Surface Chemistry Toolkit

Making sense of the role played by 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 = π (d2/4) Add emulsifier and shake to form particles with a diameter of x cm: Pvol = (4/3) π (x3/8) Number of particles (N) = V/Pvol Total surface area (S) = 4 π (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

Rat

io

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 (∆G) during preparation of the dispersion is described by the 2nd law of thermodynamics

∆G = γ A – T∆S

γ is the interfacial tension (emulsion), A is the ‘new’ interfacial area, T is temperature and ∆S is the entropy contribution (mixing)

• Driving force for instability is determined by the

magnitude of ∆G.

• 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 (∆E)

Preferred pathway

∆E

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 (ρm - ρp) g t 9ηm ηm = viscosity of the continuous phase ρm = density of continuous phase ρp = 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 (∆ρ ~ 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 ∆ρ 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 (ζ)

Electric Potential (Ψ)

Zeta potential (ζ)

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 ζ – 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

+

+

η

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

γGL

γOL

γOG

Spreading • What happens when a liquid droplet (oil) is placed on a surface?

O

• It can reside as a droplet or….

θ

S = γGS - (γOG + γOS )

S is -ve S is + ve

The surface tension of the fluid (γOG) <<< critical surface tension (CFT (γGS)) 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 (γ)

• The contact angle (θ) 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

θ

Formation of contact angle (θ)

Contact angle

θ

Contact angle (θ) 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

θ

θ = contact angle γ = surface tension

γOL

γOS γSL

At equilibrium: γOS + γOLCOS θ - γSL = 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 (θ ~ 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 = γ x ∆A A = interfacial area γ = 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 function1-5 Water in oil emulsifier5-8 Water in oil emulsifier

Oil 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 (ε) − Gibbs Marangoni effect (rubber band)

• A =Area • γ = Surface

tension

- - - - γ1

- - -

- -

- -

- - - - - - -

γ1 γ1 γ2

f f

- - - - -

γ1

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

γ

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 γO/L << γSurface

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

α-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 (γ)

Surfactant concentration (ln C)

Critical micelle concentration (CMC)

Γ= Gibbs surface excess

Γ= - 1 dγ RT dln C

Gradient

Area of adsorbed surfactant molecule = 1 (NA Γ) 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 (Lα)

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

www.solvay.com

E-mail: kevan.hatchman@solvay.com