Surface Chemistry - Difference between colloids and classical chemistry

• Introduction:
  • Surface chemistry is the branch of chemistry that deals with the study of phenomena occurring at the surface or interface of materials.
  • Colloids are a special class of substances that exhibit characteristics between those of a solution and those of a suspension.
• Size of particles:
  • In classical chemistry, particles are generally in the range of molecules or ions.
  • In colloids, particles are larger than most molecules but smaller than particles in suspensions.
• Nature of particles:
  • Classical chemistry deals with particles that are usually in the form of molecules or ions.
  • Colloids consist of particles that can be in the form of aggregates, clusters, or macromolecules.
• Dispersion medium:
  • In classical chemistry, the dispersion medium is usually a solvent.
  • Colloids have a dispersed phase (colloidal particles) and a dispersion medium (continuous phase) that can vary.
• Stability:
  • In classical chemistry, there is no specific consideration for stability.
  • Colloids can be stable or unstable depending on various factors such as temperature, concentration, and presence of electrolytes.

Surface Chemistry - Surface tension and its significance

• Definition of surface tension:
  • Surface tension is defined as the force per unit length acting perpendicular to an imaginary line drawn on the surface of a liquid.
• Key points about surface tension:
  • Surface tension arises due to the cohesive forces between the liquid molecules.
  • It causes a liquid surface to behave like a stretched elastic membrane.
  • The SI unit of surface tension is Newton per meter (N/m) or dyne per cm (dyn/cm).
• Significance of surface tension:
  • Surface tension gives rise to various important phenomena in chemistry, such as:
    1. Capillarity: The rise or fall of liquids in narrow tubes due to surface tension.
    2. Formation of droplets: Liquid droplets tend to have a spherical shape due to surface tension.
    3. Wetting of surfaces: Surface tension determines whether a liquid will spread or bead up on a surface.
• Measurement of surface tension:
  • Surface tension can be measured using various methods, including:
    1. Capillary rise method: Determining the height to which a liquid rises in a capillary tube.
    2. Du Nouy ring method: Measuring the force required to detach a platinum ring from the liquid surface.
    3. Pendant drop method: Measuring the shape and size of a liquid droplet hanging from a needle.
• Examples of surface tension in everyday life:
  • Water droplets forming on a freshly waxed car.
  • Insects walking on the surface of water due to surface tension.
  • Bubbles forming and floating on the surface of a soapy solution.

11. Examples of colloids:

• Milk: Milk is a colloidal dispersion of fat globules in water.
• Fog: Fog is a colloidal dispersion of water droplets in air.
• Blood: Blood is a colloidal dispersion of various particles in plasma.
• Gelatin: Gelatin is a colloidal dispersion of protein molecules in water.
• Aerosols: Aerosols are colloidal dispersions of solid or liquid particles in a gas.

12. Factors affecting stability of colloids:

• Electric double layer: The presence of charged particles and ions surrounding colloidal particles affects stability.
• Surface charge: The charge on the surface of colloidal particles determines the stability of the colloidal dispersion.
• Presence of electrolytes: Addition of electrolytes can neutralize the charges and cause coagulation or precipitation.
• Temperature: Temperature changes can affect the stability of colloidal dispersions.
• pH: pH changes can affect the charge on the colloidal particles and thus their stability.

13. Methods of purification of colloids:

• Dialysis: Colloidal particles can be separated from electrolytes or smaller molecules using a semipermeable membrane.
• Ultrafiltration: Colloids can be purified by passing them through a porous membrane that selectively retains the larger particles.
• Electrophoresis: Colloidal particles can be separated by subjecting them to an electric field, which causes them to migrate towards electrodes.
• Precipitation: Colloids can be precipitated by adding suitable chemicals or adjusting conditions to induce aggregation or coagulation.

14. Applications of colloids in daily life:

• Food industry: Colloids such as mayo, ice cream, and whipped cream enhance the texture and stability of food products.
• Medicine: Colloids are used in drug delivery systems to improve drug solubility and targeted delivery.
• Paints and inks: Colloidal dispersions provide color and viscosity stability in these products.
• Cosmetics: Colloids are used in creams, lotions, and makeup products to improve their texture and shelf life.
• Photography: Colloidal silver is used in photographic film and papers.

15. The Tyndall effect:

• The Tyndall effect is the scattering of light by colloidal particles or particles in a colloidal dispersion.
• When a beam of light passes through a colloidal dispersion, the path of the light is visible due to the scattering of light by the particles.
• This effect is used to distinguish between a true solution (no scattering) and a colloidal dispersion (scattering).

16. Coagulation and peptization:

• Coagulation (precipitation) refers to the clumping together of colloidal particles to form larger aggregates or precipitates.
• Peptization is the process of breaking down larger aggregates or precipitates into smaller colloidal particles.
• Coagulation can be induced by adding electrolytes, adjusting pH, or heating, while peptization can be achieved by adding suitable agents.

17. Emulsions:

• Emulsions are a type of colloidal dispersion where one liquid is dispersed in another immiscible liquid.
• Examples of emulsions include milk (fat droplets dispersed in water) and mayonnaise (oil droplets dispersed in water).
• Emulsions can be classified as oil-in-water (o/w) or water-in-oil (w/o) depending on the continuous phase.

18. Micelles:

• Micelles are formed when surfactant molecules (amphiphiles) are added to a solution above a certain concentration called the critical micelle concentration (CMC).
• In a micelle, the hydrophobic tails of surfactant molecules are clustered together, shielding them from the surrounding water.
• This formation of micelles helps in solubilizing hydrophobic substances, reducing surface tension, and acting as detergents.

19. Adsorption:

• Adsorption is the process of accumulation of molecules or ions on the surface of a material (adsorbent) from a gas or liquid phase (adsorbate).
• Types of adsorption include physisorption (weak van der Waals forces) and chemisorption (strong chemical bonds).
• Adsorption is influenced by factors such as temperature, pressure, surface area, and nature of adsorbent and adsorbate.

20. Applications of adsorption:

• Activated carbon is used for adsorption of impurities in water and air purification.
• Adsorbents like silica gel are used for drying air and moisture-sensitive materials.
• Catalysts often involve adsorption of reactants on solid surfaces to enhance chemical reactions.
• Gas masks and filters utilize adsorption to remove harmful gases and particles from the air.
• Chromatography relies on adsorption to separate and analyze mixtures.

21. Properties of colloids:

• Brownian motion: Colloidal particles exhibit random movement due to bombardment by molecules of the dispersion medium.
• Osmotic pressure: Colloidal dispersions can exert osmotic pressure, which depends on the number of particles present.
• Diffusion: Colloidal particles can diffuse, but at a slower rate compared to smaller particles in a true solution.
• Heterogeneity: Colloidal dispersions may show non-uniform distribution or heterogeneity due to the presence of larger particles.

22. Preparation methods of colloids:

• Dispersion method: Colloids can be formed by breaking down larger particles into smaller ones. For example, grinding, milling, or ultrasonic dispersion.
• Condensation method: Colloids can be formed by the condensation of smaller particles into larger ones. For example, mixing of reactants or precipitation.
• Electrical disintegration: Colloids can be formed by subjecting substances to an electric discharge or arc.
• Chemical methods: Colloids can be formed by chemical reactions, such as reduction, oxidation, and hydrolysis.

23. Classification of colloids:

• Based on the dispersed phase:
  1. Solid sols: Colloids where the dispersed phase is a solid and the dispersion medium is a liquid. Example: Starch in water.
  2. Liquid sols: Colloids where the dispersed phase is a liquid and the dispersion medium is also a liquid. Example: Emulsions.
  3. Gaseous sols: Colloids where the dispersed phase is a gas and the dispersion medium is a liquid. Example: Foam.
• Based on the nature of interaction between particles:
  1. Lyophilic colloids: Colloids in which the dispersed phase has a strong affinity for the dispersion medium. Example: Gum Arabic in water.
  2. Lyophobic colloids: Colloids in which the dispersed phase has a weak affinity for the dispersion medium. Example: Gold sol in water.

24. Emulsifying agents:

• Emulsifying agents are substances that help in dispersing and stabilizing immiscible liquids as emulsions.
• Examples of emulsifying agents include:
  1. Surfactants: Substances that can lower the surface tension between two immiscible liquids. Example: Soap.
  2. Proteins: Some proteins have emulsifying properties, such as egg yolks and casein. Example: Mayonnaise.
  3. Synthetic emulsifiers: Chemically synthesized molecules used in the food industry, cosmetics, and pharmacy to stabilize emulsions.

25. Applications of emulsions:

• Food industry: Emulsions like mayonnaise, salad dressings, and sauces provide texture and taste enhancement.
• Cosmetics: Emulsions are used in creams, lotions, and makeup products for smooth application and prolonged shelf life.
• Pharmaceuticals: Emulsions are used to deliver drugs, especially hydrophobic ones, in a more easily absorbed form.
• Paints and inks: Emulsions provide the desired consistency, stability, and spreading properties in these products.
• Fuel industry: Emulsions of fuel and water are used for more efficient combustion and reduced emissions in some applications.

26. Gibbs adsorption isotherm:

• The Gibbs adsorption isotherm describes the relationship between the surface excess of an adsorbate and its concentration in the bulk phase.
• It is given by the equation: Γ = RT ln(C/C°), where Γ is the surface excess, C is the molar concentration, C° is the bulk concentration, R is the gas constant, and T is the temperature in Kelvin.
• The Gibbs adsorption isotherm helps in understanding the behavior of adsorbates at the interface and estimating surface areas.

27. Applications of adsorption in industry:

• Catalysts: Adsorption plays a vital role in heterogeneous catalysis by facilitating reactant adsorption and providing an active surface for the reaction.
• Gas separation: Adsorbents like activated carbon are used in gas separation processes to remove impurities and selectively adsorb specific gases.
• Water purification: Adsorbents like activated alumina and activated carbon are used to remove contaminants, odors, and colors from water.
• Soil fertility: Adsorption of nutrients and ions on soil particles helps in retaining and releasing them slowly for plant uptake.
• Energy storage: Adsorption-based technologies like adsorption heat pumps and adsorption refrigeration systems offer efficient and eco-friendly cooling and heating.

28. Langmuir adsorption isotherm:

• The Langmuir adsorption isotherm describes the adsorption of a gas on a solid surface with a limited number of available sites.
• It assumes that adsorption occurs in a monolayer, and there is no interaction between adsorbed molecules.
• The Langmuir equation is given by the equation: θ = (K * P) / (1 + K * P), where θ is the fractional coverage, K is the adsorption equilibrium constant, and P is the pressure of the adsorbate.

29. Colloids in medicine and drug delivery:

• Liposomes: Colloidal vesicles made up of lipids are used for drug delivery, targeting specific tissues or organs.
• Nanoparticles: Colloidal particles in the nanometer range are being explored for targeted drug delivery and diagnostic imaging.
• Microemulsions: Thermodynamically stable colloidal systems consisting of oil, water, surfactant, and co-surfactant are used as carriers for poorly soluble drugs.
• Protein-based colloids: Colloidal systems using proteins as the dispersed phase are being investigated for controlled drug release and tissue engineering applications.
• Hydrogels: Colloidal networks of crosslinked polymers can serve as drug depots, providing sustained release of therapeutics.

30. Impact of colloids on the environment:

• Dispersion of pollutants: Colloidal particles can contribute to the dispersion and transport of pollutants in the environment.
• Water treatment: Colloidal particles influence the effectiveness of water treatment processes, such as coagulation and flocculation.
• Ecotoxicology: Colloids can interact with living organisms and affect their behavior, toxicity, and bioaccumulation of certain pollutants.
• Fate and transport of contaminants: Colloids can adsorb and transport contaminants like heavy metals, organic pollutants, and nanoparticles in soil and groundwater.
• Environmental monitoring and analysis: Colloid chemistry plays a role in the development of analytical techniques and tools for studying environmental processes.