Colloid Science Flashcards
1
Q
What is a colloid?
A
(i) A Colloid is a dispersed phase within a continuous phase where the dimensions of the dispersed phase lie between 1 nm and 1000 nm
(ii) Colloids are systems with relatively high surface-to-volume (or surface-to-weight) ratios (due to small size of the dispersed phase)
(iii) Colloids are barely affected by gravitational forces
(only applicable to certain colloids, e.g. S/L, L/L, G/L) (but on sufficient time scales, gravitational forces cannot be neglected).
2
Q
What combination of dispersed/continuous phase is allowed?
A
> All S/L/G combinations allowed except from G/G (due to rapid inter-diffusion)
3
Q
What are the names of some colloids and what are some examples?
A
> Aerosol (L/S dispersed phase and G continuous phase), fogs, mists, aerosol sprays, smokes, smogs
> Foam (G dispersed phase and L continuous phase), shampoo, washing up liquid
> Emulsion (L dispersed phase L continuous phase), milk, mayonnaise
> Dispersion (S dispersed phase and L continuous phase), inks, latex paints, soil, mud, clays sols, ceramics
> Solid Foam (G dispersed phase and S continuous phase), expanded polystyrene foam
> Solid Dispersion (S/L dispersed phase and S continuous phase), alloys, pigmented plastics, opals, pearls, butter, cream, ice cream.
4
Q
How do you calculate specific surface area (As)(area per gram) of a colloid?
A
> Sphere:
As= Area/Mass = A/pV = 4πR2/p.4/3πR3
As=3/p.R where p=density (typically 2.0g cm-3= 2x 106 g m-3) and R=radius of colloidal particle
5
Q
What are the 2 main types of S/L colloids?
A
> Lyophillic and Lyophobic.
> Lyophillic colloids have strong interactions with the continuous phase (solvent) e.g. synthetic polymer chains usually form random coils in good solvents. The radius of gyration, Rg typically lies between 1 and 100nm.
> Lyophillic colloids are thermodynamically stable (indefinitely).
> Lyophobic colloids have no interaction with continuous phase (apart from the first monolayer). Ever-present short-range attractive (van der Waals) forces between these particles eventually leads to particle aggregation/coagulation/precipitation unless repulsive forces are also operating (over sufficiently long range).
> Lyophobic colloids can be either kinetically or thermodynamically stable.
6
Q
What is a sol?
A
A dispersion of inorganic particles e.g. silica, gold, sulphur.
7
Q
What is a latex?
A
A dispersion of (organic) polymer particles e.g. polystyrene, natural rubber, poly (methyl methacrylate), polypyrrole.
8
Q
What is flocculation?
A
Aggregation of colloidal particles without loss of the original particle
morphology (can be reversible).
9
Q
What is coagulation?
A
Aggregation of colloidal particles with irreversible loss of the original
particle morphology.
10
Q
What is agglomeration?
A
Aggregation of colloidal particles due to ‘sticky’ collisions.
11
Q
What is a gel?
A
A three-dimensional macroscopic network of aggregated particles or chains in a liquid.
12
Q
What is coalescence?
A
Aggregation of liquid droplets (or gas bubbles) to form larger droplets
(or bubbles).
13
Q
What is a surfactant?
A
A surface-active agent e.g. sodium dodecyl sulfate (SDS) or C12H25SO4Na
14
Q
What is an amphiphile?
A
A molecule that contains both polar and non-polar components
e.g. either a surfactant or an AB diblock copolymer.
15
Q
What is a micelle?
A
Weakly aggregated structure of surfactant molecules in aqueous solution (a.k.a. ‘association colloid’).
16
Q
What is an emulsion?
A
A L/L colloid e.g. oil-in-water (o/w) or water-in-oil (w/o). The droplet size range is typically 100 nm to 100 μm. Only kinetically stable.
17
Q
Why are spherical morphologies most common?
A
> Maximum surface area → minimum surface energy
> Spheres are also the easiest morphologies to handle mathematically
18
Q
How is Dn (number average particle diameter) of a colloid obtained?
A
> Dn is obtained from electron microscopy (SEM, TEM)
19
Q
How is Dw (weight average particle diameter) of a colloid obtained?
A
> Dw is obtained from disc centrifuge (DCP)
20
Q
How is Dz (z average particle diameter) of a colloid obtained?
A
> Dz is obtained from dynamic light scattering (DLS)
21
Q
How is the polydispersity index (PI) calculated?
A
PI= Dw/Dn
22
Q
What values of PI correspond to perfectly-mono-/near-mono-/poly disperse?
A
> Perfectly-mono-disperse PI=1.000
> Near mono-disperse PI=1.01-1.10
> Poly disperse PI»1.10
23
Q
Describe how colloids are made by a degradation process.
A
> Grind up coarse powder (in the presence of surfactant) to get smaller particles.
> Usually get D ~ 1 – 10 mm and a skewed (Poisson) particle size distribution.
> Difficult to get D < 1 mm (colloidal) and, in the
absence of a surfactant, simply get particle
re-aggregation at end of grinding.
24
Q
Describe how colloids are made by an aggregation process.
A
> Build up from small molecules (makes it possible to access the entire colloid size range).
> Controlled precipitation is often used for inorganic sols (lyophobic colloids) e.g. silica.
> Controlled polymerisation is often used for polymer particles (both lyophobic and lyophilic) e.g. PS, PMMA latexes via free-radical emulsion or dispersion polymerisation e.g. PS, PEO, PMMA solutions via anionic or free radical solution polymerisation.
25
What are the two stages in formation of new dispersed phase (colloid)?
(i) Nucleation
| (ii) Particle Growth
26
How does the final particle size depend on relative rates of these steps?
> Get small particles (high degree of dispersion) when rate of nucleation is rapid and growth is slow.
> Get near-monodisperse polymer coils via 'living' anionic polymerisation (can control coil distribution by control of the size distribution).
27
What is Otswald Ripening and why does it occur?
> Otswald ripening occurs when large sols grow at the expense of smaller sols (lose small particles and large particles grow).
>Rationale: small sols (ions) are more soluble than large particles - they preferentially dissolve and redeposit on larger particles (depends largely on background solubility).
28
Why do S/L colloids need to be purified?
> Synthetic colloids are generally contaminated with excess ions. (when synthesising you usually know what likely contaminates are i.e. unreacted substrate or ionic byproducts).
> Natural colloids - contaminants depends on the sample origin (i.e. unknown)
29
Outline the 3 techniques used to purify colloids.
(i) EQUILIBRIUM DIALYSIS: place colloid in a semi-permeable cellulosic membrane. The membrane allows solvent/ions to pass in/out but retains the larger colloidal particle. Small particles diffuse out along chemical potential gradient.
(ii) CENTRIFUGATION/REDISPERSION: Sediment particles in a centrifuge then redisperse in pure solvent. Repeat several times. Optimum centrifugation rate and time depends on particle size and density difference between particles and fluid.
(iii) ULTRAFILTRATION: microporous membrane only allows small molecules (ions) to pass through when a pressure is applied (pressure obtained either by gravity or a pump). The larger colloids stay behind.
30
How is limit of resolution (δ) calculated (for optical microscope)?
δ= λ/2nosinθ
> We want δ to be as small as possible, therefore, we want no (refractive index) to be larger. (no = 1 in air, =1.5 in oil and =1.3 in water, therefore we use oil immersion lens to maximise δ).
31
What are the problems with optical microscopy?
> Serious errors are incurred for particle diameters less than 2 μm (2000 nm) ( ~250% error for diameter= <200 nm)
>Error in resolution (δ) increases as colloidal diameter decreases, therefore δ calculated is not useful.
>Optical microscopy also has a relatively small depth of focus.
> Light microscopy is not really used for anything smaller than 1 μm.
32
Outline the basic principles of transmission electron microscopy (TEM).
> The sample sits in a chamber ( ~1m tall) below electrons (from high energy electron gun, kv~100) which pass through the sample to give an image.
> Electrons are focussed using electromagnetic lenses - individual magnification factors multiply to give a really good overall magnification (2500 - 800 000).
> UHV conditions are ESSENTIAL to increase mean free path of electrons (otherwise they would be deflected by large air molecules). This limits the materials you can look at (UHV causes proteins to denature and oil/water to evaporate from emulsions).
> Higher energy electrons result in shorter wavelengths which results in a smaller resolution. TEM resolution = 0.25 nm to about 2 mm so covers entire colloid size range.
33
How are TEM samples prepared?
> Allow dilute colloid to dry onto carbon coated Cu grid. Get well spaced particles (carbon film - electron transparent).
34
What are some of the advantages and disadvantages of using TEM?
> Advantages:
→ Covers the entire colloid size range (1 nm to 1 mm)
→ Excellent magnification + resolution
→ Digital image analysis allows rapid data processing for particle size distributions (~103 particles per hour)
> Disadvantages:
→ Only get a two-dimensional image
→ High vacuum conditions can destroy delicate samples e.g. proteins, emulsions
→ High electron energies (100 - 300 kV) can cause beam damage to sample e.g. get melting / burning / deformation of polymer latex particles
→ Expensive to buy (£ 200 K !) and maintain (need a technician, service contract).
35
Outline the principles of the scanning electron microscope (SEM).
> SEM image is displayed synchronously with the primary electron beam as the latter is rastered across the sample (secondary lower energy electrons are detected).
> A 3D image is obtained of the sample.
36
What are some of the advantages/disadvantages of SEM?
> Advantages:
→ High magnification (~50 000) and resolution down to 2-3 nm.
→ True 3D image
→ Excellent depth of focus ( x350 - 500 better than optical microscope).
→ Easy to use (compared to TEM)
→ Can get elemental info (semi-quantitative microanalysis) via analysis of emitted xrays (for Z > 12) (extra cost).
>Disadvantages:
→ Need to sputtercoat (coat in Au) most samples (except conductors) to prevent sample-charging problems.
→ 2-3 nm resolution is often not achievable (nearer 20-30 nm).
→Expensive -up to £ 200 K if x-ray capability required as well.
→ Certain samples adversely affected by either evacuation and/or beam damage
37
What are the 2 basic types of light scattering?
(i) Static light scattering (SLS) - used to measure the weight-average molecular weight (Mw) of LYOPHILLIC colloids (i.e. polymer solutions).
(ii) Dynamic light scattering (DLS, also known as 'photon correlation spectroscopy') - used to measure particle diameter of S/L LYOPHOBIC colloids (i.e. particulate dispersions), EMULSIONS and LYOPHILLIC colloids.
All liquids scatter light due to local (molecular) density fluctuations caused by Brownian motion. Scattered light is very sensitive to particle size (larger particles scatter more). θ is variable (30°-150°), particles in the sample scatter light through 360° (you choose which angle to detect at).
Get additional, much more intense scattering from:
→ Polymer solutions - due to polymer coils
→ Colloidal dispersions - due to particles
→ Microemulsions/emulsions - due to liquid droplets.
38
How is I (in DLS) calculated?
I = Io/4πR2
| Iscatt α r6 (r = particle radius)
39
What pattern does a dispersed colloid make with random 3D light diffraction?
>Dispersed colloids cause random 3D light diffraction and give a "speckle" pattern: bright spots= constructive interference and dark background= destructive interference.
>If particles were fixed in space (e.g. a gel), the speckle pattern doesn't change over time, however, particles in solution DO move (Brownian motion) so speckle patter does change over time.
∴ time resolved speckle pattern analysis provides particle size information.
40
How can the diffusion coefficient (D, in m2 s-1) be determined by light scattering?
> Can measure 'auto-correlation' function g(T) and determine diffusion coefficient using the STOKES-EINSTEIN EQUATION:
D=kT/6π.𝜂.RH
where:
𝜂=viscosity of continuous (solution) phase (N s m-2) - KNOWN
k- Botlzmann's constant - KNOWN
T= absolute temperature (K) - KNOWN
D= MEASURED
RH= hydrodynamic particle radius, 2x RH = di~dz
>This equation is valid only for dilute, isolated, monodisperse spheres.
41
Why are particle diameters determined by TEM different to those determined by DLS?
> DLS reports an 'intensity-average' particle diameter, di (similar to dz) whereas TEM reports dn.
>dn and di are 2 different moments of the same particle size distribution, di>dn always.
>Also, DLS measures the hydrodynamic diameter of a colloid where any solvated shell is also included (usually small). TEM only 'sees' the particle core.
42
What are the advantages of DLS?
> Advantages of DLS:
→ very quick measurements (minutes)
→ applicable over (almost) entire colloid size range (3nm-1μm)
→ well suited to sizing 'near mono-disperse' colloids and also for studying aggregation processes (sensitive to small changes in particle size).
Non-perturbative and non-destructive so good for (micro)emulsions and proteins unlike TEM or SEM.
→ can get data on non-spherical particles via angular intensity measurements.
43
What are the disadvantages of DLS?
>Disadvantages of DLS:
→ very sensitive to dust particles (much larger than colloids so dominant scatterers) ∴ must rigorously eliminate dust via ultrafiltration before analysis.
→ get multiple scattering if colloid is too concentrated ∴ usually need to work with dilute (<<1%) solutions.
→ Light absorbed by coloured particles can cause local heating/convection currents.
→can get significant sedimentation if particles are large.
44
Outline the process of DIsc Centrifuge Photosedimentometry (DCP).
> DCP comprises a hollow, vertically mounted Perspex disc, sample injection point, transmission light source/detector and PC.
>Light beam passes (at 20000 rpm) through disc → large, heavier particles move faster and are detected first (by change in light intensity).
>Fractionation of particles occurs within a disc centrifuge during measurements.
Detection time,t: t= [k.𝜂.ln(Rd.Ri)]/[ω2dw2Δρ]
i.e. t is proportional to 1/Δρ.dw2
where:
k and ln(Rd.Ri) = known constants
𝜂= solution viscosity
ω= centrifugation rate, (typically 500-15000 rpm)
Δρ= density difference between the particles and the spin fluid
dw= weight-average particle diameter
Note: DCP is not suited for particle mixtures with different densities.
>When analysing larger particles, the upper limit can be extended by either increasing 𝜂 or decreasing Δρ (otherwise particles move to disc periphery too quickly).
45
What other type of source/detector can be used for DCP and what advantages/disadvantages are associated with it?
>can use an xray source/detector (instead of light)
>Disadvantages:
→more expensive
→not useful for most latexes (usually comprise only low Z atoms like C,H,N,O etc which don't scatter xrays very strongly).
>Advantages:
→ No assumptions required concerning scattering/absorption characteristics of particles.
46
What two analysis methods can be used with DCP?
1) 'Line Start' mode → differential PSD determined directly so inherently high resolution (standard analysis method).
2) 'Homogeneous start' mode→ needs larger sample volume; measures integrated PSD and calculates differential SD; better suited for broad PSDs.
47
What is the effect of coating a sterically - stabilized, micrometer-sized latex with 'sticky' overlayer? (DCP)
> induces flocculation of the latex
>DCP=sensitive to wear aggregation and doublets/triplets can be seen/confirmed.
48
What are the advantages of DCP?
>Advantages:
→ very wide dynamic range; 100nm → 60μm
→ short analysis times (10-30 minutes)
→ excellent resolution compared to DLS
→ Gives dw directly
→works well for 'hard spheres' (non-solvated particles): gives good results for silica sols, PS latex..
→ can easily assess the degree of dispersion/flocculation of dilute dispersions.
49
What are the disadvantages of DCP?
> Disadvantages:
→requires accurate value for particle density (needs helium pycnometry)
→ less good for solvated particles (sterically0stabilised latexes, microgels) since the particle density is not known precisely [low Tg latexes=problematic as they form films on internal wall of disc at end of analysis).
→assumes spherical morphology
→dynamic range depends on density difference between particles and solvent.
50
S/L colloids have higher free energy than the same macroscopic material, so how can they be stable w.r.t particle aggregation?
> Due to constant Brownian motion, particle collision rates are high →if all collisions are 'sticky' get coagulation in <1ms
τ1/2 = 3𝜂/ 4ckT
```
where:
τ1/2= half life
𝜂= solution viscosity
c=concentration
assumption: all interactions are sticky
```
>However, faradays gold sols remain stable so there must be a stabilisation mechanism.
51
What experimental evidence is there for a S/L colloid stabilisation mechanism?
(i) Aqueous S/L colloids (lyophobic) are stable in H2O, but coagulate by addition of electrolyte (e.g. NaCl) [SCHULZE-HARDY RULE]
(ii) Particles move towards electrodes in an electric field (optical microscopy and electrophoresis) .
>These observations suggest that the colloidal particles are charged.
52
Charged particles have repulsive force between them (Coulombic repulsion), what equation is used to express this?
Frep= Q1Q2/𝜀0𝜀rr2
𝜀r=1 in a vacuum and 80 in water
∴ Frep= very weak in water and CANNOT account for colloidal stabilisation.
53
What are the 4 potential origins of colloid surface charge?
1. ION ADSORPTION: e.g. Fe3+ onto silica (can be either cations or anions)
2. IONISATION: COOH (e.g. from the initiator or surfactant) or SiOH (silica) can ionise.
3. ION DISSOLUTION: At certain conditions, ions can leave resulting in a surface charge.
4. ISOMORPHOUS SUBSTITUTION: e.g. Clays surface =anionic, edges= cationic (platelets). In silicate clays, Al3+ replaces Si4+ to product a net negative surface charge.
54
What are the 2 components to the Schulze-Hardy rule?
1. Lyophobic sols can be coagulated by addition of electrolyte (salt).
2. Minimum critical concentration of added electrolyte required to induce coagulation depends on the valency of the counterion.
55
What is the electrical double layer (EDL)?
> For charged, flat plates immersed in aqueous electrolyte, there are 2 competing effects:
(i) electrostatic ordering of counter-ions near plate surface (short range).
(ii) Thermal randomisation very near (few nm)
>Observation of these effects led to the concept of the electrical double layer (EDL).
>It was found that the surface potential (ϕ0, mV) depends on surface charge (σ0):
σ0= k𝜀ϕ0
where:
𝜀= dielectric constant for solvent
k- constant where 1/k = the approximate thickness of the EDL.
>EDL concept is applicable to charged colloidal particles.
>Energetically unfavourable EDL overall generates interparticle repulsive force (Frep) ∴ EDL overlap = origin of colloid stability.
56
How does the addition of electrolyte affect the EDL?
> Adding electrolyte causes the EDL to shrink which leads to colloidal instability (1/k decreases).
57
What is DVLO theory?
> DVLO theory assumes additivity of PE curves (van der Waals attractive forces, VA and repulsive forces due to EDL overlap, VR).
>Emax= kinetic energy barrier to coagulation (doesn't allow VA to get to strong).
>For colloidal stability: Emax>> KT (thermal energy of particles).
>Adding electrolyte reduces EDL thickness ∴ lowers Emax, so thermal energy becomes sufficient to overcome repulsive forces and attractive forces take over. → get rapid coagulation due to high frequency of 'sticky' inter-particle collisions.
NOTE: charge-stabilised colloids are only kinetically stable (but can be stable for a long time).
58
Is polymer adsorption entropically or enthalpically driven?
> Adsorption = entropically enfavourable and enthalpically favourable
∴ is enthalpically driven (and usually strong and irreversible).
59
Why is it difficult for homopolymers to adsorb strongly?
> Since the monomer repeat unit either like the surface or the solvent but not usually BOTH (some exceptions).
60
Which type of polymers usually adsorb the strongest?
> Copolymers are more likely to adsorb strongly since they contain at least 2 types of monomer.
61
Outline the 5 main principles of polymer adsorption.
1) Get thicker adsorbed polymer layers (more solvated, expanded) if polymer is in a good solvent environment for loops and tails.
2) Adsorption is not always irreversible. Weakly adsorbed polymers can be desorbed from surface at high dilution using a good solvent.
3) Adsorbed amount (mg) of polymer per m2 of substrate, Г, for physically adsorbed polymers = 0.1 - 3.0 mg m-2. Г = 0.1 - 0.5 mg m-2 for polyelectrolytes and Г = 1 - 3 mg m-2 for non-ionic polymers.
4) Adsorption = promoted by reduced solvency (pH, temp), specific polymer interactions (e.g. H bonds, electrostatics), higher surface area etc → can modify specific interactions using H acceptor/donor groups.
5) Adsorption is usually confined to a monolayer. Multilayer adsorption (e.g. PVA) can occur due to H-bonded multi-molecular aggregates in solution.
62
What are the 2 general methods for preventing aggregation of S/L colloids?
1) Charge stabilisation
| 2) Steric stabilisation (involves colloidal particles coated with adsorbed polymer - more generally applicable).
63
What are the basic principles of steric stabilisation?
> There are two opposing forces; (i) Long-range, ever-present van der Waals attraction (Va) and (ii) Short-range steric repulsion (Vs).
> Interpenetration of adsorbed polymer layers of thickness δ is enthalpically AND entropically unfavourable due to desolvation and constraints (in a good solvent) → occurs at 2δ where Δεmin << kT.
>Potential energy minimum (Δεmin) is usually too shallow relative to thermal energy (kT): PARTICLES ARE EASILY SEPARATED AFTER CLOSE APPROACH i.e. get 'elastic' collisions (there is enough energy to get to 2δ and get away again).
> If adsorbed polymer layer is thinner, any two particles can now approach each other within a distance δ.
> If solvent is poor (χ < 0.50) then polymer-polymer interactions are preferred over polymer-solvent interactions. Δεflocc>Δεmin>kT. Thermal energy of particles is no longer sufficient to overcome Δεflocc ∴ interparticle collisions are 'sticky' and the dispersion is FLOCCULATED.
64
How do steric stabilisation and charge stabilisation compare?
> Steric stabilisation:
→ Poor theoretical understanding atm
→works for aqueous and non-aqueous systems
→insensitive to added electrolyte
→works well at high colloid concentration (>60%)
>Charge stabilisation:
→ DLVO theory= well developed
→works in aqueous or mix aqueous systems only (charge needs high ε)
→ very sensitive to added electrolyte (lowers the kinetic barrier Δεmax).
→ Not very effective at high colloid concentration (<10%).
OVERALL: Steric stabilisation is generally more useful.
65
What are the criteria for effective steric stabilisation?
(i) ΔHads = large and negative.
(ii) Thick adsorbed layer (large δ).
(iii) Complete coverage of colloidal particles.
(iv) Good solvency for adsorbed polymer layer.
NOTE: (i) and (iv) = mutually exclusive for homopolymers (∴ generally don't make efficient steric stabiliser).
66
What are the best steric stabilisers?
> Usually statistical or block/graft copolymers.
67
How is surface tension,ϒ calculated?
ϒ= (dF/dA)T/V = Free energy change F per unit area A.
units = mJ m-2 (millijoules per metre squared) OR mN m-1.
68
How is surface tension, ϒ measured and what difficulties are there?
> ϒ is measured using Pt ring detachment from aqueous solution. Ring detachment force (F) = directly related to surface tension:
ϒ= (𝛃 x F)/ (4πR)
where:
𝛃 = known constant.
R= ring radius.
>Measuring surface tension is difficult as it is very susceptible to impurities (need ultra-pure liquids and ultra-clean surfaces).
69
What does the value of the surface tension tell you about a liquid?
> ϒ = high for liquids with strong intermolecular interactions
i.e. ϒ (H2O) = 72.8 (due to extensive H bonding).
ϒ (Mercury) = 485 (due to meta-metal bonding).
70
What is the consequence of a high ϒ value?
> Liquids tend to form spherical droplets.
71
During adsorption at the air-water interface, where is the surface?
> Introduce a 2nd component that is surface active. The surface becomes defined so there is no net adsorption or depletion of H2O molecules.
72
What is the Gibb's adsorption isotherm equation?
dϒ/dlnc2 = -RYГ2/c2
Г2 can be positive (adsorption) or negative (depletion).
73
How is surface excess concentration calculated?
Г= csurface - cbulk
74
What are the 4 general classes of surfactant?
1) Anionic: anionic head group
2) Cationic: cationic head group (has antibacterial properties)
3) Non-ionic: overall charge= neutral
4) Zwitterionic: neutral overall
75
What evidence suggests a phase change from colloids to micelles?
> There is a discontinuation in the solution properties of a surfactant at the SAME CONCENTRATION which suggests a phase change. (properties such as osmotic pressure, turbidity, surface tension and molar conductivity).
> This is because aggregation/clustering occurs in aqueous solution at the critical concentration x.
76
Describe key aspects of surfactant micellisation.
> The core is close packed (but no overlapping of the tails) and the micelle radius = length of surfactant.
> χ= critical micelle concentration (CMC). When surfactant concentration is above this value, get free surfactant and micelles in coexistence (not just micelles).
>The 'free' surfactant is in dynamic equilibrium wit surfactant inside the micelle. Mean residence time of surfactant within micelle is 10^-1 - 10^-6 s at 25 degrees. i.e. mean exchange frequency is ~ 10^3-10^6 s-1.
>Hydrophobic alkyl chains 'escape' from H2O by hiding in micelle core. This provides a mechanism to lower free energy of surfactant solution.
77
What is the effect of increasing the surfactant concentration on interfacial and bulk solution properties?
> Adding surfactant to a beaker of water results in a decrease of surface tension as surfactant adsorbs at the air/water interface and forms a monolayer. (also adsorbs at the glass surface to avoid water as much as possible).
78
What factors affect CMC?
> Alkyl chain length: e.g. for CnH2n+1SO4-Na+, increasing n (length of R group) decreases CMC (micelles form more easily and the larger hydrophobic tail = the driving force to form a micelle.
>Temperature: CMC increases with temperature (ordered micelle structure is broken up by thermal energy).
>Electrolyte: e.g. NaCl - decrease CMC, additional ions 'screen' ionic repulsive forces between surfactant head groups on micelle exterior - non-ionic surfactants are much less sensitive to addition of salt.
> Binary mixture: e.g. anionic/neutral or cationic/neutral surfactants. Synergistic effect - get lower CMC than for pure surfactant (because mixing surfactants results in very non-ideal behaviour which favours micellation and neutral head groups have a screening effect).
>Organic molecules: depends on the effect on the water structure. Sugars aid H bonding ('structure-makers') so CMC decreases . Formamide and Urea disrupt H bonding ('structure breakers') so CMC increases. .
79
What is the critical micelle temperature? (CMT)
> Below the CMT ('Krafft temp), surfactant solubility os too low to form micelles.
>Above the CMT, a surfactant is much more soluble - mainly as micelles.
>As n (length of alkyl chain) increases, CMT increases.
80
Why do block co-polymers make useful surfactants?
> can tune both the head and the tail.
81
Discuss the shape and size of micelles.
> Micelles are relatively mono-disperse (DLS gives micelle diamter and SLS gives micelle mass).
>Micelle shape changes as [surfactant] increases: spherical → cylindrical → bilayer → reverse micelles → inverted hexagonal phase.
82
What dictates micelle morphology?
> packing parameter, P:
P=V/a0 l
where:
V= volume oh hydrophobic tail
a0 = hydrophillic head group area
l= length of hydrophobic chain
>P subtly changes as [surfactant] changes - morphology affects viscosity.
83
What is micelle aggregation number and how can it be calculated?
> micelle aggregation number = number of surfactant molecules per micelle.
n=micelle mass/ molar mass of surfactant
