The demulsification principle of emulsions and the selection of emulsifiers

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I. Delaying milk production

Demulsification refers to the complete breakdown of an emulsion, resulting in two immiscible phases. Essentially, demulsification eliminates the conditions that would otherwise stabilize the emulsion.

The process by which dispersed droplets aggregate and separate into layers.
1. Demulsification process

The process of breaking down an emulsion typically involves two steps.

The first step is the flocculation process.
During this process, the dispersed phase particles aggregate into clusters, but each particle still exists. The flocculation process is reversible, meaning that the aggregated particles can separate under external influences, existing in a dynamic equilibrium of formation and dissociation. If the density difference between the flocs and the medium is large enough, it will accelerate stratification; if the concentration of the emulsion is high enough, its viscosity will increase significantly.

Flocculation process diagram

        raw water flocculent                      condensation                     There are two mechanisms for the formation of sedimentation and flocculation.

(1) Charge neutralization mechanism

When charged dispersed particles encounter a polymer with the opposite charge, the electrostatic attraction between opposite charges neutralizes some of the charge, reducing the electrostatic repulsion between particles and resulting in flocculation.

       Inlay model diagram

(2) Bridging mechanism

This flocculation is mainly caused by the bonding or anchoring of certain chain segments in the surfactant molecules on the surface of the liquid droplets. This flocculation is related to the length of the chain segments in the surfactant molecules, the strength of their mutual affinity, and the number of adsorption centers.

• Long-chain polymer molecular segments adsorbed on the surfaces of two or more colloidal particles, forming bridges.
  
  

• polymer bridging adsorbed on the surfaces of two colloidal particles
  
  is aggregation.
  The aggregated droplets merge into larger droplets, the interfacial film breaks down, leading to a reduction in the number of droplets and the eventual complete breakdown of the emulsion.

Bad, the process is irreversible, and is often referred to as the final destructive stage of “aggregation”.

Emulsion coagulation and demulsification is a process consisting of two consecutive reactions: flocculation precedes aggregation, and the overall rate is controlled by the slow reaction. (Figure 7-4: Several ways in which

emulsions
are unstable)

(a) Agglomeration; (b) Demulsification; (c) Flocculation or aggregation; (a) Layering
2. Basic principles of demulsification

It is generally believed that the breakdown of emulsions involves a process of stratification, flocculation, and aggregation. Based on research results, the currently accepted demulsification mechanisms are as follows:

(1) Phase transfer-inverse phase transformation mechanism

When a demulsifier is added, a phase inversion occurs, resulting in an emulsion of the opposite type to that formed by the emulsifier. Such demulsifiers are called anti-phase demulsifiers. These demulsifiers react with the hydrophobic portion of the emulsifier to form a complex, thereby causing the emulsifier to lose its emulsifying properties.

(2) Displacement effect of demulsifiers

Because demulsifiers have low surface tension and good surface activity, they are easily adsorbed on the oil-water interface, replacing the original emulsifier at the interface. Since demulsifier molecules cannot form a strong interfacial film, the interfacial film is easily destroyed and demulsified under heating or mechanical stirring.

(3) Addition of electrolytes

For dilute emulsions that are mainly stabilized by the repulsive effect of the diffuse electric double layer, the addition of an electrolyte can compress the electric double layer, which is conducive to the occurrence of aggregation. Generally, high-valence counterions with opposite charges to the outer phase surface have a better demulsification effect, and the electrolyte concentration used for demulsification is relatively high.

(3) Destroy the emulsifier

This is a type of method that destroys the emulsifier of a stable emulsion, the most common of which is chemical destruction. For example, when soap is used as an emulsifier, adding acid generates fatty acids with low surface activity, thereby destroying the emulsion; when sodium fatty acid or potassium fatty acid is used as an emulsifier, adding high-valence metal salts destroys the chemical structure of the emulsifier, thus achieving the purpose of demulsification. In addition, for some natural products and emulsions with macromolecular substances as emulsifiers, microbial demulsification can be used, that is, some microorganisms grow by consuming surfactants and exert a bioallosteric effect on the emulsifier, thereby destroying the emulsion.
( 4 ) Wetting effect

For emulsions stabilized by solid powders, wetting agents with good wetting properties can be added to change the hydrophilicity and lipophilicity of the solid powders, causing the solid powders to desorb from the interface and enter the aqueous or oil phase, thereby disrupting the emulsion.

( 5 ) Flocculation-aggregation

Because nonionic demulsifiers have a relatively large molecular weight, under heating and stirring, the dispersion of these larger molecular weight demulsifiers in the emulsion causes the flocculation of fine droplets, resulting in the aggregation of the dispersed phase droplets into loose aggregates. Within these aggregates, the individual fine droplets remain. This flocculation process is reversible. The subsequent aggregation process irreversibly agglomerates these loose aggregates into larger droplets, leading to a reduction in the number of droplets. Once the droplets grow to a certain diameter, the water and oil separate due to the difference in their relative densities.

( 6 ) Collision-induced demulsification of the interfacial membrane

The amount of high molecular weight and ultra-high molecular weight demulsifiers added is only 10-6 g/L, while the area of the interfacial film is quite large. If 10 ml of water is dispersed in crude oil, the total area of the oil-water interfacial film of the formed water-in-oil emulsion can reach 6~600 m2 ^. Therefore, a small amount of additive is difficult to replace such a large interfacial film. The proposed mechanism is that under heating and stirring conditions, the demulsifier has more opportunities to collide with the liquid droplet interfacial film or replace a small part of the active substances, breaking the interfacial film and greatly reducing the stability of the interfacial film, thus causing flocculation and aggregation.
(8) Demulsification of interfacial film wrinkling

Microscopic studies of emulsion droplets revealed that generally stable W/O type emulsions possess two or more water rings, with an oil ring between the two water rings. The aforementioned theories struggle to explain the demulsification of this type of emulsion. A newly proposed mechanism suggests that under heating and stirring, a demulsifier adsorbs onto the interfacial film, causing the film to wrinkle, deform, and become brittle, leading to its destruction. At this point, the internal water rings of the droplets connect and begin to coalesce, then aggregate with other droplets, resulting in demulsification.

(9) Solubilization mechanism

The demulsifier used can form micelles with just one or a few molecules; these polymer coils or micelles can solubilize emulsifier molecules.

It can cause the emulsion to break down.