With asphaltenic-surfactant molecules, the side chains can extend considerably into the oil phase and steric repulsion can maintain the interface at a distance sufficient to inhibit coalescence.
Both of these effects oppose film drainage and inhibit coalescence. The state of asphaltenes in the crude oil has an effect on its emulsion-stability properties. While asphaltenes stabilize emulsions when they are present in a colloidal state not yet flocculated , there is strong evidence that their emulsion-stabilizing properties are enhanced significantly when they are precipitated from the crude oil and are present in the solid phase. The effect of polar fractions primarily asphaltenes on the film properties was investigated by Stassner.
Adding the precipitated asphaltenes back to the deasphalted oil in increasing quantities resulted in the formation of rigid or solid films and increasingly stable emulsions. This was further substantiated by Bobra.
Emulsification tendencies reduce with increasing aromatic content of the crude oil. Asphaltenes, apart from stabilizing emulsions themselves, alter the wettability of other solids present and make them act as emulsifying agents for water-in-oil emulsions.
They are heterocompounds, like asphaltenes, that contain:. Molecular weights of resins range from to 2, As Fig. As Figs. It appears that the asphaltene-resin ratio in the crude oil is responsible for the type of film formed solid or mobile and, therefore, is directly linked to the stability of the emulsion. Waxes are high-molecular-weight alkanes naturally present in the crude oil that crystallize when the oil is cooled below its "cloud point.
There are two types of petroleum waxes :. Waxes by themselves are soluble in oil and, in the absence of asphaltenes, do not form stable emulsions in model oils. Therefore, waxes can interact synergetically with asphaltenes to stabilize emulsions.
The physical state of the wax in the crude oil also plays an important role in emulsion stabilization. Waxes are more apt to form a stable emulsion when they are present as fine solids in the emulsion; thus, waxy emulsions are more likely at lower temperatures. Waxes, being oil-wet, have a tendency to stabilize water-in-oil emulsions. Crudes that have a high cloud point generally have a greater tendency to form stable and tight emulsions than crudes with low cloud points.
Similarly, lower temperatures generally enhance the emulsion-forming tendencies of crude oils. Fine solid particles present in the crude oil are capable of effectively stabilizing emulsions. The effectiveness of these solids in stabilizing emulsions depends on factors such as [13] [18] [19] :. Furthermore, solid particles at the interface may be electrically charged, which may also enhance the stability of the emulsion.
Particles must be much smaller than the size of the emulsion droplets to act as emulsion stabilizers. Typically these solid particles are submicron to a few microns in diameter. The wettability of the particles plays an important role in emulsion stabilization. Wettability is the degree to which a solid is wetted by oil or water when both are present. If the solid remains entirely in the oil or water phase, it will not be an emulsion stabilizer.
For the solid to act as an emulsion stabilizer, it must be present at the interface and must be wetted by both the oil and water phases. In general, oil-wet solids stabilize a water-in-oil emulsion.
Oil-wet particles preferentially partition into the oil phase and prevent the coalescence of water droplets by steric hindrance. Similarly, water-wet solids stabilize a water-continuous or an oil-in-water emulsion. Examples of oil-wet solids are:. Water-wet particles can be made oil-wet with a coating of heavy organic polar compounds. When solids are wetted by the oil and water intermediate wettability , they agglomerate at the interface and retard coalescence. These particles must be repositioned into either the oil or water for coalescence to take place.
This process requires energy and provides a barrier to coalescence. The role of colloidal solid particles in emulsion stability and the mechanisms involved are summarized in the following points. This film provides steric hindrance to the coalescence of water droplets.
The presence of solids at the interface also changes the rheological properties of the interface that exhibits viscoelastic behavior. This affects the rate of film drainage between droplets and also affects the displacement of particles at the interface. It has also been demonstrated [6] that for asphaltenes and waxes to be effective emulsifiers, they must be present in the form of finely divided submicron particles. Temperature can affect emulsion stability significantly.
Temperature affects the physical properties of oil, water, interfacial films, and surfactant solubilities in the oil and water phases. These, in turn, affect the stability of the emulsion. Perhaps the most important effect of temperature is on the viscosity of emulsions because viscosity decreases with increasing temperatures Fig.
This decrease is mainly because of a decrease in the oil viscosity. When waxes are present the temperature of the crude is below its cloud point and are the source of emulsion problems, application of heat can eliminate the problem completely by redissolving the waxes into the crude oil.
Temperature increases the thermal energy of the droplets and, therefore, increases the frequency of drop collisions. It also reduces the interfacial viscosity, which results in a faster film-drainage rate and faster drop coalescence. However, even at higher temperatures, a kinetic barrier to drop coalescence still exists. Temperature influences the rate of buildup of interfacial films by changing the adsorption rate and characteristics of the interface.
It also influences the film compressibility by changing the solubility of the crude oil surfactants in the bulk phase. Slow degassing removal of light ends from the crude oil and aging lead to significant changes in the interfacial film behavior at high temperatures. The films generated by this process remain incompressible and nonrelaxing solid films at high temperatures at which emulsion resolution is not affected by heating.
Emulsion droplet sizes can range from less than a micron to more than 50 microns. Droplet-size distribution is normally represented by a histogram or by a distribution function. Emulsions that have smaller size droplets will generally be more stable. For water separation, drops must coalesce—and the smaller the drops, the greater the time to separate. The droplet-size distribution affects emulsion viscosity because it is higher when droplets are smaller.
Emulsion viscosity is also higher when the droplet-size distribution is narrow i. The pH of water has a strong influence on emulsion stability. Adding inorganic acids and bases strongly influences their ionization in the interfacial films and radically changes the physical properties of the films.
The pH of water affects the rigidity of the interfacial films. It was reported [3] that interfacial films formed by asphaltenes are strongest in acids low pH and become progressively weaker as the pH is increased. In alkaline medium, the films become very weak or are converted to mobile films.
Natural surfactants are gaining interest as environmental and health issues are of concern. Polyoxyethylene surfactants are widely utilized in industrial applications, like coatings, food products, agriculture formulations, personal care and petroleum, where emulsion and foam stabilization is important. Alcohol ethoxylates are replacing more toxic alkylphenol ethoxylates that have been traditionally used as emulsifiers in many applications.
However, straight-chain alcohol ethoxylates do not demonstrate as good emulsifier properties as alkylphenol ethoxylates, mainly due to the bulky tail of the latter. This is due to differences in the size of the polar head and the hydrocarbon tail that hinder the formation of a closely packed film at the interfaces.
The adsorption and surface rheology properties of two technical-grade non-ionic surfactants based on C 10 -Guerbet alcohol differing in the numbers of EO groups have been studied. Surface pressure isotherms for both surfactants were fitted to the reorientation model. However, surface rheology data have to be interpreted differently. It will be shown that C 10 EO 6 can be interpreted in the framework of diffusional relaxation process, whereas C 10 EO 14 deviates from a diffusional controlled process.
Its surface rheology response is closer to that of non-ionic polymer surfactants. The solid and dashed lines are the best fit to experimental data obtained from a diffusional model. It is shown that only the experimental data values of C 10 EO 6 surfactant are in good agreement with the proposed model.
Figure 1. Solid and dashed lines are the best fit of the experimental data to the diffusional model. Figure 2. Nevertheless, the adsorbed films behave differently depending on the number of oxyethylene groups.
Thus, for the smaller surfactant, the adsorption and rheological data are well fitted to a diffusional model, whereas the larger C 10 EO 14 surfactant shows a surface behavior closer to that of polymeric surfactants. Moreover, comparison of the dilatational elasticity and viscosity of both surfactants indicates that elasticity is enhanced by increasing the number of EO groups. The elasticity of adsorbed surfactant films is directly related to foam and emulsion stability.
Therefore, the C 10 EO 14 surfactant is likely to form more stable layers against coalescence than the shorter C 10 EO 6 surfactant. With the exception of some microemulsions an emulsion is always thermodynamically unstable. That is because the interfacial tension is always greater than zero so it always costs energy to increase the interfacial area.
Emulsions exist only because they are kinetically stable. If you calculate the energy as two emulsion droplets approach and merge it will look something like this:. At intermediate distances between the drops the energy goes up, and this creates a repulsive force that keeps the droplets apart. The barrier can have many causes. For example emulsions in water are often charged - typically the droplets carry a negative charge. The droplets then repel each other due to this electrostatic repulsion.
Another common phenomenon is steric stabilisation where the droplets have some large floppy polymer adsorbed at the surface. Your system is one of the rarer cases because it is a Pickering emulsion. The mustard particles adsorb at the oil water interface and they create the barrier that stops the droplets from merging. Sign up to join this community. The best answers are voted up and rise to the top.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. What causes an emulsion to be stable or unstable? Ask Question. Asked 5 years, 10 months ago.
0コメント