What Does A Surfactant Do? · PDF fileWhat Does A Surfactant Do?. . . Cleans by helping...
Transcript of What Does A Surfactant Do? · PDF fileWhat Does A Surfactant Do?. . . Cleans by helping...
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Surfactant AggregationBackground
What Is A Surfactant?
S u r f a c t a n t
. . . A chemical that, when dissolved in water, moves toward "surfaces"
Surface active agent
What Does A Surfactant Do?
. . . Cleans by helping dissolve things that aren't water soluble
. . . Lathers by "solubilizing air" in water
. . . Many biological processes (membranes, digestion, etc.)
How Do They Do It?
. . . TEAMWORK
. . . Individual surfactant molecules do nothing, they spontaneously group together to work "Self-Aggregation"
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Surfactant AggregationBackground
What Is Self-Aggregation?
. . . Aggregate gathered together into a mass so as to constitute a whole
. . . Self-aggregation forces exist that direct this process What are these forces?
Why Do Surfactants Self-Aggregate?
. . . There are only two "rules of the game":
Hydrophobic Interactions
H2O
Oil
OilH2O
Electrostatic Effects
-
+
-
+
Repulsion
AttractionAttraction
Repulsion
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Surfactant AggregationBackground
Why Do Surfactants Self-Aggregate (continued)?
. . . Surfactant Structure:
"Amphiphilic"
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2
"Tail" Group
Hydrophobic - Water Hating
Lipophilic- Oil Loving
-
3
SO3OH
COO-
N+R
"Head" Group
Hydrophilic - Water Loving
Lipophobic- Oil Hating
. . . Molecular inhomogeneity leads to molecular recognition and organization.
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The Origin of the Surface Tension of Water
Behavior of Surfactants in Water
O
HH+
-
+
O
HH+
-
+
O
HH+
-
+O
HH+
-
+
Water Molecules Hydrogen-Bond to Each Other:
Hydrogen-Bonding Forces are Un-Balanced at the Surface:
Bulk Water Molecules
Surface Water Molecules
Unbalanced Inter-molecular Forces Result in Surface
Tension
Surfactant Molecules Adsorbing at the Surface Reduce Surface Tension
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HO2
Air
Air-WaterInterface
Addition ofSurfactant to
Water
SURFACTANT AGGREGATION
Monolayer Formation
HydrophobicForces Repel
Water
Water Interface
HO2
Tailgroup in air
Headgroup
in water
SurfactantMoves to Air-
(”surface active”)
Interface
Monolayer atAir-Water
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Monolayer atAir-WaterInterfaceSaturated
SURFACTANT AGGREGATIONMicelle Formation
AdditionalSurfactant
Dispersed inBulk Water
Enoughsurfactantto pass“CMC”
SurfactantAggregates in
Spherical Micelles
Head groupson “outside
A SurfactantMicelle
Hydrophobic“pocket”
CMC = Critical Micelle Concentration= Concentration at which micelles begin to form
- Balance of hydrophobic and electrostatic forces- Environment-dependent
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If hair (80 )was as
wide as afootball field …
… A micelle (0.005 )would be 6mm in
diameter!!
How Small is a Micelle?
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MICELLE
M + M (n-1) Mn
Many equilibrium steps, so we approximatenM Mn
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H2OSurface
Air
70
60
50
40
30
Surf
ace
Tens
ion
(mN
/mor
dyn
/cm2
)Behavior of Surfactants in Water
Surface Tension Reduction: Measurement of CMC's
H2OH2O
Surfactant at Surface Surface Saturated
H2O
Critical Micelle Concentration
(CMC)
Micelles Formed
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lower CMC
morestable
micelles
Factors Affecting the Critical Micelle Concentration (CMC)
Factors Affecting Head-to-Head Repulsion Factors Affecting Chain/Chain Attraction• Head Group Charge - Ionic > Nonionic = Anionic 10-3M = Nonionic 10-4M - pH can influence
• Head Group Position - Non-terminal position increases CMC relative to terminal position
• Ethoxylation [R-(OCH2CH2)n-O-R'] - Nonionics: increases CMC - Anionics: decreases CMC
• Counterion of Ionic Surfactants - Anionics = CMC deceases in order: Li+ > Na+ > K+ > Cs+ > N(CH3)4+ > Ca2+
- Cationics = CMC deceases in order: F- > Cl- > Br- > I-
• Hydrocarbon Chain Length (nC) - CMC decreases logarith. as nC increases - Empirical relationship: log10CMC = A-BnC = For anionics, A 1.7, B 0.3 = For nonionics, A 3.5, B 0.05
• Hydrocarbon Structure - Branching = Carbon atoms off of a main chain contribute 1/2 that of main chain carbons. - Unsaturation = Increases CMC = Cis isomer increases CMC more than trans
• Other Main Chain Moities - Benzene Ring = Accounts for 3.5 carbon atoms - Propoxylation = Each propylene oxide 0.4 methylene carbons
• Secondary Polar Functions - C's between 2° polar group and 1° polar group 1/2 as effective as those w/o substitution
• Electrolyte - Decreases CMC (more for ionics than nonionics) - Effectiveness: = 1/2 SO42- > F- > Cl- > Br- > NO3- > I-
= NH4+ > K+ > Na+ > Li+ > 1/2 Ca2+
•Organic Additives - Low Water Solubility: Slight decrease in CMC - High Water Solubility: Minimal effects (below "co-solvent" levels) - Partially Soluble Materials: Decreases CMC
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Surfactant AggregationOverview of Aggregate Types (The "Surfactant Menagerie")
Increasing Surfactant Concentration in Water
Unorganized Monomers
Monolayer at Air-Water Interface
Spherical Micelles
Cylindrical Micelles (random)
Vesicles
Planar Bi-Layers
HexagonalPhase
(Middle)
Dilute Solution Concentrated Solution
"Particles" Liquid Crystal Phases
H2OH2O H2OH2O
H2OH2O H2OH2O
Lamellar Phase (Neat)
Anhydrous Crystal
Crystal Hydrate
Crystal Phases
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Surfactant AggregationOverview of Aggregate Types (The "Surfactant Menagerie")
Increasing Surfactant Concentration in Water
Unorganized Monomers
Monolayer at Air-Water Interface
Spherical Micelles
Cylindrical Micelles (random)
Vesicles
Planar Bi-Layers
HexagonalPhase
(Middle)
Dilute Solution Concentrated Solution
"Particles" Liquid Crystal Phases
H2OH2O H2OH2O
H2OH2O H2OH2O
Lamellar Phase (Neat)
Anhydrous Crystal
Crystal Hydrate
Crystal Phases
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Surfactant AggregationOverview of Aggregate Types (The "Surfactant Menagerie")
Increasing Surfactant Concentration in Water
Unorganized Monomers
Monolayer at Air-Water Interface
Spherical Micelles
Cylindrical Micelles (random)
Vesicles
Planar Bi-Layers
HexagonalPhase
(Middle)
Dilute Solution Concentrated Solution
"Particles" Liquid Crystal Phases
H2OH2O H2OH2O
H2OH2O H2OH2O
Lamellar Phase (Neat)
Anhydrous Crystal
Crystal Hydrate
Crystal Phases
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Surfactant AggregationOverview of Aggregate Types (The "Surfactant Menagerie")
Increasing Surfactant Concentration in Water
Unorganized Monomers
Monolayer at Air-Water Interface
Spherical Micelles
Cylindrical Micelles (random)
Vesicles
Planar Bi-Layers
HexagonalPhase
(Middle)
Dilute Solution Concentrated Solution
"Particles" Liquid Crystal Phases
H2OH2O H2OH2O
H2OH2O H2OH2O
Lamellar Phase (Neat)
Anhydrous Crystal
Crystal Hydrate
Crystal Phases
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Surfactant AggregationOverview of Aggregate Types (The "Surfactant Menagerie")
Increasing Surfactant Concentration in Water
Unorganized Monomers
Monolayer at Air-Water Interface
Spherical Micelles
Cylindrical Micelles (random)
Vesicles
Planar Bi-Layers
HexagonalPhase
(Middle)
Dilute Solution Concentrated Solution
"Particles" Liquid Crystal Phases
H2OH2O H2OH2O
H2OH2O H2OH2O
Lamellar Phase (Neat)
Anhydrous Crystal
Crystal Hydrate
Crystal Phases
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Surfactant AggregationOverview of Aggregate Types (The "Surfactant Menagerie")
Increasing Surfactant Concentration in Water
Unorganized Monomers
Monolayer at Air-Water Interface
Spherical Micelles
Cylindrical Micelles (random)
Vesicles
Planar Bi-Layers
HexagonalPhase
(Middle)
Dilute Solution Concentrated Solution
"Particles" Liquid Crystal Phases
H2OH2O H2OH2O
H2OH2O H2OH2O
Lamellar Phase (Neat)
Anhydrous Crystal
Crystal Hydrate
Crystal Phases
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How Does pH Affect Surfactants in Solution?
Sulfate or Sulfonate:
|| ||R-O-S-OH
O||
O
R-O-S-O-O||
O||
R-S-OH
O||
O
R-S-O-O||
O||
pH
1 3 5 9 11 137
1.0
0.5
0.0
Prop
ortio
n Pr
esen
t
or
or
pKa
§
|
0
0.5
1
1 3 5 7 9 11 13
Prop
ortio
n Pr
esen
t
pH
R-N-CH2C-O- Na+
R
| ||O
|
H+
R-N-CH2C-OHR
| ||OH+
R-N-CH2C-O- Na+
R
||O
|
Amphoteric (amphoacetate):
pKa pKa
Carboxylate (Soap):
pH
1 3 5 9 11 137
1.0
0.5
0.0
Prop
ortio
n Pr
esen
t
R-C-OH
O||
R-C-O-O||
pKa†
§§ The protonated form of the
surfactant does not really behave as a "surfactant" (unless there is more than one polar group)
† Micellar surfactants can have pKa ’s~4 units higher than their monomer counterparts (unless electric effects dictate otherwise)
a
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SURFACTANT FUNCTIONALITY
•Wetting– Water is not attracted on an oily/greasy surface. However, the
presence of surfactants facilitate the wetting process• Similar as in emulsification
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SURFACTANT FUNCTIONALITY
•SOLUBILIZATION– Micelles act as solubilizers– Hydrophobic molecules are taken into the lipophilic
core of the micelle• An important application is to make a solvent such as hydrocarbon
dissolve in water and surfactant
+
micelle
oil
solubilized oil
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SURFACTANT FUNCTIONALITY•EMULSIFICATION
– Adsorption at the water – oil interface results in dispersion of one phase into the other depending on the properties of the system• Emulsions water / oil or oil / water
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SURFACTANT FUNCTIONALITY
•DETERGENCY– It involves wetting, emulsification, solubilization and micellization
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Basic Surfactant Chemistry- What is a surfactant- Behavior of surfactants in water
Surfactant Type Overview- Structural variables- Surfactant synthesis and range of materials available
Surfactant-Influenced Performance Properties- Lather- Mildness- Rinse feel- Deposition of Actives- Cleaning
Surfactant-Influenced Product Physical Properties- Product Rheology- Stabilization of Dispersed Phases
Surfactant Information Resources
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What is Lather?The Structure of Lather
Air
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AirAir
Air
Air-Water
Interface
Air-Water
Interface
Liquid
Lamella
GravityDrainage
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AirAir
Air
ImpededDrainage
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Salt (Electrolyte) Level
Lather Volume
(Stability)
Impact of Electrolyte Level on Lather Volume
-
-
-
-
-
-
-
-
-
-
Electrostatic RepulsionsElectrostatic Repulsions
Closer packing of head groups increases hydrodynamic friction => slower lamellar drainage.
-
--
--
--
-
--
--
--
repulsion =>Minimization of electrostatic
across the barrierconstriction and eventual rupture.
s--------
--
----------
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Impact of Co-Surfactants on Lather
-
-
-
-
-
-
-
-
-
-
Electrostatic RepulsionsElectrostatic Repulsions
-
-
-
-
-
-
-
-
-
-
+ Non-Ionic
Surfactant
1. Reduced drainage (steric blockage)
2. Tighter interfacial packing* (reduced electrostatic repulsion)
-+
-
-
-
-
-
-
-
-
-
-
++
++
++
--
-
-
--
+AmphotericSurfactant
1. Tighter interfacial packing* (reduced electrostatic repulsion)
2. Reduced drainage (steric blockage)
* Also lowers CMC, see mildness impact.
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Effects of "Soils" on Lather
SolubilizedSoil
Depleted Level of "Free"Surfactant
EmulsifiedSoil Particle
Plus Soil
Examples of Soils: • Fatty Acids • Fatty Alcohols • Triglycerides • Hydrocarbons
Important Variables: • Length of hydrocarbon chain • Particle Size • Melting Poin t
Soils Can Be Part of the Product Formula or Part of the Surface Being Cleaned
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Impact of Particulates on Lather
-
-
-
-
-
-
-
-
-
-
0.01 - 1.0 µ
-
-
-
-
-
-
-
-
-
-
+ Small Particles
Drainage impeded by particle "dam" => better lather stability
Important factors • Optimum Size • Particle shape • Concentration • Surface hydrophilicity
-
-
-
-
-
-
-
-
-
-
+Large Particles
Particle spanning lamellaruptures bubbles => decreased lather stability
Important factors • Optimum size • Surface hydrophilicity
* Particles can be ingredients or form in situ (e.g., rxn. with Ca 2+)
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Surfactant Influences in Active Deposition
Types of Actives
Soluble Materials
Triclosan(TCS)
Cationic Polymers
Glycerin
+++
+
CH2
CH
CH2
OH
OH
OH
O
Cl
Cl
Cl
OH
Insoluble Particles
Trichlocar-ban (TCC)
Zinc Pyrith-ione (ZPT)
NH
CNH
O
Cl
ClCl
N S
O
2
-Zn2+
Dispersed Liquids
Petrolatum Oils Silicones
CH2
CH
CH2
OR
OR
OR
H3 C
Si
O
H3 C CH3
Si
OSi
CH3
CH3H3 C
H3 C CH3
n
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Most actives (except water-soluble non-polymers) associate with surfactants prior to deposition:
Water Soluble Polymers:Cationic: Nonionic:- --
---+ + +
Polymer Hydrophobic Domains
- - - - - -
Insoluble Solid Particles:Relatively Hydrophilic: Hydrophobic:
--
--
----
--
- -
-- -
-
-
-
---
--
-
Dispersed Liquid Droplet :
-- -
-
-
-
---
--
-
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Why Does Surfactant Adsorption Affect Deposition of Actives?
Exposure (adsorption) :
-- -
-
-
-
---
--
-
-- -
-
-
-
---
--
-Keratinous Surface
- - - --
-- -
- --
---+ + +
Poor adsorption if .... • Species charge
same as surface charge.
• Species surface hydrophilic
Rinse (retention):
-
---
--
-
-
+
+
+
-
---
--
Wa t er
If adsorbed species hydrophilic, will be carried away with rinse water. • Rinse water ions
(Ca2+) may alter. • Phase separation
upon dilution (complex coacerva.)
Keratinous Surface- - - -
-
-- -
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Typical conditions: • Huge surfactant excess over active, and • Surfactant mostly anionic
To maximize deposition: • Minimize surfactant level, • Minimize surfactant interaction with actives
- Particle size - Dissimilar surfaces
• Have surfactant charge be opposite that of surface, • Use Ca2+ from rinse water • Employ complex coacervation
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SURFACTANT INFLUENCED PRODUCT PHYSICAL
PROPERTIES
VISCOSITY
STABILIZATION OF DISPERSED PHASES
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10
100
1000
Vis
cosi
ty (P
)Viscosity of Surfactant Solutions
10 20 30 40 50 60 70Concentration (%)
Na C12-14E2S
1Spherical Micelles
Cylindrical Micelles
Hexagonal Hexagonal+ Viscous Isotropic
Viscous Isotropic
LamellarPhase
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Effects of Additives on Surfactant Rheology
Factors that decrease surf ace charge d ensity of micelle allow closer packing of surfactants and shift phase sequence to lower concentrations:
Concentration (%)
Vis
cosi
ty (P
)
+ Electrolyte
Examples of Additives: - Electrolytes (salt) - Co-surfactants
= Alkanolamides
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Role of Surfactants in Stabilization of Dispersed Phases
For small particles ( < 1µ), “molecular” stabilization can occur:
-- -
-
-
-
---
--
-
Electrostatic stabilization by ionic surfactants:
-- -
-
-
-
---
--
-
Steric stabilization by nonionic surfactants:
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For large particles, rheological stabilization relevant:
Structured Liquid
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SURFACTANTS IN PERSONAL CARE PRODUCTSPRODUCT TYPE LEVEL IN
PRODUCTFUNCTIONS LEVEL
IN USEIN PRODUCT IN USEPersonal Cleansing
Bars ~ 60% product structure lather cleaning rinse feel active deposition mildness
5-10%Body Wash ~ 15% viscosity control
stabilize dispersedphases
5-10%
Hair CareShampoos ~ 20% viscosity control
stabilize dispersephases
2-3%
Dentrifice 2-5% solubilize flavor /oils foaming / cleaning 1-2%Moisturizers 2-5% emulsifiers 1-7%
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SURFACTANTS IN FABRIC and HOME CARE PRODUCTS
PRODUCT TYPE LEVEL IN PRODUCT
FUNCTIONS LEVEL IN USEIN PRODUCT IN USE
Fabric CareGranules 12-25% cleaning
sudsing0.02%
Liquids 16-30% viscosity control 0.04%Dish Care
Liquids ~30% viscosity control cleaning, sudsing 0.06%Automatic nil
Hard SurfaceFloor Cleaners 2-22% cleaning 0.04%Gel Cleansers 2-10% gel agent cleaning 2-10%Abrasive cleansers 1-3% cleaning ~0.5%
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SURFACTANT CLASSIFICATION
• ANIONIC– Are negatively
ionised
RSO3-
• NONIONIC– Do not contain ionisable groups
RO
OO
OH
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SURFACTANT CLASSIFICATION• Cationic
– Are positively ionised R N
• Amphoteric– Have a pH dependent ionisation profile. e.g. Anionic at high
pH, cationic at low pH and neutral somewhere in between.– Zwitterionic surfactants are charged but have a net neutral
charge over most pH’s.
R NH
CO2
R NH2
CO2
R NH2
CO2H
High pH - negative
Middle pH- net neutral
Low pH - positive
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SOLUBILIZING GROUPS• Anionic• Sulfonate Sulfate Carboxylate
• Nonionic• Poly(ethylene oxide) Sugar derived (e.g. glucose)
• Cationic• Quaternary ammonium
• Amphoteric• Betaine Sulfobetaine
SO
OO R Na NaR O S
O
OO NaR
O
O
RO On OH
OH
OH
OH
OHR
R N R' Cl
R N CO2R N SO3
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HYDROPHOBHIC GROUPS
• The hydrophobic group has very little interaction with the solvent.
– Most common solvent is water, therefore the lyophobic part receive the name of hydrophobic
• For example for detergents usually the hydrophobic part contains between 8-20 atoms of carbon
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HYDROPHOBIC GROUPS• 1. ALIPHATIC HYDROCARBON
– Straight Chain• Trivial names are given to chain length mixtures derived
from natural sources. ‘Coco’, ‘Tallow’, ‘Palm’.
– Branched Chain• Introducing branching can deliver unique properties• Watch out is rate of biodegradation
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HYDROPHOBIC GROUPS
•2. ALKYLBENZENES– Generally present as a mixture of isomers
R
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Basic Surfactant Chemistry- What is a surfactant- Behavior of surfactants in water
Surfactant Type Overview- Structural variables- Surfactant synthesis and range of materials available
Surfactant-Influenced Performance Properties- Lather- Mildness- Rinse feel- Deposition of Actives- Cleaning
Surfactant-Influenced Product Physical Properties- Product Rheology- Stabilization of Dispersed Phases
Surfactant Information Resources