Colloids Flashcards
(141 cards)
1
Q
what are colloidal materials composed of
A
2 different phases - one phase dispersed within a continuous phase
2
Q
what property must the dispersed phase of a colloid have
A
at least one dimension must be within the colloidal domain
3
Q
what is the colloidal domain
A
between 1 nm and 1000 nm
4
Q
define “phase”
A
a region of space in which physical properties are uniform
5
Q
when do colloidal crystals self assemble and what behaviour do they show
A
when they are the same size
scattering behaviour
6
Q
what happens when light wavelength is similar to the surface plasmon resonance
A
when particle size is similar to wavelength the electron cloud can be moved - this absorbs and scatters light
7
Q
how can the colour given by the surface plasmon resonance be modified
A
using nanorods of varied lengths
8
Q
what is a solid-solid colloid called
A
solid sol
9
Q
what is a liquid-solid colloid called
A
sol
10
Q
what is a gas-solid colloid called
A
aero-sol
11
Q
what is a solid-liquid colloid called
A
gel
12
Q
what is a liquid-liquid colloid called
A
liquid emulsion
13
Q
what is a gas liquid colloid called
A
liquid aerosol
14
Q
what is a solid gas colloid called
A
solid foam
15
Q
what is a liquid gas colloid called
A
foam
16
Q
why cant a gas gas colloid form
A
all gases are miscible
17
Q
what are the two types of emulsion
A
oil in water (O/W)
water in oil (W/O)
18
Q
what does monodisperse mean
A
all particles are the same size
19
Q
what does polydisperse mean
A
particles are of multiple sizes
20
Q
which colloid is more common mono or polydisperse
A
the majority of colloids are polydisperse
21
Q
what does a size distribution plot look like for a monodisperse colloid
A
1 sharp peak
22
Q
what does a size distribution plot look like for a polydisperse colloid
A
wide peak (bell shaped) - the width varies with sample
23
Q
what are the 3 types of distribution plot
A
number/size
surface area/size
volume/size
24
Q
when does a multimodal distribution occur
A
when there are 2 distinct populations in the sample
25
how does surface area/volume ratio vary for different particles
smaller particles have a greater surface area per volume
26
why is surface behaviour so important for small particles
surface molecules make up a significant proportion of the total particle
27
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
28
why do surface molecules have higher energy than those in the bulk
molecules at the surface experience less attractions than those in the bulk
29
define surface energy and what is its formula
energy required per unit area to increase surface size
| gamma(o) = ^G/^A
30
what does the term surface refer to
liquid-gas boundary
31
define interface
liquid-liquid boundary
| solid-liquid boundary
32
what size droplets are most energetically favourable
large droplets
33
what is a practical way to measure surface
| tension
-immerse wire frame in liquid
-frame pulled with force F
ST = gamma = F/2l
34
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
```
35
how is surface tension related to intermolecular forces
ST increases as intermolecular forces increase
36
how is surface tension related to temperature
ST decreases with increasing temperature
37
when do layers form between 2 phases
when cohesive interactions are stronger than adhesive interactions
38
what are cohesive interactions
interactions within a phase (between the same molecules)
39
what are adhesive interactions
interactions between different molecules
40
what is surfactant a contraction of
surface active agent
41
what are 2 properties of surfactants
have oil and water loving character
| absorb at an interface and lower surface tension
42
how and why do surfactants assemble at interfaces
hydrophobic end in oil
hydrophilic part in water
most energetically favoured - hydrophobics and hydrophilic together
43
what happens when there is not enough space for surfactants at the surface
self assemble to form micelles
44
what are the 2 main types of surfacants
ionic
| non-ionic
45
what are examples or anionic ends of ionic surfactants
carboxylate
sulfate
sulfonate
46
what are examples of cationic ends of ionic surfactants
ammonium
| quaternary ammonium
47
give examples of hydrophobic ends on non ionic surfactants
alkyl groups
| propylene glycol
48
give examples of hydrophilic ends on non ionic surfactants
polyethylene glycol
| polyols
49
why do micelles form
to minimise H-bonding disruption caused by non polar groups
50
at what concentration do micelles form
critical micelle concentration (CMC)
51
describe a plot of ST vs surfactant conc
ST decreases with increased conc until CMC
52
what are the 3 ways that surfactants can self assemble
bilayer sheet
micelle
liposome
53
what is the structure of a liposome
bilayered micelle
54
how can assembly of surfactants be predicted
using the packing parameter
55
what is the packing parameter
P=v/al
v = volume of tail
a = area of head
l = maximum extended length of tail
56
what does it mean when P>1
inverse
- water in oil
- oil soluble micelles
57
what does it mean when P~1
balanced
| forms a bilayer
58
what does it mean when P<1
water soluble micelles
59
what is the P value for phospholipids in cells
0.84
60
how can non ionic surfactant behaviour be predicted and what is the formula
using HLB
| 20 x hydrophilic end weight/total weight
61
when is the effect of HLB limited
when there are multiple hydrophilic sections
62
what range on the HLB scale indicates hydrophilicity
near to 20
63
why doesnt a bubble of air pop in liquid
due to laplace pressure
64
what is the formula for laplace pressure
^p=2γ/R
r = particle radius
p = pressure
65
how does laplace pressure depend on particle size
pressure is greater for smaller droplets
66
how can laplace pressure be reduced
lowering ST
67
how does laplace pressure affect chemical potential
leads to increased chemical potential
68
what is the formula for chemical potential
```
μi = μinfinity + 2γ Vi / R
μi = chemical potential
```
69
how does matter flow with respect to chemical potential
flows from high to low
70
what is ostwald ripening
large laplace pressure in small drops drives them to form larger droplets
71
what are 3 ways that ostwald ripening can be reduced
use narrow size distribution in sample
use insoluble material
use hydrophobes
72
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
73
what are 3 possible outcomes if attaractive forces are greater than replusive forces
coagulation
flocculation
coalescence
74
what is coagulation
irreversible aggregation of particles
75
what is flocculation
reversible weak aggreagation
76
what is coalescence
merging of droplets
77
what are london forces
attractions caused by induced dipoles
78
how do london forces affect colloids and why is it favourable
pulls them together
| reduces surface size per volume
79
how are attractive forces affected by distance
proportional to 1/D^2
80
why do colloids experience electrostatic repulsion
they are charged at their surfaces
81
what is the inner helmhotz layer
strongly attracted particles near to the surface
82
what is the outer helmhotz layer
fairly strongly attracted particles outside of inner layer
83
what is the diffuse layer and how does it differ from helmhotz layers
less attracted particles
| cant move around
84
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
85
what is repulsion proportional to
thickness of the 2 helmhotz layers
86
how can repulsion be reduced
adding salt reduces the thickness of the 2 helmhotz layers and therefore reduces repulsion
87
what does the total interaction energy comprise of
attractive + repulsive potentials
88
what can the total interaction energy predict
stability
89
at what distances do london forces dominate
large distances and very short distances
90
why are particles kinetically stable
must overcome energy barrier to aggregate
91
what happens to the energy barrier if salt is added
The energy barrier lowers and forms a second minimum - flocculation occurs here
92
what happens if there is no energy barrier for particles to overcome
immediate coagulation
93
how do non ionic surfactants interact with particles
hydrophobic end binds to particle
| hydrophilic stretches out
94
what formula can be used to describe the stability of the nonionic 2 layer overlap
ΔGmix = ΔH - TΔS
95
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)
96
why do particles with steric stabilisation not mix
ΔH and ΔS unfavoured, ΔG is positive
97
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
98
how does steric repulsion change relative to D
increases rapidly when D<2t
99
how do longer surfactant chain lengths effect steric repulsion
repulsion at larger separation before attractions become stronger
100
what is required to achieve steric stabilisation
```
high surface coverage
thick polymer layer
strong adsorption
good solvent to stabilise chain
low free polymer concentration
```
101
how are nonionic surfactants effected by addition of salt
they are not salt sensitive
102
why are electrostatic surfactants not ideal for use in the body
they are sensitive to salt
103
what forces are acting on a stationary particle in a colloid
buoyancy
| gravity
104
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
105
what is the formula for the gravitational force acting on a particle in a colloid
Pp x V x g
| Pp = density of particle
106
what is creaming
when the particles in a colloid rise to the top
107
what is sedimentation
when the particles in a colloid fall to the bottom
108
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
109
what causes motion in Brownian motion
collisions of colloids and water
110
why do particles not fall to the bottom of the mixture
diffusion pulls the particles to areas of low concentration
111
why do colloids appear white
contain particles smaller than the wavelength of light
112
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
113
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
114
how does the size of a particle effect the amount of scattering
larger particle = more scattering
115
how does wavelength effect scattering
shorter wavelength = more scattering
116
how does dynamic light scattering work
measures change in scattering at a single angle
117
what can be calculated by dynamic light scattering
diffusion angle and therefore radius
118
why do larger particles have a smaller diffusion constant
they move more slowly
119
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
120
what is a problem with intensity weighted distributions
large particles are much more intense so small particles aren't seen
121
what are 3 advantages of DLS
simple sample prep
fast measurement
measures large number of particles
122
what are 3 disadvantages of DLS
- distribution plot favours large particles
- quite low resolution
- upper size limit is 1000 nm
123
what is one of the major challenges with light scattering experiments
dust scatters a lot of light
124
briefly describe how MIE theory works
uses relative refractive index to predict scattering intensity of light
scattering intensity ahs a complex relationship with angle
125
what happens when light passes through an aperture
diffraction occurs
126
how does the magnification of a lens tell us about the aperture
higher magnification = smaller aperture
127
what is the abbe limit and what does it mean
λ/2 - limit of resolution
128
what is the best possible resolution for visible light
200 nm
129
how can the wavelength of an electron be calculated
de broglie duality formula
| λ = h/p
130
what are the 2 types of electron microscopy
SEM - surface
| TEM - transmission
131
how does SEM work
beam rasters over surface
| detects secondary and back scattered electrons
132
what is a secondary electron
an electron that has been knocked out of the conduction layer of the sample
133
what is back scattering
an electron that have elastically collided with the surface - these are more common
134
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
135
how do the electron energies vary in SEM and TEM
higher energy e- in TEM
136
what property must a sample have to undergo TEM and why
must be thin to allow an electron to pass through it
137
what are some advantaged of electron microscopy
high resolution
| allows visualisation of shape and surface
138
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
139
what is the zeta potential
potential at edge of 2 Helmholtz layers
140
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
141
how do the zeta potential relate to electrostatic repulsion
greater zeta potential = greater repulsion
