Lyophilic and Lyophobic

Lyophilic and Lyophobic

Introduction

What is Surface Chemistry?
Surface chemistry is a branch of chemistry that deals with the study of the nature of surfaces, and physical and chemical reactions occur at the interface of two phases. It is closely related to colloid science. The foundation of colloidal chemistry was laid down by Thomas Graham, a Scottish scientist.
What are Colloids?
Colloids are the dissolved state of substances that either do not pass, or pass very slowly through a parchment paper or animal membrane. A few examples of colloids are starch, glue and  gelatin.
A colloid is not a substance, but it depicts a particular state of a substance that depends upon the size of its particles. The size of a particle in a colloidal system is  between 1-100 nm. A colloidal system is a two phase heterogeneous system in which one phase is called the dispersed phase and the other is called the dispersion medium.
Dispersed phase: It is the component present in a small proportion.
Dispersion medium: It is the component present in excess.
For example, in a colloidal solution of silver in water, silver is the dispersed phase and water is the dispersion medium.
How do we classify colloids?
Based on the physical state of the dispersed phase and dispersion medium, colloids can be classified into different types.
One important class of colloidal system is sols. In sols, the dispersed phase is solid and dispersion medium is liquid.
Depending upon the nature of the interaction between the dispersed phase and dispersion medium sols can be classified into two types.

1. Lyophilic sols

1. Lyophobic sols

What are Lyophilic Sols?
The word meaning of lyophilic means ‘liquid-loving’ or ‘solvent- attracting’. This means that in this colloidal solution there is a strong attraction between the dispersed phase and dispersion medium, i.e., the dispersed phase has great affinity for the dispersion medium that results in the extensive solvation of the colloidal particles. In such solids, the dispersed phase does not easily precipitate and the sols are quite stable. These sols are reversible in nature. The dispersed phase obtained by the evaporation can be easily converted to the sol state by simply agitating it with the dispersion medium. Additional stabilisers are not required during their preparation. If water is used as the dispersion medium, lyophilic sols are called hydrophilic sols. Starch, gum, gelatin, egg albumin etc. are examples of lyophilic sols.

• Starch Sol

Starch forms lyophilic sol when water is used as the dispersion medium. The formation of sol is accelerated by heating. Starch sol can be prepared by heating it and water at 100 °C. It is quite stable and is not affected by the presence of any electrolytic impurity.

• Gum Sol

Like starch gum also form lyophilic sol with water. Instead of boiling water, warm water is used to for the preparation of sol because gum is quite soluble in warm water.

• Egg Albumin Sol

Egg albumin which is obtained from eggs forms lyophilic sol with cold water. The sol is quite stable and is not affected by the presence of traces of impurities.
What are Lyophobic Sols?
The word lyophobic means ‘liquid-hating’. That means in these sols, there is little or no interaction between the dispersed phase and the dispersion medium ie, dispersed phase has little affinity for dispersion medium. These sols are easily precipitated by the addition of small amounts of electrolyte, by heating or by shaking, therefore these sols are relatively less stable than lyophilic sols. They need stabilising agents for their preparation. If water is used as the dispersion medium, lyophobic sols are called hydrophobic sols. Examples of lyophobic sols include sols of metals and their insoluble compounds like sulphides and oxides.

• Ferric Hydroxide Sol

Ferric hydroxide forms lyophobic sols on treatment with water. Ferric hydroxide sol is prepared by the hydrolysis of ferric chloride with boiling distilled water. The reaction takes place is as follows.

The hydrolysis reaction produces insoluble ferric hydroxide particles which undergo agglomerisation to yield bigger particles of colloidal dimensions. These particles absorb Fe3+ ions preferentially from the solution to give positive charge to the sol particles. Stability of sol is due to the charge on the sol particles. Hydrochloric acid produced during hydrolysis must be removed from the sol because it destabilizes the sol. HCl can be removed from the sol by dialysis process otherwise sol will not be stable.

• Aluminium Hydroxide Sol

It is also hydrophobic in nature and is obtained by the hydrolysis of aluminium chloride.

Hydrochloric acid produced during the hydrolysis is removed by dialysis because aluminium hydroxide sol is affected by the presence of ionic impurities.

• Arsenious Sulphide Sol

It is a lyophobic sol obtained by the hydrolysis of arseniuos oxide with boiling distilled water, followed by passing H₂S gas through it.









Comparison between Lyophilic and Lyophobic sols
Differences between lyophilic and lyophobic sols are as follows:

PROPERTIES	LYOPHILIC SOLS	LYOPHOBIC SOLS
Ease of preparation	Prepared easily by directly mixing with the liquid dispersion medium.	Cannot be prepared directly. Prepared by special methods only.
Solubility	They are quite stable and are not easily precipitated or coagulated.	They are easily precipitated by addition of a small amount of a suitable electrolyte.
Hydration	They are highly hydrated.	They are not much hydrated.
Reversible and irreversible nature	They are reversible in nature i.e. once precipitated can return the colloidal sol by simply remixing with the dispersion medium.	They are irreversible in nature i.e. once precipitated cannot return the colloidal sol by simply addition of the dispersion medium.
Nature of substance	These sols are usually formed by the organic substances like starch, gum, proteins etc.	These sols are usually formed by the inorganic materials like metals, their sulphides etc.
Viscosity	Their viscosity is much than that of the medium.	Their viscosity is almost the same as that of the medium.
Surface tension	Their surface tension is usually lower than that of the dispersion medium.	Their surface tension is nearly small than that of the dispersion medium.
Visibility	Their particles are neither visible nor detected easily by the ultra microscope.	Their particles though not visible, can be detected easily by the ultra microscope.
Migration in an electric field	Their particles may migrate in either direction.	Their particles migrate only in one direction of electric field.