Colloids and Colloidal Systems

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1. Introduction to Colloidal Systems: The Intermediate State Definition and Nature: A colloidal system, often referred to simply as a colloid, is a heterogeneous mixture where one substance (the dispersed phase) is uniformly dispersed in another substance (the dispersion medium) in the form of very fine particles. Unlike true solutions, where solute particles are molecularly dispersed, or suspensions, where particles are large and settle, colloids represent an intermediate state of matter. They appear homogeneous to the naked eye but are microscopically heterogeneous. Particle Size Range: The defining characteristic of a colloid is the size of its dispersed particles. These particles typically have dimensions ranging from approximately $1 \text{ nanometer (nm)}$ ($10^{-9} \text{ m}$) to $1000 \text{ nm}$ ($10^{-6} \text{ m}$). Particles smaller than $1 \text{ nm}$ belong to true solutions. Particles larger than $1000 \text{ nm}$ are characteristic of suspensions. Key Distinguishing Characteristics: Heterogeneous but Stable: Although heterogeneous, colloidal particles are small enough that they do not readily settle out under gravity, giving them a degree of stability not seen in suspensions. Visibility: Individual colloidal particles are too small to be seen with the naked eye or even with an ordinary microscope. However, their presence can be detected and their movement observed using an ultramicroscope or electron microscope. Optical Properties: They exhibit the Tyndall effect, visible scattering of light. Kinetic Properties: They undergo Brownian motion, a random zig-zag movement. Electrical Properties: Colloidal particles often carry an electric charge. 2. Components of a Colloidal System: Dispersed Phase and Dispersion Medium Every colloidal system is composed of two primary components: Dispersed Phase (DP): This is the component that is distributed throughout the dispersion medium in the form of colloidal particles. It is present in a smaller proportion and is analogous to the solute in a true solution. The physical state of the dispersed phase can be solid, liquid, or gas. Dispersion Medium (DM): This is the continuous phase in which the colloidal particles are spread or suspended. It is present in a larger proportion and is analogous to the solvent in a true solution. The physical state of the dispersion medium can also be solid, liquid, or gas. 3. Classification of Colloids: A Multifaceted Approach Colloids can be classified based on several criteria, providing a comprehensive understanding of their diverse nature. 3.1. Classification Based on Physical State of Dispersed Phase and Dispersion Medium Since both the dispersed phase and dispersion medium can be solid, liquid, or gas, there are $3 \times 3 = 9$ possible combinations. However, a gas mixed with another gas forms a true solution (a homogeneous mixture), not a colloid. Thus, there are 8 possible types of colloidal systems. Dispersed Phase (DP) Dispersion Medium (DM) Name of Colloidal System Distinguishing Features & Examples Solid Solid Solid Sol Solid particles dispersed in a solid medium. Often formed by cooling molten mixtures. Examples: Colored glasses (e.g., ruby glass, where gold particles are dispersed in glass), Gemstones (e.g., milky glass, some alloys). Solid Liquid Sol Solid particles dispersed in a liquid medium. This is one of the most common types. Examples: Paints (pigments in oil), Cell fluids (proteins, lipids in water), Gold sol, Sulfur sol, Starch sol, Inks. Solid Gas Aerosol (Solid) Solid particles dispersed in a gaseous medium. Examples: Smoke (carbon particles in air), Dust (fine soil particles in air), Industrial chimney exhausts. Liquid Solid Gel Liquid droplets dispersed in a solid medium. Gels have a semi-rigid structure. Examples: Cheese, Butter, Jellies, Agar-agar, Gelatin. Liquid Liquid Emulsion Liquid droplets dispersed in another immiscible liquid. Requires an emulsifying agent for stability. Examples: Milk (fat in water), Hair cream, Cod liver oil (water in oil). Liquid Gas Aerosol (Liquid) Liquid droplets dispersed in a gaseous medium. Examples: Fog, Mist, Clouds (water droplets in air), Insecticide sprays, Hair sprays. Gas Solid Solid Foam Gas bubbles dispersed in a solid medium. Examples: Pumice stone (volcanic rock with gas pockets), Foam rubber, Bread. Gas Liquid Foam Gas bubbles dispersed in a liquid medium. Often stabilized by surface-active agents. Examples: Froth, Whipped cream, Soap lather, Fire extinguisher foams. 3.2. Classification Based on Nature of Interaction between Dispersed Phase and Dispersion Medium This classification primarily applies to liquid dispersion media (lyo-meaning solvent). If the medium is water, they are called hydrophilic/hydrophobic. Lyophilic Colloids (Solvent-Loving): Definition: These are colloidal systems where there is a strong affinity or attraction between the particles of the dispersed phase and the molecules of the dispersion medium. This strong interaction leads to significant solvation (or hydration if the medium is water) of the dispersed particles. Ease of Preparation: They are relatively easy to prepare. Simply mixing the dispersed phase substance with the dispersion medium usually results in the formation of a colloidal solution. For example, dissolving gelatin in warm water. Stability: Highly stable due to strong solvation and often an electric charge. The protective layer of the dispersion medium around the dispersed particles prevents them from aggregating. Reversible Nature: If the dispersion medium is removed (e.g., by evaporation), the dispersed phase can often be redissolved in the medium by simple mixing, thus reforming the colloid. They are reversible colloids. Viscosity: The viscosity of lyophilic sols is generally much higher than that of the dispersion medium alone, due to the extensive solvation and the larger effective volume of the dispersed particles. Surface Tension: The surface tension of lyophilic sols is typically lower than that of the dispersion medium. Tyndall Effect: They show a less pronounced Tyndall effect compared to lyophobic sols because the difference in refractive indices between DP and DM is smaller. Charge: Particles may carry a charge, but their stability is primarily due to solvation. Examples: Gum arabic, Gelatin, Starch, Proteins, Rubber (in benzene), Albumin. Lyophobic Colloids (Solvent-Hating): Definition: These are colloidal systems where there is little or no affinity or attraction between the dispersed phase and the dispersion medium. The particles tend to repel the dispersion medium. Preparation Difficulty: They cannot be prepared by simple mixing. Special methods are required to break down larger particles or to condense smaller particles into the colloidal range. Stability: Less stable than lyophilic colloids. Their stability primarily depends on the presence of an electric charge on the colloidal particles, which causes mutual repulsion and prevents aggregation. If this charge is neutralized, they coagulate easily. Irreversible Nature: Once precipitated or coagulated, it is difficult or impossible to redisperse the particles into the colloidal state by simply adding the dispersion medium. They are irreversible colloids. Viscosity: The viscosity of lyophobic sols is usually very close to that of the dispersion medium, as there is little or no solvation. Surface Tension: The surface tension of lyophobic sols is generally similar to that of the dispersion medium. Tyndall Effect: They show a strong Tyndall effect due to a significant difference in refractive indices between DP and DM. Charge: Particles invariably carry an electric charge, which is crucial for their stability. Examples: Gold sol, Silver sol, Ferric hydroxide sol, Arsenious sulfide sol. 3.3. Classification Based on Type of Particles of the Dispersed Phase Multimolecular Colloids: Formation: These colloids are formed by the aggregation of a large number of atoms or small molecules (which individually have diameters less than $1 \text{ nm}$) to form particles of colloidal size. The aggregation occurs due to weak intermolecular forces, such as Van der Waals forces. Particle Nature: The individual aggregated particles are typically not macromolecular but are collections of many smaller units. Stability: Often stabilized by an electric charge developed through adsorption of ions. They are generally lyophobic in nature. Molecular Mass: The molecular masses of these aggregated particles are usually not very high, though larger than individual molecules. Examples: Gold sol: Consists of aggregates of many gold atoms. Sulfur sol: Consists of aggregates of $S_8$ molecules. Macromolecular Colloids: Formation: These colloids consist of individual large molecules (macromolecules) that are themselves of colloidal dimensions. When these macromolecules are dissolved in a suitable solvent, they form a colloidal solution. Particle Nature: The dispersed phase consists of single, very large molecules. Behavior: They often behave like true solutions in many respects (e.g., stability, homogeneity at certain levels), but their large size places them in the colloidal range. Stability: Generally lyophilic in nature, and thus quite stable. Molecular Mass: Possess very high molecular masses, often in the range of thousands to millions. Examples: Natural Macromolecules: Starch, Cellulose, Proteins, Enzymes, Gelatin, Nucleic acids. Synthetic Macromolecules: Nylon, Polystyrene, Polyethylene. Associated Colloids (Micelles): Definition: These are substances that behave as normal electrolytes (forming true solutions) at low concentrations, but at higher concentrations, they aggregate to form particles of colloidal size. These aggregated particles are called micelles. Amphiphilic Nature: The molecules forming associated colloids are typically amphiphilic, meaning they have both a long hydrophobic (water-repelling) hydrocarbon tail and a hydrophilic (water-loving) polar head. Examples include soaps ($\text{RCOO}^-\text{Na}^+$) and detergents. Critical Micelle Concentration (CMC): Micelles form only above a specific concentration known as the Critical Micelle Concentration. Below the CMC, the substance exists as individual ions or molecules in a true solution. Kraft Temperature ($T_k$): Micelle formation also occurs only above a certain temperature, characteristic for a given surfactant, known as the Kraft temperature. Mechanism of Formation (in aqueous medium): At low concentrations, the amphiphilic molecules exist separately. Above CMC, the hydrophobic tails aggregate together to minimize contact with water, forming a core. The hydrophilic heads orient outwards, interacting with the aqueous medium. In non-aqueous media, the orientation is reversed: hydrophilic heads are in the core, and hydrophobic tails project outwards. Hydrophobic Core Aqueous Medium Hydrophilic Heads Hydrophobic Tails Aggregation Number: The number of molecules that aggregate to form a micelle is called the aggregation number, which can range from 60 to 100 or more. 4. Preparation of Colloidal Solutions The methods used for preparing colloids depend largely on whether they are lyophilic or lyophobic. Lyophilic sols are easy to prepare, while lyophobic sols require special techniques classified into dispersion and condensation methods. 4.1. Condensation Methods (for Lyophobic Colloids) These methods involve bringing together molecular or ionic species to form particles of colloidal size. They start from true solutions and build up to colloids. Chemical Methods: Involve chemical reactions that produce insoluble products in colloidal dimensions. Double Decomposition: Two soluble reactants form an insoluble product. Example: Preparation of arsenious sulfide sol. When hydrogen sulfide gas is passed through a dilute solution of arsenious oxide, a yellow colloidal sol of arsenious sulfide is formed. $ \text{As}_2\text{O}_3 \text{ (aq)} + 3\text{H}_2\text{S} \text{ (g)} \to \text{As}_2\text{S}_3 \text{ (sol)} + 3\text{H}_2\text{O} \text{ (l)} $ Oxidation: An oxidizing agent reacts to form a colloidal product. Example: Preparation of sulfur sol. Hydrogen sulfide is oxidized by a mild oxidizing agent like sulfur dioxide or nitric acid. $ 2\text{H}_2\text{S} \text{ (g)} + \text{SO}_2 \text{ (aq)} \to 3\text{S} \text{ (sol)} + 2\text{H}_2\text{O} \text{ (l)} $ Reduction: A reducing agent converts metal ions into metal atoms that aggregate to form a sol. Example: Preparation of gold sol. Gold chloride solution is reduced by stannous chloride, formaldehyde, or hydrazine. $ 2\text{AuCl}_3 \text{ (aq)} + 3\text{HCHO} \text{ (aq)} + 3\text{H}_2\text{O} \text{ (l)} \to 2\text{Au} \text{ (sol)} + 3\text{HCOOH} \text{ (aq)} + 6\text{HCl} \text{ (aq)} $ Hydrolysis: Hydrolysis of certain salts can yield colloidal hydroxides. Example: Preparation of ferric hydroxide sol. Ferric chloride solution is hydrolyzed by boiling with water. $ \text{FeCl}_3 \text{ (aq)} + 3\text{H}_2\text{O} \text{ (boiling)} \to \text{Fe(OH)}_3 \text{ (sol)} + 3\text{HCl} \text{ (aq)} $ Electrical Disintegration (Bredig's Arc Method): Principle: This method is used to prepare sols of metals such as gold, silver, platinum, copper, etc., which are difficult to prepare by chemical methods. It involves both dispersion and condensation. Procedure: An electric arc is struck between two electrodes of the metal immersed in the dispersion medium (e.g., water, oil, or an organic solvent containing a stabilizer). The intense heat of the arc vaporizes some of the metal, and the vapor immediately condenses to form particles of colloidal size. An ice bath is often used to keep the dispersion medium cool to facilitate condensation and prevent rapid coagulation. Electric Arc Dispersion Medium Outer container (Ice Bath) Metal Electrodes Peptization: Definition: The process of converting a freshly prepared precipitate into a colloidal sol by shaking it with a dispersion medium in the presence of a small amount of electrolyte. The electrolyte used is called a peptizing agent. Mechanism: The peptizing agent provides ions that are preferentially adsorbed onto the surface of the precipitate particles. This adsorption leads to the development of a positive or negative charge on the surface of the particles. The similarly charged particles then repel each other, breaking down the precipitate into colloidal size and preventing re-aggregation. Example: A freshly precipitated ferric hydroxide ($\text{Fe(OH)}_3$) can be converted into a red-brown colloidal sol by adding a small amount of ferric chloride ($\text{FeCl}_3$) solution. The $\text{Fe(OH)}_3$ adsorbs $\text{Fe}^{3+}$ ions from $\text{FeCl}_3$, acquiring a positive charge and dispersing into the colloidal state. 4.2. Dispersion Methods These methods involve breaking down larger particles of a substance into colloidal size. They start from coarse suspensions and reduce the particle size. Mechanical Disintegration: Principle: This method uses mechanical means to grind coarse particles into colloidal dimensions. Colloidal Mill: The primary apparatus is a colloidal mill, which consists of two heavy metal discs rotating at very high speeds (e.g., 7000 revolutions per minute) in opposite directions, with a very small gap between them. The coarse suspension of the substance in the dispersion medium is fed into this gap. The strong shearing forces generated by the rotating discs, along with friction, reduce the particles to colloidal size. Applications: Used in the preparation of paints, inks, varnishes, toothpastes, and some pharmaceutical preparations. Ultrasonic Disintegration: Principle: High-frequency ultrasonic waves (sound waves above the human hearing range) are used to break down larger particles. Mechanism: These waves create regions of high and low pressure in the liquid, causing cavitation (formation and collapse of tiny bubbles). The collapse of these bubbles generates intense local shockwaves that can shatter larger particles into colloidal ones. Applications: Can be used to prepare sols of metals (like mercury), oxides, and sulfides, as well as emulsions. 5. Purification of Colloidal Solutions Colloidal solutions prepared by various methods often contain impurities, primarily excess electrolytes and other soluble substances. These impurities can destabilize the sol and cause coagulation. Purification methods aim to reduce the concentration of these impurities to a safe level. Dialysis: Principle: Based on the difference in diffusion rates through a semi-permeable membrane. Colloidal particles are too large to pass through such a membrane, while smaller ions and molecules (electrolytes, sugar, urea) can readily diffuse through. Process: The impure colloidal solution is placed in a bag made of a suitable semi-permeable membrane (e.g., parchment paper, cellophane, collodion). This bag is then suspended in a vessel through which fresh distilled water is continuously flowing. The small impurity molecules/ions diffuse out of the bag into the flowing water, leaving the purified colloidal solution behind. Dialyser: The apparatus used for dialysis. Impure Colloid Sol Water Out (with impurities) Distilled Water In Dialysing Membrane Electro-dialysis: Principle: An accelerated form of dialysis. If the dissolved impurities are electrolytes (ions), their removal can be hastened by applying an electric field. Process: The colloidal solution in the dialyser bag is placed between two electrodes. The ions migrate rapidly towards the oppositely charged electrodes, passing through the membrane and speeding up the purification process. This method is not effective for non-electrolyte impurities. Ultrafiltration: Principle: Involves separating colloidal particles from solvent and dissolved solutes by forcing the mixture through specially prepared filters called ultrafilters. These filters have pores small enough to retain colloidal particles but large enough to allow solvent and small solute molecules/ions to pass through. Ultrafilters: Ordinary filter paper has pores too large for ultrafiltration. Ultrafilters are made by impregnating ordinary filter paper with collodion (a $4\%$ solution of nitrocellulose in a mixture of alcohol and ether) or gelatin, and then hardening it with formaldehyde. This reduces the pore size. Process: Since the pores are very fine, the filtration process is slow. To speed it up, pressure or suction is applied. Applications: Used for concentrating sols and separating different sizes of colloidal particles. 6. Properties of Colloidal Solutions: Unveiling their Unique Behavior Colloidal systems exhibit a range of distinct physical and chemical properties that differentiate them from true solutions and suspensions. 6.1. Optical Properties: Interaction with Light Tyndall Effect: Definition: The scattering of light by colloidal particles when a beam of light is passed through a colloidal solution. This scattering makes the path of the light beam visible, appearing as a luminous cone against a dark background (known as the Tyndall cone). Observation: This effect is not observed in true solutions because the particles are too small to scatter light effectively. In suspensions, the particles are large enough to block or reflect light, not scatter it in a discernible path. Conditions for Tyndall Effect: The diameter of the dispersed particles must not be much smaller than the wavelength of the light used. If particles are too small (like in true solutions), scattering is negligible. There must be a significant difference in the refractive indices of the dispersed phase and the dispersion medium. This difference is essential for the light to be scattered rather than simply passing through. Lyophobic sols show a more pronounced Tyndall effect than lyophilic sols due to a greater difference in refractive indices. Applications: Used to distinguish between true solutions and colloidal solutions. Forms the basis of the ultramicroscope, which allows observation of the scattered light from individual colloidal particles as bright points of light against a dark background. Explains the blue color of the sky (scattering of sunlight by atmospheric particles) and the visibility of light beams in dusty rooms. True Solution (Light Passes) Colloidal Sol (Light Scatters - Tyndall Cone) Light Source 6.2. Kinetic Properties: Motion of Colloidal Particles Brownian Movement: Discovery: First observed by Robert Brown in 1827 with pollen grains in water. Definition: The continuous, random, haphazard, zig-zag motion of colloidal particles suspended in a dispersion medium. Cause: This motion is not due to the colloidal particles themselves, but rather due to the unequal and unbalanced bombardment of the colloidal particles by the rapidly moving molecules of the dispersion medium. As the particles are small, the impacts from the medium molecules are not uniform on all sides, leading to a net force that causes the erratic movement. Factors Affecting: Particle Size: Brownian motion decreases with an increase in the size of the colloidal particles. Larger particles experience more uniform bombardment. Viscosity of Medium: Brownian motion decreases with an increase in the viscosity of the dispersion medium, as a more viscous medium offers greater resistance to particle movement. Temperature: Increases with increasing temperature (due to increased kinetic energy of medium molecules). Significance: Brownian motion provides significant stability to colloidal sols. The constant movement prevents the colloidal particles from settling down under gravity, thereby counteracting the force of gravity and keeping them dispersed. Diffusion: Colloidal particles, being in constant motion, tend to diffuse from a region of higher concentration to a region of lower concentration, but at a much slower rate than true solution particles due to their larger size. Sedimentation: While Brownian motion prevents rapid settling, colloidal particles do have a tendency to settle down very slowly under the influence of gravity. This process can be significantly accelerated by using an ultracentrifuge, which generates very high centrifugal forces. 6.3. Electrical Properties: Charge and Stability Charge on Colloidal Particles: Fundamental Property: A crucial characteristic of most colloidal particles is that they invariably carry an electric charge. All particles in a given colloidal sol carry the same type of charge (either positive or negative). Importance: This like charge on all particles leads to mutual electrostatic repulsion, which prevents the particles from coming too close and aggregating (coagulating). This repulsion is a primary factor contributing to the stability of lyophobic sols. Origin of Charge: The charge on colloidal particles can arise from several mechanisms: Preferential Adsorption of Ions: This is the most common cause. Colloidal particles have a large surface area and a tendency to adsorb ions from the surrounding electrolyte solution. They preferentially adsorb ions that are common to their own lattice or that lead to a more stable configuration. Example 1: When $\text{AgNO}_3$ solution is added to $\text{KI}$ solution, $\text{AgI}$ precipitate is formed. If $\text{KI}$ is in excess, $\text{AgI}$ particles preferentially adsorb $\text{I}^-$ ions from the medium, forming a negatively charged sol: $\text{AgI} / \text{I}^-$. Example 2: If $\text{AgNO}_3$ is in excess, $\text{AgI}$ particles preferentially adsorb $\text{Ag}^+$ ions, forming a positively charged sol: $\text{AgI} / \text{Ag}^+$. Example 3: Ferric hydroxide sol, prepared by hydrolysis of $\text{FeCl}_3$, is positively charged due to adsorption of $\text{Fe}^{3+}$ ions. Dissociation of Surface Molecules: For some colloids, the surface molecules themselves can dissociate to produce charged groups. For instance, protein molecules have both acidic ($\text{-COOH}$) and basic ($\text{-NH}_2$) groups, which can ionize to form $\text{-COO}^-$ and $\text{-NH}_3^+$ groups, respectively, depending on the pH of the medium. Frictional Electrification: In some cases, charge can be acquired through friction between the dispersed phase and the dispersion medium. Examples of Charged Sols: Positively Charged Sols: Hydrated metallic oxides (e.g., $\text{Fe(OH)}_3$, $\text{Al(OH)}_3$, $\text{Cr(OH)}_3$), Basic dye stuffs (e.g., Methylene blue sol), Hemoglobin, Oxides (e.g., $\text{TiO}_2$). Negatively Charged Sols: Metallic sulfides (e.g., $\text{As}_2\text{S}_3$, $\text{CdS}$), Acid dye stuffs (e.g., Eosin, Congo red sol), Metals (e.g., Gold, Silver, Platinum sols), Starch, Clay, Gum. Electrophoresis (Cataphoresis): Definition: The phenomenon of movement of colloidal particles under the influence of an applied electric field. Positively charged colloidal particles migrate towards the cathode, while negatively charged particles migrate towards the anode. Apparatus: Typically conducted in a U-tube containing the colloidal solution, with electrodes immersed at each end and connected to a DC power supply. Applications: Determining the sign of the charge on colloidal particles. Separation of different colloidal particles based on their charge and mobility. Electro-deposition of rubber onto molds. - + Colloidal Particles (e.g., negative) Movement towards Anode Electro-osmosis: Definition: If the movement of colloidal particles is prevented (e.g., by placing a semi-permeable membrane that is permeable to the medium but not the particles), the dispersion medium itself starts to move under the influence of an electric field. This phenomenon is called electro-osmosis. Principle: It's essentially the reverse of electrophoresis, where the stationary phase is the dispersed particles and the moving phase is the dispersion medium. Coagulation (Flocculation or Precipitation): Definition: The process by which the colloidal particles aggregate to form larger particles that eventually settle down under gravity. This occurs when the stability of the colloid is lost, typically by neutralizing the charge on the particles. Methods of Coagulation: By Electrophoresis: When colloidal particles migrate to the electrodes, they lose their charge and get discharged, leading to their aggregation and precipitation. By Mixing Two Oppositely Charged Sols: When two sols of opposite charges are mixed, they neutralize each other's charge, leading to mutual coagulation. Example: Mixing ferric hydroxide sol (positive) with arsenious sulfide sol (negative) causes both to precipitate. By Boiling: Heating a sol increases the kinetic energy of the particles, leading to more frequent collisions. This can disrupt the adsorbed layer of the dispersion medium (in lyophilic sols) or reduce the charge on the particles, leading to coagulation. By Persistent Dialysis: Prolonged dialysis removes all traces of electrolytes, which are essential for stabilizing lyophobic sols. Removing these stabilizing ions can lead to coagulation. By Addition of Electrolytes: This is the most common and effective method. The addition of an electrolyte introduces ions into the colloidal solution. The ions carrying a charge opposite to that of the colloidal particles are attracted to the particles, neutralizing their charge and causing them to aggregate. This ion is called the flocculating or coagulating ion. Hardy-Schulze Rule (for Electrolyte Coagulation): This rule describes the effectiveness of different ions in causing coagulation. The coagulating power of an electrolyte is directly proportional to the valence (charge) of the active ion (the flocculating ion) that carries a charge opposite to that of the colloidal particles. The higher the valence of the flocculating ion, the greater is its power to cause coagulation. Examples of Hardy-Schulze Rule: For a negatively charged sol (e.g., $\text{As}_2\text{S}_3$ sol), the effectiveness of cations in causing coagulation follows the order: $\text{Al}^{3+} > \text{Ba}^{2+} > \text{Na}^+$ (e.g., from $\text{AlCl}_3 > \text{BaCl}_2 > \text{NaCl}$). For a positively charged sol (e.g., $\text{Fe(OH)}_3$ sol), the effectiveness of anions in causing coagulation follows the order: $[\text{Fe(CN)}_6]^{4-} > \text{PO}_4^{3-} > \text{SO}_4^{2-} > \text{Cl}^-$ (e.g., from $\text{K}_4[\text{Fe(CN)}_6] > \text{Na}_3\text{PO}_4 > \text{Na}_2\text{SO}_4 > \text{NaCl}$). Coagulation Value (Flocculation Value): This is the minimum concentration of an electrolyte (expressed in millimoles per liter) required to cause the coagulation of a sol within a specified time period (usually 2 hours). A lower coagulation value indicates a higher coagulating power of the electrolyte. Protection of Colloids: Concept: Lyophilic colloids are inherently much more stable than lyophobic colloids because of their strong solvation layer in addition to any charge. Lyophobic sols are easily coagulated by electrolytes. However, the stability of lyophobic sols can be increased by adding a small amount of a lyophilic colloid. Protective Colloids: The lyophilic colloid added to stabilize a lyophobic sol is called a protective colloid. Examples include gelatin, gum arabic, albumin, starch. Mechanism: The particles of the lyophilic colloid form a thin, protective layer around the particles of the lyophobic sol. This protective layer shields the lyophobic particles from the electrolytes, preventing their coagulation. Gold Number: This is a measure of the protective power of a lyophilic colloid. It is defined as the minimum weight (in milligrams) of a protective colloid required to prevent the coagulation of $10 \text{ mL}$ of a standard gold sol when $1 \text{ mL}$ of $10\%$ $\text{NaCl}$ solution is added to it. A smaller gold number indicates a greater protective power of the lyophilic colloid. For example, gelatin has a very low gold number (0.005-0.01), indicating high protective power. 7. Emulsions: Liquid-Liquid Colloids Definition: Emulsions are colloidal systems in which both the dispersed phase and the dispersion medium are immiscible or partially miscible liquids. One liquid is finely dispersed in the other. Types of Emulsions: Oil in Water (O/W) Emulsions: In these emulsions, oil (or any non-polar liquid) is the dispersed phase, and water (or any polar liquid) is the dispersion medium. This means oil droplets are dispersed in water. Examples: Milk (fat dispersed in water), Vanishing cream, Mayonnaise. Water in Oil (W/O) Emulsions: In these emulsions, water (or any polar liquid) is the dispersed phase, and oil (or any non-polar liquid) is the dispersion medium. This means water droplets are dispersed in oil. Examples: Butter (water dispersed in fat), Cod liver oil, Cold cream. Emulsifying Agents (Emulsifiers or Stabilizers): Role: Emulsions are inherently unstable and tend to separate into two distinct liquid layers over time (demulsification). To stabilize them, a third component called an emulsifying agent is added. Mechanism: Emulsifying agents are typically surface-active substances (surfactants) that form an interfacial film between the dispersed phase and the dispersion medium. This film acts as a mechanical barrier and also helps to stabilize the droplets by providing an electric charge, preventing them from coalescing. Examples: For O/W emulsions , water-soluble emulsifiers are used. These include proteins, gums, agar, soaps, detergents. They form a protective layer around oil droplets, making them water-loving. For W/O emulsions , oil-soluble emulsifiers are used. These include heavy metal salts of fatty acids, long-chain alcohols, lampblack. They form a protective layer around water droplets, making them oil-loving. Properties of Emulsions: They exhibit the Tyndall effect and Brownian movement due to their colloidal nature. They can be coagulated or broken by heating, freezing, centrifuging, or by the addition of electrolytes. The type of emulsion can be identified by dilution test (O/W can be diluted with water, W/O cannot), dye test (water-soluble dye stains O/W, oil-soluble dye stains W/O), or conductivity test (O/W conducts electricity better than W/O). Demulsification: Definition: The process of breaking an emulsion into its constituent liquids. Methods: Mechanical Methods: Heating, freezing, centrifuging, or vigorous shaking can break the interfacial film. Chemical Methods: Adding demulsifying agents that destroy the emulsifying agent or alter its properties, or adding electrolytes that cause coagulation. Electrical Methods: High voltage electric fields can coalesce droplets. 8. Gels: Solid-like Colloids Definition: Gels are colloidal systems in which the dispersed phase is a liquid and the dispersion medium is a solid. They possess a semi-rigid, jelly-like structure. Essentially, a gel is a highly viscous sol that has set into a semi-solid mass. Formation: Gels are often formed by cooling certain sols (e.g., gelatin sol on cooling sets to a gel) or by chemical reactions that lead to the formation of a network structure. Characteristics: They are usually translucent or opaque. They have a high degree of elasticity and rigidity. They can absorb large amounts of liquid without dissolving. Types of Gels: Elastic Gels: These are reversible gels. They can be converted back into a sol by heating (solation) and then revert to a gel on cooling (gelation). This process is reversible. Example: Gelatin, agar-agar, starch. Non-elastic Gels: These are irreversible gels. Once formed, they cannot be converted back into a sol by simple heating. Example: Silica gel, ferric hydroxide gel. Special Phenomena Associated with Gels: Thixotropy: The phenomenon where certain gels (especially those with a particulate network structure) become fluid (sol) upon shaking or mechanical agitation and then revert to a solid-like gel state upon standing. This is a reversible sol-gel transformation. Example: Ferric oxide gels, some clay suspensions, printing inks, paints. Syneresis (Weeping): The spontaneous exudation (squeezing out) of small amounts of liquid from a gel. This occurs as the gel contracts over time, expelling some of the dispersion medium. Example: Old gelatin desserts, cheese. Imbibition: The process of absorption of a liquid by a solid or a gel, leading to swelling without forming a solution. This is a characteristic feature of hydrophilic gels. Example: Swelling of seeds when soaked in water. 9. Applications of Colloids: Impact on Everyday Life and Industry Colloidal systems play a vital role in various aspects of our daily lives, natural phenomena, and industrial processes due to their unique properties. Medicine and Pharmacy: Colloidal medicines are more effective because their large surface area allows for better absorption and assimilation in the body. Examples: Argyrol (a silver sol) is used as an eye lotion. Colloidal antimony is used in the treatment of kala-azar. Colloidal gold is used for intramuscular injection. Milk of magnesia (an emulsion) is used for stomach disorders. Antibiotics and cough syrups are often prepared in colloidal form. Water Purification: Alum ($\text{K}_2\text{SO}_4 \cdot \text{Al}_2(\text{SO}_4)_3 \cdot 24\text{H}_2\text{O}$) is commonly added to turbid water. The $\text{Al}^{3+}$ ions from alum neutralize the negative charge on the suspended clay and mud particles (which are colloidal), causing them to coagulate and settle down, making the water clearer and potable. Smoke Precipitation (Cottrell Precipitator): Smoke is a solid aerosol (carbon particles in air). Smoke particles are electrically charged. A Cottrell precipitator is an industrial device used to remove smoke particles from industrial exhaust gases before they are released into the atmosphere. The smoke is passed through a chamber containing highly charged metal plates, which carry a charge opposite to that of the smoke particles. The smoke particles lose their charge upon contact with the plates, coagulate, and settle down, effectively cleaning the air. Tanning of Leather: Animal hides (leather) are essentially protein fibers which are positively charged colloids. When soaked in tannin solution (tannin is a negatively charged colloidal substance), mutual coagulation occurs. The positive hide particles and negative tannin particles neutralize each other, leading to precipitation. This process hardens the leather and makes it less susceptible to putrefaction, converting it into durable leather. Cleansing Action of Soaps and Detergents: Soaps and detergents are associated colloids that form micelles above their Critical Micelle Concentration (CMC). The hydrophobic tails of the micelles entrap grease and oil (which are insoluble in water), while the hydrophilic heads remain in the water, effectively emulsifying the dirt and allowing it to be washed away with water. Photographic Plates and Films: Photographic emulsions are colloidal dispersions of light-sensitive silver halides (e.g., AgBr) in gelatin (a protective colloid). This emulsion is coated on glass plates or celluloid films. Rubber Industry: Latex, the raw material for natural rubber, is a colloidal dispersion of negatively charged rubber particles in water. Rubber is obtained by coagulating latex, usually by adding an acid like acetic acid, which neutralizes the charge on the rubber particles, causing them to aggregate and precipitate. Formation of Delta at River Mouths: River water often carries fine particles of clay and sand in colloidal form, which are typically negatively charged. When river water meets sea water, the electrolytes (salts like $\text{NaCl}, \text{MgCl}_2$) present in sea water cause the coagulation of the negatively charged clay particles. The aggregated particles settle down, leading to the formation of fertile land known as a delta. Artificial Rain: Clouds are colloidal dispersions of very fine water droplets or ice particles in air. Artificial rain can be induced by spraying charged sand particles or silver iodide (which acts as a coagulating agent or condensation nucleus) over clouds. This causes the tiny water droplets to coalesce and grow large enough to fall as rain. Blue Colour of the Sky: The blue color of the sky is primarily due to the scattering of sunlight by the fine dust particles and water vapor molecules (which are of colloidal dimensions) present in the Earth's atmosphere. This is an example of the Tyndall effect, where shorter wavelength blue light is scattered more effectively than longer wavelength red light. 10. Distinction between True Solutions, Colloids, and Suspensions Understanding the fundamental differences between these three types of mixtures is crucial for grasping the unique nature of colloids. Property True Solution Colloidal Solution Suspension Particle Size Extremely small, $ Intermediate, between $1 \text{ nm}$ and $1000 \text{ nm}$. Large, $> 1000 \text{ nm}$. Nature Homogeneous mixture (uniform composition throughout). Heterogeneous, but appears homogeneous to the naked eye. Heterogeneous mixture. Visibility Particles are invisible even under a powerful microscope. Particles are invisible to the naked eye and ordinary microscope, but visible under an ultramicroscope or electron microscope. Particles are visible to the naked eye or under a low-power microscope. Settling Particles do not settle down under gravity. Very stable. Particles do not settle down under gravity (stable due to Brownian motion and charge), but can be sedimented by ultracentrifugation. Particles settle down readily under gravity. Unstable. Filtration Particles pass through both ordinary filter paper and animal membranes (e.g., cellophane, parchment). Particles pass through ordinary filter paper but are retained by animal membranes and ultrafilters. Particles do not pass through ordinary filter paper. Tyndall Effect Does not show the Tyndall effect (no scattering of light). Shows the Tyndall effect (scatters light, path visible). Does not show true Tyndall effect; particles block or reflect light. Brownian Movement Not exhibited by solute particles. Exhibited by colloidal particles. May be exhibited by very small suspension particles, but generally not by larger ones. Diffusion Particles diffuse rapidly. Particles diffuse slowly. Particles do not diffuse. Appearance Usually clear and transparent. Usually translucent or opaque. Opaque. Examples Salt solution, Sugar solution, Air, Vinegar. Milk, Blood, Starch solution, Ink, Gelatin, Fog, Smoke. Sand in water, Muddy water, Flour in water, Paint (before drying).