Dissolution is the process where a solute in gaseous, liquid, or solid phase dissolves in a solvent to form a solution.
Solubility is the maximum concentration of a solute that can dissolve in a solvent at a given temperature. At the maximum concentration of solute, the solution is said to be saturated. The units of solubility can be provided in mol/L or g/L.
Factors that affect solubility include:
- The concentration of the solute
- The temperature of the system
- Pressure (for gases in solution)
- The polarity of the solute and the solvent
The rate of dissolution is represented by the Noyes-Whitney equation: dm/dt = D*A*(Cs - C)/h
- dm/dt represents the rate of dissolution
- D represents the diffusion coefficient for the compound
- A represents the surface area available for dissolution
- Cs represents the solubility of the compound
- C represents the solute concentration in bulk solution at time t
- h represents the thickness of the dissolution layer
Effect of temperature on liquid and solid solutes
As temperature increases, the solubility of a solid or liquid can fluctuate depending on whether the dissolution reaction is exothermic or endothermic.
Increasing solubility with increasing temperature
- In endothermic dissolution reactions, the net energy from breaking and forming bonds results in heat energy being absorbed into the system as the solute dissolves. When the temperature of the system increases, additional head energy is introduced into the system.
- So according to Le Chatelier’s Principle, the system will adjust to this increase in the heat by promoting the dissolution reaction to absorb the added heat energy. Increasing the temperature will therefore increase the solubility of the solute.
- An example of a solute whose solubility increases with greater temperature is ammonium nitrate, which can be used in first-aid cold packs. Ammonium nitrate dissolving in solution is an endothermic reaction. As the ammonium nitrate dissolves, heat energy is absorbed from the environment causing the surrounding environment to feel cold.
Decreasing solubility with increasing temperature
- In exothermic reactions, heat energy is released when the solute dissolves in a solution. Increasing temperature introduces more heat into the system. Following Le Chatelier’s Principle, the system will adjust to this excess heat energy by inhibiting the dissolution reaction. Increasing temperature, therefore, decreases the solubility of the solute.
- An example of a solute that decreases in solubility with increasing temperature is calcium hydroxide, which can be used to treat chemical burns and as an antacid.
Effect of temperature on gas solutes
In general, heat energy is released as gas dissolves in solution, meaning the dissolution reaction is exothermic. As such, a gas becomes less soluble as temperate increases.
Increasing temperature results in increased kinetic energy. Gas molecules with greater kinetic energy move more rapidly resulting in the intermolecular bonds between the gas solute and solvent breaking.
Pressure: Henry’s law
The solubility of gas is affected by changes in pressure on the system. A gas dissolves in liquids to form solutions. This results in equilibrium in the system where a proportion of gas molecules is dissolved in liquid while the rest remains in gaseous phase above the liquid.
Henry’s law states that: “At constant temperature, the amount of gas that dissolves in a volume of liquid is proportional to the partial pressure of the gas in equilibrium with the liquid.”
Henry's law results in the following equation: C = kP
- C represents the solubility of the gas at a certain temperature in a specific solvent
- K represents Henry’s law constant
- P represents the partial pressure of the gas i.e. the pressure the gas exerts on the system at a given volume and temperature
Hence as the pressure of the gas above the liquid in the system increases, the gas molecules become more soluble in the solvent. Likewise, if the pressure of the gas in the system decreases, gas becomes less soluble in the solvent.
Issues of Concern
Limitations of Henry’s Law on gas solubility:
- Only applies if the gas molecules are in equilibrium
- Does not apply if there is a chemical reaction between the solvent and the solute
- Does not apply to gas at high pressures
Methods to enhance dissolution to improve oral bioavailability include: 
- Micronization to increase surface area for dissolution
- Salt formation of the active ingredient
- Use of co-solvents and micelle solutions to aid solubilization
- Complexation through the use of cyclodextrins
- Use of lipidic systems (for lipophilic drugs)
Le Chatelier’s principle:
If stressors like pressure and heat are applied to the equilibrium, the system will respond by adjusting to minimize the effects of the stress.
For example, if pressure is applied to a system, the dissolution reaction will respond to minimize this stress by reducing the pressure in the system.
Heat of solution
Solids and liquids form as a result of individual particles being held together by inter-particulate bonds. To form a solution, energy is required to break the bonds between the particles within the solid or liquid. Heat energy is also required to break the bonds in a solvent to insert one of the molecules into the solution. Both of these processes are endothermic. Heat energy is released when the solute molecules form bonds with the solvent molecules i.e. this process is exothermic.
Depending on whether more energy is used to break the bonds within the solute and solvent or is released when new bonds are formed between the solute and solvent, the reaction overall can be exothermic or endothermic.
- If more energy is required to break the bonds within the solute and solvent than is released when new bonds are formed between the solute and solvent, the reaction is considered endothermic.
- If more energy is released when new bonds are formed between the solute and solvent than is required to break the bonds within the solute and solvent, the reaction is considered exothermic.
The total amount of heat energy released from or absorbed by the system = sum of heat energy absorbed when bonds are broken – the sum of heat energy released when bonds are formed
- If the total amount of heat energy released/absorbed from the system is greater than zero, the reaction is endothermic.
- If the total amount of heat energy released/absorbed from the system is less than zero, the reaction is exothermic.
Dissolution is important for health practitioners because, for drugs to be absorbed and have a physiological effect in the human body, they must be in solution. For solid preparations, such as tablets and suppositories, the rate of dissolution affects how fast a drug is absorbed in the body.
Aqueous solubility is often considered when formulating drugs. Poorly soluble formulations provide difficulties in the development of pharmaceuticals. Chloramphenicol, phenytoin, and digoxin are some examples. Drugs, particularly those for oral administration, may have poor aqueous solubility. This may result in low bioavailability leading to insufficient exposure and physiologic effect in the body.