Module VI: Colloids and Interfacial Science Macromolecular Science IPreparation, properties and applications of colloidal system in study of molecules, Microemulsions, Emulsions, Suspensions. Thermodynamics of Interfaces, Surfactants and its properties.

Session ObjectivesColloidsClassification of colloidsPreparation of colloidal solsPurification of colloidal solsImportant properties of colloidal solsEmulsions Identification of emulsionPreparation of emulsionsApplications of colloids

COLLOIDS

ColloidsThomas Graham in (1860) has observed that some material which passes through the ordinary filter paperbut retained by perchment membrane. These materialsare amorphous in nature. He called this material as Colloids. Kolla means glue and eoid means likeNaCl in benzene behave as colloid.Soap in alcohol behave as crystalloid.Colloid is a state of mater

Classification of colloidsClassification is based on following criteriaPhysical state of dispersed phase and dispersion medium.Nature of interaction between dispersed phase and dispersion medium.Types of particles of the dispersed phase.

Classification based on physical state of dispersed phase and dispersion medium Eight types of colloidal systems are possible.Pumice stone, foam rubber

Classification based on nature of interactionHydrophilic (Irreversible)Hydrophobic (Reversible)Lyophilic (Solvent loving)Lyophobic(Solvent hating)1) Inorganic sol, As2S3, Fe(OH)3,Organic substances:Ag, Au, Pt solsgum, gelatin, Albumin2) Unstable: coagulated easilyStable: not coagulated easily3) Surface tension, viscositysurface tension, viscosity remain almost samechanges4) Gel formation suddenlygel formation easily5) particles are changedparticles may or may not be charged

Classification based on type of particles of the dispersed phaseMultimolecular colloids : Consists of aggregates of a large number of atoms or smaller molecules whose diameter is less than 1 nmMacromolecular colloids: In these colloids, the molecules have sizes and dimensions comparable to colloidal particles. For example,proteins, starch, cellulose.

Dispersed Systems:Dispersed systems consist of particulate matter (dispersed phase), distributed throughout a continuous phase (dispersion medium).

They are classified according to the particle diameter of the dispersed material:

True solution: Molecular dispersions (less than 1 nm)Particles invisible in electron microscopePass through semipermeable membranes and filter paper- Particles do not settle down on standingUndergo rapid diffusionE.g. ordinary ions, glucose

Dispersed Systems:2- Colloidal dispersions (1 nm - o.5 um)Particles not resolved by ordinary microscope, can be detected by electron microscope.Pass through filter paper but not pass through semipermeable membrane.Particles made to settle by centrifugationDiffuse very slowlyE.g. colloidal silver sols, natural and synthetic polymers

3- Coarse dispersions (> 0.5 um)Particles are visible under ordinary microscopeDo not pass through filter paper or semipermeable membrane.Particles settle down under gravityDo not diffuseE.g. emulsions, suspensions, red blood cells

Comparisons between True solution, Suspension and Colloidal solution

Size and shape of colloids:Particles lying in the colloidal size have large surface area when compared with the surface area of an equal volume of larger particles.Specific surface: the surface area per unit weight or volume of material.The possession of large specific surface results in:1- platinium is effective as catalyst only when found in colloidal form due to large surface area which adsorb reactant on their surface.2- The colour of colloidal dispersion is related to the size of the paticlese.g. red gold sol takes a blue colour when the particles increase in size

Size and shape of colloids:The shape of colloidal particles in dispersion is important: The more extended the particle the greater its specific surface the greater the attractive force between the particles of the dispersed phase and the dispersion medium. Flow, sedimentation and osmotic pressure of the colloidal system affected by the shape of colloidal particles.

Particle shape may also influence the pharmacologic action.

Different shapes of colloids

Preparation of colloidal systems Dispersion methodCondensation methodBreaks CondenseLarge particles ------Small molecules ------------a) Mechanical dispersiona) Excessive coolingb) Ultrasonic dispersionb) Lowering of solubility by exchange of solventc) Electro dispersionc) Passing vapor of an element into a liquidd) Peptization d) Chemical actionDouble decompositionOxidationReductionHydrolysis

Colloid preparationMechanical method :-GrindingDiscLarge particle ------------- Suspension -------------- Colloidmill Ultrasonic vibration: H2O Oil

Colloid preparationPrepared by Electrodispersion:I. Physical method (Bridges arc method)- This method is employed for obtaining colloidal solutions of metals e.g. silver, gold, platinum

iceDispersion medium(Water + kOH)

I. Physical method (Bridges arc method)

An electric current is struck between two metallic electrodes placed in a container of water.The intense heat of the arc converts the metal into vapours which condensed immediately in the cold water bath.This results in the formation of particles of colloidal size.

Peptization (Reverse of coagulation) Dispersion of coagulated sol: Freshly prepared ferric hydroxide on treatment with small amount of ferric chloride. Coagulation: Aluminium hydroxide by addition of small amount of HCl in quantitative analysis.

Milk Paneer (Coagulation by acid or salt) e. g. lemon or Calcium lactate

Chemical method:i)Excessive cooling : The colloidal solution of ice in CHCl3

ii) Lowering of solubility by exchange of solvent :- alcoholwaterSulphur/ Phosphorous/ resins etc -----------solution --------------ColloidPhenolphthatein in alcohol -------- white (Colloid)iii) passing vapor of an element into the liquid :- Coldvapours (Hg) -----------------Water(sulphur) Stabilising agent ammonium salts or citrates

Chemical methodiv) Large number of molecules grow to colloidal size

Double decompositionAs2O3 + H2S As2S3 + 3H2O BoilCool Pass H2SAs2O3 (1gm) + H2O ------------------------------------ colloid (As2S3)GasOxidation: OxidationH2S ------------------------H2O + S colloidHNO3

Oxidation: [O]Sodium thiosulphate (Na2S2O3) ---------------- S colloidHNO3Br2 + H2S S + 2 HBr

HNO3 + H2S H2O + NO2 + S

Colloid preparation by CoagulationReduction :- Reduction Gold chloride -- 2AuCl3 + 3(SnCl2) -------Au + 3SnCl4 ReductionH2S + SO2 -----------2H2O +3S

Hydrolysis : - FeCl3 + 3H2O ---------- Fe(OH)3 + 3HCl

Colloid preparationC- Association / amphiphilic colloids- Certain molecules termed amphiphiles or surface active agents, characterized by two regions of opposing solution affinities within the same molecule.

Types of colloids:At low concentration: amphiphiles exist separately (subcolloidal size)At high concentration: form aggregates or micelles (50 or more monomers) (colloidal size)

Association colloids

Association Colloids