Generation and stability of bulk nanobubbles: A review and perspective

Bulk nanobubbles (BNBs) are submicron gaseous domains dispersed in solutions, which are supposed to survive for several hours or even days. In recent years, there has been a rapid growth in the research and extraordinary applications of BNBs. Conventional theories based on gas diffusion and Laplace pressure, however, predicted that nanoscale gas bubbles in water should dissolve within microseconds, presenting a modern-day paradox in current nanobubbles researches. Also, it is still challenging to efficiently produce BNBs and determine their gaseous nature with the available techniques. In this review, we start from a general introduction and brief history of nanobubbles researches and revisit the current progress on the generation methods and detection techniques. Two possible formation mechanisms are suggested, and the plausibility of the proposed theories on BNBs stability is discussed with some suggestions for future studies on bulk nanobubbles.     

Differentiating bulk nanobubbles from nanodroplets and nanoparticles

History has shown that it is not as easy as one might think to differentiate between bulk nanobubbles and nanodroplets or nanoparticles. It is generally easy to detect colloids (i.e. something that looks different, e.g. scatters light differently than its surrounding solvent), but less easy to determine the nature of these colloids. This has led to misinterpretations in the literature, where nanodroplets or nanoparticles have mistakenly been assumed to be nanobubbles. In this paper, we review a multitude of experimental methods and approaches to prove the existence of bulk nanobubbles. We conclude that combinations of optical detection with physical perturbations such as pressure or ultrasound, or phase-sensitive holographic methods are the most promising and convenient approaches.     

Spontaneous chemical reactions between hydrogen and oxygen in nanobubbles

Bulk nanobubbles (NBs) generated electrochemically by short voltage pulses of alternating polarity behave differently from those produced by regular methods. Only bubbles smaller than 200 nm are formed in the process, and their concentration is very high. Moreover, the bubbles containing both H₂ and O₂ gases disappear fast via the combustion reaction, although the reaction in such a small volume cannot happen according to the classical combustion theory. Experimental facts about these unusual NBs are reviewed, and current understanding of the observed phenomena is provided. Visualization methods of a cloud of NBs above the electrodes are briefly discussed. Experimental signatures demonstrating the reaction between the gases in NBs are considered. A surface-assisted mechanism proposed for the combustion reactions in restricted volumes with a high surface-to-volume ratio is discussed. It is explained how the same mechanism is able to describe the explosion of microbubbles that is observed at special conditions.     

Role of additives in the water vapor transport through block co-poly(amide/ether) membranes: effects on surface and bulk polymer properties

In the last years there has been a growing industrial interest in modifying the performance of traditional polymers by using additives, working as modifiers for processing, rheological, transport, bonding, and pigmentation properties. This work was focused on the understanding of the relationships between chemical structure and water vapour transport through a polymer matrix modified by different additives regarding hydrophilicity and molecular structure. A screening of the changes in surface energies and bulk morphology, as a function of the chemical nature and weight percent modifier, allowed estimating the effects on the water vapour transport through polymer membranes. Static and dynamic contact angle measurements explained the difference in surface wettability and affinity to polar species such as water molecules. Modifiers having polar groups improved the surface hydrophilicity, enhancing the breathability of the membranes, while hydrophobic components such as aromatic structures led to a reduction of the water vapour mass uptake onto the membrane surface. On the other hand, thermal analyses showed a tendency of the polymer structure to reduce its own mobility with consequent slowdown of the diffusion through polymer matrix. Modification with large and bulky structures disrupted the polymer packing density, but simultaneously increased the stiffness of the polymer chains, inhibiting the penetrant migration. As a result, balancing the effects due to modifier polarity and bulky structure, it is possible to change the performance of a polymer in terms of transport, going from breathable membranes to barrier films.     

Accessing the spatiotemporal heterogeneities of single nanocatalysts by optically imaging gas nanobubbles

Catalysts that catalyze the generation of products in the gas phase, especially those involved in the hydrogen evolution reaction (HER), hold great promise for ecofriendly and sustainable energy development. In general, gas chromatography is widely used to measure catalytic activity. Unfortunately, it gives an averaged output that washes out the heterogeneities among individuals. To assess the unique catalytic properties at the single nanoparticle level, various methods based on single particle catalysis have been proposed. Over the past fifteen years, tremendous breakthroughs have been achieved, which uncovered hidden spatial and temporal heterogeneities. Although powerful, effectively quantifying the activities of single HER nanocatalysts remains challenging because of the fast diffusion of hydrogen (H₂). In 2017, a novel approach based on a nanobubble indicator was proposed to correlate the kinetics of gas bubble evolution with the catalytic activities of individual nanoentities during the HER process. Since then, a plethora of optical microscopy techniques have been utilized to monitor dynamically evolved nanobubbles and to measure the catalytic activities of single HER catalysts. In this minireview, we summarized state-of-the-art optical microscopy for in operando imaging of dynamic nanobubbles involved in gas-generating reactions while highlighting some important discoveries, including the blinking photocatalytic activity and heterogeneous distribution of active sites. Finally, challenges and future perspectives in this promising field were identified.     

Impact of bulk and surface properties of some biocompatible hydrophobic polymers on the stability of methylene chloride-in-water mini-emulsions used to prepare nanoparticles by emulsification-solvent evaporation

The emulsifying and stabilizing ability of several hydrophobic (insoluble in water and soluble in volatile organic solvents) polymers, such as Eudragit RL, Eudragit RS, PLGA, PCL, and their mixtures, with regard to the methylene chloride (MC)-in-water mini-emulsions, has been compared to the viscosity of MC solutions and to the properties of adsorption and spread monolayers of these polymers.

Eudragits RS and RL contain 2.5 and 5 mol% of pendent cationic trimethylammonium (TMA) groups per 164 g/mol segments, whereas PLGA and PCL contain 1 and 2 polar carbonyl groups per 130 and 114 g/mol, respectively. The electrostatic attraction between the dipoles, formed by TMA groups and the condensed counter ions in the MC solutions, leads to the contraction of macromolecular coils of Eudragits, whereas the PLGA and PCL macromolecules, interacting by low polar carbonyl groups (with dipole moment μ = 2.7 D) retain more extended conformation in MC. This explains why the characteristic viscosities [η] of MC solutions are much lower for the former polymers (0.1 dL/g) with regard to PLGA and PCL solutions whose [η] is equal to 0.3 and 0.6 dL/g, respectively.

The ionization of TMA groups in contact with the water phase leads to the irreversible adsorption of Eudragits at the MC/water interface and to high decrease of the interfacial tension γ (down to 4 mN/m for the 5% MC solutions). Whereas PLGA and PCL possessing low polar carbonyl groups adsorb poorly at the MC/water interface exhibiting γ  28 mN/m. Higher stability of spread monolayers of Eudragits (π*  40 mN/m) with regard to PLGA and PCL (π* < 20 mN/m) correlates well with higher interfacial activity of the former with regard to the later. The higher surface potential ΔV of Eudragits (0.9 V) with regard to PLGA (0.3 V) and PCL (0.4 V) is explained by the formation of electric double layer (DL) by the former, whereas the later contribute to the ΔV only by cumulative dipole moments of carbonyl groups. The experimental values of surface potentials correlate well with the Gouy–Chapman model of the DL and the Helmholtz model of the monolayer.

The ensemble of experimental results leads to the conclusion that higher emulsifying and stabilizing ability of Eudragits with regard to PLGA and PCL is due to higher adsorption activity of the former which form the corona of polymeric chains with ionized TMA groups around the droplets. It can be postulated that Eudragit polymers have good surface active properties which may allow manufacturing of biocompatible nanoparticles by emulsification–solvent evaporation method without surfactants.     

Nanobubbles and nanoparticles

Gas saturated solutions have attracted great attention in the past two decades with reports of stable nanobubbles in solutions. The fundamental interest focus arises from the surprising stability which opens up a wide range of potential applications where the interactions between particles and nanobubbles are important. Here, we review the current state of knowledge on systems involving both nanobubbles and nanoparticles. As nanoparticles and nanobubbles are found together in many circumstances, particularly those involving applications of nanobubbles, knowledge of these systems is important. This includes examining the formation of nanoparticles from nanobubbles, the nucleation of nanobubbles from nanoparticles, and the interactions between nanobubbles and nanoparticles. It is clear that further work is required to more fully understand these systems, in particular on the problem of nanobubble nucleation and nanobubble–nanoparticle interactions at the submicron scale.     

Bulk and surface modifications of polylactide

This article reviews various methods of modifying the bulk and surface properties of poly(lactic acid) (PLA) so that the polymer may be used as a drug carrier in a drug delivery system (DDS) and as a cell scaffold in tissue engineering. Copolymerization of lactide with other lactone-type monomers or monomers with functional groups such as malic acid, copolymerization of lactide with macromolecular monomer such as poly(ethylene glycol) (PEG) or dextran, as well as blending polylactide and natural derivatives and other methods of bulk modification are discussed. Surface modifications of PLA-type copolymers, such as surface coating, chemical modification, and plasma treatment are described. Cell culture technology proves the efficiency of bulk and surface modification and the potential application of PLA in tissue engineering.     

Effect of surface hydrophobicity distribution on retention of ribonucleases in hydrophobic interaction chromatography

The effect of surface hydrophobicity distribution of proteins on retention in hydrophobic interaction chromatography (HIC) was investigated. Average surface hydrophobicity as well as hydrophobic contact area between protein and matrix were estimated using a classical thermodynamic model. The applicability of the model to predict protein retention in HIC was investigated on ribonucleases with similar average surface hydrophobicity but different surface hydrophobicity distribution. It was shown experimentally that surface hydrophobicity distribution could have an important effect on protein retention in HIC. The parameter "hydrophobic contact area," which comes from the thermodynamic model, was able to represent well the protein retention in HIC with salt gradient elution. Location and size of the hydrophobic patches can therefore have an important effect on protein retention in HIC, and the hydrophobic contact area adequately describes this.     

Protein surface area and retention in hydrophobic interaction chromatography

Summary Molecular surface areas accessible to a 4 ? diameter spherical probe were calculated from crystallographic data for five proteins: α-chymotrypsinogen A, lysozyme, trypsinogen, ribonuclease A and ribonuclease S. The retention factors of various proteins were measured on stationary phases having polyether- and phenylligates and with aqueous eluents containing (NH₄)₂SO₄, Na₂SO₄ or NaCl at pH 7.0. The logarithmic retention factors were plotted against the salt molality and the hydrophobic interaction parameters evaluated from the limiting slopes of the plots at high salt concentrations for the proteins in the chromatographic systems investigated. The hydrophobic interaction parameters thus obtained were linear in both the molecular surface areas of the proteins and the molal surface tension increments of the salts. The experimental results obtained with these relatively simple proteins of known molecular structure, which were available in high purity, support earlier theoretical predictions for the dependence of the hydrophobic interaction parameter on the surface area of the protein and the surface tension raising effect of the salt.     

In situ preparation and surface modification of magnesium hydroxide nanoparticles

Hydrophobic magnesium hydroxide (Mg(OH)₂) nanoparticles were successfully synthesized via a one-step solution precipitation method with octadecyl dihydrogen phosphate (n-C₁₈H₃₇OPO₃H₂, ODP) as a surface modifier. The ODP was used to control the growth of crystal in the c direction (direction perpendicular to the layers) and to modify the surface property of the Mg(OH)₂ particles produced from the precipitation. Measurements of relative contact angle and active ratio indicated that Mg(OH)₂ samples were hydrophobic. The samples were characterized by using field emission scanning electron microscope (SEM), X-ray diffraction (XRD), infrared analysis (IR). The mechanism of influence of ODP on the surface property and morphology of Mg(OH)₂ was discussed. Furthermore, from the results of the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) tests, it was confirmed that small amount of magnesium alkyl phosphate on the surface of Mg(OH)₂ particles enhanced the thermal stability of the low density polyethylene(LDPE)/Mg(OH)₂.     

The role of ligands on protein retention in adsorption chromatography: A surface energetics approach

Protein adsorption onto hydrophobic chromatographic supports has been investigated using a colloid theory surface energetics approach. The surface properties of commercially available chromatographic beads, Toyopearl Phenyl 650‐C, and Toyopearl Butyl 650‐C, have been experimentally determined by contact angle and zeta potential measurements. The adsorption characteristics of these beads, which bear the same backbone matrix but harbor different ligands, have been studied toward selected model proteins, in the hydrated as well as dehydrated state. There were two prominent groups of proteins observed with respect to the chromatographic supports presented in this work: loosely retained proteins, which were expected to have lower average interaction energies, and the strongly retained proteins, which were expected to have higher average interaction energies. Results were also compared and contrasted with calculations derived from adsorbent surface energies determined by inverse liquid chromatography. These results showed a good qualitative agreement, and the interaction energy minima obtained from these extended Derjaguin, Landau, Verwey and Overbeek calculations were shown to correlate well with the experimentally determined adsorption behavior of each protein.     

Influence of polypropylene membrane surface porosity on the performance of membrane distillation process

Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below 1 μm) caused that the air permeability was reduced from 1.365 to 0.863 dm³/m² s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO₃ deposit formed during distillation of tap water. This deposit was removed every 30–70 h by rinsing the modules with a 2–5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores.     

Atomistic modeling of surface and bulk properties of Cu, Pd and the Cu–Pd system

The BFS method for alloys is applied to the study of the Cu–Pd system. A variety of issues are analyzed and discussed, including the properties of pure Cu or Pd crystals (surface energies, surface relaxations), Pd/Cu and Cu/Pd surface alloys, segregation of Pd (or Cu) in Cu (or Pd), concentration dependence of the lattice parameter of the high temperature fcc CuPd solid solution, the formation and properties of low temperature ordered phases, and order–disorder transition temperatures. Emphasis is made on the ability of the method to describe these properties on the basis of a minimum set of BFS universal parameters that uniquely characterize the Cu–Pd system.     

Determination of Exemestane in bulk and pharmaceutical dosage form by HPTLC

An HPTLC method for analysis of Exemestane in bulk and pharmaceutical formulation has been established and validated. The analyte was separated on aluminium plates precoated with silica gel 60 F₂₅₄. The mobile phase was chloroform:methanol 9.2:0.8 (v/v). Quantification was done by densitometric scanning at 247 nm. Response was a linear function of Exemestane concentration in the range of 100–500 μg mL⁻¹. The limit of detection and quantification for Exemestane were 5.8 and 17.58 μg mL⁻¹, respectively. Average recovery of Exemestane was 100.1, which shows that the method was free from interference from excipients present in the formulation. The established method enabled accurate, precise, and rapid analysis of Exemestane in bulk as well as pharmaceutical formulation.     

Effects of interface mobility on the dynamics of colliding bubbles

At its core, the outcome of the collision between air bubbles is determined by the hydrodynamic interaction forces, which in turn are strongly dependent on the tangential mobility of the gas–liquid interfaces. A clean gas–liquid interface is tangentially mobile, whereas the presence of surfactant contaminants can immobilise the interface. Bubbles with mobile surfaces coalescence much easier because of the low hydrodynamic resistance to drainage of the thin liquid film separating the colliding bubbles. In this opinion, we highlight recent experimental and numerical simulations demonstrating that in addition to the expected faster coalescence, mobile-surface bubbles can produce a much stronger rebound from a mobile liquid interface compared to an immobile one. The stronger rebound is explained by the lower viscous dissipation during collisions involving mobile surfaces. The role of the surface mobility in controlling the stability of gas or liquid emulsion should be reassessed in the light of these new findings.     

富锂正极材料的衰减机理及循环稳定性提升的研究进展

富锂正极材料xLi2MnO3·(1-x)LiMO2(M=Ni,Co,Mn等,0＜x＜1)具有容量高(可达300 mA·h/g以上)、成本低的巨大优势,被誉为是可能的最为重要的下一代锂离子电池正极材料,受到了各国的高度重视和广泛研究.目前,这种材料尚存在初始(首圈)库仑效率低、循环性能差、电压衰减严重等问题,严重阻碍了材...     

Ab initio study of phase transition and bulk modulus of NaH

The phase transition of NaH from NaCl- to CsCl-type structure is investigated by an ab initio plane-wave pseudopotential density functional theory method with the norm-conserving pseudopotential scheme in the frame of the generalized gradient approximation correction; the isothermal bulk modulus and its first and second pressure derivatives of the NaCl- and CsCl-type structures under high pressure and temperature are obtained through the quasi-harmonic Debye model. The phase transition obtained from the usual condition of equal enthalpies occurs at the pressure of 32 GPa, which is consistent with the experimental and other calculated values. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of cell volume V and lattice constant a on temperature T at zero pressure, the isothermal bulk modulus B₀ and its pressure derivatives B₀′and B₀″ on pressure P along isotherms 0, 300, and 600 K, are also successfully obtained.     

Transport and separation of carboxylic acids through bulk liquid membranes containing tributylamine

In this study, transport and separation of carboxylic acids (formic, acetic, propionic, and butyric acids) from their aqueous solutions through bulk liquid membrane (BLM) containing tributylamine (TBA) and the parameters affecting the transport were investigated. The influence of the parameters on the separation process such as the stirring of membrane phase, the stripping phase type and concentration, the feed phase type, and the feed:membrane:stripping phase ratio (F/M/S phase ratio) were examined. In the experiments, 10% (w/w) acid solutions (formic, acetic, propionic, and butyric acids) were used as the feed phase, different concentrations of NaOH solutions within the range of 0–2?N were used as the stripping phase, and 0.5?mol/lt TBA, dissolved in oleyl alcohol, was used as the membrane phase. It was determined that the stirring of the membrane phase increases the transport of acids. In the case of 2 N NaOH solution in the stripping phase and F/M/S phase ratio 1:2:1 gave the best recovery (96.75%) for butyric acid. It was observed that BLM was an effective technique for the separation of carboxylic acids from aqueous solutions.