Colloids Flashcards
What is the difference between colloids and suspensions?
Colloids:
- small
-filtration possible
-do not settle
Suspensions:
-large size
-filtration possible
-always settle
What are gelling agents?
Pharmaceutical excipient employed in making gels
Capable of undergoing high degree of cross-linking and entanglement in dispersion medium
Do gelling agents increase or decrease viscosity of gelling agent?
Increase
What is a two-phase system gelling agent?
Mostly inorganic gels - tendency to cross-link and solidify upon standing and liquify under stress like shaking
What is a one-phase system gelling agent?
Common gels in pharmacy - linear or branched polymer macromolecules that dissolve in water are used as gelling agents
What is the usual concentration of gelling agents?
0.5-5%
Some up to 10%
What are the types of gelling agents?
Natural polymers
Semi-synthetic polymers
Synthetic polymers
examples of natural polymers
Acacia, alginic acid and tragacanth
Examples of semi-synthetic polymers
Cellulose derivatives like methylcellulose
Examples of synthetics polymers
Carbomers or polaxamers
Higher gelling agent = _____
Stiffer (more viscous) gel
Alginic acid is obtained from____
Seaweed
What strength of gels is alginic acid used in and how long does it take to disperse in water?
1-5% and 30 minutes
What is the strength of sodium alginate?
10%
What must be added to alginic acid?
Preservatives because they are prone to microbial growth
Tragacanth is used for gels that are stable at pH ____
4-8
Tragacanth requires addition of ______
Preservatives
How do we prevent lumps from forming for powdered tragacanth in water?
Aqueous dispersions are prepared or wetting agents like glycerin are used
At what pH do semi-synthetic agents maintain viscosity?
3-11
EXCEPT carboxylmethyl cellulose - 7-9
What kind of gels does methylcullulose make?
Thinner gels
What does methylcellulose have a high tolerance for?
Added drugs and salts
Methylcellulose has good compatibility with _____
Water, alcohol and propylene glycol
What does cooling Methylcellulose for an hour do?
Improves clarity and viscosity
What kind of gels does hydroxypropylmethyl cellulose(HPMC) make?
Thicker gels
What is HPMC compatible with?
Water and alcohol
HPMC disperses good in _____
Cool water
What is HPMC a good gelling agent for?
Time release preparations
What are carbomers?
High bulk density and forms acidic aqueous solutions (pH 3)
At what pH do carbomers thicken?
5-6, and become 1000x original volume
For carbomers:
When dispersed - solution of pH is ____ - ______ to increase pH- _____ viscosity
Low
Neutralizer
Increase
What are poloxamers?
Copopolymers of polyethylene and polyoxypropylene
It is an absorption enhancing topical gel
What is poloxamer gel base widely used for?
Extemporaneous compounding
What do poloxamers form?
Reverse thermal gels (dissolve in cold water or by cooling overnight)
What concentration are poloxamers used in?
15-50%
When are gels low viscosity and high viscosity?
Low temp = liquids (low viscosity)
High temp = gels (high viscosity)
What is PLO gel?
Poloxamer combined with lecithin and isopropyl palmitate
What is the most common compound made in a pharmacy using PLO gel?
Diclofenac 10%
For PLO gel what is the aqueous phase and the organic phase?
Aqueous phase = pluronic
Organic phase = lecithin
How does clumping occur in the preparation of gels?
Too rapid addition without adequate mixing
Outer molecule of the gelling agent contact the medium first
They hydrate forming a layer with the gelled surface that is difficult for medium to penetrate
How can we minimize the problem of clumping?
Sift the powder into the vortex of stirring medium
Levitate the powder with water miscible non solvent like alcohol, glycerin or propylene glycol
Use a blender to homogenously mix the powder and solvent
How long do most gelling agents require to completely hydrate and reach maximum viscosity and clarity?
24-48 hours
Should the gel be formed first, or can you add the drug first and then the gel be formed?
The active drug may be added before or after the gel is formed
BUT preference is prior to forming the gel
Characteristics of lyophobic/hydrophobic colloids:
No or little affinity to the dispersion medium (water)
No change in systems viscosity
Maintain dispersion due to mutual repulsion and Brownian movement
Generally inorganic molecules
Thermodynamically unstable
What are examples of lyophobic colloids?
Colloidal gold (sol)
Colloidal sulfur - used as antimicrobial in anti-acne mask
Colloidal silver - toxic
