Transcript Document

Colloid & Interface Science
Case Study Model Answers
Distance Learning Course in Cosmetic Science
Society of Cosmetic Scientists
Common Features
• Formulations were examples of lyophobic colloidal systems
• Dispersed and continuous phases are not compatible
• Interfacial properties are relevant
• Size of interfacial area is important
• Van der Waals forces will play a role at the interface
• We are creating new interface/interfacial area during the
processing of the formulation
• 2nd law of thermodynamics
The interface
Liquid (
)
Liquid (
)
A broad diffuse boundary
region separates the two
immiscible liquids
Liquid (
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
)
Solid (
)
Characteristic Features Of Colloids
• Surface-to-volume ratio (S/V) 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 (S/V)
Volume = 25 cm3
S/ V ratio: variation with particle size
60000
d
50000
S/V Ratio
40000
30000
Oil
20000
10000
Water
0
0.0001
0.001
0.01
0.1
1
Particle diameter (cm)
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
S/V Ratio = S/V
Total surface area (S) = 4 p (d2/4) N
V = volume of the continuous phase
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
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)
Add emulsifiers to reduce interfacial tension and
create ‘energy’ barrier (steric and electrostatic
repulsions). Work needs to be done to overcome
interactions (DE)
DE
Preferred
pathway
Rate is determined by
the thinning and
rupturing of
the film separating the
two droplets
Two
Droplets
Film
Rupture
One
Droplet
Time (t)
Routes To Instability - Kinetic Mechanisms
Creaming
Coalescence
Colloidal dispersion
Flocculation
Sedimentation
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 - 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)
Case Studies – Main Points To Remember
• Shower gel & Liquid Foundation Formulations
• Krafft point (viscosity problem) – anionic surfactants
• Alkyl sulphates are prone to become insoluble at low
temperatures
• Use hydrotrope
• Variation of viscosity with temperature
• Micelle shape changes
• Loss of rod micelle network (shower gel/shampoo)
• Packing of the surfactant molecules within the micelle
Case Studies – Main Points To Remember
• Foaming problems caused by creaming of the conditioner
from the formulation
• Will behave as an antifoam
• How can we stop the problem?
• Understand the properties of foam
• Lyophobic colloidal dispersion
• Polydisperse bubbles (cells)
• Pressure differences (Laplace) are important
• Drainage mechanisms (gravity, pressure pump)
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
Life Cycle Of Foams
Time
Gas bubbles
trapped in liquid
Liquid drains from the films
surrounding the gas
bubbles (honeycomb
structure)
Polyhedral structure
is eventually formed
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 (GibbsMarangoni effect)
• Maintain equililibrium interfacial tension – foams
can be deformed, i.e. stretchy
Film Elasticity (e) - Gibbs Marangoni Effect (Rubber
Band)
-
-
-
-
-
-
-
-
-
-
g1
-
-
-
f
-
-
• A =Area
• g = Surface
tension
f
g1
-
-
dA
g1
-
-
-
dg
-
-
ε = 2A
-
g2
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)
Foam Prevention - Antifoams
Air
Liquid
Air
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
Spreading
What happens when an oil drop is placed on a clean
liquid surface?
Remains as a drop (lens on the surface)
gGL
gOG
Gas
Oil
gOL
Liquid
Or spreads as a thin (duplex) film
Gas
Liquid
Oil layer
Spreading
•
What happens when a liquid droplet (oil) is placed on a surface?
S is -ve
S is + ve
O
q
•
It can reside as a droplet or….
•
Form a thin layer (spreading)
•
The contact angle (q) of the fluid in contact with the surface will change over time
•
We can predict whether the droplet will spread on the surface by considering the
Initial Spreading Coefficient (S)  interfacial tension (g)
S = gGS - (gOG + gOS )
The surface tension of the fluid (gOG) <<< critical surface tension (CFT (gGS)) for the
liquid to spread along the interface (liquid or solid)
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
• Size matters!
• Large oil droplets
Oil droplets
(macroemulsions) forms occlusive
layer on surface of the substrate
(e.g. skin) – delivery triggered by
Stratum
rubbing
corneum
• 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)
Case Studies – Main Points To Remember
• Cosmetic foundation
• Flocculation caused by insufficient dispersion of the
solid particulates
• Reduce particle size
• Interfacial properties become critical
• S/V ratio increases
• Need to appreciate how dispersions behave and are
made!
• Wetting of the interface
• Dispersant choice (anionic vs nonionic surfactants, or
polymers)
• Steric vs electrostatic stabilisation
Properties Of Colloidal Dispersions
Increase in
surface area
leads to better
absorption
properties,
e.g.
sunscreens
© BASF
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
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)
Wetting – the Young Equation
Spreading and wetting can be explained by the Young
equation (1800’s).
gOL
Liquid (or air)
gSL
At equilibrium:
Oil
q
Substrate
gOS + gOLCOS q - gSL = 0
gOS
q = contact angle
g = surface tension
Pigment Dispersions
Input of energy – high shear, grinding, milling
Breakdown of
agglomerates
Aggregates of
primary particles
Initial wetting of
agglomerates by
dispersant
Primary pigment
particles
Increase in interfacial area
Electrostatic Interactions – The Electrical
Double Layer
Electric Potential (Y)
Surface potential
Stern layer
Cation
-ve
Zeta potential
(z)
Distance (x)
Zeta potential (z)
Boundary of double
layer in contact with
the solution (‘slipping
plane’)
Stern layer
Surface potential
Electrical double layer
described by Guoy
Chapman or Stern models
z – magnitude affected by
pH
DLVO Theory – Electrostatic Stabilisation
Potential energy (VT)
+ve
VR
Repulsive electrostatic
(electrical double layer)
interactions
A
X
B
Resultant interaction
Energy
barrier
Particle Separation (X)
Vv
Van der Waals attractive
interactions
Primary minimum
-ve
VT = Vv + VR
Steric Stabilisation - Oil In Water (O/W)
Emulsion
Polymer chains act as ‘barrier’
to coalescence.
Oil
Oil droplets stabilised by anchored polymer chains
Oil
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
Pigment
Reduce
particle size
Bridging flocculation
Steric Stabilisation – Conformation Effects
Tail
Loop
Water phase
Oil phase
Train
Hydrophobic
group
Steric Stabilisation – Conformation Effects
Polymer ‘mushroom’
Radius of gyration
Polymer ‘brush’
Polymer chains extend
into solvent owing to
interactions with
neighbouring molecules
at high concentrations
HO
Limited penetration of the polymer
chains occurs during collision
Adsorbed layers of polymer are
fully extended into the solvent
H1
Compression of the
polymer chains
prevents the particles
from coalescing and
flocculating
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
‘Bad’ solvent
• ‘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
Stabilisation Method – Pro’s and Cons
Electrostatic
Steric
Need to add stabilising agent (polymer)
n Not reversible
n Sensitive to temperature changes (solvent
quality)
n Operates in aqueous and non-aqueous
systems
n
Easier to control
n Reversible
n Change ionic strength
n Predominantly aqueous
based
n
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
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!
www.solvay.com