25. What is the critical micelle concentration of surfactant?
Answer Surfactant molecules are composed of hydrophilic groups and hydrophobic groups. When surfactants are dissolved in water, the hydrophilic groups and water molecules have a greater attraction, while the hydrophobic groups and water molecules repel and point toward the air. In this way, surfactant molecules are arranged in an orderly manner on the surface of the liquid, forming directional adsorption at the interface between water and air. As a result of the adsorption, the original water-air interface is gradually replaced by a hydrophobic group-air interface, changing the direct contact between water molecules and air molecules, thus reducing the surface tension of water. If the concentration of surfactant increases to a certain level, more surfactant molecules gather at the air-water interface and are densely distributed on the interface without gaps, forming a tight monomolecular film, that is, the interface adsorption reaches Saturated state, that is, the surface tension of water has dropped to a low level. At this time, if the surfactant concentration is increased, in order to exist stably in the water, the surfactant molecules can gather together and arrange into micelles with the hydrophobic groups facing in and the hydrophilic groups facing out.
The low concentration at which surfactant forms micelles is called critical micelle concentration (CMC). When the surfactant concentration is greater than the critical micelle concentration, the number of micelles increases and the surface tension of water no longer decreases.
The critical micelle concentration is an important limit. When it is higher or lower than the critical micelle concentration, the surface tension of the aqueous solution and many other physical properties are very different. Different surfactants have different critical micelle concentrations, which are related to many factors. For example, the longer the hydrophobic group, the stronger the hydrophobic effect and the smaller the critical micelle concentration value. The more hydrophilic groups there are, the stronger the hydrophilicity, or the hydrophilic group is located in the middle of the hydrophobic group, the greater the critical micelle concentration value. If electrolytes are added, surfactants are prone to aggregation, causing the critical micelle concentration to decrease.
The critical micelle concentration of surfactants is generally not high, ranging from 0.001 to 0.02 mol/L or 0.02% to 0.4%. When used, it should generally be slightly greater than the critical micelle concentration to fully exert its effect. Wetting and penetration only require adsorption at the interface, and the surfactant molecule (or ion) solution concentration is slightly higher than CIVIC to show its effect; while emulsification, dispersion, solubilization, and cleaning require a large number of micelles, so the solution concentration It must exceed CIVIC by a lot to be effective.
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