1
Q
Characteristics of a Hookean Solid
A
Perfect elastic behaviour.
- An applied stress produces instantaneous strain.
- Strain (𝛾) is proportional to stress (𝜎).
- The constant of proportionality is the elastic modulus, G.
2
Q
Examples of stress of hookean solids
A
- Extensional stress
- Compressional stress
- Shear stress
In all cases, stress is independent of the rate of strain.
3
Q
Stress in Hookean solids
A
Instantaneous and reversible response
4
Q
Idealised liquid behaviour
A
Newtonian Liquid - Constant viscosity
5
Q
Characteristics of a Newtonian Liquid
A
- An applied shear stress, 𝜎, produces liquid flow. The strain rate is proportional to stress.
- The constant of proportionality is the viscosity, η.
6
Q
What is viscosity?
A
the liquid’s resistance to flow
7
Q
Stress in Newtonian Liquids
A
Delayed and irreversible
8
Q
What is Rheology?
A
the study of flow
9
Q
Newtonian fluids
A
viscosity is independent of strain rate
10
Q
Non-Newtonian fluids
A
viscosity changes with strain rate
1. Shear- thinning fluids: viscosity decreases with increasing strain rate.
2. Shear-thickening fluids: viscosity increases with increasing strain rate.
3. Thixotropic fluids: properties depend on how long they have been sheared.
4. Bingham plastic: viscosity appears infinite until a certain stress is applied.
11
Q
How can soft matter behave?
A
It can show liquid and solid-like behaviour (i.e. it has both an elastic and viscous response) = viscoelastic
12
Q
How does a viscoelastic material respond to stress?
A
in a time dependent matter
13
Q
What is the Deborah number, De?
A
the relaxation time of system/ timescale of experiment
i.e. De = 𝝉 / t
where 𝝉 is the time for the fluid response to appear
14
Q
What does a high Deborah number mean?
A
relaxation time is very large compared to the time scale of the experiment
15
Q
How are viscoelastic materials time-dependent?
A
Applied stress initially makes the material respond as a solid, with an instantaneous elastic modulus, G, but after a certain time, 𝝉, it begins to flow like a viscous liquid.
16
Q
Static experiments: what can viscoelastic materials show?
A
They dissipate energy, therefore can show hysteresis, stress relaxation and creep
17
Q
What is the Maxwell model?
A
spring and dashpot in series
- same stress through each element
- linear addition of strain
18
Q
What is the Kelvin-Voigt model?
A
spring and dashpot in parallel
- same strain through each element
- linear addition of stress
19
Q
What does the storage modulus account for?
A
(it is when stress and strain are in phase)
- it accounts for the energy that is stored in deformation, i.e. elastic behaviour
20
Q
What does the loss modulus account for?
A
(it is when stress and strain are in phase)
- it accounts for energy that is lost in viscous dissipation
21
Q
Maxwell model: what do low frequencies mean in oscillating shear experiment?
A
(Long repeat times), the dashpot can respond fully to the applied oscillating stress and so takes most of the strain.
Therefore has liquid behaviour.
22
Q
Maxwell model: What do high frequencies mean in the oscillating shear experiment?
A
(Short repeat times), the dashpot cannot move fully in response to the oscillating stress; the spring can and so takes most of the strain.
Therefore has solid behaviour.
23
Q
Voigt model: what do low frequencies mean in oscillating shear experiment?
A
(Long repeat times), the dashpot can respond fully to the applied oscillating stress with the piston moving through the dashpot to relax its stress. Spring takes most of the stress therefore solid behaviour.
24
Q
Voigt model: what do high frequencies mean in oscillating shear experiment?
A
(Short repeat times), the dashpot cannot move fully in response to the oscillating stress and so takes most of the stress. Therefore liquid behaviour.
25
Example system to approximate the Maxwell model
Starch solution
Short times: starch molecules (polymer) entangle → solid-like
Long times: molecules slide past each other → liquid-like
Relaxation time is determined by the time taken for chain to diffuse past each other.
26
Example system to approximate the Voigt model
Clay suspension
- Plate-like clay particles form a 'house of cards' → gel (solid-like), lowest energy structure.
- Shear can align clay particles with the flow → liquid-like behaviour
Low frequencies: 'house of cards' gel structure has time to develop.
Relaxation time determined by time taken for clay particles to rotate.
27
Macromolecules
high molecular weights, many many monomers
28
What are the different possible structures of polymers?
- Homopolymers
- Random Polymers
- Block Polymers
- Grafted Polymers
29
Homopolymers
1 repeat unit (polyethylene oxide)
30
Random polymers
2 or more repeat units in a random sequence
31
Block polymers
2 or more repeat units ordered in blocks AAABBB.....
32
Grafted polymers
main chain with branches attached
33
What is cross-linking?
Polymers form bonds between neighbouring groups that alter material properties
34
What is a random-flight polymer?
(or random-coil)
A polymer in which the direction of each segment is random relative to the previous segment in the chain
- The direction of different segments are uncorrelated
- The length of each segment is fixed at l
35
Is higher R (end-to-end distance) more favourable?
No because largest R is fully extended polymer chain so there is only one conformation which is entropically unfavourable as the polymer wants to adopt as many conformations as possible.
36
Why can’t we usually measure the end-to-end distance of a polymer directly?
Polymer conformations constantly change due to thermal motion.
37
What quantity is normally used to describe polymer size?
Radius of gyration Rg
38
What is the radius of gyration?
The RMS distance of polymer segments from the centre of mass
39
What shape is the accessible volume of a polymer chain?
Approximately spherical.
40
What assumption does the random-flight model make?
Each segment orientation is independent of the previous one.
41
What determines polymer configurations in the ideal model?
Entropy only
42
Free energy expression for an ideal polymer?
F=−TS
(no enthalpic contribution)
43
What happens to entropy when end-to-end distance increases?
Entropy decreases.
44
What law describes polymer elasticity at small extension?
Hooke’s Law.
45
Why is polymer elasticity called entropic elasticity?
Force arises from entropy changes, not bond stretching
46
Why do real polymers deviate from the random-flight model?
Geometric and steric constraints.
47
What are short-range correlations?
Constraints from bond angles and local geometry.
48
Example of geometric constraint in polymers?
sp³ carbon bond angle ≈ 109.5°
49
What are long-range correlations?
Distant chain segments cannot occupy the same space.
50
What effect causes long-range correlations?
Excluded volume effect
51
What does the characteristic ratio measure?
Chain stiffness relative to an ideal random chain
52
What does a larger C∞ mean?
A stiffer polymer.
53
What increases end-to-end distance in real polymers?
- Bond angle constraints
- Dihedral angle preferences
- Steric repulsion
54
What is correlation length?
Distance required for the chain to forget its initial direction
55
Why is DNA considered a stiff polymer?
Very large characteristic ratio
56
Persistence length of DNA?
~50 nm
57
Why doesn’t DNA form a simple random coil in cells?
Its stiffness makes predicted coil size larger than a cell
58
What is supercoiling?
Twisting DNA to compact its structure
59
What helps DNA form supercoils in cells?
Histone proteins
60
Why is supercoiling biologically important?
Allows long DNA molecules to fit inside cells.
61
What is excluded volume?
Polymer segments cannot occupy the same spatial region.
62
Effect of excluded volume on polymer size?
Expands the polymer coil.
63
Entropy change due to excluded volume?
Fewer available configurations → entropy loss
64
Two competing effects determining coil size?
1. Entropic elasticity (shrinks coil)
2. Excluded volume (expands coil)
65
Equilibrium condition?
dF/dR=0.
66
Ideal polymer elasticity is ___ in origin.
Entropic
67
Increasing temperature makes polymers ___ elastic
More elastic (stronger restoring force)
68
Excluded volume makes polymer coils ___?
Larger
69
How can a polymer in solution be approximated geometrically?
As occupying a spherical volume of radius equal to the radius of gyration Rg
70
What fraction of coil volume is actually occupied by polymer, and what does this imply?
~ N-1/2
This implies that polymer coils are mostly solvent (~99% solvent for large N)
71
What 3 conformations can a polymer adopt in solution?
- Expanded coil
- Ideal random coil
- Collapsed globule
72
What determines polymer size in solution?
Competition between excluded volume repulsion and polymer–polymer attraction
73
What is excluded volume?
Polymer segments cannot occupy the same space simultaneously
74
Effect of excluded volume on coil size?
Expands the polymer coil
75
Why does excluded volume favour expansion?
Small R gives large energy penalty
76
What parameter describes solvent quality?
Flory–Huggins parameter, 𝜒
77
What does 𝜒 compare?
Polymer–polymer vs polymer–solvent interactions
78
Why is there a negative sign in the interaction energy contribution?
Attractions lower free energy → coil collapse
79
What does the sign of Fnon-ideal determine?
Polymer expansion or collapse
80
(Solvent Quality)
Good Solvent
Condition for a good solvent?
𝜒 < 1/2
81
(Solvent Quality)
Good Solvent
Polymer structure?
Expanded coil
82
(Solvent Quality)
Good Solvent
Size scaling?
R ∝ N0.6
83
(Solvent Quality)
Theta (𝜃) solvent
Condition?
𝜒 = 1/2
84
(Solvent Quality)
Theta (𝜃) solvent
Behaviour?
Ideal random coil
85
(Solvent Quality)
Theta (𝜃) solvent
Scaling
R ∝ N1/2
86
(Solvent Quality)
Poor Solvent
Condition?
𝜒 > 1/2
87
(Solvent Quality)
Poor solvent
Structure?
Collapsed globule
88
(Solvent Quality)
Poor solvent
Scaling?
R ∝ N1/3
89
Definition of dilute polymer solution?
Polymer coils do not overlap
90
Chain interaction in dilute solution?
Independent motion
91
Viscosity dependence in dilute solution?
η∝ϕ
(polymer volume fraction)
92
What is C*?
The critical overlap concentration.
It is the conc where polymer coils start touching.
93
Behaviour below C*?
Dilute solution
94
Behaviour above C*?
Semi-dilute solution
95
What happens to chains in semi-dilute solutions?
Overlap and entangle
96
Consequences of structure in semo-dilute solutions?
- Rapid viscosity increase
- Non-Newtonian behaviour
- Cooperative motion
97
What is the structure of concentrated solutions?
Strong entangled network
98
What is the mechanical behaviour of concentrated solutions?
Viscoelastic or elastic-like
99
What is the behaviour of dilute polymer solutions?
viscous
100
What is the behaviour of semi-dilute polymer solutions?
viscoelastic
101
What is the behaviour of concentrated polymer solutions?
elastic-like
102
What does intrinsic viscosity measure?
Polymer size, shape, and hydrodynamic volume.
103
Why do polymers strongly thicken liquids?
Large coils disturb fluid flow significantly
104
Shear-Thinning Behaviour
Region I - Low Shear
Chain configuration?
Random coils, entangled
105
Shear-Thinning Behaviour
Region I - Low Shear
Viscosity behaviour?
Constant
106
Shear-Thinning Behaviour
Region II - Intermediate Shear
What happens to chains?
Stretch and disentangle
107
Shear-Thinning Behaviour
Region II - Intermediate Shear
Effect on viscosity?
Decreases
108
Why does shear thinning occur?
Flow aligns chains → less resistance to motion
109
What is the ideal random coil scaling?
R ∝ N1/2
110
What is the self-avoiding polymer scaling?
R ∝ N0.6
111
What is the collapsed globule scaling?
R ∝ N1/3
112
What governs polymer size overall?
Free-energy competition:
entropy + excluded volume vs attractive interactions
113
What is a colloid?
A dispersion of finely divided material in another medium, typically with particle sizes < 1 μm
114
What distinguishes colloids from molecular solutions?
Colloids contain dispersed particles rather than individual molecules.
115
Why do colloids have unusual properties compared to bulk materials?
They have a very large surface-to-volume ratio
116
Why is surface area important in colloids?
Creating surface requires breaking intermolecular bonds, making colloids higher-energy systems
117
What is a sol?
Solid particles dispersed in a liquid
118
What is an emulsion?
Liquid droplets dispersed in another immiscible liquid
119
What is a foam?
Gas bubbles dispersed in a liquid or solid
120
What is an aerosol?
Solid or liquid particles dispersed in a gas
121
What are bicontinuous structures?
Structures where both phases form continuous networks
122
Why do colloids have large surface areas?
Particles are extremely small.
123
What is the energetic consequence of large surface area?
Colloids are higher energy than bulk materials.
124
What drives aggregation in colloids?
Systems tend to reduce surface area to lower energy
125
What factors cause colloids to aggregate?
- Gravity
- van der Waals attractions
- Polymer interactions
126
What keeps colloidal particles apart?
- Polymers
- Thermal motion (Brownian motion)
- Charged surfaces
127
What is sedimentation?
Particles settling downward under gravity
128
What is creaming?
Particles rising upward due to buoyancy (e.g., oil droplets in water)
129
What determines particle distribution under gravity?
The balance between gravitational potential energy and thermal energy
130
What is decay length 𝛼?
Characteristic height where particle concentration decreases to 1/𝑒
131
What does a large decay length mean?
Particles remain well dispersed
132
When does terminal velocity occur?
When drag force equals buoyancy force
133
What factors increase sedimentation rate?
- Larger particles
- Higher density difference
- Lower viscosity
134
How can sedimentation be accelerated experimentally?
By centrifugation (increasing effective gravity)
135
What type of materials resist sedimentation by structure?
Gels and clay particle networks ("house of cards" structures)
135
How can sedimentation be slowed or prevented?
- Stirring or mixing
- Increasing viscosity
- Structuring the fluid (gels)
136
Why are colloidal systems often unstable?
They contain high-energy interfaces.
137
What determines whether a colloid remains stable?
Balance between attractive forces and stabilising effects
138
Why must colloidal particles be very small?
Thermal motion must overcome gravitational settling
139
What type of attractive force commonly causes colloids to aggregate?
van der Waals interactions
140
What causes van der Waals forces between colloids?
Instantaneous dipoles induced between particles
141
What determines the strength of van der Waals attraction?
The Hamaker constant, AH
142
What does the Hamaker constant depend on?
The material properties of the interacting particles and medium
143
How does van der Waals interaction energy depend on separation distance, D?
The attraction increases strongly as distance decreases.
144
What happens between identical materials in van der Waals interactions?
They always attract each other
145
Why do colloidal particles aggregate when they approach closely?
van der Waals attraction becomes strong enough to overcome thermal motion
146
What happens if particles make direct contact due to van der Waals attraction?
Irreversible aggregation (coagulation) may occur
147
What is flocculation?
Reversible aggregation where particles remain slightly separated
148
How far apart are particles typically in flocculated systems?
A few nanometres apart
149
What is bridging flocculation?
Aggregation caused when polymer chains attach to multiple colloidal particles
150
When does bridging flocculation occur most easily?
At low polymer coverage
150
Why does low polymer coverage cause aggregation?
One polymer chain can bind to two particles simultaneously
151
What is the driving force for bridging flocculation?
Polymer adsorption onto particle surfaces
152
What is depletion flocculation?
Attraction between particles caused by non-adsorbing polymers
153
When does depletion attraction occur?
When particle separation D < 2Rg (polymer size)
154
Why are polymers excluded from the region between particles?
There is insufficient space for polymer coils
155
What force pushes particles together in depletion flocculation?
Osmotic pressure from surrounding polymer solution
156
Why does depletion attraction lower system free energy?
Solvent molecules gain entropy when excluded volume decreases
157
What is steric stabilisation?
Stabilisation of colloids using polymer chains attached to particle surfaces
158
What happens when two polymer-coated particles approach each other?
Polymer layers overlap and repel each other
159
Why is polymer overlap unfavourable?
It reduces polymer configurational entropy
160
What effect results from polymer confinement between particles?
Entropic repulsion
161
What are grafted polymers?
Polymer chains chemically attached to surfaces at one end
162
Why do grafted polymers produce strong stabilisation?
Chains extend into solution forming protective layers
163
What are adsorbed polymers?
Polymer chains attached by multiple weak interactions with surfaces
163
What structural features do adsorbed polymers form?
- trains (adsorbed segments)
- loops
- tails
164
What happens at high surface coverage of adsorbed polymers?
Strong steric repulsion occurs
164
Why does high polymer coverage prevent coagulation?
Overlapping polymer layers generate repulsive forces
165
What is a polymer brush?
A dense layer of grafted polymer chains extending away from a surface
166
When do polymer brushes form?
At high grafting densities.
167
How do polymer brushes stabilise colloids?
Strong steric repulsion between stretched chains
168
What two forces control polymer brush structure?
- Osmotic pressure (swelling force)
- Elastic stretching of polymer chains
169
Why do polymer chains stretch in brushes?
High grafting density forces chains away from the surface
170
What special mechanical property do polymer brushes provide?
Extremely low friction between surfaces
171
Where is this property important biologically for polymer brushes and friction?
in knee joints
172
Why does solvent quality affect steric stabilisation?
Polymer chains must extend away from the surface to create repulsion
173
What happens in a good solvent?
Polymer chains swell and provide strong steric stabilisation
174
What happens in a poor solvent?
Polymer chains collapse toward the surface
174
Why does poor solvent reduce steric stabilisation?
Polymer–polymer attraction increases and chains shrink
174
Why do polymer-coated surfaces repel each other?
Overlap of polymer chains creates osmotic and entropic repulsion
175
What happens when brushes overlap strongly?
Large repulsive forces prevent particle aggregation
176
What is Brownian motion?
Random motion of particles caused by collisions with solvent molecules
177
What causes Brownian motion?
Continuous bombardment by rapidly moving solvent molecules.
178
Why do micron-sized colloidal particles move randomly in solution?
Because solvent molecules constantly collide with them.
179
Why do particles “forget” their initial direction of motion?
Momentum is rapidly lost due to collisions with solvent molecules
180
What type of motion describes the path of a Brownian particle?
Random walk
181
What is diffusion?
Movement of particles caused by random Brownian motion
182
What parameter determines how fast particles diffuse?
Diffusion coefficient 𝐷
183
What does the Stokes–Einstein equation show about particle size?
Larger particles diffuse more slowly.
184
What energy scale drives Brownian motion?
Thermal energy, kBT
185
What energy competes with thermal motion in colloids?
van der Waals attraction
186
Why can nanoparticles stick together irreversibly?
Attractive forces may exceed thermal energy
187
Why do colloids in water often carry surface charge?
Surface chemical processes generate charge
188
What mechanisms create surface charge?
- Surface group ionisation
- Differential ion solubility
- Isomorphous replacement
- Specific ion adsorption
189
What is the electrical double layer?
Region of counterions surrounding a charged surface
190
Why must counterions exist near a charged colloid?
To maintain overall charge neutrality
191
What two effects determine ion distribution near the surface?
- Electrostatic attraction
- Entropy (thermal motion)
192
What does the Gouy–Chapman model describe?
Distribution of ions in the electrical double layer.
193
Key assumptions of the Gouy–Chapman model?
- Ions are point charges
- Solvent is a dielectric continuum
- Mean-field approximation
- No specific ion interactions
194
What is the Debye length, 𝑟𝐷?
Characteristic distance over which electrostatic potential decays
195
What does Debye length measure physically?
Range of Coulomb interactions in electrolyte solution
196
What happens to Debye length when salt concentration increases?
It decreases
197
hat happens when two charged surfaces approach?
Their electrical double layers overlap.
198
What effect does overlap of double layers produce?
Osmotic repulsion
199
Why does osmotic pressure increase between surfaces?
Ion concentration becomes higher between the surfaces.
200
What force pushes charged surfaces apart?
Osmotic pressure from trapped counterions
201
How is the free energy of interaction related to pressure?
It is the work done against osmotic pressure as surfaces approach
202
How does electrostatic repulsion decay with distance?
Exponentially
203
How does salt concentration affect electrostatic repulsion?
Higher salt reduces the range of repulsion.
204
What does DLVO theory describe?
Stability of colloidal particles
205
What two interactions determine colloid stability in DLVO theory?
- van der Waals attraction
- electrostatic double-layer repulsion
206
What is the total interaction energy?
Sum of attractive and repulsive forces
207
What is the primary maximum?
Energy barrier preventing particle aggregation
208
What is the primary minimum?
Deep attractive well where particles stick irreversibly
209
What happens to DLVO stability when electrolyte concentration increases?
Double-layer repulsion decreases.
210
What is the secondary minimum?
Weak attraction causing reversible aggregation (flocculation).
211
What happens if repulsion becomes too weak?
Particles coagulate
211
What happens at low electrolyte concentration?
Large energy barrier → stable colloid
212
What happens at intermediate electrolyte concentration?
Secondary minimum appears → weak flocculation
213
What stabilises charged colloids in water?
Electrical double layer repulsion
214
What determines the range of electrostatic interactions?
Debye length
215
What two forces compete in DLVO theory?
Electrostatic repulsion and van der Waals attraction
216
What happens when salt concentration increases?
Debye length decreases and colloids become less stable
217
What determines the stability of a colloid according to DLVO theory?
The balance between van der Waals attraction and electrostatic double-layer repulsion
218
What does a potential energy barrier in DLVO theory represent?
A barrier preventing particles from aggregating, leading to kinetic stability of the colloid
219
What happens if the energy barrier disappears in DLVO theory?
Rapid coagulation/aggregation of colloidal particles occurs.
220
221
What is the primary maximum in the DLVO potential curve?
The energy barrier caused by electrostatic repulsion preventing particles from coming together.
222
What is the secondary minimum in DLVO theory?
A weak attraction region where particles can loosely aggregate (flocculation)
223
What is the primary minimum?
A deep energy well at very short distances where particles irreversibly aggregate due to strong van der Waals attraction.
224
What happens at low electrolyte concentration?
- Large electrostatic repulsion
- Large primary maximum
- Colloid remains stable
225
What happens at intermediate electrolyte concentration?
- Reduced repulsion
- Secondary minimum forms
- Weak reversible flocculation
226
What happens at high electrolyte concentration?
- Double layer compressed
- Energy barrier disappears
- Rapid irreversible coagulation
227
What causes electrostatic repulsion between colloidal particles?
Overlap of electrical double layers surrounding charged particles.
228
What happens when double layers overlap?
Repulsive osmotic pressure pushes particles apart.
229
How does ionic strength affect the double layer thickness?
Double layer thickness decreases with increasing ionic strength.
230
What is the Debye length?
The characteristic thickness of the electrical double layer around a charged particle.
231
What does the Hamaker constant describe?
The strength of van der Waals attraction between particles.
232
How do electrolytes influence colloidal stability?
They compress the electrical double layer, reducing repulsion and promoting coagulation
233
Why do multivalent ions destabilize colloids more effectively?
They neutralise surface charge more efficiently and compress the double layer more strongly.
234
What is the critical coagulation concentration (CCC)?
The minimum electrolyte concentration required for rapid coagulation
235
According to the Schulze–Hardy rule, what determines CCC?
The valency (charge) of the counter-ion.
236
What does the Schulze–Hardy rule imply about ion valency?
Higher valency ions coagulate colloids at much lower concentrations.
237
Which ion destabilizes a colloid most effectively: Na⁺, Ca²⁺, or Al³⁺?
Al³⁺, because higher valency ions strongly reduce the electrostatic barrier.
238
What can strong double-layer repulsion cause in colloids?
Self-assembly into colloidal crystals
239
What structure do colloidal crystals resemble?
Atomic crystal lattices, but made of micron-sized particles
240
What is a surfactant?
A surface-active agent that contains both hydrophilic and hydrophobic regions (amphiphilic molecule)
241
What are the two structural parts of a surfactant?
- Hydrophilic head group
- Hydrophobic hydrocarbon tail
242
Why do surfactants accumulate at interfaces?
Their amphiphilic structure lowers the free energy of the interface.
243
What are the four main classes of surfactants?
- Anionic
- Cationic
- Nonionic
- Zwitterionic
244
What type of head group does an anionic surfactant have?
A negatively charged head group.
245
What type of head group does a cationic surfactant have?
A positively charged head group.
246
What defines a nonionic surfactant?
A polar but uncharged head group.
247
What is a zwitterionic surfactant?
A surfactant containing both positive and negative charges.
248
What are examples of biological surfactants?
- Phospholipids
- Hydrophobin proteins
- Surfactin
249
What biological structure is formed by phospholipids?
Cell membranes (lipid bilayers).
250
What are association colloids?
Colloids formed by self-assembly of amphiphilic molecules (surfactants) or polymers.
251
What common spherical structure do surfactants form in solution?
Micelles.
252
What other structures can surfactants form?
- Sheets
- Rods
- Spheres
- Vesicles
253
What is the internal structure of a micelle?
- Hydrophobic tails inside
- Hydrophilic heads outside
254
What is a block copolymer?
A polymer consisting of two or more blocks of different monomers
255
How do amphiphilic block copolymers form micelles?
The hydrophobic block forms the core, and the hydrophilic block extends into solution.
256
What are block copolymer micelles useful for?
Dispersing hydrophobic particles or oil droplets.
257
How do block copolymers stabilise particles?
Through steric stabilisation.
258
What does adsorption of surfactants do to an interface?
Lowers the free energy of the interface.
259
Where do surfactants adsorb?
- Air–water interfaces
- Oil–water interfaces
- Solid surfaces
260
What properties can surfactants modify when adsorbed on particles?
- Wettability
- Dispersion stability
- Surface tension
261
Why can surfactants stabilise microemulsions?
They lower interfacial free energy.
262
What thermodynamic quantity describes surface energy?
Surface tension (γ).
263
What is γ (gamma)?
Surface tension or surface free energy per unit area.
264
What causes surface tension?
Imbalance of intermolecular forces at a surface.
265
What direction does surface tension act in?
Opposes an increase in surface area.
266
Why do liquids minimise surface area?
Surface molecules have higher free energy, so systems minimise surface area to reduce energy.
267
Why do bubbles or droplets coalesce?
Coalescence reduces total surface area and lowers free energy.
268
How do surfactants affect surface tension?
They reduce surface tension.
269
What does Γ represent in the Gibbs adsorption equation?
Surface excess concentration (amount of surfactant adsorbed per area).
270
What does 𝑚 represent in the Gibbs equation?
Surfactant concentration (monomer concentration)
271
What information does the Gibbs adsorption equation provide?
How much surfactant is adsorbed at an interface
272
What is plotted in a surface tension isotherm?
Surface tension (γ) vs surfactant concentration.
273
What does the slope of the surface tension plot give?
Surface excess concentration (Γ).
274
Why must Gibbs analysis be done below the CMC?
Because above CMC surfactant forms micelles instead of adsorbing at the surface.
275
What is the Critical Micelle Concentration (CMC)?
The concentration at which micelles begin to form.
276
How does the surface tension curve behave at the CMC?
It reaches a plateau.
277
What model often approximates surfactant adsorption?
Langmuir adsorption isotherm.
278
How does increasing hydrocarbon chain length affect surfactant behaviour?
- Stronger adsorption
- Lower CMC
- Greater surface tension reduction
279
What are two measures used to compare surfactants?
- Effectiveness – maximum lowering of surface tension
- Efficiency – concentration needed to achieve a certain surface tension reduction
280
What is the critical micelle concentration (cmc)?
Concentration above which micelles form.
281
What happens below cmc?
only monomers exist
282
What happens above cmc?
micelles form; monomer concentration stays constant
283
How does hydrophobic chain length affect cmc?
Longer chain → lower cmc
284
How do ionic surfactants compare to nonionic?
higher cmc due to electrostatic repulsion
285
What effect does adding salt have on the cmc?
decreases cmc (charge screening)
286
What is the Krafft point?
the minimum temperature for micelle formation (ionic surfactants)
287
What is the cloud point?
temperature where nonionic surfactants phase separate
288
What drives micelle formation?
entropy increase of water (hydrophobic effect)
289
Why is micelle formation entropy-driven?
water becomes less ordered when hydrophobic tails aggregate
290
What is aggregation number N?
number of surfactant molecules in a micelle
291
Are micelles monodisperse?
No, they have a size distribution
292
What is CPP?
The Critical Packing Parameter - a dimensionless geometric ratio used to predict the shapes of self-assembles surfactant structures
293
What is spontaneous curvature, c0?
preferred curvature of a surfactant aggregate
294
What is c0 for spherical micelles?
295
Why do cylindrical micelles form?
When packing constraints prevent spherical shape
296
What is the continuous phase in inverted micelles?
oil
297
What are lyotropic liquid crystals?
ordered structures formed at high surfactant concentration
298
What determines the structure of lyotropic liquid crystals?
surfactant concentration + temperature
299
What defines curvature?
principal radii, R1, R2
300
When is bending energy important?
when interfacial tension is low
301
What happens if bending energy is low?
flexible structures like vesicles form
302
What are microemulsions?
Thermodynamically stable mixtures of oil and water
303
Why are microemulsions stable?
very low interfacial tension
304
When are microemulsions stable?
when interfacial energy ≈ thermal energy
305
Winsor I system?
o/w microemulsion + excess oil
306
Winsor II system?
w/o microemulsion + excess water
307
Winsor III system?
three phases (oil, water, microemulsion)
308
Winsor IV system?
single homogeneous phase
309
What are emulsions?
Thermodynamically unstable dispersions
310
Why are emulsions unstable?
high interfacial energy
311
What is creaming/ sedimentation?
separation due to density differences
312
What is coalescence?
droplets merge to reduce surface area
313
What is Ostwald ripening?
small droplets dissolve, large ones grow
314
What drives Ostwald ripening?
higher solubility of smaller droplets (Laplace pressure)
315
How to reduce Ostwald ripening?
- Lower solubility oil
- Increase viscosity
- Reduce interfacial tension
- Add insoluble components
316
What causes phase inversion in nonionic surfactants?
Increase in temperature
317
Why does phase inversion happen?
headgroup dehydration → CPP increases
Core Chemistry 3
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