Colloids Classes of solutions True Solutions Suspensions Colloidal

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Colloids

Colloids

Classes of solutions True Solutions (Suspensions) Colloidal Solutions Suspensions: q Heterogeneous mixtures q Relatively

Classes of solutions True Solutions (Suspensions) Colloidal Solutions Suspensions: q Heterogeneous mixtures q Relatively large particles e. g. whole blood many medicines (Shake well before using)

Colloids: q Heterogeneous mixtures (micro) q Dispersed particles: 1 to 500 -1000 nm Hydrophilic

Colloids: q Heterogeneous mixtures (micro) q Dispersed particles: 1 to 500 -1000 nm Hydrophilic colloids (eucolloids) Hydrophobic colloids (aggregation)

Colloids q. Colloids are mixtures of a solvent and suspended particles. q. Particles are

Colloids q. Colloids are mixtures of a solvent and suspended particles. q. Particles are too small to see but are larger than molecules. q. Due to their small size they do not settle out of solution. q. There are several types of colloid: §aerosol (gas + liquid or solid, e. g. fog and smoke), §foam (liquid + gas, e. g. whipped cream), §emulsion (liquid + liquid, e. g. milk), §sol (liquid + solid, e. g. paint), §solid foam (solid + gas, e. g. marshmallow), §solid emulsion (solid + liquid, e. g. butter), §solid sol (solid + solid, e. g. pearl, opal).

Types of Colloids Dispersed Phase Gas Continuous Phase Liquid Name Gas Solid Foam Liquid

Types of Colloids Dispersed Phase Gas Continuous Phase Liquid Name Gas Solid Foam Liquid Solid Gas Liquid Solid Aerosol Emulsion Gel Aerosol Solid sol Foam

Properties: Brownian motion Tyndall Effect (Reflection and light scattering) Coagulation Dialysis peptization

Properties: Brownian motion Tyndall Effect (Reflection and light scattering) Coagulation Dialysis peptization

Tyndall Effect q. Tyndall effect: ability of a Colloid to scatter light. q. The

Tyndall Effect q. Tyndall effect: ability of a Colloid to scatter light. q. The beam of light can be seen through the colloid.

Removal of Colloidal Particles q. Colloid particles are too small to be separated by

Removal of Colloidal Particles q. Colloid particles are too small to be separated by physical means (e. g. filtration). q. Colloid particles are coagulated (enlarged) until they can be removed by filtration. q. Methods of coagulation: qheating (colloid particles move and are attracted to each other when they collide); qadding an electrolyte (neutralize the surface charges on the colloid particles). q. Dialysis: using a semipermeable membranes separate ions from colloidal particles.

Hydrophilic & Hydrophobic Colloids q. Focus on colloids in water. q“Water loving” colloids: hydrophilic.

Hydrophilic & Hydrophobic Colloids q. Focus on colloids in water. q“Water loving” colloids: hydrophilic. q“Water hating” colloids: hydrophobic. q. Molecules arrange themselves so that hydrophobic portions are oriented towards each other. q. If a large hydrophobic macromolecule (giant molecule) needs to exist in water (e. g. in a cell), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water.

Hydrophilic and Hydrophobic Colloids q. Most dirt stains on people and clothing are oil-based.

Hydrophilic and Hydrophobic Colloids q. Most dirt stains on people and clothing are oil-based. Soaps are molecules with long hydrophobic tails and hydrophilic heads that remove dirt by stabilizing the colloid in water. q. Bile excretes substances like sodium stereate that forms an emulsion with fats in our small intestine. q. Emulsifying agents help form an emulsion.

Hydrophilic and Hydrophobic Colloids q. Sodium stearate has a long hydrophobic tail (CH 3(CH

Hydrophilic and Hydrophobic Colloids q. Sodium stearate has a long hydrophobic tail (CH 3(CH 2)16 -) and a small hydrophobic head (-CO 2 -Na+). q. The hydrophobic tail can be absorbed into the oil drop, leaving the hydrophilic head on the surface. q. The hydrophilic heads then interact with the water and the oil drop is stabilized in water.

Particle Sizes Become Larger Solutions Colloidal Dispersions Suspensions All particles are on the order

Particle Sizes Become Larger Solutions Colloidal Dispersions Suspensions All particles are on the order of atoms, ions, or small molecules (0. 1 -1 nm) Particles of at least one component are large clusters of atoms, ions, or small molecules, or are very large ions or molecules (1 -1000 nm) Particles of at least one component may be individually seen with a lowpower microscope (over 1000 nm) Most stable to gravity Less stable to gravity Unstable to gravity Most homogeneous Also homogeneous, but borderline Homogeneous only if well stirred

Solutions Colloidal Dispersions Suspensions Transparent (but often colored) Often translucent or opaque, but may

Solutions Colloidal Dispersions Suspensions Transparent (but often colored) Often translucent or opaque, but may be transparent Often opaque but, may appear translucent No Tyndall effect Not applicable (suspensions cannot be transparent) No Brownian movement Particles separate unless system is stirred Cannot be separated by filtration Homogeneous Can be separated by filtration ———— to ———— Heterogeneous ——>

Colloids n Tyndall effect – Light is scattered by particles suspended in water –

Colloids n Tyndall effect – Light is scattered by particles suspended in water – Use the Tyndall effect to differentiate between a true solution and a suspension § True solution - a homogeneous mixture – ions and molecules are too small to scatter visible light § Suspension - particles are suspended in water – the particles are too large to be dissolved, and so are large enough to scatter light.

Colloids n Colloid – aka a colloidal dispersion – tiny particles are suspended in

Colloids n Colloid – aka a colloidal dispersion – tiny particles are suspended in some medium § the particles are single large molecules or § the particles are groups of molecules or ions from 1 to 1000 nm.

Colloids n Type of colloid depends on the medium and the dispersed phase

Colloids n Type of colloid depends on the medium and the dispersed phase

Colloids

Colloids

Colloid n What stabilizes a colloid? – Electrostatic repulsion § A colloid is neutral

Colloid n What stabilizes a colloid? – Electrostatic repulsion § A colloid is neutral § BUT, when a colloid is placed in an electric field, the particles all migrate to the same electrode. § The colloid attracts ions of the same charge which surround the colloid particle. § Ions of the opposite charge surround the first layer of ions. § Being surrounded by ion, the colloid particles repel other colloid particles, and so are unable to aggregate to precipitate out

Colloids n To destroy a colloid (coagulation) – heat the colloid § heating increases

Colloids n To destroy a colloid (coagulation) – heat the colloid § heating increases velocities of particles § particles can collide with enough energy to knock off the ion barriers, thus allowing the colloid particles to aggregate and precipitate out.

Colloids n To destroy a colloid (coagulation) – add an electrolyte § the electrolyte

Colloids n To destroy a colloid (coagulation) – add an electrolyte § the electrolyte will neutralize the ion layers § Ex: deposition of clay where a river reaches the ocean. The high salt content of the seawater causes the suspension of clay particles to coagulate.