1. True Solutions:
A true solution is a homogeneous solution in which the solute particles have diameters between 0.1 nm to 1 nm i.e., the solute particles are of molecular dimensions.
Such dispersed particles dissolve in solution to form a homogenous system. These do not settle down when the solution is left standing. The particles are invisible even under powerful microscopes and cannot be separated through filter paper, parchment paper or animal membranes. For example, sodium chloride in water is a true solution.
Most ionic compounds form true solutions in water. Organic compounds like sugar and urea also form true solutions in water.

2. DISPERSED SYSTEMS
A substance is dispersed if it is distributed throughout another phase, which is commonly called the external phase or dispersion medium.
Dispersion is defined as a homogeneously mixed state of small particles in a continuous liquid medium.

3. Suspensions
Suspensions consist of particles of a solid suspended in a liquid medium. Suspensions are systems with two distinct phases. The particles in suspensions are bigger than 100 nm to 200 nm across. The particles of a suspension may not be visible to the naked eye but are visible under a microscope. Suspensions are heterogeneous systems. They stay only for a limited period i.e. these are not stable as the particles have a tendency to settle down under the influence of gravity. The particles of a suspension can neither pass through ordinary filter paper nor through animal membranes.
Examples of suspensions are sodium chloride in benzene, silver chloride, barium sulphate or sand in water.

4. Colloidal solution or colloidal state or colloidal dispersion
They represent an intermediate kind of a mixture between true solution and suspension. The size of a colloidal particle lies roughly between nm. Colloids are also a two-phase heterogeneous system consisting of the dispersed phase and dispersion medium.

5. Colloidal solution or colloidal state or colloidal dispersion
Since the dispersed phase in a colloidal system is uniformly distributed in the dispersion medium, the colloidal state appears homogenous to the naked eye or even an ordinary microscope (due to particles being invisible). However it is a heterogeneous dispersion of two immiscible phases and this is proved by viewing it under an ultra-microscope, where the light reflected by colloidal particles can be seen. Colloidal particles do not settle down under gravity: a colloidal solution of gold prepared by Faraday over 125 years ago continues to be in excellent condition even today. Colloids can pass through ordinary filter paper but do not pass through animal membranes.

6. • Dispersed phase: The phase forming the particles • Dispersion medium: The medium in which the particles are distributed • Disperse system: The whole system Dispersed Phase + Dispersion Medium = Dispersed System
Types of Dispersed System
Size of Particles
Coarse dispersions
10–50 μm
Fine dispersions
0.5–10 μm
Colloidal dispersions
1 nm–0.5 μm
Molecular dispersions
<1 nm

8. Difference Between True Solutions, Suspensions and Colloidal Solutions

10. The particles of the dispersed phase may vary widely in size from large particles visible to the naked eye down to particles of colloidal dimensions.
Example of coarse dispersion, if we pour some fine chalk powder into a beaker of water and agitate gently, the chalk powder will become uniformly distributed, or dispersed, in the water. The dispersed particles are coarse, which means they are much larger than the particles of the dispersion medium.
Coarse dispersions are unstable; if we discontinue agitation of the chalk dispersion, the powder will soon separate, or settle out, because the chalk powder is specifically heavier than the water.
Example of molecular dispersions, is sugar, which will readily dissolve in the water. The dispersed sugar particles are roughly of the same size as the particles of the dispersion medium.
Between these two examples of dispersions there is colloidal dispersion, in which the particles are smaller than the coarsely dispersed particles but larger than the molecularly dispersed particles.

11. Coarse dispersions
Coarse dispersions characterized by lack of uniformity which can be determined by visual inspection.
They are thermodynamically unstable and will reduce the surface of dispersed phase exposed to continuous phase by either coalescence or sedimentation.
Their formulation, preparation, and use require attention to their inherent physical instability through reduction of surface energy, manipulation of viscosity and application of shear stress (shaking) to disperse or redisperse.

12. Coarse dispersions are thermodynamically unstable, because any free energy associated with the large interfacial area between the dispersed phase and the continuous phase can be decreased by the aggregation or coalescence of the dispersed phase.
If the particles of the dispersed phase are solid materials that are insoluble in the dispersion medium, the dispersed system is called suspension.
In the case of emulsions, the dispersed phase is a liquid that is neither soluble nor miscible with the liquid of the medium.

13. Electronic microscope
Coarse Dispersion
Colloidal Dispersion
True Solution
Optical properties
Scatters light
Tyndall effect
Passes light
Colligative properties
None
Small
High
Visibility
Optical microscope
Electronic microscope
Motion
Gravitational
Brownian
Thermal
Separation
Paper filter
Membrane filter
Sedimentation
Centrifuge
Ultracentrifuge

14. Colloids

15. colloid (kŏl'oid) [Gr.,=glue like], a mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance.
The mixture is also called a colloidal system, colloidal solution, or colloidal dispersion.
Familiar colloids include fog, smoke, homogenized milk.

16. 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).

17. A colloid is a subvisible particle or macromolecule between 1 and 500 nm (0.5 μm) in diameter.
In some cases, colloidal dispersions lack of homogeneity.
The distinguishing feature of colloidal dispersions is the size of their dispersed phase and the resulting large surface that they present to the continuous phase.
Hydrophilic polymers form colloidal dispersions with water, which appear to be molecular solutions until their light scattering and non-Newtonian flow properties are examined.
Colloidal solutions of hydrophilic polymers are used as gel bases for drugs to be applied to the skin and as vehicles for drug solutions or suspensions that require viscosity.
Particles of colloidal dimensions may be wettable with solvent but not dissolve. They form suspensions in which the particle size is small enough (colloidal) to prevent sedimentation.

18. Blood is a specialized fluid that delivers vital substances such as oxygen and nutrients to various cells and tissues in the body.
blood is a complex bodily fluid that is an example of the three types of dispersed systems.
The dispersion medium in blood is plasma, which is 90% water.
Blood is composed of more than one dispersed phase.
Nutrients such as peptides, and glucose are dissolved in plasma forming a molecular dispersion or true solution.
Oxygen, is carried to cells and tissues by red blood cells. Given the size of red blood cells (~6 μm in diameter and 2 μm in width) they would be considered to form a coarse dispersion in blood.
White blood cells such as leukocytes and platelets are the other major cells types carried in blood.
Serum albumin >1 nm, which puts them into
the colloidal dispersion group.

20. Colloidal solutions are quite stable
Colloidal solutions are quite stable. The colloidal particles do not settle at the bottom under the influence of gravity. This is because of the constant motion of colloidal particles.
Colloidal particles do not pass through ultrafilter papers, animal and vegetable membranes. The large pore size of ordinary filter paper enables colloidal particles to pass through.

21. COLLOIDAL DISPERSION
In a true solution, (e.g. urea or sucrose), the particles of solute distributed in the solvent are of molecular size. However, a suspension or emulsion (e.g. milk) contains particles large enough to be visible to the naked eye, or at least in the microscope, distributed in a liquid medium. Between these two examples are the colloidal systems.
The essential characteristic of the colloidal state is the existence of particles that are larger than molecules but not large enough to be seen in the microscope (about 0.2 μm).
Particles in the colloidal size range possess an increase in surface area compared with the surface area of an equal volume of larger particles.

22. A cube having a 1-cm edge and a volume of 1 cm3 has a total surface area of 6 cm2. If the same cube is subdivided into smaller cubes each having an edge of 100 μm, the total volume remains the same, but the total surface area increases to 600,000 cm2. This represents a 105-fold increase in surface area.

23. Because of their size, colloidal particles can be separated from molecular (sub-colloidal)particles by dialysis.
The technique of separation, known as dialysis, uses a semipermeable membrane of collodion or cellophane.
The pore size of the membrane will prevent the passage of colloidal particles, yet permit small molecules and ions, such as urea, glucose, and sodium chloride, to pass through.

24. Dialysis is utilized in the kidney, which removes low–molecular-weight impurities from the body by passage through a semipermeable membrane.
When dialysis is used to remove charged impurities such as ionic contaminants, the process can be hastened by the use of an electric potential across the membrane. This process is called electrodialysis.

25. Shapes of colloidal particles
Spheres and globules, short rods and prolate ellipsoids, oblate ellipsoids and flakes, long rods and threads, loosely coiled threads, and branched threads.
Shapes may influence colloidal properties as flow, sedimentation, and osmotic pressure
Particle shape may also influence pharmacological action.

29. Lyophilic Colloids (Lyophilic= solvent-loving)
Colloidal systems containing particles that have high affinity for the dispersion medium.
Lyophilic colloidal systems are usually obtained simply by dissolving the material in the solvent being used.
Solvation: the attachment of solvent molecules to the molecules of the dispersed phase.
In the case of hydrophilic colloids, in which water is the dispersion medium, this is termed hydration.
For example, the dissolution of organic materials such as acacia or gelatin in water (hydrophilic colloids).
Rubber and polystyrene form lyophilic colloids in non-aqueous, organic solvents (lipophilic colloids).

30. Lyophilic sols
are thermodynamically stable, they are reversible systems.
Stable generally in presence of electrolytes, may be salted out by high concentration of very soluble electrolytes
Molecules of the dispersed phase are solvated
Stability controlled by charge and solvation of particles (layer of solvent)
Molecules disperse spontaneously to form colloidal solution
Viscosity of the dispersion medium increased by the presence of dispersed phase
Ex, proteins, tragacanth and methylcellulose in water

31. Lyophobic Colloids Lyophobic (solvent-hating).
Colloidal systems containing particles that have little or no attraction for the dispersion medium.
Lyophobic colloids are generally composed of inorganic particles dispersed in water. Examples of such materials are gold, silver, sulfur, arsenous sulfide, and silver iodide.
Preparation methods are (a) dispersion methods, in which coarse particles are reduced in size, and (b) condensation (aggregation) methods, in which materials of subcolloidal dimensions are caused to aggregate into particles within the colloidal size range.
Supersaturation...
Lyophobic colloids are relatively less stable as weak forces of interactions exist between colloidal particles and liquid medium which result in the absence of a solvent sheath around the particle.
Emulsions are lyophobic colloids

32. Lyophobic sols
are thermodynamically unstable, the particles tend to aggregate, they are irreversible systems.
Unstable in presence of even small conc. of electrolytes leading to aggregation, depends on type, valency and concentration of counter ion of electrolyte
Stability controlled by charge of particles
Little interaction between particles and dispersion medium.
Viscosity of the dispersion medium is not affected by the presence of lyophobic colloidal particles.
 Ex, water-insoluble drugs, kaolin and oils form lyophobic dispersions

33. Association or amphiphilic colloids
Amphiphiles or surface-active agents.
The concentration of monomer at which micelles form is termed the critical micelle concentration (CMC).
The number of monomers that aggregate to form a micelle is known as the aggregation number of the micelle.
On dilution these colloids revert back to individual monomers.
The individual molecules forming the micelle are in dynamic equilibrium with those monomers in the bulk and at the interface.
Viscosity of the system increases as the conc. of micelles increase.
In aqueous solutions, the cmc is reduced by the addition of electrolytes

35. The surface active substances such as soaps and detergents belong to this class. For soaps the CMC is 10-4 to 10-3 mol/L.
Generally a lower CMC is preferable for parenteral drug administration as this will ensure that the micelles are not destabilised on dilution.
Lipid based surfactant polymorphism.

37. Surfactants are amphiphilic molecules that have a hydrophobic segment and a hydrophilic segment. At concentrations above their critical micellar concentration, surfactant molecules assemble into spherical associations called micelles.
HLB = hydrophilic lipophilic balance.
Dispersion phase consists of aggregates (micelles) of small ions whose size individually is below the colloidal range.
Hydrophilic or lipophilic portion of the molecule is solvated, depending on the dispersion medium
Because of their affinity for water and tendency to form micelles which are of colloidal dimension, they form hydrophilic colloidal sol in water but are usually classified separately as Association colloids
Colloidal aggregates are formed spontaneously when concentration of amphiphile exceeds the critical micelle concentration (cmc)
Viscosity of the system increases as the conc. of micelles increase.
In aqueous solutions, the cmc is reduced by the addition of electrolytes.

38. Functions of surfactants based on HLB no
HLB no = hydrophilic lipophilic balance number.

39. Common surfactants used in micellar formulation are either amphoteric (e.g., Lecithin), nonionic (e.g., Tween-80, Cremophor EL, Solutol HS 15, TPGS), block copolymers (e.g., poloxamers), or ionic (e.g., sodium lauryl sulfate). The amphiphilic polyethoxylated castor oil derivative Cremophor EL is one of the most frequently used surface-active formulation ingredients in parenteral dosage forms.
Surfactant micelles form only above their CMC, and rapidly break apart on dilution, which can result in premature leakage of the drug and its precipitation.
Polymeric micelles are generally much more stable than surfactant micelles, exhibiting lower CMCs, slower rates of dissociation, and longer retention of loaded drugs.

40. Disadvantages of surfactant micelles:
1- Toxicity associated with surfactants. In general, ionic surfactants have more toxic effects compared to nonionic.
Cremophor EL is known to produce hypersensitivity reactions in human.
2- On intravenous administration, owing to their surface activity, surfactant molecules at high concentration have the potential to penetrate and disrupt biological membranes and can be hemolytic.