Chapter 8. Dispersion and Flocculation of Surfactants

2006.05.20.

§ 1.

Introduction

1.

   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

§ 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 (1) 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 (2) 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

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 AgNO 3 to possess positive charge and the I aq. ion in KI aq. to possess negative charge.

(c) Metallic oxide & Indissoluble hydroxid – e.g. SiO 2 , TiO 2 , ZnO 2 , and etc – possess Zero Electric Point (ZEP):

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

(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 (1) Helmholtz EDL - Plate Model Surface potential  0  = (4  - surface charge density /D)   - thickness of Plate EDL,  very little ,no displaying electricity , neutral ,

(2) Gouy-Chapman diffusion EDL (a) 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  N A e 2  C j e = 4.80

× 10 -10 , k= 1.38 × 10 -16 Z j 2 ) 1/2 erg/k N A = 6.623 × 10 23 mol -1 , D = 78.3 (H 2 O,25 ° C)  -1 = 4.20

× 10 -8 /(  C j Z j 2 ) 1/2 = 4.20

× 10 -8 /(2C j ) 1/2

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

(3) Stern Model – Helmholtz & Gouy- Chapman (a) 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 -

 OHP – Outer Helmholtz Plane – hydrated c ounter 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  =  S exp  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

(e)  S and  -Potential   -Potential can be determined, but  S  |  S |  |  |  cannot.

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 )

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

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

§ 3. Dispersion of solid 1.

DLVO theory – stability theory of colloid independently by Derjaguin and Landau(Soviet Union) in 1945 and Verwey and Overbeek (Holland) in 1948 (1) The potential energy of attraction between particles V A  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

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

(2) The potential energy of repulsion between particles V R = (rDU 2 /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= V A + V R (a) r «  -1  The site of first minimum –  agglutination The site of second minimum –  flocculation V M – maximum V M /kT  15-25 stable colloid

 Bron repulsion energy (b) r »  -1 Instable V M  0 (c) Total potential energy  V A  , stability   V R  , stability  V T = V A + V R = -Ar/12H + (rDU 2 /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 + Anions – F > IO 3 > Rb + > K + >H 2 PO 4 > Na + >BrO 3 > Li + >Cl >ClO 3 >Br >I >CNS -

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

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

3. Applications (1) 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 

(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 (1) Wetting of the powder – driving force:spreading works   S L/S =  SV  SL Adsorption of solution surface  LV  LV   C Adsorption of S/L interface  SL  >  LV

(2) De-aggregation ( 解聚集 ) of Fragmentation ( 劈裂 ) of particle clusters ( 团粒 ) – mechanisms (a) By being adsorbed in “microcracks” ( 微裂纹 ) in the solid – permeation ( 渗透 ) – to reduce self-healing ability particles  < 90 °  P = 2  LV cos  /R  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)

(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 E electric  & E steric  5. Dispersing of surfactants (1) Aqueous dispersion (a) Nonpolar powders – e.g. black carbon (low energy surface) – addition surfactants  LV   C >  LV (b) 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

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

(3) Dispersants (a) 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 ( alcohol , and organosilicon 脂肪胺 ) ,

(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

    Adsorbed conformation Tails – steric barrier Loops – steric barrier loops trains tails 1.

Trains – bonding § 3. Flocculation Mechanisms of flocculation (1) Neutralization or reduction of the potential at the Stern Layer of the dispersed particles – addition of electrolyte – electrical barrier  – agglomeration (2) Bridging ( 架桥 ) – addition of flocculants – flocculation (a) A long surfactants containing functional groups at various points in the molecule.

(b) The bridging by interaction of the extended portions attached to different particles may occur.

2. Flocculation (1) Classes – cationics , nonionics , anionics , and zwitterionics (2) Properties (a) 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

(b) Molecular structure  Copolymers – random , block copolymers  Linear structure – effective  Charge density - mildness  Macroionic (electrical potential tunnel 电位隧道 counter ions can freely flow on macroionic )– the (3) Flocculants (a) Polymer flocculants of counter ions  Electrical interaction – 镶嵌作用  - flocculation  Bridging – 架桥作用 - M(>25 × 10 4 ) & charge density  Low charge density – loops & tails – cross linking  High charge density – trains – no bridging

(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 (1) Essential condition (a) Solubility in medium (b) Bridging functional groups in flocculants and particles (c) Straight chain – swelling conformation - bridging

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