Flashcards in Lecture 10 Deck (69)
What are disperse systems?
systems which consist of 2 phases:
1) dispersed/ internal phase which is dispersed throughout the continuous phase
2) continous phase
How can dispersed systems be classified?
on the basis of their particle size or on the nature of the dispersed phase
What can particle size range from?
atomic, molecular to mm size
What kinds of dispersion systems are there?
What is a molecular dispersion
where particle sieze fast diffusion e.g. Oxygen molecules, glucose
What is a colloidal dispersion?
Particle size 1nm-500nm
cannot be seen by ordinary microscope but can be seen through electron microscopes.
Particles can Pass through filter paper but not through semi permeable membrane --> slow diffusion
E.g. Cheese butter paint milk
What is a corase dispersion?
Particle size >500nm.
Easily visible in ordinary microscope,
Can't pass through filter paper or semipermeable membrane as particle sizes are too big, cannot diffuse,
e.g, pharmaceutical emulsions and suspensions
How are colloidal systems dispersed?
uniformly in the external phase
what are colloidal systems of topical interest?
most drug systems are colloidal carriers
how can colloidal systems be classifieid?
on the nature of the dispersed phase and the nature of the dispersed medium
What are liquid aerosols?
a colloidal system in which the dispersion medium is a gas and the dispersed phase is a liquid
e.g. fog, clouds
what are solid aerosols?
a colloidal system in which the dispersion medium is a gas and the dispersed phase is a solid
what are foams?
a colloidal system where the dispersed medium is liquid and the dispersed phase is a gas
e.g. whipped cream, soda water
what is an emulsion?
a colloidal system in which the dispersed medium and the dispersed phase are both liquids e.g. milk, cream
What is a sol?
a colloidal system in which the dispersed medium is a liquid and the dispersed phase is a solid
e.g. paint, cell fluids
What is a solid foam?
a colloidal system in which the dispersed medium is a solid and the dispersed phase is a gas
e.g. pumice, plastic foams
what is a gel?
a colloidal system in which the dispersed medium is a solid and the dispersed phase is a liuid
What is a solid sol?
a colloidal system in which the dispersed medium and the dispersed phase are both solids
e.g. ruby glass, pearls, opals.
How can colloids be separated from molecular dispersions?
by dialysis which uses a semipermeable or cellophane membrane
the colloids cant pass through, but the molecular particles can.
What are the applications of dialysis?
used to remove impurities from the blood
often done artificially for people suffering kidney malfunction
the glomerular apparatus is like a semipermeable memebrane that does this naturally
How can the speed of dialysis be enhanced?
by applying external forces e.g. an electric current.
this is now called electrodialysis.
can also use pressure. (this is now called ultrafiltration)
What causes a colloidal particle to take on multiple forms?
the type of environment it is in.
particles in a friendly environment will disperse properly, but in an unfriendly environment, will reduce its surface area
WHat types of colloidal systems are there?
lyophobic, lyophilic and amphiphilic
What are lyophilic colloidal systems?
dispersed phase: large organic molecules within colloidal size
preparation: spontaneous by dissolving in solvent and simple mixing
viscosity: increases as concentration increases. (gels may form at a certain concentration)
Effect of electrolytes: stable in the presence of electrolytes. if high concentration of electrolyes, deviation/salting out may occur
easy to make
solvates fast due to attraction between 2 phases
If dispersion medium is water, the phenomena is hydration
can also be used for organic molecules e.g. acacia or gelatine in water
What is lyophobic colloidal system?
Dispersed phase: Inorganic particles dispersed in water. E.g. Gold or silver
Solvation: little (The dispersed phase has very little affinity for the dispersion medium.)
Preparation: requires special procedure (condensation/aggregation or dispersion method)
Viscosity: not greatly increased due to unsolvation
Effect of electrolytes: sensitive to added electrolyte
What is the dispersion method?
method of colloidal preparation where you try to reduce coarse particle to colloidal range by grinding particles using a colloidal mill.
This consists of 2 blades rotating in opposite direction.
The coarse particles are placed inside and grinded to form lyophobic colloids.
What is the condensation/aggregation method?
Method of colloidal preparation where we use the materials which are in a sub colloidal range and try to make them aggregate to colloidal range.
The main conditions required is supersaturation which is achieved by reducing the temperature
This is followed by formation and growth of nuclei.
Can use this method to prepare the colloids, as well as change the dispersion
What are association/amphiphilic colloids?
Dispersed phase: aggregates of small organic molecules or ions below the colloidal size.
Solvation: hydrophilic OR lipophilic portion of the molecule is solvated depending on if the medium is aqueous or not.
preparation: spontaneous when concentration of amphiphiles exceed the CMC
Viscosity: increases as concentration of amphiphiles increase and as micelle numbers increase and become asymmetric
Effect of electrolytes: CMC is reduced and salting out occurs at high salt concentrations
Have characteristics of having 2 different types of component in the same molecule.
These have opposing solution affinities within the same molecules or ions
e.g. Head is hydrophilic in nature whereas the tail is hydrophobic in nature so has an affinity for the non aqueous phase.
How do amphiphiles act in low concentrations?
they exist separately
What happens to amphiphiles if concentrations are increased?
they will form aggregates aka micelles
What dictates which form the micelles will be in?
if environment is aqueous, the micelles will orient so that the heads point towards the outside as that is the hydrophilic portion, and the hydrophobic tails orient towards the inside.
If the environment is non polar, the tail will protrude out and the heads will be in the centre
What causes the formation of laminar micelles?
when you further increase the concentration, they will form an arrangement like a sheet
What types of amphiphiles are there?
anionic, cationic, non ionic, amphilytic (both + and _ charge
What is CMC?
critical micelle concentration
this is the concentration of surface active agents at which micells will form.
How do micelles form?
As you add the surfactant into the solvent, it will try to attach to the air water interphase if it is water.
The head is facing down so it is solving loving.
First it tries to clump at the interphase.
But when concentration is increased, they will try to aggregate.
When CMC is achieved then they will start aggregating in spherical globules called micelles
How is viscosity increased?
if you increase the concentration, the viscosity also increases.
at a certain concentration it will start forming a gel
What is the tyndall effect?
light scattering by particles in a colloid or particles in a fine suspension.
What is turbidity?
fractional decrease in intensity of scatters light as the incident light passes through 1 cm of solution
the total colloidal system appears turbid because it scatters light which is shone through it. It can do this because of the particles which are dispersed in a continuous medium
Why can the path of light only be seen in colloidal dispersions?
other solutions like water, have no particles so cant see the path of light
What are some examples of the tyndal effect?
when the sun rays pass through the window and you can see dust particles
in a foggy day you can see a beam of light from the car headlight
dust particles dispersed in the air can scatter the beam of light
How is the tyndall effect applicable to pharmaceutical science?
It helps to estimate molecular size as in a given sample the turbidity is proportional to the molecular weight
there are a lot of instruments based on this principle of light scattering
What are the different kinds of motion a dispersed medium can be subjected to?
what is brownian motion?
the random movement of particles in the dispersion medium
When dispersed, they tend to colloide witheach other and move in an irregular zigzag path.
What is the relationship between velocity and particle size?
velocity increases as particle size decreases and viscositiy decreases
what happens if you increase the viscosity of the dispersion medium?
it can decrease the brownian motion and eventually stop it
What causes diffusion?
as a result of brownian motion, particles will diffuse from a region of high concentration to a region of low concentration
What does the negative sign of the diffusion equation mean?
it indicates that diffusion occurs down a concentration gradient
What is sedimentation?
In the dispersed system where the particles are dispersed in continuous phase, there is some chance the particles will settle down
If particle size is small, the brownian motion is significant and will prevent sedimentation as well
But if the particle size increases due to aggregation of sampling they will try to settle down and result in sedimentation.
why is it difficult to sediment colloidal solutions if they are in the colloidal range?
particles are in constant motion all the time
What can we do if we want to measure something which requires sedimentation first?
we can promote sedimentation by using an ultracentrifuge (more than 10,000rpm)
What is viscosity?
the resistance to flow of a system under an applied pressure.
What happens if a colloidal dispersion is very viscous?
more force is required to make it flow
How is viscosity useful?
This provides useful information about the shape and molecular weight of the colloidal particles
What causes electrical properties of a colloidal dispersion?
the presence of charge on the surface of colloidal particles
What is surface charge caused by
being in contact with aqueous medium.
Can be caused by ion dissolutions, ionisation or ion adsorption
What is ion dissolution?
like surface charges, colloidal particles are controlled by the charges of the ions present in excess in the medium
e.g. silver iodide, if silver ions are more abundant in concentration, there will be an overall positive charge
if iodide ions are in excess, there will be an overall negative charge
What is ionisation?
drugs in solution tend to be ionised
what is ion adsoprtion?
anything which is stuck onto the surface
in general, cations are more hydrated in nature, so stay in the bulk water
anions are less hydrated, so are adsorbed onto the particle surface
What constitutes a stable colloidal system?
one where the dispersion phase does not aggregate and have a long shelf life
There should be a balance between attractive and repulsion forces,
if all particles have mutual repulsion, they will remain stable,
What happens if particles have little or no repulsive forces?
they may aggregate, and their properties will change
What is aggregation?
the collection of particles into groups
what are the 2 types of aggregation?
what is coagulation?
if particles are closely aggregated, they are difficult to redisperse
what is flocculation?
where the aggregate have an open structure in which the particles remain a small distance from one another
How can a colloidal system be stabilised?
by providing the dispersed phase with some form of charge. E.g. Adding an electrolyte
We can also Surround each particle with protective sheath that prevents mutual adherence so they remain repelled and do not attract each other
What is solubilisation?
The ability of micelles to increase the solubility of materials that are normally insoluble or partially soluble in the dispersion medium. Particularly drugs.
Why can micelles solubilise?
due to their hydrophilic head and hydrophobic tail.
The amount of material that can be solubilised by a given amount of surfactant which form micelles is a function of the polar non Polar characteristics of the surfactant HLB and the molecule being solubilised.
What is HLB?
hydrophilic lipophilic balance.
Describes the relative polarity of a Surfactant
Determined by calculating values for the different regions of the molecule