Emulsions food cosmetics pharmaceutics biological systems bituminous carpet (asphalt) etc.
Emulsion suitable for intravenous injection. Emulsions Balm: Water in oil emulsion Sodas: Oil in Water emulsion Milk: Oil in Water emulsionDodecane droplets in a Mayonnaise: Oil incontinuous phase of Water emulsionwater/glycerol mixture.
Emulsions encountered in everyday life! pesticide asphalt skin creammetal cutting oils margarine ice cream Stability of emulsions may be engineered to vary from seconds to years depending on application
IntroductionEmulsion – Suspension of liquid droplets (dispersed phase) of certain size within a second immiscible liquid (continuous phase).Classification of emulsions- Based on dispersed phase Oil in Water (O/W): Oil droplets dispersed in water Water in Oil (W/O): Water droplets dispersed in oil- Based on size of liquid droplets 0.2 – 50 mm Macroemulsions (Kinetically Stable) 0.01 – 0.2 mm Microemulsions (Thermodynamically Stable)
Emulsifying agentsStable suspensions of liquids constituting the dispersed phase, in animmiscible liquid constituting the continuous phase is brought aboutusing emulsifying agents such as surfactantsSurfactants must exhibit the following characteristics to beeffective as emulsifiers- good surface activity- should be able to form a condensed interfacial film- diffusion rates to interface comparable to emulsion forming time
Common Emulsifying AgentsSurfactantsAnionic – Sodium stearate, Potassium laurate Sodium dodecyl sulfate, Sodium sulfosuccinateNonionic – Polyglycol, Fatty acid esters, LecithinCationic – Quaternary ammonium salts, Amine hydrochloridesSolidsFinely divided solids with amphiphilic properties such assoot, silica and clay, may also act as emulsifying agents(Pickering emulsions: attribute of high stability)
Making emulsions surfactantoil droplet in water oil droplet in (unstable) water (stabilized) po lym solid er particles oil droplet in water (stabilized)
∆G = γ H ∆A >> 0 emulgeation requires large energy input ∆G = γ H ∆A << 0drop coalescence proceeds continuously∆G = γ H ∆A + desorption energy high desorption energy prevents/hinders coalescence
Surfactant Packing Parameter• Conceptual framework that relates molecular parameters (head group area, chain length and hydrophobic tail volume) and intensive variables (temperature, ionic strength etc.) to surfactant microstructures• Critical Packing Parameter / Packing Parameter v CPP or P = l ⋅ a0 v: volume of hydrocarbon core l: hydrocarbon chain length a0: effective head group area
Surfactant Packing Parameter v CPP or P = l ⋅ a0v: volume of hydrocarbon chain= 0.027(nc + nMethyl)l: hydrocarbon chain length= 0.15 + 0.127ncwhere nc = number of carbon atoms without the methyl groupnMethyl = number of methyl groupsao: effective head group area: difficult to calculate.
Packing Parameter is inversely related to HLB mid point of packing parameter P=1 analogous to HLB 10 at P = 1/ HLB = 10, surfactant has equal affinity for oil and water
W/O vs. O/W emulsionsBancrofts ruleEmulsion type depends more on the nature of the emulsifying agentthan on the relative proportions of oil or water present or themethodology of preparing emulsion.The phase in which an emulsifier is more soluble constitutes thecontinuous phaseIn O/W emulsions – emulsifying agents are more soluble in waterthan in oil (High HLB surfactants).In W/O emulsions – emulsifying agents are more soluble in oil thanin water (Low HLB surfactants).
optimum for O/W emulsions HLB oil optimum for waterW/O emulsions water oil
The type of emulsion (O / W or W / O) is affected by:• the ratio of the oil to water (non-polar to polar) phase;• the chemical properties and the concentration of the emulsification agent; • the temperature; the presence of additives; • for solid particles as the stabilizing agents (Pickering emulsions) the wetting conditions (contact angles of the oil and water phases on the solid) Bancroft’s rule (1912): the dispersion medium of an O+W emulsion is the phase in which the solubility of the emulsifying agent is higher. HLB APPLICATIONS 1-3 antifoaming agents; inverse micelles 3-8 W/O emulsifiers 7-9 wetting agents 10-16 O/W emulsifiers 13-16 detergents 15-18 solubilizers Application of surfactants on the basis of their HLB
Pickering emulsions oilθ θ víz water oil oil water water
HLB values for typical nonionic surfactants structures tenzid kereskedelmi név HLB
Bancroft’s Rule: Relation to HLB & CPP of Surfactant Surfactant SurfactantOil Water Oil WaterSurfactant more soluble in Surfactant more soluble in oilwater (CPP < 1, HLB > 10) (CPP > 1, HLB < 10) O/W emulsion W/O emulsion
Bancroft’s Rule: Relation to HLB & CPP of Surfactant Surfactant Surfactant Packing Parameter = 1Oil Water Oil Water MicroemulsionSurfactant more soluble in Surfactant more soluble inwater (CPP < 1, HLB > 10) oil (CPP > 1, HLB < 10) O/W emulsion W/O emulsion
Tests for emulsion type (W/O or O/W emulsions ?)Based on the Bancroft’s rule, many emulsion properties aregoverned by the properties of the continuous phase1. dye test2. dilution test3. electrical conductivity measurements4. refractive index measurement5. filter paper test
Emulsions are kinetically stable!Rate of coalescence – measure of emulsion stability.It depends on:(a) Physical nature of the interfacial surfactant filmFor Mechanical stability, surfactant films are characterizedby strong lateral intermolecular forces and high elasticity(Analogous to stable foam bubbles) Mixed surfactant system preferred over single surfactant.(Lauryl alcohol + Sodium lauryl sulfate: hydrophobic interactions) NaCl added to increase stability (electrostatic screening)
Emulsions are kinetically stable!(b) Electrical or steric barrierSignificant only in O/W emulsions.In case of non-ionic emulsifying agents, charge may arise due to (i) adsorption of ions from the aqueous phase or (ii) contact charging (phase with higher dielectric constant is charged positively)No correlation between droplet charge and emulsion stability in W/O emulsionsSteric barrier – dehydration and change in hydrocarbon chain conformation.
Emulsions are kinetically stable!(c) Viscosity of the continuous phaseHigher viscosity reduces the diffusion coefficientStoke-Einstein’s EquationThis results in reduced frequency of collision and thereforelower coalescence. Viscosity may be increased by addingnatural or synthetic thickening agents.Further, η ↑ as the no. of droplets↑(many emulsion are more stable in concentrated form than when diluted.)
Emulsions are kinetically stable!(d) Size distribution of droplets Emulsion with a fairly uniform size distribution is more stable than with the same average droplet size but having a wider size distribution(e) Phase volume ratio As volume of dispersed phase ↑ stability of emulsion ↓ (eventually phase inversion can occur)(f) Temperature Temperature ↑, usually emulsion stability ↓ Temp affects – Interfacial tension, D, solubility of surfactant, Brownian motion, viscosity of liquid, phases of interfacial film.
Phase inversion in emulsionsBancrofts ruleEmulsion type depends more on the nature of the emulsifyingagent than on the relative proportions of oil or water presentor the methodology of preparing emulsion.Based on the Bancroft’s rule, it is possible to change anemulsion from O/W type to W/O type by inducing changesin surfactant HLB / CPP.In other words...Phase Inversion May be Induced.
Phase inversion induced by the change in the HLB / CPP Na-soap Ba-soap + BaCl2 water oil O/W W/O O/W W/O
Why does phase inversion take place for system with surfactants? Surfactant SurfactantOil Water Oil Water O/W emulsion W/O emulsion temperature for non ionics, salting out electrolytes for ionics
Bancroft’s rule: manifested in response of surfactant solubility temperature for non ionics, salting out electrolytes for ionics O/W emulsion W/O emulsion Temperature and electrolytes disrupt the water molecules around non-ionic and ionic surfactants respectively, altering surfactant solubility in the process – Also reflected by change in curvature of the interface
Inversion of emulsions (phase inversion) O/W→ W/O1. The order of addition of the phases W →O + emulsifier → W/O O →W + emulsifier → O/W2. Nature of emulsifier Making the emulsifier more oil soluble tends to produce a W/O emulsion and vice versa.3. Phase volume ratio Oil/Water ratio↑ →W/O emulsion and vice versa
Inversion of emulsions (phase inversion)4. Temperature of the system ↑Temperature of O/W (polyoxyethylenated nonionic surfactant) makes the emulsifier more hydrophobic and the emulsion may invert to W/O.5. Addition of electrolytes and other additives. Strong electrolytes to O/W (stabilized by ionic surfactants) may invert to W/O Example. Inversion of O/W emulsion (stabilized by sodium cetyl sulfate and cholesterol) to a W/O type upon addition of polyvalent Ca.
Creaming of emulsionsDroplets larger than 1 mm may settle preferentially to the top or the bottom under gravitational forces.Creaming is an instability but not as serious as coalescence or breaking of emulsionProbability of creaming can be reduced if 4 3 πa ∆ρgH 〈〈 kT 3a - droplet radius, ∆ρ - density difference,g - gravitational constant, H - height of the vessel,Creaming can be prevented by homogenization. Also by reducing ∆ρ, creaming may be prevented. This is achieved by producing a polyphase emulsion
Methods of destabilizing emulsions1. Physical methods (i) Centrifuging (ii) Filtration – media pores preferentially wetted by the continuous phase (iii) Gently shaking or stirring (iv) Low intensity ultrasonic vibrations2. Heating Heating to ~ 700C will rapidly break most emulsions.
Methods of destabilizing emulsions3. Electrical methods • Most widely used on large scale • 20 kV results in coalescence of entrained water droplets (W/O) e.g. in oil field emulsions and jet fuels. (mechanism – deformation of water drops into long streamers) • For O/W, electrophoretic migration of charged groups to one of the electrodes. Ex. Removing traces of lubricating oil emulsified in condensed water.
Selection of emulsifiersCorrelation between chemical structure of surfactants andtheir emulsifying power is complicated because (i) Both phases oil and water are of variable compositions. (ii) Surfactant conc. determines emulsifier power as well as the type of emulsion.Basic requirements:1. Good surface activity2. Ability to form a condensed interfacial film3. Appropriate diffusion rate (to interface)
General guidelines:1. Type of emulsion determined by the phase in which emulsifier is placed.2. Emulsifying agents that are preferentially oil soluble form W/O emulsions and vice versa.3. More polar the oil phase, the more hydrophilic the emulsifier should be. More non-polar the oil phase more lipophilic the emulsifier should be.
General guidelines1. HLB method – HLB indicative of emulsification behavior. HLB 3-6 for W/O 8-18 for O/WHLB no. of a surfactant depend on which phase of the final emulsion it will become.Limitation – does not take into account the effect of temperature.
General guidelines2. PIT method – At phase inversion temperature, the hydrophilic and lipophilic tendencies are balanced. Phase inversion temperature of an emulsion is determined using equal amounts of oil and aqueous phase + 3-5% surfactant. For O/W emulsion, emulsifier should yield PIT of 20-600C higher than the storage temperature. For W/O emulsion, PIT of 10-400C lower than the storage temperature is desired.
General guidelines3. Cohesive energy ratio (CER) method Involves matching HLB’s of oil and emulsifying agents; also molecular volumes, shapes and chemical nature. Limitation – necessary information is available only for a limited no. of compounds.
Winsor-microemulsionsphase inversion may be generated by the variations of temperature/salinity nonionic surfactants: temperature increases ionic surfactants: electrolyte (NaCl) concentration increases
Winsor-microemulsions pure oil pure water O/W bicontinuous W/OWinsor-I Winsor-III Winsor-II