Colloid Flashcards
what is the definition of colloids?
a dispersed phase within a continuous phase
can be solid, liquid or gas
has a high surface-to-volume or weight ratio d
why are colloids barely affected by gravity
the mass is too small for gravitational force to be negligible but long lengths of time, gravity cannot be neglected.
what are lyophilic and lyophobic colloids
lyophilic colloids have strong interaction with continuous-phase thermodynamically stable
lyophobic have no interaction with continuous phase, can be kinetically or thermo stable
what is the specific surface area (As) equation
area divided by density x volume
name the two processes to make S/L colloids
degradation (grinding up coarse powder to get smaller particles)
Aggregation process (build-up from small molecule eg polymerisation)
for the dispersion phase, nucleation and particle growth
what is Ostwald ripening
the formation of large sols
small sols are more soluble than large sol so small sols dissolve preferably and redeposit onto the larger particles
CaCO3 can dissolve in water and is an example
name the 3 purification methods of s/l colloids with a brief explanation for each
1) equilibrium dialysis
colloids in a semi-permeable cellulosic membrane and the porous membrane only allows solvent/ions to pass
2) centrifugation
the sample is centrifugated and colloids clump together and the solvent is removed and replaced with a fresh solvent, the colloids redispose
3) ultrafiltration
microporus membrane is placed and only ions can pass
how does transmission electron microscopy work
- electrons are beamed at the sample
- the electron hit the sample and scatter
- the scattered electrons are detected by detector
the high vacuum chamber increases the mean free path of electron
pro and cons of transmission electron microscopy
-covers colloids size
-good magnification and resolution
- rapid processing
-only 2D image
-vacuum can destroy the sample
- very expensive
what are the two ways of light scattering
static light scattering (used to measure weight averaged molecular weigh)
dynamic light scattering (used to measure particle diameter)
why does dynamic light scattering oversize the diameter compared to TEM
DLS measures the hydrodynamic diameter so when the colloid is in the wet state, the polymer chain on the outside of the colloid, extend and in TEM they are in the dry state and the chain are more compacted
pros and cons of dynamic light scattering
-very quick
-covers the entire colloid size #
-well suited to sizing near monodispersed
-non-destructive
-sensitive to dust particles
-multiple scattering if the colloid is too concentrated
-light absorption by coloured particles can cause heating
what is the reason for high particle collision rates and what does it lead to
due to constant brownian motion and it leads to the possibility of sticky collisions which lead to coagulation
what are the three origins of surface charge
1) ion adsorption
2) ionisation
3) ion dissolution
what is the schulze-hardy rule
lyophobic sol xan coagulated by the addition of electrolyte
minimum critical concentration of added electrolyte required to induce coagulation depends on valency of counter ion
Al 3+ very little is needed to cause coagulation compared to Li+
