Consistent+Terminology+for+Solvation

=Consistent Terminology for Solvation= toc There seems to be a need of a clear defined vocabulary to name and differentiate between the processes of hydration, solvation and dissolvation. Particular when handling solvation free energy. Even in literature (and especially in papers) seems to be no consistent use of such terms.

That is why this collection of terms should be obligatory for our group, to be consistent when working with each other and writing paper.

This page is merely a compendium for terms to use. If you like to get some deeper insight about the physics, you may want to read some good book or visit this page for a [|short overview about solution].

solution
A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes. When, as is often but not necessarily the case, the sum of the mole fractions of solutes is small compared with unity, the solution is called a dilute solution. [|iupac]

//remark//: it is worth to stress the fact, that the distinction between //solvent// and //solute// in merely arbitrarily.

solute
The minor components of a solution which are regarded as having been dissolved by the solvent. [|iupac]

solvent
The major components of a solution which are regarded as having dissolved a solute.

//remark//: the terms //solute// and //solvent// can refer to one or many different molecular species.

dilute solution
Solution in which the sum of mol fractions of all the solutes is small compared to 1.

//remark//: If you dilute a solution by adding solvent, you do **not** get a dilute**//d//** solution (although it would be correct english) but a //dilute// solution.

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solvation
Any stabilizing interaction of a solute (or solute moiety) and the solvent or a similar interaction of solvent with groups of an insoluble material (i.e. the ionic groups of an ion-exchange resin). Such interactions generally involve electrostatic forces and van der Waals forces, as well as chemically more specific effects such as hydrogen bond formation. [|iupac]

hydration (physical)
The physical meaning of the term hydration is important for us.

The term is also used for the process: A (gas) --> A (aqueous solution) cf. the use of the term in inorganic/physical chemistry to describe the state of the ions of an electrolyte in aqueous solution. [|iupac]

//remark//: hydration can be seen as a special case of solvation, where the solvent is water. //2nd remark//: also consider the difference in //solvation// shell and //hydration// shell.

As you may guessed, //hydration// refers **not** to hydrogen atoms, but explicitly to water.

hydration (chemical)
The chemical meaning of hydration is introduced for the sake of completeness and to avoid misunderstandings. Addition of water or of the elements of water (i.e. H and OH) to a molecular entity. For example, hydration of ethene: CH2 = CH2 + H2O --> CH3-CH2-OH [|iupac]

dissolution
The mixing of two phases with the formation of one new homogeneous phase (i.e. the solution). [|iupac]

//remark//: For the dissolution of an crystal in a solvent, dissolution include the whole process of removing the ions from the crystal lattice and solvate them in the solvent.

sublimation
The direct transition of a solid to a vapour without passing through a liquid phase. Example: The transition of solid CO2 to CO2 vapour. [|iupac]

microsolvation
Clusterication of a solute molecule with a few solvent molecules.

//remark//: This is no rigorous defenition from IUPAC but more a picture to have in mind, when reading this term.

A remark on Verbs
There is also a pitfall using the associated verbs.

A solute is //dissolved// within a solvent, forming the solution. A mathematical equation is //solved//, giving also a solution. A solute can be //solvated// in a solvent, and //hydrated// in water.

Definition of (Gibbs) Free Energy
As mentioned above, when handling free energy differences, a precise definition is necessary. In an ideal case, for every calculated free energy difference should be a definition of initial and final state to avoid any misunderstandings (with you and the reader or with you and the auditory).

//Free energy// for itself is misleading, because there are two energies termed with free energy: the //Gibbs free energy// G [|see wiki] (or //Gibbs function// [|iupac]) and the //Helmholtz free energy// A (or F) [|see wiki] (or //Helmholtz function// [|iupac]). In the context of solvation, we will use //free energy// for the //Gibbs free energy//, but it may be a good idea to always use the full term, give a clear definition beforehand or add the corresponding symbol (like G).



solvation (free) energy **(ΔGsolv)**
The change in Gibbs energy when an ion or molecule is transferred from a vacuum (or the gas phase) to a solvent. The main contributions to the solvation energy come from: 1. the cavitation energy of formation of the hole which preserves the dissolved species in the solvent; 2. the orientation energy of partial orientation of the dipoles; 3. the isotropic interaction energy of electrostatic and dispersion origin; and 4. the anisotropic energy of specific interactions, e.g. hydrogen bonds, donor-acceptor interactions etc. [|iupac]

//remark//: In contrast to IUPAC, we will term this energy //solvation **free** energy//.

Below: a scheme, illustrating the procedure of solvation which may be used in computer simulation, taken from //J. Phys. Chem. B 2007, 111, 1872-1882//.

**Hydration free energy (ΔGhyd)**
is the change of the Gibbs free energy that accompanies the transfer of a solute from gaseous phase to //water//.

**Free energy of solution (or dissolution) (ΔGsoln)**
is the change of the Gibbs free energy that accompanies the transfer of a solute from crystal to a solvent.

//remark//: the term //dissolution// is more associated with a kinetic property, whereas we are mostly interessted in the properties of equilibrated states. That's why //free energy of solution// is preferable to use.

**Sublimation Free energy (ΔGsub)**
is the change of the Gibbs free energy that accompanies the transfer of a solute from crystal to gaseous phase.


 * Naturally** ΔGsoln = ΔGsub + ΔGsolv.

standard condition and standard state
The //Gibbs free energy// is a function of a system at a specific //state//. It is natural to define a //standard state// to correspond. This was done by IUPAC, but in a quite complex way:

State of a system chosen as standard for reference by convention. Three standard states are recognized: For a gas phase it is the (hypothetical) state of the pure substance in the gaseous phase at the [|standard pressure], assuming ideal behaviour. For a pure phase, or a mixture, or a solvent in the liquid or solid state it is the state of the pure substance in the liquid or solid phase at the [|standard pressure]. For a solute in solution it is the (hypothetical) state of solute at the standard molality, [|standard pressure] or [|standard concentration] and exhibiting infinitely [|dilute solution] behaviour. For a pure substance the concept of standard state applies to the substance in a well defined state of aggregation at a well defined but arbitrarily chosen [|standard pressure]. [|iupac]

In most cases, we study the system under //standard conditions// (STP). But even there, some deviations exist. There is a nice table in wikipedia [|table of STP]. The most convenient way would be, to always state your specific conditions of your simulation/ experimental data instead of mention 'under standard condition'.

If the notation ΔG 0 is used, the change in the Gibbs free energy between one state and a state under standard condition is meant. A state under standard condition in this context is //a pure compound A at a temperature T and a pressure P.// For example when dissolving a pure crystal NaCl under STP in water, the change in Gibbs free energy would be notated with ΔG0.

//remark//: the definition and usage of ΔG0 has to be extended.

Common Units of Gibbs free energy
As a thermodynamical potential, the Gibbs free energy is given in units of energy, say kJ, kcal or eV. More commonly used for changes in the Gibbs free energy of systems is the energy value per mol, where 1 mol = 6.022 × 10^23 molecules. When considering solvation free energy of single molecules the unit eV per //molecule// is also used. The unit calorie (cal) is no SI unit and has different definitions [|see wiki].

1 eV/molecule = 96.485 kJ/mol 1kcal = 2.611 × 10^19 eV = 4.184 J