Solubility and Partition Coefficient
Start with the big picture
Solubility is the maximum equilibrium concentration that dissolves in a solvent under specified conditions. Intrinsic solubility describes the unionized drug, while total solubility also reflects ionization: weak acids generally become more soluble at alkaline pH, and weak bases at acidic pH. Temperature, crystal form, particle size, and techniques such as co-solvents or surfactants can also affect solubility, though particle size reduction speeds dissolution without increasing intrinsic solubility. Partition coefficient compares unionized drug concentrations in octanol and water; logP expresses lipophilicity. LogD includes ionized species and varies with pH. Together with pKa, these measures help frame the balance between aqueous solubility and permeability and inform formulation and drug-design decisions.
What you'll learn
- Define solubility and distinguish intrinsic solubility from total solubility.
- Explain how pH and ionization affect weak-acid and weak-base solubility.
- Describe how temperature, solid state, and particle size influence solubility or dissolution.
- Differentiate partition coefficient (logP) from pH-dependent distribution coefficient (logD).
- Relate solubility and lipophilicity to permeability and drug-design considerations.
Continue your study
Work through the complete notes and reinforce the topic with the study tools available in the full lesson.
