Transcript MICELLES

SURFACTANTS
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
Phobic=Scared
Hydrophobic
Philic=Friendly
Hydrophilic
Phobic=Scared
+
Philic=Friendly
Phobic=Scared
Lyo=Dissolve +
Philic=Friendly
Lipophobic
Lipophilic
Lyophobic
Lyophilic
BASIC TERMINOLOGY
Lyophilic in oil
Hydrophobic
Lipophilic
Lyophobic in water
Lyophobic in oil
Hydrophilic
Lipophobic
Lyophilic in water
INTRODUCTION
• Surfactants are molecules that preferentially adsorb at an
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.
SURFACTANT STRUCTURE
Surfactants have amphipathic structure
Tail
head
•Tail or hydrophobic group
Little affinity for bulk solvent. Usually hydrocarbon
(alkyl/aryl) chain in aqueous solvents. Can be linear or
branched.
• Head or hydrophilic group
Strong affinity for bulk solvent. Can be neutral or charged.
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simply
add soap
What is the relationship amongst soap,
detergent and surfactant?
Surfactant
Detergent
Soap
SURFACTANT CLASSES
• Carboxylic acids and their salts including various fatty
acids tall oil acids, and hydrolyzed proteins:
• Sulfonic acids and their salts, including hydrocarbon
backbones of alkylbenzene, benzene, naphthalene, toluene,
phenolm lingin, olefins, diphenyloxide, petroleum cuts,
succinate esters etc.
O
R
O- M+
C
O
R
O- M+
S
O
O
• Sulfuric acid or salts including sulfated primary alcohols,
R
R-O
sulfated polyxyalkylenated alcohols etc.
O
O
• Alkyl xanthic acids:
R
O
O- M+
S
S- M+
C
S
R
O
• Alkyl or aryl dithiophosporic acids:
P
R
S- M+
O
• Polymeric anionics involving repeated groups containing
carboxyl acid functionality:
O
C
O- M+
R
Anionic (~ 60% of industrial surfactants)
n
SURFACTANT CLASSES
Anionic (~ 60% of industrial surfactants)
SURFACTANT CLASSES (contd.)
• Long chain amines derived from animal and
vegetable acids, tall oil and synthetic amines:
R
NH2
• Diamines and polyamines including ether amines
and imidazolines:
R
H
N
R
'
N
H
2
H
• Quaternary ammonium salts including tertiary mines
and imidazolines:
R
N+
R' M-
H
R"
• Quaternized and unquartenized polyoxyalkylenated
long chain amines:
R''''
O
R
n
R'''
N+
R"
R
• Amine oxides derived from tertiary amines oxidized
with hydrogen peroxide:
R'
N
O
R"
Cationic (~ 10% of industrial surfactants)
R' M-
SURFACTANT CLASSES (contd.)
• Polyoxyethylenated alcohols, alkyl phenols, alcohol
ethoxylates including derivatives from nonyl phenol,
coconut oil, tallow, and synthetic alcohols:
R''''
O
R
• Polyoxyethylenated glycols:
R''''
O
CH2
R''''
O
n
OH
CH2
CH2
n
OH
CH
OH
• Polyoxypropylenated glycols:
CH3
O
n
• Esters of carboxylic acids and alkyene oxides:
R
C
O
R"
NH
R"
O
• Alkanolamine condensates with carboxylic acids:
R
• Polyoxyalkylenated mercaptans:
R'
C
S
Non-ionic (~ 25% of industrial surfactants)
O
OH
R
n
OH
SURFACTANT CLASSES (contd.)
O
• Acrylic acid derivatives with amine functionality:
R'
NH
R
O- M+
C
O
O
• Subsituted alkylamides:
R
C
H
N
R"
C
O- M+
R"
C
O- M+
R'
O
R'
• n-Alkyl betaines:
X-
N+
R
O
R"
C
R'
R'
• n-Alkyl suffobetaine:
X-
R
N+
O
R"
R'
• Thio alkyl amines and amides:
R
S
R
'
O-M+
S
O
N
H
2
Amphoteric or zwitterionic:
(~ 10% of industrial surfactants). Generally expensive “specialty chemicals”.
O -M +
HYDROPHILIC-LIPOPHILIC BALANCE
• Griffin (1949): the hydrophilic-lipophilic balance (HLB) of a
surfactant reflects its partitioning behavior between a polar
(water) and non-polar (oil) medium.
• HLB number, ranging from 0-40, can be assigned to a
surfactant, 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).
What is HLB of
a surfactant?
The Hydrophilic-lipophilic balance [HLB]of a surfactant is a
measure of the degree to which it is hydrophilic or lipophilic,
determined by calculating values for the different regions of the molecule.
HYDROPHILIC-LIPOPHILIC BALANCE
-- Effect of Structure -Coil
oil
water
Cwater
C6H13COO-
C10H21COO-
C8H17COO-
HLB decreases
Surfactant
HLB
Sodium laury sulfate, C12H25SO4-Na+
40
Potassium oleate, C17H35COO-K+
20
Sodium oleate, C17H35COO-Na+
18
Oleic acid, C17H35COOH
1
n-butanol, C4H9OH
7
cetyl alcohol, C16H33OH
1
HLB value –
HLB Value
1
2
significance
3
4
5
Water in oil
emulsifier
6
7
8
9
10
1
1
12
1
3
1
4
15
16
1
7
1
8
Oil in water Emulsifiers
Use
Wetting Agents
Detergent
s
Solubilizer
HYDROPHILIC-LIPOPHILIC BALANCE
HLB
USE
4-6
Water-in-oil emulsions
7-9
Wetting agents
8-18
Oil-in-water emulsion
13-15
Detergents
15-18
Solubilizing
A value of 10 represents a “mid-point” of HLB.
HYDROPHILIC-LIPOPHILIC BALANCE
Translucent to
clear solution
No
dispersibility
in water
0
2
poor dispersibility
in water
4
6
Water in oil
emulsifier
triglycerol monooleate: Cream
and ointment stabilizers
8
Milky
dispersion;
unstable
10
Wetting agent
Insecticidal
sprays
Clear solution
12
14
Detergent
16
18 HLB
Solubilizer
Oil-in-water
emulsifier
Polysorbate 20
MICELLES
•If concentration is sufficiently high, surfactants can form
aggregates in aqueous solution
 micelles.
• Typically spheroidal particles of 2.5-6 nm diameter.
+
-
+
+
+
- - - -
+
Water
Layer
Hydrocarbon
Layer
Water
Layer
-+
- - - -+
Hartley Spherical Micelle
McBain Lamellar Micelle
+
oil
H2O
Oil in Water Micelle
H2O
oil
Water in Oil Micelle
(Klimpel, Intro to ChemicalsUsed in Particle Systems,p. 29, 1997, Fig 21)
H2O
Surfactant Micelle
MICELLES
--CMC--
• Onset of micellization observed by sudden change in
measured properties of solution at characteristic surfactant
concentration
 critical micelle concentration (CMC).
(Klimpel, Intro to ChemicalsUsed in Particle Systems,
p. 29, 1997, Fig 20)
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 salt
concentration.
(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.
MICELLES
--Driving Force--
• Hydrophobic groups can perturb solvent structure and
increase free energy of system. Surfactant will  concentrate at
S/G interface to remove hydrophobic groups from solution and
lower DGo.
AIR
WATER
MICELLES
--Driving Force--
• DGo can also be decreased by aggregation into micelles
such that hydrophobic groups are directed into interior of
structure and hydrophilic groups face solvent.
• Decrease in DGo for removal of hydrophobic groups from
solvent contact by micellization may be opposed by:
(i) loss in entropy of surfactant
(ii) electrostatic repulsion for charged headgroups
• Micellization is  a balance between various forces which
can be influenced by certain phenomena (Mukerjee and
Mysels, 1971).
+
MICELLES
--Example: Mayonnaise-+
Oil
+
+
lecithin
+
Water matrix
containing fat droplets.
The surfactant
(emulsifier) is lecithin.
It can contain up to 12
g of fat in 15 ml
++
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
2 μm
Water matrix
http://wilfred.berkeley.edu/~gordon/BLOG-images/mayo15.jpg
MICELLES
--Headgroup and Chain Length-• Klevens (1953): surfactants with linear alkyl chains, CMC is
related to number of carbons by;
log10CMC = b0 - b1mc
Where:
mc is number of carbons in chain
b0 and b1 are constants
(Hunter, Foundations of Colloid Science, p. 569, 1993, Fig 10.2.1)
MICELLES
--Headgroup and Chain Length-Surfactant
Temp (C)
b0
b1
Na carboxylates
20
2.41
0.341
K carboxylates
25
1.92
0.290
alkyl sulfonates
40
1.59
0.294
alkyl sulfates
45
1.42
0.295
alkylammonium chlorides
25
1.25
0.265
• Branching or addition of double bonds or polar groups to alkyl chain
generally increases CMC.
• Addition of benzene ring equivalent to addition of ~ 3.5 carbons
(methylene groups).
• Replacement of hydrogens in alkyl chain with fluorine initially
increases CMC, followed by marked decrease as fluorine
substitution goes to saturation.
(Hunter, Foundations of Colloid Science, p. 569, 1993, Fig 10.2.1)
MICELLES
--Temperature and Pressure--
• For ionic surfactants there exists a critical temperature above which
solubility rapidly increases (equals CMC) and micelles form
 Kraft point or Kraft temperature (TK),
• Below TK solubility is low and no micelles are present.
(Klimpel, Intro to Chemicals Used in Particle Systems, p. 30, 1997, Fig 22)
What is cloud point & pour point?
 The
Cloud point of a fluid is the temperature at which dissolved
solids are no longer completely soluble, precipitating as a second
phase giving the fluid a cloudy appearance.
 The
pour point of a liquid is the lowest temperature at which it will
pour or flow under prescribed conditions.
What is cloud point & pour point?
MICELLES
--Temperature and Pressure--
surfactant
crystals
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 Krafft
point.
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)
b2 and 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.
MICELLES
--Electrolyte-• If energy required is increased by electrolyte, activity
coefficient 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- > CNSand
NH4+ > K+ > Na+ > Li+
MICELLES
--Electrolyte-• For ionic surfactants;
log10CMC = b4 + b5log10Cs
where b4 and b5 are constants for a specific ionic head group at a
particular temperature.
• Depression of CMC with increasing salt due to double layer
compression around charged head group and charge screening
effect between head groups in micelle.
• Different salts vary in their effectiveness, e.g. for sodium laurate,
CMC depression follows:
PO42- > B4O72- > OH- > CO32- > SO42- > Cl-
MICELLES
--Electrolyte--
The effect of added salt on the CMC of SDS and dodecylamine
hydrochloride (DHC). (From Stigter 1975a,with permission)
(Hunter, Foundations of Colloid Science, p. 572, 1993, Fig 10.2.2)
MICELLES
--Organic Molecules-Small amounts of organic molecules can affect the CMC, e.g. in
aqueous solution of SDS, dodecanol (hydrolysis product of SDS)
causes minimum in surface tension measurement.
Surface Tension
go
CMC
Surfactant Concentration
• Solubilization of impurity in micelles causes rise in surface tension.
Very important for detergency, stabilization and dispersion.
MICELLES
--Organic Molecules-Solubilization characterized by large increase in solubility of
lipophilic (hydrophobic) organic species above surfactant CMC.
• Lipophilic organics can aid or oppose micelle formation. Two
classes based on mode of action.
• Group A (or Type I):
Adsorb within micelle and reduce CMC. Typically polar
molecules, e.g. alcohols and amides.
Effective at low concentrations.
Short chain members adsorb near micelle-water interface.
Longer chain members adsorb in core
 can influence micelle shape.
MICELLES
--Organic Molecules-Free energy of micellization lowered by screening repulsion
between charged head groups (ionic surfactants) and/or reducing
steric hindrance (non-ionic surfactants).
CMC depression greatest for linear species
 maximum when chain length approaches that of
surfactant.
• Group B (or Type II):
Modify bulk water structure around surfactant or micelle,
usually at higher concentrations than Group A molecules.
Structure breakers disrupt water structure about hydrophobic
tails and increase entropy. Entropy increase upon micelle
formation  reduced
 CMC is increased.
MICELLES
--Organic Molecules-Examples of structure breakers are urea, formamide and guanidinium
salts. Most effective on non-ionic surfactants of PEO type.
• Structure makers promote structuring of water, e.g. xylose and
fructose. Conversely, CMC is reduced due to enhanced entropy
increase upon micellization.
• At high bulk concentrations, species such as dioxane, esters,
short-chain alcohols and ethylene glycol can increase solubility of
monomeric surfactant, thus opposing micellization and raising
CMC.
MICELLES
--Aggregation Number-• Formation of micelles from association of n surfactant
monomers can be described by;
k1
nS
k-1
Mn
where n is number of surfactant monomers needed to form a
micelle
 aggregation number
k1 and k-1 are rate constants for forward and reverse reactions
• Equilibrium constant, K, can be expressed as:
k1 M 
K
 n
k 1 S 
MICELLES
--Aggregation Number--
If Cs and Cm are concentrations of surfactant monomer
and micelle, respectively;
Variation of dCm/dCT with total surfactant concentration for different values of
aggregation number, n. C0 is the critical micellization concentration and Cm the
concentration of micelles.
(Hunter, Foundations of Colloid Science, p. 572, 1993, Fig 10.3.1)
MICELLES
--Aggregation Number--
t1
t2
Micelle size distribution. Mn is the number of aggregates of size n. The
aggregates on the left side of the minimum (L) are called submicellar,
those on the right-hand side (proper) micelles with mean size of n, and
the width of their size distribution is given as σ.
(Hunter, Foundations of Colloid Science, p. 572, 1993, Fig 10.7.1)
MICELLES
--Aggregation Number Trends-(1) For same polar group, n increases with increasing chain
length.
(2) For constant alkyl chain length, n decreases with increasing
number of ethylene oxide groups in surfactant molecule.
(3) Oils or long chain alcohols increase n.
(4) For ethoxylated non-ionic surfactants, n drastically increases
with temperature.
(5) For anionic surfactants, n increases when NaCl is replaced
with MgCl2 or CaCl2.
(6) In aqueous solution, n ranges from 50-5,000, while in
organic solvents, n usually < 10.
(Hunter, Foundations of Colloid Science, p. 572, 1993, Fig 10.7.1)