1.Surface tension
The contraction force of arbitrary unit length of liquid surface is called surface tension, and the unit is N • m-1.
2, surface active and surfactant
The property that can reduce the surface tension of a solvent is called surface activity, and the substance with surface activity is called a surface active substance.
Surface active substances that can associate molecules in aqueous solution and form associations such as micelles, have high surface activity, and also have wetting, emulsifying, foaming, and washing functions are called surfactants.
3. Molecular structure characteristics of surfactants
Surfactants are organic compounds with special structures and properties. They can significantly change the interfacial tension between two phases or the surface tension of a liquid (usually water). They have wetting, foaming, emulsifying, and washing properties.
As far as structure is concerned, surfactants have a common feature, that is, the molecule contains two groups of different properties, one end is a long-chain non-polar group, which is soluble in oil but not water, so-called Hydrophobic groups or hydrophobic groups, such hydrophobic groups are generally long-chain hydrocarbons, and sometimes organic fluorine, silicone, organic phosphorus, organic tin chains and so on. The other end is a water-soluble group, that is, a hydrophilic group or a hydrophilic group. The hydrophilic group must be sufficiently hydrophilic to ensure that the entire surfactant is soluble in water and has the necessary solubility. Because surfactants contain hydrophilic and hydrophobic groups, they are soluble in at least one phase of the liquid phase. This hydrophilic and lipophilic nature of a surfactant is called amphiphilic.
4.Types of surfactants
Surfactant is an amphiphilic molecule with both hydrophobic and hydrophilic groups. The hydrophobic groups of surfactants are generally composed of long-chain hydrocarbons, such as straight-chain alkyl groups C8 to C20, branched chain alkyl groups C8 to C20, alkylphenyl groups (the number of alkyl carbon atoms is 8 to 16), etc. . The difference between hydrophobic groups is mainly in the structural change of the hydrocarbon chain, the difference is small, and there are more types of hydrophilic groups, so the properties of surfactants are mainly related to the size and shape of the hydrophobic groups. It is also related to hydrophilic groups. The structure of the hydrophilic group changes more than the hydrophobic group, so the classification of surfactants is generally based on the structure of the hydrophilic group. This classification is based on whether the hydrophilic group is ionic, and it is divided into anionic, cationic, nonionic, zwitterionic, and other special types of surfactants.
5. Characteristics of surfactant aqueous solution
① Adsorption of surfactant on the interface
Surfactant molecules have lipophilic and hydrophilic groups and are amphiphilic molecules. Water is a strongly polar liquid. When a surfactant is dissolved in water, its hydrophilic groups are soluble in water because of their hydrophilicity and water phase, based on the principles of similar polarity and repulsion. The phase repels and leaves the water. As a result, the surfactant molecules (or ions) are adsorbed on the interface of the two phases, which reduces the interfacial tension between the two phases. The more surfactant molecules (or ions) are adsorbed on the interface, the greater the decrease in interfacial tension.
②Some properties of adsorption membrane
Surface pressure of the adsorption membrane: The surfactant is adsorbed on the gas-liquid interface to form an adsorption membrane. For example, if a frictionless movable float is placed on the interface, the adsorption membrane is pushed along the solution surface by the float, and the membrane generates a pressure on the float. This pressure is called surface pressure.
Surface viscosity: Like surface pressure, surface viscosity is a property exhibited by insoluble molecular films. Suspend a platinum ring with a thin metal wire, make its plane contact the water surface of the sink, rotate the platinum ring, the platinum ring is hindered by the viscosity of the water, and the amplitude gradually decreases. Based on this, the surface viscosity can be measured by firstly on the surface of pure water An experiment was performed to measure the amplitude attenuation, and then measure the attenuation after the surface film was formed. The viscosity of the surface film was obtained from the difference between the two.
The surface viscosity is closely related to the firmness of the surface film. Because the adsorption film has surface pressure and viscosity, it must be elastic. The higher the surface pressure and the higher the viscosity of the adsorption film, the greater its elastic modulus. The elastic modulus of the surface adsorption film is of great significance in the stabilization process.
③ Formation of micelles
The dilute solution of rhenium surfactant obeys the rules followed by the ideal solution. The adsorption amount of surfactant on the solution surface increases with the increase of the solution concentration. When the concentration reaches or exceeds a certain value, the adsorption amount no longer increases. These excessive surfactant molecules are disordered in the solution, or somehow There is a regular way. Practice and theory have shown that they form associations in solution, which are called micelles.
Critical micellar concentration: The minimum concentration of a surfactant to form micelles in a solution is called the critical micelle concentration.
④ cmc value of common surfactants.
6. Hydrophilic-lipophilic balance
HLB is an abbreviation of hydrophile lipophile balance, and represents the hydrophilic-lipophilic balance value of the hydrophilic group and lipophilic group of the surfactant, that is, the HLB value of the surfactant. A large HLB value indicates that the molecule is strongly hydrophilic and weakly lipophilic; otherwise, the lipophilicity is strong and the hydrophilicity is weak.
①Requirement of HLB value
The HLB value is a relative value, so when setting the HLB value, as a standard, the HLB value of paraffin wax without hydrophilic property is specified as 0, and the HLB value of sodium lauryl sulfate with strong water solubility is 40. Therefore, the HLB value of the surfactant is generally within the range of 1 to 40. Generally speaking, emulsifiers with an HLB value less than 10 are lipophilic, while emulsifiers greater than 10 are hydrophilic. Therefore, the turning point from lipophilicity to hydrophilicity is about 10.
7.Emulsification and solubilization
Two kinds of mutually insoluble liquids, one is formed by microparticles (droplets or liquid crystals) dispersed in the other is called emulsion. Because the interfacial product of the two liquids increases when the emulsion is formed, this system is thermodynamically unstable. To stabilize the emulsion, a third component, an emulsifier, needs to be added to reduce the interfacial energy of the system. Emulsifiers are surfactants whose main function is to act as a milk. The phase in which the droplets exist in the emulsion is called the dispersed phase (or internal phase, discontinuous phase), and the other phase connected into one piece is called the dispersion medium (or external phase, continuous phase).
①Emulsifiers and emulsions
Common emulsion, one phase is water or aqueous solution, and the other phase is water-immiscible organic matter, such as oil and wax. Emulsions formed by water and oil can be divided into two types according to their dispersion: oils are dispersed in water to form oil-in-water emulsions, which is expressed in O / W (oil / water): water-in-oil forms water-in-oil Emulsion is expressed in W / O (water / oil). In addition, it is also possible to form complex water-in-oil-in-water W / O / W and oil-in-water-in-oil O / W / O multi-component emulsions.
Emulsifiers stabilize emulsions by reducing interfacial tension and forming a monomolecular interface film.
Requirements for emulsifiers in emulsification: a: the emulsifier must be able to adsorb or enrich at the interface of the two phases to reduce the interfacial tension; b: the emulsifier must give the particles a charge and cause electrostatic repulsion between the particles, or A stable, highly viscous protective film is formed around the particles. Therefore, substances used as emulsifiers must have amphiphilic groups in order to effect emulsification. Surfactants can meet this requirement.
② Preparation method of emulsion and factors affecting emulsion stability
There are two methods for preparing emulsions: one is to use mechanical methods to disperse liquids as fine particles in another liquid, which is mostly used in industry to prepare emulsions; the other is to dissolve liquids in a molecular state In another liquid, it is then properly aggregated to form an emulsion.
Emulsion stability refers to the ability to resist particle aggregation leading to phase separation. Emulsions are thermodynamically unstable systems with large free energy. Therefore, the so-called emulsion stability actually refers to the time required for the system to reach equilibrium, that is, the time required for a liquid to separate in the system.
