I. Several concepts about surfactants.
The property that can reduce the surface tension of the solvent is called surface activity, and the substance with surface activity is called surface active substance. The surface active substance that can associate molecules in aqueous solution and form micelles and other associates, and has high surface activity, and also has the functions of wetting, emulsification, foaming and washing is called surfactant. The contraction force of any unit length on the surface of a liquid is called surface tension, and the unit is N-m-1.
Second, the molecular structure characteristics of surfactants.
Surfactants are organic compounds with special structures and properties that can significantly change the interfacial tension between two phases or the surface tension of a liquid (generally water), and have properties such as wetting, foaming, emulsification and washing. In terms of structure, surfactants all have a common feature that their molecules contain two groups with different properties, one end is a long-chain non-polar group, which is soluble in oil but insoluble in water, also known as hydrophobic group or water-repellent group. The other end of the spectrum is water-soluble groups, namely hydrophilic groups or hydrophilic groups. The hydrophilic group must be sufficiently hydrophilic to ensure that the entire surfactant is soluble in water and has the necessary solubility. Since surfactants contain both hydrophilic and hydrophobic groups, they are thus soluble in at least one of the liquid phases. This property of surfactants that are both hydrophilic and lipophilic is called amphiphilicity.
There is also a special type of biphilic surfactant on the market, of which kynurenine glycol surfactant is one of the representative products. It has two pairs of hydrophobic and hydrophilic groups structurally bonded by a symmetrical and relatively “rigid” functional group in the middle. The baryonic surfactants are less stable foam and have specific outstanding dynamic wettability.
Types of surfactants.
Surfactant is an amphiphilic molecule with both hydrophobic and hydrophilic groups. The hydrophobic groups of surfactants are generally composed of long chains of hydrocarbons, such as straight-chain alkyl C8 to C20, branched-chain alkyl C8 to C20, alkylbenzene groups (alkyl carbon atoms number 8 to 16), etc. The difference of hydrophobic groups is mainly in the structural changes of hydrocarbon chains, which is smaller, while the variety of hydrophilic groups is larger. Therefore, the properties of surfactants are mainly related to hydrophilic groups in addition to the size and shape of hydrophobic groups. The structure of hydrophilic groups varies more than that of hydrophobic groups, so the classification of surfactants is generally based on the structure of hydrophilic groups. This classification is based on whether the hydrophilic group is ionic or not, and it is divided into anionic, cationic, nonionic, amphoteric and other special types of surfactants.
IV. Characteristics of surfactant aqueous solution.
1. Surfactant adsorption at the interface. Surfactant molecules have lipophilic and hydrophilic groups, which are amphiphilic molecules. Water is a strong polar liquid, and when surfactant is dissolved in water, its hydrophilic group is attracted to water and dissolved in water according to the principle of polarity similarity and repulsion, and its lipophilic group is repelled from water and leaves water, as a result, surfactant molecules (or ions) are adsorbed at the interface of two phases, so that the interfacial tension between two phases is reduced. The more surfactant molecules (or ions) are adsorbed at the interface, the greater the reduction in interfacial tension.
2、Some properties of adsorption membrane.
Surface pressure of adsorption film: surfactant is adsorbed at the gas-liquid interface to form an adsorption film, such as placing a frictionless movable float at the interface to push the adsorption film along the solution surface, the film generates a pressure on the float, and this pressure is called surface pressure.
Surface viscosity: the same as the surface pressure, surface viscosity is a property exhibited by the insoluble molecular film. Suspension of a fine metal wire platinum ring, so that its plane contact with the water surface of the tank, rotate the platinum ring, the platinum ring by the viscosity of water obstruction, amplitude gradually decay, according to which the surface viscosity can be measured, the method is: first in pure water surface experiments, measured amplitude decay, and then measured the formation of surface film decay, from the difference between the two to find the viscosity of the surface film. The surface viscosity is closely related to the solidity of the surface film, as the adsorption film has surface pressure and viscosity, it must have elasticity. The higher the surface pressure and the higher the viscosity of the adsorbed film, the higher its elastic modulus. The elastic modulus of the surface adsorption film has an important significance in the process of stabilizing the bubble.
3、The formation of micelles.
The dilute solution of surfactant obeys the law followed by the ideal solution. The amount of surfactant adsorbed on the surface of the solution increases with the concentration of the solution, and when the concentration reaches or exceeds a certain value, the amount of adsorption no longer increases. These excess surfactant molecules are disordered in the solution or exist in some regular way. Both practice and theory suggest that they form associates within the solution, which are called micelles. The minimum concentration of surfactants in solution to form micelles is called the critical micelle concentration (CMC).
HLB is the abbreviation of hydrophile lipophile balance, which indicates the hydrophilic and lipophilic balance of the hydrophilic and lipophilic groups of the surfactant, i.e. the HLB value of the surfactant. a large HLB value indicates a strong hydrophilic molecule and a weak lipophilic molecule; conversely, a strong lipophilic molecule and a weak hydrophilic molecule. The HLB value is a relative value, so when the HLB value is set, the HLB value of paraffin wax, which has no hydrophilic properties, is set at 0, while the HLB value of sodium dodecyl sulfate, which is more water-soluble, is set at 40. Generally speaking, emulsifiers with HLB values less than 10 are lipophilic, while those with values greater than 10 are hydrophilic. Therefore, the turning point from lipophilic to hydrophilic is about 10.
According to the HLB value of the surfactant, the possible uses can be roughly understood, as shown in the table on the left, the HLB value of the surfactant suitable for use as a water-in-oil emulsifier is 3.5 to 6, while the HLB value of the water-in-oil emulsifier is 8 to 18.
Fifth, the role of emulsification and solubilization.
Two mutually insoluble liquids, one with particles (droplets or liquid crystals) dispersed in the other to form a system called emulsion. The formation of emulsion due to the increase in the boundary area of the two liquids, so this system is thermodynamically unstable, in order to make the emulsion stable need to add a third component – emulsifier to reduce the interfacial energy of the system. Emulsifier belongs to the surface active agent, its main function is to play the role of emulsion. Emulsion in the presence of droplets of that phase is called the dispersed phase (or internal phase ﹑ discontinuous phase), connected to another phase called the dispersion medium (or external phase ﹑ continuous phase).
1, emulsifiers and emulsions. Common emulsion, one phase is water or aqueous solution, the other phase is not miscible with water organic substances, such as grease, wax, etc.. Water and oil emulsions formed, according to its dispersion can be divided into two kinds: oil dispersed in water to form an oil-in-water emulsion, to O / W (oil / water): water dispersed in oil to form an oil-in-water emulsion, to W / O (water / oil) said. Complex water-in-oil-in-water W/O/W type and oil-in-water-in-oil O/W/O type multiple emulsions may also be formed.
Emulsifiers are used to stabilize emulsions by reducing interfacial tension and forming single-molecule interfacial films. In the emulsification of the emulsifier requirements: a, the emulsifier must be able to adsorb or enrich the interface between the two phases, so that the interfacial tension is reduced; b, the emulsifier must give the particles to the charge, so that electrostatic repulsion between the particles, or the formation of a stable ﹑ viscosity around the particles of a particularly high protective film. Therefore, the substance used as an emulsifier must have amphiphilic groups in order to emulsify, and surfactants can meet this requirement.
2, emulsion preparation methods and factors affecting the stability of emulsions.
There are two methods of preparing emulsions: one is to use the mechanical method to disperse the liquid as tiny particles in another liquid, which is mostly used in industry to prepare emulsions; the other is to dissolve the liquid in a molecular state in another liquid, and then make it gather appropriately to form an emulsion.
The stability of an emulsion is the ability to resist particle aggregation that would lead to phase separation. Emulsions are thermodynamically unstable systems with large free energies. Therefore the so-called stability of an emulsion is actually the time it takes for the system to reach an equilibrium state, i.e. the time it takes for separation of one of the liquids in the system to occur. When the interfacial membrane has polar organic molecules such as fatty alcohols, fatty acids and fatty amines in it, the membrane strength increases significantly. This is because, in the interfacial adsorption layer of emulsifier molecules and alcohols, acids and amines and other polar molecules to form a “complex”, so that the interfacial membrane strength increased.
More than two kinds of surfactants consisting of emulsifier called mixed emulsifier. Mixed emulsifier adsorbed at the water/oil interface, intermolecular action can form complexes. Due to the strong intermolecular action, the interfacial tension is significantly reduced, the amount of emulsifier adsorbed at the interface is significantly increased, the formation of interfacial film density increases, the strength increases.
The charge of the liquid beads has a significant effect on the stability of the emulsion. Stable emulsions, the liquid beads are generally charged. When using ionic emulsifier, adsorbed at the interface of the emulsifier ion its lipophilic group inserted into the oil phase, hydrophilic group in the water phase, thus making the liquid beads charged. As the emulsion of the liquid beads with the same charge, they repel each other, not easy to agglomerate, so that the stability is increased. It can be seen that the more emulsifier ions adsorbed on the beads, the greater the charge, the greater the ability to prevent the beads from agglomeration, the more stable the emulsion system.
The viscosity of the emulsion dispersion medium has a certain influence on the stability of the emulsion. Generally, the higher the viscosity of the dispersion medium, the higher the stability of the emulsion. This is because the viscosity of the dispersion medium is large, which has a strong effect on the Brownian motion of the liquid beads and slows down the collision between the liquid beads, so that the system remains stable. Usually the polymer substances that can be dissolved in emulsions can increase the viscosity of the system and make the stability of emulsions higher. In addition, polymers can form a strong interfacial film, making the emulsion system more stable.
In some cases, the addition of solid powder can also stabilize the emulsion. Solid powder is in the water, oil or interface, depending on the oil, water on the wetting ability of the solid powder, if the solid powder is completely wetted by water, and can be wetted by oil, will only be retained in the water-oil interface. Solid powder does not make the emulsion stable because the powder gathered at the interface enhances the interface film, which is similar to the interface adsorption emulsifier molecules, so the more closely the solid powder is arranged at the interface, the more stable the emulsion.
The surfactant has the ability to significantly increase the solubility of insoluble or slightly soluble organic substances after forming micelles in aqueous solution, and the solution is transparent at this time, and this effect of micelles is called solubilization. The surfactant that can produce solubilization is called solubilizer, and the organic matter that is solubilized is called solubilized matter.
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