The term “Disperse system” refers to a system in which one substance (dispersed phase) is distributed, in discrete units, throughout a second substance/phase called (continuous phase). or
Dispersed systems consist of at least two phases, the substance that is dispersed known as the dispersed (or) internal phase, and a continuous (or) external phase.
Dispersion system include following preparation used in pharmaceutical products.
Classification of Dispersion system
Based on the particle size of the dispersed phase, dispersions are generally classified as molecular dispersions, colloidal dispersions, and coarse dispersions.
Molecular dispersions have dispersed particles lower than 1.0 nm in size. Colloidal dispersions have particle sizes between 1 nm and 1mm. Microemulsions, nanoparticles, microspheres are some of the examples of colloidal dispersions. Coarse dispersions have a particle size greater than 1nm, which includes suspensions and emulsions.
In pharmacy, dispersions are found in a wide variety of dosage forms and in nearly all routes of drug administration. Examples range from solutions of very large molecules (macromolecules) such as albumin and polysaccharides to liquid suspensions of “nano”-sized crystals (nanocrystals) and of “micro”-sized droplets (microemulsions), to coarse (larger particle) emulsions and suspensions.
Coarse dispersions have a particle size greater than 1nm, which includes suspensions and emulsions.
Suspensions are a class of dispersed system in which a finely divided solid is dispersed uniformly in a liquid dispersion medium.
Typically, the suspensions with particle size greater than 1 μm are classified as coarse suspension, while those below 1 μm are classified as a colloidal suspension. When the particles constituting the internal phase of the suspension are therapeutically active, the suspension is known as pharmaceutical suspension. Depending on their intended route of delivery, pharmaceutical suspensions can be broadly classified as parenteral suspension, topical suspensions, and oral suspensions.
Ideally, the internal phase should be dispersed uniformly within the dispersion medium and should not sediment during storage.
An emulsion is a dispersion of at least two immiscible liquids, one of which is dispersed as droplets in the other liquid, and stabilized by an emulsifying agent. Two basic types of emulsions are the oil-in-water (O/W) and water-in-oil (W/O) emulsion. However, “double emulsions” or “multiple emulsions” can be made. These emulsions have an emulsion as the dispersed phase in a continuous phase and they can be either water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O)
The size of the dispersed droplet generally ranges from 1 to 100 μm, although some can be as small as 0.5 μm or as large as 500 μm. Emulsions are subdivided arbitrarily such as macro, mini, and microemulsions, based on the droplet size. In macroemulsions, the droplet size usually exceeds 10mm. In the case of mini emulsions, the droplets are in the size range of 0.1–10 μm, and in microemulsions, the droplets are below 100 nm.
Colloidal systems contain micelles, self-assembling colloidal systems with particle sizes normally ranging from 5 to 100 nm.
Micelles are spontaneously formed when amphiphilic molecules are placed in water at a certain concentration and temperature i.e. CMC (Critical Micelles Concentration).
At a low concentration, the molecules exist separately in a solution. When the concentration is increased, the molecules quickly self-assemble to form spherical micelles. (See Fig below). The hydrophobic portions of the molecules condense to form the core, whereas the hydrophilic portions constitute the shell or corona of the micelle.. The concentration at which micellar association start is called the critical micelle concentration (CMC) and the temperature below which amphiphilic molecules exist separately is known as critical micellization temperature (CMT).