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CHEM426 > Colloids > Flashcards

Colloids Flashcards

1
Q

what are colloidal materials composed of

A

2 different phases - one phase dispersed within a continuous phase

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2
Q

what property must the dispersed phase of a colloid have

A

at least one dimension must be within the colloidal domain

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3
Q

what is the colloidal domain

A

between 1 nm and 1000 nm

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4
Q

define “phase”

A

a region of space in which physical properties are uniform

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5
Q

when do colloidal crystals self assemble and what behaviour do they show

A

when they are the same size

scattering behaviour

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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

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7
Q

how can the colour given by the surface plasmon resonance be modified

A

using nanorods of varied lengths

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8
Q

what is a solid-solid colloid called

A

solid sol

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9
Q

what is a liquid-solid colloid called

A

sol

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10
Q

what is a gas-solid colloid called

A

aero-sol

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11
Q

what is a solid-liquid colloid called

A

gel

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12
Q

what is a liquid-liquid colloid called

A

liquid emulsion

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13
Q

what is a gas liquid colloid called

A

liquid aerosol

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14
Q

what is a solid gas colloid called

A

solid foam

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15
Q

what is a liquid gas colloid called

A

foam

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16
Q

why cant a gas gas colloid form

A

all gases are miscible

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17
Q

what are the two types of emulsion

A

oil in water (O/W)

water in oil (W/O)

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18
Q

what does monodisperse mean

A

all particles are the same size

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19
Q

what does polydisperse mean

A

particles are of multiple sizes

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20
Q

which colloid is more common mono or polydisperse

A

the majority of colloids are polydisperse

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21
Q

what does a size distribution plot look like for a monodisperse colloid

A

1 sharp peak

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22
Q

what does a size distribution plot look like for a polydisperse colloid

A

wide peak (bell shaped) - the width varies with sample

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23
Q

what are the 3 types of distribution plot

A

number/size
surface area/size
volume/size

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24
Q

when does a multimodal distribution occur

A

when there are 2 distinct populations in the sample

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25
Q

how does surface area/volume ratio vary for different particles

A

smaller particles have a greater surface area per volume

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26
Q

why is surface behaviour so important for small particles

A

surface molecules make up a significant proportion of the total particle

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27
Q

why do particles naturally form into spheres

A

the surface is at a higher energy, a sphere has the least possible amount of surface per volume

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28
Q

why do surface molecules have higher energy than those in the bulk

A

molecules at the surface experience less attractions than those in the bulk

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29
Q

define surface energy and what is its formula

A

energy required per unit area to increase surface size

gamma(o) = ^G/^A

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30
Q

what does the term surface refer to

A

liquid-gas boundary

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31
Q

define interface

A

liquid-liquid boundary

solid-liquid boundary

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32
Q

what size droplets are most energetically favourable

A

large droplets

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33
Q

what is a practical way to measure surface

tension

A

-immerse wire frame in liquid
-frame pulled with force F
ST = gamma = F/2l

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34
Q

how do surface energy and surface tension relate

A 
SE, gamma=ΔG/ΔA
units - J/m^2
ST, gamma=F/2l
units - N/m
they are equivalent
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35
Q

how is surface tension related to intermolecular forces

A

ST increases as intermolecular forces increase

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36
Q

how is surface tension related to temperature

A

ST decreases with increasing temperature

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37
Q

when do layers form between 2 phases

A

when cohesive interactions are stronger than adhesive interactions

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38
Q

what are cohesive interactions

A

interactions within a phase (between the same molecules)

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39
Q

what are adhesive interactions

A

interactions between different molecules

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40
Q

what is surfactant a contraction of

A

surface active agent

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41
Q

what are 2 properties of surfactants

A

have oil and water loving character

absorb at an interface and lower surface tension

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42
Q

how and why do surfactants assemble at interfaces

A

hydrophobic end in oil
hydrophilic part in water
most energetically favoured - hydrophobics and hydrophilic together

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43
Q

what happens when there is not enough space for surfactants at the surface

A

self assemble to form micelles

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44
Q

what are the 2 main types of surfacants

A

ionic

non-ionic

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45
Q

what are examples or anionic ends of ionic surfactants

A

carboxylate
sulfate
sulfonate

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46
Q

what are examples of cationic ends of ionic surfactants

A

ammonium

quaternary ammonium

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47
Q

give examples of hydrophobic ends on non ionic surfactants

A

alkyl groups

propylene glycol

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48
Q

give examples of hydrophilic ends on non ionic surfactants

A

polyethylene glycol

polyols

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49
Q

why do micelles form

A

to minimise H-bonding disruption caused by non polar groups

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50
Q

at what concentration do micelles form

A

critical micelle concentration (CMC)

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51
Q

describe a plot of ST vs surfactant conc

A

ST decreases with increased conc until CMC

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52
Q

what are the 3 ways that surfactants can self assemble

A

bilayer sheet
micelle
liposome

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53
Q

what is the structure of a liposome

A

bilayered micelle

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54
Q

how can assembly of surfactants be predicted

A

using the packing parameter

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55
Q

what is the packing parameter

A

P=v/al
v = volume of tail
a = area of head
l = maximum extended length of tail

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56
Q

what does it mean when P>1

A

inverse

  • water in oil
  • oil soluble micelles
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57
Q

what does it mean when P~1

A

balanced

forms a bilayer

58
Q

what does it mean when P<1

A

water soluble micelles

59
Q

what is the P value for phospholipids in cells

A

0.84

60
Q

how can non ionic surfactant behaviour be predicted and what is the formula

A

using HLB

20 x hydrophilic end weight/total weight

61
Q

when is the effect of HLB limited

A

when there are multiple hydrophilic sections

62
Q

what range on the HLB scale indicates hydrophilicity

A

near to 20

63
Q

why doesnt a bubble of air pop in liquid

A

due to laplace pressure

64
Q

what is the formula for laplace pressure

A

^p=2γ/R
r = particle radius
p = pressure

65
Q

how does laplace pressure depend on particle size

A

pressure is greater for smaller droplets

66
Q

how can laplace pressure be reduced

A

lowering ST

67
Q

how does laplace pressure affect chemical potential

A

leads to increased chemical potential

68
Q

what is the formula for chemical potential

A 
μi = μinfinity + 2γ Vi / R
μi = chemical potential
69
Q

how does matter flow with respect to chemical potential

A

flows from high to low

70
Q

what is ostwald ripening

A

large laplace pressure in small drops drives them to form larger droplets

71
Q

what are 3 ways that ostwald ripening can be reduced

A

use narrow size distribution in sample
use insoluble material
use hydrophobes

72
Q

how to hydrophobes stop ostwald ripening

A

as the hydrophilic component leaves due to high laplace pressure, this leaves smaller droplets with hydrophobes - so not net laplace decrease

73
Q

what are 3 possible outcomes if attaractive forces are greater than replusive forces

A

coagulation
flocculation
coalescence

74
Q

what is coagulation

A

irreversible aggregation of particles

75
Q

what is flocculation

A

reversible weak aggreagation

76
Q

what is coalescence

A

merging of droplets

77
Q

what are london forces

A

attractions caused by induced dipoles

78
Q

how do london forces affect colloids and why is it favourable

A

pulls them together

reduces surface size per volume

79
Q

how are attractive forces affected by distance

A

proportional to 1/D^2

80
Q

why do colloids experience electrostatic repulsion

A

they are charged at their surfaces

81
Q

what is the inner helmhotz layer

A

strongly attracted particles near to the surface

82
Q

what is the outer helmhotz layer

A

fairly strongly attracted particles outside of inner layer

83
Q

what is the diffuse layer and how does it differ from helmhotz layers

A

less attracted particles

cant move around

84
Q

how does repulsion work

A

when particles come together the ionic atmospheres overlap
high density of ions
water try’s to diffuse and pushes layers apart

85
Q

what is repulsion proportional to

A

thickness of the 2 helmhotz layers

86
Q

how can repulsion be reduced

A

adding salt reduces the thickness of the 2 helmhotz layers and therefore reduces repulsion

87
Q

what does the total interaction energy comprise of

A

attractive + repulsive potentials

88
Q

what can the total interaction energy predict

A

stability

89
Q

at what distances do london forces dominate

A

large distances and very short distances

90
Q

why are particles kinetically stable

A

must overcome energy barrier to aggregate

91
Q

what happens to the energy barrier if salt is added

A

The energy barrier lowers and forms a second minimum - flocculation occurs here

92
Q

what happens if there is no energy barrier for particles to overcome

A

immediate coagulation

93
Q

how do non ionic surfactants interact with particles

A

hydrophobic end binds to particle

hydrophilic stretches out

94
Q

what formula can be used to describe the stability of the nonionic 2 layer overlap

A

ΔGmix = ΔH - TΔS

95
Q

what happens when the layers of surfactants around a particle interact and what are the energetics

A

release of bound solvent ΔH +ve (unfavoured)

loses configurational entropy ΔS -ve (unfavoured)

96
Q

why do particles with steric stabilisation not mix

A

ΔH and ΔS unfavoured, ΔG is positive

97
Q

what is repulsion due to steric stabilisation proportional to

A

a (particle radius)
Γ^2 - particle surface coverage
(1/2 - χ) - interaction parameter
(1-D/2t)^2 - t = layer thickness, D = separation

98
Q

how does steric repulsion change relative to D

A

increases rapidly when D<2t

99
Q

how do longer surfactant chain lengths effect steric repulsion

A

repulsion at larger separation before attractions become stronger

100
Q

what is required to achieve steric stabilisation

A 
high surface coverage
thick polymer layer
strong adsorption
good solvent to stabilise chain
low free polymer concentration
101
Q

how are nonionic surfactants effected by addition of salt

A

they are not salt sensitive

102
Q

why are electrostatic surfactants not ideal for use in the body

A

they are sensitive to salt

103
Q

what forces are acting on a stationary particle in a colloid

A

buoyancy

gravity

104
Q

what is the formula for the buoyancy force acting upon a particle in a colloid

A

Pc x V x g

Pc density of continuous phase

105
Q

what is the formula for the gravitational force acting on a particle in a colloid

A

Pp x V x g

Pp = density of particle

106
Q

what is creaming

A

when the particles in a colloid rise to the top

107
Q

what is sedimentation

A

when the particles in a colloid fall to the bottom

108
Q

what is the formula for the drag force applied to a particle in a colloid

A

F = 6pi η R nu
η = viscosity coefficient
R = radius
ν = velocity

109
Q

what causes motion in Brownian motion

A

collisions of colloids and water

110
Q

why do particles not fall to the bottom of the mixture

A

diffusion pulls the particles to areas of low concentration

111
Q

why do colloids appear white

A

contain particles smaller than the wavelength of light

112
Q

describe the process of light scattering

A

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
Q

what is the formula for rayleigh scattering

A

sigma(s) is proportional to - R^6 n^2 x 1/λ^4
sigma(s) = cross section of scattering
n = relative refractive index

114
Q

how does the size of a particle effect the amount of scattering

A

larger particle = more scattering

115
Q

how does wavelength effect scattering

A

shorter wavelength = more scattering

116
Q

how does dynamic light scattering work

A

measures change in scattering at a single angle

117
Q

what can be calculated by dynamic light scattering

A

diffusion angle and therefore radius

118
Q

why do larger particles have a smaller diffusion constant

A

they move more slowly

119
Q

how can size be determined from DLS data

A

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
Q

what is a problem with intensity weighted distributions

A

large particles are much more intense so small particles aren’t seen

121
Q

what are 3 advantages of DLS

A

simple sample prep
fast measurement
measures large number of particles

122
Q

what are 3 disadvantages of DLS

A
  • distribution plot favours large particles
  • quite low resolution
  • upper size limit is 1000 nm
123
Q

what is one of the major challenges with light scattering experiments

A

dust scatters a lot of light

124
Q

briefly describe how MIE theory works

A

uses relative refractive index to predict scattering intensity of light
scattering intensity ahs a complex relationship with angle

125
Q

what happens when light passes through an aperture

A

diffraction occurs

126
Q

how does the magnification of a lens tell us about the aperture

A

higher magnification = smaller aperture

127
Q

what is the abbe limit and what does it mean

A

λ/2 - limit of resolution

128
Q

what is the best possible resolution for visible light

A

200 nm

129
Q

how can the wavelength of an electron be calculated

A

de broglie duality formula

λ = h/p

130
Q

what are the 2 types of electron microscopy

A

SEM - surface

TEM - transmission

131
Q

how does SEM work

A

beam rasters over surface

detects secondary and back scattered electrons

132
Q

what is a secondary electron

A

an electron that has been knocked out of the conduction layer of the sample

133
Q

what is back scattering

A

an electron that have elastically collided with the surface - these are more common

134
Q

what is one of the conditions that must be met for SEM

A

surface must be coated with a conductive layer to stop charge build up

135
Q

how do the electron energies vary in SEM and TEM

A

higher energy e- in TEM

136
Q

what property must a sample have to undergo TEM and why

A

must be thin to allow an electron to pass through it

137
Q

what are some advantaged of electron microscopy

A

high resolution

allows visualisation of shape and surface

138
Q

what are some disadvantages on electron microscopy

A

sample must be dry
sample could be sensitive to prep methods
only measures small amount of particles
time consuming

139
Q

what is the zeta potential

A

potential at edge of 2 Helmholtz layers

140
Q

how is the zeta potential obtained and why does this work

A

electrophoretic mobility measurement

- a charge particle experiences a force associated with a magnetic field

141
Q

how do the zeta potential relate to electrostatic repulsion

A

greater zeta potential = greater repulsion

CHEM426 (4 decks)

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