Information AboutMicelle |
| CATEGORIES ABOUT MICELLE | |
| supramolecular chemistry | |
| colloidal chemistry | |
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Individual surfactant molecules that are in the colloid but are not part of a micelle are called "monomers." In water, the hydrophilic "heads" of surfactant molecules are always in contact with water, regardless of whether the surfactants exist as monomers or as part of a micelle. However, the hydrophobic "tails" of surfactant molecules have less contact with water when they are part of a micelle. In a micelle, the hydrophobic tails of several surfactant molecules assemble into an oil-like core that has less contact with water. In contrast, surfactant monomers are surrounded by water molecules that create a "cage" of molecules connected by hydrogen bonds. This water cage is similar to a Clathrate and has an Ice -like Crystal structure. In a Nonpolar solvent, the hydrophilic groups form the core of the micelle, and the hydrophobic groups remain on the surface of the micelle (so-called reverse micelle). Micelles only form when the concentration of surfactant is greater than the critical micelle concentration (CMC), and the temperature of the system is greater than the critical micelle temperature, or Krafft temperature. The formation of micelles can be understood using because of a balance between Entropy and Enthalpy . In water, the Hydrophobic Effect is the driving force for micelle formation, despite the fact that assembling surfactant molecules together reduces their entropy. Broadly speaking, above the CMC, the entropic penalty of assembling the surfactant molecules is less than the entropic penalty of the caging water molecules. Also important are enthalpic considerations, such as the electrostatic interactions that occur between charged (or ionic) surfactants. Micelles composed of ionic surfactants are surrounded by a "cloud" of tightly-bound ions. Because these ions have a charge opposite or counter to the charge of the ionic surfactant, they are called counterions. Although the bound counterions partially neutralize a charged micelle (by up to 90%), the effects of micelle charge may be important relatively far from the micelle, and ionic micelles can influence many properties of the mixture, including its electrical conductivity. However, adding salt to a colloid containing micelles can decrease the strength of electrostatic interactions and lead to larger ionic micelles. When surfactants are present above the CMC, they can act as Emulsifiers . For instance, micelles formed by Detergent s in water assist in the cleaning by incorporating poorly soluble hydrophobic material (such as oil, grease, or dirt) into their core. Detergent also helps clean by lowering the Surface Tension of water, making it easier to remove dirt from a surface. For example, when you wash your hands with Soap or detergent, the surfactant makes it easier to remove dirt when you rub your hands together. The micelles then trap the dirt so that it doesn't redeposit on your hands. The emulsifying ability of surfactants is also the basis for Emulsion Polymerization . SEE ALSO |
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