Colloids
Objective
At the end of this lecture, student will be able to
• Classify colloids with examples
• Compare different forms of colloidal sols
• Explain the types of colloids
• Explain the significance of association colloids
• Describe the methods of preparation of lyophobic colloids
• Discuss the physical properties of colloidal dispersion
• Discuss the optical properties of colloidal dispersion
• Explain the kinetic properties of colloids
• Explain the electrical properties of colloids
• Discuss sedimentation potential of colloidal dispersion
• Describe Donnan membrane effect
• Discuss the stability of colloids
• Explain the purification methods of colloids
• Explain the analysis of colloids
• Define Hardy-Schulze rule
COLLOIDS
Colloids, often referred to as colloidal dispersions or colloidal systems, are heterogeneous mixtures where tiny particles or droplets of one substance are dispersed within another medium. The particles in a colloid are larger than individual molecules but smaller than those in suspensions, making them intermediate in size.
Examples of colloidal systems from daily life
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:
1- Molecular dispersions (less than 1 nm)
• Particles invisible in electron microscope
• Pass through semipermeable membranes and filter paper
• Particles do not settle down on standing
• Undergo rapid diffusion
• E.g. ordinary ions, glucose
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 centrifugation
• Diffuse very slowly
• E.g. colloidal silver sols, naural and synthetic polymers
3- Coarse dispersions (> 0.5 um)
• Particles are visible under ordinary microscope
• Do not pass through filter paper or semipermeable membrane.
• Particles settle down under gravity
• Do not diffuse
• E.g. emulsions, suspensions, red blood cells
Classification of Colloids Based on Size
|
Class |
Size |
Examples |
|
Molecular dispersion |
< 1.0 nm |
Oxygen gas, ordinary ions, glucose |
|
Colloidal dispersion |
1.00 nm to 0.5 micro meter |
Silver sols, natural and synthetic polymer lactices |
|
Coarse dispersion |
> 0.5 micro meter |
Sand, pharmaceuticals emulsions and dispersions, Red blood cells |
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 particles
• 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.
Classification of Colloids Based on the State of the Dispersed Phase and Dispersion Medium
|
Dispersion Medium |
Dispersed phase |
Type of colloid |
Example |
| Gas | Liquid | Aerosol | Fog, clouds |
| Gas | Solid | Aerosol | Smoke |
| Liquid | Gas | Foam | Soda water |
| Liquid | Liquid | Emulsion | Milk, hair cream |
| Liquid | Solid | Sol | Paints, cell fluids |
| Solid | Gas | Foam | Plastic foams |
| Solid | Liquid | Gel | Jelly, cheese |
| Solid | Solid | Solid Sol | Ruby glass |
Types of colloids
• The nature of interaction between dispersed phase and dispersion medium.
A-Lyophilic colloids (solvent attracting) (solvent loving) –
The particles in a lyophilic system have a great affinity for the solvent.
• If water is the dispersing medium, it is often known as a hydrosol or hydrophilic.
• Readily solvated (combined chemically or physically, with the solvent) and dispersed, even at high concentrations; More viscid
• Examples of lyophilic sols include sols of gum, gelatin, starch, proteins and certain polymers (rubber) in organic solvents.
• The dispersed phase does not precipitate easily
• The sols are quite stable as the solute particle
surrounded by two stability factors:
a- Negative or positive charge
b- Layer of solvent
• If the dispersion medium is separated from the dispersed phase, the sol can be reconstituted by simply remixing with the dispersion medium. Hence, these sols are called reversible sols.
• Prepared simply by dissolving the material in the solvent being used e.g. dissolution of acacia in water.
B-lyophobic (solvent repelling) (solvent hating) –
The particles resist solvation and dispersion in the solvent.
• The concentration of particles is usually relatively low.
• Less viscid
• These colloids are easily precipitated on the addition of small amounts of electrolytes, by heating or by shaking
• Less stable as the particles surrounded only with a layer of positive or negative charge
• Once precipitated, it is not easy to reconstitute the sol by simple mixing with the dispersion medium. Hence, these sols are called irreversible sols.
• Examples of lyophobic sols include sols of metals and their insoluble compounds like sulphides and oxides. e.g. gold in water
C- Association / amphiphilic colloids
• Certain molecules termed amphiphiles or surface active agents, characterized by two regions of opposing solution affinities within the same molecule.
• At low concentration: amphiphiles exist separately (subcolloidal size)
• At high concentration: form aggregates or micelles (50 or more
Association colloids
Comparison of colloidal sols
| Lyophilic | Associated | Lyophobic |
| Dispersed phase (large organic mole. With colloidal size) | Dispersed phase (micelles of organic molec. Or ion – size below the colloidal range) |
Dispersed phase (Inorganic particles as gold) |
| Molec. of dispersed phase are solvated Formed spontaneously |
Hydrophilic and lyophilic portion are solvated , Formed at conc. Above CMC |
Not formed spontaneously |
| The viscosity ↑ with ↑ the dispersed phase conc. | The viscosity ↑ with ↑ the micelles conc. | Not greatly increase |
| Stable dispersion in presence of electrolytes | CMC↓ with electrolytes | Unstable dispersion in presence of electrolytes |
Critical micelle concentration (C.M.C): the concentration at which micelle form
• The phenomenon of micelle formation can be explained:
1- Below C.M.C: amphiphiles are adsorbed at the air/water interface
2- As amphiphile concentration is raised: both the interphase and bulk phase become saturated with monomers (C.M.C)
3- Any further amphiphile added in excess: amphiphiles aggregate to form micelles
• In water: the hydrocarbon chains face inwards into the micelle forming hydrocarbon core and surrounded by the polar portions of the amphiphile associated with water molecules.
• In non-polar liquid: the polar heads facing inward and the hydrocarbon chains are associated with non-polar liquid.
• At concentrations close to C.M.C à spherical micelles
• At higher concentrations à lamellar micelles
KRAFT POINT – The temperature at which the solubility of surfactant = CMC (Micelle formation)
Shapes of surfactant aggregates
Methods to prepare Lyophobic colloids
A) Dispersion methods:
Coarse particles are reduced in size to colloidal particles
1) Milling and Grinding – Using Colloidal mill
• Substance ground to coarser form using a dispersion medium – then passed through colloidal mill, that has two steel discs, having a small aperture between them.
• These discs are rotated at high speeds in opposite directions
• Process is repeated until the desired size is achieved
Colloidal mill
• Coarse to colloidal
• Material sheared between 2 rapidly oppositely rotating close plates.
• Low efficiency & reduce the size of small proportion of particles
• Stabilizers added to control the size (gums, gelatin)
• Eg: collidal kaolin, zno
2) Ultrasonic Generator
• Dispersion achieved by High intensity Ultrasonic waves at frequency more than 20,000 cycles/second that produce alternate cavity and compression of the medium
• Stabilizers: surfactants are added to prevent reunion of particles
3) Peptization
Breaking up of aggregates into colloidal sized particles
• Removal of electrolytes
• Addition of surfactants
Peptizing agents: glycerin, sugar, lactose – They promote size reduction but don’t interfere
4) Electric arc method – Bridge‘s arc method
• An electric current is struck between two metallic electrodes placed i container of water.
• The intense heat of the arc converts metal into vapours which conden immediately in the cold water bath.
• This results in the formation of par of colloidal size.
• KOH – stabilizer
• Silver, platinum and Gold colloidal solutions
B) Condensation methods –
Particles of sub colloidal range are made to aggregate into colloidal particles
1) Chemical reaction
• For lyophobic colloids
• By oxidation, reduction or hydrolysis
• Eg: Sulphur solution is obtained by bubbling H2S gas through the solution of an oxidizing agent like HNO3 or Br2 in water
• Only for inorganic substances
• Not used much in Pharmaceuticals currently
2) Addition of non-solvent
• Concentrated alcoholic solution of S2 ——– Added into excess of water
• S2 present in molecular state in alcohol will precipitate as finely divided particles
• These particles grow rapidly and form a colloidal dispersion
• Why it grows – based on solubility – super saturation of sulphur – either crystal growth or precipitation
• Crystallization – Nuclei growth and nucleation, but nucleation will not be stable
• Molecules and ions adsorbed onto nuclei and forms colloidal dimension
Colloids – Properties
The main properties of Colloidal Solutions are as follows:
(1) Physical properties
(i) Heterogeneous nature: Colloidal sols are heterogeneous in nature. They consists of two phases; the dispersed phase and the dispersion medium.
(ii) Stable nature: The colloidal solutions are quite stable. Their particles are in a state of motion and do not settle down at the bottom of the container.
(iii) Filterability: Colloidal particles are readily passed through the ordinary filter papers. However they can be retained by special filters known as ultrafilters (parchment paper).
Optical Properties of Colloids
1-Faraday-Tyndall effect
• When a strong beam of light (IR light) is passed through a colloidal sol, the path of light is illuminated (a visible cone formed)
• This phenomenon resulting from the scattering of light by the colloidal particles
• This is due to interaction of particles with light
• This scattered beam is called as Tyndall beam
• Most visible when observed in dark
2- Electron microscope
• Electron microscope is capable of yielding pictures of actual particles size, shape and structure of colloidal particles
• Electron microscope has high resolving power, as its radiation source is a beam of high energy electrons
Electron Microscope
3- Ultra Microscope – Dark field microscope
4- Turbidity method
• Used to determine the concentration of dispersed particles and molecular weight of solute
• By Spectrophotometer or Nephelometer
• Turbidity is measured based on the intensity of transmitted light in Spectrophotometer
I/I0 = e lt
• I0 = intensity of incident light
• I = intensity of transmitted light
• l = length of sample
• t = turbidity
• Turbidity is measured based on the intensity of scattered light at right angles to the direction of incident light in Nephelometer
5- Light Scattering
• Used to study proteins, polymers and association colloids
• Spherical particles – light scattering in all directions
• Rod shaped particles – light scattering will be right angle to direction of flow
• Turbidity is measured from scattered light at a particular angle is given by equation
T = 16∏R/3
R = Intensity of incident and scattered light
R = Ir2/Is
r = Distance from the scattered particle to point of observation
T = Turbidity
• When light scattering due to random motion or difference in refractive index, molecular weight is calculated by DEBYE equation
Hc / T = 1/M + 2Bc
T: turbidity
C: conc of solute in gm / cc of solution
M: molecular weight
B: interaction constant
H: constant for a particular system
Hc / T Vs Concentration gives STRAIGHT LINE, 2B is slope and 1/M is intercept
Kinetic Properties of Colloids
1-Brownian motion
• Colloidal particles do not settle due to its size
• The zig-zag movement of colloidal particles continuously and randomly due to thermal energy
• This brownian motion arises due to the uneven distribution of the collisions between colloid particle and the solvent molecules and with walls of container
• Brownian movement was more rapid for smaller particles
• It decrease with increase the viscosity of the medium
• Observed under light microscope
2- Diffusion
• Particles diffuse spontaneously from a region of higher concentration to lower concentration until equilibrium is established
• Diffusion is a direct result of Brownian motion
• Fick’s first law used to describe the diffusion: The amount of Dq of substance diffusing in time dt across a plane of area A is directly proportional to the change of concentration dc with distance traveled
dq = -DA (dc / dx) dt
D à Diffusion coefficient
• The amount of the material diffused per unit time across a unit area when dc/dx (conc. gradient) is unity
D à Diffusion coefficient of a polymer and its molecular weight is estimated by the formula
M = molecular weight
v = Partial specific volume of particles
N = Avagadro’s number
3- Osmotic pressure – Van’t Hoff equation
π = cRT
• Can be used to determine the molecular weight of colloid in dilute solution
• Replacing c by C / M (where C = the grams of solute / liter of solution,
M = molecular weight)
π /C = RT/M
π = osmotic pressure; R= molar gas constant
4- Sedimentation
• The velocity of sedimentation is given by Stokes‘Law:
dst = diameter of particles
ρs = density of solid
ρl = density of liquid
g = gravitational constant
η = viscosity of medium
h = height
t – time interval
5- Viscosity:
• It is the resistance to flow of system under an applied stress. The more viscous a liquid, the greater the applied force required to make it flow at a particular rate
• The viscosity of colloidal dispersion is affected by the Shape of particles of the disperse phase:
Spherocolloids à dispersions of low viscosity
Linear particles à more viscous dispersions
Viscosity of colloidal dispersion is influenced by
• Affinity of particles – Linear particles when kept in a medium of low affinity viscosity decreases
• Type of colloids – Lyophilic colloids – more viscous than dispersion medium
Lyophobic colloids – equal viscosity of dispersion medium
• Molecular weight of particles – higher the molecular weight, greater the viscosity
Electric Properties of Colloids
• The particles of a colloidal solution are electrically charged and carry the same type of charge, either negative or positive
• The colloidal particles therefore repel each other and do not cluster together to settle down
• The charge on colloidal particles arises because of the dissociation of the molecular electrolyte on the surface
• E.g. As2S3 has a negative charge, During preparation of colloidal As2S3 , H2S is absorbed on the surface and dissociate to H+ (lost to the medium) and S-2 remain on the surface of colloid
• Fe(OH)3 is positively charged, Due to self-dissociation and loss of OH- to the medium, so they become [Fe(OH)3] Fe+3
• The distribution of ions of a charged particle is explained by Electrical Double Layer
• When particles moves, this electrical double layer also moves
• Electrical potential in the plane of shear of the charged particle is called as ZETA POTENTIAL, which is used to predict the stability surface of colloid
• Zeta potential is measured by Electrophoresis
Electrophoresis
• Electrophoresis is the most known electrokinetic phenomena
• It involves motion of charged particles through a fluid under the influence of an applied electric field, using ELECTROPHORETIC CELL
• If an electric potential is applied to a colloid, the charged colloidal particles move toward the oppositely charged electrode
• If the particle moves towards anode, the charge of the particle is negative
• If the particle moves towards cathode, the charge of the particle is positive
• Rate of migration depends on charge of particles
• As the particle is located at “Tightly Bound Layer”, potential determined is ZETA POTENTIAL
• Sign and magnitude of migration can be determined
• As the potential gradient across the electrodes increases, the velocity of migration of a particle increases
• Based on this Zeta potential is calculated by the formula
ɕ = v4
π ƞ/EƐ
E – Electric potential in volts; Ɛ – Dielectric constant
Electro-osmosis
• It is the opposite in principal to that of electrophoresis
• When electrodes are placed across a clay mass and a direct current is applied, water in the clay pore space is transported to the cathodically charged electrode by electro-osmosis
• Electro-osmotic transport of water through a clay is a result of diffuse double layer cations in the clay pores being attracted to a negatively charged electrode or cathode
• As these cations move toward the cathode, they bring with them water molecules that clump around the cations as a consequence of their dipolar nature
Sedimentation potential
• The sedimentation potential also called the Donnan effect
• It is the potential induced by the fall of a charged particle under an external force field
• It is analogous to electrophoresis in the sense that a local electric field is induced as a result of its motion
• If a colloidal suspension has a gradient of concentration (such as is produced in sedimentation or centrifugation), then a macroscopic electric field is generated by the charge imbalance appearing at the top and bottom of the sample column
Passive distribution – Donnan equilibrium
• The ratio of positively charged permeable ions equals the ratio of negatively charged permeable ions
• Mathematically expressed:
• At equilibrium
Outside (o) Inside (I)
Na+ Na+
Cl- Cl-
R-
• Applying the condition of electroneutrality, the number of positive charges must equal the number of negative charges on each side of the membrane
• The presence of impermeable negatively charged molecules requires more positively charged molecules inside the cell.
So Outside: [Na+]0 = [Cl–]o ……………………… (1)
Inside: [Na+]I = [R–]i + [Cl–]I ……………………… (2)
• According to the principle of Escaping tendency of electrolytes, concentrations must be equal.
[Na+]0 * [Cl–]o = [Na+]i * [Cl–]i ……………………… (3)
Substituting equation 1 and 2 in 3, we get
[Cl–]0 * [Cl–]o = [Cl–]i {[R–]i+ [Cl–]o}
[Cl–]2 o = [Cl–]2i + [Cl–]I * [R–]I ……………………… (4)
Divide both sides by [Cl–]2i
Eq.5 , on rearrangement
……………………. (6) — Donnan Membrane Equilibrium
This equation is used to calculate the ratio of concentration of diffusible anion outside and inside the membrane at equilibrium
Stability of colloids
• Stabilization serves to prevent colloids from aggregation
• The presence and magnitude, or absence of a charge on a colloidal particle is an important factor in the stability of colloids
• Two main mechanisms for colloid stabilization:
1-Steric stabilization i.e. surrounding each particle with a protective solvent sheath which prevent adherence due to Brownian movement
2-Electrostatic stabilization i.e. providing the surface of particles with electric charge
A- Lyophobic Colloids
• Thermodynamically unstable and forms aggregates
• Explained by DLVO (Derjaguin, Landau, Verway, Overbeek) Theory,
Particle-Particle interactions
• Two types of interactions – Attraction and Repulsion
• Types of forces involved – Van der Waals forces of attraction and Electrostatic repulsive forces
• Combination for both the above forces is Net Energy of Interaction VT
Potential Energy Vs Interparticle distance
Van der Waals forces of attraction
• Depends on chemical nature and size of the particle
• Forces cannot be altered easily
• Potential Energy is VA
Electrostatic repulsive forces
• Depends on density, surface charges and thickness of bulk layer Indicates the magnitude of Zeta Potential
• Potential Energy is VR
• Primary minimum – particles are very close – Increase in potential energy – Precipitation
• Secondary minimum – particles are too long – Aggregation due to more attractive forces
• Net energy peak – at intermediate distance – particles in Brownian motion – Good stability due to positive Zeta potential – Can be estimated by height of maximum in potential energy curve (VM)
• Value of VM is 10-20kt which is equivalent to 50 mV
Reasons for Coagulation
• Removal of electrolytes – causes primary minimum
• Addition of excess electrolytes – results in accumulation of opposite ions and decrease zeta potential
• Electrolytes of opposite charge – causes secondary minimum. The charge and valency of electrons are described by “Hardy-Schulze Rule”
• Addition of oppositely charged electrolytes – Decrease in ZP below 50 volts and causes secondary minimum
Hardy-Schulze Rule
• Coagulation of colloidal dispersions can be brought about by addition of electrolytes which reduces the zeta potential
• The effectiveness of an electrolyte to cause precipitation depends not only on concentration but also on the valence of the active ion
• The precipitating power of an ion on a dispersed phase of opposite charge increases with the increase in valence or charge of the ion
• Greater is the valency of the oppositely charged ion of the electrolyte being added, the faster is the coagulation
Eg: For ferric hydroxide colloid, phosphate ions are more effective than sulphate ions
B- Lyophilic association colloids
• Stable
• Present as true solution
• Addition of moderate amounts of electrolytes not cause coagulation (opposite lyophobic)
• Stability is based on electrical charge and hydration
Reasons for coagulation
1- Addition of large
amounts of electrolytes
• Anions arranged in a decreasing order of precipitating power: citrate > tartrate > sulfate > acetate > chloride> nitrate > bromide > iodide
• Cations = Mg > Ca > Ba > Na
• The precipitation power is directly related to the hydration of the ion and its ability to separate water molecules from colloidal particles
• When excess electrolytes are added ions get hydrated and water is not available for hydration which leads to salting out or flocculation
2- Addition of less polar solvents/nonsolvent
• e.g. alcohol, acetone
• Addition of this leads to dehydrated forms of lyophilic colloids and stability depends on charge they possess
• It becomes unstable
3 – Addition of oppositely charged colloids
• Shell of tightly bound water molecules prevents flocculation
• Electrostatic attractions of oppositely charged particles hold particle together
• Particles separate from the dispersion to form a layer rich in the colloidal aggregates
Sensitization and protective colloidal action
• Sensitization: the addition of small amount of hydrophilic or hydrophobic colloid to a hydrophobic colloid of opposite charge tend to sensitize (coagulate) the particles
• Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions.
• For example, negatively-charged colloidal silica particles can be flocculated by the addition of a positively-charged polymer
• Acacia with Gelatin
• Protection: Addition of large amount of hydrophilic colloid (protective colloid) carrying opposite charges to a hydrophobic colloid they tend to stabilize the system
• This is due to: hydrophilic colloid gets adsorbed to hydrophobic particles and forms a protective layer around it
• This prevents coagulation by stopping the precipitating ions reaching the colloidal particle
• The colloid which helps to stabilize the other colloid is called as protective colloid and this property is expressed as GOLD NUMBER
GOLD NUMBER
• It is the measure of protective ability of hydrophilic colloid
• Minimum weight in mgs of a protective colloid required to prevent a color change from red to violet in 10ml of gold solution on addition of 1ml of 10% NaCl solution
• Lower the gold number, more the protective action
Method of determination of Gold Number
• Series of test tubes – 10ml of gold sol – add 1ml of 10% NaCl solution
• At high conc. Gold sol dos not change its color – but at low conc.
• Color changes from red to violet – test tube having min. amt. of colloid which prevents the change in color is the gold number of the protective colloid
• Eg: Albumin – 0.1, Sodium oleate – 3, Tragacanth – 2
Purification of colloidal solutions
• When a colloidal solution is prepared is often contains certain electrolytes which tend to destabilize it. The following methods are used for purification:
1- Dialysis:
• Semipermeable cellophane membrane prevent the passage of colloidal particles, yet allow the passage of small molecules or electrolytes
2- Electrodialysis
• In the dialysis unit, the movement of ions across the membrane can be speeded up by applying an electric current through the electrodes induced in the solution.
• The most important use of dialysis is the purification of blood in artificial kidney machines.
• The dialysis membrane allows small particles (ions) to pass through but the colloidal size particles (haemoglobin) do not pass through the membrane.
Ultra filtration
• Colloidal particles an pass through ordinary filter paper due to its pore size.
• Filter papers when impregnated with collodion (Nitrocellulose) are called as “ULTRAFILTERS” where the pore size is reduced
• It removes all electrolytes and colloidal particles are retained on the filter paper
• Collected and dispersed in a dispersion medium to get a Sol
• Process is slow, so pressure or suction can be applied
Applications of colloidal solutions
1- Therapy
• Colloidal system are used as therapeutic agents in different areas.
e.g- Silver colloid-germicidal Copper colloid-anticancer Mercury colloid-Antisyphilis
2- Stability
• e.g. lyophobic colloids prevent flocculation in suspensions. e.g- Colloidal dispersion of gelatin is used in coating over tablets and granules which upon drying leaves a uniform dry film over them and protect them from adverse conditions of the atmosphere.
3- Absorption
• As colloidal dimensions are small enough, they have a huge surface area. Hence, the drug constituted colloidal form is released in large amount.
e.g- sulphur colloid gives a large quantity of sulphur and this often leads to sulphur toxicity
4-Targeted Drug Delivery
• Liposomes are of colloidal dimensions and are preferentially taken up by the liver and spleen.
5- Photography
• A colloidal solution of silver bromide in gelatine is applied on glass plates or celluloid films to form sensitive plates in photography.
6- Clotting of blood
• Blood is a colloidal solution and is negatively charged.
• On applying a solution of Fecl3 bleeding stops and blood clotting occurs as Fe+3 ions neutralize the ion charges on the colloidal particles.
Summary
• 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
• Flow, sedimentation and osmotic pressure of the colloidal system affected by the shape of colloidal particles.
• Particle shape may also influence the pharmacologic action
• Examples of lyophilic sols include sols of gum, gelatin, starch, proteins and certain polymers (rubber) in organic solvents.
• The dispersed phase does not precipitate easily
• The sols are quite stable as the solute particle surrounded by two stability factors namely – negative or positive charge and layer of solvent
• Colloids are classified as lyophobic, lyophilic and association colloids
• 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
• Flow, sedimentation and osmotic pressure of the colloidal system affected by the shape of colloidal particles.
• Particle shape may also influence the pharmacologic action
• Examples of lyophilic sols include sols of gum, gelatin, starch, proteins and certain polymers (rubber) in organic solvents.
• The dispersed phase does not precipitate easily
• The sols are quite stable as the solute particle surrounded by two stability factors namely – negative or positive charge and layer of solvent
• Colloids are classified as lyophobic, lyophilic and association colloids
• The phenomenon of micelle formation can be explained as : below C.M.C: amphiphiles are adsorbed at the air/water interface, as amphiphile concentration is raised: both the interphase and bulk phase become saturated with monomers and any further amphiphile added in excess: amphiphiles aggregate to form micelles
• The main properties of colloidal solutions are Physical
properties, Heterogeneous nature, stable nature, Filterability
• Optical properties of colloids include Faraday Tyndall effect, electron microscope and light scattering effect
• The phenomenon of micelle formation can be explained as: below C.M.C: amphiphiles are adsorbed at the air/water interface, as amphiphile concentration is raised: both the interphase and bulk phase become saturated with monomers and any further amphiphile added in excess: amphiphiles aggregate to form micelles
• The main properties of colloidal solutions are Physical properties, Heterogeneous nature, stable nature, Filterability
• Optical properties of colloids include Faraday Tyndall effect, electron microscope and light scattering effect
• The particles of a colloidal solution are electrically charged and carry the same type of charge, either negative or positive.
• The colloidal particles therefore repel each other and do not cluster together to settle down
• The sedimentation potential also called the Donnan effect
• It is the potential induced by the fall of a charged particle under an external force field.
• It is analogous to electrophoresis in the sense that a local electric field is induced as a result of its motion.
• Brownian motion – The zig-zag movement of colloidal particles continuously and randomly.
• Diffusion – Particles diffuse spontaneously from a region of higher conc. To one of lower conc. Until the conc. of the system is uniform throughout
• Van’t Hoff equation = π = cRT is used to determine the molecular weight of colloid in dilute solution
• The particles of a colloidal solution are electrically charged and carry the same type of charge, either negative or positive.
• The colloidal particles therefore repel each other and do not cluster together to settle down
• The sedimentation potential also called the Donnan effect
• It is the potential induced by the fall of a charged particle under an external force field.
• It is analogous to electrophoresis in the sense that a local electric field is induced as a result of its motion.
• Stabilization serves to prevent colloids from aggregation.
• The presence and magnitude, or absence of a charge on a colloidal particle is an important factor in the stability of colloids.
• Sensitization: the addition of small amount of hydrophilic or hydrophobic colloid to a hydrophobic colloid of opposite charge tend to sensitize (coagulate) the particles.
• Colloids are analysed by dialysis, electro dialysis and ultrafiltration
• Colloidal system are used as therapeutic agents in different areas.
• Stability – lyophobic colloids prevent flocculation in suspensions.
• Coagulation of colloidal dispersions can be brought about by addition of electrolytes which reduces the zeta potential
• Gold number is the measure of protective ability of hydrophilic colloid and it is defined as minimum weight in mgs of a protective colloid required to prevent a color change from red to violet in 10ml of gold solution on addition of 1ml of 10% NaCl solution.
• Stabilization serves to prevent colloids from aggregation.
• The presence and magnitude, or absence of a charge on a colloidal particle is an important factor in the stability of colloids.
• Sensitization: the addition of small amount of hydrophilic or hydrophobic colloid to a hydrophobic colloid of opposite charge tend to sensitize (coagulate) the particles.
• Colloids are analysed by dialysis, electro dialysis and ultrafiltration
• Colloidal system are used as therapeutic agents in different areas.
• Stability – lyophobic colloids prevent flocculation in suspensions.
• Coagulation of colloidal dispersions can be brought about by addition of electrolytes which reduces the zeta potential
• Gold number is the measure of protective ability of hydrophilic colloid and it is defined as minimum weight in mgs of a protective colloid required to prevent a color change from red to violet in 10ml of gold solution on addition of 1ml of 10% NaCl solution.
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