Transcript Document 7149335
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 )