Gels and Magmas

Pharmacist Ghada Hamid

Gels and Magmas
They are defined as semisolid systems consisting of dispersions made up of either small inorganic particles or large organic molecules enclosing and interpenetrated by a liquid.
They are also defined as semi-rigid systems in which the movement of the dispersing medium is restricted by an interlacing three dimensional network of particles or solvated macromolecules of the dispersed phase.
Gels may be
• Clear as water or,
• Turbid because of that ingredients may not be molecularly dispersed or they may form aggregates which disperse light.

Terminology related to gels:
• Imbibition: is the taking up of a certain amount of liquid without a measurable increase in volume.
• Swelling: is the taking up of a liquid by a gel with an increase in volume, only liquids that solvate a gel can cause swelling.
• Synersis: occurs when the interaction between particles of the dispersed phase becomes so great that on standing, the dispersion medium is squeezed out in droplets and the gel shrinks, it depends on the conc. of gelling agent.
• Aging: is the slow spontaneous aggregation which results in formation of denser network of gelling agent.
Separation of solvent phase occur because of the elastic contraction of polymeric molecules, in the swelling process during gel formation the macromolecules become stretched and the elastic forces increase as a swelling proceeds. At equilibrium the restoring force of the macromolecules is balanced by the swelling forces determined by osmotic pressure, if the osmotic pressure decreases as on cooling, water may be squeezed out of the gel.

Classification and types of gels:
We have two classification, the first one:
? Single phase gel: gels in which macromolecules are distributed so that no apparent boundaries exist between them, it also called organic gels e.g carbopol, tragacanth.
? Two phase gel: gels consist of floccules of small, distinct particles, and frequently called a magma gels or inorganic gels e.g Aluminium hydroxide gel , bentonite magma.

Another classification is based on the type of continuous phase:
? Hydrogels : includes ingredients that are dispersible as colloidals or soluble in water they include organic hydrogels, natural and synthetic gums and inorganic hydrogels.
? Organogels include hydroxylcarbons, animal and vegetable fats, and hydrophilic organogels.
? Xerogels: gels in which vehicle has been removed leaving a polymer network.

Consideration of pharmaceutical gels:
1. Choice of vehicles.
2. Inclusion of buffers.
3. Preservatives.
4. Antioxidants.
5. Flavoring and coloring agents.

Gelling agents: these are substances which when added to an aqueous mixture leads to increase its viscosity without modifying its other properties. They are of 3 types:
• Natural polymers e.g proteins and polysaccharides.
• Semisynthetic polymers e.g cellulose derivatives.
• Synthetic polymers.