Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles

Mineral trioxide aggregate in membrane form as a barrier membrane in guided bone regeneration

Background/purpose

In the field of conservative dentistry and endodontics, mineral trioxide aggregate (MTA), commonly used, possesses advantages such as biocompatibility, antimicrobial properties and osteogenic potential. This study investigated the feasibility of utilizing membrane form mineral trioxide aggregate (MTA) as a barrier membrane in guided bone regeneration (GBR) procedures.

Materials and methods

Membranes were electrospun from three different formulations: 15 w/v% Polycaprolactone (PCL), 13 w/v% PCL + 2 w/v% MTA (2MTA), and 11 w/v% PCL + 4 w/v% MTA (4MTA). Physicochemical and mechanical properties of the electrospun membrane were compared, encompassing parameters such as surface morphology, fiber diameter distribution, chemical composition, phase identification, tensile stress, pH variation, and water contact angle. Moreover, the antimicrobial properties against of the electrospun membranes were assessed through direct exposure to streptococcus aureus (S. aureus) and candida albicans (C. albicans). Additionally, on the 7th day, biocompatibility and cell attachment were investigated with respect to L929 (fibroblast) and MC3T3 (pre-osteoblast) cells. Inhibition of L929 cell infiltration and the expression of osteogenic related genes including osteocalcin (OCN), alkaline phosphatase (ALP), and runt related transcription factor 2 (RUNX2) in MC3T3 cells on 7th and 14th days were also investigated.

Results

PCL, 2MTA, and 4MTA exhibited no statistically differences in fiber diameter distribution and tensile stress. However, as the MTA content increased, wettability and pH also increased. Due to the elevated pH, 4MTA demonstrated the lowest viability S.aureus and C.albicans. All membranes were highly biocompatibility and promoted cell attachment, while effectively preventing L929 cell infiltration. Lastly 4MTA showed increase in OCN, ALP, and RUNX2 expression on both 7th and 14th day.

Conclusion

The membrane form MTA possessed characteristics essential for a novel barrier membrane.

Ultra-stable pickering emulsion stabilized by anisotropic pea protein isolate-fucoidan conjugate particles through Maillard reaction

Bio-based emulsifiers hold significant importance in various industries, particularly in food, cosmetics, pharmaceuticals and other related fields. In this study, pea protein isolate (PPI) and fucoidan (FUD) were conjugated via the Maillard reaction, which is considered safe and widely used in the preparation of food particle. The PPI-FUD conjugated particles exhibit an anisotropic non-spherical structure, thereby possessing a high detachment energy capable of preventing emulsion coalescence and Ostwald ripening. Compared to emulsions previously prepared in other studies (< 500 mM), the Pickering emulsion stabilized by PPI-FUD conjugate particles demonstrates outstanding ionic strength resistance (up to 5000 mM). Furthermore, when encapsulating curcumin, the Pickering emulsion protects the curcumin from oxidation. Additionally, the formulated emulsions demonstrated the capability to incorporate up to 60 % (v/v) oil phase, revealing remarkable performance in terms of storage stability, pH stability, and thermal stability.

Sequential Approach for Water Purification Using Seashell-Derived Calcium Oxide through Disinfection and Flocculation with Polyphosphate for Chemical Pollutant Removal

Safe water supply is usually inadequate in areas without water treatment plants and even in a city under emergency conditions due to a disaster, even though safe water is essential for drinking and other various purposes. The purification of surface water from a river, lake, or pond requires disinfection and removal of chemical pollutants. In this study, we report a water purification strategy using seashell-derived calcium oxide (CaO) via disinfection and subsequent flocculation with polyphosphate for chemical pollutant removal. Seashell-derived CaO at a concentration (2 g L-1) higher than its saturation concentration caused the >99.999% inactivation of bacteria, mainly due to the alkalinity of calcium hydroxide (Ca(OH)2) produced by hydration. After the disinfection, the addition of sodium polyphosphate at 2 g L-1 allowed for the flocculation of CaO/Ca(OH)2 particles with adsorbing chemical pollutants, such as Congo red, dichlorodiphenyltrichloroethane, di(2-ethylhexyl)phthalate, and polychlorinated biphenyls, for removing these pollutants; purified water was obtained through filtration. Although this purified water was initially highly alkaline (pH ∼ 12.5), its pH decreased into a weak alkaline region (pH ∼ 9) during exposure to ambient air by absorbing carbon dioxide from the air with the precipitating calcium carbonate. The advantages of this water purification strategy include the fact that the saturation of CaO/Ca(OH)2 potentially serves as a visual indicator of disinfection, that the flocculation by polyphosphate removes excessive CaO/Ca(OH)2 as well as chemical pollutants, and that the high pH and Ca2+ concentrations in the resulting purified water are readily decreased. Our findings suggest the usability of seashell-derived material-polymer assemblies for water purification, especially under emergency conditions due to disasters.

Effect of chemicals on the phase and viscosity behavior of water in oil emulsions

Due to population growth, the need for energy, especially fossil fuels, is increased every year. Since the costs of exploring new reservoirs and drilling new wells are very high, most reservoirs have passed their first and second periods of life, and it is necessary to use EOR methods. Water-based enhanced oil recovery (EOR) methods are one of the popular methods in this field. In this method, due to the possibility of emulsion formation is high, and by creating a stable emulsion, viscosity and mobility improved. In this study, the parameters affecting the stability and viscosity of the emulsion have been investigated step by step. In the first step, 50% (v/v) of water has been selected as the best water cut. The type of salt and its best concentration was evaluated in the second step by measuring the average droplets size. The third step investigated the effect of SiO2 nanoparticles and surfactant (span80) on emulsion stability and viscosity. According to the results, the best amount of water cut was 50% due to the maximum viscosity. In salts the yield was as follows: MgCl2 > CaCl2 > MgSO4 > Na2SO4 > NaCl. The best yield was related to MgCl2 at a concentration of 10,000 ppm. Finally, it was shown that the synergy of nanoparticles and surfactants resulted in higher stability and viscosity than in the case where each was used alone. It should be noted that the optimal concentration of nanoparticles is equal to 0.1% (w/w), and the optimal concentration of surfactant is equal to 200 ppm. In general, a stable state was obtained in 50% water-cut with MgCl2 salt at a concentration of 10,000 ppm and in the presence of SiO2 nanoparticles at a concentration of 0.1% and span 80 surfactants at a concentration of 200 ppm. The results obtained from this study provide important insights for optimal selection of the water-based EOR operation parameters. Viscosity showed a similar trend with stability and droplet size. As the average particle size decreased (or stability increased), the emulsion viscosity increased.

Experimental investigations on the vertical distribution and properties of oil-mineral aggregates (OMAs) formed by different clay minerals

After oil spills, the floating oil may interact with suspended minerals to form the oil-mineral aggregates (OMAs) in turbulent environments. In this work, a flume was used in conjunction with a settling device to investigate the vertical distribution and properties of OMAs formed by different clay minerals. The density and size of OMAs depend on the density and surface properties of the constituent particles, which also affect the vertical distribution of dispersed oil. Density of oil-montmorillonite aggregates increased from 1165 to 1897 kg/m3 within 6 h test. Among the four minerals, montmorillonite displayed the highest affinity with dispersed oil and the most significant modification of oil-water interfacial tension. Oil dispersion efficiency was significantly greater and reached 39.3% in the presence of montmorillonite at 300 mg/L compared with the control group (17.6%). Particle concentration is the most important factor for the capture of oil and participation of particles during the OMA formation, while the zeta potential and hydrophobicity have nonsignificant effect on the two processes. Cation exchange capacity has a moderate effect on the sunken oil formation, which is also the second main factor governing the particle participation. Particle size plays a second leading role in governing the sunken oil formation but with a minor contribution of the particle participation.

Advanced Switchable Molecules and Materials for Oil Recovery and Oily Waste Cleanup

Advanced switchable molecules and materials have shown great potential in numerous applications. These novel materials can express different states of physicochemical properties as controlled by a designated stimulus, such that the processing condition can always be maintained in an optimized manner for improved efficiency and sustainability throughout the whole process. Herein, the recent advances in switchable molecules/materials in oil recovery and oily waste cleanup are reviewed. Oil recovery and oily waste cleanup are of critical importance to the industry and environment. Switchable materials can be designed with various types of switchable properties, including i) switchable interfacial activity, ii) switchable viscosity, iii) switchable solvent, and iv) switchable wettability. The materials can then be deployed into the most suitable applications according to the process requirements. An in-depth discussion about the fundamental basis of the design considerations is provided for each type of switchable material, followed by details about their performances and challenges in the applications. Finally, an outlook for the development of next-generation switchable molecules/materials is discussed.

Responsive Emulsions for Sequential Multienzyme Cascades

Multienzyme cascade biocatalysis is an efficient synthetic process, avoiding the isolation/purification of intermediates and shifting the reaction equilibrium to the product side.. However, multienzyme systems are often limited by their incompatibility and cross-reactivity. Herein, we report a multi-responsive emulsion to proceed multienzyme reactions sequentially for high reactivity. The emulsion is achieved using a CO2 , pH, and thermo-responsive block copolymer as a stabilizer, allowing the on-demand control of emulsion morphology and phase composition. Applying this system to a three-step cascade reaction enables the individual optimal condition for each enzyme, and a high overall conversion (ca. 97 % of the calculated limit) is thereby obtained. Moreover, the multi-responsiveness of the emulsion allows the facile and separate yielding/recycling of products, polymers and active enzymes. Besides, the system could be scaled up with a good yield.

Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion

Recently, there have been increasing demand for the application of Pickering emulsions in various industries due to its combined advantage in terms of cost, quality and sustainability. This review aims to provide a complete overview of the available methodology for the physical characterization of emulsions that are stabilized by solid particles (known as Pickering emulsion). Current approaches and techniques for the analysis of the formation and properties of the Pickering emulsion were outlined along with the expected results of these methods on the emulsions. Besides, the application of modelling techniques has also been elaborated for the effective characterization of Pickering emulsions. Additionally, approaches to assess the stability of Pickering emulsions against physical deformation such as coalescence and gravitational separation were reviewed. Potential future developments of these characterization techniques were also briefly discussed. This review can act as a guide to researchers to better understand the standard procedures of Pickering emulsion assessment and the advanced methods available to date to study these emulsions, down to the minute details.

Improving Stability and Accessibility of Quercetin in Olive Oil-in-Soy Protein Isolate/Pectin Stabilized O/W Emulsion

Herein we report a soy protein isolate/pectin binary complex particle to stabilize emulsion (olive oil served as dispersed phase) containing quercetin. FTIR was conducted to confirm successful preparation of emulsion before and after embedding quercetin. CLSM was used to determine the microstructure and zeta-potential, rheological behavior, storage stability and freeze-thaw stability were analyzed and were correlated with pH condition. Olive oil-soy protein isolate/pectin emulsion at pH 3.0 can remain stable after 30 days’ storage and exhibited greatest freeze-thaw stability after 3 cycles. Quercetin availability was evaluated by in vitro gastrointestinal digestion experiments and it reached 15.94% at pH 7.0.

Novel amphiphilic cellulose nanocrystals for pH-responsive Pickering emulsions

Development of a green, recyclable emulsifier for pH-responsive Pickering emulsion would be of great importance to many industries. To this end, a novel emulsifier, benzyl-polyethyleneimine modified cellulose nanocrystals (Ben-PEI-CNCs), was developed via the periodate oxidation of cellulose nanocrystals and reductive amination. Ben-PEI-CNCs possess pH-responsive amphiphilicity due to the existence of hydrophilic amino and hydrophobic benzyl groups. The Pickering emulsions stabilized by Ben-PEI-CNC2 and Ben-PEI-CNC18 are very responsive to pH changes, and adjusting the pH from 3 to 7 effectively triggers oil-water separation and emulsification. Additionally, cyclic testing establishes the robustness of this process. Overall, this study demonstrates that Ben-PEI-CNCs can promote the transition from a stable emulsion to an unstable emulsion by adjusting the pH, allowing the recovery of oil and the recycling of the emulsifier.

Influence of pH and Salt Concentration on Pickering Emulsions Stabilized by Colloidal Peanuts

Solid-stabilized emulsions commonly known as Pickering emulsions offer unique benefits such as superior stability and controlled permeability compared to conventional surfactant stabilized emulsions. In this article, the effect of pH, the electrolyte and particle concentration, homogenization speed, and volume fraction of oil on the formation, stability, and the microstructure of emulsion droplets stabilized by micron-size peanut-shaped hematite particles are investigated. The influence of surface charge of particles on emulsification is studied by varying the pH of the dispersing medium, the addition of an electrolyte or a combination of both. Stable O/W emulsions are formed only when the aqueous dispersions at intermediate pH between 4 and 11, and decane (2:1 volume ratio) are vigorously mixed. However, emulsions are not formed when the particles are highly charged that is, at pH 2 and 12. The presence of monovalent salt or high-speed homogenization assists the emulsion formation at pH 3, whereas their combination helps in emulsification at pH 2. However, neither the addition of an electrolyte nor the high-speed homogenization or their combination facilitates the formation of emulsions at pH 12. We show that the image-charge repulsion and the surface charge induced wettability change can explain the influence of both pH and salt concentrations on the formation of Pickering emulsions. Although oil-in-water emulsions typically cream because of the density difference, microscopy observations revealed the presence of a large number of small particle-covered oil droplets in the sediments of the emulsified samples. These drops are observed to be entrapped in dense-particle networks. This leads to a considerable reduction in the number of particles available for the stabilization of floating emulsion droplets and thus influences their size and surface coverage. The possibility of tailoring the stability, droplet size and, the surface coverage discussed in this article can play a crucial role in situations that demand controlled release of active components.

Pickering nano-emulsion as a nanocarrier for pH-triggered drug release

This study investigates the formulation of surfactant-free Pickering nano-emulsions able to release a drug at specific pH, in order to enhance its oral bioavailability. The stabilizing nanoparticles composed of magnesium hydroxide, were obtained by nano-precipitation method. The oil-in-water Pickering nano-emulsions stabilized with Mg(OH)₂ nanoparticles, and encapsulating a model of hydrophobic drug (ibuprofen) were formulated following a high-energy process, using a sonication probe. The experimental approach explored the impact of all formulation parameters, composition and size of Mg(OH)₂ nanoparticles, on the physico-chemical properties of the Pickering nano-emulsions. The system was characterized by DLS and transmission electron microscopy. In addition, Mg(OH)₂ has the advantage of being solubilized in an acid medium leading to the destabilization of the nano-emulsion and the release of the active ingredient orally. The acid release study (pH = 1.2) showed cumulative release as a function of initial nanodroplet loading and saturation concentration. In basic media (pH = 6.8), we found a significant release of ibuprofen from the nano-emulsions that already had saturation in an acid medium. These nano-emulsions can not only protect patients from the side effects of acid medicines through the basic properties of hydroxides but also can contribute to the increase of the bioavailability of these drugs. In addition, once in the stomach pH is increased by hydroxides and promotes the release of active ingredients such as ibuprofen whose solubility is strongly influenced by pH.

Emulsions Stabilized by Chitosan-Modified Silica Nanoparticles: pH Control of Structure-Property Relations

In food-grade emulsions, particles with an appropriate surface modification can be used to replace surfactants and potentially enhance the stability of emulsions. During the life cycle of products based on such emulsions, they can be exposed to a broad range of pH conditions and hence it is crucial to understand how pH changes affect stability of emulsions stabilized by particles. Here, we report on a comprehensive study of the stability, microstructure, and macroscopic behavior of pH-controlled oil-in-water emulsions containing silica nanoparticles modified with chitosan, a food-grade polycation. We found that the modified colloidal particles used as stabilizers behave differently depending on the pH, resulting in unique emulsion structures at multiple length scales. Our findings are rationalized in terms of the different emulsion stabilization mechanisms involved, which are determined by the pH-dependent charges and interactions between the colloidal building blocks of the system. At pH 4, the silica particles are partially hydrophobized through chitosan modification, favoring their adsorption at the oil-water interface and the formation of Pickering emulsions. At pH 5.5, the particles become attractive and the emulsion is stabilized by a network of agglomerated particles formed between the droplets. Finally, chitosan aggregates form at pH 9 and these act as the emulsion stabilizers under alkaline conditions. These insights have important implications for the processing and use of particle-stabilized emulsions. On one hand, changes in pH can lead to undesired macroscopic phase separation or coalescence of oil droplets. On the other hand, the pH effect on emulsion behavior can be harnessed in industrial processing, either to tune their flow response by altering the pH between processing stages or to produce pH-responsive emulsions that enhance the functionality of the emulsified end products.

Use of Different Agroindustrial Waste and Produced Water for Biosurfactant Production

The high and increasing environmental concern about chemical surfactants triggers attention to more eco-friendly compounds, which are capable of presenting low toxicity and biodegradable nature. This study has evaluated the emulsifying potential of broths obtained from production of rhamnolipids by Pseudomonas aeruginosa AP 029/GLVIIA using different agro-industrial wastes as carbon source, and the influence of using produced water in the culture medium then acting as an inducer. There were used residues of coconut, cashew, sugar cane, carnauba, and moringa seeds. These materials were chemically characterized and used in the production of biosurfactant broths. The emulsifying activity and the surface tension were used as indirect analysis for determining the production of rhamnolipids on biosurfactant broth. For coconut and cashew residues the pH emulsion was observed along time at different temperatures such as 4ºC, 25ºC and 40ºC The emulsification index of all broths were assessed with and without presence of produced water in the culture medium and has indicated the stability of the emulsion along time. Coconut and cashew residues showed a better stability of oil/water emulsion than the other ones, evidencing their potential surfactants. The lowest surface tension about 40 N/m and the highest contact angle (approximately 69 o) were observed for the coconut residue.

Colloidal Particles at a Range of Fluid-Fluid Interfaces

The study of solid particles residing at fluid-fluid interfaces has become an established area in surface and colloid science recently, experiencing a renaissance since around 2000. Particles at interfaces arise in many industrial products and processes such as antifoam formulations, crude oil emulsions, aerated foodstuffs, and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also, and the area is now multidisciplinary, involving research in the fundamental science and potential applications. In this Feature Article, the flavor of some of this interest is given on the basis of recent work from our own group and includes the behavior of particles at oil-water, air-water, oil-oil, air-oil, and water-water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilized emulsions, particle-stabilized aqueous and oil foams, dry liquids, liquid marbles, and powdered emulsions.

pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant

pH-responsive oil-in-water Pickering emulsions were prepared simply by using negatively charged silica nanoparticles in combination with a trace amount of a zwitterionic carboxyl betaine surfactant as stabilizer. Emulsions are stable to coalescence at pH ≤ 5 but phase separate completely at pH > 8.5. In acidic solution, the carboxyl betaine molecules become cationic, allowing them to adsorb on silica nanoparticles via electrostatic interactions, thus hydrophobizing and flocculating them and enhancing their surface activity. Upon increasing the pH, surfactant molecules are converted to zwitterionic form and significantly desorb from particles' surfaces, triggering dehydrophobization and coalescence of oil droplets within the emulsion. The pH-responsive emulsion can be cycled between stable and unstable many times upon alternating the pH of the aqueous phase. The average droplet size in restabilized emulsions at low pH, however, increases gradually after four cycles due to the accumulation of NaCl. Experimental evidence including adsorption isotherms, zeta potentials, microscopy, and three-phase contact angles is given to support the postulated mechanisms.

Kafirin Nanoparticle-Stabilized Pickering Emulsions as Oral Delivery Vehicles: Physicochemical Stability and in Vitro Digestion Profile

Kafirin nanoparticle-stabilized Pickering emulsions (KPEs) were used to encapsulate curcumin. The stability of KPEs under processing conditions and their protective effects against photo-oxidation of curcumin and lipid oxidation of oil in emulsions, as well as the digestion profiles in gastrointestinal tract, were investigated. KPEs were found to be more stable under acidic than basic environment, and elevated temperature induced their structural instability. The protective effect of KPEs on the chemical stability of curcumin was manifested when subjected to UV radiation as compared to other comparable formulations, such as bulk oil or Tween 80 stabilized emulsions (TEs). Meanwhile, the lipid oxidation rate was retarded in KPEs as compared to those of TEs. Due to hydrolysis of pepsin, KPEs could not survive through the gastric digestion process. After the intestinal digestion process, the extent of lipolysis of KPEs and the curcumin bioaccessibility fell between those of TEs and bulk oil. These results will fill the gap between the physicochemical properties of protein particle-based Pickering emulsions and their realistic applications in the oral delivery of functional food ingredients.

Effects of Ionic Strength on the Colloidal Stability and Interfacial Assembly of Hydrophobic Ethyl Cellulose Nanoparticles

Nanoparticle attachment at a fluid interface is a process that often takes place concurrently with nanoparticle aggregation in the bulk of the suspension. Here we investigate systematically the coupling of these processes with reference to the adsorption of aqueous suspensions of ethyl cellulose (EC) nanoparticles at the air-water interface. The suspension stability is optimal at neutral pH and in the absence of salt, conditions under which the electrostatic repulsion among EC nanoparticles is maximized. Nonetheless, hydrophobic attraction dominates particle-interface interactions, resulting in the irreversible adsorption of EC nanoparticles at the air-water interface. The addition of salt weakens the particle-particle and particle-interface repulsive electrostatic forces. This leads to destabilization of the suspension at ionic strengths of 0.05 M or greater but does not affect nanoparticle adsorption. The energy of adsorption, the surface tension and interface coverage at steady state, and the particle contact angle at the interface all remain unchanged by the addition of salt. These findings contribute to the fundamental understanding of colloidal systems and inform the utilization of EC nanocolloids, in particular for the stabilization of foams and emulsions.

Stimuli-responsive Pickering emulsions: recent advances and potential applications

Pickering emulsions possess many advantages over traditional surfactant stabilized emulsions. For example, Pickering emulsions impart better stability against coalescence and, in many cases, are biologically compatible and environmentally friendly. These characteristics open the door for their use in a variety of industries spanning petroleum, food, biomedicine, pharmaceuticals, and cosmetics. Depending on the application, rapid, but controlled stabilization and destabilization of an emulsion may be necessary. As a result, Pickering emulsions with stimuli-responsive properties have, in recent years, received a considerable amounts of attention. This paper provides a concise and comprehensive review of Pickering emulsion systems that possess the ability to respond to an array of external triggers, including pH, temperature, CO2 concentration, light intensity, ionic strength, and magnetic field. Potential applications for which stimuli-responsive Pickering emulsion systems would be of particular value, such as emulsion polymerization, enhanced oil recovery, catalyst recovery, and cosmetics, are discussed.

Pickering emulsions stabilized by whey protein nanoparticles prepared by thermal cross-linking

A Pickering (o/w) emulsion was formed and stabilized by whey protein isolate nanoparticles (WPI NPs). Those WPI NPs were prepared by thermal cross-linking of denatured WPI proteins within w/o emulsion droplets at 80°C for 15 min. During heating of w/o emulsions containing 10% (w/v) WPI proteins in the water phase, the emulsions displayed turbid-transparent-turbid phase transitions, which is ascribed to the change in the size of the protein-containing water droplets caused by thermal cross-linking between denatured protein molecules. The transparent stage indicated the formation of WPI NPs. WPI NPs of different sizes were obtained by varying the mixing speed. WPI NPs of 200-500 nm were selected to prepare o/w Pickering emulsions because of their good stability against coalescence. By Confocal Laser Scanning Microscopy, it was observed that WPI NPs were closely packed and distributed at the surface of the emulsion droplets. By measuring water contact angles of WPI NPs films, it was found that under most conditions WPI NPs present good partial wetting properties, but that at the isoelectric point (pI) and high ionic strength the particles become more hydrophobic, resulting in less stable Pickering emulsion. Thus, at pH above and below the pI of WPI NPs and low to moderate ionic strengths (1-10 mM), and with a WPI NPs concentration of 2% (w/v), a stable Pickering emulsion can be obtained. The results may provide useful information for applications of WPI NPs in environmentally friendly and food grade applications, notably in food, pharmaceutical and cosmetic products.

Surface chemical compositions and dispersity of starch nanocrystals formed by sulfuric and hydrochloric acid hydrolysis

Surface chemical compositions of starch nanocrystals (SNC) prepared using sulfuric acid (H₂SO₄) and hydrochloric acid (HCl) hydrolysis were analyzed by X-ray photoelectron spectroscopy (XPS) and FT-IR. The results showed that carboxyl groups and sulfate esters were presented in SNC after hydrolysis with H₂SO₄, while no sulfate esters were detected in SNC during HCl-hydrolysis. TEM results showed that, compared to H₂SO₄-hydrolyzed sample, a wider size distribution of SNC prepared by HCl-hydrolysis were observed. Zeta-potentials were −23.1 and −5.02 mV for H₂SO₄- and HCl-hydrolyzed SNC suspensions at pH 6.5, respectively. Nevertheless, the zeta-potential values decreased to −32.3 and −10.2 mV as the dispersion pH was adjusted to 10.6. After placed 48 h at pH 10.6, zeta-potential increased to −24.1 mV for H₂SO₄-hydrolyzed SNC, while no change was detected for HCl-hydrolyzed one. The higher zeta-potential and relative small particle distribution of SNC caused more stable suspensions compared to HCl-hydrolyzed sample.

Pickering emulsions prepared by layered niobate K₄Nb₆O₁₇ intercalated with organic cations and photocatalytic dye decomposition in the emulsions

We investigated emulsions stabilized with particles of layered hexaniobate, known as a semiconductor photocatalyst, and photocatalytic degradation of dyes in the emulsions. Hydrophobicity of the niobate particles was adjusted with the intercalation of alkylammonium ions into the interlayer spaces to enable emulsification in a toluene-water system. After the modification of interlayer space with hexylammonium ions, the niobate stabilized water-in-oil (w/o) emulsions in a broad composition range. Optical microscopy showed that the niobate particles covered the surfaces of emulsion droplets and played a role of emulsifying agents. The niobate particles also enabled the generation of oil-in-water (o/w) emulsions in a limited composition range. Modification with dodecylammonium ions, which turned the niobate particles more hydrophobic, only gave w/o emulsions, and the particles were located not only at the toluene-water interface but also inside the toluene continuous phase. On the other hand, interlayer modification with butylammonium ions led to the formation of o/w emulsions. When porphyrin dyes were added to the system, the cationic dye was adsorbed on niobate particles at the emulsion droplets whereas the lipophilic dye was dissolved in toluene. Upon UV irradiation, both of the dyes were degraded photocatalytically. When the cationic and lipophilic porphyrin molecules were simultaneously added to the emulsions, both of the dyes were photodecomposed nonselectively.