When the concentration of the solution reaches the micelle concentration of the surfactant

2020-04-15 16:40:26

There is a change process of surfactant forming micelles in solution with the change of concentration of aqueous solution. When the concentration of surfactant in solution is very low, that is, when the concentration of surfactant in solution is very thin, air and water are almost in direct contact. The surface tension of water drops little, approaching the state of pure water. There are only a few surfactant molecules in water. When the concentration of surfactant increases slightly, the surface activity Agent molecules will soon gather on the surface of the water. The covering of water will reduce the contact between water and air, and the surface tension of the solution will drop sharply.

Some of the surfactants in the water gather together, and the hydrophobic groups of these surfactants start to form small micelles. With the increase of surfactant concentration, when the surfactant solution reaches saturation adsorption, the surface begins to form a closely arranged monolayer. When the concentration of the solution reaches the micelle concentration of the surfactant, the surface tension of the solution drops to a low value.

If the surfactant continues to increase after the concentration of the solution reaches the critical micelle concentration, although the concentration of the solution increases, the surface tension of the solution almost no longer decreases. At this time, the micelle number and aggregation number in the solution increase, and the solution system is composed of micelles. The microreactor used for the synthesis of nano powder is the micelle at this time, Then the system gradually changes to form liquid crystal state. When the concentration of surfactant in aqueous solution reaches CMC, the surfactant will form micelles with the increase of its concentration, which is reflected in the turning point of the curve of surfactant surface tension and concentration (r-1gc curve), while the other physical and chemical properties of the solution are not ideal.

For ionic surfactants, the micelles formed by them have higher charges. Due to the effect of electrostatic attraction, some counter ions will be attracted around the micelles, which makes some positive and negative charges cancel each other. However, the blocking force of the ion mist formed by the counter ions increases greatly after the micelle has formed a high charge, which can be used to adjust the dispersion of the nano powder. Because of these two reasons, the equivalent conductance of the solution decreases rapidly with the increase of concentration after CMC. Therefore, this point is also used to measure the critical micelle concentration of surfactant.

The structure of ionic surfactant micelle is spherical micelle, which is composed of inner core, outer shell and diffusion double electric layer. The inner core of ionic surfactant is composed of hydrocarbon chain similar to liquid hydrocarbon hydrophobic, and its diameter is about 1-2.8nm. Because the neighboring polar group - CH2 - has certain polarity, there are still some water molecules around the core, so there are more permeated water in the micelle core. At this time, this - CH2 - group is not only the core composed of liquid hydrocarbon chain, but also a part of the non liquid micelle shell.