Purification and Properties of Colloids (2025) PDF

When a colloidal solution is prepared is often contains certain electrolytes and some other soluble impurities. which tend to destabilize it (may cause its precipitation )  Dialysis Dialysis is the separation of impurities from colloidal suspension through semi-permeable membr...

When a colloidal solution is prepared is often contains certain electrolytes and some other soluble impurities. which tend to destabilize it (may cause its precipitation )  Dialysis Dialysis is the separation of impurities from colloidal suspension through semi-permeable membranes The most generally membranes used are prepared from a partially evaporated solution of cellulose nitrate in an alcohol/ether mixture which is called collodion Membranes of various known pore sizes can be obtained commercially Electro-dialysis is a further modification of the simple dialysis technique. A potential is applied between the metal screens supporting the membranes to speed up the migration of small ions to the membrane surface prior to their diffusion to the outer liquid. The important application of the dialysis process in the artificial kidney machine used for the purification of blood of patients whose kidneys have failed to work (Hemodialysis)  Ultrafiltration Ultrafiltration is achieved by the application of suction through the filtration system to force the small particles of colloid and solvent across a membrane while larger particles are retained. The membrane is supported on a highly porous sintered glass disc as shown below. "Sintered" refers to a process in which powdered materials are heated to a temperature below their melting point, causing the particles to bond together to form a solid mass. Is an effective method for wastewater treatment with the ability to reject particulates, bacteria, proteins, and polysaccharides  Ultracentrifugation Ultracentrifugation is designed for separation of colloidal particles from the impurities via the application of centrifugation forces across the colloidal suspension The tubes containing the colloidal sol are mounted and then rotated at very high speeds. The particles tend to settle down at the bottom of the tube but the impurities remain soluble in solution. The settled particles are then mixed with an appropriate dispersing medium to regenerate the sol  Colligative Properties Colligative properties depend upon the number of moles of solute present in given mass of solvent. It does not depend on nature of solute particles. All colloidal dispersions give low value of osmotic pressure, freezing point and Elevation in boiling point Kinetic Properties Which refers to the motion of particles within the dispersion medium The laws governing the motion of particles through liquids 1. Brownian motion 2. Diffusion 3. Sedimentation rate 9 The random motion of particles arises from continuous changes in direction, primarily caused by collisions with the molecules of the surrounding medium, result in the irregular movement characteristic of Brownian motion. Each particle moves in an irregular path called zig-zag motion as shown here 9 0 Brownian displacement 𝒙− of a particle from its original position a long a given axis after time t is given by The diffusion coefficient D represents the measure of how fast particles diffuse through a medium. According to Einstein’s law of diffusion, the diffusion coefficient of suspended particles is related to their frictional coefficient 𝑓 For the spherical particles, Where is 𝑵𝑨 Avogadro‘s constant and 𝐾 is Bolzmann constant 10 1 In Brownian motion, the particle moves in a zigzag path. The displacement is inversely dependent on the particle radius. Importance of Brownian motion 2 Diffusion is the process by which molecules move from regions of higher concentration to regions of lower concentration, resulting from the random motion of particles, known as Brownian motion. Fick's first law of diffusion states that the mass of a substance dm that diffuses in the x direction across an area A in a time dt is proportional to the concentration gradient dc/dx. 12  The negative sign indicates that diffusion occurs in the direction of decreasing concentration, meaning that the rate of diffusion (dm/dt) increases as the concentration decreases. The permeability of particles takes place from the concentrated solution to dilute one  3. Sedimentation rate refers to the distance a particle travels through a solution in a specified period of time, indicating how quickly particles settle out of a suspension due to gravity Consider the sedimentation of uncharged spherical particle of mass (m) in a liquid of density 𝛒𝐋. g is the local acceleration due to gravity (or a centrifugal field). The factor 𝐦(𝟏 − 𝐕𝐒𝛒𝐋)𝐠 allows for the buoyancy of the liquid. The liquid medium offers a resistance to the motion of the particle which increases with increasing velocity At equilibrium, the sedimentation force acting on a particle equals the resistance force exerted by the liquid: 𝑑𝑥 𝐦(𝟏 − 𝐕𝐒𝛒𝐋)𝐠 = 𝐟 𝑑𝑡 Where f is the frictional coefficient of the particle in the given medium. f = 6𝛑η𝐚 η the viscosity coefficient of the medium, a is the raduis of the particle and 𝛒𝐒 is the density of a spherical particle in the disprsed state 𝑑𝑥 4/3 𝜋𝑎3 (𝛒 𝐒 − 𝛒𝐋) g = 6 𝜋η 𝑎 𝑑𝑡 𝑑𝑥 2𝑎2 𝛒𝐒−𝛒𝐋 𝑔 = Stokes‘ law 𝑑𝑡 9η Assumptions of Stokes’ law 1- The colloidal suspension should be dilute 2- The particles used should have small size 3- Based on assumptions 1 and 2, the motion of the particles would be extremely slow  𝟒 𝒎 = 𝝅𝒂𝟑 𝝆𝑺 , 𝝆𝑺 is the particle density 𝟑 𝟒 𝟑 𝒅𝒙 𝝅𝒂 𝝆𝑺 𝟏 − 𝑽𝑺 𝝆𝑳 𝐠 = 𝟔𝒂 𝟑 𝒅𝒕 𝟏 𝐂𝐨𝐧𝐬𝐢𝐝𝐞𝐫𝐢𝐧𝐠 𝐦 = 𝟏 𝑽𝑺 = 𝝆𝑺 𝟒 𝟑 𝟏 𝒅𝒙 𝝅𝒂 𝝆𝑺 𝟏 − 𝝆𝑳 𝐠 = 𝟔η𝒂 𝟑 𝝆𝑺 𝒅𝒕 𝟒 𝟑 𝝆𝑺 − 𝝆𝑳 𝒅𝒙 𝝅𝒂 𝝆𝑺 𝐠 = 𝟔η𝒂 𝟑 𝝆𝑺 𝒅𝒕 𝟐 𝟐 𝒅𝒙 𝒂 𝝆𝑺 − 𝝆𝑳 𝐠 = 𝟐η 𝟑 𝒅𝒕 𝒅𝒙 𝟐𝒂𝟐 𝝆𝑺 − 𝝆𝑳 𝒈 = Stokes‘ law 𝒅𝒕 𝟗η Large particles sediment more rapidly than small ones. The sedimentation rate is directly proportional to particle radius and density difference Δρ between the particle and liquid, while inversely proportional to medium viscosity (η). Understanding sedimentation behavior in colloidal solutions is crucial determining their stability Since sedimentation rates are often very slow, centrifugation is used to expedite sedimentation In the context of sedimentation, what does Δρ represent? A) Viscosity of the medium B) Density difference between particle and liquid C) Mass of the particle D) Acceleration due to gravity Why is understanding sedimentation behavior important in colloidal solutions? A) To determine boiling points B) To assess stability C) To measure temperature changes D) To calculate viscosity In Stokes' law, which factor does NOT influence the resistance force on a particle moving through a viscous medium? A) Radius of the particle B) Viscosity of the medium C) Mass of the particle D) Velocity of the particle Which principle explains why larger particles diffuse more slowly than smaller particles in a liquid? A) Stokes' law B) Fick's first law C) Brownian motion D) Kinetic molecular theory What happens to the rate of diffusion as the concentration gradient increases? A) The rate of diffusion decreases. B) The rate of diffusion remains constant. C) The rate of diffusion increases. D) The rate of diffusion becomes negligible. What effect does increasing the viscosity of the medium have on the sedimentation rate of particles? A) Increases sedimentation rate B) Decreases sedimentation rate C) Has no effect on sedimentation rate D) Depends on particle size Optical Properties of Colloids The Tyndall effect results from the scattering of light by colloidal particles. The scattering of light is a primary cause of turbidity in colloidal solutions. Higher concentrations and larger sizes of particles lead to increased light scattering, resulting in higher turbidity measurements. Turbidity is defined as the cloudiness or haziness of a fluid due to suspended particles. It can be quantitatively expressed using the equation: 𝑰𝒕 = 𝑒 −𝜏𝑙 𝑰𝟎 Where I0 is the intensity of incident light, It is the intensity of transmitted light. l is the length of the sample and 𝝉 is turbidity Turbidity is measured using a turbidimeter or nephelometer, which shines light through the liquid and measures the amount of scattered light. Higher turbidity results in greater light scattering, yielding a higher turbidity reading. Nephelometric Turbidity Units (NTU) measure turbidity based on light scattering at a 90-degree angle, while Formazin Nephelometric Units (FNU) are based on scattering by a standard formazin suspension  Electrical Properties Electrophoresis Electrophoresis is a process by which particles or molecules can be separated by applying an electric potential. Each particle or molecule travels through the medium at a different rate depending on its electrical charge and molecular size When the colloidal particles are placed in an electric field, they move towards either cathode or anode depending upon their individual charge Basic dyestuffs, ferric hydroxide, aluminum hydroxide, etc., are examples of positive sols. Colloidal solutions of gums, starch, soap solution, metals (Ag, Cu, Au, Pt, etc.), metal sulfides, and some acid dyestuffs are examples of negative sols Electrodialysis is a form of dialysis in which the rate is increased by the presence of an electric potential across the membrane While Electrophoresis is the migration of electrically charged molecules through a medium under the influence of an electric field  Coagulation of colloidal sol The stability of a sol is due to the charge present on the colloidal particles. Due to similar charges, colloidal particles repel one another and are unable to combine together to form larger particles. However, if the charge on colloidal particles is destroyed, they are free to come nearer and grow in size Coagulation is the destabilization of colloidal particles by the addition of a chemical reagent called a coagulant. Flocculation is the agglomeration of destabilized particles into microfloc and after into bulky floccules which can be settled called floc.