Question

Which of the following is not a method for solubility enhancement:

• A. Crystallization
• B. Co-solvency
• C. Salt formation
• D. Hydrotropy

Hint:

When enhancing solubility, consider using co-solvency (using multiple solvents), salt formation (converting the drug into a more soluble salt), or hydrotropy (using a solubilizing agent) rather than crystallization.

Answer 1

The Correct Option is A

Crystallization is not a method for solubility enhancement. In fact, crystallization is typically used to purify compounds and reduce solubility in a solvent. In contrast, co-solvency, salt formation, and hydrotropy are methods used to improve the solubility of poorly soluble drugs.

Answer 2

The question wants the option that is NOT a technique used to increase the solubility of a poorly soluble drug. Let's check what each listed process actually does.

1. Crystallization: This is a purification technique where a compound is made to form solid crystals out of a solution, usually by cooling or evaporating solvent. Its goal is to isolate a pure solid, and it does not add anything to the drug that would make it dissolve better. In many cases a highly crystalline form is actually less soluble than an amorphous one, so this process works against solubility enhancement rather than for it.
2. Co-solvency: Here a second, water miscible solvent (such as ethanol or propylene glycol) is mixed with water to change the overall polarity of the vehicle. This makes the mixed solvent system a better match for a poorly soluble drug, so more of the drug can dissolve. This is a genuine solubility enhancement method.
3. Salt formation: A weakly acidic or basic drug is converted into its salt form (for example, converting a free acid into its sodium salt). Salts are ionic and interact more strongly with water, so they dissolve far more easily than the parent free acid or base. This is a standard and widely used enhancement method.
4. Hydrotropy: A hydrotropic agent, such as sodium benzoate or urea, is added at high concentration to break up the structured water around the drug molecule and pull more of it into solution. This is another established solubility enhancement technique.

Co-solvency, salt formation, and hydrotropy are all deliberately used to push more drug into solution, while crystallization is a separation and purification step that does not serve this purpose.

Therefore, the correct answer is Crystallization.