COLLOIDS

COLLOIDS

  • Definition: A colloid is a substance microscopically dispersed throughout another substance. The word colloid comes from the Greek word 'kolla', meaning glue; thus colloidal particles are glue-like substances.

  • Filtration and membranes: These particles pass through filter paper but not through a semipermeable membrane.

  • Centrifugation: Colloids can be made to settle by centrifugal forces.

PARTICLE SIZE AND DETECTION

  • Dispersed-phase particle size: D \,\approx\, 1\,\text{nm} \text{ to } 100\,\text{nm}.

  • Visibility: Such particles are normally invisible in ordinary optical microscopy; presence can be confirmed with an ultramicroscope or an electron microscope.

  • Colloidal system consists of two phases:

    • Dispersed phase (a discontinuous phase)

    • Dispersion medium (a continuous phase)

SOLUTIONS, COLLOIDAL MIXTURES, SUSPENSIONS

  • SOLUTIONS

    • Made up of solutes and a solvent.

    • Solvent is usually a liquid, but can be a gas.

    • Particles are atoms, ions, or molecules of very small diameter.

  • COLLOIDAL MIXTURE

    • Particles are intermediate in size: not as small as a solution, not as large as a suspension.

  • SUSPENSIONS

    • Made up of particles and a solvent; particles are larger than those in a solution.

    • Particles in a suspension can be distributed evenly by shaking; they tend to settle on standing.

SOLUTION, SUSPENSION, COLLOID – DEFINITIONS AND EXAMPLES

  • Solution: table salt (NaCl) dissolves in water to form saline water. Solute: NaCl; Solvent: water; Result: solution.

  • Suspension: flour suspended in water (appears light blue because blue light is scattered more than red light).

  • Colloid: milk is an emulsified colloid of liquid butterfat globules dispersed in a water-based liquid. Colloids are stabilized in suspension by electrostatics (mutual repulsion of like charges).

PROPERTIES COMPARISON (SOLUTIONS, COLLOIDS, SUSPENSIONS)

  • Particle size:

    • True solution: < 1\,\text{nm}

    • Colloid: 1\,\text{nm} \le D \le 100\,\text{nm}

    • Suspension: > 100\,\text{nm}

  • Appearance:

    • True solution: clear

    • Colloid: cloudy

    • Suspension: cloudy

  • Homogeneity:

    • True solution: homogeneous

    • Colloid: homogeneous or heterogeneous

    • Suspension: heterogeneous

  • Transparency:

    • True solution: transparent (but can be colored)

    • Colloid: often translucent

    • Suspension: often opaque, but can be translucent

  • Separation:

    • True solution: does not separate

    • Colloid: can be separated (e.g., by centrifugation)

    • Suspension: separates or settles

  • Filterability:

    • True solution: passes through filter paper

    • Colloid: passes through filter paper

    • Suspension: particles do not pass through filter paper

EXAMPLES OF COLLOIDS

  • Milk, synthetic polymers, fog, blood, jam, shoe polish, smoke, etc.

  • Visual examples (from imagery in the transcript): Cherry blossom, shoe polish samples, various labelled products.

CLASSIFICATION OF COLLOIDS

  • Based on:

    • Physical state of dispersed phase and dispersion medium

    • Nature of interaction between dispersed phase and dispersion medium

    • Molecular size in the dispersed phase

    • Appearance of colloids

    • Electric charge on dispersion phase

BASED ON PHYSICAL STATE OF DISPERSED PHASE AND DISPERSION MEDIUM

  • Dispersed phase / Dispersion medium / Typical name / Examples

    • Solid / Solid → Solid-Sol; Examples: Alloys, Cranberry glass

    • Solid / Liquid → Sol; Examples: Ink, Blood

    • Solid / Gas → Aerosol; Examples: Smoke, Ice cloud

    • Liquid / Solid → Gel; Examples: Jelly, Cur d

    • Liquid / Liquid → Emulsion; Examples: (common emulsions in foods/chemistry)

    • Liquid / Gas → Liquid aerosol; Examples: Cloud, Fog

    • Gas / Solid → Solid form (solid foams like pumice)

    • Gas / Liquid → Foam; Examples: Shaving cream

    • Gas / Gas → None (gases mix completely)

  • Note: All gases are miscible.

BASED ON NATURE OF INTERACTION BETWEEN DISPERSED PHASE AND DISPERSION MEDIUM

  • LYOPHILIC COLLOIDS (intrinsic colloids)

    • Dispersed phase has a great affinity for the dispersion medium.

    • Tend to pass into colloidal solution when in contact with dispersion medium.

    • If dispersion medium is water, they are called hydrophilic or emulsoids.

    • Lyophilic colloids are generally self-stabilized; reversible in nature and heavily hydrated.

    • Examples: starch, gelatin, rubber, protein, etc.

  • LYOPHOBIC COLLOIDS (extrinsic colloids)

    • Dispersed phase has no affinity for the dispersion medium.

    • Do not tend to pass into colloidal solution when in contact with dispersion medium.

    • Relatively unstable and irreversible by nature; stabilized by small amounts of electrolyte.

    • Poorly hydrated.

    • If dispersion medium is water, they are called hydrophobic or suspenoids.

    • Examples: sols of metals (Au, Ag), sols of metal hydroxides, sols of metal sulfides.

BASED ON MOLECULAR SIZE IN THE DISPERSED PHASE

  • MULTI MOLECULAR COLLOIDS

    • Individual dispersed particles consist of aggregates of atoms or small molecules with diameter < 10^{-7}\,\text{cm}.

    • Held together by weak van der Waals forces.

    • Examples: gold sol, sulfur sol.

  • MACROMOLECULAR COLLOIDS

    • Dispersed phase particles are large enough to be colloidal in size; these are natural polymers.

    • Terms include large polymeric colloids.

ASSOCIATED COLLOIDS (MICELLES)

  • Behavior: Act as normal electrolytes at low concentrations but behave as colloids at higher concentrations.

  • Also called micelles.

  • Example: Sodium stearate (C18H35NaO2) behaves as an electrolyte in dilute solution but as a colloid in higher concentrations.

  • Examples: Soaps, higher alkyl sulphonates, polythene oxide.

BASED ON APPEARANCE OF COLLOIDS

  • SOLS (sols)

    • A colloidal solution that appears as a fluid.

    • When dispersion medium is water, the sol is called hydrosol/aquosol.

    • When dispersion medium is alcohol/benzene, called alcosol/benzosol respectively.

  • GELS

    • A colloidal solution that appears solid.

    • Rigidity varies by substance.

    • Examples: jelly, butter, cheese, curd.

BASED ON ELECTRICAL CHARGE ON DISPERSION PHASE

  • POSITIVE COLLOIDS

    • Dispersed phase carries a positive charge.

    • Examples: metal hydroxides like Fe(OH)3, Al(OH)2, methylene blue sol, etc.

  • NEGATIVE COLLOIDS

    • Dispersed phase carries a negative charge.

    • Examples: Ag sol, Cu sol.

PREPARATION (CHEMICAL METHODS)

  • Oxidation: Addition of oxygen and removal of hydrogen; formation of a colloid by oxidation.

    • Example: Colloidal sulfur prepared by oxidizing aqueous H2S with an oxidizing agent such as bromine water:

    • H2S + Br2 \rightarrow 2HBr + S}

    • H2S + SO2 \rightarrow 2H_2O + 3S}

  • Reduction: Addition of hydrogen and removal of oxygen.

    • Example: Gold sol prepared by reducing a dilute aqueous solution of gold with stannous chloride:

    • 2AuCl3 + 3SnCl2 \rightarrow 3SnCl_4 + 2Au}

  • Hydrolysis: Break down of water; sols of ferric hydroxide and aluminium hydroxide prepared by boiling the aqueous solution of the corresponding chlorides.

    • Example: FeCl3 + 3H2S \rightarrow Fe(OH)_3 + 3HCl

  • Double Decomposition: Insoluble inorganic salts sol formed by double decomposition reactions.

    • Example: Arsenous sulphide sol prepared by passing H$_2$S gas through a dilute aqueous solution of arsenous oxide:

    • As2O3 + 3H2S \rightarrow As2S3(OH)3 + 3H_2O}

PURIFICATION OF COLLOIDS

  • Common purification methods:

    • Dialysis

    • Electrodialysis

    • Ultrafiltration

DIALYSIS

  • Process: Separation of colloid from crystalloids by diffusion through a suitable membrane.

  • Principle: Colloidal particles cannot pass through a cellophane membrane, ions of electrolytes can.

  • Impurity removal: Impurities diffuse out of the bag; keep the bag in distilled water to prevent back-diffusion of crystalloids.

  • Example use: Removing HCl from ferric hydroxide sol.

ELECTRO-DIALYSIS

  • Dialysis pace is slow; speeded up by applying an electric potential.

  • This process is called electro-dialysis.

APPLICATIONS OF ELECTRO-DIALYSIS

  • Artificial kidney machines (hemodialysis):

    • Hemodialyzer as the filtering unit; filtered blood returns to the body.

    • Blood flows to the dialyzer; filtered blood returns.

ULTRAFILTRATION

  • Definition: High-pressure filtration through a semipermeable membrane; colloidal particles are retained while small solutes and solvent pass through due to hydrostatic pressure.

APPLICATION OF ULTRAFILTRATION

  • Vital role in kidneys: glomerular ultrafiltration forms part of urine production (Bowman’s capsule, filtration at the glomerulus, efferent/afferent arterioles involved in ultrafiltrate).

PROPERTIES OF COLLOIDS

  • PHYSICAL PROPERTIES

    • Heterogeneity: Colloidal solutions consist of two phases (dispersed phase and dispersion medium).

    • Visibility: Dispersed particles are not visible to the naked eye; they appear homogeneous.

    • Filterability: Colloidal particles pass through ordinary filter paper but can be retained by animal membranes, cellophane membranes, and ultrafilters.

    • Stability: Lyophilic sols (and lyophobic sols in the absence of substantial electrolyte concentrations) are typically quite stable.

    • Colour: Colour depends on particle size; larger particles absorb longer wavelengths and transmit shorter wavelengths, affecting the observed color.

OPTICAL PROPERTIES OF COLLOIDS

  • TYNDALL EFFECT

    • When a concentrated beam of light passes through a colloidal solution in darkness, the path is illuminated with a bluish light (Tyndall cone).

    • Caused by scattering of light by colloidal particles.

    • Not exhibited by true solutions because particles are too small to scatter light.

    • Used to distinguish colloids from true solutions; forms the basis for detecting colloidal dimensions with ultramicroscopy.

MECHANICAL PROPERTIES OF COLLOIDS

  • BROWNIAN MOVEMENT

    • Continuous zigzag movement of colloidal particles in the dispersion medium.

    • Due to unequal bombardments by moving molecules of the dispersion medium.

    • Decreases with increasing colloidal particle size; suspensions do not show this movement.

ELECTRICAL PROPERTIES OF COLLOIDS

  • ELECTROPHORESIS

    • Movement of colloidal particles toward a particular electrode under an electric field.

    • Positively charged particles move toward the cathode; negatively charged toward the anode.

  • ELECTROOSMOSIS

    • Movement of the dispersion medium under an electric field when the movement of colloidal particles is hindered by a membrane.

    • The dispersion medium moves toward the oppositely charged electrode while colloidal particles are restricted.

ADVANTAGES AND REAL-WORLD APPLICATIONS OF COLLOIDS

  • Advantages

    • Allow dispersion of normally insoluble materials (e.g., metallic gold or fats) for easier use or absorption.

    • Colloidal gold can be used in medicine to carry drugs/antibiotics due to its non-reactivity and low toxicity.

    • Pharmaceutical industry uses colloidal solutions in medicines; many medicines are emulsions.

    • Paint industry uses colloids in pigment dispersion and stability.

  • Real-world examples

    • In milk, fat droplets form a colloidal suspension that prevents thickening and aids nutrient absorption.

    • Sewage water contains colloidal dirt particles that carry electrical charges; electrophoresis can remove them.

    • The sky appears blue due to scattering by colloidal dust particles (Tyndall effect).

    • Sugars in milk ferment to lactic acid; ions destroy surface charge on colloids, causing coagulation (curd formation).

    • Soap solutions are colloidal in nature; dirt particles can be removed by adsorption or emulsification.

    • Many everyday foods (milk, butter, ice cream, etc.) are colloidal in nature.

COLLOIDAL GOLDS AND OTHER COLLOIDAL PREPARATIONS (EXAMPLES)

  • Colloidal gold and colloidal silver preparations used in various products and applications.

  • Advertisements and product labels in the transcript reference colloidal gold (20 ppm) and colloidal silver (30 ppm).

SUMMARY REMARKS

  • Colloids occupy a unique size range between true solutions and suspensions, with distinctive optical, mechanical, and electrical properties that enable diverse applications in science, medicine, and industry.

REMEMBERED TERMINOLOGY

  • Sol: solid dispersed in liquid.

  • Gel: liquid dispersed in solid.

  • Emulsion: liquid dispersed in liquid.

  • Aerosol: solid or liquid dispersed in gas.

  • Foam: gas dispersed in liquid.

  • Aerogel: gas dispersed in solid.

  • Lyophilic: solvent-loving (intrinsic) colloids; self-stabilizing.

  • Lyophobic: solvent-fearing (extrinsic) colloids; stabilized by electrolytes.

  • Tyndall effect: light scattering by colloidal particles; used to identify colloids.

  • Dialysis / Electrodialysis / Ultrafiltration: purification methods for colloids.

  • Brownian movement: random motion of colloidal particles due to molecular collisions.

  • Electrophoresis / Electroosmosis: movement of particles/medium under an electric field.

  • Micelles: associated colloids that behave differently at different concentrations (e.g., soaps).

A colloid is a substance microscopically dispersed throughout another substance, with particle sizes typically ranging from 1\,\text{nm} to 100\,\text{nm}. Unlike true solutions, colloidal particles can pass through filter paper but not semipermeable membranes, and they are too small for ordinary optical microscopy but visible with an ultramicroscope or electron microscope. Suspensions, in contrast, have larger particles that settle and do not pass through filter paper.

Properties Comparison (Solutions, Colloids, Suspensions)

  • Particle Size: True solution (< 1\,\text{nm}); Colloid (1\,\text{nm} \le D \le 100\,\text{nm}); Suspension (> 100\,\text{nm}).

  • Appearance: True solution (clear); Colloid (cloudy/translucent); Suspension (cloudy/opaque).

  • Homogeneity: True solution (homogeneous); Colloid (homogeneous or heterogeneous); Suspension (heterogeneous).

  • Separation: True solution (no separation); Colloid (can be separated, e.g., by centrifugation); Suspension (separates/settles).

Classification of Colloids

Colloids are classified based on the physical state of dispersed phase/medium (e.g., Sol: solid in liquid; Gel: liquid in solid; Emulsion: liquid in liquid; Aerosol: solid/liquid in gas; Foam: gas in liquid), nature of interaction (Lyophilic/solvent-loving vs. Lyophobic/solvent-fearing), molecular size (Multimolecular, Macromolecular, Associated Colloids/Micelles), appearance (Sols vs. Gels), and electrical charge (Positive vs. Negative Colloids).

Preparation Methods

Colloids can be prepared by chemical methods such as oxidation (H2S + Br2 \rightarrow 2HBr + S), reduction ( 2AuCl3 + 3SnCl2 )
ightarrow 3SnCl_4 + 2Au}), hydrolysis ( \mathrm{FeCl3 + 3H2O
ightarrow Fe(OH)_3 + 3HCl}), and double decomposition (\mathrm{As2O3 + 3H2S ightarrow As2S3(OH)3 + 3H_2O}$$).

Purification Methods

Common purification methods include:

  • Dialysis: Separates colloids from crystalloids using a semipermeable membrane.

  • Electrodialysis: Speeds up dialysis by applying an electric field, used in artificial kidney machines.

  • Ultrafiltration: High-pressure filtration retaining colloidal particles.

Key Properties of Colloids

  • Tyndall Effect: Scattering of light by colloidal particles, illuminating the path of a light beam (Tyndall cone); used to distinguish colloids from true solutions.

  • Brownian Movement: Continuous zigzag motion of colloidal particles due to molecular bombardments.

  • Electrophoresis: Movement of charged colloidal particles toward an oppositely charged electrode under an electric field.

  • Electroosmosis: Movement of the dispersion medium under an electric field when particle movement is hindered.

Applications

Colloids allow dispersion of insoluble materials, are used in medicines (e.g., colloidal gold for drug delivery), paints, and are common in everyday foods (milk, butter). They also play roles in phenomena like the blue sky (Tyndall effect from dust) and sewage treatment (electrophoresis).