Solubility and Distribution Phenomena Quiz

Solubility and Distribution Phenomena

• Solubility phenomena describe the processes and principles governing the dissolution of a solute in a solvent to form a homogeneous solution.
• This involves solute-solvent interactions, leading to uniform distribution of solute particles within the solvent.
• Solubility is a fundamental concept representing the maximum amount of solute that can dissolve in a solvent at a specific temperature and pressure.
• Factors affecting solubility include:
  • Molecular interaction (hydrogen bonding, van der Waals forces, ionic interactions).
  • Temperature (most solids are more soluble at higher temperatures).
  • Pressure (gases are more soluble at higher pressures).
  • Nature of solute and solvent ("like dissolves like"): Polar solutes dissolve in polar solvents, and non-polar solutes dissolve in non-polar solvents.

Terminologies

• Solute: The dissolved component (less abundant in the solution).
• Solvent: The component in which the solute is dissolved (more abundant in the solution).
• Solution: A mixture of two or more components forming a homogeneous mixture.

Solubility Expressions

• Solubility can be expressed as percentage, molarity, and molality.
• Percent weight by weight (%w/w): Grams of solute dissolved in 100 grams of solution.
• Percent volume by volume (%v/v): Milliliters of solute dissolved in 100 milliliters of solution.
• Percent weight by volume (%w/v): Grams of solute dissolved in 100 milliliters of solution.
• Molarity: Moles of solute per liter of solution.
• Molality: Moles of solute per kilogram of solvent.
• USP (United States Pharmacopeia) solubility definitions: Provides the volume of solvent required to dissolve a specific amount of solute (e.g., 1 g of boric acid dissolves in 18 mL of water).

Types of Solubility

• Solubility of Gases in Liquids

  • The amount (or concentration) of gas dissolved in a liquid at equilibrium.
  • Some gas remains in a gaseous phase above the liquid.
  • Includes hydrochloric acid and effervescent preparations containing carbon dioxide.
  • Factors affecting solubility of gases in liquids: The mass of gas molecules, pressure, temperature, presence of salt.
  • Henry's Law: Solubility is directly proportional to the partial pressure of the gas at a constant temperature.
  • Temperature and solubility of gases: Solubility decreases as temperature increases.

• Solubility of Liquids in Liquids

  • Mixing of two or more liquids to form solutions used in pharmaceutical preparations. (e.g., alcohol and water forming hydroalcoholic solutions).

• Solubility of Solids in Liquids:

  • Most common type encountered in pharmaceutical solutions.
  • Mixtures of various solutes and solvents.

Ideal and Real Solutions

• Ideal solutions:

  • No attraction between solute and solvent molecules.
  • No change in properties other than dilution on mixing.
  • No heat absorbed or released during mixing.
  • Examples include benzene + ethylene chloride, and ethyl bromide + ethyl iodide.

• Real (Non-ideal) solutions:

  • Deviations from ideal behavior due to differences in intermolecular forces.
  • "Cohesive" forces between like molecules may be stronger than "adhesive" forces between unlike molecules.

Factors Influencing Solubility

• Temperature: Solubility of solids generally increases with temperature. The effect on gases is reversed: solubility decreases as temperature increases..

• pH: Affects the ionization of weak electrolytes.

• Particle Size (Surface Area): Smaller particles dissolve faster.

• Crystal Properties:

  • Polymorphs: Different crystalline forms of the same substance with varying physical properties (melting point, solubility).
  • Amorphous compounds: Non-crystalline; generally dissolve faster than crystalline forms

• Molecular size: Larger molecules usually are less soluble.

• Branching (Organic Compounds): More branching typically increases solubility due to reduced molecular size.

• Functional Groups: Hydrophobic (non-polar, dissolving in non-polar solvents) and hydrophilic (polar, dissolving in polar solvents) influence a molecule's overall solubility.

• Surfactants: Substances that have both hydrophobic and hydrophilic regions (amphiphilic), influence solubility by altering surface tension and interfacial forces.

Solubility Enhancement Techniques

• Complexation: Forming stable complexes with ligands—like cyclodextrins—to increase solubility.
• Cosolvency: Using a mixture of solvents to increase solubility, usually used for poorly water soluble drugs.

Distribution Phenomena

• Distribution: Refers to the distribution of solute between two immiscible liquids.
• Partition Coefficient (K or P): A ratio of the concentrations of a substance in the two phases at equilibrium; reflects lipophilicity.
• Shake Flask Method: A common technique to determine partition coefficients (e.g., octanol and water).
• Octanol is frequently used as a model for biological membranes in the determination of drug distribution.

Applications of Distribution Concepts

• Oil-water systems preservation, drug absorption/distribution, extracting active ingredients.

Important Concepts Summarized

• Solubility is the maximum amount of a substance that can dissolve in a given amount of solvent.
• Solubility is influenced by factors such as temperature, pH, particle size, the nature of the solute and solvent, and by various techniques (complexation, cosolvency) to improve solubility.
• Partition coefficient is determined to predict or quantify lipophilicity, often used to determine drug distribution.