Chapter 2 Adsorption of Surfactants at Interface

2006.3.18.

泡沫

乳液

润湿

分散

表面活性

胶束作用

吸附作用

气液界面

液液界面

固液界面

吸附机理表面电位

Gibbs吸附方程

表观吸附方程

洗涤作用

增溶作用

§1. The surface excess concentration and the Gibbs adsorption equation

1. About adsorption

(1) The interfaces of adsorption

G-L surface adsorption – foam

L-L interface adsorption – emulsion

S-L interface adsorption – wetting, dispersing

(2) Surface active and adsorption

Surface active Adsorption on surface

(3) Tow type in surface adsorption

(a) Orientation adsorption of hydrophobic groups;

(b) Orientation adsorption of hydrophilic groups.

2. Surface excess concentration

(1) Interface phase (or layer)

• two phases inter-dissolved

、 • Thickness of interface phase

a couple of molecules ~ 0.5nm

in dilute solution

(2) Surface excess

concentration

(a) If total mole

number of i component : ni0

(b) The concentration of 、 phase

Ci Ci

and Ci > Ci

(c) The boundary surface ss

total volume of 、 phase : V, V

ni = Ci V + Ci

V

(d) Surface excess ni = ni

0- ni=ni0- (Ci

V + Ci V)

(e) Surface excess concentration

i= ni/A A – area of ss-surface

意义：单位表面积上 i 组分的摩尔数比本体相中相同数量溶剂所含 i 组分摩尔数的超量。

对于表面活性剂：稀溶液区， Ci Ci

1, 即 i

组分在吸附界面上，单位面积的摩尔数。i= ni

/A = [ni0- ni]/A

=[ni0- (Ci

V + Ci V)]/A

ni0 /A

2. Gibbs adsorption equation

(1) Thermodynamics

Mono-component:

U=TS-PV

Multi-component:

U=TS-PV +ini

In surface phase:

U =TS -PV +ini + A

Total differential:

dU =TdS + S dT - PdV - VdP +i dni +n

i dI + dA + A d ………………①

Thermodynamic equation

dU =TdS - PdV +i dni + dA ………②

①-②, then

S dT - VdP +ni di + A d = 0

( )TP

ni di + A d = 0 or d = -[ni

/A]di

d = -idi

d = -idi

Two-component :d = -1d1 - 2d2

ss surface uncertain! So i – uncertain!

(2) Gibbs method:

(a) If solvent i =1, then 1 = 0

Gibbs eq. d = - 2(1)d2

(1)-solvent 1 as frame of reference

2= 20 + RTlna2

Gibbs eq. d = - 2(1)d2 = - RT2

(1)dlna2

(b) Gibbs equation

2(1)

= -(1/ RT) d/dlna2 = -(a2/ RT) d/da2

-(1/ RT) d/dlnc2 = -(c2/ RT) d/dc2

d/dc2< 0, c2, , 2(1)

> 0 positive adsorption;

d/dc2= 0, c2 , 2(1)

= 0 no adsorption;

d/dc2> 0, c2, , 2(1)

< 0 negative adsorption

(c) Multi-component

-d = idi = RT i(1)

dlna2

RT i(1)

dlnc2

§2. Surfactants adsorption at G-L interface

1. Calculation of ( )TP,Two-component:

2(1)

= -(1/ RT) d/dlna2 = -(a2/ RT) d/da2

-(1/ RT) d/dlnc2 = -(c2/ RT) d/dc2

(1) Nonionics (c2 < 10-2)

2(1)

= -(1/ RT) d/dlna2 = -(a2/ RT) d/da2

-(1/ RT) d/dlnc2 = -(c2/ RT) d/dc2

If (d/dc2)c2 is known, 2(1)

at c2 can be calculated.

(2) Ionics

(a) 1-1type ionics

RNa R- + Na+

-(d/ RT) = R-(1) dlnaR- + Na+

(1) dlnaNa+ +

OH-(1) dlnaOH-+ H+

(1) dlnaH+

Very low degree of ionization , R-(1) Na+

(1)

-(d/ RT) = R-(1) [dlnaR- + dlnaNa+]

= R-(1) [dlnaR- aNa+]

a± = a+

+a--, = R-

(1) dlna±2 = 2 R-

(1) dlna±

2 R-(1) dlnc± 2 R-

(1) dlnm±

(b) Electrolyte: Surface excess concentration homo ion e.g. NaCl no homo ion e.g. KCl , K+ and Na+ exchange

-(d/ RT) = xR-(1) dlnm±

= xR-(1) dlnmR- = xR-

(1) dlncR-

x =1 +cR-/(cR-+cs)

Cs- concentration of salt

1-1type: finite quantity cs = 0, x =2;

-(d/ RT) = xR-(1) dlnm±

Infinity quantity cs = ， x = 1.

-(d/ RT) = R-(1) dlnm±

(c) No 1-1type ionics

if 1mole ionics ionize to x mole positive and negative ions, then

-(d/ RT) = x2(1) dlna±

2. Adsorption of surfactants at solution surface(1) Langmuir adsorption isotherm

= 0 - 0ln(1+ c2)

d /d c2= -[0 /(1 + c2)]

2(1)

-(c2/ RT) d/dc2

= (0 / RT)[ c2/(1 + c2)]

= (1) [ c2/(1 + c2)]

(2) The Surface excess concentration 2(1) &

(1)

unit: mole/m2

(3) The area per molecule A & A

A = 1018/NA 2(1) (nm2)

lauryl sodium sulfate十二烷基硫酸钠

The area of C12H25O(C2H4O)nH(55ºC)

The area of C16H33O(C2H4O)nH(55ºC)

§3. Surfactants adsorption at L-L interface

1. L-L interface(1) L-L two phases(2) Distribution of surfactants in

L-L two phases

2. Adsorption of PEO nonionics at coal oil-water i

nterface(1)T<TP(Fig. a)(2) T>TP(Fig. b)(3) benzene PEO in water PPO in benzene

§4. Interfacial Adsorption & Surfactivity

1. Efficiency( 效率 ) and Effectiveness( 效能 ) of Surface Adsorption

(1) What are the Efficiency ( 效率 ) and the Effectiveness ( 效能 ) ?

Efficiency( 效率 ) – the effects produced per wastage

Effectiveness( 效能 ) – the most effects

(2) Efficiency( 效率 ) of Surface Adsorption

I/ci – adsorption per-concentration

Two-component Gibbs eq.: 2/c2= - (1/RT)[d/dc2]

If - [d/dc2] , then 2/c2

(3) Effectiveness( 效能 ) of Surface Adsorption

- saturated adsorption excess concentration

(4) Some factors of influence to them

(a) Hydrophobic groups:

hydrophobicity(R, or SiR or YR), 2/c2

if R>C16, then

(b) Hydrophilic groups: 2/c2: Nonionics > Ionics (same R)

: Nonionics > Ionics (coulomibic repulsion)

Nonionics: n↑, ↓

(c) Additives

• Electrolyte , Ionic Strength: I=(1/2)CiZi2 ,

hydrophilicity , surface activity , 2/c2

the radius of ionic atmosphere , • Regulator of water structure( 水结构调节剂 )Promoters – fructose ， xylose ； 2/c2Breakers – urea,lower alcohol; 2/c2

no marked affect

(d) Temperature if T, then

• Ionics: water-soluble, 2/c2; repulsion ,

• Nonionics: water-soluble, 2/c2;

2. Efficiency( 效率 ) and Effectiveness( 效能 ) of Surface Tension Reduction

(1) Efficiency( 效率 ) of Surface Tension Reduction(a) Traube rule Surface Pressure( 表面压 ) = 0 - Efficiency( 效率 ) : /c2 , Efficiency(b) PC20 = - log10 c = 20mN/m , Efficiency(2) Effectiveness( 效能 ) of Surface Tension Reduction

CMC = 0 - CMC , Effectiveness(3) Some Factors of Influence to Them(a) Efficiency of Surface Tension Reduction:

~2 ~ Efficiency of Surface adsorption

• Fluorocarbons > Silicones > Hydrocarbon > Branched Hydrocarbon

• Nonionics > Zwitterion > Ionics

• I , PC20

(b) Effectiveness(CMC ) of Surface Tension Reduction

From Gibbs Eq. d = -d = xRTdlnC

= 2 - 1 = xRT lnC = xRT (lnC2-lnC1)

If C1=C20, 1 = 20mN/m; C2= CMC, 2 = CMC , then

CMC= 20+xRT ln(CMC/C20)

x – mole number dissociated by1 mole ionics

(CMC/C20), C20 or CMC, Surface Tension Reduction > Micelle

(CMC/C20), C20 or CMC, Surface Tension Reduction < Micelle

Generally CMC ~ , but some special case ,e.g.

Branched Chain Surfactants:

branching degree , , CMC , (CMC/C20)

CMC

The Branched Chain Surfactants is a Surfactants of Surface Tension Reduction.

Efficiency( 效率 ) and Effectiveness( 效能 ) ofSurfactants at Interface

Efficiency( 效率 ) and Effectiveness( 效能 ) ofSurfactants at Interface

Efficiency( 效率 ) and Effectiveness( 效能 ) of

Surfactants at Interface

Efficiency( 效率 ) and Effectiveness( 效能 ) ofSurfactants at Interface

Efficiency( 效率 ) and Effectiveness( 效能 ) ofSurfactants at Interface

Efficiency( 效率 ) and Effectiveness( 效能 ) ofSurfactants at Interface

CMC/C20 Ration of some Surfactants

CMC/C20 Ration of some Surfactants

3. Insolubility Monomolecular Membrane

(1) Formation of Monomolecular Film

1769’, Franklin Spread a cup of olive oil ( 橄榄油 80% oleic acid) on 2000m2 of pool, than the wave of pool was calm immediately.

(2) Every Stations of Monomolecular Film

(a) Gaseous film

ideal gas: -d/dc = - /c2 = -(-0)/(c2-0)

= (0-)/c2 = /c2

From Gibbs equation: 2(1)

-(c2/ RT) d/dc2

= /RT = /N0kT

/N0 2(1)

= A = kT

(b) Liquid film: expand film(L1) & condensed film(L2)

L1: A ~ 50Å2 ( - 0)(A – A0) = kT

L1L2: transition region , condensability.

L2: A A = b - a

(c) Solid film(S): A ~ 20Å2

1. Adsorptive capacity and its determination from solution:

(1) Adsorbents( 吸附剂）(2) Adsorbate ( 吸附质 )

(3) Apparent Absorbency( 表观吸附量 ): = x/m = (C0-C)V/m mole/g

2. Mechanisms of Adsorption at S-L interface

L-S Interface may be Electrified, Adsorption at S-L interface is comparatively complex.

(1) Ion Exchange adsorption

§5. Surfactants adsorption at S-L interface

(2) Ion Pairing

(3) Hydrogen bonding

(4)Acid-Base Interaction

(5) Adsorption by Polarization of Electrons

(6) Adsorption by Dispersion Forces

(7) Hydrophobic bonding

3. Factors Affecting the Adsorption at S-L Interface(1) Adsorbate ( 吸附质 )(a) Hydrophobic Groups

hydrophobicity (e.g. R), fluocarbon chains > siloxane > hydrocarbon chains

(b) Hydrophilic GroupsIonics with different charge of interface >Nonionics > Ionics with same charge of interface

(2) Temperature

(a) Ionics: T, (b) PEO Nonionics: T , (3) pH

(a) Surface charge of adsorbents( 吸附剂 ):IEP, ZEP

pH, negative surface charge

pH, positive surface charge

(b) Charge of adsorbates( 吸附质 ): IEP

(4) Additives

• Electrolyte , I=(1/2)CiZi2 , radius of ionic atm

osphere , hydrophilicity , ,

• Regulator of water structure( 水结构调节剂 ) Promoters – fructose ， xylose ； /c , (sm

all) Breakers – urea,lower alcohol; /c, (small)

(6) Adsorbents( 吸附剂）(a) Adsorption from aqueous solution onto adsorbent

s with strongly charged sites Such substrates as wool and other polyamides at p

H above and below their isoelectric points; Such oxides as alumina at pH above and below th

eir points of zero charge;

cellulosic and silicate surfaces at high pH e.g. Ion Exchange, Ion Pairing, Hydrogen bonding adsorption S-shaped adsorption isotherm for an ionics on an oppositely charged substrute. ①ion exchange ②interaction of hydrophobic chains. The conc. Well below the CMC,-hemimicelle formation or cooperative adsorption

(b) Adsorption from aqueous solution onto nonpolar, hydrophobic dsorbents

e.g. carbon and polyethylene or polypropylene Adsorption of sodium dodecyl sulfate o

nto Graphon at 25ºC(0.1MNaCl aq.)

Adsorption of dodecyltrimethylammoniumbromide onto Graphon at 25ºC(0.1MNaBr aq.)

(c) Adsorption from aqueous solution onto polar adsorbents without strongly charged sites

such as cotton, polyesters and polyamides in neutral solution

by a combination of hydrogen bonding and adsorption or dispersion forces

Langmuir adsorption type