Dispersions in liquids: suspensions, emulsions, and …

1 Ian Morrison 2008 steric stabilization Dispersions in liquids: suspensions, emulsions, and foamsACS National MeetingApril 9 10, 2008 New OrleansLecture 4 - steric stabilization1 Ian Morrison 2008 Rates of flocculation Strength of interparticle forces 31/ 28 = dWtkT1120exp = UdHWdkT HThe time for half the particles to flocculate is:Since flocculation is a change in average particle size, the half life can be measured. And W, the stability ratio, be stability ratio depends on the interparticle forces:Lecture 4 - steric stabilization2 Ian Morrison 2008 Hamaker model for the attraction between particlesHMolecules in particle 1 Molecules in particle 2 The intermolecular attraction is due to London ( dispersion ) energies:6111132Ur = rLecture 4 - steric stabilization3 Ian Morrison 2008 Hamaker equations for dispersion force attraction1111212 =AGH111124 =AdGHFor two flat plates (per unit area):For two spheres (per pair).

2 The A11are the Hamaker 4 - steric stabilization4 Ian Morrison 2008 Hamaker constants for some materialsSubstance A11 (10-20 J) Graphite Gold , , Silicon carbide 44 Rutile (TiO2) 43 Silver , Germanium , Chromiun Copper Diamond Zirconia (n-ZrO2) 27 Silicon , Metals (Au, Ag, Cu) 25 40 Iron oxide (Fe3O4)

3 21 Selenium , Aluminum , 14, Cadmium sulfide Tellurium Polyvinyl chloride Magnesia , Polyisobutylene Mica 10, Polyethylene Polystyrene , , , , Polyvinyl acetate Polyvinyl alcohol Natural rubber Polybutadiene Polybutene-1 Quartz Polyethylene oxide Polyvinyl chloride Hydrocarbon (crystal)

4 CaF2 7 Potassium bromide Hexadecane Fused quartz Polymethylmethacrylate Potassium chloride Chlorobenzene Dodecane , Decane Toluene 1,4-Dioxane n-Hexadecane Octane , Benzene n-Tetradecane Cyclohexane , Carbon tetrachloride , Methyl ethyl ketone Water , , Hexane Diethyl ether Acetone , Ethanol Ethyl acetate Polypropylene oxide Pentane , PTFE liquid He Lecture 4 - steric stabilization5 Ian Morrison 2008 The affect of liquid between the particlesThe effect of an intervening medium calculated by the principle of Archimedean buoyancy.

5 1211122122 AAA A=+ Introducing the approximation:[]1/ 2121122 AAA Which leads to:()()()21/ 21/ 212111221/ 21/ 21/ 21/ 212311223322 AAAandAAAAA= = Lecture 4 - steric stabilization6 Ian Morrison 2008 Lifshitz theoryLimitation of Hamakertheory:all molecules act independentlyLifshitztheory:the attractions between particles area result of the electronic fluctuationsin the describes the electronic fluctuations in the particle?the absorption spectra: uv-vis-irResult:The Lifshitz constant depends on the absorption spectra of the = nrnrAGHL ecture 4 - steric stabilization7 Ian Morrison 2008 Data for Lifshitz calculationsThe absorption spectra is measured. Often a single peak in the UV and an average IR is sufficient. That is two amplitudes and two dielectric spectrum is calculated from the absorption spectrum. The only additional information needed is the static dielectric 4 - steric stabilization8 Ian Morrison 2008 Lifshitz calculations()12321233013 2mnmkTAm == = ()()() ()()()() ()123212321232 and nnnnnnnniiiiiiii = = + + 24nkTnh =where kis the Boltzmann constant, Tis the absolute temperature, his Planck's constant, and the prime on the summation indicates that the n= 0 term is given half weight.

6 At 21 C, 1is 1014rad/s, a frequency corresponding to a wavelength of light of about m. As nincreases, the value of increases and the corresponding wavelength decreases, hence takes on more values in the ultraviolet than in the infrared or visible. The Lifshitz constant is a double summation of products of dielectric functions:The dielectric functions are differences in dielectric constants over a series of frequencies:The frequencies are:Lecture 4 - steric stabilization9 Ian Morrison 2008 Lifshitz calculation vs measurementForce - separation for TiO2 at the PZCS eparation (nm)-100102030405060F/R ( N/m)-200-150-100-50050100direction (0) IR(rad/s) CIR UV(rad/s) CUV perpendicular 86 1 x 1014 80 x 1015 parallel 170 1 x 1014 163 x 1015 Larson, I.

7 ; et alJACS,1993, 115, 4 - steric stabilization10 Ian Morrison 2008 Colloidal stability requires a repulsion force:Electrostatically stabilizedSterically stabilizedLecture 4 - steric stabilization11 Ian Morrison 2008 steric stabilizationWork is required to push polymer layers into each < When 2 then 0 HtGLecture 4 - steric stabilization12 Ian Morrison 2008 dispersion attraction between spheres12112124 AdGH =kT2tLecture 4 - steric stabilization13 Ian Morrison 2008 Criterion for steric stabilization (1storder)12148 AtdkT>12148 AdkTt>The kinetic energy must be greater than the attractive energy:12124 AdkTH>Especially when the polymer layers just touch:orFor example: A121 (x 1020) J A121/48kT Oil-water Polystyrene-water Carbon-oil TiO2 water Lecture 4 - steric stabilization14 Ian Morrison 2008 Polymer thickness for steric stabilizationcarbon/oiltitania/waterpoly styrene/wateroil/waterLecture 4 - steric stabilization15 Ian Morrison 2008A simple theory for polymer thickness Radius of gyration for linear polymers.

8 Molecular weight "Length" (nm) 1/ 22r 1,000 2 10,000 6 100,000 20 1,000,000 60 1/ 221 rMW A reasonable assumption is that the surface coating has a thickness equal to the radius of 4 - steric stabilization16 Ian Morrison 2008 The Sizeof polymers in solution[]1/31/ 22025 MWrN = []1*c =A polymer forms a random coil in solution. The polymer increases the viscosity of the solution in a manner approximately dependent onmolecular polymer size can be calculated from the intrinsic viscosity: []01lim1solutioncsolventc = The intrinsic viscosity is gotten by.

9 Where lis the Kuhn MWis molecular weight and N0is Avogadro s c* is the concentration where the viscosity is not linear in 4 - steric stabilization17 Ian Morrison 2008 steric stabilization a better modelSteric repulsionDispersion attractionCoildiameterHTG +-Lecture 4 - steric stabilization18 Ian Morrison 2008 Configurations of adsorbed polymersHomopolymersRandom copolymersBlock copolymersGrafted polymersBrushAnchorTw o or t hr e e se g m e nt sare may beattached to or grownfrom the 4 - steric stabilization19 Ian Morrison 2008 Polymers in solution - Phase DiagramsConcentrationTemperature L UOne phase regionTwo phase regionTwo phase regionSterically stabilized Dispersions are stable when the polymer is soluble the one phase 4 - steric stabilization20 Ian Morrison 2008 steric stabilization !AqueousEthylacetate141 nmsilica particles- with grafted were taken at 0 C and 60 particles phase-transfer with the change in polymer solubility.