OIA1008 W1-W4 DISPERSE SYSTEMS [QUIZLET]

Disperse System

System that consist of particulate matter, known as the dispersed phase (yellow), distributed throughout a continuous/dispersion medium.

Classification of Disperse Systems

Classification based on particle size dispersed: Molecular dispersion, Colloidal dispersion, Coarse dispersion.

Molecular Dispersion

Range of particle size: < 1 nm. Characteristics: Particles are invisible in electron microscope. Particles pass through ultra filter and semi-permeable membrane. Particles undergo rapid diffusion. Dispersion forms true solution because of its homogenous character. Examples: oxygen molecules, glucose.

Colloidal Dispersion

Range of particle size: 1nm - 1µm or 1 nm - 0.5 µm. Characteristics: Particles are visible in electron microscope, often visible in ultra microscope but not visible under the light microscope. Particles pass through ordinary filter paper but not the semi-permeable membrane. Particles diffuse very slowly. Examples: colloidal silver sols, polymeric (natural & synthetic) dispersions.

Coarse Dispersion

Range of particle size: >>1 µm or >> 0.5µm. Characteristics: Particles are visible under the light microscope and often visible to the naked eye. Particles do not pass through the filter paper or dialyze through the semi-permeable membrane. Particles do not diffuse. Examples: sand grains, red blood cells, most pharmaceutical emulsions & suspensions.

Colloids

They are dispersed system in which one or more of the dispersed phase has at least one dimension within the range of about 1 nm - 1 µm and includes shape such as spheres, cubes, ellipsoids, rods, discs and random coils, where other dimensions may be significantly larger than 1 µm.

Classification of Colloidal Systems

2 types classification, based on: State of matter/physical state of the dispersed phase and the dispersed medium; Interaction between dispersed phase and the dispersed medium.

Interaction between dispersed phase and dispersed medium

Mainly refers to solid in liquid dispersions.

Lyophilic Colloids

"Lyophilic" means solvent-loving. It becomes hydrophilic if the dispersion medium is water or lipophilic if the dispersion medium is organic solvents.

A material which forms a lyophilic colloidal system in one liquid eg. water, may NOT do so in another liquid eg. benzene.

There is considerable attraction between the dispersed phase and the dispersed medium.

They are thermodynamically stable.

Considered as reversible systems.

Lyophilic Colloids (example)

Most lyophilic colloids are organic molecules eg. acacia, gelatin, tragacanth, methylcellulose, albumine, insuline.

Lyophilic Colloids (formation)

A material which forms a lyophilic colloidal system in one liquid eg. water, may NOT do so in another liquid eg. benzene.

Preparation of Lyophilic Colloids

Prepared by dispersing the material in the solvent used eg. acacia/gelatin in the water or gum rubber/polystyrene in non-aqueous solvents, such materials form colloidal dispersions or sols.

Sol

Refers to a colloidal dispersion of a solid in a liquid or gaseous medium.

Hydrosol

Indicates the dispersion medium is water.

Alcosol

Indicates the dispersion medium is alcohol.

Aerosol

Indicates the dispersion medium is gas.

Sols (mechanical strength)

Sols are fluid, they do not possess any mechanical strength unlike gel.

Lyophobic Colloids

"Lyophobic" means solvent-hating.

There is very little attraction between the dispersed phase and the dispersed medium.

Dispersed particles tend to aggregate.

Thermodynamically unstable.

Considered as irreversible systems.

Colloids that do not easily disperse in a solvent.

Examples of Lyophobic Colloids

Egs. are aqueous dispersions of oleophilic materials (such as polystyrene or gum rubber or steroids or sulphur or gold or silver chloride in water) or w/o emulsions.

Dispersion Methods

Methods used to reduce the size of coarse particles to colloidal dimensions.

Mechanical Disintegration

Using colloid mills, micronizers, ball mills, homogenizers, and ultrasonic generators to produce fine particles or fine droplets in fine emulsions.

Colloid Mill

A device used for mechanical disintegration to produce colloidal particles.

Homogenizers

Equipment used to create a uniform mixture by reducing particle size.

Ball Mill

A grinding device that uses balls to crush and grind materials into fine particles.

Peptization

Breaking up of aggregates or secondary particles into smaller primary particles in the colloidal size range.

Primary Particles

Particles which are not formed of smaller ones.

Flocculating Agents

Substances that promote the clumping of particles, usually electrolytes.

Deflocculating Agents

Substances like surfactants or water-soluble polymers that prevent particle aggregation.

Sodium Lauryl Sulfate

A deflocculating agent used at 0.1% concentration to disintegrate aggregated granules of powdered activated charcoal in water.

Condensation Methods

Methods involving rapid production of supersaturated solutions of colloidal material followed by formation and growth of nuclei.

Supersaturation

A state achieved through chemical reaction, change in solvent, or reduction in temperature that allows for colloidal sol formation.

Chemical Reactions

Processes such as hydrolysis of chlorides that result in the formation of colloidal sols.

Hydrolysis Reaction Example

NaCl + AgNO3 — AgCl (colloid) + NaNO3.

Double Decomposition Example

(NH4)2S + NiCl2 — NiS (colloid) + 2NH4Cl.

Change in Solvent

A method where a saturated solution of a substance is poured into a different solvent to create a colloidal dispersion.

Colloidal Dispersion Example

Saturated solution of sulphur in acetone poured into hot water results in a colloidal dispersion of sulphur.

Association Colloids

Colloids formed by amphiphiles or surface active agents that contain both hydrophobic and hydrophilic groups.

Amphiphiles (structure)

Molecules that contain large hydrophobic moieties along with strongly hydrophilic groups.

Surfactants

Agents that reduce surface tension and can form colloids when present in low concentrations.

Subcolloidal Range

The size range of particles that are smaller than colloidal dimensions.

Amphiphiles

Molecules that can exist separately in low concentration in a liquid medium, typically in the subcolloidal size range.

Critical Micelle Concentration (cmc)

The concentration at which amphiphiles aggregate to form micelles, which may contain 50 monomers or more.

Micelles

Aggregates formed by amphiphiles at or above the critical micelle concentration, lying within the colloidal size range.

Dispersed System (or)

A system in which particles are dispersed in a continuous medium.

Colloids (size)

Mixtures where one substance is dispersed evenly throughout another, typically with particle sizes in the range of 1 nm to 1 µm.

Examples of Colloids

Examples include milk, fog, and ink.

Class of Dispersed Systems

Different categories of dispersed systems based on the size and nature of the dispersed phase and the continuous medium.

Methods of Preparing Colloids

Various techniques used to create colloidal systems, such as dispersion, condensation, and chemical synthesis.

Optical Properties of Colloids

Characteristics of colloids related to light interaction, including absorption, scattering, and transmission.

Faraday-Tyndall Effect

A phenomenon where a strong beam of light passed through a colloidal sol produces a visible cone due to light scattering by colloidal particles.

Magnitude of Turbidity/Opalescence

Depends on the nature, size, and concentration of the colloidal particles.

Ultramicroscope

An instrument used to examine the turbidity or opalescence of colloidal systems.

Turbidity Measurement

Described in terms of turbidity, quantified by the equations It = Io exp(-TL) or T = 1/L ln(Io/It).

Intensity of Transmitted Light Beam (It)

The intensity of light that passes through the colloidal sample.

Intensity of Incident Beam (Io)

The intensity of the light beam before it enters the colloidal sample.

Turbidity (T)

A measure of the cloudiness or haziness of a colloidal solution.

Length of Sample (L)

The distance that the light travels through the colloidal sample.

Hydrophilic Colloidal Systems

Colloidal systems where the Tyndall effect is weaker compared to lyophobic colloidal dispersions.

Lyophobic Colloidal Dispersion

Colloidal systems that exhibit a stronger Tyndall effect compared to hydrophilic systems.

Spectrophotometer

An instrument used to measure the turbidity of colloidal solutions.

Photoelectric Colorimeter

An instrument used to measure the intensity of light in a colloidal solution.

Nephelometer

An instrument used to measure the scattering of light in a colloidal solution.

Optical Constant (H)

A constant specific to a particular system used in the relationship between turbidity and molecular weight.

Concentration of Solute (C)

The amount of solute present in the colloidal solution, measured in g/cc.

Weight Average Molecular Weight (M)

The average molecular weight of the particles in the colloidal system, measured in g/mole or daltons.

Interaction Constant (B)

A constant that describes the interactions within the colloidal system.

Constant for particulate solute/solvent system and dependent on the degree of interaction between the solvent & solute molecules.

HC/T

1/M + 2BC

H

optical constant for a particular system

C

concentration of the solute (g/cc) in the sol

M

wt. average molecular weight (g/mole or daltons)

Intercept in HC/T vs. C plot

1/M

Slope in HC/T vs. C plot

2B

Thermally induced motion

thermal motion, e.g., Brownian movement, diffusion, osmosis

Gravitationally induced motion

gravitational motion (with or without force), e.g., sedimentation

Brownian Movement

random collisions of colloidal particles with molecules of the dispersion medium resulting in erratic motion

Effect of viscosity on Brownian motion

Increase in viscosity will decrease the Brownian motion

Effect of particle size on velocity

Decrease in particle size will increase velocity of motion

Diffusion

spontaneous movement of particles from higher concentration to lower concentration until uniform

Osmotic Pressure (definition)

pressure necessary to balance osmotic flow when a solution and solvent are separated by a semi-permeable membrane

van't Hoff equation variables

π = cRT, where c = cg/M, cg = gram of solute per liter of solution, M = mw, R = molar gas constant, T = absolute temperature

Osmotic Pressure for colloidal dispersion

π/cg = RT(1/M + Bcg), where B = constant for particulate solute/solvent system

Plot of π/cg vs. cg

yields 1 of 3 lines, depending on whether the system is ideal or real

Osmotic Pressure Equation (ideal)

For ideal solutions: π = (cg/M) RT

Colloidal Dispersion

For a colloidal dispersion: π/cg = RT(1/M + Bcg)

Line I

Slope, B = 0, reflects dilute spherocolloidal system.

Line II

B is large linear, reflects a linear colloid in a solvent for which it has a poor affinity → linear lyophobic colloidal system.

Line III

B is larger, reflects a linear colloid in a solvent for which it has a high affinity → linear lyophilic colloidal system.

Molecular Weight Determination

Determination of molecular weight of 'the polymer' by Osmotic method.

Sedimentation Velocity

Velocity of sedimentation of spherical particles is given by Stokes' Law: v = 2r²(ρ - ρo)g / 9ηo

Stokes' Law Variables

r: radius of the particle, ρ: density of particle, ρo: density of medium, ηo: viscosity of medium, g: acceleration due to gravity (980.665 cm sec⁻²)

Lower Size Limit

The lower size limit of particles obeying Stokes' Law is about 5 µm.

Centrifugal Force

Force stronger than gravity must be applied to bring about sedimentation → centrifugal force.

Ultracentrifuge

Ultracentrifuge can produce a force of 10⁶ g to sediment colloidal particles.

Viscosity (definition)

Viscosity is an expression of resistance to flow of a system under an applied stress.

Viscosity of Colloidal Dispersions

Viscosity of colloidal dispersions is affected by the shape of the dispersed phase (colloids).

Spherocolloids

Spherocolloids form dispersions of relatively low viscosity.

Linear Colloidal Particles

Linear colloidal particles form more viscous dispersions.

Einstein's Equation of Flow

η = η0 (1 + 2.5Φ)

Viscosity Variables

η: viscosity of dispersion, η0: viscosity of dispersion medium, Φ: volume fraction of colloidal particles present.

Volume Fraction Definition

Φ defined as volume of particles divided by total volume of dispersion. Φ is equivalent to a concentration term.