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S U R F A C T A N T S

AMIT M. GUPTALECTURER,

AGNIHOTRI COLLEGE OF PHARMACY, WARDHA

S U R F A C T A N T S

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BASIC TERMINOLOGY

Hydrophilic: A liquid/surface that has a high affinity to water.

Hydrophobic: A liquid/surface that has very low affinity to water

Lipophilic: A liquid/surface that has a high affinity to oil.

Lipophobic: A liquid/surface that has a very low affinity to oil.

Hydro=Water

Lipo=Oil

Philic=Friendly

Phobic=Scared

Philic=Friendly

Phobic=Scared

+

+Hydrophilic

Hydrophobic

Lipophilic

Lipophobic

Lyo=DissolvePhilic=Friendly

Phobic=Scared+

Lyophilic

Lyophobic

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BASIC TERMINOLOGY

Hydrophobic Lipophilic

Lyophilic in oil

Lyophobic in water

Hydrophilic Lipophobic

Lyophobic in oil

Lyophilic in water

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Surfactants

A molecule that contains a polar portion and a non polar portion.

A surfactant can interact with both polar and non polar molecules.

A surfactant increases the solubility of the otherwise insolublesubstances.

In water, surfactant molecules tend to cluster into a spherical geometry

non polar ends on the inside of the sphere

polar ends on the outside

These clusters are called micelles

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Surfactants are molecules that preferentially adsorb atan interface, i.e. solid/liquid (froth flotation),

liquid/gas (foams), liquid/liquid (emulsions).

Significantly alter interfacial free energy (work

needed to create or expand interface/unit area).

Surface free energy of interface minimized by

reducing interfacial area.

INTRODUCTION

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Surfactants have amphipathic structure

Tail or hydrophobic group

Little affinity for bulk solvent. Usually hydrocarbon

(alkyl/aryl) chain in aqueous solvents. Can be linear or

branched.

Heador hydrophilic group

Strong affinity for bulk solvent. Can be neutral or

charged.

Tail

head

SURFACTANT STRUCTURE

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SURFACTANT CLASSES

Anionic (~ 60% of industrial surfactants)

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SURFACTANT CLASSES (contd.)

Cationic (~ 10% of industrial surfactants)

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Non-ionic (~ 25% of industrial surfactants)


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Amphoteric or zwitterionic (~ 10% of industrial surfactants).

Generally expensive specialty chemicals.

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HYDROPHILIC-LIPOPHILIC BALANCE

Griffin (1949): the hydrophilic-lipophilic balance (HLB) of asurfactant reflects its partitioning behavior between a polar

(water) and non-polar (oil) medium.

HLB number, ranging from 0-40, can be assigned to asurfactant, based on emulsification data. Semi-empirical only.

Strongly hydrophilic surfactant, HLB 40

Strongly lipophilic surfactant, HLB 1

HLB dependent upon characteristics of polar and non-polar

groups, e.g. alkyl chain length, headgroup structure (charge,

polarity, pKa).

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-- Effect of Structure --

oil

water

Coil

Cwater

C6H13COO- C8H17COO

- C10H21COO-

HLB decreases

Surfactant HLB

Sodium laury sulfate, C12H25SO4-Na+

Potassium oleate, C17H35COO-K+

Sodium oleate, C17H35COO-Na+

Oleic acid, C17H35COOH

n-butanol, C4H9OH

cetyl alcohol, C16H33OH

40

20

18

1

7

1

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A value of 10 represents a mid-point of HLB.

HLB USE

4-6

7-9

8-18

13-15

15-18

Water-in-oil emulsions

Wetting agents

Oil-in-water emulsion

Detergents

Solubilizing

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0 2 6 8 10 12 14 16 184

Nodispersibility

in waterpoor dispersibility

in water

Water in oil

emulsifierWetting agent

Milkydispersion;

unstable

Translucent to

clear solution

Clear solution

Detergent Solubilizer

Oil-in-water

emulsifier


triglycerol monooleate: Cream

and ointment stabilizersPolysorbate 20

Insecticidal

sprays

HLB

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MICELLES

If concentration is sufficiently high, surfactants can form

aggregates in aqueous solution micelles.

Typically spheroidal particles of 2.5-6 nm diameter.

(Klimpel,Intro to ChemicalsUsed in Particle Systems,p. 29, 1997, Fig 21)

McBainLamellarMicelle Hydrocarbon

Layer

WaterLayer

WaterLayer

Hartley

SphericalMicelle

+

+

+

+

+

+

+

+

- - - - --

---

- - -

---

-

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Micelle Structure of a Surfactant

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Hydrophilic head Hydrophobic tail

Amphiphiles

Coarse-grainedModel

beads connected by an anharmonic spring.

Interactions between every two beads are

governed by a Lennard-Jones (LJ) andFENE potentials.

2 3h t

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MICELLES--CMC--

Onset ofmicellization observed by sudden change in

measured properties of solution at characteristic surfactantconcentration

critical micelle concentration (CMC).

(Klimpel,Intro to ChemicalsUsed in Particle Systems,

p. 29, 1997, Fig 20)

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MICELLES--CMC Trends--

(1) For the same head group, CMC decreases with increasing

alkyl chain length.

(2) CMC of neutral surfactants lower than ionic

(2) CMC of ionic surfactants decreases with increasing saltconcentration.

(3) For the same head group and alkyl chain length, CMC

increases with increase in number of ethylene oxide groups.

(4) For mixed anionic-cationic surfactants, CMC much lower

compared to those of pure components.

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MICELLES--Example: Mayonnaise--

+

+ +

+ + + +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

Water matrix containing fat

droplets. The surfactant(emulsifier) is lecithin. It can

contain up to 12 g of fat in

15 ml

Water matrix

Oil+

+

+

+

http://wilfred.berkeley.edu/~gordon/BLOG-images/mayo15.jpg

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MICELLES--Headgroup and Chain Length--

(Hunter, Foundations of Colloid Science, p. 569, 1993, Fig 10.2.1)

Surfactant Temp (C) b0 b1

Na carboxylates

K carboxylates

alkyl sulfonates

alkyl sulfates

alkylammonium chlorides

20

25

40

45

25

2.41

1.92

1.59

1.42

1.25

0.341

0.290

0.294

0.295

0.265

Branching or addition of double bonds or polar groups to alkyl chaingenerally increases CMC.

Addition of benzene ring equivalent to addition of ~ 3.5 carbons(methylene groups).

Replacement of hydrogens in alkyl chain with fluorine initiallyincreases CMC, followed by marked decrease as fluorine

substitution goes to saturation.

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Cloud point versusKrafft point surfactants

The Krafft point is defined as the temperature at whichthe solubility of the surfactant equals the critical

micellization concentration (cmc) = melting point ofhydrated surfactant

The Cloud Point is the LCST MW PEO = 5 106

[CmH2m+1NHCO(CH2)nOSO3Me, abbreviated asm-n-Me (Me=Na, 0.5 Ca)]

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MICELLES--Temperature and Pressure--

For ionic surfactants there exists a critical temperature above whichsolubility rapidly increases (equals CMC) and micelles formKraft pointor Kraft temperature (TK),

Below TK solubility is low and no micelles are present.

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MICELLES--Temperature and Pressure--

surfactantcrystals

TK

Temperature

Surfactants much less effective below Kraft point, e.g. detergents.

For non-ionic surfactants, increase in temperature may result in

clear solution turning cloudy due to phase separation. This critical

temperature is the cloud point.

Cloud point transition is generally less sharp than that of Krafftpoint.

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MICELLES--Electrolyte--

Addition of electrolyte significantly affects CMC, particularly

for ionic surfactants.

For non-ionic and zwitterionic surfactants;

log10CMC = b2 + b3Cs

where Cs is salt concentration (M)

b2and b3 are constants for specific surfactant, salt and

temperature.

Change in CMC attributed to salting in or salting out

effects. Energy required to create volume to accommodate

hydrophobic solute is changed in electrolyte solution due to

water-ion interactions

change in activity coefficient.

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MICELLES--Electrolyte--

If energy required is increased by electrolyte, activitycoefficient of solute is increased and salting out occurs

micellization is favored and CMC decreases.

Conversely, for salting in, CMC increases.

Effects of electrolyte depend on radii of hydrated anions and

cations and is greater for smaller hydrated ions, i.e. follow

lyotropic series.

CMC depression follows order:F

- > BrO3-> Cl

-> Br

-> NO3

-> I

-> CNS

-

and

NH4+ > K+ > Na+ > Li+

A R i Mi ll

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In a typical surfactant system, bulk concentration, surface

concentration -- until cmc is reached.

cmc (critical micelle concentration)surfactant conc. where

micellization occurs.

A. Review: Micelles

log CB

Surface

tension,

CMC

TCd

d

ln Surface excess

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Forces driving micelle formation:

a) hydrophobic forceb) entropy

Forces opposing micelle formation:a) concentration gradient

b) thermal (Brownian) motion

c) charge repulsion between ionic polar heads

Note: cmcs of nonionic surfactants are much lower than those of

ionic surfactants. Why?

B Mi ll Ki ti

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B. Micellar Kinetics

Micelles are NOT static structures.

Micelles are unstable structures with two characteristic relaxationtimesfast relaxation time (1) and slow relaxation time (2)

+ +

Fast relaxation time, microseconds

Slow relaxation time, milliseconds to minutes

+

t1

t2

C T h i U d t M Mi ll Ki ti

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C. Techniques Used to Measure Micellar Kinetics

Pressure-Jump (conductivity or optical detection)

Temperature-Jump (optical detection)

Stopped-Flow (conductivity, optical detection and

fluorescence) Ultrasonic Absorption

Fluorescence

Shock-Tube

D Effect of Surfactant Conc on Micelle Lifetime

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D. Effect of Surfactant Conc. on Micelle Lifetime

It has been shown that

micelle slow relaxationtime, 2, is a function of

surfactant concentration

For all surfactants that

form micelles, 2increases to a certain

maximum value

For ionic surfactants, 2

begins to decrease fromthe maximum value

0.001

0.01

0.1

1

10

0 100 200 300 400 500 600 700

SDS Concentration (mM)

CMCSlowR

elaxation

Time,

2

(sec)

0.001

0.01

0.1

1

10

0 100 200 300 400 500 600 700

SDS Concentration (mM)

CMCSlowR

elaxation

Time,

2

(sec)

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For nonionic surfactants, 2 remains constant at a maximum

value.

Remember: nonionic surfactants have much longer micellar

relaxation times (2) than ionic surfactantson the order of

seconds to minutes!

E T h l i l P

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E. Technological Processes

Foaming (foamability & foam stability)

Fabric Wetting

Solubilization

Emulsification

Importance of Micellar Kinetics in

Technological Processes

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The Importance of Micelle Break-up

in Foaming

Thin Liquid Film

Air

Air

Air

Surfactant

solution

More stable micelles Less monomer flux

Lower foamability

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The Importance of Micelle Break-up

in Emulsification

OilDroplet

Water

More stable micelles Less monomer flux

Higher interfacial tension Larger droplet size

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Geometrical Effects in

aggregationPacking Parameter

V

al=

J. N. Israelachvili, Intermolecular and surface forces, Academic, New York (1985).

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Critical Micelle Concentration

(CMC)Definition of theCMC

Brownian Dynamics Results for amphiphiles

with different sizes of the hydrophilic group

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Effect of the large head on micelle

size distribution The geometry of amphiphileeffects the Cluster Distribution

Bigger head group moleculesform smaller clusters withnarrower distribution

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