chapter 9. detergency of surfactants

Chapter 9. Detergency of Surfactants

2006.6.6.

§1. Introduction • Cleaning process is a very complex process;• No surfactants by self can clean a surface;• Detergency (洗涤作用 ) – this term, when applied to a

surfactants, means the special property it has of enhancing the cleaning power of a liquid.

• Detergency – a combination of effects involving adsorption at interface, alteration of interfacial tension, solubilization, emulsification, and the formation and dissipation of surface charges;

• Detergency – involving mechanical action, biochemical action, and fabric softening.

§2. Mechanisms of the cleaning process

1. Three elements in cleaning process

(1) The substrate (the surface that is to be cleaned) Hard surface – glass , metals, and etc Soft surface – fabric , fiber

(2) The soil (the material that is to be removed from the substrate in the cleaning process)

Liquid soils – oil or fat Solid soils – black carbon, dust, and etc

(3) The bath or cleaning solution.

2. Force between substrates and soils

(1) Mechanical force – soft & porous substrates, particle diameter r of the particles , then force,

r < 100nm, then difficult to wash off

(2) Electrostatic force – surface potentail

wool/water= - 48mV

cotton/water= - 38mV

silk/water= - 1mV

cation bridge (阳离子桥 ) - polyvalency ions

Soil---

++- - - - - - - - - - - - - - - -

2. Force between substrates and soils

(1) Mechanical force – soft & porous substrates, particle diameter r of the particles , then force,

r < 100nm, then difficult to wash off

(2) Electrostatic force – surface potentail

wool/water= - 48mV

cotton/water= - 38mV

silk/water= - 1mV

cation bridge (阳离子桥 ) - polyvalency ions

Electrostatic force > Mechanical force

Soil---

- - - - - - - - - - - - - - -

++++

(3) Chemical force – polar soils proteins , fats , oxides etc

Hydrogen bond and electrovalent bond

(4) Van Der Waals force – interactions between polar & polar or polar & non-polar or non-polar & non-polar

Two cleaning process: removal of the soil from the substrates suspension of the soil in the bath and prevention of its re

deposition (再沉淀 )

3. Removal of the soil from the substrates

It not cover the removal of soil by mechanical work, or chemical reagents (e.g. bleaches, reducing agents) or enzymes

(1) Removal of liquid soil – roll-back or roll-up

(a) Young equation:

SO = SB- BO Cos

Cos = (SB- SO)/BO

SO , Cos , , then roll-up

SB , Cos , , then spreading

(b) Roll-up force R:Initial state: R = (SO-SB) + BOCos , < 0, < 90° ,Cos > 0

R= + BO Cos = 0 , force balanceIf surfactants is added and only adsorbed on interface S/

B and O/B, then SB , OB , then > 0, < 90°, BOCos > 0, R > 0, force imbalance, roll-up and ,

> 90°, > 0, BOCos < 0, R = 0, anew force balance

(c) Three kinds of roll-up Complete removal Spontaneously roll-up

(自动卷离 ) = 180°

e.g. anionics: only S/B,O/B By mechanism work

(作功卷离 )90°< <180°

e.g.nonionics: S/B,O/B,S/O Incomplete removal

SO < SB , Cos > 0

< 90°

Complete removal of oil droplets from substrate

Rupture and incomplete removal of large oil droplets from substrate

(2) Removal of solid soil

(A) Mechanism

(a) Liquefiable (可液化 ) soil – roll-up

(b) Particulate soil Wetting of the substrate and the soil particles by the bath Spreading coefficient

particles SB/P=PV- PB- BV > 0

substrates SB/S=SV- SB- BV > 0

Additive pressure – gap capillary (间隙毛细管 )

P = LVCos/R

Adhesion of solid soil

WSP= SB+ PB- SP > 0

(B) Potential curve

DLVO theory V=VA+VR

A state – adsorbed state

B state – transition state

C state – separating state

Esoil removal - energy barrier

of soil removal

Ere-soiling- energy barrier

of re-soiling

4. Suspension(悬浮作用 ) of the soil in the bath and prevention of re-deposition (预防再沉积 )

(1) Solid particulate soil: formation of electrical and steric barrier

(2) Liquid oily soil

(a) Solubilization – C > CMC Ionic surfactants C < CMC (only adsorption) Nonionic surfactants C > CMC (adsorption a

nd solubilization)

(b) Macro-emulsification

5. Dry cleaning – hydrocarbon or chlorinated hydrocarbon

(1) Thickness of electrical double layer is very thin (very low dielectric constant) – low electrical barrier

(2) Surfactants are probably adsorbed with polar head oriented toward the substrate and soil – high steric barrier

§3. Detergents

1. Composition of detergents

(1) Surfactants – enhancing the cleaning power

(2) builders – increasing the efficiency and effectiveness of surfactants as detergents and supplement their beneficial effects on soil removal (协助去污 )

(3) Other auxiliary agent

2. The relation of the chemical structure of the surfactant to its detergency

(1) Effect of soil and substrate – hydrophilic head oriented toward the bath or substrates

(a) Oily soil hydrophobic substrates (e.g. polyester, nylon) – P

OE hydrophilic substrates (e.g. cotton, wool) – negati

ve charge at neutral or alkaline pH – anionics > nonionics (partly hydrogen bond) > cationics

(b) Particulate soil – anionics > nonionics > cationics

(2) Effect of the hydrophobic group of the surfactants – extent and orientation of adsorption (efficiency of adsorption) ; solubility in bath.

(a) R, surface activity; R, solubility e.g. carboxylate: the optimum Rn

Used temperature: 38C – R12~ R14; 55 C – R16

Counter ion: valence number , Rn Hardness of water , Rn

(b) Straight > branching chains

(3) Effect of the hydrophilic group of the surfactant(a) Anionics(b) Nonionics (c) Zwitterionic surfactants3. Builders – chelation (螯合 ), buffering (缓冲 ), di

spersion (分散 )

(1) Sodium tripolyphosphate (Na5P3O10 STPP)(a) Chelation power of Ca++-

Water –102 CaCO3 mg/g

STPP – 468 CaCO3 mg/g

Polyacrylate (PAA) – 466 CaCO3 mg/g

(b) Alkaline buffering power – pH adjusting

Water – neutralizing 0.15 ml 0.1M NaOH aq.

STPP – neutralizing 5.20 ml 0.1M NaOH aq.

PAA – neutralizing 4.60 ml 0.1M NaOH aq.

(c) Dispersing power – dispersion of particles

Water – dispersing 8.0 ppm Mn

STPP – dispersing 13.0 ppm Mn

PAA – dispersing 11.0 ppm Mn

Disadvantage : – pollute environment

(2) Zeolite molecular sieve (4A型沸石分子筛 ) –

NaOAl2O3 nSiO2H2O

A type : Al2O3: SiO2(mole) = 1.3 - 2.4

(a) Mechanism Ion exchange – power of Ca++exchange 300mg Ca

CO3/g

Coprecipitation with soil (与污垢共沉淀 ) Safety (安全 )

(b) Disadvantage Low rate of exchange No dispersibility

(3) Other builders Polyacrylate Polymaleate (聚马来酸 )

4. Other auxiliary agents(1) Bleacher (漂白剂 ) – peroxide , sodium perbora

te (过硼酸钠 ), sodium percarbonate(过碳酸钠 ), etc

(2) Fluorescent bleachng agent (荧光剂 ) – adsorption 300-400nm ultraviolet radiation

radiation 400-500nm visible light(3) Enzyme – protease (蛋白酶 ), amylase(淀粉酶 ),

lipase (脂肪酶 ), cellulase (纤维素酶 )(4) Finishing agents - antistatic agents (抗静电剂 ),

anti-bacterial agents (抗菌剂 ), softening agents (柔软剂 )

5. Standard washing powder

Surfactants 15%

Builder (STPP) 17%

Sodium silicate(硅酸钠 ) 10%

Sodium carbonate(碳酸钠 ) 3%

Sodium sulfate(硫酸钠 ) 18%

CMC 1%