Differences between nanoemulsion, microemulsion, and ordinary emulsion and a brief introduction to the preparation method of nanoemulsion
Emulsion, in daily life, common lotions include food milk and cosmetic lotions. Lotions are usually a mixture of two immiscible liquids. In the emulsion, one of the two liquids exists in the form of droplets in the other liquid. The droplet phase is called the dispersed phase or internal phase, and the other is called the continuous phase or external phase. The immiscible two phases in common emulsions are generally water phase and oil phase, which can be divided into water-in-oil (W/O) emulsions and oil-in-water (O/W) emulsions according to the different dispersed phases. In practice, oil-in-oil (O/O) emulsions formed by two mutually immiscible organic solutions also exist, but their applications are rare. Double and multiple emulsions have emerged in the past three decades, which means that there may be more complex structures in a system, such as W/O/W type or O/W/O type. The former is an immiscible oil phase separating the two waters, and the latter is an immiscible water phase separating the two oils. The structures of ordinary emulsions and multiple emulsions are shown in Figures 1 and 2.
Figure 1 Schematic diagram of oil-in-water emulsion (O/W) and water-in-oil emulsion (W/O)
Figure 2 Schematic diagram of water-in-oil-in-water emulsion (W/O/W) and oil-in-water-in-oil emulsion (O/W/O) Schematic diagram of emulsions
classified according to the particle size of the internal phase
traditional emulsions, nanoemulsions and microemulsions
Traditional emulsions, sometimes referred to as conventional emulsions, emulsions or giant emulsions, usually refer to dispersion systems with droplet diameters between 300 nm and 100 μm. Judging from the diameter range of the droplets, most of them belong to a coarse dispersion system. This type of colloidal system is kinetically unstable, meaning that the dispersed oil and water phases have lower free energy than the emulsion itself. Therefore, the free energy of the oil-water interface is positive, and there is a high surface tension between the two phases. We know that a negative free energy means that the reaction process is spontaneous. A positive free energy is not conducive to the interaction between the two phases, because strong hydrogen bonds will form between water molecules in the water phase, but they will not interact with molecules in the oil phase. This is the general term hydrophobic effect, so traditional emulsions always have a tendency to break emulsion over time. In addition, these emulsions tend to be opaque because the droplet diameter is in a similar range to the wavelength of light and strongly reflects light (provided that the difference in refractive index between the aqueous phase and the oil phase is not very close to zero).
In contrast, some colloidal dispersion systems have better physical and chemical stability and bioavailability due to their greatly reduced particle size compared to traditional emulsions. This is commonly referred to as microemulsions and nanoemulsions.
In 1943, researchers first reported the dispersion system of microemulsion. Mixing water and oil with large amounts of surfactants and co-surfactants (typically medium chain length alcohols) spontaneously forms a transparent or translucent system, which can be oil-in-water (O/W) or water-in-oil (W/O). It is a thermodynamically stable isotropic liquid, microscopically composed of droplets of one or two liquids stabilized by a surfactant interface film. The droplets of the dispersed phase are spherical, but the radius is very small, usually in the range of 10-100 nm. However, it should be noted that the microemulsion can only remain kinetically stable under a specific composition ratio and environmental conditions. Once these conditions change, it may be unstable (such as dilution, addition of components, or changing temperature, etc.)
Nanoemulsions generally refer to a homogeneous system formed by two immiscible phases (oil and water) by adding suitable emulsifiers and using some means. The droplet size of nanoemulsions is generally between 20 and 300 nm, and the particle size distribution is relatively narrow. Compared with traditional emulsions, nanoemulsions have many advantages that are conducive to food processing. For example, nano-sized particle sizes can stabilize the system well and reduce emulsion instability caused by aggregation or gravity. Compared with microemulsions, nanoemulsion systems require lower amounts of surfactants, and can use biocompatible biomacromolecular surfactants, such as proteins, etc. At the same time, nanoemulsions use less organic co-solvents than microemulsions, so the current food and beverage industry pays more attention to the production of nanoemulsions than microemulsions. In addition, from the production process, nanoemulsions usually require high-energy external forces to form (such as shearing, homogenization, etc.). Compared with self-assembly or nanocarriers formed by covalent bonds, the encapsulated food functional factors are released after digestion. It is more likely to be absorbed. See Figure 3 for a comparison of the physical properties of several colloidal systems.
Figure 3 Relationship between particle size and stability of different types of emulsions.
Emulsion preparation methods
depend on the physical and chemical mechanism of potential droplet rupture. There are two main methods for the formation of nanoemulsions: high-energy external force emulsification (high-pressure homogenization, micro-jet, sound, etc.) and low-energy emulsification (e.g. emulsion phase inversion (EPI), spontaneous emulsification).
High-pressure homogenization method
High-pressure homogenization is when the material is subjected to strong shear force and pressure under high pressure to achieve crushing and emulsifying effects. The main reasons for shear force and pressure behavior are the cavitation effect, laminar flow effect and Rui flow effect. The high-pressure homogenizer can provide the energy needed by the system in a short time and obtain nanoemulsions with conventional properties.
Sonic emulsification Sonic
emulsification is a method that uses an acoustic device and uses the action of sound waves to crush and disperse the immiscible liquid in the system uniformly, and form an emulsion with the surrounding liquid. During the acoustic process, a large number of bubbles are generated in the liquid. These small bubbles gradually grow and increase with the acoustic vibration, and then suddenly split and break. The split small bubbles continue to grow and break. This phenomenon is called acoustic cavitation effect. Compared with ordinary emulsification methods, sonic emulsification has small emulsion particle size, stable emulsion system and small power required. Although sonification has a significant effect in reducing particle size, it can only be used in laboratory research or small-batch production, so its application is limited.
微射流乳化法
The microjet emulsification method usesmicrojet high-pressure homogenization equipment and uses a high-pressure pump to allow materials to enter the reaction chamber. In the reaction chamber, the raw material is divided into multiple streams to form a high-speed fluid, and enters the impact zone of the reaction chamber in a high-speed jet state, performing high-frequency shearing, and at the same time, the jets undergo more intense vertical collision. Most of the energy is released during the collision, creating a 90% pressure drop. In the impact zone, shearing and mutual impact occur between the materials, which makes the liquid droplets in the material highly broken, achieving homogeneous emulsification of the material. Studies have shown that the particle size of emulsions prepared by microjet emulsification method is usually less than 200nm, and it is also common to be less than 100nm. As small as research reports that the particle size of microjet emulsions is 7nm, and the distribution is concentrated (PDI is generally less than 0.2). Strong repeatability, compared with the emulsions prepared by this method, the use of emulsifiers can be reduced, and the products prepared are clearer, transparent, and have higher stability. The microjet homogenization method has been used by some international brands in the preparation of high-end products. nbsp;
Preparation of nanoemulsions by low-energy emulsification Method
Low-energy emulsification is a method of preparing nanoemulsions by utilizing the internal energy within the system itself. Low-energy emulsification methods include phase transition temperature method (P method), inverse phase emulsification method (PIC method) and self-emulsification method.
Phase transition temperature method (PIT method)
The phase transition temperature method is a method based on the change of molecular geometric characteristics of non-ionic surfactants with temperature changes. In a ternary system containing water-oil-surfactant, when the temperature of the system rises to the phase transition temperature, the spontaneous curvature of the surfactant is close to 0, and the surface tension is low at this time. Then the system is cooled instantaneously to obtain a nanoemulsion with a smaller particle size. At low temperatures, the spontaneous curvature of the surfactant is a large positive value, which is conducive to the formation of a stable oil-in-water (O/W) emulsion. At high temperatures, the spontaneous curvature of the surfactant is large negative, thus facilitating the formation of stable water-in-oil (W/O) emulsions. Inverse
emulsification (PIC method)
Reverse emulsification method is a method of gradually dripping water into a certain ratio of oil-surfactant solution at a constant temperature. With the increase of water content, the surfactant undergoes reverse transformation and finally obtains O/W nanoemulsion. Therefore, this method is called reverse solidification method. Studies have shown that during the emulsification process, with the increase of water content, the continuous phase of the system changes from oil phase to layered liquid crystal phase, then from layered liquid crystal phase to gel phase, and finally from gel phase to water continuous phase nanoemulsion. In addition, the research shows that there is a better HLB range for the reverse emulsion method. Within this HLB range, the particle size of the nanoemulsion obtained by the system is as small as, while the particle size of the nanoemulsion prepared by the PIT turning method has a small relationship with the HLB value. Figure 4 shows the process of forming nanoemulsions by reverse emulsification.
Figure 4 Reverse emulsification method Forming nanoemulsions
Self-emulsification method Self-emulsification
method is also the microemulsion dilution method. The method of forming nano-particle sizes by using chemical energy released during the microemulsion dilution process is called the microemulsion dilution method. During the dilution process, there is no phase change in the system, that is, there is no positive or negative transition in the spontaneous curvature of the surfactant. This is an important difference between the microemulsion dilution method and the reverse emulsification method and the phase transition temperature method. During the process of diluting the microemulsion, part of the co-surfactant enters the water phase from the oil phase. Since there is not enough surfactant concentration on the surface of the droplet to maintain a low interfacial tension, the original microemulsion cannot maintain thermodynamic stability, resulting in a kinetically stable nanoemulsion. Figure 5 shows the process of forming nanoemulsion by self-emulsification. Some studies have pointed out that the formation mechanism of nanoemulsions is that during the dilution process, the change of water content causes the spontaneous curvature of the concentrated liquid system to change, which reduces the system's ability to solubilize the oil phase, and causes the oil phase originally solubilized in the bicontinuous microemulsion phase to be supersaturated and self-emulsified. Nanoemulsions are prepared by the microemulsion dilution method. The addition of alcohol can change the softness of the surfactant layer and reduce the interfacial tension of the system, thus contributing to the formation of the microemulsion region.
图5 自乳化法形成纳米乳液
For more information, please refer to:
1.& nbsp; Application and differences between high-pressure microjet homogenizer and high-pressure homogenizer in the preparation of nanoemulsions and liposomes.
2.& nbsp;Brief introduction to the working principle and characteristics of microjet high-pressure homogenizer.
3.& nbsp; Genizer Microjet Diamond Interactive Cavity Use Guide.
more details, please contact
 | manager Wang Tel: 13020218906 Email: biotech@willnano.com Website: www.willnano.com Suzhou Microfluidic Nanobiotechnology Co., Ltd. |
苏州微流纳米Vic
