12.6
Micelles
Micelles form from amphiphilic molecules that contain both hydrophilic and hydrophobic regions.
These molecules, commonly surfactants in soaps and detergents, arrange themselves in water so that hydrophilic heads face the surrounding solvent while hydrophobic tails avoid contact with it, forming micelles.
However, only when the surfactant concentration exceeds a threshold known as the critical micelle concentration or CMC, surfactant molecules form micelles, causing significant changes in the solution's physical properties.
For instance, osmotic pressure levels off because many individual surfactant molecules combine into fewer, larger aggregates.
Molar conductivity also decreases sharply, since charged surfactant molecules become part of large micelles rather than moving freely in solution.
Ionic surfactants typically have higher CMC values than non-ionic surfactants with identical hydrophobic tails.
Micelles can take various shapes. As the concentration increases, ionic micelles can change from spherical to cylindrical due to reduced repulsions between surface head groups. These cylindrical micelles can form hexagonal arrays, which finally lead to the formation of lamellar micelles.
Micelle formation is an intricate process that hinges on the properties of amphiphilic or amphipathic molecules and the conditions of the system in which they are found. Amphiphilic molecules, which have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts, play a critical role in this process.
In aqueous environments, these molecules arrange themselves such that their hydrophilic heads are turned towards the water phase, while their hydrophobic tails are oriented away from it. This orientation minimizes the contact of the hydrophobic parts with water, leading to the formation of micelles. However, this self-assembly into micelles only occurs when the concentration of surfactant molecules exceeds a particular threshold known as the critical micelle concentration (CMC).
The structure of the surfactant molecule significantly influences micelle formation. The length and branching of the hydrophobic tail, for instance, can determine the size and shape of the micelle. Longer hydrophobic chains typically result in larger micelles, while branched or bulky hydrophobic groups may create smaller micelles.
The hydrophilic head of the surfactant also affects micelle formation. Ionic surfactants generally have higher CMCs compared to non-ionic surfactants with the same hydrophobic tail. Moreover, surfactants with multiple hydrophilic heads have larger CMCs than those with a single hydrophilic head.
Various factors can affect the CMC, thereby influencing micelle formation. For example, the presence of electrolytes in the solution can reduce the CMC for certain types of surfactants due to the decrease in the thickness of the ionic atmosphere surrounding the ionic head groups in the micelle.
The introduction of organic materials can also modify the CMC. Class I organic materials like alcohols and amides can integrate into the micelle, reducing the CMC. Class II organic materials such as urea and polyhydric alcohols can alter the interactions between the surfactant and the solvent, affecting the CMC at higher concentrations.
Effect of temperature on the CMC is complex. An initial increase in temperature decreases the hydration of the hydrophilic group, favoring micellization. However, a further increase in temperature can distort the structure of water surrounding the hydrophobic group, inhibiting micellization.
In summary, micelle formation is a dynamic and complex process influenced by a range of factors. Understanding these factors and their interplay is essential for various applications, including detergency and the solubilization of water-insoluble materials.
Micelles form from amphiphilic molecules that contain both hydrophilic and hydrophobic regions.
These molecules, commonly surfactants in soaps and detergents, arrange themselves in water so that hydrophilic heads face the surrounding solvent while hydrophobic tails avoid contact with it, forming micelles.
However, only when the surfactant concentration exceeds a threshold known as the critical micelle concentration or CMC, surfactant molecules form micelles, causing significant changes in the solution's physical properties.
For instance, osmotic pressure levels off because many individual surfactant molecules combine into fewer, larger aggregates.
Molar conductivity also decreases sharply, since charged surfactant molecules become part of large micelles rather than moving freely in solution.
Ionic surfactants typically have higher CMC values than non-ionic surfactants with identical hydrophobic tails.
Micelles can take various shapes. As the concentration increases, ionic micelles can change from spherical to cylindrical due to reduced repulsions between surface head groups. These cylindrical micelles can form hexagonal arrays, which finally lead to the formation of lamellar micelles.
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