Evaluation of drops dimensions in time and rheological properties of the multiple emulsion
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Results of drops dimension and rheological properties of the produced multiple emulsion are presented in this paper. Sunflower oil (oil phase), distilled water (liquid phase), and lecithin from soya beans were used as materials to produce emulsion. Emulsions were produced in vessel with inner diameter D = 0.1 m and liquid height H = 0.5D. Smith turbine has been used to stir the emulsions. Examination of the drops dimension and rheological properties were conducted for emulsions collected after 15 min and 45 min of stirring as well as after the 3rd and 30th day. There were 6 measurements altogether. To do the analysis of drops dimension, in each series 700–800 inner oil-phase drops were taken. The results were presented in a form of graphics as a distribution of drops. The dimensions of the drops obtained d were also presented as a minimal diameter dmin, arithmetic mean diameter da, median dm, and Sauter mean diameter d32. The results of the research of rheological emulsions were presented graphically as a viscosity curve and they were described by Herschel–Bulkley model (Eq. 4)
KeywordsMultiple emulsion Microscopic analysis Rheological measurements
List of symbols
Width of the baffles (m)
Inner diameter of the vessel (m)
Diameter of the stirrer (m)
Diameter of the drop (µm)
Arithmetic mean diameter of the drop (µm)
Mediana diameter of the drop (µm)
SAUTER mean diameter (µm)
Volume-weighted mean diameter (µm)
Liquid height in the vessel (m)
Number of baffles
Number of stirrer blades (s−1)
Shear stress (Pa)
Shear rate (s−1)
Double emulsions are systems, where the inner phase contains outer-phase droplets. That is why it is possible to distinguish two types of double emulsions: emulsion water in oil in water (W/O/W) and emulsion oil in water in oil (O/W/O). In emulsions W/O/W the water phase is additionally dispersed in oil phase, whereas in O/W/O emulsions the oil phase is additionally dispersed in water phase. In both cases, the interim phase creates a peculiar kind of membrane between two identical phases. Another kind of double emulsions is the one in which two different water phases marked as W1/O/W2 or two different oil phases (O1/W/O2) were applied. It is necessary to remember, that liquids used during emulsions creation cannot dissolve in themselves.
where xi—number of particles.
Perez-Moral et al. (2014), Schuch et al. (2013, 2014) and Ursica et al. (2005) papers also present W/O/W type of emulsion. Perez-Moral et al. (2014) investigated the role of different emulsifiers and a simple novel approach to gel the internal aqueous droplets to improve the stability to heat, shear, and the presence of salt. In their study polyglycerol polyricinoleate (PGPR 4125) was used as emulsifier. Another emulsifier was used in their study than lecithin. Lecithin was found to be the most stable emulsifier to heating. The main objective of the work Schuch et al. (2013) was to investigate a structural parameter, namely the dispersed phase content of the inner emulsion, on breakup of double-emulsion droplets. The inner emulsions were rheologically characterised to explain the breakup behaviour. They think that breakup of double-emulsion drops was comparable to breakup of simple-emulsion droplets. According to them the breakup behaviour can be described and predicted by the viscosity ratio (viscosity inner and outer emulsion). The comparison of different emulsification devices for the production of W/O/W double emulsion was presented by Schuch et al. (2014). Ursica et al. (2005) presented the analysis of drops dimensions for seven multiple emulsions W/O/W as distribution histogram. The particle size analysis was accomplished by the microscopic images of the samples. In their paper, the evaluation of the median diameter in time for seven different multiple emulsion was presented.
The topic connected with emulsions W/O/W was extensively presented by Hino et al. (2000), Okochi and Nakano (2000), and Jimenez-Colmenero (2013). Hino el al. (2000) presented stabilisation of W/O/W emulsion and its application to transcatheter arterial embolization therapy. The preparation and application of W/O/W type emulsions entrapping vancomycin were presented by Okochi and Nakano (2000). Jimenez-Colmenero (2013), in the article offers a detailed review of the specific possibilities and food applications of multiple emulsions for the development of healthier food, including functional foods.
The research with double emulsions, where the oil phrase is additionally dispersed in water phrase (O/W/O), were also of Pal (1996) and Dłuska and Markowska-Radomska (2010) interest. Pal (1996) presented the rheological behaviour of simple oil in water in oil emulsions. Dłuska and Markowska-Radomska (2010) described the effect of operating parameters in the Couette–Taylor flow (CTF) contactor on multiple emulsion appearance, drop size, packing, and rheological behaviour.
Wide usage of emulsions in different industry branches contributes to conducting research on multiple emulsions formation, their stability and defining their structure and determining their properties. To create emulsion, it is necessary to lower the phase-to-phase tension on the border of phases. That effect is ensured thanks to a presence of the appropriate surfactant. In cosmetic or pharmaceutic emulsions it seems to be good to use lecithin as an emulsifier. Lecithin belongs to a group of natural, amphoteric emulsifiers. It is obtained from such plants as soya, sunflower, and rape (Molski 2009; Brud and Glinka, 2001).
The aim of the paper was to define structure changes, including drops dimensions during multiple emulsion production using soya lecithin in the vessel equipped with standard planar baffles and Smith’s turbine. The character and dimension of emulsion drops, which were put aside for 3 days and then mixed by shaking or Smith turbine mechanical mixing, were specified. The process of drops dimension measurement was repeated after 30 days. In this paper the rheological properties of the emulsion produced and the emulsion in the 30 days of its production were specified.
In the studies sunflower oil (refined sunflower oil produced for Jeronimo Martins Polska S.A.) was used as oil phase and distilled water as water phase. Lecithin from soya beans (producer: Vemica) was used as an emulsifier. Sudan III (producer: EUROCHEM BGD Sp. z o.o.) was used to the colour oil phase.
Experimental set up
Procedure of the emulsion production
The part of individual phase contained 70 vol % of oil phase and 30 vol.% of water phase. In distilled water phase 8 g soya lecithin was solved (it corresponds to 6.4 mass % of lecithin related to water phase) and stirred with magnetic agitator until mixture homogeneity was obtained. Such prepared water phase was poured to the vessel with Smith’s turbine. Sunflower oil, earlier coloured with Sudan III, was poured to the mixture of distilled water and soya lecithin. The emulsion was produced by stirring water phase and oil phase with Smith’s turbine, rotation frequency 500 rpm. Samples to describe emulsion produced and determined drops dimension and rheological properties were collected after 15 and 45 min of stirring. After 45 min the produced emulsion was divided into two samples which were set aside for 3 and 30 days. The coalescence effect leading to stages stratification was observed. After 3 as well as after 30 days before conducting the research emulsions were stirred and shaken to obtain homogeneity.
Rheological measurements and microscopic analysis
Rheological measurements were performed using rheoviscometer type RT 10 by firm Haake with the system of two co-axial cylinders (DG 41). Measurements were conducted for the shear rate < 50 1/s and temperature 23 °C.
Microscopic analysis of the investigated samples was conducted to gain information about the multiple characteristics of the emulsion. Images were taken with diagnostic inverted microscope OPTA-TECH MW – 100, with software and digital camera OPTA-TECH 5MP.
Results and discussion
Analysing data presented in Fig. 3a, b it was possible to observe, that the biggest collection of inner oil-phase drops being 37% of the whole population in question, is in the range of diameters from 2 to 4 μm, for the sample collected after 15 min of stirring (Fig. 3a) and in the range of 2.19–4.38 μm for the sample stirred for 45 min (Fig. 3b), where the number of drops appearance equals 41% of the whole inner oil-phase drops tested. The 13% bigger appearance of inner oil-phase drops number of diameter smaller than 2.19 μm was visible too, when the emulsion was stirred for 45 min in comparison to the emulsion stirred for 15 min It leads to the conclusion that the prolonging stirring time influences the occurrence of the bigger number of smaller-diameter inner oil phase drops.
A different distribution of drops is visible in values of data analysed of inner oil phase of the produced emulsion diameters obtained, which after 30 days was re-shaken to gain homogeneity (Fig. 7b). The existence of significant, 63% population of drops analysed was spotted, drops of the d < 2.35 μm diameter. The number of drops of diameters belonging to another range 2.36 μm < d < 4.7 μm was only 18% of the analysed population. Thanks to such drops distribution, the value of mean alternative drops diameter was calculated and equals da = 3.95 μm. This value is smaller than the value of mean drops diameter da = 5.06 μm for emulsion, which was re-stirred before the analysis.
The sizes and values of population of inner oil-phase drops of the emulsion produced
3 day stirred
3 day shaken
30 day stirred
30 day shaken
Herschel–Bulkley model presented by the Eq. (4) describes the experimental data with the mean error 3.5%, for the shear speed range 1 < , 1/s < 50.
On the basis of research and data obtained during emulsion production in the vessel of the D = 0.1 m and H = 0.5D, with Smith’s turbine (ds = 0.33D), it stated that multiple emulsion oil in water in oil (O/W/O) was produced. It was possible for O/W/O emulsion consisting of 70 vol % sunflower oil and 30 vol % distilled water in which 8 g soya lecithin (it corresponds to 6.4 mass.% of lecithin related to water phase) was solved. Such emulsion was produced in 15 and 45 min of stirring, with the rotation frequency equal to 500 rpm. The emulsion produced is not stable and stratifies in time. However, mechanical re-stirring or re-shaking causes the return to the previously described type of emulsion O/W/O.
Prolonging the stirring time with Smith’s turbine affects the change of mean alternative drop diameter, decreasing its value (for 15 min d = 6.37 μm, and for 45 min d = 5.49 μm). In case of mean Sauter diameter its value increases with time of stirring used at work (d32 = 26.11μm for 15 min stirring, d32 = 32.37μm for 45 min stirring).
The results received after 3 and 30 days show that, according to size distribution analyses it is obvious that re-shaking of emulsion leads to better dispersion than re-stirring.
The rheological properties of the emulsion produced during 15 and 45 min of stirring were described by Herschel–Bulkley model presented by the Eq. (4) appropriate for the range of strain speed 1 < , 1/s < 50.
The rheological properties of emulsion practically do not change when the emulsion is re-stirred or re-shaked manually after 30 days.
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