Emulsion Droplet Size Distribution Research Articles

Kafirin nanoparticles-stabilized Pickering emulsions: Microstructure and rheological behavior

Kafirin, the prolamin protein from sorghum, was fabricated into spherical nanoparticles and then introduced as a fully natural interfacial stabilizer for Pickering emulsions. Kafirin nanoparticles produced via anti-solvent precipitation method exhibited water over oil wetting preference. The resultant oil-in-water emulsions exhibited resistance against coalescence with internal oil phase fraction ranging from 58.8% to 78.6%. Emulsion droplet size distribution and rheological properties of emulsions stabilized by kafirin particles were influenced by the applied particle concentration, oil fraction and ionic strength in aqueous phase. Basic trends included: increasing particle concentration in aqueous phase led to increase in emulsified phase volume, decrease in oil droplet sizes and increase in storage moduli for the viscoelastic responses; As oil phase fraction increased, the emulsified phase volume fraction and droplet size increased, and the rheological properties shifted from fluid dominant to elastic dominant. The addition of salt in aqueous phase promoted the migration process and lifted the stiffness of gel-like emulsions. Cryo-scanning electron microscopy (cryo-SEM) and optical microscopy under fluorescence field evidenced the attachment of kafirin particles on oil-water interface. Overall, kafirin nanoparticles offer promising prospects in fulfilling the demands of “surfactant-free” emulsions with tunable characteristics.

Starch-based Pickering emulsions for topical drug delivery: A QbD approach

Pickering emulsions are stabilized by solid particles instead of surfactants and have been widely investigated in pharmaceutical and cosmetic fields since they present less adverse effects than the classical emulsions. A quality by design (QbD) approach was applied to the production of w/o emulsions stabilized by starch. A screening design was conducted to identify the critical variables of the formula and the process affecting the critical quality properties of the emulsion (droplet size distribution). The optimization was made by establishing the Design Space, adjusting the concentration of starch and the quantity of the internal aqueous phase. The emulsion production process was, in turn, adjusted by varying the time and speed of stirring, to ensure quality and minimum variability. The stability was also investigated, demonstrating that an increase in starch concentration improves the stability of the emulsion. Rheological and mechanical studies indicated that the viscosity of the emulsions was enhanced by the addition of starch and, to a higher extent, by the presence of different lipids. The developed formulations was considered non-irritant, by an in vitro assay using human cells from skin (Df and HaCat) with the cell viability higher than 90% and, with self-preserving properties. Finally, the QbD approach successfully built quality in Pickering emulsions, allowing the development of hydrophilic drug-loaded emulsions stabilized by starch with desired organoleptic and structural characteristics. The results obtained suggest that these systems are a promising vehicle to be used in products for topical administration.

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.

Role of Counterions in Controlling the Properties of Ultrasonically Generated Chitosan-Stabilized Oil-in-Water Emulsions.

An oil-in-water emulsion was ultrasonically prepared in aqueous chitosan solutions containing different counterions. Tetradecane was used as the oil phase in order to mimic nonpolar substances used in food processes. Various acids were used to dissolve chitosan, and we found that conjugate bases of the acids used, which act as counterions to neutralize the positive charges of ammonium ions present in the chitosan backbone, played a significant role in controlling the size, size distribution, and stability of chitosan-encapsulated tedradecane emulsion droplets (microspheres). The counterion effect is also found to be strongly dependent upon tetradecane (TD)/chitosan (CS) ratio and ultrasonic power. Key observations are: (i) for a given TD/CS ratio, the size and size distribution decrease when the acid is varied from nitric acid to benzenesulfonic acid at high TD/CS ratio, and the effect becomes less significant at low TD/CS ratio; (ii) for a given acid, the size and size distribution increase with an increase in TD/CS ratio; and (iii) at low TD/CS ratio the size and size distribution are significantly influenced by the viscosity of the system. A possible mechanism for the observed counterion effect is proposed. The role of counterions, solution viscosity, and ultrasonic power in controlling the physical and functional properties of ultrasonically generated chitosan-stabilized tetradecane microspheres is discussed in detail. The key new finding of this study is that it is possible to form stable emulsions without the addition of external emulsifiers and stabilizers, but only using chitosan with different acids to dissolve chitosan. This strategy could be used in the generation of stable food emulsions.

Demulsifying water-in-oil emulsions by ethyl cellulose demulsifiers studied using focused beam reflectance measurement

Removal of residual water droplets from produced fluids of petroleum is a challenging task because the emulsified water droplets with size of a few micrometers are extremely stable. A focused beam reflectance measurement (FBRM) was used for on-line monitoring of emulsified water droplet size distribution in petroleum emulsion systems and chemical demulsification process. After establishing the feasibility of using FBRM to measure droplet size distributions in petroleum emulsions, factors influencing the average size of water droplets were investigated to evaluate the demulsification dynamics by ethyl cellulose demulsifiers. The FBRM is proven to be a valuable tool for monitoring demulsification process in situ in real time to understand demulsification mechanisms, which sheds the lights on improving the current technology of demulsifying water-in-oil emulsions of petroleum industry.

Formulation and optimization by experimental design of eco-friendly emulsions based on d-limonene

d-Limonene is a natural occurring solvent that can replace more pollutant chemicals in agrochemical formulations.In the present work, a comprehensive study of the influence of dispersed phase mass fraction, ϕ, and of the surfactant/oil ratio, R, on the emulsion stability and droplet size distribution of d-limonene-in-water emulsions stabilized by a non-ionic triblock copolymer surfactant has been carried out.An experimental full factorial design 32 was conducted in order to optimize the emulsion formulation. The independent variables, ϕ and R were studied in the range 10–50wt% and 0.02–0.1, respectively. The emulsions studied were mainly destabilized by both creaming and Ostwald ripening. Therefore, initial droplet size and an overall destabilization parameter, the so-called turbiscan stability index, were used as dependent variables. The optimal formulation, comprising minimum droplet size and maximum stability was achieved at ϕ=50wt%; R=0.062.Furthermore, the surface response methodology allowed us to obtain the formulation yielding sub-micron emulsions by using a single step rotor/stator homogenizer process instead of most commonly used two-step emulsification methods. In addition, the optimal formulation was further improved against Ostwald ripening by adding silicone oil to the dispersed phase.The combination of these experimental findings allowed us to gain a deeper insight into the stability of these emulsions, which can be applied to the rational development of new formulations with potential application in agrochemical formulations.

Formation of Nanoemulsions by Stirred Media Milling

Emulsions are widely used in the pharmaceutical and food industry. In recent years an increasing use of emulsion-based delivery systems for bioactive substances involving small particle sizes is notable. Apart from spontaneous formation of microemulsions, in general energy has to be introduced into the system, i.e. the formation process of nanoemulsions (thermodynamically metastable systems) is not spontaneous and the droplet-breakup process needs application of external energy input. Within this account we show that highly stable nanoemulsions can be produced by an alternative and scalable process applying stirred media milling. Alkane (hexadecane, dodecane)-Tween85-water-systems have been used as model emulsion systems to demonstrate the feasibility of the approach. The influence of fundamental process parameters such as emulsion formulation (c.f. oil mass fraction and oil-to-emulsifier mass ratio), stressing conditions (grinding bead size and stirrer tip speed), process time and especially temperature on the emulsion droplet size and size distribution has been studied in detail. Low oil-to-emulsifier ratios are most appropriate with respect to obtaining small mean droplet sizes and narrow droplet size distributions. It is shown that the smallest mean droplet sizes (at constant oil-to-emulsifier ratio) are obtained at the lowest oil mass concentrations. Product emulsions of Sauter diameters x1,2 below 25nm have been obtained in the hexadecane-Tween85-water system using ZrO2 grinding beads of 100μm and 400μm in diameter, respectively. Interestingly, working at high stress intensities is not appropriate: higher stress intensities lead to a more intense mixing that increases the coalescence rate. Remarkably, lowering the process temperature below the solidification temperature of the oil phase, i.e. ‘grinding’ of the solid oil, is advantageous as compared to emulsification of the liquid oil: Coalescence and ripening can be significantly reduced. Smallest product droplets with x1,2 < 10nm are obtained in the hexadecane-Tween85-water system after a short processing time of clearly below 1 hour. The feasibility of the process proposed to produce nanoemulsions using oil phases loaded with the hydrophobic organic compounds pyrene and trans-stilbene is demonstrated.

Characterizing Emulsions by Observation of Single Droplet Collisions—Attoliter Electrochemical Reactors

We report an electrochemical study of the collisions of single droplets in an emulsion by two methods. In the first method, an electroactive redox species, for example, ferrocene, inside a toluene-in-water emulsion droplet (but not in the continuous phase) is measured by chronoamperometry during a collision with an ultramicroelectrode (UME). Here, a blip or spike type of collision signal is observed, representing electrolysis of the droplet contents. In the second method, electrochemical oxidation of an electroactive redox species in the continuous aqueous phase is hindered by a droplet blocking collision. In this case, a staircase current decrease is observed. From an analysis of single soft particle collision data, one can find the emulsion droplet size distribution and the droplet contents.

Moderately stable emulsions produced by a double syringe method

Different emulsification techniques lead to emulsions with different targeted properties specific to their intended use. In this study, emulsions are generated using several emulsification techniques. They are compared as regards the component droplet size distribution and their stability in time. A particular technique based on the “Tessari” (known as double syringe system – DSS, as well) method – used originally for foam generation – is evaluated for emulsion production. Results show that DSS method leads to emulsions with mean droplet size about two orders of magnitude larger than in the case of emulsions obtained by conventional high speed and high pressure homogenizers (HPH). However, DSS emulsions have comparable viscosity behavior with the emulsions obtained by conventional methods and are stable for time intervals of a couple of hours which is convenient for specific medical applications. Furthermore, this study evaluates the effect of small quantities of some emulsion stabilizers on the mean droplet size distribution of emulsions. In the examined narrow range of values no significant modifications of the mean droplet size distribution was observed.

Droplet size distributions in turbulent emulsions: Breakup criteria and surfactant effects from direct numerical simulations

Lattice Boltzmann simulations of water-in-oil (W/O) type emulsions of moderate viscosity ratio (≃1/3) and with oil soluble amphiphilic surfactant were used to study the droplet size distribution in forced, steady, homogeneous turbulence, at a water volume fraction of 20%. The viscous stresses internal to the droplets were comparable to the interfacial stress (interfacial tension), and the droplet size distribution (DSD) equilibrated near the Kolmogorov scale with droplet populations in both the viscous and inertial subranges. These results were consistent with known breakup criteria for W/O and oil-in-water emulsions, showing that the maximum stable droplet diameter is proportional to the Kolmogorov scale when viscous stresses are important (in contrast to the inviscid Hinze-limit where energy loss by viscous deformation in the droplet is negligible). The droplet size distribution in the inertial subrange scaled with the known power law ~d(-10/3), as a consequence of breakup by turbulent stress fluctuations external to the droplets. When the turbulent kinetic energy was sufficiently large (with interfacial Péclet numbers above unity), we found that turbulence driven redistribution of surfactant on the interface inhibited the Marangoni effect that is otherwise induced by film draining during coalescence in more quiescent flow. The coalescence rates were therefore not sensitive to varying surfactant activity in the range we considered, and for the given turbulent kinetic energies. Furthermore, internal viscous stresses strongly influenced the breakup rates. These two effects resulted in a DSD that was insensitive to varying surfactant activity.

Emulsifying properties of a novel polysaccharide extracted from basil seed (Ocimum bacilicum L.): Effect of polysaccharide and protein content

Emulsifying properties of basil seed gum (BSG) were evaluated in terms of emulsion droplet size distribution, rheological properties, visual phase separation, and adsorption properties. Soya oil-in-water emulsions (30% wt/wt) were formulated and stabilised by different concentrations of BSG (0.03–1% wt/wt). The effects of BSG concentration as well as purification (removal of proteins from BSG) on 0.3% (wt/wt) BSG-stabilised oil-in-water emulsions were tested. Emulsions with monomodal droplet distributions and with oil droplet size below 1.0 μm (d32) were formed with as little as 0.3% (wt/wt) BSG using a two-stage high pressure homogenizer at 35/8 MPa. The rheological properties of BSG emulsions appeared to be dependent on gum concentration and purification. Surface/interfacial tension measurements confirmed the adsorption of BSG at the oil-water interfaces. All gum preparations (crude, purified and protein-depleted) exhibited an ability to lower the interfacial tension. Microstructures confirmed that the purification of the gum reduces the adsorption properties, resulting in a higher degree of droplet aggregation/coalescence. Nevertheless, the BSG emulsions remained stable against phase separation for at least one month. BSG demonstrated excellent emulsifying and stabilising properties when compared to other polysaccharides. Protein-depleted BSG produced larger droplets than crude and purified BSG, but still it produced stable emulsions, comparable to other gums, such as sugar beet pectin. This suggests that the emulsifying and stabilising mechanism of the gum may not be only ascribed to the surface-active protein moiety, but could also be attributed to the hydrophobic character of the polysaccharide itself. Overall, BSG is a promising gum, which can be considered as a novel hydrocolloid emulsifier.

Application of a Laser Diffraction Method for Determination of Stability of Dispersion Systems in Food and Chemical Industry

Laser diffraction was used for determination of particle size of various emulsions. Mean particle size, fractions' number, and dispersion coefficients of tested systems are presented. Stability of greasing emulsions in relation to their pH was also tested. It was found out that determination of droplet size distribution in fat emulsions allows prediction of their properties (stability). The parameter may be helpful in selection of appropriate production process parameters, composition, and control of marketed emulsion systems during their storage. The purpose of the study was to determine applicability of a laser diffraction method for evaluation of stability of selected dispersion systems in food and chemical industry.

Formation of nanoemulsions in stirred media mills

Alkane–Tween85–water-systems have been used as model emulsion systems to demonstrate the feasibility of emulsification in stirred media mills. The influence of fundamental process parameters, i.e. emulsion formulation (oil mass fraction and oil-to-emulsifier mass ratio), stress conditions (grinding bead size and stirrer tip speed), process time and especially temperature on the emulsion droplet size and size distribution has been studied for this approach. By lowering the process temperature below the solidification temperature of the oil phase, coalescence and ripening can be significantly reduced. Smallest product droplets with x1,2<10nm thus are obtained even for nominal oil mass fractions up to 20wt% in the hexadecane–Tween85–water system. The feasibility of the process proposed to produce nanoscale emulsions using oil phases loaded with compounds such as pigments and other fillers is demonstrated.

Stability of oil-in-water macro-emulsion with anionic surfactant: Effect of electrolytes and temperature

This paper reports on the destabilization of oil-in-water macro-emulsions prepared with diesel oil dispersed in a continuous water phase and stabilized by an anionic surfactant—sodium dodecyl benzene sulphonate (SDBS). Phase inversion temperature (PIT) of anionic surfactant–oil–water emulsion system was determined by measuring the conductivity as a function of temperature at different surfactant to oil ratios. The influence of different kinds of inorganic salts on the PIT, electrophoretic properties and long-term stability of the o/w emulsion were studied by making measurements of conductivity, zeta potential and turbidity of the emulsion. PIT was found to be a linear function of logarithmic molar concentration of the electrolytes. Droplet size distribution of stable emulsion is found to have broader range for mono-valent electrolytes than that of higher valence cations. Larger the valence of cations, lower is the stability of the emulsion. By analyzing the evolution of emulsion droplet size, and turbidity of the resulting emulsion with time, the main destabilizing mechanisms of the o/w macro-emulsion were explained. It is found that coalescence, and Ostwald ripening could be the main destabilizing mechanisms of o/w macro-emulsion.

Use of a spectroscopic sensor to monitor droplet size distribution in emulsions using neural networks

AbstractMonitoring of emulsion properties is important in many applications, like in foods and pharmaceutical products, or in emulsion polymerisation processes, since aged and ‘broken’ emulsions perform worse and may affect product quality. This study reports the use of an ‘in‐line’ turbidity sensor coupled with a neural network model to monitor droplet size distributions of metal working fluid emulsions (MWF), a case where emulsion aging affects product quality. The data from the sensor were used to fit the model for droplet size distribution estimation. The technique was applied to monitor the destabilisation of commercially available MWF with good accuracy.

Microencapsulation of menthol by crosslinked chitosan via porous glass membrane emulsification technique and their controlled release properties

Chitosan-encapsulated menthol microcapsules were successfully prepared in an oil-in-water (o/w) emulsion using the Shirasu Porous Glass (SPG) membrane emulsification technique and high-speed dispersion technique for preparing a mixed o/w emulsion. The size of the menthol-loaded chitosan microcapsules was strongly depended on the average pore size of the SPG membrane and the amount of menthol loading in the dispersed phase. The membrane pore size of 5.2 µm was suitable for a viscous dispersed phase containing light mineral oil. The average diameter of emulsion droplets of 28.3 µm was obtained. Increasing the menthol loading in the dispersion phase from 5% to 10% w/w of chitosan decreased the emulsion droplet size with a broad size distribution. The crosslinked microcapsule size and size distribution of mixed emulsion droplets decreased with the increasing crosslinking time. The menthol release was a diffusion control which depended on the proportion of amino group in chitosan-to-tripolyphosphate molar ratio and crosslinking time. This work also demonstrated that hydrophilicity/hydrophobicity of the continuous phase and dispersion phase controlled SPG membrane emulsification efficiency and quality of the resulting emulsion droplets.

Effects of maltose on stability and rheological properties of orange oil-in-water emulsion formed by OSA modified starch

Effects of maltose on the stability and rheological properties of orange oil-in-water (O/W) emulsion formed by octenyl succinic anhydride modified waxy corn starch (OS-starch) were investigated. Emulsion samples were prepared using a jet homogenizer. Creaming behavior, droplet size distribution and rheological properties of emulsions during storage were studied. Results showed that the emulsion with no maltose was unstable over a quiescent storage period of one month, while the emulsions containing maltose appeared relatively stable. The OS-starch played a critical role in stabilizing emulsion by adsorbing at oil–water interface. Maltose assisted the emulsion against creaming, flocculation, and coalescence. Rheological results showed that the viscoelastic properties of the emulsions increased with increasing maltose content. Maltose increased the viscosity of emulsion, and enhanced the elastic structure of emulsion through interaction with OS-starch and water. Increase of viscoelasticity of the emulsion system retarded oil droplets free motion and strengthened the mutual acting force between droplets, thereby delaying creaming, flocculation, and coalescence of oil droplets.

Droplet-size distribution and stability of commercial injectable lipid emulsions containing fish oil

The droplet size of commercial fish oil-containing injectable lipid emulsions, including conformance to United States Pharmacopeia (USP) standards on fat-globule size, was investigated. A total of 18 batches of three multichamber parenteral products containing the emulsion SMOFlipid as a component were analyzed. Samples from multiple lots of the products were evaluated to determine compliance with standards on the volume-weighted percentage of fat exceeding 0.05% (PFAT(5)) specified in USP chapter 729 to ensure the physical stability of i.v. lipid emulsions. The products were also analyzed to determine the effects of various storage times (3, 6, 9, and 12 months) and storage temperatures (25, 30, and 40 °C) on product stability. Larger-size lipid particles were quantified via single-particle optical sensing (SPOS). The emulsion's droplet-size distribution was determined via laser light scattering. SPOS and light-scattering analysis demonstrated mean PFAT(5) values well below USP-specified globule-size limits for all the tested products under all study conditions. In addition, emulsion aging at any storage temperature in the range studied did not result in a significant increase of PFAT(5) values, and mean droplet-size values did not change significantly during storage of up to 12 months at temperatures of 25-40 °C. PFAT(5) values were below the USP upper limits in SMOFlipid samples from multiple lots of three multichamber products after up to 12 months of storage at 25 or 30 °C or 6 months of storage at 40 °C.

Parameter Selection of Emulsification Processes: Conditions for Nano‐ and Macroemulsions

AbstractBeside factors like nature of the emulsifier as well as rheology of the interface and continuous phase, the droplet size distribution of an emulsion governs emulsion properties such as long‐term stability over months or years, texture, and optical appearance. Consequently, emulsions with droplets in nano‐scale are of interest when well‐defined emulsion properties are needed. The formation of emulsions consisting of water, corn oil, and nonionic surfactants using disc systems and high‐pressure homogenizers was studied. The emulsion droplet size distributions were obtained by means of a laser diffraction method. The influence of parameters affecting the emulsion formation, such as emulsification time, viscosity for the disc system, pressure, and homogenizing steps for high‐pressure homogenization, was investigated. Data to determine the effect of the surfactant type and concentration were collected for both systems. The emulsification process using a disc system was evaluated in order to highlight its advantages and limits in comparison to high‐pressure homogenization.

Physical properties of emulsion-based hydroxypropyl methylcellulose films: Effect of their microstructure

The initial characteristics of emulsions and the rearrangement of the oil droplets in the film matrix during film drying, which defines its microstructure, has an important role in the physical properties of the emulsion-based films. The objective of this work was to study the effect of the microstructure (two droplet size distributions) and stability (with or without surfactant) of HPMC oil-in-water emulsions over physical properties of HPMC emulsion-based edible films. HPMC was used to prepare sunflower oil-in-water emulsions containing 0.3 or 1.0% (w/w) of oil with or without SDS, as surfactant, using an ultrasonic homogenizer. Microstructure, rheological properties and stability of emulsions (creaming) were measured. In addition, microstructure, coalescence of oil droplets, surface free energy, optical and mechanical properties and water vapor transfer of HPMC films were evaluated. Image analysis did not show differences among droplet size distributions of emulsions prepared at different oil contents; however, by using SDS the droplet size distributions were shifted to lower values. Volume mean diameters were 3.79 and 3.77μm for emulsions containing 0.3 and 1.0% without surfactant, respectively, and 2.72 and 2.71μm for emulsions with SDS. Emulsions formulated with 1.0% of oil presented higher stability, with almost no change during 5 and 3 days of storage, for emulsions with and without SDS, respectively. Internal and surface microstructure of emulsion-based films was influenced by the degree of coalescence and creaming of the oil droplets. No effect of microstructure over the surface free energy of films was found. The incorporation of oil impaired the optical properties of films due to light scattering of light. Addition of oil and SDS decreased the stress at break of the emulsion-based films. The replace of HPMC by oil and SDS produce a lower “amount” of network structure in the films, leading to a weakening of their structure. The oil content and SDS addition had an effect over the microstructure and physical properties of HPMC-based emulsions which lead to different microstructures during film formation. The way that oil droplets were structured into the film had an enormous influence over the physical properties of HPMC films.