Solubility

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 solute	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=k_hc

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 ↓

Solubility of Gas and Pressure

As P of a gas above a liquid ↑, solubility of gas ↑

Solubility Curves

                                              

Miscible Liquids

•   Liquids that are miscible in all proportions are called as Miscible Liquids

• Example of miscible liquids- Ethyl alcohol in water

• This principle of solvents are applied on aerosol products

Raoult’s Law

• The vapour pressure of liquid serves as a quantitative expression for describing the escaping tendencies of molecules

• Raoult’s law states that “the partial vapour pressure of each volatile constituents is equal to the vapour pressure of the pure constituents multiplied by its mole fraction in the solution at a given temperature”

• Raoult’s law may be mathematically expressed as

Partial vapour pressure= vapour pressure of pure liquid X mole fraction of liquid

P_A= P_A⁰ X_A………….(1)

P_B= P_B X_B…………..(2)

• When two liquids are mixed, the vapour pressure of each one is reduced by the presence of other by the extent of dilution of each phase

• Raoult’s law is appropriately suited to describe an ideal solution

Ideal Solutions

• Ideal  solution  is  defined  as  the  one  in  which  there  is  no change in the properties of the components other than dilution, when they are mixed to form a solution

• Heat is neither absorbed nor evolved during mixing

• No shrinkage or expansion when liquids are mixed

• Examples of ideal solutions are – methanol-water, benzene- toluene

• These liquids have similar properties, i.e., attractive forces are in complete uniformity

Nonideal or Real Solutions

• Most liquid mixtures show varying degree of deviation from Raoult’s law, i.e. which does not obey Raoult’s law; these solutions are real or non-ideal solutions

• When solute-solute, solute-solvent and solvent-solvent interactions are unequal, these deviations are observed

• Typical examples are- carbon tetrachloride and cyclohexane, chloroform and acetone

• Equations (1) and (2) may be modified as:

P_A= P_A⁰α_A…………(3)

P_B= P_B⁰α_B………..(4)

• αA and αB indicates are activities of components A and B respectively

Deviation from Raoult’s Law- Positive Deviation

• In some liquid systems, the vapour pressure is greater than the sum of the partial pressures of the individual components

• Such systems exhibit positive deviation from Raoult’s law

• Examples are carbon tetrachloride and cyclohexane, water and ethanol etc.

• This type of behaviour occurs when the components differ in their polarity, length of hydrocarbon chain and degree of association

Deviation from Raoult’s Law- Negative Deviation

• In some liquid systems, the vapour pressure is less than the sum of partial pressures of the individual components

• Such systems are said to exhibit negative deviation from Raoult’s law

• Examples include chloroform and acetone, pyridine and acetic acid etc.

• This type of behaviour occurs when interactions such as hydrogen bonding, salt formation and hydration occur between the components

Phase Rule

• Phase is defined as a homogenous physically distinct portion of a system that is separated by bounding surfaces from each other

• Phase rule is a device for relating the effect of the least number of independent variables upon the various phases that can exist in equilibrium system containing a given number of components

• Examples of independent variables are temperature, pressure and concentration

Phase Rule and Its Applications

• It is known as Gibb’s phase rule, and may be stated mathematically as:

F = C – P + 2……..(1)

Where, F= number of degree of freedom

C= number of components

P= number of phases

• Applications of phase rule are:

- In determining the purity of a substance

- In the solubility phenomenon

Partially Miscible Liquids

• Partially miscible liquids or conjugate liquids are defined as a two phase liquid system in which their mutual solubility in one another is limited

• As such when temperature is increased, the mutual solubility of one liquid in another increases

• Miscibility temperature is defined as the temperature at which two conjugate solutions are mutually soluble

• The miscibility temperature is identified either by the disappearance of turbidity or by reappearance of turbidity

• When solubility of one liquid in another liquid is plotted against miscibility temperature, a specific pattern is obtained

• These solubility temperature profiles are known as miscibility curves or phase diagrams

• Applications of phase diagrams are:

- To decide the proportion of two liquids to be taken during formulation of solutions

- Testing purity of a liquid

• Typical examples of partially miscible liquids are:

- Phenol-water system

- Trimethylamine-water system

- Nicotine – water system

Phenol- Water System

• The miscibility pattern of phenol-water system can be shown as below:

• The left hand side of the curve represents the % W/W of phenol in water at various temperature

• The right hand side of the curve represents % W/W of water in phenol at various temperature

• The two curves meet at a maximum temperature of 66.80C

• The critical solution temperature (CST) is defined as maximum temperature at which the two conjugate solutions (layers) merge into one layer at all proportions

• CST is also known as upper consolute temperature

• CST of phenol water system is 66.80C

• At any temperature above CST phenol and water are miscible in all proportions

• Outside the curves, phenol and water are miscible

• The Tie line is represented by the line drawn parallel to the base line from two points on the curve at any temperature in the phase diagram of partially miscible liquids

Phenol- Water System-Applications

• The phenol-water miscibility curve suggests that 76% w/w phenol corresponds to 80 % w/v solution, which should be used in dispensing

• CST is a characteristic of a system and used for testing the purity of a substance

• The method can be used to determine the percentage composition of added component in the conjugate solution

Triethylamine-Water System

• The temperature composition curve of trimethylamine and water is shown below:

• The left hand side of the curve indicates the miscibility of triethylamine in water

• The right hand side of the curve indicates the solubility of water in triethylamine

• Lower consolute temperature is defined as the minimum temperature at which the two conjugate solutions are miscible in all proportions

• The lower consolute temperature for triethylamine-water system is 18.50C

Nicotine- Water System

• The temperature composition curve of nicotine –water system is shown below:

• At room temperature, nicotine and water are miscible in all proportions

• At higher temperature, the mutual solubility decreases

• This system exhibit both lower (60.80C) and upper (2080C) critical solution temperature

Summary

• Solution-Defined as a homogenous mixture in which one substance is said to dissolve in the other

• Saturated solutions- The solution in which the dissolved solute is in equilibrium with the solvent phase, at a definite temperature

• Supersaturated solution- Solution that contains more of the dissolved solute than that it would normally contain at a definite temperature

• Whether or not a solute will dissolve in a solvent and the extent in which it will dissolve

• The amount of a solute that will dissolve in a specific solvent given condition

• When the temperature drops while you mix the solute and solvent, raising the temperature will increase solubility

• The greater the surface area per unit mass, the quicker it will dissolve

• The liquid-liquid intermolecular forces are not as strong as the intermolecular forces between solid solute particles   with the solvent.    

• Miscible liquids-Liquids that are miscible in all proportions are called as miscible liquids

• Raoult’s law –It   states that the partial vapour pressure of each volatile constituents is equal to the vapour pressure of the pure constituents multiplied by its mole fraction in the solution at a given temperature

• Mathematical expression of Raoult’s law can be given by

PA= PA0 XA

PB= PB XB

• Ideal solution-It is defined as the one in which there is no change in the properties of the components other than dilution, when they are mixed to form a solution

• Non ideal or real solution- Liquid mixtures that show varying degree of deviation from Raoult’s law, are real or non-ideal solutions

• Positive deviations from Raoult’s law- The systems in which the vapour pressure is greater than the sum of the partial pressures of the individual components                 

• Phase - It is defined as a homogenous physically distinct portion of a system that is separated by bounding surfaces from each other

• Phase rule- can be represented by:

F = C – P + 2

• Partially miscible liquids - Partially miscible liquids or conjugate liquids are defined as a two phase liquid system in which their mutual solubility in one another is limited

• Critical solution temperature - The critical solution temperature (CST) is defined as maximum temperature at which the two conjugate solutions (layers) merge into one layer at all proportions

• Lower consolute temperature - it is defined as the minimum temperature   at   which   the   two   conjugate   solutions   are miscible in all proportions                

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