Colloid & Interface Science Case Study Model Answers

Transcription

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 2 nd law of thermodynamics

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

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 cm 3 oil and 25 cm 3 water respectively Form emulsion droplets with a diameter of cm New interfacial area created 150,1681 cm 2 (~150 m 2 ) S/V ratio: ~ 60,000 50,000 times increase in interfacial area!

5 Surface Area/Volume Ratio (S/V) Volume = 25 cm 3 S/ V ratio: variation with particle size d S/V Ratio Particle diameter (cm) Oil Water Area of oil/water interface: Area = π (d 2 /4) Add emulsifier and shake to form particles with a diameter of x cm: P vol = (4/3) π (x 3 /8) Number of particles (N) = V/P vol S/V Ratio = S/V Total surface area (S) = 4 π (d 2 /4) N 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 ( G) during preparation of the dispersion is described by the 2 nd 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

9 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 ( E) E 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)

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

11 Stokes Law - Predicting Phase Separation For a spherical particle (dilute solution): Rate = x = 2r 2 (ρ m - ρ p ) g t 9η m η m = viscosity of the continuous phase ρ m = density of continuous phase ρ p = 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 ρ 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

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)

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 Gas bubbles trapped in liquid Liquid drains from the films surrounding the gas bubbles (honeycomb structure) Polyhedral structure is eventually formed

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 (ε) Gibbs Marangoni Effect (Rubber Band) ε = 2 A A =Area d d γ = Surface tension Gravity thins lamellae γ A γ f - - γ 1 γ 2 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) - - f γ 1 γ 1

20 Foam Prevention - Antifoams Air Liquid Air Oil Oil spreads on the film and displaces surfactants γ O/L << γ Surface Oil Film thins and ruptures result of change in interfacial tension between film and 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) γ GL γ OG Oil γ OL Gas Liquid Or spreads as a thin (duplex) film Gas Liquid Oil layer

22 Spreading What happens when a liquid droplet (oil) is placed on a surface? S is -ve θ O S is + ve It can reside as a droplet or. Form a thin layer (spreading) The contact angle (θ) 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 (γ) S = γ GS - (γ OG + γ OS ) The surface tension of the fluid (γ OG ) <<< critical surface tension (CFT (γ GS )) 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

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. 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). γ OL γ SL Liquid (or air) Substrate At equilibrium: γ OS + γ OL COS θ - γ SL = 0 θ Oil γ OS θ = contact angle γ = 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 (Ψ) Surface potential -ve Cation Stern layer Zeta potential (ζ) Boundary of double layer in contact with the solution ( slipping plane ) Zeta potential (ζ) Stern layer Surface potential Distance (x) Electrical double layer described by Guoy Chapman or Stern models ζ magnitude affected by ph

33 DLVO Theory Electrostatic Stabilisation Potential energy (V T ) +ve V R Repulsive electrostatic (electrical double layer) interactions A X B Energy barrier Resultant interaction Particle Separation (X) V v Van der Waals attractive interactions Primary minimum -ve V T = V v + V R

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 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

36 Steric Stabilisation Conformation Effects Loop Water phase Tail Oil phase Train Hydrophobic group

37 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

38 H O Limited penetration of the polymer chains occurs during collision Adsorbed layers of polymer are fully extended into the solvent H 1 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 Steric Electrostatic 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 (C 8/10 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!

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!

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