Pharmaceutical Solubility Concepts

Study Notes

Solubility and Dissolution

Solubility is defined as the concentration of a solute in a saturated solution at a specific temperature.
Solubility of the API, particularly in the context of pharmaceutical preparations, is often a crucial factor for preparing aqueous solutions.
For drugs with poor water solubility, a mix of water and a water-miscible solvent (such as alcohol, glycerol, polyethylene glycol, or propylene glycol) is typically used.
A drug must be in solution before it can be absorbed by the body and exert its intended biological effect.

Factors Affecting Solubility

Physicochemical Properties of the Therapeutic Agent: Includes molecular weight, volume, density, number of hydrogen bonds, and particle size.
Solubility in Water:
- High Solubility: The pharmaceutical drug can be readily incorporated into a vehicle and formulated as an oral solution.
- Moderate Solubility: Co-solvents may be needed to enhance solubility if the desired concentration exceeds the inherent solubility of the drug.
- Low Solubility: Suspensions are the preferred formulation if co-solvents or surfactants are not viable options due to their detrimental effects on the drug’s safety.
Dissolution Rate: The Noyes-Whitney Equation describes the rate of dissolution: dc/ dt = K * S * (Cs - Ct)
- Where dc/ dt represents the rate of dissolution, K is the dissolution rate constant, S is the surface area of the dissolving solid, Cs is the concentration of the drug near the dissolving particle (solubility), and Ct is the concentration of the drug in the bulk fluid.
- The dissolution process involves several steps: initial mechanical lag, wetting, penetration of the dissolution medium, disintegration, deaggregation, and dissolution of individual particles.
- Wetting is often a critical factor in the dissolution rate, as it governs the liquid's access to the solid surface.
- Surfactants can improve wettability, which in turn enhances dissolution.
- Air bubbles trapped in pores act as barriers, hindering dissolution.
Factors Affecting Dissolution Rate:
- Surface Area/Particle Size: A larger surface area promotes faster dissolution.
- Temperature: Higher temperatures generally result in faster dissolution rates.
- Agitation: Stirring or shaking speeds up the dissolution process.
- Saturation:
  - Unsaturated Solution: Can dissolve more solute.
  - Saturated Solution: Contains the maximum amount of solute that can be dissolved at a given temperature.
  - Supersaturated Solution: Contains more dissolved solute than a saturated solution (achieved at a higher temperature).
Dissolution vs. Solubility:
- Dissolution is a kinetic process, describing the rate at which a substance dissolves.
- Solubility is a dynamic equilibrium state, where the rate of dissolution equals the rate of precipitation.

Solubility Expression

Quantity per Quantity: mg/mL, g/L
Percentage: % w/w (weight/weight), % w/v (weight/volume), % v/v (volume/volume)
Molarity: mol/L (of solution)
Molality: mol/kg (of solvent)
Mole Fraction
Normality