Factors affecting critical micelle concentration and micellar size

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Chapter: Pharmaceutical Drugs and Dosage: Surfactants and micelles

1. Size and structure of hydrophobic group 2. Nature of hydrophilic group 3. Nature of counterions 4. Addition of electrolytes 5. Effect of temperature 6. Alcohol

Factors affecting critical micelle concentration and micellar size

·           Size and structure of hydrophobic group: An increase in the hydro-carbon chain length causes a logarithmic decrease in the CMC. This is because an increase in hydrophobicity reduces aqueous solubility of the surfactant and increases its partitioning into the micelles. Micellar size increases with an increase in the hydrocarbon chain length, owing to an increase in the volume occupied per surfactant in the micelle.

·           Nature of hydrophilic group: An increase in hydrophilicity increases the CMC due to increased surfactant solubility in the aqueous medium and reduced partitioning into the interface. As the propor-tion of surface/interface to bulk surfactant concentration reduces, more of added surfactant is required to achieve saturation of the sur-face before micelles can form. Thus, nonionic surfactants have very lower hydrophilicity and CMC values compared with ionic surfac-tants with similar hydrocarbon chains.

·           Nature of counterions: About 70%–80% of the counterions of an ionic surfactant (e.g., Na+ is a counterion for carboxylate and sulfonate groups, and Cl- is a counterion for quaternary amine groups) are bound to the micelles. The nature of the counterion influences the properties of these micelles. For example, size of micelles formed with a cationic surfactant increases according to the series Cl < Br < I and with an anionic surfactant according to the series Na+ < K+ < Cs+. This is a function of not only the size and electronegativity of the counterion but also the size of the hydration layer around the counterion. The weakly hydrated (smaller, highly electronegative) ions are adsorbed more closely to the micellar surface and neutralize the charge on the surfactant more effectively, leading to the formation of smaller micelles.

·           Addition of electrolytes: Addition of electrolytes, such as salt, to solu-tions of ionic surfactants decreases the CMC and increases the size of the micelles. This is due to a reduction in the effective charge on the hydrophilic headgroups of the surfactants. This tips the hydrophilic lipophilic balance toward greater lipophilicity, increases the propor-tion of surface/interface to bulk surfactant concentration below the CMC, and promotes the formation of micelles in the bulk liquid. In contrast, micellar properties of nonionic surfactants are only mini-mally affected by the addition of electrolytes.

·           Effect of temperature: Size of micelles increases and CMC decreases with increasing temperature up to the cloud point for many nonionic surfactants due to increased Brownian motion of the monomers. Temperature has little effect on ionic surfactants. This is due to stron-ger hydrogen bonding and electrical forces governing the hydrophilic interactions of ionic surfactants than nonionic surfactants.

·           Alcohol: Addition of alcohol to an aqueous solution reduces the dielectric constant and increases the capacity of the solution to sol-ubilize amphiphilic (surfactant) and hydrophobic molecules. Thus, greater surfactant solubility in the hydroalcoholic solutions decreases the surface/interface to bulk solution concentration of the surfactant, thus increasing the CMC.

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