Differential scanning calorimetry (DSC) in multiple W/O/W emulsions

Multiple water-in-oil-in-water (W/O/W) emulsions offer a huge potential as encapsulation systems in different food, cosmetic, and pharmaceutical applications. Because of their complex structure, however, it is difficult to characterize these systems. Typical measurement techniques to determine the size and stability of the inner water droplets encapsulated in the oil droplets show limitations and inaccuracies. Determining the total amount of water in the inner droplets is most often done by indirect methods to date. We describe an analytical method based on differential scanning calorimetry (DSC) for characterizing the total amount of encapsulated water droplets and their stability in W/O/W multiple emulsions. It uses the possibility to directly determine the latent heat of freezing of water droplets of the same size and composition as in the multiple emulsions. The amount of water in the inner droplets of a W/O/W emulsion can thus be calculated very accurately. It is shown that this method enables furthermore detecting multi-modalities in the size distribution of inner water droplets in W/O/W emulsions. Changes in droplet size distribution of the inner droplets occurring during the second emulsification step of processing or during storage can be detected. DSC thus offers a powerful tool to characterize the structure of multiple W/O/W emulsions.     

A single-cell encapsulation method based on a microfluidic multi-step droplet splitting system

Single cell analysis is of great significance to understand the physiological activity of organisms.Microfluidic droplet is an ideal analytical platform for single-cell analysis. We developed a microfluidic droplet splitting system integrated with a flow-focusing structure and multi-step splitting structures to form 8-line droplets and encapsulate single cells in the droplets. Droplet generation frequency reached1021 Hz with the aqueous phase flow rate of 1 m L/min and the oil phase flow rate of 15 mL /min. Relative standard deviation of the droplet size was less than 5% in a single channel, while less than 6% in all the8 channels. The system was used for encapsulating human whole blood cells. A single-cell encapsulation efficiency of 31% was obtained with the blood cell concentration of 2.5× 10~4cells/mL, and the multicellular droplet percentage was only 1.3%. The multi-step droplet splitting system for single cell encapsulation featured simple structure and high throughput.     

Characterization and Evaluation of Electrolyte Influence on Canola Oil/Water Nano‐Emulsion

Emulsion stability is controlled by the physicochemical properties of the adsorbed layers formed on the surface of the droplets. Zeta potential and droplet size measured initially and during storage can estimate O/W emulsion stability. The aim of this study was to characterize and evaluate the effects of different hydrolyzable compounds employed in pharmaceutical and cosmetic preparations on the zeta potential and droplet size of canola O/W nano‐emulsions and, consequently, the emulsion stability. The samples containing additives demonstrated significant change in zeta potential, but in spite of that, no macroscopic instability was observed. Yet the droplet size values did not undergo significant change.     

Spontaneous size selection in cholesteric and nematic emulsions

We study the spontaneous size selection in lyotropic cholesteric (W/O) and thermotropic nematic (O/W) liquid crystal emulsions. The droplet sizes have been characterized by dynamic light scattering, which indicates a narrow monomodal distribution of droplets achieved spontaneously even without emulsion filtration. Anchoring of the director, provided by the chosen surfactant on the interface, may generate a topological defect inside the droplet. Below the critical radius R = K/W, determined by the ratio of Frank elastic and the surface anchoring constants, the effective anchoring strength is weak and droplets are not topologically charged; this allows them to coalesce freely, depleting the size distribution in this range. Large droplets possess a topological charge of +1 and present a high elastic energy barrier for pair coalescence; the resulting size distribution is skewed, with R > R, and effectively frozen.     

Controlled production of monodisperse double emulsions by two-step droplet breakup in microfluidic devices

A microfluidic device having both hydrophobic and hydrophilic components is exploited for production of multiple-phase emulsions. For producing water-in-oil-in-water (W/O/W) dispersions, aqueous droplets ruptured at the upstream hydrophobic junction are enclosed within organic droplets formed at the downstream hydrophilic junction. Droplets produced at each junction could have narrow size distributions with coefficients of variation in diameter of less than 3%. Control of the flow conditions produces variations in internal/external droplet sizes and in the internal droplet number. Both W/O/W emulsions (with two types of internal droplets) and oil-in-water-in-oil emulsions were prepared by varying geometry and wettability in microchannels.     

Filler effects of oil droplets on the rheology of heat-set emulsion gels prepared with egg yolk and egg yolk fractions

Hen egg yolk is a traditional ingredient used in a wide variety of food emulsions, especially fluid sauces. Industrial processing of these sauces generally involves heat treatments in order to pasteurise or sterilise them. These heat treatments may cause undesired gelation of the emulsion, because egg yolk proteins are particularly thermosensitive. Heat gelation of oil-in-water emulsions prepared with egg yolk may differ from that of egg yolk solutions, because of the influence of oil droplets on network formation. In this study, we investigated the influence of oil droplets on the gelation of oil-in-water emulsions made with yolk. We studied three pH values: 3.0, 5.0 and 7.0 with a constant NaCl concentration: 0.55 M. Oil droplet size was controlled after emulsification, gelation of solutions and emulsions was monitored in situ by coupling heating with recording viscoelastic properties, and transmission electron microscopy was conducted in heat-set emulsion gels. In an attempt to target the proteins that impose the kinetic of gelation of egg yolk, we repeated the experiment with plasma and granules, the main fractions of yolk. In situ rheology showed that, in our experimental conditions [especially oil volume fraction (0.3) and oil droplet size (d3.2=1 &mgr;m)], emulsions made with yolk and plasma have a similar gelation process with oil droplets acting as inactive fillers. Furthermore, transmission electron microscopy showed similar network characteristics between heated emulsions made with yolk and plasma. Moreover, we demonstrated that acidic conditions provided the fastest gelation of yolk solutions and emulsions. On the other hand, in emulsions prepared with granules, oil droplets behaved as active filler particles and reinforced the gel strength.     

On the escape process during phase inversion of an emulsion

This paper deals with a phenomenon which plays an important role in the phase inversion process of emulsions. This process is governed by the interplay of coalescence of droplets, often leading to double emulsions, and the escape of those internal droplets. The latter process retards the inversion process. Coalescence has been the subject of many studies, contrary to the escape event. This paper addresses the escape process both theoretically and experimentally. The model developed analyses the rate of the escape of internal droplets from the mother droplet via a coalescence process, where the internal flow, as generated by the external flow, generates the viscous force for coalescence. Incomplete mixing in the droplet has been assumed. Experimental data on the escape rate of oil droplets from O/W/O emulsions have been analysed using a Computational Fluid Dynamics approach, where the model as indicated above has been incorporated. Experimental data and simulations compare very well. Data have been compared on varying the size of the inner droplets and the rotational speed of the vessel where the double emulsion has been formed and where the escape took place.     

Demulsification of water-in-oil emulsions by permeation through Shirasu-porous-glass (SPG) membranes

To investigate the effect of the droplet/pore size ratio on membrane demulsification, water-in-oil (W/O) emulsions with uniform-sized droplets was demulsified by permeation through Shirasu-porous-glass (SPG) membranes with a narrow pore size distribution at mean droplet/pore diameter ratios of 0.52–5.75. At transmembrane pressures above a critical pressure, the water droplets larger than the membrane pore size were demulsified, where the SPG membrane acted as a coalescer because the hydrophilic membrane surface had a high affinity for the water droplets. By contrast, at transmembrane pressures below the critical pressure, the larger water droplets were all retained by the membrane due to the sieving effect of the uniform-sized pores. When a W/O emulsion with a mean droplet diameter of 2.30 μm was allowed to permeate through a membrane with a mean pore diameter of 0.86 μm, the demulsification efficiency increased with increasing transmembrane pressure, to a maximum value of 91% at a transmembrane pressure of 392 kPa, and then decreased, while the transmembrane flux increased almost linearly with increasing transmembrane pressure. The demulsification efficiency was higher for higher water phase content and lower concentration of the surfactant, tetraglycerin condensed ricinoleic acid ester, in the emulsions due to the reduction of the emulsion stability.     

Influence of composition on the encapsulation properties of P/O/W multiple emulsions for Vitamin C

Abstract

The aim of this work was to study the encapsulation properties of polyols-in-oil-in-water (P/O/W) multiple emulsions for Vitamin C (Vc). The influence of formulation factors, including the concentration of lipophilic emulsifier, hydrophilic emulsifier, salt and glycerol had been investigated. The results indicated that the encapsulation stability could be improved by increasing the lipophilic emulsifier concentration which could strengthen the interfacial film. In contrast, the excess of hydrophilic emulsifier destabilized the emulsion. The presence of glycerol in the outer aqueous phase accelerated the phase transfer, thus reduced the encapsulation rate. The addition of salt in inner polyols phase had little effect on encapsulation rate while markedly affected the morphology and stability of this system. P/O/W multiple emulsions showed better encapsulation stability than the W/O/W multiple emulsions as the former’s encapsulation rate could remain more than 75% after 2?weeks while the latter only remained less than 60%. Meanwhile, the P/O/W emulsions exhibited higher storage modulus (G’), bigger loss modulus (G’’) and broaden linear viscoelastic regions than W/O/W emulsions.     

Influence of oil polarity on droplet growth in oil-in-water emulsions stabilized by a weakly adsorbing biopolymer or a nonionic surfactant

The influence of oil type (n-hexadecane, 1-decanol, n-decane), droplet composition (hexadecane:decanol), and emulsifier type (Tween 20, gum arabic) on droplet growth in oil-in-water emulsions was studied. Droplet size distributions of emulsions were measured over time (0-120 h) by laser diffraction and ultrasonic spectroscopy. Emulsions containing oil molecules of low polarity and low water solubility (hexadecane) were stable to droplet growth, irrespective of the emulsifier used to stabilize the droplets. Emulsions containing oil molecules of low polarity and relatively high water solubility (decane) were stable to coalescence, but unstable to Ostwald ripening, irrespective of emulsifier. Droplet growth in emulsions containing oil molecules of relatively high polarity and high water solubility (decanol) depended on emulsifier type. Decanol droplets stabilized by Tween 20 were stable to droplet growth in concentrated emulsions but unstable when the emulsions were diluted. Decanol droplets stabilized by gum arabic exhibited rapid and extensive droplet growth, probably due to a combination of Ostwald ripening and coalescence. We proposed that coalescence was caused by the relatively low interfacial tension at the decanol-water boundary, which meant that the gum arabic did not absorb strongly to the droplet surfaces and therefore did not prevent the droplets from coming into close proximity.     

Acoustic and electroacoustic spectroscopy of water-in-diluted-bitumen emulsions

Water-in-oil emulsions of Athabasca bitumen diluted with toluene have been studied using the latest advances in acoustic and electroacoustic spectroscopy. From the sound attenuation spectra of emulsions, the water droplet size distribution is measured. The electrical surface charge density of the water droplets is obtained from the colloid vibration current. In the case of freshly prepared water-in-oil emulsions, the droplet size increased while the surface charge density decreased with time. The time-dependent relaxation of the surface charge ranges from several hours to 3 days, and it is probably due to the slow adsorption/desorption kinetics of bituminous components at the water-oil interface. This study illuminates the contribution of the electrostatic interactions to the stability of water-in-oil emulsions.     

Interface composition of multiple emulsions: rheology as a probe

We have investigated the dynamic rheological properties of concentrated multiple emulsions to characterize their amphiphile composition at interfaces. Multiple emulsions (W1/O/W2) consist of water droplets (W1) dispersed into oil globules (O), which are redispersed in an external aqueous phase (W2). A small-molecule surfactant and an amphiphilic polymer were used to stabilize the inverse emulsion (W1 in oil globules) and the inverse emulsion (oil globules in W2), respectively. Rheological and interfacial tension measurements show that the polymeric surfactant adsorbed at the globule interface does not migrate to the droplet interfaces through the oil phase. This explains, at least partly, the stability improvement of multiple emulsions as polymeric surfactants are used instead of small-molecule surfactants.     

Improved formulation of W/O/W multiple emulsion for insulin encapsulation. Influence of the chemical structure of insulin

W/O/W multiple emulsions are systems of potential interest in the oral administration of insulin. Although it has been shown that a single oral administration of insulin-loaded W/O/W multiple emulsion to diabetic rats led to the significant decrease of blood glucose levels (Silva Cunha et al., 1998, Int J Pharm 169:33), repeated administrations displayed unpleasant side effects such as diarrhoea and steatosis. These unwanted effects were attributed to the high oil concentration used for their preparation. In the present study, attention was focused on the reduction of oil concentration in the formulation of these systems and on the encapsulation of two different insulins. The physical properties and stability of the multiple emulsions over long periods of time were assessed by conductivity measurements, and granulometric and microscopic analyses. The encapsulation in the inner aqueous phase of two insulins, Umulin and Humalog, differing only by the transposition of one amino acid, had non-negligible effects on the formation and stability of W/O/W multiple emulsions. Both insulins were shown to improve the formation of the multiple emulsions. Circular dichroism studies and surface tension measurements evidenced the contribution of insulin conformation and surface properties in multiple emulsion formation and stability.     

Effect of Emulsifier Type on the Characterization of O/W Emulsions Using a Spectroscopy Technique

The droplet size distribution (DSD) of emulsions is the result of two competitive effects that take place during emulsification process, i.e., drop breakup and drop coalescence, and it is influenced by the formulation and composition variables, i.e., nature and amount of emulsifier, mixing characteristics, and emulsion preparation, all of which affect the emulsion stability. The aim of this study is to characterize oil-in-water (O/W) emulsions (droplet size and stability) in terms of surfactant concentration and surfactant composition (sodium dodecyl benzene sulphonate (SDBS)/Tween 80 mixture). Ultraviolet-visible (UV-vis) transmission spectroscopy has been applied to obtain droplet size and stability of the emulsions and the verification of emulsion stability with the relative cleared volume technique (time required for a certain amount of emulsion to separate as a cleared phase). It is demonstrated that the DSD of the emulsions is a function of the oil concentration and the surfactant composition with higher stability for emulsions prepared with higher SDBS ratio and lower relative cleared volume with the time. Results also show that smaller oil droplets are generated with increasing Tween 80 ratio and emulsifier concentration.     

Factors Affecting the Droplet Size of Water‐in‐Oil Emulsions (W/O) and the Oil Globule Size in Water‐in‐Oil‐in‐Water Emulsions (W/O/W)

Multiple emulsions of the W₁/O/W₂ type are promising tools for encapsulating bioactive ingredients in the inner aqueous droplets. It is necessary, however, to control the factors influencing their encapsulation efficiency. One important factor is the particle size because it determines the surface area available for mass transport. Because of the coexistence of water and oil droplets in multiple emulsions, there are numerous factors that have an impact on particle size, for example, oil phase composition, interfacial properties, and viscosity of the phases. The purpose of this study was to systematically investigate the effect of these factors on particle sizes in multiple emulsions.     

Water-in-carbon dioxide emulsions stabilized with hydrophobic silica particles

W/C emulsions were stabilized using hydrophobic silica particles adsorbed at the interface, resulting in average droplet diameters as low as 7.5 microm. A porous cross-linked shell was formed about a hydrophilic (colloidal and fumed) silica core with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethyoxysilane, to render the particles CO(2)-philic. The stability of emulsions comprising equal weights of CO(2) and water was assessed with visual observations of settling fronts and the degree of emulsion coalescence, and the average drop size was measured by optical microscopy. The effect of CO(2) density on both emulsion stability and droplet size was determined quantitatively. The major destabilizing mechanism of the emulsions was settling, whereas Ostwald ripening and coalescence were not visible at any density, even over 7 days. Flocculation of the settling droplets did not occur, although gelation of the emulsions through particle interactions resulted after longer periods of time. CO(2)-philic particles offer a new route to highly stable W/C emulsions, with particle energies of attachment on the order of 10(6)kT, even at CO(2) densities as low as 0.78 g ml(-1). At these low densities, surfactants rarely stabilize emulsions as the result of poor surfactant tail solvation.     

Unique crystal morphologies of glycine grown from octanoic acid-in-water emulsions

We have obtained both porous and dendritic, intricate morphology crystals of beta-glycine by the novel and simple method of emulsion droplet adhesion and encapsulation. By using octanoic acid emulsified with nonionic surfactants, the adhesion of the emulsion droplets can be so strong that, remarkably, crystal growth often proceeds around the droplets, leading to their inclusion within the single crystals. Consequently, porous single crystals can be produced with the pore diameters ( approximately 10-25 mum) corresponding to the emulsion droplet sizes. Highly intricate, dendritic morphologies for glycine were obtained by increasing the surfactant concentration in the emulsions to 50%. In this case, partial droplet encapsulation results in crystal dendrites growing on either side of adsorbed droplets, with the complex morphologies developing due to the high density of dendritic branches that can occur. These intricate morphologies are in stark contrast to the facetted crystals that normally develop at these low supersaturations in the absence of octanoic acid droplets. This study demonstrates that complex architectures can be attained by using simple emulsion systems and tuning the degree of droplet adhesion.     

Influence of interfacial characteristics on Ostwald ripening in hydrocarbon oil-in-water emulsions

The influence of the nature of the interfacial membrane on the kinetics of droplet growth in hydrocarbon oil-in-water emulsions was investigated. Droplet growth rates were determined by measuring changes in the droplet size distribution of 1 wt % n-tetradecane or n-octadecane oil-in-water emulsions using laser diffraction. The interfacial properties of the droplets were manipulated by coating them with either an SDS layer or with an SDS-chitosan layer using an electrostatic deposition method. The emulsion containing SDS-coated octadecane droplets did not exhibit droplet growth during storage for 400 h, which showed that it was stable to Ostwald ripening because of this oils extremely low water-solubility. The emulsion containing SDS-coated n-tetradecane droplets showed a considerable increase in mean droplet size with time, which was attributed to Ostwald ripening associated with this oils appreciable water-solubility. On the other hand, an emulsion containing SDS-chitosan coated n-tetradecane droplets was stable to droplet growth, which was attributed to the ability of the interfacial membrane to resist deformation because of its elastic modulus and thickness. This study shows that the stability of emulsion droplets to Ostwald ripening can be improved by using an electrostatic deposition method to form thick elastic membranes around the droplets.     

We the droplets: A constitutional approach to active and self-propelled emulsions

The field of active matter, and particularly active emulsions, is growing rapidly, with significant progress made recently on both theoretical and experimental fronts. Here, we summarize experimental research progress related to active droplets. The constitution of active droplets, in particular the chemical compositions and structure of interfaces, is critical. We discuss how emulsion properties such as mechanism of motion, speed, trajectory, interaction strength, and lifetime are related to the droplet composition. We consider not only traditional single emulsions but also more complex variants, such as Janus droplets, Pickering emulsions, and multiple emulsions. Active behavior of isolated droplets as well as pairwise and multibody interactions between droplets is described. The influence of physical barriers that shape the local chemical gradients and fluid flow is also highlighted. This review provides perspective on the past, current, and promising future experimental research directions in active droplet research.     

Ultrasonic attenuation spectroscopy study of flocculation in protein stabilized emulsions

The extent and kinetics of droplet flocculation in emulsions was studied using ultrasonic attenuation spectroscopy. Flocculation in 10 wt.% soybean oil-in-water emulsions, stabilized by whey protein isolate (0.75 wt.%), was controlled by adjusting the pH (between 3 and 7) to alter the electrostatic interactions between the droplets. Droplet flocculation was then monitored by measuring the ultrasonic attenuation spectra (1–150 MHz) and by using laser light scattering. Extensive droplet flocculation was observed in the emulsions around the isoelectric point of the proteins (pH 3.5–5.5). Flocculation caused an appreciable change in the ultrasonic attenuation spectra, which was in good qualitative agreement with a theory recently developed to describe the ultrasonic properties of flocculated emulsions. Our results indicate that ultrasonic spectroscopy is a powerful tool for monitoring both the extent and kinetics of flocculation in concentrated emulsions in situ.