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122 Cards in this Set
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Define a colloid |
A colloid is a dispersed phase (S/L/G) within a continuous phase (S/L/G) with a relatively high surface area to volume ratio, where the dimensions of the dispersed phase lie between 1-1000nm |
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Both the dispersed and continuous phases can be solid, liquid or gas in any combination except gas/gas. Why is gas/gas not possible? |
Cannot get gas/gas collisions due to rapid inter-diffusion (diffsuing or mixing freely so as to produce a homogenous mixture) |
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What is the nomenclature for a solid dispersed phase and a gas continous phase? (e.g. smokes) |
Aerosol |
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What is the nomenclature for a liquid dispersed phase and a gas continous phase? (e.g. clouds) |
Aerosol |
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What is the nomenclature for a gas dispersed phase and a liquid continous phase? (e.g. shampoo) |
Foam |
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What is the nomenclature for a liquid dispersed phase and a liquid continous phase? (e.g. milk) |
An emulsion |
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What is the nomenclature for a solid dispersed phase and a liquid continous phase? (e.g. ink, mud) |
Dispersion |
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What is the nomenclature for a gas dispersed phase and a solid continous phase? (e.g. expanded polystyrene foam) |
A solid foam |
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What is the nomenclature for a solid dispersed phase and a solid continous phase? (e.g. alloys, pearls) |
Solid dispersion |
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What is the nomenclature for a liquid dispersed phase and a solid continous phase? (e.g. butter) |
Solid dispersion |
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What can be noted about the surface to volume and surface to weight ratios of colloids?
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They are very high due to the small size of the dispersed particles |
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Show and explain why a 1cm cube has a surface area 100 times smaller than when it is divided into 10^6 smaller cubes of dimensions 10^-2cm.
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There is an exposure of a lot of internal surfaces. Area of a cube face: 1cm*1cm = 1cm^2 Total cube surface area: 6*1cm^2 = 6cm^2 Area of mini-cube face: 10^-2 * 10^-2 = 10^-4cm^2 Surface area of mini-cube: 6*10^-4cm^2 Total surface area of mini-cubes: 10^6*6*10^-4cm^2 = 600cm^2 |
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How do you calculate the specific surface area (As) of a sphere? What general rule can be made in terms of the relationship between As and R? |
As= Area/mass As=3/pR (where p is the density and R is the radius of the sphere) Therefore as As~1/R and as R tends to 0, As tends to infinity |
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Sedimentation (creaming) of less dense (lighter) dispersed phases from continuous liquid phases only occurs on very long time scales. Why? It is important to note that gravitational forces cannot be ignored on a longer timescale, give an example. |
Because the individual colloid particle has such a small mass that it is barely affected by gravitational forces. Over longer timescales it will have an effect, e.g. the setting of an old tin of paint over a timescale of months/years. |
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What are the two main types of S/L colloids and outline the main difference between them.
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Lyophilic colloids have strong interaction with the continuous phase (i.e. the solvent) Lyophobic colloids have no interaction with the continuous phase (except the first monolayer). Particles are dispersed and not dissolved. |
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Outline all differences between lyophilic and lyophobic colloids |
Lyophilic colloid synthetic polymers form random coils in good solvents with Rg 1-100nm. Biopolymers (e.g. DNA) have more specific conformations. Lyophilic colloids are thermodynamically stable. Lyophobic colloids can be either kintetically or thermodynamically stable. Attractive Van Der Waals forces eventually lead to coagulation over time unless repulsive repulsive forces are present resulting in an indefinite stable colloid. |
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Define dispersion, suspension |
A colloid of solid particles within a liquid phase (S/L) |
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Define sol |
A dispersion of inorganic particles (e.g. silica, gold) |
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Define a latex
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A dispersion of (organic) polymer particles (e.g. polystyrene, natural rubber etc.) |
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Define a flocculation |
An aggregation of colloidal particles without loss of original morphology (can be reversible) |
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Define coagulation |
Aggregation of particles with irreversible loss of inidividual particle morphology |
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Define agglomeration |
Aggregation of particles due to 'sticky' collisions |
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Define a gel |
A 3D macroscopic network of aggregated particles or chains in a liquid |
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Define coalescence |
Aggregation of liquid droplets (or gas bubbles) to form larger droplets (or bubbles) |
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Define surfactant: |
A surface-reactive reagent (e.g. SDS) |
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Define an amphiphile |
A molecule that contains both polar and non-polar components (e.g. a surfactant or an AB diblock copolymer) |
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Define a micelle |
A weakly aggregated structure of surfactant molecules in aqueous solution (i.e. an association colloid) |
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Define emulsion: |
A L/L colloid, e.g. oil in water (or visa/versa). Only kinetically stable with droplet size typically 100nm-100,000nm (100um) |
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Define a microemulsion |
A thermodynamically stable emulsion of fine droplets in the size range 10-100nm |
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Define a microgel |
A lightly cross-linked latex particle that can become swollen under certain conditions. Often either pH or thermally responsive |
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Why are spherical particle morphologies most common? |
Because they have a maxiumum surface area and therefore they will gave a minimum surface energy |
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Particles are not usually the same size, therefore must take a particle size distribution (PSD). Explain how Dn, Dw and Dz can be determined experimentally and give the equation for calculating PDI |
Dn: Electron microscopy (TEM, SEM) Dw: Disc centrifuge (DCP) Dz: Dynamic light scattering (DLS) PDI = Dw/Dn |
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There are two ways to make S/L (dispersion) colloids. Outline the degradation process and explain the disadvantage: |
Grind up a coarse particle in the presence of a surfactant to get smaller particles. Usually get D~1-10um and a skewed PSD as difficult to get D<1 (colloidal) and in abscence of surfactant particles simply re-aggregate. |
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There are two ways to make S/L (dispersion) colloids. Outline the aggregation process and explain the advantage: |
Build-up of small molecules, so possible to access entire colloid size range. Controlled precipitation often used for inorganic sols (lyophobic colloids). Controlled polymerisation often used for polymer particles and solution (both lyophilic and lyophobic colloids). |
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What are the two stages in the formation of a new dispersed phase (colloid), and what are their relative rates determined by? |
1) Nucleation 2) Particle growth Get small particles (high degree of dispersion) when rate of nucleation is fast and particle growth is slow |
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What must be ensured to get a near-monodisperse colloid?
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Must restrict nucleation to a relatvely short period at the start of sol (or latex) formation |
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How can you get a near-monodisperse polymer latex particle mixture? |
Emulsion polymerisation |
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How can you get a near-monodisperse polymer coils? |
Living anionic polymerisation |
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Give 3 examples of uses for near-monodisperse colloids |
1) Comercial use for calibration standards for particle sizing and particle sieves 2) Near-monodisperse silicas are used as anti-reflective lens coating 3) Colloids with narrow PSD's are useful in camera film |
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Why do some large sols grow at the expense of smaller sols? E.g. for CaCO3 why does the PSD shift over time as small particles dissapear? |
Because the smaller caCO3 particles will be more soluble and therefore will dissolve and be re-deposited on the larger particles |
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Name the 3 techniques for colloid purification and explain why theyh are necessary
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Equilibirum dialysis, centrifugation/redispersion, ultrafiltration.
Synthetic colloids generally contaminated with excess ions or organic species. Natural colloids have contaminants that depend on the sample origin |
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Outline what occurs in equilibrium dialysis |
Place a colloid in a semi-permeable cellulosic membrane. The membrane will allow the aolvent and ions to diffuse but retain the larger colloidal particles. Leave in equiblibrium with large solvent resevoir for several days and change solvent regularly |
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Outline what occurs in centrifugation/redispersion |
Sediment particles in a centrifuge or ultracentrifuge. Decant, redisperse in fresh solvent. Optimum centrifugation rate depends on particle size and density different between particles and fluid. |
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Outline what occurs in ultrafiltration |
Only ions (or small organics) can pass through and colloids stay behind |
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How can the resolution limit of optical light microscopy be deduced? How can it be improved? What can be expected for particles of a size smaller than the resolution and hence explain why optical microscopy is not very good for sizing colloids? |
Limit of resolution δ = λ / (2 n0sinθ) Can be improved by increasing refractive index by using oil immersion lens. Visible lightis only 600nm so gives δ ~ 200nm, smaller particles will lead to serious error calculations. Also has relatively small depth of focus. |
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What are the basic principles of Transmission Electron Microscopy (TEM)? |
Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through it. 1)Electrons are focused using electromagnetic lenses. 2) High vaccum chamber essential as prevents delfection from air molecules (mean free path). 3) Wavelength of electron is ~0.01nm, much shorter than visible light, so gives 0.25nm resolution limit 4) Overall magnification is 2,500-800,000, covers entire colloid range 5) TEM sample prepared by allowing dilute colloid to dry onto a carbon-coated copper grid, causing particles to separate |
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What are the advantages of TEM? |
1) Covers entire colloid range 2) Excellent magnification and resolution 3) Digital imagin analysis allows rapid data processing for particles in relation to size distribution |
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What are the disadvantages of TEM? |
1) Only get a 2D image 2) High vacuum can destroy delicate samples (e.g. proteins and emulsions) 3) High energy electron beam can damage samples (e.g. polymer latexes) 4) Expensive to buy and maintain |
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Draw a scanning electron microscope |
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Outline the principles of SEM |
A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that can be detected and that contain information about the sample's surface topography and composition. The electron beam is generally scanned in a raster scan pattern, and the beam's position is combined with the detected signal to produce an image. The most common SEM mode is detection of secondary electrons emitted by atoms excited by the electron beam. The number of secondary electrons that can be detected depends, among other things, on the angle at which beam meets surface of specimen i.e. on specimen topography. By scanning the sample and collecting the secondary electrons with a special detector, an image displaying the topography of the surface is created. |
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What are the 5 advantages of SEM? |
1) Good magnification (approx 50,000) and resolution down to 2-3nm 2) Get true 3D image 3) Excellent depth of focus 4) Relatively easy to use 5) Get semi-quantitative microanalysis from x-ray analysis |
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What are the 4 disadvantages of SEM? |
1) Need to sputter coat most samples to prevent sample-charging problems 2) 2-3nm resolution often not achievable (20-30nm) 3) Expensive 4) Some samples adversely affected by evacuation or beam damage |
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What is sputter coating and why is it essential for SEM? |
Sputter coating is the standard method for preparing non-conducting or poorly conducting specimens for observation in a scanning electron microscope (SEM) by covering them with conducting material (e.g. ultathin layer of gold). Prevents charging of sample by the electron beam. |
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All liquids scatter light due to local molecular density fluctuations caused by Brownian motion. What are the two basic types of light scattering and what are they used for in colloid science?
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1) Static light scattering, used to measure molecular weight average (Mw) of lyophilic colloids 2) Used to measure particle diameter of lyophobic colloids |
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What 3 types of substances will tend to give more intense scattering? |
1) Polymer solutions due to polymer coils 2) Colloidal dispersions due to particles 3) Micro-emulsions/emulsions due to liquid droplets. |
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Outline how dynamic light scattering works |
1) Need a coherent laser light source (e.g. He-Ne) 2 )Dispersed colloid causes random 3D diffraction giving a 'speckle' pattern 3) Bright spots indicate constructive interfecrence, dark spots destructive interference. 4) If particles were stationary speckle pattern would be unchanged with respect to time, but particles do move, so time-resolved speckle pattern analysis gives information on particle size 5) Measure the parameter known as the auto-correlation function and hence determine the diffusion coefficient D |
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How can measuring the diffusion coefficient, D, by dynamic light scattering result in calculation of the particle diameter? |
Using the Stokes-Einstein equation where RH is the hydrodynamic particle radius and 2*RH = di (intensity-average particle diameter) |
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Under what circumstances is the Stokes-Einstein equation applicable?
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Only for dilute, monodisperse, isolated spheres |
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Why do particle diameters determined by TEM and DLS usually disagree? |
1) dn (TEM) and di(DLS) are different moments of the same particle size distribution. For a size distribution of finite polydispersity di>dn 2) Because DLS measures the hydrodynamic diameter, so the solvated shell is included whereas TEM sees only the particle core |
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What are the 5 advantages of DLS? |
1) Very quick measurements 2) Applicable over a wide range of collloids 3) Non-pertubative or destructive (good for emulsions and proteins unlike TEM) 4) Well suited for near-monodisperse colloids and also for studying aggregation processes 5) Can also get useful data on non-spherical particles via angular intensity measurement (e.g. length/width ration) |
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What are the 4 disadvantages of DLS? |
1) Very sensitive due to dust particles (must rigoursly eliminate by ultrafiltration) 2) Multiple scattering if concentration of colloid is too high 3) Light absorption by coloured particles can lead to degredation 4) Can get significant sedimentation on the DLS timescale if particles large |
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Disc Centrifuge Photosedimentometry (DCP) is a high resolution particle sizing technique, it is quick, reliable, convenient and the preferred sizing technique for hard particles. How does it work? |
Particles are thrown out radially to the disc periphery and detected by change in light intensity. The fractionation of particles occurs within a disc centrifuge during measurement and dw can be calculated from the detection time. Much higher resolution thn DLS |
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What are the two methods of analysis for disc centriguge photosedimentometry? |
1) Line start mode: differential PSD determined directly so inherently high resolution; standard analysis method 2) Homogenous start mode; needs larger sample volume, measures integrated PSD, better suited for broad PSD's. |
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What are the 6 advantages of DCP? |
1) Very wide dynamic range (10-100nm) 2) Short analysis times (10-30 mins) 3) Excellent resolution compared to DLS 4) Gives weight average particle diameter 5) Works well for 'hard spheres' (non-solvated particles) 6) Can easily assess the degree os dispersion/flocculation of dilute dispersions |
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What are 4 disadvantages of DCP? |
1) Requires accurate value for particle density 2) Less good for solvated particles (sterically stabilised latexes, microgels) since the particle density is not known precisely 3) Assumes spherical morphology 4) Dynamic range depends on density difference between particles, colvent |
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Label each line to identify which particle sizing technique is best for their respective ranges
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S/L Colloids have a higher free energy than the same macroscopic material, so how can they be relatively stable with respect to particle aggregation? |
Due to high particle collision rates because of constant Brownian motion, if all collisions were sticky then would get coagulation in less than a millisecond. There must be a colloid stabilisation mechanism, colloid particles are charged, but mere columbic repulsion not enough to stabilise colloids in aquous solution. |
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What are the 4 possible origins of S/L colloid surface charge |
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What empirical experimental observations prodvide evidence of S/L colloid surface charge known as the Schulze-Hardy rule? |
1) Lyophobic sols can be coagulated by addition of electrolyte 2) Minimum critical concentration of added electrolyte reuiqred to induve soagulation depends strongly on valency of the counter ion |
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What is the origin of colloid stability? |
Electric double layer (EDL) forms, with competing short range electrostatic ordering of counter-ions and long range thermal randomisation. Colloid stabilised by unenergetically favourable EDL overlap due to generation of inter-particle repulsive force. EDL shrinks in the presence of salt (electrolyte), therefore colloid stability decreases. |
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What is the main prediction of DLVO theory? Which is the most efficient ion for destabilising negatively charged particles? |
Critical coagulation concentration for added electroyle is inversely proportional to the valency of the counter ion. I.e. much less electrolyte is required to coaggulate the colloid if the electrolyte has multi-valent ions of opposite sign to the particle surface charge. Kinetic energy barrier to coagulation normally prevents colloids from coagulating. Ag3+ most efficient ion for destabilisation of negatively charged particles. |
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Summarise DVLO theory |
In colloids there is competition between long range Van der Waals attractive forces and repulsion due to EDL overlap. The kinetic energy barrier for coagulation results in the colloid being stabilised, but the addition of electrolye reduces EDL thickness and there is enough energy to overcome repulsive force rresulting in rapid coagulation due to high frequency of sticky particle collisions.
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Explain the formation of river deltas (deposition of sediment as the river meet the sea). |
![Charged clay particles exist as a colloid. In fresh water the NaCl concentration is low so colloidly stable. As the river meets the sea [NaCl] increases, reuslting in the shrinking of the clay edl and coagulation.](https://images.cram.com/images/upload-flashcards/54/60/93/15546093_m.png)
Charged clay particles exist as a colloid. In fresh water the NaCl concentration is low so colloidly stable. As the river meets the sea [NaCl] increases, reuslting in the shrinking of the clay edl and coagulation. |
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With few exceptions, most polymers absorb onto most surfaces to some extent. If it is known that the change in entropy when a polymer is absorbed is less than 0 (ΔSads<0), what can be deduced about the entropy of the reaction?
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ΔG = ΔH -TΔS If ΔS is negative, then the -TΔS term will be positive overall. For the adsorbtion to be favourable ΔG must be a negative value, therefore ΔH must be highly negative. ΔHads<<0, reaction is ethalpically driven. |
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What can be noted about the loops and the tails? What are the arrows pointing to? |
The loops and tails are solvated. The arrows refer to the 'trains', which are polymer segments adsorbed onto the surface. |
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Why are copolymers likely to adsorb more strongly than homopolymers? |
Because homopolymer monomer repeat unit will either prefer the solvent or the surface but not both. Copolymers will adsorb more strongly because they contain two types of monomer. One comonomer can be selected to promote adsorption and the other conomoer can be designed to remain solvated. |
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What are the 5 principles of polymer adsorption? |
1) Adsorbed polymer layer is thicker if the polymer is a good solvent environment for loops and tails 2) Some weakly adsorbed polymers can be desorbed from surface at high dilution using a good solvent. 3) Adsorbed amount Γ is 0.1 - 3.0 mg m^-2 for a phyically adsorbed polymer 4) Adsorption is promoted by reduced solvency, specific polymer-surface interactions (e.g. H-bonds, electrostatics), increased surface area etc. 5) Adsorption is usually confined to a single polymer layer. Multilayer adsorption (e.g. PVA) is known, but probably due to H-bonded multi-molecular aggregates present in solution. |
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What are two general methods for preventing aggregation of S/L colloids? |
a) Charge stabilisation b) Steric stabilisation |
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Explain the basdic principles of steric stabilisation of S/L colloids
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Colloid particles are coated in adsorbed polymer. Two opposing forces for coaggulation are the long range Van der Waals attractive forces, and the steric repulsive forces. Interpenetration of adsorbed polymer layersof thickness δ is energetically unfavourable in a good solvent. The potential energy minimum Δε(mins) is usually too shallow relative to thermal energy, so particles are easily separated on close approach. Hence get elastic rather than sticky collisions. |
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Explain the occurence of weakly flocculated particles in sterically stabilised S/L colloids |
If the adsorbed polymer layer is thinner, any two particles can approach each other at a shorter distance. The thermal energy of the particles is no longer sufficient to overcome the Δε(flocc). Inter-particle collisions become sticky and flocculation occurs. Also, if there is a poor solvent for the polymer, then polymer-polymer interactions are preferred over polymer-solvent interactions. |
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Give the 4 criteria for effective steric stabilisation and show why homopolymers do not make efficient steric stabilisers. Which are the best steric stabilisers? |
1) Strong (exothermic) adsorption (ΔHads large and -ve) 2) Thick adsorbed layer (large δ) 3) Complete coverage of colloidal particles 4) Good solvency for adsorbed polymer layer 1 and 4 are mutually exclusive for homopolymers. Best stabilisers are block/graft copolymers. |
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Compare steric and charge stabilisation |
Steric stabilisation much mroe useful |
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Give 2 definitions of the surface tension γ and the equation to calculate it.
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1) The free energy change F per unit area A in mJm m^-2) 2) The surface force per unit length (mN m^-1) |
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What can be noted about these surface tension values at 20 degrees? |
Water has a high surface tension due to extensive H-bonding. Organic solvents all have similar surface tensions and mercury has a very high surface tensions due to metal-metal bonding. |
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What is a consequence of surface tension? |
Liquids minimse thier surface area by forming spherical droplets. Measuing surface tensions experimentally is difficult. |
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How can surface tensions be meausured experimentally? |
Pt ring detachment from aqueous solution. F is the minimum force required for detachment of Pt ring from surface of solution. |
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Where is the surface in an air-water interface? |
Where A1=A2 defines the Gibbs dividing surface |
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Give Gibbs absorption isotherm equation |
Γ2 can be +ve (adsorption) or –ve (depletion, or ‘negative’ adsorption) |
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Explain what this graphd emonstrates |
Long-chain alcohols are high surface active, they are strongly adsorbed at air-surface interface to give close-packed monolayers |
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What are 4 types of surfactant and examples of each? |
Anionic: SDS, Soap Cationic: Dodecylamine hydrochloride Non-ionic: Lauryl PEO Zwitterionic: Dedocyl carboxybetaine |
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What is occuring at point X to the properties of SDS? |
Discontinuity at X suggests a phase change. Get aggregation/clustering (a.k.a. micellisation) of surfactant molecules in aqueous solution at certan critical concentration X. |
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What are the 4 basic principles of surfactant micellisation? |
1) X is the critical micelle concentration (CMC) 2) The 'free' surfactant is in dynamic equilibrium with surfactant inside micelle. 3) Hydrophobic alkyl chains 'escape' from H2O by hiding in micelle core. Provides a mechanism to lower free energy of surfactant solution 4) At surfactant concentrations > X, get free surfactant micelles + surfactant in coexistence (NOT just micelles only) |
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What is the Effect of Increasing the Surfactant Concentration on Interfacial and Bulk Solution Properties? |
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What are the 5 factors affecting CMC (critical micelle concentration)? |
1) Alkyl chain length (increasing R group decreases CMC) 2) Temperature, CMC increases with T (micelle structure broken up) 3) Add electrolye, ions screen ionic repulsive forces between surfactant head groups on micelle exterior - lowers CMC 4) Surfactant binary mixtures, synergistic effect, get a lower CMC for binary mixture that for either pure surfactant alone 5) Organic molecules, sugars aid H bonding so CMC decreases, wheras formamide disrupts H bonding so CMC increases |
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What is micellar catalysis? |
Surfactants used to get faster reaction rates because of microcompartmentalisation - micelles act as mini-reactors making organic reactions occur faster within a hydrophobic interior (like emulsion polym?) |
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What is the critical micelle temperature (CMT)? |
The temparature below which the surfactant solubility is too low to form micelles. Above CMT a surfactant is more soluble, mainly as micelles |
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What is interesting about a block co-polymer such as PEO - PPO ? (Synthesised by anionic polymerisation) |
Can act as a surfactant by tuning both head and tail. |
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Why are micelles a good system to study by dynamic light scattering? What is measured by DLS and SLS respecitvely? |
Because they are relatively monodisperse. DLS measures micelle diameter whilst SLS measures micelle mass |
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How can the micelle aggregation number, (number of surfactant molecules per micelle), be calculated? |
n = micelle mass / molar mass of surfactant |
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What can be noted about micelle shape as the [surfactant] increases? |
It changes from unimers, to spherical micelles, to cylindrical micelles, to bilayer lamellae, to reverse micelles to inverted hexagonal phase |
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What dictates the micelle morphology? |
The packing parameter P: P=V/a*l V = volume of hydrophobic tail a = hydrophilic head group area l = length of hydrophobic tail |
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How can micelle morphology be related to biology?
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In living cells get a micelle lipid bilayer, exists as two-tailed zwitterionic surfactant that favours bilayer formation |
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Explain why the oil will 'roll up' in water upon addition of surfactant |
Consider balance of forces at triple boundary P given by Young’s equation:γso = γsw + γowcosθw. For oil-stained fibre in water, θw is ~ 180º so cosθw ~ -1 and γso + γow ~ γs. Addition of small amount of surfactant results in its adsorption at S/W & O/W interfaces. Adsorbed surfactant lowers γsw and γow butγso is not affected since surfactant is not oil-soluble. To keep forces balanced, cosθw must increase (i.e. θw decreases & θO increases): Hence oil stain begins to roll u. Surfactant concentration increases until θw approaches zero and oil stain is almost completely removed. This is aided by the surface pressure as surfactant spreads across O/W interface. Mechanical agitation then displaces oil droplet from fibre substrate (e.g. during spin cycle). |
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Describe the properties of L/L colloids: Emulsions |
Emulsions: 0.10-100 mm diameter; usually polydisperse; can be o/w or w/o; stabilised by emulsifiers; emulsions have a ‘milky’ appearance |
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What are 5 ways of emulsion paticle sizing? |
1) Optical Microscopy ( > 1 mm) 2) Coulter Counter (change in electrical resistance proportional to droplet size) 3) Disc Centrifuge Photosedimentometry 4) Dynamic Light Scattering (< 1 mm) 5) Laser Diffraction (Malvern Mastersizer) |
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How can you tell if something is an o/w or w.o emulsion? |
Depends on oil. water volume fractions (higher fraction is usually the continuous phase). 1) o/w solution feels creamy (e.g. milk), w/o emulsion feels greasy (e.g. butter) 2) If water-soluble dye dissolves readily then o/w emulsion, if oil-soluble die dissolves readily then w/o 3) o/w >> w/o emilsion in terms of electrical conductivity of solution |
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In the absence of an emulsifier, emulsions get phase separations within a few seconds. What are the 4 types of emulsifiers? |
1) Surfactants, e.g. SDS 2) Finely divided solids e.g. SiO2, CaCO3, BaSO4, carbon black etc. (wide range of lyophobic colloids known as Pickering emulsifiers) 3) Synthetic polymers e.g. PVA 4) Naturally occuring biopolymers e.g. albumin, cellulose, proteins, gelatin |
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What possible steps are there between an emulsion tranisitioning to being phase separated?
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Breaking emulsions (demulsification) is important for making butter (destabilising milk), tertiary oil recovery, preventing oil corrosion etc. How can it be achieved? |
Centrifugation, distillation, freezing, filtration, electrical fields |
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Microemulsions are much smaller than emulsions: typically 10 – 100 n. How can they be made? |
Need interfacial tension γ ow ~ 0 Use surfactant + co-surfactant (or block copolymer) Typically need lots of emulsifier(ionic surfactant e.g. SDS, or non-ionic surfactant) e.g. depending on whether o/w or w/o use:10 - 70 % oil + 10 - 70 % water + 5 - 40 % emulsifier (cf. 1 - 2 % for emulsions) |
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Why do microemulsions require much more emulsifier? |
Much more emulsifier needed for microemulsions, since they have much higher surface area than conventional emulsions |
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Explain why emulsions appear milky where as microemulsions appear transparent |
Emulsions appear milky because of light scattering by relatively large emulsion droplets. Small droplet size in microemulsions leads to minimal light scattering, hence transparency. |
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Why can microemulsions be considered as swollen micelles? |
Because they are an intermediate between emulsions and micelles |
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Distinguish between the thermodynamic properties of emulsions and microemulsions |
Microemulsions are thermodynamically stable as opposed to kinetically stable, and are formed spontaneously without the need for mechanical aggitation |
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Why does the polymerisation of vinyl monomers such as styrene, methyl methacrylate, vinyl acetate, vinyl chloride, acrylonitrile and various acrylates tend to be problematic? What is the solution to these problems? |
1) Vinyl polymerisations tend to be highly exothermic 2) Bulk vinyl polymerisations usually become very viscous (which leads to stirring and heat dissipation problems) 3) Solution viscosity can be lowered by using a suitable solvent, but these are expensive, toxic and cost energy to remove Solution is emulsion polymerisation: Synthesis of microscopic water-insoluble polymer particles in water |
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Consider the polymerisation of styrene. What are the typical lab-scale formulations for emulsion polymerisation? |
25-50 g Styrene Monomer ('oil') 2-4 g Emulsifier (SDS) 0.5-1.0 g Water-Soluble Free-Radical Initiator (e.g. K2S2O8) 200 g Water (continuous phase) 0-0.2 g Optional Chain Transfer Agent (e.g. C12H25-SH) |
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What is occurring in emulsion polymerisation? |
Water-soluble initiator diffuses into micelles, polymerises styrene monomer in situ. Locus of polymerisation is > 99 % inside monomer-swollen micelles. As monomer polymerises, further monomer diffuses from droplets to micelles. Only one polymer radical (growing chain) per micelle (otherwise get annihilation). Eventually get stable colloidal particles of insoluble polymer latex of 100 - 500 nm. |
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Why are the resulting latexes from emulsion polymerisation colloidally stable? |
Either charge or steric stabilisation. Syrface charge can arise from ionic surfactant, whilst steric stabilisers include PNVP, PVA |
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What is the overall process for styrene emulsion polymerisation? |
Styrene-in-water emulsion (L/L colloid), styrene polymerisation in micelles (association colloid), charge-stabilised or sterically stabilised latex (S/L colloid) |
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What are the 7 advantages of emulsion polymerisation? |
1) HIGH molecular weight polymer formed quickly at high solids (up to 50 %) 2) LOW solution viscosity with GOOD thermal control using only water as a CHEAP, NON-TOXIC 'solvent‘ so a very environmentally-friendly process 3) Very low residual monomer (> 99.9 % conversion) so highly efficient 4) Can regulate (reduce) polystyrene molecular weight using chain transfer agents (e.g. thiols) if desired 5) Can also make latexes with well-defined core-shell morphologies by a two-stage polymerisation process 6) Such core-shell latexes used to tune mechanical properties e.g. impact modifiers for rubber 7) Can get latexes with very narrow particle size distributions |
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What are microgels? |
Microgels are intermediate between non-solvated (lyophobic colloids) latexes and solvated random coils (lyophilic colloids) They are lightly cross-linked (typically 0.1- 1.0 %) and are of colloidal dimensions |
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PolyNIPAM has inverse temperature solubility in H2O: its cloud point is 32 Ci.e. polyNIPAM is water-soluble below 32 C and water-insoluble above 32 C. What occurs at 60 C? |
Gives polyNIPAM latex, not water-soluble polymer. Can tune this latex-to-microgel thermal transition by copolymerisation with various other acrylamides |
