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Emulsification of surfactants

The molecular structure of surfactants is amphoteric

One end is a hydrophilic group and the other end is a hydrophobic group.1 Hydrophilic groups are often polar groups, such as carboxylic acids, sulfonic acids, sulfuric acids, amino or amine groups and their salts, and hydroxyl groups, amide groups, ether bonds, etc. can also be used as polar hydrophilic groups.2 Hydrophobic groups are often non-polar hydrocarbon chains, such as hydrocarbon chains with more than 8 carbon atoms. Its universal applications are all based on the basic role of surfactants, i.e., the ability to adsorb at various interfaces and the formation of various ordered combinations in the liquid phase (even at interfaces) derived from a variety of functional roles. It is no exaggeration to say that surfactants have penetrated into various fields of application, and the common and general ones are wetting, solubilization, emulsification, emulsion breaking, dispersion, aggregation, foaming, defoaming, decontamination, washing, surface modification of solids, etc. Translated with www.DeepL.com/Translator (free version)

Emulsification, in which two immiscible liquids are obtained to form an emulsion, is probably the most versatile property of surfactants in practical applications. Products such as coatings, insecticides, margarine, ice cream, cosmetics, metal cleaners, nanoparticles, and textile fiber oils are often involved in emulsification technology, so the main focus here is on the study of the role of surfactants in emulsification phenomena. An emulsion is a significantly stable dispersion formed by a liquid droplet dispersed in the form of droplets of a certain size in another liquid that is immiscible with it. Depending on the size of the mass point of the dispersed phase, emulsions can be classified into three types.

1 Ordinary emulsions, i.e. the most common kind of opaque looking emulsions with particle size > 400 nm (0.4 μm), which can be easily observed under a microscope.

2 microemulsions with transparent appearance and particle size <100nm (0.1μm).

3 nm or fine emulsion with bluish-white appearance and particle size between the first two 100 nm ≤ particle size ≤ 400 nm.

Definition of emulsifier

Two mutually immiscible pure liquids cannot form an emulsion. To make the dispersion of one liquid in another liquid species also has sufficient stability, that is, can form an emulsion, then a third component must be added to stabilize the system. The third component becomes an emulsifier, which is usually a surfactant, but of course some systems of fine solid particles can also be used as emulsifiers. Conventional type of emulsifier does not have to be a single substance, and in practice, the most effective emulsifier is usually a combination of two or more substances. no.1 ordinary emulsion according to the nature of the dispersed phase, ordinary emulsion can be divided into “oil in water” (O / W) type emulsion and “oil in water “(W/O) type. The type of emulsion formed by oil and water depends mainly on the nature of the emulsifier used and to some extent on the process of preparation and the ratio of the oil and water phases. Generally, if the emulsifier dissolves more readily in the water phase than in the oil phase, an O/W type emulsion is likely to be formed; conversely, if the emulsifier dissolves more readily in the oil phase, a W/O type emulsion is likely to be formed.

The two types of lotions are easy to distinguish

  • 1 Emulsions are easily diluted by the external phase but not by the internal phase, so O/W emulsions are easily dispersed in water, while W/O emulsions are easily dispersed in oil.
  • 2 O/W emulsions have conductivity close to that of water, while W/O emulsions have insignificant conductivity.
  • 3W/O emulsions can be dyed by oil-soluble dyes, while these fuels stain O/W emulsions very weakly, but can be dyed by water-soluble dyes.
  • 4 If the two phases have different refractive indices, then the droplet can be observed with a microscope to determine its type. If the refractive index of the droplet is greater than that of the continuous phase, the droplet darkens when focused upward, and if the relative refractive indices of the two phases are known, then the substance in the droplet can be determined by this method.
  • 5 Filter paper test: drop the emulsion on the filter paper, a drop of O/W emulsion can immediately form a wider wetting area; while W/O emulsion cannot.

Formation of emulsions Ordinary emulsions are formed when one of two immiscible solutions is broken up and dispersed in a droplet state in the second one. Since the interfacial tension between droplets is always greater than zero, it is always extremely unstable thermodynamically. The role of emulsifiers is to keep these intrinsically unstable systems stable for an appropriate period of time. They do this by adsorbing at the liquid/liquid interface and forming a directional interfacial film to accomplish the stabilizing effect. Oriented interfacial films have two roles: to reduce the thermodynamic instability and to reduce the rate of agglomeration between liquid beads.

Factors Affecting Emulsion Stability The stability of an emulsion usually refers to the effect of the emulsion on the agglomeration that occurs against dispersed droplets. Slight rise or settling (stratification) of droplets due to density differences between the droplets and the continuous phase is not usually considered to be instability. Flocculation or coalescence between dispersed droplets, although also a form of instability, is not considered as severe as agglomeration and emulsion breaking as long as the liquid inside the droplets does not agglomerate. It has been found that the rate at which the liquid droplets of an ordinary emulsion agglomerate into larger droplets and cause the emulsion to break depends on a number of factors, roughly divided into the following six points.

  • 1 Physical properties of the interfacial film: the mechanical strength of the interfacial film is one of the main factors, and the formation of liquid crystals can also stabilize the emulsion.
  • 2 The presence of electrical and site-resistant agglomeration barriers on the dispersed beads: If an electrical charge exists on the dispersed beads, it will form an electrical barrier that prevents the dispersed beads from approaching each other, and it is generally believed that this factor is a significant factor only for O/W emulsions.
  • 3 continuous phase viscosity: under the specific concentration of oil, water and emulsifier, the formation of continuous phase viscosity increases also liquid crystal phase, which can greatly improve the stability of emulsions. Special ingredients can be added to the emulsion “thickener” to improve the viscosity.
  • 4 Liquid bead size distribution: the narrower the range of bead sizes, the more stable the emulsion. Emulsions with a fairly uniform particle size distribution are more stable than emulsions with a wide particle size distribution.
  • 5 phase volume ratio: As the dispersed phase in ordinary emulsion increases, the area of the interfacial film needs to keep increasing to wrap the dispersed liquid beads, so the instability of the system increases.
  • 6 temperature: changing the temperature will cause a series of factors in the system to change, including the interfacial tension between the two phases, the nature and viscosity of the interfacial film, the relative solubility of the emulsifier in the two phases, the vapor pressure and viscosity of the liquid, and the thermal motion of the dispersed phase mass points, etc. In particular, the increase in vapor pressure of the liquid caused by the increase in temperature will lead to an increase in the flow of molecules through the interfacial film, which will also reduce the stability of the emulsion. Phase transition can be made by changing certain emulsification conditions to change a normal emulsion from W/O to O/W type or vice versa.

These conditions include.

  • 1 the order of addition of the two phases (adding water to oil dissolved with emulsifier may result in a W/O type emulsion, while adding oil to water dissolved with the same emulsifier may result in an O/W type emulsion).
  • 2 the nature of the emulsifier (oil-soluble emulsifiers tend to form W/O-type emulsions, while water-soluble emulsifiers tend to form O/W-type emulsions).
  • 3 volume ratio of the two phases (increasing the oil/water ratio tends to produce W/O type emulsions, and conversely increasing the water/oil ratio tends to produce O/W type emulsions).
  • 4 dissolved emulsifier phase (the addition of hydrophilic surfactants as emulsifiers in the aqueous phase facilitates the formation of O/W-type emulsions).
  • 5 temperature of the system (for POE non-ionic surfactant-stabilized O/W emulsions, elevated temperature leads to increased lipophilicity of the surfactant and possible conversion of the emulsion to W/O; on the other hand, cooling may lead to stable conversion of some ionic surfactants to W/O).
  • 6 The amount of electrolytes or other additives can also lead to transphasing. Multiple emulsions People have studied multiple emulsions that

In part, this makes them potentially useful for the following reasons.

  • 1 Delivering drugs to designated parts of the body without causing toxic side effects to other organs.
  • 2 Delay the release of drugs with very short biological half-lives. Multiple emulsions are multilayered emulsions formed by dispersing one emulsion in another continuous phase. Common multiple emulsions are divided into two main categories: water-in-oil-in-water (W/O/W), and oil-in-water-in-oil (O/W/O).

Theory of emulsion type

1► Qualitative theory that qualitatively explains the formation of O/W and W/O type emulsions, the more current theory is that the interfacial tension (or interfacial pressure) between the two sides of the interfacial region formed by the surface active molecules adsorbed at the liquid-liquid interface and arranged in a directional manner is different. That is, the interfacial tension between the hydrophilic end of the surfactant and water is the same (or the interfacial pressure between hydrophilic head groups) and the interfacial tension between the pro-friend segment of the surfactant and the oil phase (or the interfacial pressure between the oleophilic base ends) is different. Thus, during emulsion formation, the interfacial region tends to bend toward the side with the higher interfacial tension (or the side with the lower interfacial pressure) to minimize the interfacial free energy. If the interfacial tension on the oleophilic side is greater than that on the hydrophilic side, the film will bend toward the oil phase to reduce the interfacial area on the oleophilic side, resulting in the oil being wrapped by water, thus forming an O/W emulsion. Conversely, the formation of W / O emulsion. Obviously, the water-soluble emulsifier in the water phase side to produce a lower interfacial tension, the formation of O / W emulsion; and oil-soluble emulsifier in the oil phase side to produce a lower interfacial tension, resulting in the formation of W / O type emulsion.

2► Kinetic theory of ordinary emulsion types Davies proposed a quantitative theory of ordinary emulsion types. According to this theory, when the oil and water phases are stirred in the presence of an emulsifier, the type of ordinary emulsion depends on the relative rates of two competing processes: agglomeration of oil beads and agglomeration of water beads. Assuming that stirring breaks both the oil and water phases into liquid beads, and the emulsifier is adsorbed at the interface around the beads, the one phase with the greater rate of agglomeration will become the continuous phase; if the rate of agglomeration of water beads is much greater than that of oil beads, an O/W type emulsion is formed; conversely, a W/O emulsion is formed. No.2 Microemulsion Microemulsion is referred to as microemulsion, which is an isotropic and transparent colloidal dispersion system formed spontaneously by two liquids that are not miscible in the presence of one or more amphiphilic compounds. Microemulsion dispersed phase beads are generally 10-100 nm in size, roughly between surfactant micelles and common hydrophobic colloidal particles, and much smaller than the size of ordinary emulsion beads. Similar to common emulsions, the main types of microemulsions are also O/W and W/O types, in addition to a toned continuous phase (also called microemulsion-in-phase), i.e., both water and oil are continuous in the bicontinuous phase for the emulsion.

The choice of surfactant used as an emulsifier in the emulsification method, the nature of the oil phase, water phase after a certain, the most important preparation of relatively stable emulsion is the choice of emulsifier. Because of the variability of the oil and water phase components, as well as the different types of emulsions required to be formed, it is practically impossible to find a universal good emulsifier. That is, after specifying the composition and properties of the oil phase, the water phase and the type of emulsion required, a suitable method is used to select the relatively best emulsifier. In spite of this, there are still some general rules that can be provided for the application of emulsifier selection.

  • 1 has good surface activity and the ability to reduce surface tension. This allows the emulsifier to adsorb well at the interface.
  • 2 emulsifier molecules or other additives at the interface can form a tightly arranged cohesive film, in which the molecules have strong lateral interactions.
  • 3 emulsifier emulsification performance and its affinity for the oil phase or water phase is related. Oil-soluble emulsifier easy to get W / O emulsion, water-soluble emulsifier easy to get O / W emulsion. Oil-soluble and water-soluble emulsifiers can sometimes have a better emulsification effect when mixed with two kinds of emulsifiers. Accordingly, the greater the polarity of the oil phase, the greater the requirement for emulsifiers to be hydrophilic; the smaller the polarity of the oil phase, the stronger the requirement for emulsifiers to be hydrophobic.
  • 4 Appropriate external phase viscosity to reduce the rate of droplet aggregation.
  • 5 special use (such as food emulsions, emulsion drugs) when choosing a non-toxic emulsifier.
  • 6 To be able to achieve the emulsification effect at the minimum concentration and the minimum cost.

There are 2 main methods currently used to select emulsifiers.

HLB method (hydrophilic-lipophilic equilibrium method) and PIT (phase transition temperature) method. HLB method is applicable to all types of surfactants, and PIT method is a supplement to HLB method and is only applicable to nonionic surfactants. Hydrophilic-lipophilic equilibrium (HLB method for short) The hydrophilic-lipophilic equilibrium method, or HLB method for short. The molecules of surfactants are all amphiphilic molecules containing hydrophilic and lipophilic groups, and the size and strength of hydrophilic and lipophilic groups in different emulsifier molecules are different. The HLB value is the balance of the size and ability of the two groups in the molecule, hydrophilic and lipophilic, and the total result of the hydrophilic-lipophilic balance of these groups, the HLB value is a relative value, and the HLB value of paraffin waxes with strong lipophilicity (completely non-hydrophilic) is 0. The smaller the HLB value, the stronger the lipophilicity; conversely, the stronger the hydrophilicity.1 The HLB value is additive.2 The nature of the oil phase is different, and the HLB value of the emulsifier has different requirements. The HLB value of the emulsifier should be consistent with the value required for the emulsified oil phase. 3 HLB value has certain limitations, because the HLB value does not give the best emulsification effect when the emulsifier concentration, but also does not predict the stability of the resulting emulsion.

Phase transition (PIT) method The water synthesis of hydrophilic groups (especially polyoxyethylene chains) of nonionic surfactants all decreases with increasing temperature, and the hydrophilicity of surfactants decreases, and their HLB values also decrease. In other words, the HLB value of nonionic surfactants is temperature dependent: when the temperature increases, the HLB value decreases; when the temperature decreases, the HLB value increases. This makes it possible to form O/W emulsions at low temperatures when using nonionic surfactants as emulsifiers, which may change to W/O emulsions at higher temperatures, and vice versa. For a certain oil/water system, each nonionic surfactant has a phase transition temperature PIT, at which the hydrophilic and lipophilic balance of the surfactant is just right. The emulsifier can be selected according to the PIT: above the PIT to form W/O emulsions, below the PIT to form O/W emulsions. no.4 Breaking the emulsion In some cases, the emulsification of oil/water phases is undesired. When mixing two phases that are not miscible, such as in reservoir extraction, crude oil is always combined with water or brine and contains natural emulsifiers such as bitumen and resin. These natural emulsifiers, mixed with other components of the crude oil such as paraffin, form a thick viscous interfacial film around the water droplets with the polar groups toward the water and the non-polar extremes toward the oil. This interfacial film has a high viscosity, resulting in the formation of a stable viscous W/O emulsion. It causes difficulty in extraction. Therefore, the main role of the emulsion breaker is to eliminate the effective role of the emulsifier, the choice of emulsion breaker should be for the characteristics of the emulsifier. Select emulsion breakers have several basic principles: 1 have a good surface activity, emulsifiers in the emulsion from the interface top down; 2 emulsion breakers in the oil / water interface to form an interfacial film, the role of external conditions or droplet collision is easy to break. 3 ionic emulsifiers can make droplets charged and stable, the choice of anti-number charge of ionic emulsion breakers can make the droplet surface charge neutralization. 4 large molecular weight of non-ionic or polymeric emulsion breaking agent dissolved in the continuous phase can be bridged by the role of droplet aggregation, and then aggregation, and broken emulsion. 5 solid powder emulsifier stable emulsion, you can choose a good wetting agent for solid powder as an emulsion breaker to make the powder completely wet and enter the aqueous phase or oil phase.

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