Solubility
Contents of
This Chapter
• Concepts of solution, saturated and supersaturated
solution
• Applications of solubility
• Methods of expressing solubility
• Factors affecting solubility
• Factors influencing solubility of solid in liquids
• Concepts of Raoult’s law
• Ideal and real solutions
• Positive and negative deviations from Raoult’s law
• Concepts of partially miscible liquids and critical
solution temperature
• Concept of phase rule and its applications
• Concepts of partially miscible liquids Phenol water
system, critical solution temperature and its applications
• Triethylamine- water system
• Nicotine- water system
Learning
Objectives
• At the end of this lecture, student will be able to
– Define solubility,
saturated and unsaturated solution
– Discuss the
applications of solubility
– Describe the
methods to express solubility
– Describe the factor
Affecting Solubility
– Explain the
influence of solubility of solids in liquids
– Explain the
influence of solubility of liquids in liquids
– Explain the
influence of solubility of gases in liquids
– Describe the
concepts and applications of Raoult’s law
– Explain the
concepts, ideal and real solutions
– Describe the positive and negative deviations from
Raoult’s law
– Describe the principle and applications of partially
miscible liquids and critical solution temperature
– Explain phase rule and its applications
– Describe the principle and applications of partially
miscible liquids
– Describe the
concept of phenol water system and critical solution temperature
– Explain the applications of phenol- water system and
trietylamine-water system
Solubility-
Definitions
• A solution can be defined as a homogenous mixture in which
one substance is said to dissolve in the other
• In quantitative terms, solubility is defined as the
concentration of solute in a saturated solution at a certain temperature
• Quantitatively, solubility is defined as a spontaneous
interaction of two or more substances to form a homogenous molecular dispersion
• Saturated solutions are the solution in which the
dissolved solute is in equilibrium with the solvent phase, at a definite
temperature
• An unsaturated solution is the solution containing the
dissolved solute in concentration below that is necessary for saturation, at a
definite temperature
• A supersaturated solution is the one that contains more of
the dissolved solute than that it would normally contain at a definite
temperature
• A supersaturated solution can be applied for
crystallisation process
Solubility-
Applications
• For the manufacture of liquid orals such as syrups and
elixirs
• For the preparation of intravenous, intramuscular and
subcutaneous injections
• For the dissolution of drugs in GIT
• The release and absorption of a drug from an ointment or
an intramuscular injection
• It serves as a standard test for purity
• It provides information regarding intermolecular forces of
attraction
• Saturated solution theory is important for the
crystallization of drugs from solvents
• Principles of solubility are used for determining
physicochemical properties
• Differences in solubility in various solvents often serve
as a useful means of separating one component from the other and for
purification process
|
Descriptive phases in ml/g of |
Approximate volume of solvent
|
|
Very soluble |
less than 1 |
|
Freely soluble |
from 1 to 10 |
|
Soluble |
from 10 to 30 |
|
Sparingly soluble |
from 30 to 100 |
|
Slightly soluble |
from 100 to 1000 |
|
Very slightly soluble |
from 1000 to 10000 |
|
Insoluble or practically insoluble |
more than 10000 |
Methods of
Expression of Solubility
• The other methods of expressing solubility are:
– Weight per cent
– Volume per cent
– Normality – is a function of equivalents
Normality = (equivalents of X)/Liter
– Molarity – Number of moles of a solute dissolved/liter of
solution
– Molality – Number of moles of solute dissolved/kilogram of
solvent.
– Mole fraction – It
is equal to the moles of one component divided by the total moles in the
solution or mixture
– Mole per cent – all
the mole percents of a mixture add up to 100 mole percent. Mole percent can be
converted to mole fraction by dividing by 100.
– Equivalent weight
Factors
Influencing Solubility of Drugs
• Influence of particle size, shape and surface area
• Influence of physicochemical properties of drugs
• Influence of solvents
• Influence of pH of the medium
• Influence of co-solvents
• Influence of temperature
• Influence of other ingredients
• Influence of surfactants
Factors
that affect solubility
• The nature of the solute and solvent
• Temperature
• Pressure (only applicable to gases)
Nature of
Solute and Solvent
• Polar Solvent- a liquid made up of polar molecules
• Non-polar Solvent- a liquid made up of non-polar molecules
• When two substances are similar they can dissolve in each
other
– Polar solutes dissolve in polar solvents
– Non-polar solutes tend to dissolve in non-polar solvents
• “like dissolves like”
– Two liquids dissolve in each other because their molecules
are alike in polarity
• Ionic compounds are made up of charged ions similar to
polar compounds
• Ionic compounds are more soluble in a polar solvent than
in a non-polar solvent
|
Solute |
Polar Solvent |
Non-polar solvent |
|
Polar |
Soluble |
Insoluble |
|
Non-Polar |
Insoluble |
Soluble |
|
Ionic |
Soluble |
Insoluble |
Temperature
• Solutions of gases in liquids are affected by temperature
– As temperature increases, the solubility of a GAS in a
liquid decreases
• WHY?
– As temperature increases, the kinetic energy of the solute
gas increases and the gas can escape
• Solubility of SOLIDS in liquids: total opposite
– The solubility of a solid increases as the temperature
increases (there are a few exceptions)
• Temperatures Affecting the Solubility as the Solution is
Formed
– When the temperature drops while you mix the solute and
solvent, raising the temperature will increase solubility
– If the temperature stays neutral, the temperature will
have minimal or insignificant effect either way
– If the temperature is increased when the solute and
solvent are mixed, raising the temperature will decrease solubility
Pressure
• When the pressure is increased over the SOLVENT, the
solubility of the gas is increased.
• Why?
– Pressure increases as gas molecules strike the surface to
enter solution is increased
• Henry’s Law:
Solubility of gas is directly proportional to the partial pressure of the gas
above the liquid
p=khc
p= partial pressure
kh= gas constant
p=khc
c= concentration of the solute
Surface
Area
• Dissolving solutes happen in the surface area of the solvent
• Speed up the process by increasing the surface area
• The greater the surface area per unit mass, the quicker it
will dissolve
Stirring
• Dissolving happens at the surface of the solvent
• Contact between the solvent and the solute is increased
Solubility of most liquids
is not greatly affected by temperature. Why?
The liquid-liquid intermolecular forces are not as strong as
the intermolecular forces between solid solute particles with the solvent.
Solubility
of Gases in Liquid
Solubility of gas in liquid
α 1/T
– or –
As T of liquid ↑, solubility of gas ↓