What is dissolution?

Summary

In this video, we explore the process of dissolution, where a solid dissolves into a solution, with an emphasis on its relevance to pharmaceuticals. The video explains solubility, dissolution steps, and the thermodynamic concepts involved, such as enthalpy of fusion and enthalpy of mixing. It also highlights how factors like the solid's crystalline structure, temperature, and interactions with solvents affect dissolution rates. Key distinctions are made between amorphous and crystalline materials, with insights on how these differences impact drug development. The video concludes with a focus on the two key steps in dissolution and their role in determining the rate of the process.

Takeaways

• 😀 Solubility refers to the maximum concentration of a solute that can dissolve in a solvent, typically water, and is determined when the solution reaches saturation.
• 😀 Dissolution is the process of a solid dissolving into a solvent, which is essential in determining how substances like drugs dissolve in the body.
• 😀 The first step of dissolution involves molecules breaking free from the solid, which requires energy to overcome the bonds in the crystalline structure. This is an endothermic process.
• 😀 The energy required to break the bonds in the solid lattice is called the enthalpy of fusion, which is similar to the energy needed to melt a crystalline material.
• 😀 Solids with different melting temperatures will dissolve differently due to varying bond strengths, which is important for drug development.
• 😀 Amorphous materials dissolve faster than crystalline ones because they lack a solid lattice structure and therefore require no energy to break bonds.
• 😀 The second step in dissolution is solvation, where the solvent molecules surround and interact with the solute molecules, forming a shell around each solute particle.
• 😀 Solvation can be either exothermic (releasing energy) or endothermic (requiring energy), depending on the nature of the interaction between the solvent and solute molecules.
• 😀 The overall energy change during dissolution is the sum of the enthalpy of fusion and the enthalpy of mixing, which together determine whether the process will be favorable or not.
• 😀 For dissolution to occur efficiently, the enthalpy of solution should be negative, meaning the interaction between the solute and solvent is energetically favorable.
• 😀 Dissolution rates are controlled by either the breaking of bonds (interfacially-controlled dissolution) or the solvation process (diffusion-controlled dissolution), with one typically being the rate-limiting step.

Q & A

What is solubility and how is it determined?

Solubility is the maximum concentration of a given solute that can be attained in a particular solvent, usually water in the pharmaceutical world. It is determined by the equilibrium between a saturated solution and undissolved solid, with excess solid indicating the solute's solubility limit.

What is dissolution?

Dissolution is the process by which a solid dissolves into a solvent to form a solution. It involves two main steps: breaking away molecules from the solid and then solvating those molecules with the solvent.

What happens in the first stage of dissolution?

In the first stage of dissolution, molecules must break away from the surface of the dissolving solid. For crystalline solids, the lattice energy holding the molecules together must be overcome, which is an endothermic process similar to melting.

Why is the process of dissolution endothermic in the first stage?

The process is endothermic because energy is required to break the bonds holding molecules together in a crystalline solid. This is similar to the energy required to melt a material, which overcomes the lattice energy.

What is the enthalpy of fusion?

The enthalpy of fusion is the energy required to break the bonds in a crystalline solid and is the same as the energy required to melt the material. This energy plays a role in the dissolution of solids.

How does the dissolution of polymorphs differ?

Polymorphs of the same material can dissolve differently because they have different melting temperatures. This implies that solids with different polymorphs may dissolve at different rates, affecting drug development.

Why do amorphous materials dissolve faster than crystalline materials?

Amorphous materials dissolve faster because they lack a regular crystalline structure, and therefore do not have lattice energy to overcome. This makes it easier for the molecules to break away and dissolve.

What happens in the second stage of dissolution?

In the second stage, the molecules that have broken away from the solid surface are surrounded by solvent molecules, forming a solvation shell. This step involves the formation of weak bonds between the solute and solvent molecules.

What determines whether dissolution will proceed rapidly or slowly?

The rate of dissolution depends on whether the solute and solvent molecules 'like' each other. If they form favorable bonds (exothermic interaction), dissolution proceeds quickly. If not, the interaction is unfavorable (endothermic), and dissolution slows or may not occur.

What is the enthalpy of mixing and how does it affect dissolution?

The enthalpy of mixing refers to the energy change when solvent molecules interact with solute molecules. For dissolution to be favored, the enthalpy of mixing must be large and negative (exothermic). A positive enthalpy of mixing (endothermic) would slow down or prevent dissolution.

What is the difference between interfacially-controlled and diffusion-controlled dissolution?

Interfacially-controlled dissolution occurs when the first stage (breaking bonds in the solid) is the slowest step, while diffusion-controlled dissolution occurs when the second stage (solvation and diffusion) is the slowest. The rate-limiting step depends on which part of the process takes more time.

How does the overall energy of dissolution affect its success?

For dissolution to proceed efficiently, the overall enthalpy of solution (sum of enthalpy of fusion and enthalpy of mixing) should ideally be negative. This indicates that both steps of dissolution are energetically favorable and will proceed efficiently.