Colloid & Interface Science

Case Study Model Answers

Distance Learning Course in Cosmetic ScienceSociety of Cosmetic Scientists

• 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

Common Features

Liquid ( )Liquid ( )

Liquid ( )Liquid ( )

Liquid ( )Liquid ( )

Solid ( )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

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 cm150,1681 cm2 2 (~150 m(~150 m22))• S/V ratio: ~ 60,000S/V ratio: ~ 60,000

• 50,000 times increase in interfacial area!

Surface Area/Volume Ratio (S/V)

Oil

Water

d

Area of oil/water interface:Area = (d2/4)

Add emulsifier and shake to form particles with a diameter of x cm:Pvol = (4/3) (x3/8)

Number of particles (N) = V/Pvol Total surface area (S) = 4 (d2/4) NS/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

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

G = A – TS

• 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)TwoDroplets

One Droplet

FilmRupture

Rate is determined by the thinning and rupturing ofthe 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

Creaming Coalescence

Flocculation Sedimentation

Colloidal dispersion

Routes To Instability - Kinetic Mechanisms

Stokes’ Law - Predicting Phase Separation

For a spherical particle (dilute solution):

Rate = x = 2r2 (m - p) g t 9m

m = viscosity of the continuous phasem = density of continuous phase

p = density of dispersed phaser = radius of spherical particlet = 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 is small

• ‘Weighting’ the oil phase (changing the density)

• Increasing the viscosity

• Surfactant system (phase behaviour)

• Polymers

• Inorganics (clays, silicas)

• 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)

Case Studies – Main Points To Remember

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 Gibbs Marangoni Effect (Rubber Band)

• A =Area = Surface

tension

- - - - 1

-- -

--

- -- -

- -- --

112

f f

- - - - -

1

• 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

d

dε A2

Foam Prevention - Antifoams

Oil

Oil

Oil spreads on the film and displaces surfactants O/L << Surface

Film thins and ruptures – result of change in interfacial tension between film and oil

Foam collapses

Air

Liquid

Air

Spreading

What happens when an oil drop is placed on a clean liquid surface?

Remains as a drop (lens on the surface)

Gas

LiquidOil

Or spreads as a thin (duplex) film

Oil layerLiquid

Gas

GL

OL

OG

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

O

• It can reside as a droplet or….

S = GS - (OG + OS )

S is -ve S is + ve

The surface tension of the fluid (OG) <<< critical surface tension (CFT (GS)) 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 ()

• The contact angle () of the fluid in contact with the surface will change over time

• Form a thin layer (spreading)

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)

• 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

Case Studies – Main Points To Remember

Properties Of Colloidal Dispersions

© BASF

Increase in surface area leads to better absorption properties, e.g. sunscreens

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).

OilLiquid (or air)

Substrate

= contact angle = surface tension

OL

OSSL

At equilibrium:

OS OS + + OLOLCOS COS -- SLSL = 0 = 0

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

Electrostatic Interactions – The Electrical Double Layer

-ve

Cation

Surface potential

Stern layer

Zeta potential ()

Electric Potential ()

Zeta potential ()

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

– 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

Steric Stabilisation - Oil In Water (O/W) Emulsion

OilOil

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

TailLoop

TrainOil 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

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!

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