colloid chemistry Flashcards
Adsorption is spontaneous which means that adsorption is…….
Enthalpically driven and is very strong and irreversible.
S<0 G<0 therfore H«0
what is Г
adsorbed amount (mg) of polymer per m2 of substrate
why dont homopolymers adsorb strongly to surfaces?
monomer repeat unit prefers either solvent or surface not both
why do copolymers adsorb more strongly to surfaces
contain 2 monomers in which 1 is selected to promote adsorption and the other to remain solvated
What the principles of polymer adsorption?
- thicker polymer layers obtained if polymer is in a good solvent environment for loops.
- weakly adsorbed polymers can be desorbed at high dilution by using a good solvent.
- Г for an adsorbed polymer is between 0.1-0.3 mg m-2
- Ads promoted by less solvency, specific poly- surface int and increased surface area.
- ads confined to a single layer. get multi layer ads when H bonded multi molecular aggregates in sol
what methods are used to prevent aggregation of S/L colloids?
i) charge stabilisation
ii) Steric stabilisation (involves coating colloid particles with adsorbed polymer)
what are the two opposing forces in steric stabilisation?
Long ranger - wins when R is large eg VDW
Short range - wins when R is small eg steric repulsion
in steric stabilisation what happens if a poor solvent is used?
get poly - poly int rather than poly- solvent int
why in steric stabilisation are the dispersion flocculated and inter particle collisions are sticky?
Thermal energy of partcles is no longer sufficient in overcoming Δεflocc
advantages/dis of steric stabilisation
poor theoretical understanding at present
works well for aq and non aq systems
insensitive to electrolytes
Works well at v high colloid conc
advantages/dis of charge stabilisation
DVLO theory is well developed
works well in aq and aq mixed systems
v sensitive to electrolytes
therefore steric stabilisations more useful
what is the criteria for effective steric stabilisation?
strong exothermic adsorption thick adsorbed layer complete coverage of colloidal particle good solvency for adsrobed polymer layer (1 and 4 are mutually exclusive for homopolymers so these generally do not make good steric stabilisers)
what polymers make good steric stabilisers?
statistical, block or graft polymers
describe the basic principles of surfactant micellisation
- X is the CMC
- surfactant is in dynamic equilibrium with surfactant inside micelle
- Hydrophobic alkly chains escape water by hiding inside micelle core
- at conc above X get free surfactant molecule and micelle in co existence.
what is the effect on increasing surfactant conc on micellisation of surfactants?
lowers surface tension, after x surfactants that adsorb at air/water interface get micellisation.
how does alkyl chain length affect CMC
Increase R decreases CMC
How does temp affect CMC?
High temps cause CMC to increase. Micelle structure disrupted at higher temp
how does the addition of an electrolyte affect CMC?
decreases CMC. additional ions screen repulsive forces between surfactant head and groups
how does binary mixtures effect CMC of surfactant?
synergistic effect: Lower CMC for binary mixture than for either pure surfactant
how do organic molecules effect CMC?
Depends on effect on water structure: If it aids H bonding then decreases CMC
destorys H bonding then CMC increases
why are reactions fact in micelles?
Microcompartmentalisation - micelles act as mini reactors. organic reactions faster within hydrophobic interior.
what is CMT?
critical micelle temp - above this surfactant is soluble
how are DLS and SLS used to analyse micelles?
DLS measures micelle diameter
SLS measures micelle mass
how can the micelle aggregation number be calculated?
Micelle mass/ mass of surfactant
how does surfactant conc change micelle shape?
C < CMC - unimers
C = CMC spherical
C»_space; CMC cylindrical
what is the packing parameter?
Dictates micelle morphology P = V/ ao L V is volume of hydrophobic tail L is length of hydrophibic tail Ao is area of hydrophillic head
revise why interfacial tension is important
look at book
How is emulsion particle size achieved?
DLS DCP Optical microscopy Coulter counter - change in electrical resistance Laser diffraction
what is a colloid?
A Colloid is a dispersed phase within a continuous phase where the dimensions of the dispersed phase lie between 1 nm and 1000 nm
Can you get gas/gas colloids?
Cannot get gas/gas colloids due to rapid inter-diffusion
what is the equation for the specific surface area of a sphere?
As= 3/p.R p = density
true or false: colloids are barely affected by gravitational forces even on very long timescales?
false. over long periods of time gravitational forces acting on colloid can no longer be ignored. sedimentation can occur with denser dispersed phases.
Give the difference between lyophiliic and lyophobic solid liquid colloids.
- Lyophillic colloids have strong interactions with the continuous phase whereas lyophobic colloids do not have an interaction with the continous phase
2, lyophillic colloids are thermodynamically favourable
whereas lyophobic colloids are thermo and kinetically favourable. - in good solvents lyophillic will form random coils
describe the effect on attractive and repulsive forces on lyophobic colloids
ever present attractive forces eg VDW between particles eventually leads to particle aggregation/coagulation/precipitation
if repulsive forces present over a long range then the particles remain dispersed indefinitely ie stable colloid
what is a micelle?
Weakly aggregated structure of surfactant molecules in aqueous solution
what is a microgel?
Lightly crosslinked latex particle that can become highly swollen under certain conditions
what is the equation for polydisperisty index
PDI: Dw/Dn
Dw is obtained from DCP
Dn is obtained from electron microspray (TEM) and (SEM)
what is the degradation process?
to make S/L colloids. grind up into a coarse powder in the prescence of a surfactant to get smaller particles
what are the two stages in the formation of new dispersed phase?
- nucleation
- particle growth
the rates of these determine the size of the colloid
how do you get near monodispersed polymer coils?
living’ anionic polymerisation
what is ostwald ripening?
larger sols grow at the expense of smaller sols. over time the particle size distribution shits.
what is the rationale behind the ostwald ripening?
small sols are more soluble than larger particles hence small particles dissolve more preferentially and are redeposited on large particles
how does equilibrium dialysis work?
Place colloid in a semi-permeable cellulosic membrane
This membrane allows solvent / ions to pass in & out,
but retains the larger colloidal particles.
Leave in equilibrium with large solvent reservoir for several days, change solvent frequently
describe the process of centrifugation/redispersion?
sediment particles in a centrifuge or ultracentrifuge and then redisperse in pure solvent. repeat several times
what is ultrafiltration?
only small ions can pass through the microporous membrane, the larger colloids are retained
why is optical microspray not useful for sizing colloids?
Optical Microscopy has a relatively small depth of focus
what are the basic principles on transmission electron microspray?
- the electrons are focused using an electromagnetic lense
- under high vacuum to increase the mean free path of electrons
- the resolution of TEM covers the entire colloid range
- the wavelength is shorter than that of visible light
- to prep the sample: dilute the sample and dry on a carbon coated copper grid. this gives good separation
what are the advantages and disadvantages of TEM?
adv: covers entire colloid range
good magnification and resolution
digital image analysis allows rapid data for PSD
dis: only 2d images
high vacuum can destroy delicate samples
high energy electron beam can damage sample too eg by deformation
expensive to buy and maintain
advantages of SEM?
- high magnification and good resolution, 2-3 nm (theory)
- 3d images
- good depth of focus. 500 x better than the optical microscope
- easier to use than TEM
what are the disadvantages on SEM?
- resolution is often not achievable
- expensive
- sample has to be sputter coated to prevent sample charging problems
- certain samples may be affected by beam damage or evacuation
what is static light scattering used for?
used to measure molecular weight of lyophillic colloids
what is the dynamic light scattering used for?
to measure particle diameter of lyophibic S/L colloids. microemulsions and lyophillic colloids.
how do you get additional light scattering?
polymer solutions - polymer coils
colloidal dispersions
microemulsion and emulsions
why does particle diameter determined by DLS and TEM differ?
DLS and TEM are different moments of the same particle size distribution.
DLS measures the intense average diameter whereas TEM measures the average diameter. If latex in not perfectly monodisperse then DLS will oversize
what are the advantages of DLS?
- covers entire colloid size range
- very quick measurements
- suited in studying near monodisperse colloids and aggregation processes
- non perturbative and non destructive so good for emulsions and proteins
- can get useful data on non spherical particles via angular intensity measurements.
what are the disadvantages of DLS?
- very sensitive to dust and since dust particles are bigger they can become the dominant scatterers
- Can get multiple scattering if colloid is too concentrated
- light absorption on coloured particles can cause convection and heating effects
- can get significant sedimentation over dls timescales especially if particles are large
how does DCP work?
particles are thrown out radially to the disk periphery and detected by changes in light intensity.
an X-ray source/detector can be used instead of a light source/detector in DCP, evaluate this?
No assumptions required about particle characteristics but this not useful for latexes as they compromise of low Z atoms. also an x ray source and detector is more expensive
what are the two analysis methods in DCP?
Line start mode - directly measures differential PSD, high resolution
homogenous start mode: measures the integrated PSD and calculates the differential PSD. this is better suited to broad PSD.
why do core shell nanocomposite particles of finite polydispersity exhibit artifactual narrowing of their DCP size distribution?
Superimposed density distribution
True or false: with sterically stabilised diblock copolymer nanoparticles of finite polydispersity that have high density and low density shells you get atrifactual broadening
true
what are the pros of DCP?
- short analysis time
- excellent resolution compared to DLS
- gives dw directly
- works well for hard spheres, silica shells and PS latec
- wide dynamic range
what are cons associated with DCP?
- assumes spherical morphology
- accurate particle density needs to be known
- less good for solvated particles since particle density is not known precisely.
how can you tell the difference between an o/w and w/o emulsion?
o/w feels creamy w/o feels greasy
o/w if water sol dye dissolves
w/o if oil sol dye dissolves
o/w»_space; w/o in terms of electrical conductivity of solution
How do surfactants stabilise emulsions?
hydrophobic tails interact with oil
what do emulsifiers do and give 4 types?
They stabilise emulsions
- surfactant
- finely divided solids - pickering emulsifiers
- synthetic polymers eg PVNP - these can be water or oil soluble
- biopolymers - naturally occurring
describe some of the changes that could happen to emulsions?
flocculation - particles are sticky
coalescence - oil is less dense than water
creaming - water molecules move up dense oil, this is a physical change
Ostwald ripening - loss of smaller particles
what is are microemulsions?
intermediate between emulsions and micelles
what do you need to make microemulsions?
interfacial tension
surfactant and co surfactant
lots of emulsifier
why do microemulsions need more emulsifiers?
higher surface area than conventional emulsion
True or false:
Emulsions are thermodynamically stable so are difficult to break down. So DLS studies are suitable
They are not formed spontaneously so mechanical agitation is needed.
True
false - they are formed spontaneously
why are microemulsions transparent but emulsions are not?
small droplet size leads to minimal scattering whereas in emulsions light is scattered due to large droplets
how do you synthesise microscopic water insoluble polymer particles in water
emulsion polymerisation
describe the process of emulsion polymerisation?
Water insol monomer, water, surfactant and water sol initiator. polymerisation occurs within micelle.
water sol initiator generates radicals which diffuse into micelles and start polymerisation in situ. monomer droplets deplete as polymer swells . eventually you get stable colloidal particles of insoluble polymer latex of 100 - 500 nm
why are the resulting latex particles colloidally stable?
either charge or steric stabilisation
surface charge can arise from ionic surfactant or charged chain ends
what are the benefits of emulsion polymerisation?
- get latexes with narrow PSD
- Good solution viscosity with good thermal control
- water is non toxic and non flammable and plentiful
- high mol weight polymer formed quickly
- low residual polymer so highly efficient
what is a microgel?
intermediate between non solvated latexes (lyophibic colloids) and solvated random colloid (lyophilic colloids) and are lightly crosslinked and are of colloidal size.
why is the microgel PolyNipam insoluble above its cloud point?
crosslinks in the gel prevent solubilisation
how do you tune the latex to microgel thermal transition?
copolymerisation with various other acrylamides
why do DLS and TEM give different particle diameters in light scattering experiments for latex particles?
due to the effect of polydispersity (DLS is more sensitive to the larger particles in the latex size distribution and so ‘oversizes’ relative to TEM
what is creaming?
Dispersed phase separates from an emulsion
What is coalescence?
Fusion of droplets to form larger droplets with reduced surface area
what Ostwald ripening?
The formation of larger droplets at the expense of smaller ones
What is the essential criterion for the colloid stability of aq lyophibic colloids? Why does the addition of salt cause coagulation of such dispersions? State one industrial application for this phenomenon.
The essential criterion for colloid stability of such particles is that εmax»_space; kT, where Δε is the kinetic energy barrier to coagulation and kT is the thermal energy.
The addition of salt causes coagulation since it reduces the electrical double layer and hence lowers the εmax.
One industrial application of the salt-induced coagulation of an aqueous latex is the manufacture of latex gloves (used by all good experimental chemists!).
