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Surface Chemistry Toolkit
Making sense of colloid science in cosmetics & personal care
Distance Learning Course in Cosmetic Science
Society of Cosmetic Scientists
Dr Kevan Hatchman
Introduction • The toolkit brings together elements of the Colloid Science &
Surfactant modules:
• What is surface chemistry?
• The colloidal state & the role of the interface
• Practical applications: ‘Looking for clues’
• Product instability, appearance (homogeneous), sensory
(‘feel’)….
• Providing sensible solutions - application of surface chemistry
• Surfactants (micelles & phase behaviour)
• Polymers (steric stabilization, rheology, interactions)
What is surface chemistry?
• Colloid & Interface Science
• Size is important (surface area interfacial area) – dispersed phases
• Reduce particle size, the total surface area to volume ratio of the ‘system’ increases – affects performance
• ‘The world of neglected dimensions’ - Wolfgang Ostwald (1915)
• Welcome to the ‘twilight zone’…..
What is a colloid ?
• ‘Colloid’ : term introduced by Thomas Graham
(1861)
Dispersed phase
Continuous phase
• It is comprised of one phase dispersed in another
• May be comprised of several different types – liquid, gas or solid Multiple
combinations, e.g. w/o/w
What is a colloid ?
• Describing colloidal systems is not easy, but it is possible to characterise them according to
the following behaviours:
• Lyophilic or solvent ‘loving’, i.e. the dispersed phase appears to be miscible with the
continuous phase
• Polymer colloids (‘swell’ in the solvent)
• Spontaneously form and are stable
• With respect to thermodynamics and kinetics (time)
• Lyophobic or solvent ‘hating’, i.e. dispersed phase is immiscible in the continuous
phase
• Majority of personal care and cosmetic products fall into this category
• Requires energy to make them
− Not stable: Thermodynamics and kinetics
− Composition will change with time
• How can we differentiate a colloid from a dispersion?
• It is purely down to the dimensions of the dispersed phase!
How Are Colloids Made?
• It usually involves an energy change
• 2nd law of thermodynamics
• Creation of ‘new’ interface
• Achieved by….
• Communition
– High Shear mixing
– Two immiscible liquids
– Dispersing particles in a liquid
– Breaking up large particles in a ball mill
• A phase change
– Nucleation & growth
• Sols
• Polymer matrices
The colloidal state – properties of the
dispersed phase
Flat plate (clay)
Cylinder (fibre)
Sphere (oil droplet)
Colloidal dimension (1 – 1000 nm)
Cube (abrasive)
Dealing with systems
comprised of phases
with dimensions of the
order of 10-9 m to 10-6 m
Putting size into perspective!
© BASF • Particulate size of the dispersed phase is important interfacial area
• Affects appearance and performance of the product, e.g. opacity, rheology (phase volume)
• Product trends towards ‘nanotechnology’ – properties of the interface become very relevant
• Is Nanotechnology really new?
• Nature has being doing it for millions of years!
Characteristic features of colloids • Surface-to-volume ratio is high
• Potentially, colloidal systems may have interfacial areas comparable in
size to a football pitch!
• 6 cm diameter jar containing 25 cm3 oil and 25 cm3 water respectively
• Form emulsion droplets with a
diameter of 0.0001 cm
• New interfacial area created
• 150,1681 cm2 (~150 m2)
• S/V ratio: ~ 60,000
• 50,000 times increase in interfacial area!
Surface area/volume ratio
Oil
Water
d
Area of oil/water interface:
Area = p (d2/4)
Add emulsifier and shake to form particles with a diameter of x cm:
Pvol = (4/3) p (x3/8)
Number of particles (N) = V/Pvol Total surface area (S) = 4 p (d2/4) N
S/V Ratio = S/V V = volume of the continuous phase
S/ V ratio: variation with particle size
0
10000
20000
30000
40000
50000
60000
0.0001 0.001 0.01 0.1 1
Particle diameter (cm)
S/V
Ratio
Volume = 25 cm3
Properties of colloidal dispersions
© BASF
Increase in
surface area
leads to better
absorption
properties,
e.g.
sunscreens
Characteristic features of colloids
• The dispersed phase has an affect on the properties of the formulation,
e.g. rheology or the phase volume (emulsions)
Monodisperse system (uniform
droplets) : phase volume ~ 0.75
max Polydisperse system (non-uniform
droplets): phase volume > 0.75
Characteristic features of colloids
Stratum
corneum
Oil droplets
• Size matters!
• Large oil droplets (macroemulsions)
forms occlusive layer on surface of the
substrate (e.g. skin) – delivery triggered
by rubbing
• Small oil droplets (microemulsions)
penetrate surface of skin
• Improve deposition of silicones on hair, e.g. polydimethylsiloxane (PDMS)
• Increase molecular weight (viscosity) or use cationic emulsifiers
• Tailor particle size distribution
• Increase particle size to improve deposition
• Deposition is poor for very small particulate sizes (microemulsions)
though can be improved by presence of cationic polyelectrolytes and
anionic surfactants (coacervates)
The interface
What is an interface?
It is the transition region separating two or more immiscible phases
The following interfaces are involved in cosmetic science:
Gas/liquid – foams, aerosols
liquid/liquid – emulsions
solid/liquid – pastes, slurries, suspension emulsion systems
Gas/solid – aerosols, foams
Properties of the interface affect the performance of the product:
Surface tension (gas/liquid or gas/solid)
Interfacial tension (liquid/liquid or solid/liquid), e.g. wetting
and spreading
Related to physical characteristics of the interface:
Composition (polarity – hydrophilic or hydrophobic)
Surface roughness (solids)
Governed by intermolecular interactions (Van der Waals forces)
Liquid ( )
Liquid ( )
Liquid ( )
Solid ( )
A broad diffuse boundary
region separates the two
immiscible liquids
The composition of the boundary
region is not the same as the
liquid/liquid or gas/solid interface.
There is an abrupt transition from one
phase to another at the point
separating them
The interface
Formulating cosmetic and personal care products
What happens when we put a formulation together?
Assess the properties/identify the colloidal system
What types of interfaces are we dealing with…
Interfacial area increases during preparation particle size
distribution (dispersed phase)
The processes required to make it….
Do we need an input of energy?
What happens when it goes wrong….
Storage…
Performance…
What steps are needed take to rectify any problems….
Formulating cosmetic and personal care
products
Raw materials
Compatibility
Choice - are they really up to the job?
Quality - what you put in is what you get out!
Understand the problem
Stability – manner of phase separation
Performance (foaming, conditioning or cleansing)
Look for clues, colloid science can help to find the solution
Looking for clues….
• We know most personal care and cosmetic
formulations are ‘lyophobic’ colloids
• The dispersed and continuous phases are not
compatible with each other – i.e. immiscible
• Not stable - will separate very quickly into two
or more phases to reduce interfacial area
(thermodynamics)
• Overcome Van der Waals attractive forces
(‘balancing act’)
© Nanosight
Colloid Stability
• Colloidal systems are quite energetic
• The particles in the continuous phase are always moving
• We call it Brownian Motion….
Colloid Stability
• Notice anything about the way the particles were moving?
• Particles are moving in a random manner
• Rate is determined by a number of factors
• Temperature
• The viscosity of the medium
• Collisions between particles will happen….
• They can bounce off each other….
• Or stick together….
• But that’s another story!
Feel the force….
• The stability of cosmetic and personal care formulations (lyophobic colloids) are influenced by the following intermolecular interactions:
• Van der Waals attractive forces
• Leads to product instability
• Electrostatic and steric interactions
• Stabilise the dispersion
‘Do not underestimate the power of
the force….’
– Darth Vader
Van der Waals attractive forces
• Forces with the greatest effect are :
• London Dispersion Forces or Universal Attractive Forces.
• Keesom or Orientation Forces (Dipole-Dipole Interactions), e.g. hydrogen bonding
• Debye Forces (Dipole Induced Dipole Interactions).
• Magnitude of the interactions affect properties such as surface/interfacial tension
Interfacial forces – surface tension
Surface (gas)
Liquid
Net Force
Molecules in bulk interact equally in all directions with each other
Limited interaction at surface
• The properties of the surface/interface are dictated by the Van der Waals forces operating at
the surface and in the bulk material
• The surface tension of a liquid is a product of the attractive interactions between the gas and
liquid molecules at the surface (weak) and within the interior (stronger)….
Looking for clues….
• Lyophobic colloids require mechanical energy - mixing
• High shear mixer (Silverson)
• Stability of systems governed by thermodynamics and
defined by kinetics (time – reference point)
• Possible for ‘unstable’ formulations (thermodynamics)
to be ‘stable’ for several years
• Performance of the product will be determined by the
properties of the dispersion, i.e. phase separation
• Instability arises from random particle-particle
collisions (Brownian motion)
• State where intermolecular forces are in ‘balance’ is
often called ‘metastable’
Thermodynamics – the fly in the ointment
• Energy changes (DG) during preparation of the
dispersion is described by the 2nd law of
thermodynamics
DG = g A – TDS
g is the interfacial tension (emulsion), A is the ‘new’
interfacial area, T is temperature and DS is the
entropy contribution (mixing)
• Driving force for instability is determined by the
magnitude of DG.
• Reason why interfacial area plays an important role
Energy changes : emulsion stability
Free Energy (G)
Time (t) Two
Droplets One
Droplet Film
Rupture
Rate is determined by
the thinning and
rupturing of
the film separating the
two droplets
Add emulsifiers to reduce interfacial tension and
create ‘energy’ barrier (steric and electrostatic
repulsions). Work needs to be done to overcome
interactions (DE)
Preferred
pathway
DE
Anionic emulsifiers - charge stabilisation
-
-
-
•Adding a nonionic surfactant allows closer packing at the
interface and contributes to stabilizing the interface
Nonionic surfactant
Anionic surfactant (charge repulsions)
Mixed or paired emulsifiers (HLB)
• Use of mixed surfactants allows more surfactant to pack
effectively at the oil - water interface.
• This produces lower interfacial tensions and therefore a more
stable emulsion (steric stabilisation)
High HLB - more water soluble
Low HLB - more oil soluble
Routes to instability – kinetic mechanisms
• ‘Lyophobic’ colloidal systems are not stable
• 2nd law of thermodynamics
• We can, however, stabilise the dispersion by the creation of an energy barrier
• Adsorption of surface active agents or polymers
• There are a number of pathways through which a colloidal system can breakdown
• The preferred route however depends on the composition of the dispersion
• Density and rheological changes due to temperature effects
• Compatibility of ingredients, i.e. solubility
• And the properties of the interface…..
Phase separation
• Density changes alter the composition of the formulation
• Change in temperature
• Densities of the continuous and disperse phases do not ‘match’
• Appearance of the colloid will change over time….
Creaming Sedimentation
(caking) • ‘Stable’ colloidal dispersions can irreversibly separate
• Large particles will move much faster than smaller ones
• We can slow down the rate of separation by observing a few simple rules….
Interfacial Effects
• Phase separation is also influenced by the molecular interactions at the interface
• The inter-particle interactions are affected by the magnitude of the intermolecular forces
• It is possible for appearance of the colloid to change with time
• The particles can stick together to form floccs comprised of discrete particles
• Or fuse together to form larger ones coalescence
• It is possible to retard the process but we need to know a little more about the interface….
Flocculation Coalescence
Stokes’ law - predicting phase separation
For a spherical particle (dilute solution):
Rate = x = 2r2 (rm - rp) g
t 9hm hm = viscosity of the continuous phase
rm = density of continuous phase
rp = density of dispersed phase
r = radius of spherical particle
t = time taken to move specified distance (x)
g = acceleration due to gravity
Relevance – suspending pearlescent agents or pigments in
cosmetic formulations
Stokes’ Law
Decrease
particle
size
Match
densities
(Dr ~ 0)
Use
polymers
surfactants
waxes
clays
Structure the continuous phase (increase viscosity) to slow movement of the particles
Stokes’ law - problem solving
• Phase separation prevented by determining the
mechanism
• Matching the density of the dispersed and
continuous phase – ensure Dr is small
• ‘Weighting’ the oil phase (changing the
density)
• Increasing the viscosity
• Surfactant system (phase behaviour)
• Polymers
• Inorganics (clays, silicas)
Ion adsorption (electrostatic repulsions)
Oil Oil
Ionic surfactants adsorb at the interface and affect the
resultant surface charge
+ve -ve
Cationic surfactant Anionic surfactant
Electrostatic interactions – the electrical
double layer
-ve
Cation
Surface potential
Stern layer
Zeta potential (z)
Electric Potential (Y)
Zeta potential
(z)
Stern layer
Surface potential
Distance (x)
Boundary of double
layer in contact with
the solution (‘slipping
plane’)
Electrical double layer
described by Guoy
Chapman or Stern models
z – magnitude affected by
pH
Potential energy (VT)
Primary minimum
Van der Waals attractive
interactions
Particle Separation (X)
X
Repulsive electrostatic
(electrical double layer)
interactions
Resultant interaction
Energy
barrier
-ve
+ve
A B
DLVO theory – electrostatic stabilisation
VT = Vv + VR
VR
Vv
Potential energy (VT)
+
-
+
-
+
-
a) No electrolyte b) Electrolyte added
c) High electrolyte concentration
Secondary minimum
(weak flocculation)
Primary minimum
Primary minimum
EB EB
Energy barrier (EB) decreases as
the electrical double layer is
compressed and is eventually
neutralised
Distance
(x)
Distance
(x)
Distance
(x)
Potential energy (VT)
Potential
energy (VT)
+ +
+ + + + + + +
+ + +
- - -
- - - - - -
- - -
- - Negative charged surface
Positive
charged
surface
‘House of cards’ structure
Shearing force
- - - - -
- - -
+ + +
+
+
+
Dispersed
phase trapped
within the
structure
- - - - -
- - - - -
+ +
- - - - - - - - - - - - - - - - - - - - -
- - - - -
+ + +
Particles slide over each other
(electrostatic repulsions) – low
viscosity
Clay particle
- - - - - - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - -
+ + +
+ +
+ + +
+
+
+
Making use of electrostatic interactions
+
+
h
pH 0 2 4 6 8 10 12 14
-
- -
-
-
-
-
-
-
Ionic non-associative thickeners
Polyacrylic acid chain untangles as a result of ionised groups repelling each
other
Thickening effect is greatest for high molecular weight polymers and is
sensitive to changes in pH
O
H O
O
H O l o w e r p H
r a i s e p H
O
O
O
O
Steric stabilisation - oil in water (O/W)
emulsion
Oil Oil
Oil droplets stabilised by anchored polymer
chains.
Polymer chains act as ‘barrier’
to coalescence.
Steric stabilisation – performance engineering
Molecular weight and chemical structure are important
Dispersing agents
Anchor to substrate to provide stability
(hydrophobic or ionic interactions with surface)
Conformation is important (loops & tails)
Electrostatic/steric stabilisation
Select dispersant for the application, e.g.
molecular weight
Problems:
Poor adsorption (solvent quality), e.g. depletion
flocculation
Particle size is very small, bridging flocculation
may become an issue – assess particle size
distribution (photon correlation spectroscopy
(PCS)
‘Comb’ polymer
Bridging flocculation
Reduce
particle size
Pigment
Tail Loop
Train Oil phase
Water phase
Steric stabilisation – conformation effects
Hydrophobic
group
Radius of gyration
Polymer ‘brush’
Polymer ‘mushroom’
Polymer chains extend
into solvent owing to
interactions with
neighbouring molecules
at high concentrations
Steric stabilisation – conformation effects
Compression of the
polymer chains prevents
the particles from
coalescing and
flocculating
Limited penetration of the polymer
chains occurs during collision
Adsorbed layers of polymer are fully
extended into the solvent
HO
H1
Solvent concentration gradient
between bulk phase and adsorbed
polymer layer. Polymer prefers
solvent and particles are forced to
part, allowing the chains to be
solvated
Steric stabilisation - solvent effects
‘The Good, The Bad And The Theta!’
• ‘Good’ solvent
• Polymer chain segments extended in solvent producing an open configuration (polymer is miscible).
• ‘Bad’ solvent
• Polymer chain collapses into a more compact form.
• Transition occurs at the theta (q) temperature
• Polymer separates from solution, e.g. cloud point of PEGs
‘Good’ solvent ‘Bad’ solvent
Stabilisation method – pro’s and cons
Need to add stabilising agent (polymer)
Not reversible
Sensitive to temperature changes (solvent
quality)
Operates in aqueous and non-aqueous
systems
Easier to control
Reversible
Change ionic strength
Predominantly aqueous
based
Steric Electrostatic
Dealing with liquid/solid interfaces
• Dispersing solids in a liquid phase
• Cleansing product
• Make-up
• Applying a product to the skin
• Sensory (‘feel’) and penetration
• The properties of the interface dictates how
the formulation will behave
• Wetting and spreading
Wetting and spreading – an historical
perspective
The Ancient Egyptians used oils to
make coloured cosmetics
They found it was easier to disperse
coloured pigments
Why? – surface tension of the oils
were comparable to the critical surface
tensions of the pigments. It was easier
to ‘wet’ the solid and therefore aided
their dispersion in the oil.
The oils also permitted the
formulation to spread easily on the
skin. Egyptian cosmetic jar (2000 - 1000 BC)
Wetting and spreading – an historical perspective
The Romans also understood
spreading. They found oils were good for
cleaning the skin whilst bathing or as a
moisturiser.
Why? Surface tensions of oils were
similar to that of skin – easily spreads on
the surface
They also used oils and fats as
lubricants – formation of protective layers
on surfaces
Pliny wrote about fishermen pouring oil
onto the sea to form lenses to look for
fish
Wetting • Why does a droplet of water refuse to form a film on a greasy
surface?
• What causes a material to absorb a fluid, whilst another repels it?
• We are dealing with the properties of the interface and…
• Balancing the ‘driving’ forces of cohesion and adhesion
• Cohesive forces are result of the Van der Waals interactions between the molecules in the liquid
• Adhesive forces are the result of Van der Waals interactions between the molecules residing at the interface, i.e. fluid and substrate
• Wetting is purely: Adhesion >> Cohesion
Wetting
• Wetting is the displacement from a surface of one fluid by another
• Involves three phases - at least two must be fluids (liquid or gas) or a solid
• Wetting must take place before:
• Spreading, dispersing and emulsification, e.g. detergency (cleansing)
Spreading
What happens when an oil drop is placed on a clean
liquid surface?
Remains as a drop (lens on the surface)
Gas
Liquid
Oil
Or spreads as a thin (duplex) film
Oil layer
Liquid
Gas
gGL
gOL
gOG
Spreading • What happens when a liquid droplet (oil) is placed on a surface?
O
• It can reside as a droplet or….
q
S = gGS - (gOG + gOS )
S is -ve S is + ve
The surface tension of the fluid (gOG) <<< critical surface tension (CFT (gGS)) for the
liquid to spread along the interface (liquid or solid)
• We can predict whether the droplet will spread on the surface by considering the Initial Spreading Coefficient (S) interfacial tension (g)
• The contact angle (q) of the fluid in contact with the surface will change over time
• Form a thin layer (spreading)
What happens when a liquid is in contact
with a solid surface ?
Complete wetting
Incomplete wetting
q
Formation of contact angle (q)
Contact angle
q
Contact angle (q) decreases as droplet spreads
Substrate can affect contact angle (chemical nature or surface
roughness)
Contact angle results from a balance of interfacial surface
tensions acting at the point of contact (Young’s equation)
Wetting – the Young Equation
Spreading and wetting can be explained by the Young
equation (1800’s).
Oil Liquid (or air)
Substrate
q
q = contact angle
g = surface tension
gOL
gOS gSL
At equilibrium:
gOS + gOLCOS q - gSL = 0
140o 90o
30o
Oil
Oil
Best effect obtained for
hydrophobic particles that form
a contact angle around 90o
(partially embedded) - will
flocculate in either phase
Particle completely wetted by oil
phase (q ~ 0o)
Electrostatic repulsions
help to stabilise the
emulsion
Particle size
smaller than oil
droplet
Relevance of contact angle – Pickering emulsions
Emulsions
• Classified into two types:
• Oil in water (O/W) and water in oil (W/O)
O/W W/O
10-9 – 10-3 m
The type formed is determined by the relative proportions of
the components
Particle size – macroemulsions, nanoemulsions and
microemulsions
Emulsions
• An input of energy (work) is required to form the emulsion
Work = g x DA
A = interfacial area g = interfacial tension
• The lower the interfacial tension, less work is required to form an emulsion with a specific droplet size/interfacial area
Emulsion Stability
• Emulsifier (surfactant) lowers interfacial tension
• Surfactant adsorbed around droplet and acts as a physical barrier (can form liquid crystalline phase around oil droplet)
• Electrostatic repulsion (ionic surfactants)
• Steric repulsions (nonionic surfactants)
• Polymers stabilise emulsions by steric interactions
• Surfactant selection is important
• Mixed surfactant systems are beneficial (packing of the surfactant molecules at the interface)
• Use HLB numbers (Griffin) to select emulsifier
Hitting the target: HLB system (1940’s)
• HLB = Hydrophilic Lipophilic Balance
• Aids selection of nonionic emulsifiers
(surfactants) by characterising their
solubility in oil and water
• Assign number, defines water-liking and
oil-liking properties of a surfactant
• Arbitrary scale 0-20
• 0 - totally oil soluble
• 20 - totally water soluble
HLB system (1940’s)
• Many oils are assigned required HLB values
• This allow you to select appropriate emulsifiers for it
• Paired or mixed emulsifiers desirable
• Low and high HLB values
• Closer matching to actual HLB
• Gives more stable emulsions (packing at the interface)
• The HLB values assigned to surfactants are related to their structure
• Determine by calculation or experiment
The HLB of a nonionic surfactant gives an
indication of its role
HLB value Surfactant function
1-5 Water in oil emulsifier
5-8 Water in oil emulsifierOil in water stabiliserWetting agent
8-12 Oil in water emulsifierWetting agent
12-15 Oil in water emulsifierDetergent and solubiliser
15-20 Oil in water emulsifierDetergent and solubiliser
Hydrophile-Lipophile Balance (HLB)
Nonionic Surfactants HLB
1 18 4 6 8 10 13 15
Emulsifier
O/W
Functions for cleaning formulations
• Mixture of ‘low’ & ‘high’ HLB surfactants
• Emulsion stability (HLB range) can be affected by:
• Temperature
• Alcohol ethoxylate solubility in water decreases with increasing temperature
− The cloud point
− Electrolytes
− ‘Salting out’ electrolytes, e.g. NaCl, can affect the solubility of surfactants in water
Emulsifier selection - summary
• Points to consider
• Emulsion type O/W or W/O
• Selection based upon HLB
• Preferential solubility of the emulsifer in the oil or
aqueous phase dictates which type of emulsion
will be formed (Bancroft’s rule)
• Use of paired emulsifiers
• pH range
• Temperature range (nonionic surfactants)
• Compatibility with salts & actives
• Surfactant level, aim for 10% of oil concentration
(macroemulsions)
Emulsion instability
• Emulsions can be stabilized by:
• Using the correct combination of surfactants (steric stabilisation), e.g. HLB system
• Creating charge repulsions between oil droplets (ionic surfactants)
• Thickening the continuous phase - e.g. polymer
• Thickening (strengthening the interface) with waxes - e.g. liquid crystals formed with long chain alcohols
Dispersion
• Surfactant (dispersant) wets the surface of the solid and displaces any adsorbed fluids, e.g. gas.
• Solid disperses more readily in liquid.
Solid not wetted by
surfactant
Pigment dispersions
Increase in interfacial area
Input of energy – high shear, grinding, milling
Initial wetting of
agglomerates by
dispersant
Breakdown of
agglomerates Aggregates of
primary particles
Primary pigment
particles
Detergency
• Detergency is the removal of a soil (matter) by mechanical and chemical
action (pH) under favourable conditions (temperature) in the presence of a
surfactant
• Combination of the following functions
• Wetting
• Dispersing
• Solubilisation
• Emulsification
• Oily droplet on substrate….
No surfactant With surfactant
Detergency
Hair
Sebum
Wetting
Emulsification
Dispersion
Foams – gas/liquid interface
• Personal cleansing products formulated to give long lasting creamy foam
• Consumers will buy products that will produce copious amounts of foam
• Foams deliver actives to the skin or hair and help to remove oils and dirt
• Generated with the aid of surfactants
• Formulations that produce the most foam with the minimum quantity of surfactant are desirable
What is foam ?
Dispersion of a gas in a liquid
Trap gas by mechanical action
(agitation)
Can be a problem (industrial
processes)
Not stable (lyophobic colloid)….
Foam is a collection of bubbles
Stabilise using surface active agents –
surfactants, polymers, particulates
Time
Gas bubbles
trapped in liquid
Liquid drains from the films
surrounding the gas
bubbles (honeycomb
structure)
Polyhedral structure
is eventually formed
Life cycle of foams
Foam instability
Gravitational force - drainage
Capillary pressure (squeeze liquid from film
separating bubbles) – liquid flows to regions of low
pressure, i.e. separating cells (Plateau regions)
Diffusion of gas across foam lamellae (bubble
disproportionation)
Leads to bursting of bubbles and rearrangement of
foam lamellae
Foam persistance
• Prevent drainage and diffusion of gas across foam lamellae (increase viscosity or retard fluid drainage by presence of liquid crystals)
• Polyelectrolytes bind to surfactant at interface – impart mechanical rigidity
• Close packing of surfactants at the interface
• Maintain low interfacial tension
• Ionic surfactants (electrostatics) – can be screened by electrolytes and affect stability
• Annealing of foam lamellae by surfactant (Gibbs-Marangoni effect)
• Maintain equililibrium interfacial tension – foams can be deformed, i.e. stretchy
Film elasticity (e) - Gibbs Marangoni effect
(rubber band)
• A =Area
• g = Surface
tension
- - - - g1
- - -
- -
- -
- - - - - - -
g1 g1 g2
f f
- - - - -
g1
Gravity thins lamellae
Gibbs-Marangoni effect (combination of two separate processes) restores equilibrium
(fills holes in the film) - lowers surface tension
Concentration dependent (migration of surfactant to the interface from bulk solution)
A
g
d
d ε A 2 =
Gas
Liquid
Gas
Anionic
surfactant
Cationic
polymer
Polymer
binds to the
surfactant to
make the
film more
rigid
Polymer forms ‘bridge’
between neighbouring films
Polymer-surfactant interactions – foam stabilisation
Foam performance
• Foam performance of cleansing formulations containing surfactants depends on:
• Surfactant ratio (primary: secondary) and concentration
• Presence of additives, e.g. oils, polymers
• We can assess the foaming ability using a combination of different techniques which includes:
• Rotary foam measurements (Beh-James) – screen several formulations at a time
• Beating/pouring (Hart De George)
• Static methods, e.g. Ross-Miles
• Dynamic foam test
• Instrumental (e.g. Foam Scan)
When foam is a problem !!!!
• Presence of foam may not be desirable
• Severe agitation
• Need to use some kind of control
Foam inhibition
• Why is this beer is flat?
• Oil slicks ! (grease….)
• Provides some form of foam control
• Care needed when formulating products
with oils
• Too much can prevent foam from
forming, e.g. oils with ‘low’ surface
tensions can spread along the interface
(antifoam)
Antifoams
• Compounds that inhibit foam formation are called antifoams or defoamers
• Antifoam compounds include:
• Silicones (e.g. polydimethylsiloxane) - laundry/industrial processes
• Branched alcohols (affects packing of the surfactant molecules at the interface)
• Oils, fats and waxes - may form solid particles (contact angle)
Foam prevention - antifoams
Oil
Oil
Oil spreads on the film and
displaces surfactants
gO/L << gSurface
Film thins and ruptures –
result of change in interfacial
tension between film and oil
Foam collapses
Air
Liquid
Air
What is a surfactant?
• It is a “surface active agent”
• A chemical compound that combines oil soluble and water soluble properties
• Surfactants are “active” at a surface or interface
Oil soluble portion
Lipophillic Water soluble portion
Hydrophillic
Surfactants - Four Types
No charge
+
-
+/-
- + Acidic
Alkaline
+ve charge
Anionic
Cationic
Nonionic
Amphoteric
- ve charge
Natural vs synthetic routes
• Feedstocks for the hydrophobe (alkyl chain) obtained from two main sources.
• ‘Natural’ or renewable sources – animals or plants
• ‘Synthetic’ or non-renewable sources – oil and coal
• Both require processing to obtain either the fatty acids or triglycerides and olefins.
• It is energy intensive.
• Synthetic routes need more processing steps, e.g. cracking
Natural vs synthetic routes
• Fatty alcohols are one of the most important feedstocks
• Natural – oils and fats are purified before conversion to fatty acids or methyl esters. The products are then distilled/fractionated to give the desired cut. Fatty alcohols are obtained by hydrogenation of fatty acids with a catalyst.
• Synthetic – olefins are converted to the fatty alcohol by
• ‘OXO’ process
• Ziegler process
• The fatty alcohols prepared by the different routes have different properties
Oils & Fats
Purification
Transesterification
Glycerol
Esterification
Hydrolysis
Fatty acid
Glycerol
Methyl ester
Hydrogenation
Fatty alcohols
Distillation Fractionation
Purification carried out
at high temperatures &
high pressure
Crude oil & natural gas
Ethylene n-paraffins
a-Olefins i-Olefins
OXO process
Oxo-alcohols
Ziegler process
n-Alkanols
Natural vs synthetic routes
• The hydrocarbon feedstocks are then processed further with hazardous chemicals to produce the surfactants
• Hydrophiles used to make the surfactants fall into two groups
• Inorganic – H2SO4/SO2, SO3 and P2O5
• Organic – Ethylene oxide/propylene oxide, polyols and alkanolamines
• Performance of the surfactant is influenced by the relative ‘strengths’ of the hydrophilic and hydrophobic groups
Oils and fats Fatty acid
methyl ester Alkanolamides
Ethoxylated
alkanolamides
Fatty acids
Glycol + glyceryl
esters
Amphoacetates
Fatty alcohol
Alkyl
sulphates Alkyl
dimethylamines
Alkylamine oxides,
betaines + quats
Fatty acid
isethionates N-acyl derivatives
Alcohol
ethoxylates
Alkyl
polyglycoside
Alkyl ether + ester
carboxylates
Alkyl ether
sulphates
Phosphate
esters
Sulfo-succinate
Alkyl amido betaines
Integrated oleochemical routes
Hydroxy-
sultaines
Micelles – association colloids
• Breaking up and reforming
• Comprised of 100s of molecules
• Surfactant molecule structure - affects micelle shape (sphere, rod….)
• Micelle shape and size can effect the rheology and behaviour of
detergent systems
Sphere Rod Disc
CMC - critical micelle concentration
Surface
tension (g)
Surfactant concentration (ln C)
Critical micelle concentration (CMC)
G= Gibbs surface excess
G= - 1 dg
RT dln C
Gradient
Area of adsorbed surfactant molecule = 1
(NA G) NA is Avogadro’s number
Critical micelle concentration (CMC)
• CH3(CH2)nX
• CMC decreases as n increases (Traube’s rule)
• CMC decreases from being ionic to nonionic
• CMC at minimum where X is at the end of the molecule
• When X=(EO)m , CMC decreases as m decreases
• CMC decreases as the surfactant becomes less soluble (Krafft point)
Importance of CMC
Low High
Concentration
Nonionic surfactant Ionic surfactant
Poor solubility in water
(Krafft point)
Soluble in water
Mild Irritant
CMC can be reduced by additives, e.g. polymers, amphoteric surfactants
Micelle
Surfactant
molecules bind to
polymer chain
Micelles force polymer chain
to open and expand
(repulsions between
aggregates)
Micelles form a ‘string of
pearls’ arrangement along
the chain
Polymer-surfactant interactions – hydrophobic
polymer
Precipitation region
Polymer (polyquaternium)
and anionic surfactant
(negative) below critical
micelle concentration (cmc)
Deposition of coacervate
(complex) from solution
Coacervate structure expands
as micelles form and the
complex is solubilised
Micelles form
along the chain
Increasing surfactant concentration
Polymer-surfactant interactions - conditioning
Speed is everything - interfacial properties
• Surfactants readily adsorb at interfaces
• Rate determined by:
• Diffusion of molecules from bulk solution to surface
• Size of the molecule
• Orientation of the molecule into preferred packing arrangement at
the interface
• Crucial for:
• Wetting, emulsification and dispersing (detergency)
• Foaming, liquid aerosols (sprays)
The Krafft Point
• The Krafft phenomena is the temperature dependent solubility of ionic surfactants
• Below the Krafft point the surfactant exists as hydrated crystals - turbid appearance at low temperature
• Krafft point increases with increasing chain length
• Addition of salting out electrolytes increases the Krafft point
The Krafft Point
• Krafft point is lowered by branched chains
• Unsaturation (double bonds)
• Insertion of EO groups between alkyl chain and the head
group - alkyl ether sulphates have lower Krafft points than
alkyl sulphates
• Hydrotropes - enhance solubility of surfactants in water,
e.g aryl sulphonates, short chain (C8/10 phosphate ester,
APG...), amphoteric surfactants
Micelle shape (critical packing parameters)
‘Driving force’ for different micelle structures – head
& tail interactions
lc
P = v
lc a
P = critical packing parameter
a = cross sectional area of the head
group
v = volume of hydrocarbon tail
lc = all ‘trans’ length of tail
a
Micelle shape (critical packing parameters)
Surfactants molecules have different geometries – affects
packing at interfaces
P > 1 P ~ 1 1/3 < P < 1/2
Micelle Shape (critical packing parameters)
Critical packing Packing shape Structure
factor (P)
P < 1/3 Cone Spherical micelles
1/3 < P < ½ Truncated cone Rod micelles
½ < P < 1 Truncated cone Vesicles
P ~ 1 Cylinder Bilayer micelles
P > 1 Inverted truncated cone Inverse micelles
Head group size: hydrophilic character
Hydrophobe group: lipophilic character
Importance of surfactant molecular structure
Micelle shape (critical packing parameters)
Weak head group repulsions
- ‘Salting out’ electrolytes for ionic
surfactants
Small head group, large bulky tail (branching,
unsaturation (kinky), di-alkyl derivatives)
Low curvature structure (disc shaped micelles),
P ~ 1 – favoured for microemulsions
Bulky tails favour ‘reverse’ structures – w/o emulsions or liquid crystalline
phases (bicontinuous cubic (V2 & I2) and hexagonal (H2))
Low or planar curvature – ideal for multi-lamellar vesicles
Micelle shape (critical packing parameters)
Strong head group repulsions (electrostatic or
steric)
Large head group, small tail
Micelle has highly curved structure (spheres
and rods)
‘Gaps’ at interface – o/w emulsions
Strengthen film (low interfacial surface tension) with mixture of different
molecular structures (mixed HLB’s)
‘Salting out’ electrolyte (%)
Viscosity build relies on
entanglement of
rod/cylindrical micelles
Viscosity
Rod micelles
Surfactant phase behaviour – rheology
Surfactant phase behaviour – lyotropic liquid
crystals
Surfactants form micelles in
aqueous/polar media
Cubic phase (I1)
Increase concentration micelles form organised structures called liquid crystals
Three main types – cubic (I and V), hexagonal and lamellar
Exhibit birefringence and have defined crystal lattice spacings (x-ray)
Phase diagrams are used to ‘map’ the regions where these structures are found
Surfactant lyotropic liquid crystalline phases
Hexagonal phase
(H1)
Lamellar phase
(La)
Small Angle X-ray Scattering (SAXS)
Importance of lyotropic liquid crystals -
emulsion stability
Oil droplet is coated by layers of
lamellar phase (multi-lamellar vesicle
structure)
Droplets appear as ‘maltese’ crosses
when viewed with a polarized light
microscope
Oil droplet
Liquid crystal provides a ‘barrier’ to
coalescence
Summary
• Use principles of colloid and surface chemistry to solve the
problem
• Identify causes and their effect on the formulation –
evaluate/performance indicators
• Problems can be caused by more than one process
• Need to bear in mind….
‘Nae cannae change the laws
of physics’
Montgomery Scott
Thermodynamics rules ok!
Solutions… • More than one solution….
• Increase the viscosity of the continuous phase
• Polymers, surfactants….
• Adapt the formulation e.g. Krafft point, tolerant to water
hardness…
• Reduce level of oils (emollients) if they are suspected of
acting as a defoamer or remove them completely
• Replace immiscible components, e.g. compatibility issues
• Evaluate performance (rheology, tests…)
• Carry out storage tests…
Summary
• Use the INCI listings on back of products as a
guide
• Review patents
• Raw materials - careful selection what you put
in is what you get out!
• Contact raw material manufacturers!
Further reading
• ‘Basic Principles of Colloid Science’, D H Everett, RSC (1987)
• ‘Introduction to Colloid and Surface Chemistry’, D J Shaw, Butterworth Heinemann, 4th ed (2000)
• ‘Surfaces, Interfaces & Colloids : Principles & Applications’, D Myers, Wiley & Sons (1999)
• ‘Interfacial Science’, M W Roberts, Blackwell Science (1997)
• ‘Introduction to Soft Matter: Polymers, Colloids, Amphiphiles and Liquid Crystals’, I W Hamley, J Wiley & Sons (2000)
Further reading
• ‘Colloid Science, Principles, Methods and Applications’, Ed T Cosgrove,
Blackwell (2005)
• ‘A guide to the Surfactants World’, X Domingo, Proa (1995)
• ‘Surfactants in Cosmetics’, ed. M M Rieger and L D Rhein, 68, Surfactant
Science Series, Marcel Dekker Inc (1997)
• ‘Surfactants and Polymers in Aqueous Solution’, B Jonnson, B Lindman, K
Holmberg and B Kronberg, John Wiley & Sons (1998)
Further reading
• M Garvey, Chemistry in Britain, 2003, February, 28
• J Mufti, D Cernasov, R Macchio, HAPPI, 2002, February, 71
• R Y Lochhead, L R Huisinga, Cosmetics & Toiletries, 2004, 119(2), 37
• R E Stier, Cosmetics & Toiletries, 2004, 119(12), 75
• R Y Lochhead, S Jones, HAPPI, 2004, July, 67
• R Y Lochhead, L R Huisinga, Cosmetics & Toiletries, 2005, 120 (5), 69
‘Size matters……’
Va, Va voom!….’
Thierry Henry
