SOLUTIONS
AND
SOLUBILITY
Phase Solubility Analysis: (cont.)
Steps of determination:
shaken at
constant (T,P)
Equilibrium
1gm
2gm
3gm
4gm
Separate solid from solution
1gm



2gm
3gm
4gm
Determine amount dissolved
Plot Y axis (solution conc)
Plot X axis (system conc)
Gibbs phase Rule:
F= C – P + 2
F= Degree of freedom (T, P, C)
C= Number of components
P= number of phases
At constant T and P
(F= C- P)

Phase solubility curves:
A)For pure substance:
F=0
Saturation
F=1
A-
Phase solubility diagram for a pure substance
B: Conc is below saturation
 B- C: Conc is above saturation
 B – C: has no slope, indicating purity
Point D: Solubility of pure substance.
B) For non- pure substance: (one impurity)
F=0
F=
F=11
F=2
•Phase solubility curve for substance contain one impurity
A- B: Conc is below saturation for both (1 phase)
At B: Saturation with major component
From B to C: Conc is above saturation with major component and below
saturation for minor one (2 phases)
Section C – D: Saturation with both components (3 phases)
 Value of AE: Solubility of major component.
Value of EF: Solubility of minor component
At BC: Pure solid major
The Process of Dissolution
1. The solute is separated from other similar molecules
+
Step 1
W22
2. The solvent molecules are separated sufficiently from other
molecules to create space to accommodate the solute
molecule.
Step 2
W11
3. The solute molecule becomes surrounded by solvent
molecules
+
Step 3
-W12
The free energy change of solution is (w11 + w22 -w12)
The Process of Dissolution
The free energy change of solution is (w11 + w22 -w12)
W11+ W22 > W12
Endothermic
W11+W22 < W12
Exothermic
The Process of Dissolution
1- Separation of solute from similar molecules to
become surrounded by solvent molecules.
2- Separation of solvent from similar molecules to
create space to accommodate the solute.
3- Placing the solute molecule in the solvent cavity
requires a number of solute-solvent contacts
4- Dissolution occurs if solute-solvent attraction
overcomes:
Solute-solute interaction
Solvent-solvent interaction.
Prediction of solubility in aqueous medium
(Dilute solution)

Predicting the solubility of solutes in aqueous
media depends on:
1.
Molecular surface area of solute (substituents)
2.
Nature of the key chemical groups in the solute.
Prediction of solubility in aqueous medium
(Dilute solution)

The solubility
A.
The larger the solute molecule, the larger the cavity
required.
The greater the number of contacts created.
B.
with of molecular surface area.

The term w12 is a measure of solute-solvent
interactions (SOLVATION).
A.
Interactions involving the non-polar part of the solute.
Interactions with the polar portion of the solute.
B.
Solvation and hydration

Solvation is the process of binding of solvent
to solute molecules.

If the solvent is water, the process is hydration
Hydration of non-electrolytes
A non-electrolyte does not
provide ions in a solution
and therefore current does
not flow through such
solution
e.g. Carbohydrates
Hydration of non-electrolytes

In a solution of Sucrose, six water molecules are
bound to each sucrose molecule as a one unit

Mannitol, sorbitol and inositol are sugar alcohols
have very different affinities for water.

The solubility of Sorbitol in water is about 3.5
times that of Mannitol.
(sorbitol has an equatorial –OH group on pyranose sugar).


Compatibility of the equatorial –OH with the
structure of water in bulk.
Axial hydroxyl -OH groups cannot bond with water
without distorting it.
Axial -OH
Equatorial -OH

Differences in the Lattice Energies of the crystals may
also contribute.
AXIAL -H
Equatorial -H
A cyclohexane molecule
Hydration of electrolyes and ionic groups
An electrolyte provides ions
in a solution and therefore
current flow through such
solution
e.g. Salts as NaCl
Hydration of electrolyes and ionic groups
 All ions in water possess a layer of “Tightly Bound Water”
 “Four “ water molecules are in the bound layer of most
monovalent, monatomic ion.
 The firmly held layer can be regarded as being in a ‘Frozen’
condition around a positive ion.
Orientation of water molecules around the ion
 Primary solvent sheath in which the water
molecules could be oriented with all the
hydrogen atoms of the water molecules
pointing outwards. (tightly bound layer)
 An intermediate layer of water around the
bound layer which is less ordered than bulk
water.
 Bulk water layer
Ionic species Hydration
Water Structure Breakers and Structure Makers




Ions, which include all the alkali and halide ions
except Li+ and F-, are called structure breakers.
Li < Na < K < Rb < Cs in size (Monovalent
alkali metals)
Li+ and F-, and many polyvalent ions, for example
Al3+, increase the structured nature of water
beyond the immediate hydration layer, and are
therefore structure makers.
Na+ is considered as weak structure maker.
Hydration numbers

Hydration number: the number of water molecules in he
primary hydration layer. (tightly bound layer)

Solvation number: the number of solvent molecules in the
primary layer
(zero in the case of large ions such as:
(iodide, caesium, tetraalkylammonium ions).

The solvation numbers decrease with increase of ion size the
?
Ionic force field diminishes with increasing radius.
Polar
Solvent (H2O)
I) Electrolytes
II) Non-electrolytes
I) Electrolytes
Strong (NaCl) and weak (AgCl) electrolytes interact with water
(polar), through dipole-dipole interactions, where

Ionization occurs first

Followed by hydration
II) Non-electrolytes
e.g. phenols, alcohols, aldehydes, ketones, amines. The hydration
occurs through the formation of solute/solvent unit through
dipole-dipole interactions.
Non-polar
Solvent
I) Induced dipole
Induced dipole
II) Dimerization
I) Induced dipole-induced dipole:
They are able to dissolve other non-polar in which bonds are weak.
e.g. hydrocarbon in hydrocarbon- oil, fat in petroleum ether.
II) Dimerization:
e.g. acetic acid in CCl4.
(Consumption of dipole)
Semi-polar
Solvent
I) Permanent dipole
-induced dipole
II) Intermediate solvent
I) Permanent dipole-induced dipole:
e.g. alcohol (semi-polar) dissolves in benzene (non-polar) through
induction of temporary dipole in benzene molecule.
Examples of semi-polar solvents are: alcohols and ketones.
II) Intermediate solvent (Cosolvency):
e.g. acetone increases the miscibility of ether (non-polar) in water
(polar).