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Chapter 8. Dispersion and Flocculation of Surfactants

Chapter 8. Dispersion and Flocculation of Surfactants. 2006.05.20. §1. Introduction . Dispersion – multi-phase dispersing system S/G – dust , smoke, and so on ; S/L – suspension ( 悬浮液 ) ; colloids ( 胶体 ) dispersephase ( 分散相 ) – dispersed solids dispersed medium ( 分散介质 ) – water

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Chapter 8. Dispersion and Flocculation of Surfactants

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  1. Chapter 8. Dispersion and Flocculation of Surfactants 2006.05.20.

  2. §1. Introduction • Dispersion – multi-phase dispersing system • S/G – dust , smoke, and so on ; • S/L – suspension (悬浮液) ; colloids (胶体) • dispersephase (分散相) – dispersed solids • dispersed medium (分散介质) – water • thermodynamic unstable systems – dispersants 2. Flocculation – destabilization (失稳定) of colloids • Static interactions between colloids • Steric interactions between colloids • Flocculating agents or Flocculants

  3. §2. Interfacial potential at interface of solid-liquid Interface potential – properties of S/L - electric double layer 1. The electrification (带电) at interface of solid-liquid • Ionization at interface solid-water – e.g. proteins , ion exchange resin , etc. protein possess isoelectric points as IEP pH > IEP to ionize anions or negative charge at S/L pH < IEP to ionize cations or positive charge at S/L • Adsorb the ions from bulk phase - in preference to adsorb anions or cations to electrify at S/L (a) Adsorption on Low Energy Surface - in preference to adsorb anions to possess negative charge. E.g. oil and

  4. Fat, synthetic fibres , and carbon etc. Reason : the cations are hydrated easier and more stable than anions in bulk water phase, so the anions are adsorbed easier than cations. (b) Indissoluble salts (难溶盐)– Fajans rule - homo ions (同离子)are adsobed easier by ionic crystal e.g. AgI colloid adsorbs the Ag+ ion in AgNO3 aq. to possess positive charge and the I- ion in KI aq. to possess negative charge. (c) Metallic oxide & Indissoluble hydroxid – e.g. SiO2, TiO2, ZnO2, and etc – possess Zero Electric Point (ZEP):

  5. If pH > ZEP, then in preference to adsorb OH- to possess negative charge on interface S/L If pH < ZEP, then in preference to adsorb H+ to possess positive charge on interface S/L (3) Triboelectrification (摩擦起电) – not only Solid – Water but also Solid – Organic medium. Reason : according to difference electron affinity between(电子亲合力) two phase, the electrons are ejected from one phase to another. Dielectric constant (介电常数) , electron affinity  Positive charge , contrarily negative charge e.g. glass = 5-6, benzene =2, water=81, aceton=21 G/Water, G/aceton – negative ; G/benzene – positive

  6. (4) Replace of crystal lattice – e.g. kaoline (高岭土), montmorillonite (蒙脱土), etc Mg++, Ca++ Al+++ negative charge 2. Electrical Double Layer Model • Helmholtz EDL - Plate Model Surface potential 0 = (4/D)  • - surface charge density • - thickness of Plate EDL,  very little ,no displaying electricity , neutral ,

  7. (2) Gouy-Chapman diffusion EDL • Outline as follow • Electrification at S/L interface, • Counter ions as a particle in solution phase • Diffusion EDL • Thermodynamics potential 0 • Interfacial potential  = 0 e-x • Thickness of diffusion EDL • = 1/ = (1000DkT/4NAe2CjZj2)1/2 e = 4.80×10-10, k= 1.38 ×10-16 erg/k NA= 6.623 ×1023mol-1, D = 78.3 (H2O,25°C) -1 = 4.20×10-8/(CjZj2)1/2 = 4.20×10-8/(2Cj)1/2 1:1

  8. (b) Disadvantage • The point charge hypothesis n0- the concentration of positive and negative ions in bulk phase in diffusion layer : Counter ions: nc= n+=n0exp(Ze/kT) Homo ions: nh= n-=n0exp-(Ze/kT) If n0 and 0  nc » nh may be reasonless • Only static interaction between ions and interfacial of solid-liquid

  9. (3) Stern Model – Helmholtz & Gouy- Chapman • Outline as follow • The ions which includes hydrate water possess size ; • Not only static interaction , but also dispersion force between ions and interfacial of solid-liquid • Stern layer & Diffusion layer (b) Stern layer • IHP – Inner Helmholtz Plane • Counter ions – static interaction mostly • Homo ions – dispersion force mostly e.g. surfactants – characteristic adsorption • Partially hydrated ions -

  10. OHP – Outer Helmholtz Plane – hydrated counter ions (c) Diffusion Layer – same with G-C Model (d) Surface potential • Surface Thermodynamic Potential 0- from S/L interface to bulk phase: 0= 0(T,P) • Surface Stern Potential S- from Stern Layer to bulk phase: Diffusion potential = Sexp-x • Adsorbed counter ions |S| < |0| until showing reversal (相反)of surface potential • Adsorbed homo ions |S| > |0| • -Potential – from plane of shear at S/L to bulk phase – electrokinetic potential

  11. (e) S and -Potential • -Potential can be determined, but S cannot. • |S|  ||  the plane of shear is more far from the S/L interface than Stern Plane • Small electric potential gradient (电位梯度): |S|  || • If ion strength (I) or Stern potential (|S| ) is low, and • Thickness of diffusion EDL(-1) is long, ales |S| » || (4) Zeta potential (a) Mensuration • Electro-phoresis (电泳) • Electro-osmosis (电渗)

  12. (b) Factors effecting Zeta potential • Characteristic adsorption – ionics • Electrolyte – the electrical double layer is compressed, electric potential gradient is increased , and ||

  13. Relations of Zeta potential and Гof cationics on bentonite (膨润土) Relations of Zeta potential and Гof SMP on bentonite

  14. §3. Dispersion of solid • DLVO theory – stability theory of colloid independently by Derjaguin and Landau(Soviet Union) in 1945 and Verwey and Overbeek (Holland) in 1948 • The potential energy of attraction between particles VA • The potential energy of attraction between molecule Van Der Waals’s energy (force) : = --6 including induction (Debye) force , dipole (Keesom) force , and dispersion (Landon) force

  15. H r • The potential energy of attraction between particles VA= - (A r/12H) If H « r as a plane particle VA= - (A r/12H2) A – Apparent Hanaker constant A = [(A2)1/2 – (A1)1/2]2 A1,A2- Hamaker constant of particle and dispersion medium

  16. (2) The potential energy of repulsion between particles VR= (rDU2/2) ln [1+exp-H] D –dielectric consrant of dispersion medium U – potential between adsorbed layer and diffusion layer -1- thickness of diffusion DEL (3) The Total potential energy V= VA+ VR • r « -1 • The site of first minimum – agglutination • The site of second minimum – flocculation • VM – maximum VM/kT15-25 stable colloid

  17. Bron repulsion energy (b) r » -1 Instable VM0 (c) Total potential energy VA, stability VR, stability VT= VA + VR = -Ar/12H + (rDU2/2)ln [1+exp-H] • -1, D, U, and A then stability • I , -1, ||, then stability • Counter Ions – the radius of hydrated ions, ability • Cations – H+ > Cs+ > Rb+ > K+ > Na+ > Li+ • Anions – F- > IO3- >H2PO4- >BrO3- >Cl- >ClO3->Br->I->CNS-

  18. (4) Limitations of the DLVO Theory The stability of lyophobic dispersion is limited to the effect of surface potential of the particles. • A decease in the contact angle of dispersing medium on solid may increase dispersibility; • Surfactants that are polymeric or that have long POE chains may form non-electrical steric barriers; • In liquids of low dielectric constant, surfactants may produce steric barriers to aggregation; • For highly solvated particles in particular the Zeta potential may be quite different from s.

  19. 2. Steric Forces – stability and flocculation of polymers & POE nonionicsh (1) An entropic effect – due to restriction of the motion of the chains extending into the liquid phase when adjacent particles approach each other closely. When H  -1， the effects becomes particularly important. (to see a) (2) A mixing ffect – due to solvent-chain interactions and the high concentration of chains in the region of overlap. if chain-chain>solvent-chain, overlap, G, dispersion if shain-chain<solvent-chain, overlap, G, flocculation (3) Both effects Number of adsorbed chain effect Length of adsorbed chain effect

  20. 3. Applications • The addition of a cationic surfactants to a negatively charged colloidal dispersion. • First step – cationic surfactants , ||, ||,stability reaching to the point of zero charge and a minimum, • Second step – cationic surfactants, changing to positive sign , ||, ||, stability • Third step - cationic surfactants, compressing to the electrical double layer (2) The addition of a polymeric ionic surfactants to a colloidal dispersion of same sign • First step - surfactants, potential & stability  • Second step – surfactants, plane of shear away from the surface ||, steric barrier, stability 

  21. (3) The addition of a POE nonionic surfactants to an aqueous dispersion carried a small negative charge • The stability increased sharply as adsorption of the nonionic surfactants • The stability at this point is very high even when the electrical double layer is compressed by I or pH 4. Role of the surfactants in the dispersion process • Wetting of the powder – driving force:spreading works SL/S=SV - SL - LV • Adsorption of solution surface - LV  C > LV • Adsorption of S/L interface - SL 

  22. (2) De-aggregation (解聚集) of Fragmentation (劈裂) of particle clusters (团粒) – mechanisms • By being adsorbed in “microcracks” (微裂纹) in the solid – permeation (渗透) – to reduce self-healing ability particles P = 2LVcos/R • < 90° P > 0 then penetrable, else cannot cos  = (SV - LS)/LV Addition surfactants SV &LV cos  ,   (b) By being adsorbed an ionic surfactants onto the particles in clusters – acquire an electrical charge of similar sign – to reduce the energy required to rupture solid : homo-ionics > nonionics > counter ionics (instable and flocculation)

  23. (3) Prevention of reaggregation (阻止再聚集) (a) Reduce the thermodynamic instability of dispersion LS×A, LS  (b) Increase the dynamic stability of dispersion Eelectric & Esteric  5. Dispersing of surfactants • Aqueous dispersion • Nonpolar powders – e.g. black carbon (low energy surface) – addition surfactants LV  C> LV • Charged and Polar powders – e.g. metallic oxide (high energy surface) • Homoions – electrical barrier, stability  • Counter ions – first step , flocculation second step hydrophobic adsorption, ,dispersion

  24. (2) non-aqueous dispersion • Inorganic powders – high energy surface A=[(A2)1/2-(A1)1/2]2– surface modification – low energy surface – e.g. TiO2 ZEP=5.8 surface negative potential in neutral – TiO2+aluminium salts (positive potential) + carboxylate surfactant (anionics) - oriented adsorption of hydrophilic groups – the hydrophobic chains as a steric barrier on surface of particles. • Organic powders – low energy surface – surface modification – e.g. organic pigments + stearic amine - oriented adsorption of hydrophilic groups • Steric barrier • Hamaker constant

  25. (3) Dispersants • Water diapersants • Anionic – naphthaline dispersants (NNO), lignosulfonate (木质素磺酸盐), and polymer (polyacrylic acid ester) • Nonionic – Tween series, alkyl alcohol ether , alkyl phenolic ether etc • Zwitterionic – amino acidic , betaine (甜菜碱) (b) Organic medium dispersants • Inorganic particles - aliphatic amine (脂肪胺) , alcohol , and organosilicon

  26. (c) Super-dispersants – nonaqueous – e.g. Pigmento-philic – Lyophilic (亲颜料亲液）system • Characteristics and mechanism of dispersion: • M=1000 – 10000 ; • Bonding groups (electrovalent bond, hydrogen bond, Van Der Waals force, and etc) – 锚固机理 • Lyophilic chains (steric barrier, length – 10-15nm) – 稳定机理 • Molecular structure • Single functional endgroup polymers • Double functional endgroup polymers • A-B or A-B-A block co-polymers • Comb(梳) or Graft(接枝) or Random co-polymers

  27. loops tails trains • Adsorbed conformation • Tails – steric barrier • Loops – steric barrier • Trains – bonding §3. Flocculation • Mechanisms of flocculation • Neutralization or reduction of the potential at the Stern Layer of the dispersed particles – addition of electrolyte – electrical barrier – agglomeration • Bridging (架桥) – addition of flocculants – flocculation (a) A long surfactants containing functional groups at various points in the molecule.

  28. (b) The bridging by interaction of the extended portions attached to different particles may occur. 2. Flocculation • Classes – cationics , nonionics , anionics , and zwitterionics (2) Properties • Molecular weight and its distribution • Middling MW and narrow distribution–ideal flocculants • Middling and low MW – cationics , negative colloid • High MW – anionics , van der Waals force • Wide distribution – cationcs flocculants

  29. (b) Molecular structure • Copolymers – random , block copolymers • Linear structure – effective • Charge density - mildness • Macroionic (electrical potential tunnel 电位隧道)– the counter ions can freely flow on macroionic (3) Flocculants • Polymer flocculants of counter ions • Electrical interaction – 镶嵌作用- - flocculation • Bridging – 架桥作用- M(>25×104) & charge density • Low charge density – loops & tails – cross linking • High charge density – trains – no bridging

  30. (b) Polymer flocculants of homo ions • Possess positive electric charge area on negative solid surface • Higher molecular weight – packing 包裹作用 • Electrical potential tunnel – counter ions flow from the bulk phase into electrical double layer of particles – to compress the electrical double layer 3. Polymeric flocculants • Essential condition • Solubility in medium • Bridging functional groups in flocculants and particles (c) Straight chain – swelling conformation - bridging

  31. (d) High molecular weight – bridging (e) Bridging site with polymer on the surface of particles (2) Commercially available flocculants • Cationics – quaternary ammonium salt of poly-acrylate or poly-acrylamide, and etc • Anionics – poly-acrylic acid, poly-maleate , and etc • Nonionics – poly-acrylamide , PVA, and etc • Zwitterion • Natural polymers – cationic starch (阳离子淀粉), chitosan, and etc • Bio-polymers – negative charge polysaccharide (多糖)

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