István Bányai, University of Debrecen Dept of Colloid and Environmental Chemistry

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1 Colloid stability István Bányai, University of Debrecen Dept of Colloid and Environmental Chemistry (Stability of lyophilic colloids see: macromolecular solutions)

2 Stabilities 1. Stability of lyophobic colloids ( colloid stability, kinetic stability Electrostatic Steric Mixed 2. Stability of lyophilic colloids (thermodynamic stability) Making unstable

3 Colloidal stability requires a repulsion force: Lyophobic colloid may be stabilized by lyophilic colloid V R V S

4 Molecular origins of van der Waals Attraction (between particles in vacuum.) V A Attraction between atoms/ molecules in vacuum r const. r r 6 Dispersion attraction between atoms / molecules is additive so it effects in case of macroscopic bodies too. depends on geometry! V A H A H 2 A Hamaker constant H a V A H Aa 12H

5 Hamaker model: calculates the attraction between particles from molecular attractions Molecules in particle 1 Molecules in particle 2 depends on geometry! The attraction of bodies arises from London (dispersion) attraction of molecules (all molecules act independently). The effect is additive; that is, one molecule of the first colloid has a van der Waals attraction to each molecule in the second colloid. This is repeated for each molecule in the first colloid, and the total force is the sum of all of these. An attractive energy curve is used to indicate the variation in van der Waals force with distance between the particles.

6 Attraction: effective Hamaker constant H V A H Aa 12H Hamaker constant: A in vacuum depends on material properties: density, polarizability The effective Hamaker constant A eff also depends on the dispersion medium H An attractive energy curve is used to indicate the variation in van der Waals force with distance between the particles. V H, J A V A H Aa 12H

7 Similar charged particles: zeta potential ψ St ζ exp x St St Positive particle with negative ion atmosphere x ~ distance from surface Plane of shear

8 Electrostatic repulsion between overlapping double layers H ~ distance between surface V R The loosely held countercharges form electric double layers. The electrostatic repulsion results from the interpenetration of the diffuse part of the double layer around each charged particle. 2 VR H 0 exp H An electrostatic repulsion curve is used to indicate the energy that must be overcome if the particles are to be forced together

9 The Balance of Repulsion & Attraction is the sum of the electrostatic repulsion and the dispersion attraction, DLVO theory: Notice the secondary minimum. The system flocculates, but the aggregates are weak. This may imply reversible flocculation. V T = V A + V R V A R H Aa 12H ( ) exp V H a kt z H The point of maximum repulsive energy is called the energy barrier. Energy is required to overcome this repulsion. The height of the barrier indicates how stable the system is.the electrostatic stabilization is highly sensitive with respect to surface charge (ζ~ψ~ ph) and salt concentration (κ, z). ze St exp 1 2kT ze St exp 1 2kT

10 Total Interaction= sum of the attractive and repulsive interactions V T = V A + V R V T,V A, V R (J) the total, attractive and repulsive energy of two spherical particles at distance d (m) The height of the energy barrier depends upon the zeta potential and 1/ sol Precipitate, or cake In the secondary minimum there is a reversible flocculation: sol- gel transformation large sediment height or gel coagulation van der Waals attraction will predominate at small and at large interparticle distances. At intermediate distances double layer repulsion may predominate, depending on the actual values of the forces. In order to agglomerate, two particles on a collision course must have sufficient kinetic energy due to their velocity and mass, to jump over this barrier.

11 Electrostatic stability of dispersions 1 2 An increase in electrolyte concentration leads to a compression of the double layer (kappa increases) and so the energy barrier to coagulation decreases or disappears. If the barrier is cleared, then the net interaction is all attractive, and as a result the particles coagulate. This inner region is after referred to as an energy trap since the colloids can be considered to be trapped together by van der Waals forces. What concentration of salt (n 0 ) just eliminates the repulsive barrier? Curve 1: Low ionic strength: primary minimum and high maximum stable colloidal dispersion. Curve 2: High ionic strength: only primary minimum unstable colloidal dispersion.

12 Critical coagulation concentration What concentration of salt (n 0 ) eliminates the repulsive barrier? If the potential energy maximum is large compared with the thermal energy, kt of the particles, the system should be stable; otherwise, the system should coagulate. Counter -ion valency c.c.c (in mol/l) ~z -6 c.c.c. is the concentration of salt just eliminates the repulsive barrier.

13 Schulze Hardy Rule The Schulze Hardy Rule: the stability depends on the sixth power of the charge on the ions! c.c.c (in mol/l) ~z -6 1:1/2 6 :1/3 6 =1:0.015: What concentration of salt (n 0 or c.c.c.) just eliminates the repulsive barrier?

14 Strength of interparticle forces Rates of coagulation Rates of coagulation can be measured by the change in the number of particles, Smoluchowski equation: dn dt DaN kd N k d is the rate of the diffusion limited aggregation or rapid coagulation (no barrier, V max =0) If there is an energy barrier, V max to coagulate then a fraction (α) of collisions is unsuccessful, so the rate of coagulation slower, k s. the stability ratio: W k k d s exp V kt max The stability of dispersion is increased by: increase in particle radius, increase in surface potential (ζ >25mV), decrease in Hamaker constant, decrease in the ionic strength, decrease in temperature. t is the time, Np the numbers of single particles per unit volume, D diffusion coefficient, k D rate constant, kboltzman constant, T temperature, V max

15 Stability ratio vs. electrolyte conc. the stability ratio: W 1 W lnw 0 k k d s

16 Stable and instable systems The larger the negative voltage value of ZP, the more dispersing power it has. Can you see this happening inside our bodies? [ A low Zeta Potential will cause blood cells to clump together. It is the force that maintains the discreteness of the billions of circulating cells, which nourish the organism ]

17 Coagulation in the human blood system A low Zeta Potential will cause blood cells to clump together. Many types of cardiovascular disease are manifest in the early stages as "moderate to significant" intravascular coagulation, and in advanced stages as "heavy to very heavy" coagulation. Numerical "Grade" (arbitrary) "Degree" of Clump * (Observed in Sclera) Probable ZP of Red Blood Cells (in situ) mv 0 Absent 17 1 Slight 16 2 Moderate 15 3 Significant 14 4 Heavy 13 5 Very Heavy 12 6 Terminal (death) 11 8 Fluid gel (5 min.) 7 10 Rigid gel (10 min.) 7

18 Colloidal stability requires a repulsion force: Lyophobic colloid may be stabilized by lyophilic colloid V R V S

19 Steric stabilization (V s potential) New repulsive force can arise by adsorption of natural or industrial macromolecules or amphiphiles. These stabilizers are in interaction with the medium: hydration, solvation Consists of three main components - entropic effect (conformational S) - osmotic effect - enthalpy effect Thickness of polymer layer We need to invest work (isotherm reversible) to push them close, within the distance determined by the adsorbed polymer. No repulsion outside of the layer. Importance: Food industry, cooking (soups) fruit juice, cocoa with milk

20 Entropy effect The details of effects The degree of freedom decreases when the two layers overlap: S<0 stabilization Effective distance H < 2r Better stbilization with increasing of the chain length and of the amount adsorbed polymer There is an attractive component: restricted volume The volume available for solvent molecules encreases

21 Osmotic effects solvent RT c ln cage c bulk The sorbed macromolecules on the particles (or amphiphiles) penetrate into each other s layers and push out solvent molecules. The chemical potential of the solvent will be lower in the cage, so is the chemical potential. As a consequence osmotic pressure arises and stresses apart the two particles. Stabilization

22 Enthalpy effect If there is good solvent (from the point of view of coating molecules) present, then the water (solvent) molecules are in thermodynamically more stable state hydrating the particles It is an repulsive potential: stabilization.

23 Steric stabilization, (no other attraction beyond Van der Waals interaction) Steric stabilization (Adsorption of polymers): 1. not sensitive on the salt concentration 2. works in non-aqueous medium 3. works in concentrated colloid systems It is difficult to plan, a lot of empirical rules exist. If the energy of attraction is larger (negative) than that of the thermal motion no coagulation happens. If it is larger (negative) then the coagulation takes place

24 Conditions of the steric stability Dispersion is stable when the kinetic energy is larger than the energy of attraction in the case of collision. It is fulfilled when the distance is enough large so the attraction is weak. Energy balance (A 121 Hamaker constant particle-medium-particle) kt >A 121 d/ (48t). Therefore the thickness of the polymer, t, should be larger than a certain value: Aa VA H t > A 121 d / (48kT) 12H A 121 (10-21 ), J A 121 /48kT, nm Oil - water Polistyrene-water carbon-water TiO 2 -water

25 Titania spheres (hidroxy-propyl cellulose)

26 Steric + electrostatic stabilization Polyelectrolytes (pl. proteines, gelatin) sorption - Neutral polymers can stabilize charged colloids Thre can be opposite effect: sensitization V Teljes = V A + V R V Teljes = V A + V R + V S

27 Sensitization A combination of long polymer, small concentration good solvent, strong adsorption Application: water purification (Fe y (OH) (x-3y) x ) A few ppm cationic poly electrolites can flocculate colloids

28 Stability of lyophilic colloids: destabilization Lyophilic colloids are liquid loving colloids (Lyo means solvent and philic means loving) lyophilic colloids: Lyophilic sols stability comes from solvation + charge. If solvation interaction alone is strong enough the colloids stay stable at its isoelectric ph if it is not they coagulate at their isoelectric ph. Gelatin is stable at its isoelectric condition so called isostable colloids, but it can be precipitate with much more salt or dehydration agent (acetone, alcohol). Casein is unstable at this isoelectric ph where there is no charge, this is a isolabile protein. Casein precipitates at iep where there is no repulsion. The isoelectric point of casein is 4.6. isostable no precipitation at iep isolabile precipitation at iep The fermentation of milk sugar (lactose) produces lactic acid, which acts on milk protein to give yoghurt its gel-like texture