Questions and Answers
What is the qualitative definition of solubility?
Spontaneous interaction of two or more substances to form a homogenous molecular dispersion.
How is dissolution defined?
The transfer of molecules or ions from a solid state into solution.
Solubility of gases in liquids decreases with an increase in temperature.
True
Solubility of ___ in liquids is more commonly referred to as miscibility rather than solubility.
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What is the purpose of using co-solvents to improve the solubility of drugs?
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Which of the following are examples of weak bonds involved in complexation for improving drug solubility? (Select all that apply)
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What is the purpose of water soluble prodrugs in enhancing drug solubility?
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Dissolution is a process independent of time and a constant at constant conditions.
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The _ is the proportionality constant in the Noyes-Whitney equation for dissolution rate.
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Study Notes
Solutions and Solubility
- A dispersed system is a heterogeneous system consisting of one component (dispersed phase) distributed throughout another component (dispersion medium or continuous phase).
- Based on the size of the dispersed phase, there are three types of dispersed systems:
- Solution: a mixture of two or more components forming a homogeneous molecular dispersion.
- Colloid: a mixture of particles with sizes between 1-100 nm.
- Suspension: a mixture of particles with sizes larger than 100 nm.
Dissolution Process
- The transfer of molecules or ions from a solid state into a solution is known as dissolution.
- The dissolution process can be represented by the equation: Solute + Solvent → Solution.
- The spontaneous interaction between two or more substances to form a homogeneous molecular dispersion is known as solubility.
Qualitative and Quantitative Solubility
- Qualitative definition of solubility: the ability of a substance to dissolve in another substance to form a homogeneous molecular dispersion.
- Quantitative definition of solubility: the amount of solute that passes into solution when equilibrium is established between the solute in solution and the excess undissolved solute at a constant temperature.
Types of Solutions
- Saturated solution: a solution in which the dissolved solute is in equilibrium with the undissolved solute at a definite temperature.
- Unsaturated solution: a solution in which the concentration of dissolved solute is less than that necessary for complete saturation at a definite temperature.
- Supersaturated solution: a solution in which the concentration of dissolved solute is more than that necessary for complete saturation at a definite temperature.
Methods of Expressing Solubility
- Qualitative description: using descriptive terms such as very soluble, freely soluble, soluble, sparingly soluble, slightly soluble, and very slightly soluble.
- Quantitative description: using concentration terms such as molarity (M), percent weight in volume (%w/v), and volume percent (%v/v).
Types of Solubility
- Solubility of gases in liquids: the concentration of the dissolved gas when it is in equilibrium with some of the pure gas above the solution.
- Solubility of solids in liquids: the ability of a solid to dissolve in a liquid.
- Solubility of liquids in liquids: the ability of a liquid to dissolve in another liquid.
Factors Affecting Solubility of Gas in Liquid
- Pressure: the concentration of the dissolved gas is proportional to the partial pressure of the gas above the solution (Henry's law).
- Temperature: the solubility of gases in liquids decreases with an increase in temperature.
Solubility of Solid in Liquid
- Mechanism of solute-solvent interactions: polar solvents (e.g., water, ethanol, glycerol) dissolve polar solutes through dipole interaction and hydrogen bond formation, while non-polar solvents (e.g., benzene, mineral oil) dissolve non-polar solutes by weak Van der Waals forces.
Factors Affecting Solubility of Solid in Liquid
- Temperature: the solubility of solids in liquids is affected by temperature, with some solids having an endothermic, exothermic, or isothermic process.
- pH: the solubility of weak acids and bases is affected by pH, with ionization increasing solubility in water.
- Particle size: a decrease in particle size increases the surface area, which can increase solubility.
- Crystal structure: amorphous forms of drugs are more soluble than crystalline forms.
- Molecular structure: changes in molecular structure can affect solubility, such as salt formation, esterification, and addition of polar groups.
Methods to Improve Solubility of Drugs
- Use of co-solvents: a co-solvent is a water-miscible organic solvent that can reduce the difference in polarity between the drug and water, improving solubility.
- Complexation: a reversible association between two or more molecules to form a non-bonded product with a well-defined stoichiometry.
- Water-soluble prodrugs: pharmacologically inactive derivatives of active drugs that can be converted into active drugs within the body.
- Solubilization using surfactants: surfactants can reduce the surface tension of the solvent, allowing for better solubility of the drug.
Dissolution vs. Dissolution Rate
- Solubility: the saturation process independent of time, which is a physical property that is constant at constant conditions.
- Dissolution: the process by which a solid goes into solution.
- Dissolution rate: the rate at which a solute goes into solution, which is dependent on the stagnant layer, diffusion coefficient, and concentration gradient.
Dissolution Theory
- The stagnant layer is a thin layer of solvent surrounding the solid particle, which is saturated with the dissolved solute.
- The dissolution rate is dependent on the slowest step, which is the diffusion of the dissolved solute through the stagnant layer.
- The Noyes-Whitney equation describes the dissolution rate of a solid: Dissolution Rate (dm/dt) = DA (Cs - C)/L.
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Description
This quiz covers the basics of solubility and solutions, including definitions, types, and methods of expression. It also explores partition coefficients, diffusion, osmosis, and surface phenomena.