Critical Micelle Concentration microemulsions reverse micelles

Critical Micelle Concentration for M.Sc Chemistry and Pharmaceutics.

1. Critical Micelle Concentration Physical Chemistry Aayushi Sharma Assistant Professor (Chemistry) Department of Pharmacy Indus International University Una

2. Critical micelle Concentration What is CMC? CMC is the concentration of surfactant at which micelles begin to form. Below the CMC, surfactant molecules exist as individual molecules, while above the CMC, they aggregate to form micelles. Effect of Chain Length on CMC The chain length of a surfactant has a significant impact on its CMC. In general, as the chain length of a surfactant increases: CMC decreases: Longer-chain surfactants have a lower CMC due to the increased hydrophobic interactions between the chains. Micelle size increases: Longer-chain surfactants tend to form larger micelles, which can lead to changes in the surfactant's properties and behavior. Surfactant solubility decreases: Longer-chain surfactants tend to be less soluble in water, which can affect their ability to form micelles.

3. Quantitative Relationships Several quantitative relationships have been developed to describe the dependence of CMC on chain length: Trabue's rule : This rule states that the CMC of a surfactant decreases by a factor of 2-3 for each additional methylene group (-CH2-) in the chain. Reynolds' equation: This equation relates the CMC of a surfactant to its chain length (n) and the number of carbon atoms in the chain (N): CMC = Ae^(-BN), where A and B are constants. Exceptions and Limitations While the general trends described above hold true for many surfactants, there are exceptions and limitations: Branching and unsaturation : Branched or unsaturated chains can affect the CMC and micelle size. 2 . Headgroup effects : The size and charge of the headgroup can influence the CMC and micelle size. Solvent effects : The solvent can affect the CMC and micelle size, especially for non-aqueous solvents.

4. Micelle Shape Micelles can form various shapes, depending on the surfactant type, concentration, and solvent conditions. Common micelle shapes include: Spherical micelles: These are the most common type of micelle, with a spherical shape and a hydrophobic core. Rod-like micelles: These micelles have a cylindrical shape and are often formed by surfactants with a high molecular weight. Lamellar micelles: These micelles have a flat, disk-like shape and are often formed by surfactants with a high degree of unsaturation. Vesicular micelles: These micelles have a spherical shape with a hydrophilic interior and a hydrophobic exterior.

5. Micelle Size Micelle size can vary greatly, depending on the surfactant type, concentration, and solvent conditions. Common micelle sizes range from: 1. Small micelles: 2-5 nm in diameter, often formed by low-molecular-weight surfactants. 2. Medium micelles: 5-20 nm in diameter, often formed by medium-molecular-weight surfactants. 3. Large micelles: 20-100 nm in diameter, often formed by high-molecular-weight surfactants.

6. Factors Influencing Micelle Shape and Size Several factors can influence micelle shape and size, including: 1. Surfactant type and concentration 2. Solvent conditions (temperature, pH, ionic strength) 3. Presence of additives or impurities4. Shear rate and flow conditions

7. Thermodynamic Parameters 1. Gibbs Free Energy (ΔG): The change in Gibbs free energy is a measure of the spontaneity of micelle formation. 2. Enthalpy (ΔH): The change in enthalpy is a measure of the energy change associated with micelle formation. 3. Entropy (ΔS): The change in entropy is a measure of the disorder or randomness associated with micelle formation.

8. Thermodynamic Driving Forces 1. Hydrophobic Effect: The hydrophobic effect is the primary driving force for micelle formation. It arises from the tendency of non-polar molecules to avoid contact with water. 2. Hydrogen Bonding: Hydrogen bonding between surfactant molecules and water molecules can also contribute to micelle formation. 3. Electrostatic Interactions: Electrostatic interactions between surfactant molecules and counterions can also influence micelle formation.

9. Micelle Formation Process 1. Surfactant Monomers: Surfactant molecules exist as monomers in solution. 2. Micelle Nucleation: A critical concentration of surfactant monomers is reached, and micelle nucleation occurs. 3. Micelle Growth: The micelle grows as more surfactant monomers are added to the micelle. 4. Micelle Equilibrium: The micelle reaches equilibrium, and the rate of micelle formation equals the rate of micelle dissociation.

10. Thermodynamic Models Mass Action Model: This model assumes that micelle formation is a reversible process and that the equilibrium constant can be expressed in terms of the surfactant concentration. Pseudo-Phase Model: This model assumes that the micelle is a separate phase from the bulk solution and that the equilibrium constant can be expressed in terms of the surfactant concentration.

11. Importance of Thermodynamics in Micelle Formation Understanding Micelle Stability: Thermodynamics helps us understand the stability of micelles and how they respond to changes in temperature, concentration, and other environmental factors. Designing Surfactant Systems: Thermodynamics informs the design of surfactant systems for specific applications, such as detergency, emulsification, and solubilization. Predicting Micelle Properties: Thermodynamics helps us predict the properties of micelles, such as their size, shape, and aggregation number.

12. Microemulsions Microemulsions are thermodynamically stable, transparent or translucent mixtures of two or more immiscible liquids, such as oil and water, stabilized by a surfactant or a mixture of surfactants. Characteristics: Thermodynamic stability: Microemulsions are stable over time, unlike emulsions, which can separate into distinct phases. Transparency or translucency: Microemulsions are typically transparent or translucent, indicating that the droplet size is in the nanoscale range. High stability: Microemulsions can withstand changes in temperature, pH, and other environmental factors without separating into distinct phases.

13. Types of Microemulsions 1. Oil-in-water (O/W) microemulsions: These microemulsions consist of oil droplets dispersed in a continuous water phase. 2. Water-in-oil (W/O) microemulsions: These microemulsions consist of water droplets dispersed in a continuous oil phase. 3. Bicontinuous microemulsions: These microemulsions consist of a continuous network of both oil and water phases.

14. Applications 1. Pharmaceuticals: Microemulsions are used as drug delivery systems, as they can solubilize hydrophobic drugs and improve their bioavailability. 2. Cosmetics: Microemulsions are used in personal care products, such as creams, lotions, and shampoos, due to their ability to solubilize oils and improve skin hydration. 3. Food industry: Microemulsions are used in food products, such as sauces, dressings, and beverages, due to their ability to stabilize flavors and improve texture. 4. Environmental remediation: Microemulsions are used to clean up contaminated soil and groundwater, as they can solubilize and remove pollutants.

15. Formation and Stability 1. Surfactant selection: The choice of surfactant is critical in forming and stabilizing microemulsions. 2. Composition: The composition of the microemulsion, including the ratio of oil to water and the concentration of surfactant, can affect its stability. 3. Temperature and pH: Changes in temperature and pH can affect the stability of microemulsions

16. Characterization Techniques 1. Dynamic light scattering (DLS): DLS is used to measure the size and size distribution of microemulsion droplets. 2. Transmission electron microscopy (TEM): TEM is used to visualize the morphology of microemulsion droplets. 3. Nuclear magnetic resonance (NMR) spectroscopy: NMR spectroscopy is used to study the structure and dynamics of microemulsions.

17. Reverse micelles Reverse micelles are a type of micelle that forms when a surfactant is dissolved in a non-polar solvent, such as an organic solvent. In this case, the surfactant molecules aggregate to form a micelle with a hydrophilic (water-loving) core and a hydrophobic (water-fearing) exterior. Structure : Reverse micelles have a unique structure, with the following characteristics: 1. Hydrophilic core: The core of the reverse micelle is hydrophilic, meaning it has a strong affinity for water. 2. Hydrophobic exterior: The exterior of the reverse micelle is hydrophobic, meaning it has a strong affinity for non-polar solvents. 3. Surfactant molecules: The surfactant molecules that make up the reverse micelle have both hydrophilic and hydrophobic regions.

18. Properties Reverse micelles have several unique properties, including: 1. Solubilization: Reverse micelles can solubilize water and other polar solvents in non-polar solvents. 2. Stability: Reverse micelles are stable over a wide range of temperatures and solvent compositions. 3. Size: The size of reverse micelles can vary depending on the surfactant and solvent used.

19. Applications Reverse micelles have several applications, including: 1. Extraction: Reverse micelles can be used to extract polar compounds from non-polar solvents. 2. Catalysis : Reverse micelles can be used as microreactors for catalytic reactions. 3. Drug delivery: Reverse micelles can be used to deliver polar drugs in non-polar solvents.

20. Practical Applications: Personal Care and Cosmetics 1. Soaps and Detergents : Surfactants are used to clean and emulsify oils, making it easier to rinse away dirt and grime. 2. Shampoos and Conditioners: Surfactants help to clean and moisturize hair, reducing tangles and improving manageability. 3. Toothpaste: Surfactants help to remove plaque and bacteria from teeth, freshening breath and preventing gum disease. Pharmaceuticals and Biotechnology 1. Drug Delivery: Surfactants are used to solubilize and deliver hydrophobic drugs, improving their bioavailability and efficacy. 2. Vaccine Adjuvants: Surfactants are used to enhance the immune response to vaccines, improving their effectiveness. 3. Gene Therapy: Surfactants are used to deliver genetic material into cells, enabling the treatment of genetic diseases. Food and Beverage Industry 1. Food Emulsifiers : Surfactants are used to stabilize emulsions in foods, such as mayonnaise and ice cream. 2. Beverage Stabilizers: Surfactants are used to stabilize foams and emulsions in beverages, such as beer and soda. 3. Food Packaging: Surfactants are used to improve the wettability and printability of food packaging materials. Industrial and Institutional Cleaning 1. Industrial Cleaning Agents: Surfactants are used to clean and degrease industrial equipment and surfaces. 2. Institutional Cleaning Products: Surfactants are used in cleaning products for hospitals, schools, and other institutions. 3. Pest Control: Surfactants are used to improve the efficacy of pesticides and herbicides.

21. REFERENCES :- 1. Myers, D. (2006). Surfactant Science and Technology. John Wiley & Sons. 2. Rosen, M. J. (2004). Surfactants and Interfacial Phenomena. John Wiley & Sons. 3. Holmberg, K. (2003). Handbook of Applied Surface and Colloid Chemistry. John Wiley & Sons.