1. Colloid & Interface Science Case Study Model Answers Distance Learning Course in Cosmetic Science Society of Cosmetic Scientists

2. 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 2

3. The interface Liquid ( )
A broad diffuse boundary region separates the two immiscible liquids
Liquid ( )
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
Figure 3
Solid ( )

4. 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 cm
New interfacial area created
150,1681 cm2 (~150 m2)
S/V ratio: ~ 60,000
50,000 times increase in interfacial area!

5. Surface Area/Volume Ratio (S/V)
Volume = 25 cm3 d
Oil
Water
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

6. 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

7. 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

8. 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

9. Energy Changes : Emulsion Stability
Add emulsifiers to reduce interfacial tension and create ‘energy’ barrier (steric and electrostatic repulsions). Work needs to be done to overcome interactions (DE)
Free Energy (G) DE
Preferred pathway
Rate is determined by the thinning and rupturing of
the film separating the two droplets
Time (t)
Two
Droplets
Film
Rupture
One
Droplet

10. Routes To Instability - Kinetic Mechanisms
Creaming
Coalescence
Figure 6
Colloidal dispersion
Sedimentation
Flocculation

11. 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

12. 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)

13. 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 13

14. 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) 14

15. 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

16. Life Cycle Of Foams Time
Liquid drains from the films surrounding the gas bubbles (honeycomb structure)
Polyhedral structure is eventually formed
Gas bubbles trapped in liquid

17. 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

18. 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

19. Film Elasticity (e) - Gibbs Marangoni Effect (Rubber Band)1 2 f A g d ε 2 =
A =Area
 = Surface tension
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)

20. Foam Prevention - Antifoams
Air
Oil spreads on the film and displaces surfactants
gO/L << gSurface
Oil
Liquid
Air
Film thins and ruptures – result of change in interfacial tension between film and oil
Oil
Foam collapses

21. Spreading
What happens when an oil drop is placed on a clean liquid surface?
Remains as a drop (lens on the surface)
gOG
gGL
Gas
Oil
gOL
Liquid
Or spreads as a thin (duplex) film
Gas
Oil layer
Liquid

22. Spreading S = gGS - (gOG + gOS ) S is -ve S is + ve O
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)

23. 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

24. Characteristic Features Of Colloids
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
Oil droplets
Stratum corneum
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)

25. 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 25

26. Properties Of Colloidal Dispersions
Increase in surface area leads to better absorption properties, e.g. sunscreens
© BASF

27. Dispersion
Surfactant (dispersant) wets the surface of the solid and displaces any adsorbed fluids, e.g. gas.
Solid disperses more readily in liquid.
SOLUBILASIATION -
Induce apparent solubility of insoluble components.
Encapsulate in micelle
Premix - insoluble material should be present when micelle forms.
Eg. Picture SHOW GELS
DISPERSION
System with 2 or more phases (One continuous)
One or more phases finely dispersed.
If just had solid + Continuous phase -> agglomeration
Add surfactant which adsorbs on surface of solid particles forcing them apart and dispersing them.
DEMO
Solid not wetted by surfactant

28. 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

29. 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)

30. Wetting – the Young Equation
Spreading and wetting can be explained by the Young equation (1800’s).
gOL
Liquid (or air)
Oil
gSL
gOS q
Substrate
At equilibrium:
gOS + gOLCOS q - gSL = 0
q = contact angle
g = surface tension

31. 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

32. Electrostatic Interactions – The Electrical Double Layer
Electric Potential (Y)
Surface potential
Stern layer
-ve
Cation
Zeta potential (z)
Distance (x)
Electrical double layer described by Guoy Chapman or Stern models
z – magnitude affected by pH
Zeta potential (z)
Boundary of double layer in contact with the solution (‘slipping plane’)
Stern layer
Surface potential

33. DLVO Theory – Electrostatic Stabilisation
Potential energy (VT)
Repulsive electrostatic (electrical double layer) interactions
+ve VR A B X
Resultant interaction
Energy barrier
Particle Separation (X)
Figure 49
Van der Waals attractive interactions Vv
Primary minimum
VT = Vv + VR
-ve

34. Steric Stabilisation - Oil In Water (O/W) Emulsion
Polymer chains act as ‘barrier’
to coalescence.
Oil
Oil
Oil droplets stabilised by anchored polymer chains

35. Steric Stabilisation – Performance Engineering
‘Comb’ polymer
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)
Pigment
Reduce particle size
Bridging flocculation

36. Steric Stabilisation – Conformation Effects
Tail
Loop
Water phase
Figure 21
Oil phase
Hydrophobic group
Train

37. Steric Stabilisation – Conformation Effects
Polymer ‘mushroom’
Radius of gyration
Polymer ‘brush’
Figure 20
Polymer chains extend into solvent owing to interactions with neighbouring molecules at high concentrations

38. HO
Limited penetration of the polymer chains occurs during collision
Adsorbed layers of polymer are fully extended into the solvent H1
Figure 19
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

39. 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

40. Stabilisation Method – Pro’s and Cons
Electrostatic
Steric
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

41. 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

42. 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

43. 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….

44. ‘Nae cannae change the laws of physics’
Montgomery Scott
Thermodynamics rules ok!
Intoduce reasons why micelles are important

45. 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…

46. 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!