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Questions and Answers
What effect does pH have on the solubility of weak acids and bases?
What effect does pH have on the solubility of weak acids and bases?
Weak acids become more soluble with an increase in pH, while weak bases require a decrease in pH for enhanced solubility.
Explain the concept of cosolvency in the context of drug solubility.
Explain the concept of cosolvency in the context of drug solubility.
Cosolvency refers to increasing the solubility of a weak electrolyte or non-polar compound by adding another miscible solvent where the compound is soluble.
What criteria should be considered when choosing a cosolvent for drug formulation?
What criteria should be considered when choosing a cosolvent for drug formulation?
The chosen cosolvent should have low toxicity and irritancy, and ideally possess a dielectric constant between 25 and 80.
What is the Henderson-Hasselbalch equation used for in pharmaceutical science?
What is the Henderson-Hasselbalch equation used for in pharmaceutical science?
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Describe how solubilization helps in improving drug solubility.
Describe how solubilization helps in improving drug solubility.
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What are the potential risks of using excessive amounts of surfactants in drug formulations?
What are the potential risks of using excessive amounts of surfactants in drug formulations?
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How does temperature affect the solubilization of drugs?
How does temperature affect the solubilization of drugs?
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What types of structures can be formed by surface-active agents (SAAs) in drug formulations?
What types of structures can be formed by surface-active agents (SAAs) in drug formulations?
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Why is it essential to ensure that drug formulations do not lie close to a phase boundary?
Why is it essential to ensure that drug formulations do not lie close to a phase boundary?
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What is the role of dielectric constant in selecting cosolvent blends for drug solubility?
What is the role of dielectric constant in selecting cosolvent blends for drug solubility?
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Study Notes
Approaches to Improving Aqueous Solubility
- Strongly ionized substances are likely to dissolve readily in water.
- Weak acids and bases dissolve better at optimal pH levels.
- The concentration of a substance shouldn't be near its solubility limit (avoid cooling or evaporation).
Cosolvency
- Enhancing the solubility of a weakly charged or nonpolar substance in water can be achieved by adding another solvent miscible with water, in which the substance is also soluble.
- Mixing solvents often results in increased solubility beyond what's predicted by the individual component solubilities.
- Choosing a cosolvent involves considering toxicity, irritation, and desired administration methods (like oral or parenteral).
- Ideal cosolvent blends have dielectric constants between 25 and 80.
- Examples include water/ethanol blends, and mixtures involving sorbitol, glycerol, propylene glycol, and syrups.
- Specific examples include co-trimoxazole (PG/water) and paracetamol (alcohol/PG/syrup/water).
pH Control
- Many drugs are either weak acids or weak bases.
- The Henderson-Hasselbalch equation relates pH, pKa, and the solubility of the unionized form of the drug.
- Lowering the pH increases the solubility of weak bases.
- Increasing the pH increases the solubility of weak acids.
- The chosen pH must not compromise other product requirements, like chemical stability.
Solubilization
- Adding a surface-active agent (SAA)—a non-toxic and non-irritating substance—can increase solubility.
- SAAs form micelles, ranging from simple spheres to complex liposomes or liquid crystals.
- Nonpolar molecules dissolve within the lipophilic hydrocarbon core of the micelle.
- The amount of surfactant needs careful control to avoid cost and toxicity issues.
- Excessive amounts of surfactant can reduce drug bioavailability due to adsorption within the micelle.
- The chosen formulation should not be near a phase boundary, which can vary depending on the storage temperature.
- Solubility generally increases with temperature.
- Examples include using polysorbates to solubilize fat-soluble vitamins like phytonadione, iodine with macrogol ethers (PEG) for enhanced stability and reduced loss of active agent, and cresol/chloroxylenol with soap for enhanced solubility.
- Lysol's example demonstrates this principle via the use of potassium soaps with oleic, linoleic, and linolenic acids.
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Description
Explore the methods of enhancing the aqueous solubility of substances. This quiz covers the principles of strong ionization, pH effects, and cosolvent strategies for solubility improvement. Test your knowledge on solubility limits and cosolvent selection for various applications.
