Chapter 6 emulsification and emulsification by surfactants

2006.4.19.

§1. Introduction 1. Emulsion – immiscible liquid phase, multiphase disp

ersion, thermodynamics unstable system (from a few minutes to a few years)– e.g. milk , soybean milk, bearnaise, finishing oil etc.

2. Formation (1) Emulsification – one fluid dispersed in a second dispersephase – 分散相 , inner phase , discontinuous phas

e dispersion medium – 分散介质， outer phase ， continuo

us phase Three types: o/w, w/o, and w/o/w or o/w/o two immiscible, pure liquids cannot form emulsion.

(2) Emulsifying agents – the third component – surfactants

3. Properties

(1) Size and aspect of particles Macroemulsions: d>400nm, creamy white Miniemulsions: 400nm>d>100nm, blue-white Microemulsions: 100nm>d>50nm, translucence Nanoemulsions: 50nm<d, transparence

(2) Viscosity – Einstein’s eq. = 0 (1 + 2.5)

0 – viscosity of outer phase

- volume fraction of inner phase

(3) Electric conductivity – outer phase

o/w 》 w/o , ionics 》 nonioncs

4. Differentiation of emulsion types

(1) Dilution method

(2) Dyeing method

(3) Electric conductivity

(4) Filter paper method

(5) Light reflectance

(6) Fluorescence method

§2. Theoretics and types of emulsionsConversion in different types of emulsion

o/w w/o

1. Phase volume ratio

if particles(inner phase) are rigid ball with same size, then volume fraction of inner phase

74.02% water > 74%, then w/o o/w

water < 26%, then o/w w/o

monodisperse & rigid ball – polydisperse & elastic ball(w/o of oil=99%)

2. Wedge theory – geometry theory Match model

3. Solubility rule – hydrophilicity of surfactants

(1) Hydrophilic surfactants – o/w

(2) Lyophilic surfactants – w/o

4. Effects of wall

(1) Hydrophilic wall (high energy surface) – o/w

(2) Lyophilic wall (low energy surface) – w/o

5. Kinetic theory of emulsion type

Davies(1957)developed a quantitative theory of emulsion type relating the type of emulsion formed to kinetics of coalescence( 聚集 ) of the two types of droplets present: oil droplets & water droplets

Rate of coalescence of the particles:

dv/dt = Ae-E/kT

A – collision factor; E – energy barrier to coalescence hydrophilic surfactants: Eoli, dv/dt of oil droplets, dv/

dt of water droplets, to form o/w emulsions lyophilic surfactants: Ewater, dv/dt of water droplets,

dv/dt of oil droplets, to form w/o emulsions

6. Powder for emulsification

(1) Condition of emulsification V/L Young’s eq. SV = SL + LV cos

Spreading S = SV - SL - LV = LV (cos-1) 0

O/w Young’s eq. SO = SW + OW cosW

or SW = SO + OW cosO

W+ O=180º

liquid

vapor

SLSV

SV

w

waterO

oil

OW

SWSO

Spreading SW = SO - SW - OW = OW (cosW-1) 0

or SO = SW - SO - OW = OW

(cosO-1) 0

If so ow + sw then W 0, the powder is in water phase

If sw ow + so then o 0, the powder is in oil phase

Else the powder is adsorbed at O/W interface

0º < W < 180º or 0º < O < 180º

(2) According O/w Young’s eq.

SO = SW + OW cosW or cosW = (SO - SW)/ OW

if SO > SW, W < 90º, hydrophilic , mostly in water phase o/w

if SO < SW, W > 90º, lyophilic , mostly in oil phase w/o

if SO = SW, W = 90º, balance, o/w or w/o , but unstable

§3. Factors of effect on stability of emulsion

1.Reduces of interfacial tension between two liquids

e.g. 10ml n-octane( 正辛烷 ) is dispersed in water to 0.1 m emulsion, then its surface area is 300m2,

L/L= 50.8mJ/m2, then total surface energy=15.24J

if the surfactants are added into the disperse system, then L/L 1 mJ/m2, then total surface energy 0.3J.

Dynamic stability

2. Physical nature of the interfacial film

(a) Mechanical stability – with strong lateral intermolecular forces and high film elasticity

(b) Liquid–crystal formation – with a high-viscosity region and a steric barrier to stabilize of emulsion

3. Existence of an electrical or steric barrier to coalescence on the dispersed droplets

(a) Electrical barrier –-potential

of ionics:

-potential , stability(b) Steric barrier

4. Viscosity of the outer phase - diffusion coefficient of droplets: D=kT/6a

- viscosity of the outer phase

a – the radius of the droplets

, D, stability5. Size distribution of droplets

larger particle have less interfacial surface per unit volume than smaller droplets, so they are thermodynamically more stable than smaller ones.

emulsion with uniform size distribution is more stable.

§4. Emulsifying agent and HLB value

HLB method which was advanced by Griffin in 1949, is the most frequently used method in the selection of emulsifying agents.

1.HLB value – Hydrophilic-Lipophilic Balance

HLB = Hydrophilic portion/Lipophilic portion

= 0 ~ 40

Paraffin ~ 0, sodium dodecyl sulfate ~ 40

HLB value < 10 lipophilic

HLB value >10 hydrophilic

HLB value for typical nonionic surfactants structures

2. HLB value & application 1 ~ 3 anti-foaming agent

3 ~ 6 w/o emulsifying agents 7 ~ 9 wetting agents 8 ~ 18 o/w emulsifying agents 13 ~15 detergents 15 ~18 solubilizing agents

3. Estimate of HLB value(1)Nonionics:

HLB = mass fraction of hydrophilic group20(a) Fatty acid of many polyhydric alcohols( 多元醇 )

HLB = 20(1-S/A)S – saponification number( 皂化值 ) of the ester;A – acid number( 酸值 ) of the fatty acid in the ester.

e.g. C17H35COOCH2CH(OH)CH2OH

S=161, A=198, then HLB = 3.7

(b) Nonionics of POE & polyol

HLB = (E + P)/5

E – the weight percentage of oxyethylene content

P – the weight percentage of polyol content

e.g. C16H33(OC2H4)10OH

E = (44 10+17)/(44 10+17+225) = 67.0%;P = 0

HLB = (E+P)/5 = 67.0/5 = 0.67 20 = 13.4

(c) Logarithm: HLB=7+11.7ln(MW/MO)

MW or MO- molecular weight of hydro- or lipo-philic group: MW=441, MO=225, then HLB=14.9

(2) Ionic surfactants – group numbers could be calculated based upon group contribution accord-ing to the formula:

HLB = 7 + (group numbers)

e.g.C12H25SO4Na

HLB=7+38.7-0.47512=40

e.g. C16H33(OC2H4)10OH

HLB=7+0.33 10+0.5-0.475 16=3.20

(3) Mensuration (a) behavior in water HLB Range no dispersibility 1 – 4 poor dispersion 3 – 6 milky dispersion after vigorous agitation 6 – 8 stable milky dispersion (upper end almost translucent) 8 – 10 from translucent to clear 10 – 13 clear solution 13 +

(b) Cloud point – POE nonionics 1% aq.

TP , HLB, TP>100ºC, HLB>15,

TP HLB

(4) HLB of a mixyure

HLBmix= fiHLBi =fAHLBA+(1-fA)HLBB

fi – the weight fraction of surfactant i in mixture

4. Emulsifying agents

(1) Synthetical surfactants

(a) Anionics – HLB > 8 , o/w type

(b) Nonionics – HLB < 18, o/w or w/o type

(c) Cationics - a few

(2) Natural surfactants – lecithin( 卵磷脂 ), cholesterin( 胆甾醇 ) etc

(3) Polymeric emulsifying agents

(4) Powder emulsifying agents

5. Application - selection of surfactants using HLB value as emulsifying agents

(1) Optimal HLB value for emulsification

(A)Using the data handbook

(a) HLB value for emulsification of some oils

(b) Mixture of oils

HLBmix= Fj HLBj= FAHLBA+(1-FA)HLBB

Fj ,HLBj – weight fraction and HLB of j oil.

(B) Mensuration and check

e.g. using the Span-series (HLBSpen-60=4.3) and Tween-series (HLBTween-80=15), the optimal HLB value for emulsification

(2) Optimal emulsifying agents

– two principles

(a) The structure of

hydrophobic groups

of emulsifying agents

is close to the oils

(b) The synergistic effect

6. Phase Inversion Temperature ( 相转变温度 PIT)

(1) Disadvantage of HLB method

(a) Surfactants with same HLB value could possess different emulsifying ability

(b) It makes no allowance for the change in HLB value with change in the conditions for emulsification (temperature, nature of oil & water phase). E.g. POE T, HLB

(2) PIT – the temperature when the POE is used to emulsifying agents from o/w emulsion inversion to w/o

(3) Mensuration of PIT – emulsion : 3-5% emulsifying agents and o:w volume ratio =1

(4) Emulsion stability (a) o/w type : PIT – TS =20-60ºC TS – storage temperature ( 储藏温度 )of emulsion(b) w/o type : TS – PIT = 10-40ºC(c) Preparing temperature of emulsificatopn by the PIT Preparation at a temperature 2-4ºC below the PIT

to obtain a very fine average particles size Cooled down to storage temperature( for o/w) to in

creases it stability(5) Factors affecting PIT(a) PIT HLB selection of emulsion PIT according t

o their HLB value(b) Hydrophility of POE , PIT(c) Distribution of POE , PIT, o/w stability

(d) Addition of electrolyte in water phase, PIT(e) In oil phase addition of nonpolar organics e.g. paraffin P

IT addition of polar organics e.g. long chain, PIT short chain, PIT Mixture of oils

PITmix= i PITi = A PITA + (1- A) PITB

i – volume fraction of i oil; PITi – PIT of i oil in this emulsifying system

7. Preparation method of emulsion(1) Addition of emulsifying agents (a) in water phase(b) in oil phase(c) nascent soap( 初生皂法 ) (d) mixed films (respectively addition of hydrophilic s

urfactants in water and hydrophobic surfactants in oil phase)

(e) alternately addition of oil and water(2) Oil-Water mixing(a) The addition of water to oil - to obtain a very fine

average particles size and stable o/w emulsion(b) The addition of oil to water

(3) Methods of emulsification

(a) Direct emulsification – 70 - 75ºC

(b) Phase inversion emulsification

(4) Equipment of emulsification

(a) Simple agitating - impulse type ( 推进式 ), turbo type( 涡轮式 )

(b) Homogenizer – high pressure 6.89-34.47 Mpa

(c) Colloid mill ( 胶体磨 ) – rotor( 转子 ) and stator( 定子 ), rotate speed = 1000-20000 rpm ， high shearing force ( 高切力 )

§5. instability of emulsion and demulsification

1. Inversion of emulsion – o/w w/o

(1) Phase volume ratio

(2) Polyvalent ions

(3) Temperature T>PIT

(4) Addition of electrolytes

o/w w/oe.g. Inversion of o/w to w/o by an interface film of sodiumcetyl sulfate ( 十六烷基硫酸钠 )and cholesterol ( 胆固醇 )upon addition of polyvalent cations

2. The creaming( 分层 ),

flocculation( 絮凝 ),and

coalescence( 聚结 ) – the

reversible prelude of

emulsion breaking

(a) Creaming – difference

in gravitational

(b) Flocculation - attractive

force between droplets

(c) Coalescence – irreversible

- breaking, phase separation

3. Emulsion breaking( 破乳 ) – to break the interface films

(1) Methods (a) Physical method Heating Electrical precipitation Ultrasonic (b) Chemical method De-emulsifying agent Inorganic acid Polyvalent ion to inversion

(2) Mechanism

(a) Displacing – emulsifying agents are displaced by de-emulsifying agent

(b) Wetting – powder emulsifying agents

(c) Flocculating – cross-linking agents

(d) Collision - deemulsification

(e) Make the interface film to distortion( 变形 ) and tendering( 变脆 )

(3) Factor affecting de-emulsification

(a) pH value – e.g. carboxylate surfactant is unstable at low pH.

(b) Electrolyte Concentration – I , , stability. Valenta value – polyvalent ion , stability(c) Temperature - T, solubility of surfactants, inte

nsity of film(d) Phase volume ratio – volume fraction > 74.02%

(h) Stirring – Reynolds number ( 雷诺数 ) - 3500

§6. Micro-emulsion

Generally : surfactants & cosurfactants and etc

In 1986,the cosurfactant-free microemulsion have been prepared

• The differentia with macro-emulsion

2. Mechanism of formation

(1) Negative interfacial tension theory – Schulman and co-workers: 0 spontaneous process

it may be a transient phenomenon( 暂时 ), and at equilibrium must be zero or slightly positive.

is macroscopy property and made no sense in micro-emulsion

(2) 双重膜 (duplex film?)

surfactants + co-surfactants

+ auxiliary agent( 助剂 )If mw mo, then the films is bended to water phase, w/o

If mw < mo, then the films is bended to oil phase, o/w

(3) Swollen micelles

Mixed film

mo

mw

oil

water