Surface Chemistry

Surface chemistry is a fascinating branch of chemistry that focuses on studying physical and chemical phenomena occurring at the interfaces of two different phases. These phases can be pure chemicals or solutions; the interface is the boundary where they meet. 

1.0What is surface chemistry?

Surface Chemistry studies chemical phenomena occurring at surfaces, such as solid-liquid, solid-gas, solid-vacuum, and liquid-gas interfaces. Due to gases' complete miscibility, there is no interface between them. Accurate studies in Surface Chemistry require extremely clean surfaces. Achieving an ultra-clean metal surface is possible under a very high vacuum of 10⁻⁸ to 10⁻⁹ Pascal. These solid materials must be stored in a vacuum to prevent contamination by dioxygen and dinitrogen molecules from the air.

Surface chemistry provides insights into how substances interact at their boundaries, influencing the development of new materials, improving industrial processes, and advancing catalysis and corrosion protection technologies.

Surface chemistry properties refer to the attributes of objects' outer boundaries, including molecules. Key topics in surface chemistry include adsorption, catalysis, colloids, and emulsions.

2.0Adsorption

Several examples that demonstrate the tendency of a solid's surface to attract and retain molecules from the contacting phase. These molecules remain at the surface and do not penetrate deeper into the bulk. This accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption. The molecular species or substance that concentrates or accumulates at the surface is called the adsorbate, while the material on whose surface the adsorption occurs is called the adsorbent.

Adsorption is a surface phenomenon. Solids, particularly when finely divided, possess a large surface area, making them effective adsorbents. Materials such as charcoal, silica gel, alumina gel, clay, colloids, and finely divided metals are notable examples of good adsorbents.

Absorption

The phenomenon of absorption occurs when the molecules of a substance are consistently distributed throughout the body of a solid or liquid.

Sorption

Sorption is a phenomenon in which both adsorption and absorption occur simultaneously. For example, cotton fibers absorb dyes as well as other fibers.

Difference between adsorption and absorption 

Below given  table clearly highlights the key differences between adsorption and absorption.

Mechanism of Adsorption 

Adsorption occurs because surface particles of an adsorbent have unbalanced attractive forces compared to those inside the bulk, attracting adsorbate particles. This phenomenon increases with the surface area per unit mass of the adsorbent.

• Adsorption is exothermic, releasing heat as surface energy decreases (∆H is negative). 
• When a gas is adsorbed, its molecules' movement is restricted, reducing entropy (∆S is negative). 
• For adsorption to be spontaneous (∆G negative), ∆H must be sufficiently negative to offset the positive term -T∆S. As adsorption progresses, ∆H becomes less negative until it equals T∆S, achieving equilibrium.

Types of Adsorption

There are two main types of gas adsorption on solids: physisorption and chemisorption. Weak van der Waals' forces drive physisorption, while chemisorption involves the formation of chemical bonds, either covalent or ionic, between gas molecules and the solid surface.

Adsorption Isotherms

An adsorption isotherm is a graphical representation illustrating the correlation between the quantity of gas adsorbed onto a solid surface (adsorbent) and the pressure of the gas maintained at a consistent temperature.

These models help understand different types of adsorption behaviour and are crucial in designing and optimizing adsorption systems for various industrial applications such as gas storage, separation processes, and catalysis.

The most common adsorption isotherms include:

Freundlich Isotherm:

The given equation represents the Freundlich adsorption isotherm, an empirical model describing adsorption on heterogeneous surfaces. The equation can be expressed as:

• Freundlich adsorption isotherm equation; n ≥ 1

  and in logarithmic form 

                      $\frac{x}{m}=\mathrm{kP}{P}^{\frac{1}{n}}$

$\log \frac{x}{m}=\log \mathrm{k}+\frac{1}{n}\log \mathrm{P};0\le \frac{1}{n}\le 1\text{; in general }\frac{1}{n}\text{ is }0.1\text{ to }0.5\text{. }$

• X is the amount of gas adsorbed per unit mass of the adsorbent,
• k is the Freundlich constant indicative of the adsorption capacity,
• P  is the pressure of the gas,
• $\frac{1}{n}$ ​ is a constant related to the adsorption intensity (with n>1).

In this equation:

• K and $\frac{1}{n}$ are empirical constants that must be determined experimentally.
• A higher value of k  indicates a greater adsorption capacity.
• The value of $\frac{1}{n}$ ​​ gives insight into the adsorption process; if $\frac{1}{n}<1$, the adsorption is favorable, and if $\frac{1}{n}>1$, the adsorption process is less favourable.

This relationship is typically plotted as a curve showing the mass of gas adsorbed per gram of adsorbent against pressure. These curves indicate that physical adsorption decreases with increasing temperature and approach saturation at high pressure.

This model is particularly useful for describing adsorption processes where the surface of the adsorbent is heterogeneous, and it does not assume a uniform surface or a constant heat of adsorption.

Applications of Adsorption

The phenomenon of adsorption has numerous applications as mentioned below:            

Adsorption from the solution phase

Adsorption from solutions involves the accumulation of solute molecules on the surface of a solid adsorbent. The process is influenced by: 

3.0Catalysis

Substances that accelerate the rate of a chemical reaction without being chemically or quantitatively changed after the reaction are known as catalysts. This phenomenon is referred to as catalysis.

4.0Colloids

Colloids are mixtures where microscopically dispersed insoluble particles are suspended in another substance. The particle size in a colloid ranges from 1 to 1000 nanometers.

Characteristics of colloids include:

• Particle Suspension: The suspended particles do not settle, unlike in suspensions where particles settle at the bottom if left undisturbed.
• Tyndall Effect: Colloidal solutions exhibit the Tyndall effect, where beams of light are scattered upon interaction with the colloidal particles. This scattering makes the path of the light visible.

Classification of Colloids

Colloids can be classified based on three main criteria:

• Physical State of Dispersed Phase and Dispersion Medium:

• Nature of Interaction Between Dispersed Phase and Dispersion Medium:
• • Lyophilic Colloids: These colloids exhibit a strong affinity between the dispersed phase and the dispersion medium, leading to stability and reversibility. Examples include gum and gelatin in water. 
  • Lyophobic Colloids: TA minimal affinity between the dispersed phase and the dispersion medium results in lower stability and irreversibility. Examples include gold sol and ferric hydroxide sol.

• Type of Particles of the Dispersed Phase:
• • Multimolecular Colloids: Consist of aggregates of atoms or small molecules—for example, Sulfur sol.
  • Macromolecular Colloids: These consist of large molecules forming colloidal solutions when dispersed, such as starch proteins..
  • Associated Colloids (Micelles): Formed by the aggregation of molecules with both lyophilic and lyophobic parts. —for example, Soap and detergent solutions.

Mechanism of micelle formation

This mechanism allows soap to emulsify grease and oil, facilitating their removal from surfaces and thus performing its cleansing action.

Soap is a sodium or potassium salt of higher fatty acids, represented asRCOO⁻ Na⁺

For example, sodium stearate (CH₃(CH₂)₁₆COO^–Na⁺) is a major component of many bar soaps. When dissolved in water, soap dissociates into RCOO⁻ and Na⁺ ions.

Structure of Soap Ions:

• RCOO⁻ Ion: Composed of a long hydrocarbon chain R (non-polar 'tail') that is hydrophobic (water-repelling) and a polar group COO⁻(polar-ionic 'head') that is hydrophilic (water-loving).
• Orientation in Water: The COO⁻ group stays in the water while the hydrocarbon chain stays away from it, remaining at the surface.

Formation of Micelles:

• Critical Micelle Concentration (CMC): At a specific  concentration, called the critical micelle concentration, the soap anions aggregate into spherical structures called micelles.
• Micelle Structure: The hydrocarbon chains point inward, away from water, forming the core of the micelle, while the  COO⁻ groups remain outward, interacting with water.

Cleansing Action of Soap:

• Micelle Formation: Soap molecules form micelles around oil droplets, with the hydrophobic tails embedded in the oil and the hydrophilic heads projecting outward.
• Removal of Oil: The hydrophilic heads interact with water, pulling the oil droplet into the water and removing it from the dirty surface.

Preparation of Colloids

Chemical Methods

• Double Decomposition: Involves a reaction between two compounds resulting in colloid formation. Example: SO₂ + 2H₂S →3S + 2H₂O
• Hydrolysis: A chemical reaction with water to form colloidal particles.
• Oxidation: Involves the addition of oxygen to form a colloid.
• Reduction: Involves removing oxygen or adding hydrogen to form a colloid.

Electrical Disintegration or Bredig’s Arc Method

This method involves using metal electrodes immersed in a dispersion medium. An electric arc is struck between the electrodes, producing intense heat. The metal is vaporized and then condenses into colloidal particles. Example: Preparation of gold sol or silver sol.

Peptization

Peptization transforms a precipitate into a colloidal sol by shaking it with a dispersion medium and adding a small amount of an electrolyte, known as a peptizing agent.

The peptizing agent helps to break down the precipitate into smaller colloidal particles.

Purification of Colloidal Solution

Colloidal solutions often contain impurities, such as excess electrolytes. These impurities need to be removed to obtain a pure colloidal solution. Commonly used methods of purification are Dialysis, Electrodialysis, and Ultrafiltration.

These methods ensure that the colloidal solutions are free from unwanted impurities, making them suitable for various applications in scientific and industrial fields.