Colloids Flashcards
what are colloidal materials composed of
2 different phases - one phase dispersed within a continuous phase
what property must the dispersed phase of a colloid have
at least one dimension must be within the colloidal domain
what is the colloidal domain
between 1 nm and 1000 nm
define “phase”
a region of space in which physical properties are uniform
when do colloidal crystals self assemble and what behaviour do they show
when they are the same size
scattering behaviour
what happens when light wavelength is similar to the surface plasmon resonance
when particle size is similar to wavelength the electron cloud can be moved - this absorbs and scatters light
how can the colour given by the surface plasmon resonance be modified
using nanorods of varied lengths
what is a solid-solid colloid called
solid sol
what is a liquid-solid colloid called
sol
what is a gas-solid colloid called
aero-sol
what is a solid-liquid colloid called
gel
what is a liquid-liquid colloid called
liquid emulsion
what is a gas liquid colloid called
liquid aerosol
what is a solid gas colloid called
solid foam
what is a liquid gas colloid called
foam
why cant a gas gas colloid form
all gases are miscible
what are the two types of emulsion
oil in water (O/W)
water in oil (W/O)
what does monodisperse mean
all particles are the same size
what does polydisperse mean
particles are of multiple sizes
which colloid is more common mono or polydisperse
the majority of colloids are polydisperse
what does a size distribution plot look like for a monodisperse colloid
1 sharp peak
what does a size distribution plot look like for a polydisperse colloid
wide peak (bell shaped) - the width varies with sample
what are the 3 types of distribution plot
number/size
surface area/size
volume/size
when does a multimodal distribution occur
when there are 2 distinct populations in the sample
how does surface area/volume ratio vary for different particles
smaller particles have a greater surface area per volume
why is surface behaviour so important for small particles
surface molecules make up a significant proportion of the total particle
why do particles naturally form into spheres
the surface is at a higher energy, a sphere has the least possible amount of surface per volume
why do surface molecules have higher energy than those in the bulk
molecules at the surface experience less attractions than those in the bulk
define surface energy and what is its formula
energy required per unit area to increase surface size
gamma(o) = ^G/^A
what does the term surface refer to
liquid-gas boundary
define interface
liquid-liquid boundary
solid-liquid boundary
what size droplets are most energetically favourable
large droplets
what is a practical way to measure surface
tension
-immerse wire frame in liquid
-frame pulled with force F
ST = gamma = F/2l
how do surface energy and surface tension relate
SE, gamma=ΔG/ΔA units - J/m^2 ST, gamma=F/2l units - N/m they are equivalent
how is surface tension related to intermolecular forces
ST increases as intermolecular forces increase
how is surface tension related to temperature
ST decreases with increasing temperature
when do layers form between 2 phases
when cohesive interactions are stronger than adhesive interactions
what are cohesive interactions
interactions within a phase (between the same molecules)
what are adhesive interactions
interactions between different molecules
what is surfactant a contraction of
surface active agent
what are 2 properties of surfactants
have oil and water loving character
absorb at an interface and lower surface tension
how and why do surfactants assemble at interfaces
hydrophobic end in oil
hydrophilic part in water
most energetically favoured - hydrophobics and hydrophilic together
what happens when there is not enough space for surfactants at the surface
self assemble to form micelles
what are the 2 main types of surfacants
ionic
non-ionic
what are examples or anionic ends of ionic surfactants
carboxylate
sulfate
sulfonate
what are examples of cationic ends of ionic surfactants
ammonium
quaternary ammonium
give examples of hydrophobic ends on non ionic surfactants
alkyl groups
propylene glycol
give examples of hydrophilic ends on non ionic surfactants
polyethylene glycol
polyols
why do micelles form
to minimise H-bonding disruption caused by non polar groups
at what concentration do micelles form
critical micelle concentration (CMC)
describe a plot of ST vs surfactant conc
ST decreases with increased conc until CMC
what are the 3 ways that surfactants can self assemble
bilayer sheet
micelle
liposome
what is the structure of a liposome
bilayered micelle
how can assembly of surfactants be predicted
using the packing parameter
what is the packing parameter
P=v/al
v = volume of tail
a = area of head
l = maximum extended length of tail
what does it mean when P>1
inverse
- water in oil
- oil soluble micelles
what does it mean when P~1
balanced
forms a bilayer
what does it mean when P<1
water soluble micelles
what is the P value for phospholipids in cells
0.84
how can non ionic surfactant behaviour be predicted and what is the formula
using HLB
20 x hydrophilic end weight/total weight
when is the effect of HLB limited
when there are multiple hydrophilic sections
what range on the HLB scale indicates hydrophilicity
near to 20
why doesnt a bubble of air pop in liquid
due to laplace pressure
what is the formula for laplace pressure
^p=2γ/R
r = particle radius
p = pressure
how does laplace pressure depend on particle size
pressure is greater for smaller droplets
how can laplace pressure be reduced
lowering ST
how does laplace pressure affect chemical potential
leads to increased chemical potential
what is the formula for chemical potential
μi = μinfinity + 2γ Vi / R μi = chemical potential
how does matter flow with respect to chemical potential
flows from high to low
what is ostwald ripening
large laplace pressure in small drops drives them to form larger droplets
what are 3 ways that ostwald ripening can be reduced
use narrow size distribution in sample
use insoluble material
use hydrophobes
how to hydrophobes stop ostwald ripening
as the hydrophilic component leaves due to high laplace pressure, this leaves smaller droplets with hydrophobes - so not net laplace decrease
what are 3 possible outcomes if attaractive forces are greater than replusive forces
coagulation
flocculation
coalescence
what is coagulation
irreversible aggregation of particles
what is flocculation
reversible weak aggreagation
what is coalescence
merging of droplets
what are london forces
attractions caused by induced dipoles
how do london forces affect colloids and why is it favourable
pulls them together
reduces surface size per volume
how are attractive forces affected by distance
proportional to 1/D^2
why do colloids experience electrostatic repulsion
they are charged at their surfaces
what is the inner helmhotz layer
strongly attracted particles near to the surface
what is the outer helmhotz layer
fairly strongly attracted particles outside of inner layer
what is the diffuse layer and how does it differ from helmhotz layers
less attracted particles
cant move around
how does repulsion work
when particles come together the ionic atmospheres overlap
high density of ions
water try’s to diffuse and pushes layers apart
what is repulsion proportional to
thickness of the 2 helmhotz layers
how can repulsion be reduced
adding salt reduces the thickness of the 2 helmhotz layers and therefore reduces repulsion
what does the total interaction energy comprise of
attractive + repulsive potentials
what can the total interaction energy predict
stability
at what distances do london forces dominate
large distances and very short distances
why are particles kinetically stable
must overcome energy barrier to aggregate
what happens to the energy barrier if salt is added
The energy barrier lowers and forms a second minimum - flocculation occurs here
what happens if there is no energy barrier for particles to overcome
immediate coagulation
how do non ionic surfactants interact with particles
hydrophobic end binds to particle
hydrophilic stretches out
what formula can be used to describe the stability of the nonionic 2 layer overlap
ΔGmix = ΔH - TΔS
what happens when the layers of surfactants around a particle interact and what are the energetics
release of bound solvent ΔH +ve (unfavoured)
loses configurational entropy ΔS -ve (unfavoured)
why do particles with steric stabilisation not mix
ΔH and ΔS unfavoured, ΔG is positive
what is repulsion due to steric stabilisation proportional to
a (particle radius)
Γ^2 - particle surface coverage
(1/2 - χ) - interaction parameter
(1-D/2t)^2 - t = layer thickness, D = separation
how does steric repulsion change relative to D
increases rapidly when D<2t
how do longer surfactant chain lengths effect steric repulsion
repulsion at larger separation before attractions become stronger
what is required to achieve steric stabilisation
high surface coverage thick polymer layer strong adsorption good solvent to stabilise chain low free polymer concentration
how are nonionic surfactants effected by addition of salt
they are not salt sensitive
why are electrostatic surfactants not ideal for use in the body
they are sensitive to salt
what forces are acting on a stationary particle in a colloid
buoyancy
gravity
what is the formula for the buoyancy force acting upon a particle in a colloid
Pc x V x g
Pc density of continuous phase
what is the formula for the gravitational force acting on a particle in a colloid
Pp x V x g
Pp = density of particle
what is creaming
when the particles in a colloid rise to the top
what is sedimentation
when the particles in a colloid fall to the bottom
what is the formula for the drag force applied to a particle in a colloid
F = 6pi η R nu
η = viscosity coefficient
R = radius
ν = velocity
what causes motion in Brownian motion
collisions of colloids and water
why do particles not fall to the bottom of the mixture
diffusion pulls the particles to areas of low concentration
why do colloids appear white
contain particles smaller than the wavelength of light
describe the process of light scattering
radiation interacts with matter - absorbs
due to the inverting electric field in light an alternating electric field is generated in particles - current
oscillating electons produce electromagnetic field
what is the formula for rayleigh scattering
sigma(s) is proportional to - R^6 n^2 x 1/λ^4
sigma(s) = cross section of scattering
n = relative refractive index
how does the size of a particle effect the amount of scattering
larger particle = more scattering
how does wavelength effect scattering
shorter wavelength = more scattering
how does dynamic light scattering work
measures change in scattering at a single angle
what can be calculated by dynamic light scattering
diffusion angle and therefore radius
why do larger particles have a smaller diffusion constant
they move more slowly
how can size be determined from DLS data
larger particles are slower and have less fluctuation in the intensity picked up by the detector. This is auto correlated a the larger the particle the better the correlation
what is a problem with intensity weighted distributions
large particles are much more intense so small particles aren’t seen
what are 3 advantages of DLS
simple sample prep
fast measurement
measures large number of particles
what are 3 disadvantages of DLS
- distribution plot favours large particles
- quite low resolution
- upper size limit is 1000 nm
what is one of the major challenges with light scattering experiments
dust scatters a lot of light
briefly describe how MIE theory works
uses relative refractive index to predict scattering intensity of light
scattering intensity ahs a complex relationship with angle
what happens when light passes through an aperture
diffraction occurs
how does the magnification of a lens tell us about the aperture
higher magnification = smaller aperture
what is the abbe limit and what does it mean
λ/2 - limit of resolution
what is the best possible resolution for visible light
200 nm
how can the wavelength of an electron be calculated
de broglie duality formula
λ = h/p
what are the 2 types of electron microscopy
SEM - surface
TEM - transmission
how does SEM work
beam rasters over surface
detects secondary and back scattered electrons
what is a secondary electron
an electron that has been knocked out of the conduction layer of the sample
what is back scattering
an electron that have elastically collided with the surface - these are more common
what is one of the conditions that must be met for SEM
surface must be coated with a conductive layer to stop charge build up
how do the electron energies vary in SEM and TEM
higher energy e- in TEM
what property must a sample have to undergo TEM and why
must be thin to allow an electron to pass through it
what are some advantaged of electron microscopy
high resolution
allows visualisation of shape and surface
what are some disadvantages on electron microscopy
sample must be dry
sample could be sensitive to prep methods
only measures small amount of particles
time consuming
what is the zeta potential
potential at edge of 2 Helmholtz layers
how is the zeta potential obtained and why does this work
electrophoretic mobility measurement
- a charge particle experiences a force associated with a magnetic field
how do the zeta potential relate to electrostatic repulsion
greater zeta potential = greater repulsion
