What are the stabilizing factors of emulsification?

The emulsion is a heterogeneous dispersion system with thermodynamic instability and the droplet tends to coalesce automatically. The early theory of emulsion stability has noted that the addition of surfactants, polymers, and even solid particles plays an important role in the formation and type of emulsion, but the theoretical transverse form of stability has not been very clear. Studies of surfactant directed adsorption of monolayers at the oil/water interface reveal a possible picture of emulsion stability:

The unstable mode of emulsion: stratification, sedimentation, flocculation, aggregation, coalescence, deformation, demulsification, etc., as shown in the figure

The stabilizing factor of the emulsion

In practical application, the stability of emulsion refers to the ability of dispersed droplets to inhibit coalescence. The coalescence rate of dispersed phase droplets is the most important factor in measuring the stability of emulsion, which can be achieved by measuring the change of the number of droplets per unit volume of emulsion with time. The speed of the process is mainly related to the physical properties of the interfacial film, the electrostatic repulsion between droplets, the spatial obstruction of the polymer film, the viscosity of the continuous phase, the size and distribution of droplets, the ratio of phase to volume, and the temperature.

Among these factors, the physical properties of the interface film, the electrical action and the space barrier action are the most important.

(1) Physical properties of the interfacial film The collision of droplets of dispersed phase emulsion is the premise of coalescence. Coalescence continues, and small droplets become large droplets until the emulsion is broken. The mechanical strength of droplet interfacial film is the primary factor that determines the stability of emulsion during droplet collision and coalescence. In order to have high mechanical strength, the interface film must be a condensed film, and there is strong lateral force between the surfactant molecules forming the interface film. The interfacial film must also have good membrane elasticity so that it can be automatically repaired when the local area is damaged due to droplet collision.

(2) Electrical action The surface of the emulsion droplet can be charged for a variety of reasons: ionization of ionic surfactants, adsorption of certain ions on the droplet surface, friction between the droplet and the medium, etc. The charging of o/w emulsion droplets plays an important role in preventing droplet aggregation, coalescence and even emulsion breaking. According to the colloidal stability theory, the van der Waals force attracts the droplets to each other, and when the droplet is close to the surface, the electrostatic repulsion prevents the droplet from getting closer. Obviously, if the repulsion effect is greater than the attraction effect, the droplets are not easy to collide and coalesce, and the emulsion is stable. Otherwise, coalescence and demulsification will occur.

As for w/O type emulsions, the water droplets are less charged, and because of the small dielectric constant of the continuous medium and the thick double electric layer, the electrostatic effect has little influence on the stability of the emulsions.

(3) Spatial stabilization When the polymer is used as an emulsifier, the interface layer is thick, like forming a thick lyophilic protective layer around the liquid drop, which constitutes a spatial barrier for the proximity and contact of the liquid drop. The lyophilic nature of the polymer molecules also causes the protective layer to contain a considerable amount of continuous phase liquid, similar to a gel. Therefore, the interfacial region has high interfacial viscosity and good viscoelasticity, which will prevent droplet merging and maintain its stability. Even if some droplets coalesce, polymer emulsifiers often gather in the form of fibers or crystals at the reduced interface, making the interface film of the droplets thicker and preventing further coalescence of the droplets.

(4) Distribution of droplet size When the same volume of dispersed phase is dispersed into droplets of different sizes, the boundary area of the large droplet system is smaller than that of the small droplet system, and the interface energy is lower. Therefore, it has high thermodynamic stability. When there are both large and small droplets in the milky liquid system, the small droplets automatically decrease and the large droplets increase. If this process continues, it will eventually break the milk. Therefore, emulsions with uniform droplet size distribution have better stability than emulsions with equal droplet size distribution but wide droplet size distribution.

The change of temperature can cause changes in the interfacial tension, the properties and viscosity of the interfacial film, the relative solubility of the emulsifier in the two phases, the vapor pressure of the liquid phase, and the thermal motion of the dispersed phase droplets. These changes

It will affect the stability of the emulsion, and may even cause the emulsion to be deformed and demulsified.