Dissolution Wikipedia

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Dissolution (chemistry)This article needs additional citations for verification. Please helpimprove this articleby

adding citations to reliable sources. Unsourced material may bechallengedandremoved.(December 2009)

Making asaline watersolution by dissolvingtable salt(NaCl) inwater. The salt is the solute

and the water the solvent.

Gold, formerly dissolved in crystal ofpyrite, is left behind after thecubiccrystal of pyrite

dissolved away. Note a corner of the former cube seen in center of rock.
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Dissolution is the process by which asoluteforms asolutionin asolvent. 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., thesolubility) is governed by thethermodynamicenergies

involved, such as theheat of solutionandentropyofsolution, but the dissolution itself (akinetic process) is not. Overall thefree energymust be negative for net dissolution to occur.

In turn, those energies are controlled by the way in which differentchemical bondtypes

interact with those in the solvent.Solid solutionsoccur inmetal alloysand their formation

and description is governed by the relevantphase 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

Forionic compounds, dissolution takes place when the ionic lattice breaks up and the

separate ions are then solvated. This most commonly occurs inpolarsolvents, such aswater

orammonia.

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) Na+

(aq) + Cl-(aq)

The behavior of this system is characterised by theactivity coefficientsof the components

and the solubility product, defined as:

The ability of an ion to preferentially dissolve (as a result of unequal activities) is classifiedas thePotential Determining Ion. This in turn results in the remaining particle possessing

either a net positive/negative surface charge.

Polar compounds

Other solid compounds experience dissolution as a breakdown of their crystal lattice, and due

to their polarity, or non-polarity, mix with thesolvent. The solubility of ionic salts inwateris

generally determined by the degree ofsolvationof the ions by water molecules. Such

coordination complexesoccur by water donating spareelectronson theoxygenatom to the

ion.
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Polymers

The solubility ofpolymersdepends on the chemical bonds present in the backbone chain and

their compatibility with those of the solvent. TheHildebrand solubility parameteris

commonly used to evaluate polymer solubility. The closer the value of the parameters, the

more likely dissolution will occur.

Liquids

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 bondsplay an important role in aqueous dissolution.

Gases

Compounds in the gaseous state will dissolve in liquids dependent on the interaction of their

bonds with the liquid solvent.

Rate of dissolution

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 theNoyes-Whitney Equationor the Nernst

and Brunner equation[1]of the form:

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

substanceCs, mass concentration of the substance on the surface
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Cb, mass concentration of the substance in the bulk of the solvent

For dissolution limited bydiffusion, 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 inkg/m2s and referred to as "intrinsic dissolution rate". The intrinsic dissolution rate is defined

by theUnited 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

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) 111.

doi:10.1016/j.ijpharm.2006.07.011

http://en.wikipedia.org/wiki/Dissolution_%28chemistry%29
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