Surfactant Basics 2 (Emulsion, Emulsifiers)

First, an interface is a boundary surface that exists between two substances with different properties, and interfaces exist between liquids and solids, liquids and liquids, and liquids and gases.

Surfactants enhance performance by performing functions such as washing, emulsifying, dispersing, wetting, and penetrating at this interface.

Interface = a boundary surface that exists between two substances with different properties
Liquid and solid: cup and coffee, machine and lubricant 
Liquid and liquid: water and oil 
Liquid and gas: seawater and air, soap bubbles

Examples of roles of surfactants
Cleaning ・・・ Removing dirt 
Emulsification ・・・ Dispersion ・・ Making unmixable things easier to mix
Wetting / Penetration ・・・ Makes wetting and soaking easier

-Surfactants have different structures in their molecules with different properties: lipophilic groups (oil-fitting parts) and hydrophilic groups (water-fitting parts).

-Surfactants are broadly classified into four types according to the structure of the hydrophilic group: nonionic, anionic, cationic, and amphoteric (having both anionic and cationic groups).

It consists of seven short movies for each function. 
0:00　Introduction of surfactants functions　
0:16　Part1　Washability
1:00　Part2　Permeability
2:10　Part3　Dispersion
2:55　Part4　Foaming porperties
3:25　Part5　Defoaming properties
3:39　Part6　Smoothness
4:20　Part7　Antibacterial properties

Mixing water and oil is called emulsifying, and the resulting emulsion is called an "emulsion" or "emulsion.

 Water and oil do not mix and separate, but by adding a surfactant, the surfactant adheres to the oil or water and forms particles. Emulsification occurs when micelle particles are stabilized in water. Among emulsification, the phenomenon in which oil becomes transparent and mixes with water is called "solubilization. (In the solubilized state, the micelle particles appear transparent because their diameter is shorter than the wavelength of light.)

A liquid (water) in which other liquids exist as fine particles is called an emulsion, and a solid in which solids exist as fine particles is called a dispersion (or suspension).　In addition, emulsions and dispersions are usually distinguished according to whether they are in a liquid or solid state when mixed with water.

For example, when a liquid monomer is polymerized into fine particles in water to form solid polymer particles, this is also considered an emulsion (emulsion polymerization).

When a surfactant is added to water and oil that do not mix as they are, the surfactant comes to the contact surfaces where the water and oil repel each other, orientating hydrophilic groups in the water and lipophilic groups in the oil and preventing the water and oil from repelling (lowering surface tension).

When a drop of water drips slowly, it tries to form a sphere after falling for a while.This is because the surface of the water has a contracting force called surface tension, which causes the droplet to take the smallest surface area shape.The table below shows examples of surface tension of liquids.

Compared to water, hydrocarbons such as octane and benzene have a lower surface tension.

 Since the lipophilic groups of surfactants are mainly hydrocarbon groups, the presence of surfactants explains the decrease in surface tension at the water/air and water/oil interfaces, as shown in the figure below.

Fig. Relationship between surfactant behavior and changes in surfactant concentration

As the concentration of surfactant increases, it forms a monolayer tightly lined up on the surface of the aqueous solution, completely blocking direct contact between air and water (state (c)). This state corresponds to the point in the above figure where the curve of surface tension has dropped and is about to become horizontal, and from this concentration, at a slightly higher concentration, surfactant molecules gather in the aqueous solution by tens to hundreds, forming an aggregate (micelle) with their lipophilic groups facing inward and their hydrophilic groups facing outward. This surfactant micelle is called a "surfactant micelle. The lowest concentration at which these surfactant micelles form is called the critical micelle concentration (c.m.c.).

In order to stably emulsify oils of different properties, such as vegetable oils and agricultural oils, into water, it is necessary to select an appropriate surfactant for each application.

This selection is strongly related to the balance between hydrophilic and lipophilic properties of surfactants, and HLB (Hydrophile-Lipophile Balance) has been proposed as an indicator to show this balance. The method of calculating HLB using the organic and inorganic properties of the surfactant as indices is convenient.

In this formula, the organic and inorganic properties of an organic compound are calculated from the values shown in the table below.

Organic values: determined as 20 per carbon atom
Inorganic value: According to the value in the table above. However

(1) When used in the calculation of HLB, oxyethylene (-CH2CH2O-) is treated specially and given an inorganic value of 75 and an organic value of 40.
(2) The organic property should be added separately for carbon atoms in the inorganic group.
(3) Carbon atoms in both organic and inorganic groups are added to the organic value.
(4) Although two literature values exist, the value in parentheses () is more appropriate for emulsification in many cases.

Next, let us consider in detail why water and oil do not dissolve. The solubility parameter (SP value) is an important factor in determining whether water and oil dissolve into each other or are separated without dissolving.

This SP value is expressed as the square root of the aggregation energy density of the functional groups that make up the compound. Low molecular weight compounds can be compatible with each other even if these SP values are quite far apart, but high molecular weight compounds will not be compatible with each other if the SP values are usually about 1 or more apart.

Example:
Compound with high SP value: Water (SP value = 23.4)
Compound with low SP value: Toluene (SP value = 8.9)
Water and toluene do not dissolve each other.

Compounds such as water and toluene, which do not dissolve into each other, can be mixed in an emulsion because each compound is emulsified in water.

Note that not all emulsions can mix with each other, so a little caution is required.The ionic properties of emulsion particles are an important controlling factor in whether emulsions can mix.Ionicity of emulsion particles refers to the electrical properties of the particles, which can be divided into anionic, cationic, and nonionic properties.

When these emulsions, anionic and cationic emulsions, are mixed, the particles stick to each other and agglomerate, destroying the emulsion (if they are the same ion or a combination of nonionic emulsions, the emulsion can usually be mixed without destroying the emulsion). (If the same ions are mixed with each other or non-ionic emulsions are combined, the emulsion is usually not destroyed.)

Fig. Change of state when emulsions with different ionic properties are mixed

In the field of paints and adhesives, solvent-based and emulsion-based systems are used separately, depending on durability and the nature of the adherend.

Water-based paints and water-based adhesives are mainly composed of synthetic resin emulsions, and most of these synthetic resin emulsions are synthesized by a method called emulsion polymerization.

Emulsion polymerization is a method in which monomers with unsaturated double bonds are polymerized in micelles of an emulsifier to achieve high molecular weight. The molecular weight of the resulting resin usually exceeds 100,000, making it a very efficient method for obtaining high molecular weight resins while emulsifying them in water.

Emulsions with reactive emulsifiers, which combine unsaturated double bonds and emulsification in a single molecule, have also been developed. In emulsions using non-reactive emulsifiers, the emulsifier tends to be released from the resin when it dries to form a film, causing a decrease in adhesion to the adherend. On the other hand, this reactive emulsifier chemically bonds with the resin and is incorporated into the resin, so it is not released from the resin when the emulsion dries to form a film, resulting in excellent adhesive performance.

Clothing is made from natural fibers such as cotton, wool, and silk, and synthetic fibers such as polyester and nylon. These synthetic fibers are manufactured by synthesizing polymers through a polymerization reaction of monomers, which are then stretched into thin strips with diameters ranging from 1 to 100 μm.　Synthetic fibers are strong and can be freely modified in terms of thickness and shape to provide a variety of functions, and today their production volume exceeds that of natural fibers.

The following is a list of textile oils used in the production of synthetic fibers.

None of these functions are necessary in the final product, and since such oils sticking to the yarn cause uneven dyeing when coloring the cloth, the textile oils are usually removed in the refining process prior to dyeing.　Therefore, although the textile oil does not remain in the final product, it plays an extremely important role in the efficient production of synthetic fibers.

Now, this oil for textile is often given to yarns in the form of oil emulsified in water, or oil-in-water emulsion, mainly because of the following reasons.

(1) Components with excellent smoothness are strongly hydrophobic and usually do not dissolve in water, so in order to uniformly adhere the minimum amount required, they are made into an emulsion so that they can be freely diluted with water.

(2) Water as a dispersant has a large heat capacity and latent heat of evaporation, making it excellent for cooling yarns. In addition, water as a diluent is optimal because it is inexpensive and free from fire and other hazards.

Chemicals that impart various functions to synthetic fibers after dyeing are called textile finishing agents, which are also used in the form of emulsions.

 Examples of functions required for fibers include flexibility, texture such as resilience and elasticity, water and oil repellency, water absorbency, sewability, antistatic properties, antibacterial properties, wind resistance, anti-pilling properties, and various others.

Fig. How textile products are made

As mentioned above, textile finishing agents are agents that provide a great many functions, and no single agent alone can meet these requirements. Therefore, several types of agents must be mixed together, and this is where the emulsification function plays a major role.

In order to make a homogeneous solution, whether it is an aqueous solution or an organic solvent solution, the solute (what is dissolved in the solvent) must be dissolved in the solvent; it is easy to dissolve one solute in one solvent, but often impossible to dissolve several solutes with different properties in the same solvent.

The table above shows examples of wax and polyvinyl alcohol as solutes and their dissolution in solvents.

Wax has low polarity, so it dissolves in toluene, a non-polar solvent, but not in water, a polar solvent. On the other hand, polyvinyl alcohol has high polarity and is soluble in water but not in toluene. Thus, the properties of the solvent and the solute, i.e., the SP values, must be close to each other to ensure uniform dissolution.

 Therefore, these solutes with different properties (in this case, various finishing agents) are converted into one liquid by emulsification.

Some pesticides are called emulsions. This is a liquid form of pesticide to which an emulsifier or other agent has been added, and is designed so that the pesticide can be easily emulsified into water by pouring the emulsion into the water. While most emulsions are produced by mechanically applying shear force, pesticides are designed to be naturally emulsified (emulsified by simply adding oil to water) because they are used by farmers who do not have such machinery and by ordinary households.

Contribution of emulsifiers
(1) Lowering the interfacial tension between water and oil
(2) Giving an electric charge to the particle surface and repulsion between particles
(3) Forming an adsorption film on oil particles to protect the particle surface

As an emulsifier, a surfactant having both lipophilic and hydrophilic groups in a single molecule is usually used, so it has the action of (1). In addition, by using ionic surfactants (in combination), the action (2) can be added. In selecting the best emulsifier to emulsify a certain oil, the most difficult part is usually the optimization of the effect of (3). In order to efficiently achieve the action of (3), it is important that the emulsifier has strong interaction with oil particles and good affinity with water (balance of lipophilic and hydrophilic groups).

Cosmetics are broadly classified into skin care cosmetics, skin cleansing cosmetics, and finishing (makeup) cosmetics, This page introduces skin care cosmetics, especially cosmetic emulsions and creams, which are closely related to the emulsifying function.

The role of cosmetic emulsions and creams is to moisturize and soften the skin, especially the stratum corneum, the outermost layer of the skin, by replenishing moisture and oil.Emulsions, which emulsify oil in water, are therefore convenient because they provide moisture and oil to the skin at once and in relatively flexible proportions. This is where the emulsifying function plays an important role.

As a surfactant manufacturer, we offer a lineup of numerous emulsification and solubilization-related products.

The structure and features of our emulsifiers are shown in the table below. Nonionic and anionic emulsifiers are available, and each has its own characteristics in terms of emulsification, dispersion stability, particle size, etc. They are selected according to the purpose, and are often used in combination.

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