colloids and suspensions Flashcards
How do we make a pharmaceutical suspension?
- we need the drug ! It must have small particles of uniform size
- if the drug is water-insoluble, we may add a wetting agent. This breaks the interfacial tension, ensuring the solid particles disperse easily throughout the liquid;
- interfacial tension is an energy barrier which prevents the liquid spreading around the solid
low vs high interfacial tension for making a good or bad suspension
low interfacial tension: the liquid spreads around the particle -> this should give us a good suspension
high interfacial tension: the liquid does not spread around the particle
this will give us a bad suspension
types of wetting agents
- surfactants
- hydrophilic colloids
- simple solvents
increased wetting of hydrophobic leads to a
decrease in surface tension
wetting agents also decrease adsorption of particles to the container … how?
by applying a repellent coating to the particles in the suspension
without a wetting, particles tend to cling to the container rather than being
dispersed
once we have a suspension we need to determine if it’s […] or […]
deflocculated versus flocculated
deflocculation vs flocculation
deflocculated - where the particle remain as separate units
flocculated - where the particles exist as loose aggregates
in a flocculated system, the aggregates settles quickly/slowly
quickly
in a flocculated system, the aggregates settles, what does this lead to in the sediment
liquid entrapment in the sediment, which tends to be fairly easy to redisperse
why are flocculated suspensions are better then deflocculated ones
in a deflocculated system, the rate of sedimentation depends on the particle size, but generally is slow
-> a slow rate of settling prevents liquid entrapment in the sediment, which becomes compact ‘caked’ and is very difficult to redisperse
flocculation system vs deflocculation system:
flocculated system:
- loose aggregates of particles
- large volume of final sediment
- rapid sedimentation rate
- suspension clears quickly
- entrapment of liquid within sediment
- easy to redisperse sediment
deflocculated system:
- particles exist as discrete units
- small volume of final sediment
- slow sedimentation rate
- suspension remains cloudy for a prolonged period
- liquid entrapment in the sediment is prevented
- difficult to redisperse sediment
Flocculated system | Deflocculated systems |
what is a dispersed system
A system in which one component is dispersed as particles or droplets throughout another component.
colloidal dispersions size:
dispersions in which the size of the dispersed particles in the continuous phase is in the range of 10-9 –* 10-6 m
is it a colloid or solution? e.g. if we add a powder of a drug to water
- In a true solution, the drug molecules are dispersed on the molecular scale in the solution – each molecule is isolated from all other drug molecules;
- In a colloid, we have particles of drug – aggregates consisting of many drug molecules.
pharmaceutical suspensions vs colloidal systems:
- In a pharmaceutical suspension, the solid’s particle size (in the liquid vehicle) is generally > 1μm;
- This is different to a colloidal system, where particle size is < 1 μm by definition.
- the principles of colloidal science apply to pharmaceutical suspensions
why do we use suspensions:
- Poorly soluble drugs cannot always be made into solutions;
- taste masking: unpleasant tastes may be less noticeable in suspension than in solution (e.g. paracetamol);
- more stable - drug if formulated as a suspension instead of a solution, or the drug may be more stable as a solid so we make a suspension just before dispensing
what makes a good suspension:
- the suspension must be easy to disperse upon shaking: this is called redispersibility
- the suspension should contain particles which are small and the same size: this is to ensure patients do not find the suspension to be gritty
- the suspension must be homogenous. for the period after shaking and removing the dose, the particles need to be evenly distributed throughout the liquid to ensure the same dose is given each time
if you add particles to a clear liquid, this changes how the light is:
- absorbed
- transmitted
- scattered
by the liquid
what is the Tyndall effect
if a beam of light is passed through a true solution, there is very little scattering of the light, and so the path of the beam cannot be seen;
- In a colloid, the particles scatter the beam of light and so you can see its path.
The Tyndall effect =
colloidal systems can be assessed based on how a beam of light behaves when it comes into contact with the system!
Light scattering makes colloidal systems look
cloudy or turbid
turbidity formula
https://www.notion.so/fundamentals-of-colloid-science-13300bb3982d81edb531d0385a15d7fa?pvs=4#13300bb3982d81c2b1eace7a2c4751d9
BROWNIAN motion in colloids
- motion is a kinetic property
as particles = so small in colloids, they undergo brownian motion - there will be random movement of the dispersed particles throughout the continuous phase
what is Brownian motion
- consider a solid - in - liquid colloid
- irregular and complication zigzag pattern
- random collisions with: solvent molecules: other particles; container wall
diffusion in colloids fick’s law
https://www.notion.so/fundamentals-of-colloid-science-13300bb3982d81edb531d0385a15d7fa?pvs=4#13300bb3982d812eb6d4cac318ca2a17
sedimentation
https://www.notion.so/fundamentals-of-colloid-science-13300bb3982d81edb531d0385a15d7fa?pvs=4#13300bb3982d81b19199ee04509716e5
aggregation =
the collection of particles into groups
- coagulation arises when the particles are…
closely aggregated and difficult to redisperse
in flocculation: the aggregates have a loose structure, in which the particles are
a small distance apart, and only weakly bound into groups
what is caking
- This is the formation of a densely pack, non-dispersible, aggregate at the bottom of the container in a suspension.
- It is very bad, because it results in a patient underdosing and then overdosing.
- Caking cannot be eliminated by reduction in particle size, or by increasing the viscosity of the continuous phase;
- Viscosity reducing agents delay sedimentation and caking, but do not prevent it;
- To prevent caking, we need to consider
flocculating agents
→ think about electrical properties
if we increase viscosity (n) of the liquid phase, the rate of sedimentation is…
reduced
examples of viscosity enhancing agents
- polysaccharides
- celluloses
- hydrated silicates
- carbomers and silicon dioxide
will viscosity enhancing agents stop caking
-> sedimentation is delayed not stoped, so caking will still happen eventually
flocs are loose aggregates with a large volume: flocs are loosely bound with some distance between the particles
flocculation tends to be a good thing, whereas caking is a bad thing;
To achieve flocculation, we often need to add additional components to a suspension
what do flocculating agents do?
minimise the extent of caking in a suspension
-> ideally we want a partially deflocculated system
how do flocculating agents work / Electrical properties of colloids
- The occurrence of coagulation and/or flocculation is driven by electrical forces;
- Particles tend to have a surface charge;
- In water, this is typically a negative charge
- flocculating agents can neutralise this etc
what do the electrical properties of colloids
this means that there will be lots of electrostatic interactions a particle and the other components in the system:
the negative charge at the particle at the surface will attract positive ions in solutions
which will in turn attract negative ions
this results in the electrical double layer
flocculation depends on what […] theory
DLVO
DLVO theory
-> VT = VA + VR
Vt = total potential energy of interaction
Va = potential energy of attraction
Vr = potential energy of repulsion
what does dvlo theory predict
-> this predicts the stability of charged particles in a dispersion to aggregation
Vr determines what
whether a partially deflocculated system is stable or not
When the particles in a colloid are close together, attractive forces dominate and we get an energy minimum: At intermediate distances, what happens
the double layers repel, and give an energy maximum
If this maximum is:
- LARGE the particles will
stay dispersed;
If this maximum is SMALL the particles will
aggregate.
DLVO theory = simplified to forces of attraction and repulsion
attraction arises from van der waals
repulsion arise from electrostatic forces
we can alter this in all sorts of ways e.g. by adding a surfactant
