Question

The pKa for a typical long chain fatty acid is approximately 5. In an aqueous solution at pH>7, these fatty acids will spontaneously form micelles. In the same solution at pH>5, these fatty acids are no longer capable of micelle formation. Explain the observed difference in behavior of the fatty acid between these two conditions, specifically adressing the forces and interactions that drive micelle formation vs. non-specific aggregation of fatty acids.

Answer 1

In aqueous solution, fatty acids having both polar or charged groups and non polar regions (amphiphilic molecules) form aggregates which are called as micelles. In a micelle, polar or ionic heads form an outer shell in contact with water, while non polar tails are sequestered in the interior.

For the fatty acid to form a micelle it must have a charged polar head group as mentioned above. This will only happen when the carboxyl group of the fatty acid gets ionized, i.e., COOH -------> COO- + H+ , and this will only happen at pH values that is higher than the pKa values.

Here if pH is >7 that is pH > pKa the micelle are formed.

if pH>5 i.e. pH= pka no micelle are formed as no acid is ionized, hence no polar heads and micelles are not formed.