КОЛЛОИДНЫЙ ЖУРНАЛ, 2010, том 72, № 4, с. 560-564
УДК 541.182.43
INSTABILITY OF HIGHLY CONCENTRATED EMULSIONS WITH OVERSATURATED DISPERSED PHASE. ROLE OF A SURFACTANT
© 2010 г. N. N. Tshilumbu*, E. E. Ferg**, I. Masalova*
*Department of Civil Engineering, Faculty of Engineering, Cape Peninsula University of Technology, PO Box 652, Cape Town 8000, Republic of South Africa **Department of Chemistry, NMMU, PO Box 77000, Port Elizabeth 6031, Republic of South Africa Поступила в редакцию 12.10.2009 г.
Instability of highly concentrated emulsions of the water-in-oil type which were investigated in this work is related to the existence of the internal phase as an oversaturated salt solution in water. The principal features of crystallization of these systems were studied by as earlier. This study is devoted to the development of this investigation and based on involving different surfactants and various concentrations of surfactants. It was shown that the originally proposed mechanism of crystallization, which suggested that growing crystals break through interfacial layers, was valid for all highly concentrated emulsions under investigation. Moreover, the Kholmogorov—Avrami kinetic equation with an unusually high exponent value equal to 6 is also applicable to different systems. It was proven that the general relationship between the growth of the yield stress and the degree of crystallization can be formulated for all surfactants studied in the work. The role of a surfactant consists in varying the characteristic time constant for the rate of crystallization. This time constant is much lower for a low-molecular-weight surfactant compared to oligomeric surfactants. This constant noticeably increases with an increase of concentration and the decrease of the average droplet size.
INTRODUCTION
Highly concentrated emulsions represent a rather specials class of colloids. As with any other emulsion, they are a mixture of two incompatible liquids stabilized with a surfactant. But in contrast to standard emulsions and suspensions, the concentration of a dispersed phase in highly concentrated emulsions exceeds the limit of the closest packing of regular spheres (app. 0.74), which is possible due to the "compressed" state of droplets that have a polyhedral shape. Figure 1 illustrates the structure of such highly concentrated emulsions.
The result of this structure is that such emulsions have rather unusual rheological properties (for liquid mixtures). Their characteristic features include the transition from a fluid to a visco-plastic medium with solid-like elastic behavior at low deformations; existence of yield stress and strong non-Newtonian behavior at higher shear stresses [1—8].
This solid-like elastic behavior ofhighly concentrated emulsions was basically explained by the Princen conception concerning the increase of the surface storage energy in droplet compression by osmotic pressure [10] that was supported by further publications [11—13]. According to these ideas the interfacial tension is the dominating factor determining stability and elasticity ofhighly concentrated emulsions. Meanwhile, it was found that there are some experimental findings which show that this approach, possibly, is insufficient in describing rheol-ogy of such emulsions; it is especially true for the depen-
dencies of the yield stress and elastic modulus on the droplet size [5-8, 14-15].
In the current work, this problem has been investigated from the other side, discussing the stability of highly concentrated emulsions with different surfactants. The objects of the investigation were emulsions of the water-in-oil type used in industry as "liquid explosives". Their composition and structure are described below in the Experimental section.
Fig. 1. Microscopic image depicting the emulsion with average droplet size 15 ^m (x500).
INSTABILITY OF HIGHLY CONCENTRATED EMULSIONS WITH OVERSATURATED
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EXPERIMENTAL
The objects of this work were highly concentrated emulsions used as "liquid explosives". The main features of these materials are described in detail in former publications [5, 7, 16]. Therefore, only their basic characteristics will be briefly mentioned here.
These emulsions belong to the water-in-oil (W/O) type systems. The concentration of the dispersed phase is 90—96 wt. %. This phase is a super-cooled aqueous solution of mainly ammonium nitrate salt in water. Water comprises < 20% by mass of this phase.
The oil phase is a solution of emulsifier (surfactant) in hydrocarbon oils.
Four surfactants were used in this work:
1. Poly(isobutylene succinic) anhydride (PIBSA) reacted approx. 1 : 1 with monoethanolamine to an uncondensed amide/acid head group (this surfactant will be called PIBSA-Amide). The overall molecular weight was expected to be about 1109.
2. PIBSA-Imide. PIBSA was condensed to an N-sub-stituted pyrrolidinedione (succinimide) structure, as below. The overall molecular weight was 1091.
3. PIBSA-Urea. Its end group was a ring including NH radical. Its molecular weight was 1047.
4. Sorbitan mono-oleate (SMO). This is an ester formed by sorbitan and oleic acid. Its molecular weight was 428.
The first three surfactants therefore belonged to the same type of chemical compound, though with different end groups, and can be treated as oligomers, while SMO was a monomeric product.
In addition, mixture of PIBSA-Amide and SMO (taken in the ratio 10 : 1) was used in some cases.
Samples were provided by Lake International Technologies as surfactants dissolved in hydrocarbon-based oils at various concentrations. When originally synthe-sised, the surfactants were dissolved or dispersed in Par-prol 32, paraffinic petroleum oil with no additives. Further dilution with another hydrocarbon-based oil, named Mosspar-H was used to achieve the desired concentration and viscosity. The properties of these materials are given in Table.
The important aspect for the goals of the research was the rather close values of interfacial tension of all these surfactants.
Measuring the size of dispersed particles was carried out with the Mastersizer-2000 device (Malvern Instruments Co.). The procedure for measuring is based on sample dispersion under software control and the measurement of angle dependence of the intensity of scattering of a collimated He-Ne laser beam. With this, particle sizes in the range from 0.26 to 1500 ^m can be measured; this range is much wider than the droplet sizes in the real samples used in this work. The size distribution calculations were based on the rigorous Mie theory and using the standard software applicable to the instrument. The reg-
Properties of fuel oils
Fuel oil Parprol 32 Mosspar H
Manufacturer Engen Chemicals PetroSA
Manufacturer's description Solvent dewaxed light paraffinic distillate Isoparaffinic solvent
Boiling range, °C 290-300
¿so-paraffins, % 70 80-90
n -paraffins, % 1-10
Cycloparaffins, % 26 10-15
Aromatics, % 4 < 0.1 typical)
Density at 20°C, kg m-3 866 or 874 790-820 (792 typical)
Density at 22°C, kg m-3 (measured) 863 794
Viscosity at 40°C, cSt 28-33 2.5
Viscosity at 30°C, mPa s (measured) 42.9 2.9
Refractive index, n20 (measured) 1.478 1.443
ulation of droplet size was achieved in the technological process of emulsion preparation. This size depends on the conditions of mixing in a vessel: speed of rotation of a stirrer and time of stirring (deformation). By varying these factors, one can obtain emulsions with different droplet sizes. This method was used for preparing all samples.
The size ofdispersed particles ranged from 1 to approx. 40 ^m, and the average diameter of particles in different samples varied from 8.2 to 20.5 ^m. It is also worth mentioning that the droplet size distributions for all samples under investigation were Gaussian and practically did not depend on the nature of the surfactant that was used.
The internal phase was formed by an oversaturated aqueous solution of inorganic salts. In preparation of the emulsions, the granulated ammonium nitrate was added to distilled water at 80°C. The concentration of a salt was kept 80 wt. %. The equilibrium crystallization temperature of the solution with the chosen concentration of a salt was approx. 63°C.
All experiments were carried out at 28 ± 2°C. We are therefore dealing with super-cooled solutions. They could be stored for several weeks only because of the small size of the droplets. Earlier, it had been found that the critical droplet size which should not be exceeded in creating stable emulsions is close to 40 ^m.
However, the internal phase was a meta-stable system and visible crystallisation started after 12 to 14 weeks of storage (at room temperature). This effect was studied for different droplet sizes and different volume fractions using a single surfactant by X-ray diffraction and published in earlier publications [7, 16, 17].
30 t, day
Fig. 2. The kinetics of crystallization of emulsions prepared with (1, 2) PIBSA-Imide and (3) PIBSA-Amide/SMO. Droplet size equals (1, 3) 15 ^m and (2) 10 ^m. Solid lines show results of calculations using Kholmogorov—Avrami model.
The X-ray diffraction analysis to determine the emulsions' crystallinity was done on a Bruker D8 Advance powder diffractometer using Cu radiation. Conventional Bragg—Brentano geometry was used with a Ni filter and a 29 scan range of 10° to 60° at 0.02° step intervals and at a scan rate of 1 s/step. The emulsion samples were placed in a standard top-loading polycarbonate sample holder and supported on a horizontal rotating sample stage. Structural information obtained from the literature ofthe various solid phases of ammonium nitrate was used to do Rietveld refinement of the complete diffraction pattern and determining the degree of crystallinity was done using the Topas 3.0.
RESULTS AND DISCUSSION
The basic idea of the work was the clearing up of the role of a surfactant in the kinetics of crystallization of an oversatura
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