КОЛЛОИДНЫЙ ЖУРНАЛ, 2013, том 75, № 5, с. 633-639
УДК 541.18
EFFECT OF THE TYPE OF THE OIL PHASE ON STABILITY OF HIGHLY CONCENTRATED WATER-IN-OIL EMULSIONS
© 2013 г. I. Masalova, E. Kharatyan, N. N. Tshilumbu
Research Rheology Laboratory, Engineering Faculty, Cape Peninsula University of Technology (CPUT)
P.O. Box 8000, Cape Town, South Africa masalovai@cput.ac.za Поступила в редакцию 23.01.2013 г.
The water-in-oil high internal phase emulsions were the subject of the study. The emulsions consisted of a super-cooled aqueous solution of inorganic salt as a dispersed phase and industrial grade oil as a continuous phase. The influence of the industrial grade oil type on a water-in-oil high internal phase emulsion stability was investigated. The stability of emulsions was considered in terms of the crystallization of the dispersed phase droplets (that are super-cooled aqueous salt solution) during ageing. The oils were divided into groups: one that highlighted the effect of oil/aqueous phase interfacial tension and another that investigated the effect of oil viscosity on the emulsion rheological properties and shelf-life. For a given set of experimental conditions the influence of oil viscosity for the emulsion stability as well as the oil/aqueous interfacial tension plays an important role. Within the frames of our experiment it was found that there are oil types characterized by optimal parameters: oil/aqueous phase interfacial tension being in the region of 19—24 mN/m and viscosity close to 3 mPa s; such oils produced the most stable high internal phase emulsions. It was assumed that the oil with optimal parameters kept the critical micelle concentration and surfactant diffusion rate at optimal levels allowing the formation of a strong emulsifier layer at the interface and at the same time creating enough emulsifier micelles in the inter-droplet layer to prevent the droplet crystallization.
DOI: 10.7868/S0023291213050091
INTRODUCTION
The term "emulsion" in general describes the dispersion of two relatively immiscible liquids (such as, for example, water and oil). The study is devoted to highly concentrated water-in-oil type emulsions, where the concentration of the dispersed phase exceeded the limit of the closest packing of spherical droplets, 9* > 0.74. The emulsion consisted of industrial grade oil as the continuous phase and a super-concentrated aqueous salt solution as the dispersed phase. The result of this dispersed phase composition is that such emulsions transit from emulsion to suspo-emulsion with ageing. One of the most important characteristics of an emulsion is its stability [1—5]. Generally emulsions are recognised as thermodynam-ically unstable systems that tend to break down with time due to gravitational separation, flocculation, creaming, coalescence or Ostwald ripening [6—11]. So, the discussions on emulsion "stability" are rather relative. Nevertheless, many authors now discuss the "stability" of emulsions [4, 12, 13]. The main experiments described in this paper lie below the emulsion dispersed phase crystallization point. Therefore, all the instabilities of emulsion were considered based on the crystallization of the super-cooled salt solution that formed the emulsion dispersed phase droplets. The coalescence was not considered a destabilizing
mechanism, since it was shown in our previous publications that the dominating mechanism of our emulsion instability is crystallization of the supersaturated dispersed phase or emulsion — suspo-emulsion transition [2].
It has been noted that the stability of an emulsion can be affected by the nature of the surfactant used to stabilize the emulsion [3, 4, 14], dispersed phase fraction [15], final emulsion mechanical properties [2, 16—18] and aqueous phase composition [6—11, 19]. On the other hand, little is known about the role of the oil composition on emulsion stability, even less on wa-ter-in-oil emulsion stability and much less on highly concentrated water-in-oil emulsions with a supercooled aqueous salt solution as the dispersed phase.
In available fundamental researches regarding the influence of oil type on stability of emulsion, the investigators usually use oils that consist of a single molecular species (laboratory or analytical grade oils). In reality, most industrial emulsions contain oil that comprises of a variety of different molecular species, named industrial grade oils. This variety of different molecular species can affect the stability of the emulsion [12, 20-22].
The present research examined the combined influence of industrial grade oil properties on the stability of highly concentrated water-in-oil emulsions. We
Interfacial tension, mN/m 40 r
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Fig. 1. Typical interfacial tension vs. time dependence. Aqueous phase: 60% water solution of ammonium nitrate; oil phase: Wittol 222; and temperature 25°C.
concentrate on the influence of the oil—water interfacial tension as a function of oil polarity and oil viscosity on the droplet crystallisation in emulsions stabilized by non-ionic surfactant — sorbitan monooleate. Sorbitan monooleate is one of the most widely used surfactants in such emulsions due to its cost and availability [4, 23, 24].
Methods
The droplet size and distribution was measured using a Malvern Mastersizer 2000 instrument. The emulsion droplet size was kept the same for all oil types and equaled to 10 ^m.
The interfacial tension measurements were performed at a planar aqueous—oil boundary using a Kruss K100 Tensiometer (Wilhelmy plate method). All interfacial tension measurements were conducted at 25°C. A 60-wt. % aqueous solution of ammonium nitrate was used as an aqueous phase, since at this concentration the ammonium nitrate still remains in saturated state.
The rheological measurements were carried out with the use of a rotational dynamic rheometer MCR 300 (Paar Physica): the measuring unit geometry was "bob-in-cup" with sandblasted bob surface. The bob diameter was 27 mm and the gap distance between the cup and the bob was 1 mm. Steady state flow measuring flow curves and oscillatory measurements for measuring strain amplitude dependencies of the storage components of dynamic modules were chosen for the investigation. All the rheological measurements were conducted at 30°C.
The optical analysis was conducted by means of Leica optical microscope equipped with a digital camera. The magnification was kept at 630x. The structural changes of the water-in-oil emulsions with ageing were followed as a function of emulsion formulation.
EXPERIMENTAL
Materials
The oils under the investigation were industrial grade hydrocarbon oils: Shellsol (Shell Chemicals), Ash-H (1)/(2) (PetroSA), Parprol 32 (ENGEN), Wittol 222 (ENGEN), Mosspar-H (PetroSA), Ash 1925 (PetroSA). The detailed oil descriptions can be found at producers' web-pages. Based on the specification there is no difference between Ash-H (1) and Ash-H (2) oils, but the difference in 60% aqueous ammonium nitrate solution — Ash-H oils interfacial tension was found, so the oils were treated as two different types. Industrial grade emulsifier — sorbitan monooleate (SMO) - was used as supplied. 5 wt. % of SMO was dissolved in different types of oil and such solutions were utilized as the continuous phase in the various emulsions. Ammonium nitrate of ANFlow grade was used in order to produce the emulsion dispersed phase, which was a supersaturated aqueous solution of ammonium nitrate: the inorganic salt content was more than 80 wt. % of the dispersed phase. The above materials were used in order to prepare highly concentrated emulsions where the continuous phase did not exceed 6% of the emulsion by mass. A more detailed description of the emulsions and emulsion preparation can be found elsewhere [1-3].
RESULTS AND DISCUSSION
The influence of aqueous—oil interfacial tension as well as oil viscosity was investigated by preparing emulsions using different types of oils as the continuous phase. The effect was examined based on emulsion stability to droplet crystallization.
Characterization of Oils
In order to understand the difference between the different types of oil, the interfacial tension at the planar aqueous—oil boundary was determined for a series of oils. The typical interfacial tension vs. time dependence for all types of oil is presented in Fig. 1. It is clearly seen that at the initial stage the interfacial tension decreases with time until it reaches the relative equilibrium value. Such behaviour shows that the oils are not pure and have some molecular species responsible for such behaviour. The equilibrium interfacial tension was chosen and recorded as a characteristic one for the investigation (see Fig. 2, Table 1). The viscosity of oils was also determined and presented in Table 2. Based on the above the oils were divided into the following groups for further investigation:
♦ Oils with the constant viscosity but different interfacial tension:
- Shellsol, Ash 1925
Interfacial tension, mN/m 40
35
30
25
20
15
10 5000
QOOOOOOOOOOOOOOOOOOO
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2
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а д i <m*
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Shear stress, Pa 1000
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7000 7500 Time, s
Fig. 2. Equilibrium interfacial tensions for all oil types: (1) Shellsol, (2) Ash H (1), (3) Parprol 32, (4) Wittol 222, (5) Mosspar H, (6) Ash H (2), and (7) Ash 1925. Aqueous phase: 60-wt % solution of ammonium nitrate; temperature 25°C.
- Mosspar-H, Ash-H (1), Ash-H (2)
♦ Oils with the constant interfacial tension but different viscosity:
- Mosspar-H, Parprol 32, Wittol 222
♦ Oils with the constant interfacial tension and viscosity
- Wittol 222, Parprol 32.
Effect of oil Type on Emulsion Stability
Water-in-oil emulsions, with a continuous phase mass fraction of 6%, were prepared, where the different types of oil were used as a continuous phase in each emulsion. The stability of the emulsi
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