Surface Chemistry Toolkit

Transcription

1 Surface Chemistry Toolkit Making sense of colloid science in cosmetics & personal care Distance Learning Course in Cosmetic Science Society of Cosmetic Scientists Dr Kevan Hatchman

2 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)

3 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..

4 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

5 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!

6 How Are Colloids Made? It usually involves an energy change 2 nd 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

7 The colloidal state properties of the dispersed phase Cube (abrasive) Flat plate (clay) Colloidal dimension ( nm) Dealing with systems comprised of phases with dimensions of the order of 10-9 m to 10-6 m Sphere (oil droplet) Cylinder (fibre)

8 Putting size into perspective! Particulate size of the dispersed phase is important interfacial area BASF 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!

9 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 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!

10 S/V Ratio Surface area/volume ratio Volume = 25 cm 3 S/ V ratio: variation with particle size d Particle diameter (cm) Oil Water Area of oil/water interface: Area = p (d 2 /4) Add emulsifier and shake to form particles with a diameter of x cm: P vol = (4/3) p (x 3 /8) Number of particles (N) = V/P vol S/V Ratio = S/V Total surface area (S) = 4 p (d 2 /4) N V = volume of the continuous phase

11 Properties of colloidal dispersions Increase in surface area leads to better absorption properties, e.g. sunscreens BASF

12 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

13 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)

14 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)

15 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

16 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.

17 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

18 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 )

19 Colloid Stability Colloidal systems are quite energetic The particles in the continuous phase are always moving We call it Brownian Motion. Nanosight

20 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!

21 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

22 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

23 Interfacial forces surface tension Limited interaction at surface Surface (gas) Net Force Liquid Molecules in bulk interact equally in all directions with each other 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).

24 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

25 Thermodynamics the fly in the ointment Energy changes (DG) during preparation of the dispersion is described by the 2 nd 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

26 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)

27 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)

28 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

29 Routes to instability kinetic mechanisms Lyophobic colloidal systems are not stable 2 nd 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..

30 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 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. Sedimentation (caking)

31 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 Flocculation Coalescence 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.

32 Stokes law - predicting phase separation For a spherical particle (dilute solution): Rate = x = 2r 2 (r m - r p ) g t 9h m h m = viscosity of the continuous phase r m = density of continuous phase r 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

33 Stokes Law Decrease particle size Use polymers surfactants waxes clays Match densities (Dr ~ 0) Structure the continuous phase (increase viscosity) to slow movement of the particles

34 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)

35 Ion adsorption (electrostatic repulsions) Ionic surfactants adsorb at the interface and affect the resultant surface charge Oil +ve Oil -ve Cationic surfactant Anionic surfactant

36 Electrostatic interactions the electrical double layer Electric Potential (Y) Surface potential -ve Cation Stern layer Zeta potential (z) Boundary of double layer in contact with the solution ( slipping plane ) Zeta potential (z) Stern layer Surface potential Distance (x) Electrical double layer described by Guoy Chapman or Stern models z magnitude affected by ph

37 Potential energy (V T ) +ve DLVO theory electrostatic stabilisation 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

38 Potential energy (V T ) Potential energy (V T ) + + a) No electrolyte b) Electrolyte added E B Distance (x) EB Distance (x) - + Primary minimum Potential energy (V T ) - Distance (x) Secondary minimum (weak flocculation) Primary minimum Energy barrier (E B ) decreases as the electrical double layer is compressed and is eventually neutralised - c) High electrolyte concentration

39 Making use of electrostatic interactions House of cards structure Clay particle Negative charged surface Positive - + charged surface Shearing force Dispersed phase trapped within the structure Particles slide over each other (electrostatic repulsions) low viscosity

40 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 r a i s e p H O O O O H O H O l o w e r p H O O h ph

41 Steric stabilisation - oil in water (O/W) emulsion Polymer chains act as barrier to coalescence. Oil Oil Oil droplets stabilised by anchored polymer chains.

42 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

43 Steric stabilisation conformation effects Loop Water phase Tail Oil phase Train Hydrophobic group

44 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

45 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

46 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

47 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

48 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

49 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 ( BC)

50 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

51 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

52 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)

53 Spreading What happens when an oil drop is placed on a clean liquid surface? Remains as a drop (lens on the surface) g GL g OG Oil g OL Gas Liquid Or spreads as a thin (duplex) film Gas Liquid Oil layer

54 Spreading What happens when a liquid droplet (oil) is placed on a surface? S is -ve q O S is + ve 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 = g GS - (g OG + g OS ) The surface tension of the fluid (g OG ) <<< critical surface tension (CFT (g GS )) for the liquid to spread along the interface (liquid or solid)

55 What happens when a liquid is in contact with a solid surface? Complete wetting Incomplete wetting q Formation of contact angle (q)

56 Contact angle Contact angle (q) decreases as droplet spreads Substrate can affect contact angle (chemical nature or surface roughness) q Contact angle results from a balance of interfacial surface tensions acting at the point of contact (Young s equation)

57 Wetting the Young Equation Spreading and wetting can be explained by the Young equation (1800 s). g OL Liquid (or air) Oil g SL q g OS At equilibrium: Substrate g OS + g OL COS q - g SL = 0 q = contact angle g = surface tension

58 Relevance of contact angle Pickering emulsions Best effect obtained for hydrophobic particles that form a contact angle around 90 o (partially embedded) - will flocculate in either phase Electrostatic repulsions help to stabilise the emulsion Oil 140 o 90 o 30 o Oil Particle size smaller than oil droplet Particle completely wetted by oil phase (q ~ 0 o )

59 Emulsions Classified into two types: Oil in water (O/W) and water in oil (W/O) m O/W W/O The type formed is determined by the relative proportions of the components Particle size macroemulsions, nanoemulsions and microemulsions

60 Emulsions An input of energy (work) is required to form the emulsion Work = g x DA A = interfacial area g = interfacial tension The lower the interfacial tension, less work is required to form an emulsion with a specific droplet size/interfacial area

61 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

62 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 totally oil soluble 20 - totally water soluble

63 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

64 The HLB of a nonionic surfactant gives an indication of its role HLB value Surfactant function 1-5 Water in oil emulsifier 5-8 Water in oil emulsifier Oil in water stabiliser Wetting agent 8-12 Oil in water emulsifier Wetting agent Oil in water emulsifier Detergent and solubiliser Oil in water emulsifier Detergent and solubiliser

65 Hydrophile-Lipophile Balance (HLB) Nonionic Surfactants HLB Emulsifier O/W Mixture of low & high HLB surfactants Functions for cleaning formulations 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

66 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)

67 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

68 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

69 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

70 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

71 Detergency Sebum Hair Wetting Emulsification Dispersion

72 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

73 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

74 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

75 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

76 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

77 Film elasticity (e) - Gibbs Marangoni effect (rubber band) ε = 2 A A =Area d d g = Surface tension Gravity thins lamellae g A g f f g 1 g g 1 g 1 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)

78 Polymer-surfactant interactions foam stabilisation Polymer forms bridge between neighbouring films Gas Liquid Gas Polymer binds to the surfactant to make the film more rigid Cationic polymer Anionic surfactant

79 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)

80 When foam is a problem!!!! Presence of foam may not be desirable Severe agitation Need to use some kind of control

81 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)

82 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)

83 Foam prevention - antifoams Air Liquid Air Oil Oil spreads on the film and displaces surfactants g O/L << g Surface Oil Film thins and ruptures result of change in interfacial tension between film and oil Foam collapses

84 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

85 Surfactants - Four Types Anionic - - ve charge Nonionic Cationic + No charge +ve charge Amphoteric +/- Acidic + Alkaline -

86 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

87 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

88 Purification carried out at high temperatures & high pressure Oils & Fats Purification Hydrolysis Transesterification Esterification Fatty acid Glycerol Methyl ester Distillation Fractionation Hydrogenation Glycerol Fatty alcohols

89 Crude oil & natural gas Ethylene n-paraffins a-olefins i-olefins Ziegler process OXO process n-alkanols Oxo-alcohols

90 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 H 2 SO 4 /SO 2, SO 3 and P 2 O 5 Organic Ethylene oxide/propylene oxide, polyols and alkanolamines Performance of the surfactant is influenced by the relative strengths of the hydrophilic and hydrophobic groups

91 Integrated oleochemical routes Oils and fats Fatty acid methyl ester Alkanolamides Fatty acids Glycol + glyceryl esters Fatty acid isethionates Alkyl amido betaines Amphoacetates N-acyl derivatives Ethoxylated alkanolamides Hydroxysultaines Fatty alcohol Sulfo-succinate Alkyl polyglycoside Alkyl ether + ester carboxylates Phosphate esters Alcohol ethoxylates Alkyl ether sulphates Alkyl sulphates Alkyl dimethylamines Alkylamine oxides, betaines + quats

92 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

93 Surface tension (g) Critical micelle concentration (CMC) G= Gibbs surface excess G= - 1 dg RT dln C Gradient Area of adsorbed surfactant molecule = 1 (N A G) N A is Avogadro s number CMC - critical micelle concentration Surfactant concentration (ln C)

94 Critical micelle concentration (CMC) CH 3 (CH 2 ) n X 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)

95 Importance of CMC Low Nonionic surfactant Concentration High Ionic surfactant Poor solubility in water Mild (Krafft point) Soluble in water Irritant CMC can be reduced by additives, e.g. polymers, amphoteric surfactants

96 Polymer-surfactant interactions hydrophobic polymer Micelle Micelles form a string of pearls arrangement along the chain Surfactant molecules bind to polymer chain Micelles force polymer chain to open and expand (repulsions between aggregates)

97 Polymer-surfactant interactions - conditioning Precipitation region Micelles form along the chain Polymer (polyquaternium) and anionic surfactant (negative) below critical micelle concentration (cmc) Deposition of coacervate (complex) from solution Increasing surfactant concentration Coacervate structure expands as micelles form and the complex is solubilised

98 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)

99 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

100 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

101 Micelle shape (critical packing parameters) Driving force for different micelle structures head & tail interactions a P = v l c a l c P = critical packing parameter a = cross sectional area of the head group v = volume of hydrocarbon tail l c = all trans length of tail

102 Micelle shape (critical packing parameters) Surfactants molecules have different geometries affects packing at interfaces P > 1 P ~ 1 1/3 < P < 1/2

103 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

104 Importance of surfactant molecular structure Head group size: hydrophilic character Hydrophobe group: lipophilic character

105 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 (V 2 & I 2 ) and hexagonal (H 2 )) Low or planar curvature ideal for multi-lamellar vesicles

106 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)

107 Surfactant phase behaviour rheology Viscosity Viscosity build relies on entanglement of rod/cylindrical micelles Rod micelles Salting out electrolyte (%)

108 Surfactant phase behaviour lyotropic liquid crystals Surfactants form micelles in aqueous/polar media Cubic phase (I 1 ) 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

109 Surfactant lyotropic liquid crystalline phases Small Angle X-ray Scattering (SAXS) Hexagonal phase (H 1 ) Lamellar phase (L a )

110 Importance of lyotropic liquid crystals - emulsion stability Liquid crystal provides a barrier to coalescence Oil droplet Droplets appear as maltese crosses when viewed with a polarized light microscope Oil droplet is coated by layers of lamellar phase (multi-lamellar vesicle structure)

111 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.

112 Nae cannae change the laws of physics Montgomery Scott Thermodynamics rules ok!

113 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

114 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!

115 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)

116 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)

117 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

118 Size matters Va, Va voom!. Thierry Henry

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