the free-energy of adsorption to the graphene–water interface (highly favorable), and integration into the alkane monolayer phase (highly favorable). Furthermore, the peak-to-peak distances in AFM force profiles perpendicular to the interface (0.43–0.53 nm), agree with the distances calculated in simulations for overlayers of alkane-like molecules, but not for molecules such as N2, water, or aromatics. Taken together, these results suggest that ordered domains observed on graphene, graphite, and other hydrophobic materials in water are consistent with alkane-like molecules occupying the interface.

Hydration breaking and chemical ordering in a levitated NaCl solution droplet beyond the metastable zone width limit: evidence for the early stage of two-step nucleation.

For over two decades, NaCl nucleation from a supersaturated aqueous solution has been predicted to occur via a two-step nucleation (TSN) mechanism, i.e., two sequential events, the formation of locally dense liquid regions followed by structural ordering. However, the formation of dense liquid regions in the very early stage of TSN has never been experimentally observed. By using a state-of-the-art technique, a combination of electrostatic levitation (ESL) and in situ synchrotron X-ray and Raman scatterings, we find experimental evidence that indicates the formation of dense liquid regions in NaCl bulk solution at an unprecedentedly high level of supersaturation (S = 2.31). As supersaturation increases, evolution of ion clusters leads to chemical ordering, but no topological ordering, which is a precursor for forming the dense disordered regions of ion clusters at the early stage of TSN. Moreover, as the ion clusters proceed to evolve under highly supersaturated conditions, we observe the breakage of the water hydration structure indicating the stability limit of the dense liquid regions, and thus leading to nucleation. The evolution of solute clusters and breakage of hydration in highly supersaturated NaCl bulk solution will provide new insights into the detailed mechanism of TSN for many other aqueous solutions.

One‐Step Generation of Core–Gap–Shell Microcapsules for Stimuli‐Responsive Biomolecular Sensing

AbstractThe versatile design of stimuli‐responsive microparticles embedding valuable biomolecules has great potential in a variety of engineering fields, such as sensors, actuators, drug delivery, and catalysis. Here, results are reported on thermoresponsive core–gap–shell (TCGS) microcapsules made of poly(N‐isopropylacrylamide) (PNIPAm), which encapsulate hydrophilic payloads in a simple and stable manner. These are realized by a one‐step microfluidic approach using the phase separation of a supersaturated aqueous solution of NIPAm. Various designs of the microcapsules are achieved by individual control of the swelling or by incorporating pH‐responsive comonomers of the inner core and outer shell. The gap, i.e., the space between the inner core and outer shell, can be loaded with cargo‐like nanoparticles. The outer shell can serve as a stimuli‐responsive gateway for the transport of smaller molecules from the external solution. It is shown that the TCGS microcapsules are suitable as temperature controllable glucose sensors and hold promise in the design of controllable enzymatic reactions. The proposed platform provides an avenue for developing a new‐generation of microparticles for diverse and efficient engineering applications.

Stability of the Supersaturated State of Encapsulated Solutions

By the example of highly concentrated water-in-oil emulsions, in which droplets of a supersaturated aqueous ammonium nitrate solution are dispersed in industrial oil, it has been shown that a thermodynamically stable state is possible due to the limited volume of dispersed phase particles.

Crystals of Benzamide, the First Polymorphous Molecular Compound, Are Helicoidal.

The growth of spontaneously twisted crystals is a common but poorly understood phenomenon. An analysis of the formation of twisted crystals of a metastable benzamide polymorph (form II) crystallizing from highly supersaturated aqueous and ethanol solutions is given here. Benzamide, the first polymorphic molecular crystal reported (1832), would have been the first helicoidal crystal observed had the original authors undertaken an analysis by light microscopy. Polymorphism and twisting frequently concur as they are both associated with high thermodynamic driving forces for crystallization. Optical and electron microscopies as well as electron and powder X-ray diffraction reveal a complex lamellar structure of benzamide form II needle-like crystals. The internal stress produced by the overgrowth of lamellae is shown to be able to create a twist moment that is responsible for the observed non-classical morphologies.

Crystals of Benzamide, the First Polymorphous Molecular Compound, Are Helicoidal

AbstractThe growth of spontaneously twisted crystals is a common but poorly understood phenomenon. An analysis of the formation of twisted crystals of a metastable benzamide polymorph (form II) crystallizing from highly supersaturated aqueous and ethanol solutions is given here. Benzamide, the first polymorphic molecular crystal reported (1832), would have been the first helicoidal crystal observed had the original authors undertaken an analysis by light microscopy. Polymorphism and twisting frequently concur as they are both associated with high thermodynamic driving forces for crystallization. Optical and electron microscopies as well as electron and powder X‐ray diffraction reveal a complex lamellar structure of benzamide form II needle‐like crystals. The internal stress produced by the overgrowth of lamellae is shown to be able to create a twist moment that is responsible for the observed non‐classical morphologies.

Inhalable formulations of rifampicin by spray drying of supersaturated aqueous solutions

Tuberculosis is still one of the leading causes of death from a single infectious agent (i.e. Mycobacterium tuberculosis). First line therapy includes per oral administration of high doses of rifampicin over several months and is often times accompanied by the occurrence of unwanted side effects that might limit the patient’s adherence to the therapy. Thus, local antibiotic treatment at the site of infection i.e. the lungs is desirable. Amongst other approaches, spray drying of solutions of rifampicin has been shown as suitable method to produce respirable dry powders. In this work, we present inhalable formulations manufactured via spray drying of aqueous solutions of rifampicin. Powders manufactured were characterized for their aerodynamic and solid state properties, as well as their physical and chemical stability. The main focus of this study was to investigate the mechanism of particle formation using an acoustic levitator. Fine particle fractions of the test formulations ranged from 80 to 89% whereas a reference formulation (a spray dried isopropyl alcoholic solution of rifampicin) showed a lower fine particle fraction of 37%. Acoustic levitator and surface tension experiments showed that interfacial properties of rifampicin lead to early crust formation upon drying of the droplets, which eventually decoupled from the liquid core and formed highly collapsed, low apparent density powders with excellent aerosol properties.

Rapid hydration mechanism of carbonic acid and cement

Early-age carbonation is an attractive option to reduce the net CO2 emissions of cementitious materials. Applying early-age carbonation to the most common cementitious material, ready-mix concrete, is difficult as the process must be performed in atmosphere. A technique that could supply a large quantity of CO2 to cement in atmosphere is by mixing powdered cement with a super-saturated aqueous solution. However, this would create an acidic solution. This paper addresses the competing effects of dissolved CO2 ions and carbonic acid when mixed with cement paste or mortar. While CO2 is expected to form carbonates, carbonic acid usually decreased the strength of cement. The cement replacement fraction, and the strength and porosity of the original mix alter the relative effect of carbonic acid on compressive strength. The most likely cause of strength and workability losses are identified as a very rapid but limited hydration in the first 10 min after mixing.

Molecular Dynamic Simulation of D-Mannitol Polymorphs in Solid State and in Solution Relating With Spontaneous Nucleation

In this article, the polymorphic nucleation of D-mannitol was studied in depth by molecular simulating the solid state of different forms and the aqueous solution both in unsaturated and supersaturated conditions. In our previous work, it was found that different polymorphs of D-mannitol could nucleate in various supersaturations. Here in this work, we try to use molecular dynamic simulation as a tool to explore the system to microscopically understand the phenomenon. First, the molecular structure of the 3 D-mannitol polymorphs in the solid unit cell is studied, and it is found that the molecular conformation directly affects the stability of the polymorphs. Then, the properties of D-mannitol molecules in unsaturated and supersaturated aqueous solutions are investigated. The results reveal that D-mannitol molecules are mostly monomers in unsaturated aqueous solution, while dimers start to show as the increase of concentration and also in the supersaturated aqueous solutions. Moreover, the form of the dimer varies with the degree of supersaturation, which might be the reason why the final crystal forms are different under different supersaturations. It seems that the I-dimers in supersaturated aqueous solutions is beneficial to the formation of the delta form of D-mannitol, and the T-dimers might be likely to form alpha crystal form, while the H-type dimers should be more favorable for to nucleate the beta form.

Molecular Dynamics Simulations of Crystal Nucleation from Solution at Constant Chemical Potential.

A widespread method of crystal preparation is to precipitate it from a supersaturated solution. In such a process, control of solution concentration is of paramount importance. The nucleation process, polymorph selection, and crystal habits depend crucially on this thermodynamic parameter. When performing molecular dynamics simulations with a fixed number of molecules in the canonical ensemble, crystal growth is accompanied by a decrease in the solution concentration. This modification of the thermodynamic condition leads to significant artifacts. Inspired by the recent development of the constant chemical potential molecular dynamics simulation method by Perego et al. [ J. Chem. Phys. 2015 , 142 , 144113 ] , we develop a spherical variant of it to study nucleation from solution. Our method allows determining the crystal nucleus size and nucleation rates at constant supersaturation. As an example, we study the homogeneous nucleation of sodium chloride from its supersaturated aqueous solution.

Pulsed Laser-Induced Nucleation of Sodium Chlorate at High Energy Densities

We report on a study of laser-induced nucleation (LIN) of sodium chlorate in supersaturated aqueous solutions using focused nanosecond laser pulses at high energy densities (420 kJ cm–2). On irradiation with a single laser pulse, optical breakdown was observed in the form of a luminous plasma, and numerous microbubbles were produced. On the basis of the observations, we estimate the energy threshold for optical breakdown in the solutions to be 70 J cm–2. Remarkably, even at high energy densities, single laser pulses produced on average only one or two crystals. The mean number of crystals obtained was 1.5 (532 nm) and 1.8 (1064 nm) per sample (3 cm3). The effect of left circularly polarized (LCP) and right circularly polarized (RCP) light on the nucleation of dextrorotatory (d) versus levorotatory (l) enantiomorphs of cubic (phase I) sodium chlorate crystals was investigated. No significant correlation between the helicity of circular polarization and the chirality of enantiomorph was observed. The result...

Combining hydration and carbonation of cement using super-saturated aqueous CO2 solution

Carbonation of cementitious materials is an attractive option to reduce the environmental costs of concrete production. Firstly, carbonation can strengthen cementitious materials and therefore reduce the use of cement. Secondly, carbonation can sequester carbon dioxide (CO2) economically on a large scale. However, the major obstacle to widespread implementation of the technique is that the hydration reaction of cement occurs much faster than the carbonation reaction in atmosphere. Therefore, novel mechanisms for the delivery of CO2 are required. In this study, water used for cement hydration was replaced by a super-saturated solution of CO2 to bypass the dissolution step in the carbonation reaction of cement, and increase the carbonation rate of young age cement. Cement made with saturated CO2 aqueous solution instead of regular water was observed to generate a 1-day strength 6% higher than a pure cement pastes and 26% higher than a cement with a 12% limestone replacement fraction.

Growth, structure, thermal, electrical and optical properties of potassium aluminum sulfate dodecahydrate (potash alum) single crystal

Single crystal of potassium aluminum sulfate dodecahydrate (potash alum) was grown by the slow evaporation technique of supersaturated aqueous solution at room temperature. The obtained crystal was characterized by using powder X-ray diffraction, DSC, TGA, electrical conductivity, optical transmittance and optical reflectance measurements. Structural and thermal analyses were performed on potash alum crystal. The electrical conductivity of potash alum as electrolyte sample was 1.894 × 10−6 S/cm. The optical transition and optical band gap energy of potash alum crystal were studied for the first time. Direct and indirect optical band gap energies were 5.75 and 4.50 eV respectively. Extinction coefficient, refractive index, real and imaginary dielectric constants and optical conductivity were calculated for potash alum crystal at a range of photon energies between 1.4 and 6.2 eV. Normal dispersion parameters of potash alum crystal were calculated for the first time.

State of Oxygen Molecules in Aqueous Supersaturated Solutions.

The state of oxygen in aqueous supersaturated solutions prepared by different methods was studied using high-resolution ultrasonic spectroscopy in combination with other techniques. This allowed for nondestructive evaluation of the properties of oxygen solute particles, composed of oxygen molecules and surrounding (coordinating) molecules of water, at equilibrium, supersaturated conditions, and different temperatures and concentrations of O2. The results were compared with the behaviors of other types of solutes in water, including H2O2, which has similar molecular size and mass to O2 but is characterized by a significantly different type of interaction with water molecules. Additionally, theoretical modeling was performed to assess the ultrasonic characteristics of dispersions of oxygen nanobubbles stabilized by a surface electrical charge. The obtained data indicate a clathrate-like organization of water in the coordination shells of single molecules of O2. We did not find any signs of formation of clusters of oxygen molecules in supersaturated solutions. No quantifiable presence of oxygen nanobubbles in the solutions was detected. The state of O2 molecules was not affected by supersaturation within the analyzed concentration range of oxygen. The results also demonstrated the potential of the ultrasonic technique in precision real-time nondestructive monitoring of oxygen solubilization and outgassing processes.

Precipitation of Calcium Carbonate (CaCO3) in Water–Monoethylene Glycol Solutions

The influence of monoethylene glycol (MEG) on the precipitation process of calcium carbonate at constant supersaturation, 25 °C, and pH 8.5 was investigated. MEG aqueous solutions supersaturated with respect to all calcium carbonate polymorphs were used since they are often encountered in oil production processes where they are used as a hydrate inhibitor. Both seeded and unseeded experiments were done in aqueous supersaturated calcium carbonate solutions containing MEG over the concentration range 0–80% v/v. The presence of MEG affected the induction times and the subsequent rates of precipitation in unseeded, spontaneous precipitation of calcium carbonate. At low values of the supersaturation ratio (SRcalcite = 77.62–15 135.61 or σcalcite = 7.81–122) and for MEG concentrations in the range 30–80% v/v, the solutions were stable for 4 days. At lower than 30% v/v MEG concentrations, calcium carbonate precipitation took place past induction periods between 160 and 500 min. The morphological examination of the precipitates showed that in the presence of low MEG concentrations (10–20% v/v) the precursor vaterite was stabilized. The homogeneous nucleation and growth of calcium carbonate in the absence and in the presence of MEG was also investigated by the inoculation of stable supersaturated solutions with quartz and calcite seed crystals. In both cases nucleation and growth of calcium carbonate was significantly reduced with increasing MEG concentration in the solutions.

Insight into the Mechanisms of Scale Inhibition: A Case Study of a Task‐Specific Fluorescent‐Tagged Scale Inhibitor Location on Gypsum Crystals

AbstractScaling in reverse osmosis facilities, boilers, heat exchangers, evaporation plants, and oilfield applications is a serious problem worldwide. In order to provide a new insight into the mechanism of the scale formation and inhibition, a novel fluorescent‐tagged bisphosphonate scale inhibitor 1‐hydroxy‐7‐(6‐methoxy‐1,3‐dioxo‐1H‐benzo[de]isoquinolin‐2(3H)‐yl)heptane‐1,1‐diyl‐di(phosphonic acid), (HEDP−F) was synthesized and used for fluorescent microscope visualization of gypsum crystal formation in supersaturated aqueous solutions. The visualization of HEDP−F location on gypsum crystals has demonstrated that the bisphosphonate molecules do not act as they are expected to do according to the current scale inhibition theory. At ambient temperature the gypsum macrocrystals are found to form, and then to grow without visible sorption of bisphosphonate on the crystal edges or any other gypsum crystal growth centers. A tentative nonconventional mechanism of scale inhibition in the bulk supersaturated aqueous solutions of gypsum is proposed.

Influence of wettability on pressure-driven bubble nucleation: A potential method for dissolved gas separation

Bubble nucleation in supersaturated solutions is important for a wide range of natural and industrial processes. In this systematic experimental investigation, the effect of wettability on pressure-driven CO2 bubble nucleation from its aqueous supersaturated solution is studied. Glass surfaces of various degrees of hydrophobicities were prepared using respective surface chemical treatments with chlorinated polydimethylsiloxane (CM), chlorinated fluoroalkylmethylsiloxane (CF), and (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane (HT) for the bubble nucleation experiments. Contact angle and atomic force microscopy (AFM) measurements were conducted to quantify the wettability and surface roughness of the treated and untreated glass substrates. The average air-water contact angles of untreated, CM, CF and HT treated substrate were 33.9° ± 0.4°, 90.8° ± 0.8°, 97.8° ± 0.4° and 114.6° ± 0.75°, respectively. The average roughness (Ra) values of the untreated and treated surfaces were in the range of 1.1–18.9 nm. Supersaturated solutions of CO2 in water were prepared in a glass vial placed in a pressure cell by reducing the pressure in 100 mbar steps from the initial saturation pressure of 6000 mbar. The untreated (hydrophilic) surface did not promote bubble nucleation even when the pressure was reduced to atmospheric pressure. In contrast, the average bubble nucleation onset pressures for CM, CF and HT treated glass vials were 4840 ± 182, 5260 ± 134 and 5560 ± 89 mbar, respectively. These onset pressures for bubble nucleation were further confirmed by conducting the bubble nucleation experiments using CM, CF, and HT treated glass beads in hydrophilic vials.

Insight into the roles of two typical ion clusters and their second hydration shells: Implication for the nucleation mechanism in MgSO4 aqueous solution

In this study, the ion association characteristics and the hydration dynamics of Mg2+ ions in MgSO4 aqueous solution at various concentrations were systematically investigated using density functional theory and classical molecular dynamics simulations. The results show that contact associated (CA) and solvent separated (SS) structures of Mg2+ and SO42− ions are the dominant species and play different roles in the crystallization and gelation processes of MgSO4 aqueous solution due to their distinctive association trend and the distinctive dynamics of their second hydration shells. The CA structures are more stable and the further aggregation of these clusters is sluggish in MgSO4 aqueous solution. The supersaturation or the formation of gel state can be partially attributed to the abundance of CA structures in supersaturated MgSO4 aqueous solution. In contrast, although the ion exchange in SS structures occurs frequently, the aggregation of SS structures is active and can be greatly promoted by increasing of the concentration. The dynamics of the second hydration shells around Mg2+ ions of SS structures also becomes more active as the concentration increases. The dynamic evolution characteristics of SS structures demonstrate their aggregation tendency for nucleation in the oversaturated MgSO4 aqueous solution, which helps to understand the nucleation mechanism in MgSO4 aqueous solution and the role of ion hydration dynamics in the nucleation process.

Molecular dynamics simulation of homogeneous nucleation of supersaturated potassium chloride (KCl) in aqueous solutions

Molecular dynamics (MD) simulation is used to investigate the mechanism of crystal nucleation of potassium chloride (KCl) in a supersaturated aqueous solution at 293 K and 1 atm.