Colloidal Dispersions 1 Flashcards
What are the properties of colloidal dispersions?
Very small particles (1-500 nm) dispersed in a continuous phase
Very large specific surface (the surface area per unit weight or volume of material)
The color of colloidal dispersions is related to the size of the particles
Not pass through semi-permeable membrane (dialysis, ultrafiltration, electrodialysis)Not visible by naked eye
Have light scattering effects
Not resolve by optical microscope
Visible in electron microscope
Diffuse slowly, exhibits characteristics of molecular solutions (Brownian motion, diffusion)
Not completely dissolved in “solvent” (external phase)
What are the types of colloidal systems?
- Lyophilic (solvent loving)
- Lyophobic (solvent hating)
- Association (amphiphilic)
What is a lyophilic colloid?
Have a strong affinity with their medium
Form colloidal dispersions, or sols, with relative ease.
Thermodynamically stable
The attraction between the dispersed phase and the dispersion medium, which leads to solvation (he attachment of solvent and dispersed phase molecules )
Hydration: the attachment of media (water) molecule to the dispersed phase
Spontaneous formation
E.g. Jello from gelatin,
What is a lyophobic colloid?
Little attraction between particle and media
Thermodynamically unstable
Need a special methods to prepare
Dispersion methods: Ultrasonication, Colloidal Mill, Condensation methods:
e.g. colloidal Silver
What is an association colloid?
Formed by the association of dissolved molecules to create larger particles (colloidal dimension), micelle, liposomes and micro-emulsions
Thermodynamically stable
Micelles
Amphiphiles or surface-active agents, have two distinct regions of opposing solution affinities within the same molecule.
When present in a liquid medium at low concentrations, the amphiphiles exist separately (sub-colloidal)
As the concentration is increased, aggregation occurs over a narrow concentration range (50 or more monomers)
These aggregates called micelles
Shapes of micelles
a) spherical micelle in aqueous media
(b) reversed
micelle in non-aqueous media
(c) laminar micelle, formed at higher amphiphile concentration in
aqueous media.
Critical Micelle Concentration (CMC)
Critical micelle concentration (CMC) is the concentration of surfactant monomer at which micelle starts to forms. At CMC, the surface is saturated with surfactants and any further increase in concentration leads the formation of micelle in the bulk.
Aggregation number: The number of monomers that aggregate to form a micelle
CMC and Colloidal Properties
Surface tension Osmotic pressure Light scattering Drug solubility Conductivity Density Detergency
Amphiphiles
Amphiphile : a chemical that have both hydrophilic (water-loving, polar) and lipophilic (fat-loving) properties.
Amphiphiles may be anionic, cationic, nonionic, or ampholytic (zwitterionic)
How to predict the CMC of a mixture of amphiphiles
Mixtures of two or more amphiphiles are usual in pharmaceutical formulations Assuming an ideal mixture we are able to predict the CMC of the mixture:
1/CMC= x1/ CMC1 + x2/ CMC2
CMC1 and 2: CMC values of the pure amphiphiles
X 1and 2: mole fractions of each in the mixture
Solubilization (by micelle)
A: benzene and toluene: nonpolar
B: Salicylic acid: “amphiphilic”
C: para-hydroxy-benzene: more polar
Solubilization is a important micelle application in drug formulation
(refer to slide 23)
CERNEVIT (BAXTER)
Multivitamins for i.v. infusion
Mixed micelles formed with glycocholic acid and lecithin
Retinol palmitate. Colecalciferol DL-alpha-tocopherol Ascorbic acid Cocarboxylase tetrahydrate Riboflavin dihydrated sodium phosphate. Pyridoxine hydrochloride. Cyanocobalamin Folic acid. Dexpanthenol D-Biotin Nicotinamide
Properties of Colloids
Optical Properties of Colloids The Faraday-Tyndall effects Electron microscope Light scattering Kinetic Properties of Colloids Brownian motion Diffusion Osmotic pressure Sedimentation Viscosity Electrical Properties of Colloids Electrokinetic phenomena Stability of colloid systems Sensitization and protective colloidal action
Types of Electron Microscopy
Transmission electron microscopy (TEM)
Scanning electron microscopy (SEM)
Reflection electron microscope (REM)
Scanning transmission electron microscope (STEM)
