Dissolution (chemistry)

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Making a saline water solution by dissolving table salt (NaCl) in water. The salt is the solute and the water the solvent.
Gold, formerly dissolved in crystal of pyrite, is left behind after the cubic crystal of pyrite dissolved away. Note a corner of the former cube seen in center of rock.

Dissolution is the process by which a solute forms a solution in a solvent. The solute, in the case of solids, has its crystalline structure disintegrated as separate ions, atoms, and molecules form. For liquids and gases, the molecules must be adaptable with those of the solvent for a solution to form. The outcome of the process of dissolution (the amount dissolved at equilibrium, i.e., the solubility) is governed by the thermodynamic energies involved, such as the heat of solution and entropy of solution, but the dissolution itself (a kinetic process) is not. Overall the free energy must be negative for net dissolution to occur. In turn, those energies are controlled by the way in which different chemical bond types interact with those in the solvent. Solid solutions occur in metal alloys and their formation and description is governed by the relevant phase diagram.

Dissolution process is of fundamental importance to the description of numerous natural processes on earth, and it is commonly utilized by humans. Dissolution testing is widely used in the pharmaceutical industry for optimization of formulation and quality control.

Ionic compounds[edit]

For ionic compounds, dissolution takes place when the ionic lattice breaks up and the separate ions are then solvated. This most commonly occurs in polar solvents, such as water or ammonia:

NaCl(s) → Na+(aq) + Cl-(aq)

In a colloidal dispersed system, small dispersed particles of the ionic lattice exist in equilibrium with the saturated solution of the ions, i.e.

NaCl(aq)  \rightleftharpoons Na+(aq) + Cl-(aq)

The solubility of ionic salts in water is generally determined by the degree of solvation of the ions by water molecules. Such coordination complexes occur by water donating spare electrons on the oxygen atom to the ion.

The behavior of this system is characterised by the activity coefficients of the components and the solubility product, defined as:

a_{Na^ +  }  \cdot a_{Cl^ -  }  = K_{sp}

The ability of an ion to preferentially dissolve (as a result of unequal activities) is classified as the Potential Determining Ion. This in turn results in the remaining particle possessing either a net positive/negative surface charge.

Polar compounds[edit]

Other solid compounds experience dissolution as a breakdown of their crystal lattice, and due to their polarity, or non-polarity, mix with the solvent.

Polymers[edit]

The solubility of polymers depends on the chemical bonds present in the backbone chain and their compatibility with those of the solvent. The Hildebrand solubility parameter is commonly used to evaluate polymer solubility. The closer the value of the parameters, the more likely dissolution will occur.

Liquids[edit]

Compounds in a fluid state may also dissolve in another liquid depending on the compatibility of the chemical and physical bonds in the substance with those of the solvent. Hydrogen bonds play an important role in aqueous dissolution.

Gases[edit]

Compounds in the gaseous state will dissolve in liquids dependent on the interaction of their bonds with the liquid solvent.

Rate of dissolution[edit]

The rate of dissolution quantifies the speed of the dissolution process.

The rate of dissolution depends on:

  • nature of the solvent and solute
  • temperature (and to a small degree pressure)
  • degree of undersaturation
  • presence of mixing
  • interfacial surface area
  • presence of inhibitors (e.g., a substance adsorbed on the surface).

The rate of dissolution can be often expressed by the Noyes-Whitney Equation or the Nernst and Brunner equation[1] of the form:

\frac {dm} {dt} = A \frac {D} {d} (C_s-C_b)

where:

m, mass of dissolved material
t, time
A, surface area of the interface between the dissolving substance and the solvent
D, diffusion coefficient
d, thickness of the boundary layer of the solvent at the surface of the dissolving substance
Cs, mass concentration of the substance on the surface
Cb, mass concentration of the substance in the bulk of the solvent

For dissolution limited by diffusion, Cs is equal to the solubility of the substance.

When the dissolution rate of a pure substance is normalized to the surface area of the solid (which usually changes with time during the dissolution process), then it is expressed in kg/m2s and referred to as "intrinsic dissolution rate". The intrinsic dissolution rate is defined by the United States Pharmacopeia.

Dissolution rates vary by orders of magnitude between different systems. Typically, very low dissolution rates parallel low solubilities, and substances with high solubilities exhibit high dissolution rates, as suggested by the Noyes-Whitney equation. However, this is not a rule.

References[edit]

  1. ^ Aristides Dokoumetzidis, Panos Macheras, "A century of dissolution research: From Noyes and Whitney to the Biopharmaceutics Classification System", International Journal of Pharmaceutics 321 (2006) 1–11. doi:10.1016/j.ijpharm.2006.07.011

See also[edit]