solutions, suspensions & emulsions Flashcards
what is a solution contain molecules of
solute and solvent
what does a suspension contain particles and molecules of what?
continuous phase e.g. water
what is an emulsion formed of
one liquid dispersed in another
solution details:
- is the drug dissolved
- whats its visual appearance
- particle/droplet size
- dissolved
- clear
- non
suspension details:
- is the drug dissolved
- whats its visual appearance
- particle/droplet size
- not dissolved
- cloudy unless particles are very small (<1um)
- 0.1-10 um
emulsion details:
- is the drug dissolved
- whats its visual appearance
- particle/droplet size
- can be dissolved in oil / oil can be the drug
- transluscent to opaque, depending on droplet size
0.1-25 um
advantages of iquids:
advantages:
- easy dose adjustment
- easy to apply/inject e.g. fungal drops on nails, depot preparations
- rapid action by oral/intravenous route
- liquids are easier to manufacture
- easily swallowed
disadvantages of liquids
- bulky/inconvenient if container breaks
- taste of drug more pronounced
- media for microbial growth
- accurate dosing more difficult
- stability of ingredients = poorer
what is a solution
a molecular dispersion of a solute in a solvent
In general, the solvent is present in greater amount, but there are exceptions.
whats the difference between aq solutions and non aq solutions
Aqueous solutions: water is the most widely used solvent
Non-aqueous solutions: solvents include oils (e.g. almond oil, castor oil,
ethyl oleate), alcohols (e.g. ethanol, polyethylene glycol (PEG) 400, glycerol), liquid paraffin, isopropyl myristate, isopropyl palmitate.
examples of pharmaceutical solutions
ear drops, enemas, nasal products, elixir,mixtures, gargles, mouthwashes
drug dissolution process
- a solute molecule is removed from its crystal
- a cavity for the molecule is created in the solvent
- the solute molecule is inserted into the cavity
why is water more widely used as a solvent in pharmacy
physiologically compatible, not toxic
how can we increase drug solubility?
- pH of medium (acidic drugs dissolve in basic medium and vice versa)
- use mixed solvents
- cyclodextrins as solubilising agents (CD - enzymatically modified starches
Ring is cylindrical, outer surface hydrophilic, internal cavity non-polar.
Appropriately sized lipophilic molecules can be accommodated wholly or partially) - use of surfactants as solubilises
- use of additives like salts
what is a surfactant
compound whose structure contains 2 separate regions, a hydrophilic region and a hydrophobic region
Surfactants self-assemble in a liquid medium to shield the liquid-hating part. what is this assembly called?
a micelle
A hydrophilic/hydrophobic drug can dissolve in the hydrophobic micelle interior
hydrophobic drug
what is a suspension
a dispersion of finely divided insoluble solid particles in a fluid
characteristics of suspensions
- fluid generally water
- solid diameter up to 1um is termed a colloidal suspension
- to administer insoluble/poorly soluble drug
- large surface area of drug ensures fast dissolution and absorption
- also ok for parenteral, ophthalmic use, topical application
suspensions xtra details
- must not be too viscous
- desirable qualities
- suspended material should not settle too rapidly
- particles that do settle to the bottom of container shouldn’t form a hard mass, but be readily dispersed when container shaken
what is a stable suspension like
- particles do not aggregate
- particles remain uniformly distributed. if they do settle they should be easily redispersed by moderate shaking
sedimentation rate is governed by stokes law. what is that?
velocity of sedimentation = 2gr^2(pd-pc) / 9n
g - acceleration due to gravity
r - radius of particle
pd - density of particle
pc - density of fluid
n - viscosity of fluid
a good suspension must be re-dispersible upon shaking: this is not possible if particles have formed a cake. how do we prevent caking?
to stop caking: form flocs of the solid particles in a controlled manner - controlled flocculation
caking formation is essentially stopped by preventing the particles from coming so close to each other
if there is a larger floc, what will there also be
faster sedimentation rate, as each floc will act as one unit
-> greater sediment volume
therefore no cake as liquid entrapped
among particles, latter do not come close to one another, and the suspension is easy to redisperse
what are emulsions
opaque, consisting of two immiscible liquids, one of which is finely subdivided, and uniformly distributed as droplets throughout the other
whats the disperse phase
phase that is subdivided
whats the continuous phase
phase in which the disperse phase is distributed
phase volume ratios can be 50/50 etc.. what is the ratio of stable emulsions
generally the most stable emulsions have an internal phase which occupies 40-60% of the emulsion
w/o vs o/w
water in oil vs oil in water
multiple emulsions
w/o/w and o/w/o
microemulsions e.g.
water, oil, and amphiphile
these are clear and stable
pharmaceutical applications of emulsions
emulsions more palatable for oral application
o/w - less greasy and more cosmetically acceptable for lotioning
emulsions with lots of fats used for their high calorific value
physics instability of emulsions shows in:
creaming (opposite of sedimentation, reversible on shaking)
flocculation (less easily reversible)
coalescence (cracking/breaking): irreversible
what law governs creaming
stokes law
(2gr^2 (pd-pc)/9n)
about the chemical instability of emulsions:
- ionic surfactants are often incompatible with materials of opposite charge
- in some cases, phase inversion may occur rather than emulsification
- changes in pH may break emulsion
how changing temperature affects creaming and coalescence
↑Temp. ↓ apparent viscosity of continuous phase ↑ creaming
↑Temp. ↑kinetic motion of dispersed droplets - ↑collisions - ↑potential for creaming.
↑Temp. ↑kinetic motion of emulsifying agents at water-oil interface - a more expanded monolayer - coalescence more likely.
↑Temp certain macromolecular emulsifiers may be coagulated
effect of freezing an o/w
Freezing an o/w - aq. phase freezes - formation of ice crystals - oil droplets are pressed together and against the ice crystals with considerable pressures. This can result in rupture of interfacial film and drop coalescence. Also, any dissolved salt may become locally highly concentrated and eventually crystallise out.
Freezing an o/w - dissolved electrolyte may concentrate in the unfrozen water - affects charge density on globules.
Certain emulsifiers may precipitate at low temp.
preservatives should be used in emulsions: what characteristic do they have to have?
has to be soluble in the aqueous phase
