Disperse Systems - Surfactants and Suspending Agents Flashcards
Surface tension
Water molecules attract each other hydrogen bond pulling away from air
Surfactant
- Surface active agent
- Amphatic molecule that consists of a hydrophilic head and a hydrophobic tail
- Accumulate at interfaces
- molecule that self assembles
Surfactant uses
- In emulsions and solution
- aggregated suracant used in drug delivery vehicles
- Solublises insoluable drugs
Surfacant in oil-water interface
- Adsobption at lower interfacial tension
- Maintain dispersion of oil in to small droplets
- Maintains particles in a dispersed state
Ionic surfacant
- Anatonic (dissacosiation at high pH)
- Catatonic (dissacosiation at low pH)
Non ionic surfacant
- less toxic and irritant compared to ionic surfacant
Anionic
- Negatively charged surfacant with positive counter ion
- Freely soluable in water
- Forms self emulsifying base with Fatty alcohols
Cationic
- Positively charged surfacant and negatively charged counterion
- Freely soluable in water
- Has antimicrobial properties
Non Ionic
- uncharged
- alkyl chain linked to polyoxyethylene glycol
- Spans - Sorbitan fatty acid esters
Zwitterion
- 2 charges overall neutral
Co surfacants
- Achieve low interfacial tension, 2nd co surfacant added
- Usually short alcohols or amines
- Helps form and stablise micelles
- Enables interface to become more fluid
- Form efficiently packed film cover whole droplet
Micelles
- Non-ionic surfactants dissolved in water
- Colloidal spherical nanoparticles
- 100 sufcants into single entity
- Dynamic break up and reform - slow
- Indavidual monommer leave and reform - fast
Formation of micelles
- 60 randomly positioned molecules of polysorbate 80 cluster to form
- undergo reconstructing to yeild final equalibrium
- hydrogen bonding which causes flickering clusters and highly ordered water molecules ‘cages’
Critical packing parameter
V/lca
v = volume
lc = Extended chain length
a = cross sectional area
Dispersion of lipid in water
Each lipid molecule forces surounding water molecules to become ordered
Cluster of lipid molecule
Only lipid potion at edge of cluster are ordered few molecule order increase entropy
Micelles in water
All hydrophobic groups are sequested from water no highy ordred shells of water so high entropy
Water with hydrophilic head group
- Head group repel eachother when bought close together stops growth of micelle
- Strong repulsion in ionic smaller micelles
- weaker repulsion in non-ionic larger micelles
Critical micelle concentration
(CMC)
- The surfacant concentration at which micelles are formed
- Determined by measuring the surface tension at diffrent surfacants
Surface tension and CMC
Above CMC the surface tension of the solution is constent
Osmotic pressure and CMC
- Dependant on the number of molecules of water rate slowly decreases after CMC
Conductivity and CMC
- At CMC theres an abrubt decrease of the current carrier anions to decrease
Light scattering and CMC
- Sharp increase from CMC point from free surfacant to micelle only occur at micelle formation
Drug solubility and CMC
- Micelles enable drug solubility after CMC point
Purpose of micelle
- Enables material that were insoluable in water to become soluable by incorperating into a micelle
Suspending agent
- Reduce rate of settling of suspened active ingreedients
- Redily resuspended before use
- Increase viscosity of suspension
Ideal quality of suspending agent
- Readily and uniformly incorporated into formulation
- Readily dispersed in water without special techniques
- Ensure the formation of a loosely packed system which does not cake
- Does not influence the dissolution rate or absorption rate of the drug
- Be inert, non-toxic and free from incompatibilities
