Colloid chemistry. Lecture 13: Emulsions

Transcription

1 Colloid chemistry Lecture 13: Emulsions

2 Emulsions food cosmetics pharmaceutics biological systems bituminous carpet (asphalt) etc.

3 Emulsion suitable for intravenous injection. Balm: Water in oil emulsion Emulsions Sodas: Oil in Water emulsion Milk: Oil in Water emulsion Dodecane droplets in a continuous phase of water/glycerol mixture. Mayonnaise: Oil in Water emulsion

4 Emulsions encountered in everyday life! pesticide asphalt skin cream metal cutting oils margarine ice cream Stability of emulsions may be engineered to vary from seconds to years depending on application

5 Introduction Emulsion 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 mm Macroemulsions (Kinetically Stable) mm Microemulsions (Thermodynamically Stable)

6 Emulsifying agents Stable suspensions of liquids constituting the dispersed phase, in an immiscible liquid constituting the continuous phase is brought about using emulsifying agents such as surfactants Surfactants must exhibit the following characteristics to be effective as emulsifiers - good surface activity - should be able to form a condensed interfacial film - diffusion rates to interface comparable to emulsion forming time

7 Common Emulsifying Agents Surfactants Anionic Nonionic Cationic Sodium stearate, Potassium laurate Sodium dodecyl sulfate, Sodium sulfosuccinate Polyglycol, Fatty acid esters, Lecithin Quaternary ammonium salts, Amine hydrochlorides Solids Finely divided solids with amphiphilic properties such as soot, silica and clay, may also act as emulsifying agents (Pickering emulsions: attribute of high stability)

8 Making emulsions surfactant oil droplet in water (unstable) solid particles polymer oil droplet in water (stabilized) oil droplet in water (stabilized)

9 G = γ H A >> 0 emulgeation requires large energy input G = γ H A << 0 drop coalescence proceeds continuously G = γ H A + desorption energy high desorption energy prevents/hinders coalescence

10 Making emulsions O / W W/ O

11 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 CPP or P = v: volume of hydrocarbon core l: hydrocarbon chain length a 0 : effective head group area v l a 0

12 Surfactant Packing Parameter CPP or P = v l a 0 v: volume of hydrocarbon chain= 0.027(n c + n Methyl ) l: hydrocarbon chain length= n c where n c = number of carbon atoms without the methyl group n Methyl = number of methyl groups a o : effective head group area: difficult to calculate.

13 Surfactant Packing Parameter

14 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

15 W/O vs. O/W emulsions Bancroft's rule Emulsion type depends more on the nature of the emulsifying agent than on the relative proportions of oil or water present or the methodology of preparing emulsion. The phase in which an emulsifier is more soluble constitutes the continuous phase In O/W emulsions emulsifying agents are more soluble in water than in oil (High HLB surfactants). In W/O emulsions emulsifying agents are more soluble in oil than in water (Low HLB surfactants).

16 optimum for O/W emulsions HLB oil optimum for W/O emulsions water water oil

17 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 antifoaming agents; inverse micelles W/O emulsifiers wetting agents O/W emulsifiers detergents solubilizers Application of surfactants on the basis of their HLB

18 Pickering emulsions oil θ θ víz water oil oil water water

19 HLB values for typical nonionic surfactants structures tenzid kereskedelmi név HLB

20 Bancroft s Rule: Relation to HLB & CPP of Surfactant Surfactant Surfactant Oil Water Oil Water Surfactant more soluble in water (CPP < 1, HLB > 10) O/W emulsion Surfactant more soluble in oil (CPP > 1, HLB < 10) W/O emulsion

21 Bancroft s Rule: Relation to HLB & CPP of Surfactant Surfactant Surfactant Packing Parameter = 1 Oil Water Oil Water Surfactant more soluble in water (CPP < 1, HLB > 10) O/W emulsion Microemulsion Surfactant more soluble in oil (CPP > 1, HLB < 10) W/O emulsion

22 Tests for emulsion type (W/O or O/W emulsions?) Based on the Bancroft s rule, many emulsion properties are governed by the properties of the continuous phase 1. dye test 2. dilution test 3. electrical conductivity measurements 4. refractive index measurement 5. filter paper test

23 Conductivity of emulsions O / V V / O

24 Emulsions are kinetically stable! Rate of coalescence measure of emulsion stability. It depends on: (a) Physical nature of the interfacial surfactant film For Mechanical stability, surfactant films are characterized by 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)

25 Emulsions are kinetically stable! (b) Electrical or steric barrier Significant 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 emulsions Steric barrier dehydration and change in hydrocarbon chain conformation.

26 Emulsions are kinetically stable! (c) Viscosity of the continuous phase Higher viscosity reduces the diffusion coefficient Stoke-Einstein s Equation This results in reduced frequency of collision and therefore lower coalescence. Viscosity may be increased by adding natural or synthetic thickening agents. Further, η as the no. of droplets (many emulsion are more stable in concentrated form than when diluted.)

27 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.

28 Phase inversion in emulsions Bancroft's rule Emulsion type depends more on the nature of the emulsifying agent than on the relative proportions of oil or water present or the methodology of preparing emulsion. Based on the Bancroft s rule, it is possible to change an emulsion from O/W type to W/O type by inducing changes in surfactant HLB / CPP. In other words... Phase Inversion May be Induced.

29 Phase inversion induced by the change in the HLB / CPP Na-soap Ba-soap water + BaCl 2 O/W W/O oil O / W W/ O

30 Why does phase inversion take place for system with surfactants? Surfactant Surfactant Oil Water Oil Water O/W emulsion W/O emulsion temperature for non ionics, salting out electrolytes for ionics

31 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

32 Inversion of emulsions (phase inversion) O/W W/O 1. The order of addition of the phases W O + emulsifier W/O O W + emulsifier O/W 2. 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

33 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.

34 Creaming of emulsions Droplets 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 emulsion Probability of creaming can be reduced if 4 3 πa 3 ρgh kt a - 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

35 Methods of destabilizing emulsions 1. Physical methods (i) Centrifuging (ii) Filtration media pores preferentially wetted by the continuous phase (iii) Gently shaking or stirring (iv) Low intensity ultrasonic vibrations 2. Heating Heating to ~ 70 0 C will rapidly break most emulsions.

36 Methods of destabilizing emulsions 3. 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.

37 Selection of emulsifiers Correlation between chemical structure of surfactants and their 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 activity 2. Ability to form a condensed interfacial film 3. Appropriate diffusion rate (to interface)

38 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.

39 General guidelines 1. HLB method HLB indicative of emulsification behavior. HLB 3-6 for W/O 8-18 for O/W HLB 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.

40 General guidelines 2. 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 C higher than the storage temperature. For W/O emulsion, PIT of C lower than the storage temperature is desired.

41 General guidelines 3. 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.

42 Breaking emulsions 1 phase separation (creaming/sedimentation) 2 Ostwald ripening 3 aggregation processes (flocculation; coagulation; coalescence) 4 phase inversion

43 Breaking emulsions coalescence breaking primary emulsion flocculation creaming

44 Stabilization of emulsions emulsifiers: mostly surfactants hydration forces: O / W stericforces:w/ O electrostratic forces: ionic surfactants polymers: steric forces (entropy stabilization) solid powders: hydrophobic forces (+ wetting) Breaking emulsions sedimentation centrifugation filtration thermal coagulation electric treatment ultrasonication chemical additives (e.g. salting out)

45 Complex (multiphase) emulsions aqueous phase stirring step 1 oil + lipophilic surfactant W / O emulsion W / O emulsion stirring step 2 hidrophilic surfactant W/ O / W complex emulsion

46 primary emulsifier oil phase inner aqueous phase Complex (multiphase) emulsions secondary szekunder emulsifier emulgeálószer outer aqueous phase W / O / W emulsion W / O / W O / W / O 10 µm 20 µm W / O / W O / W / O

47 Hypothetic phase diagram surfactant water oil

48 unstable metastable stability stable microemulsions miniemulsions macroemulsions

49 Micelles, solubilizates, emulsions thermodynamically stable normal micelle solubilizate thermodynamically unstable microemulsion O/W macroemulsion

50 Emulsions microemulsions - internal structure O/W W/O Bicontinuous structure (µe) - bicontinuous µes do exist; - bicontinuos emulsions do not exist!

51 The interfacial tension (IFT) for microemulsions is ca times less than the IFT of O/W or W/O emulsions!!! IFT [mn/m] O / W µe W/ O 100 % water 100 % oil

52 Appearance and properties microemulsion emulsion

53 Physico-chemical properties property formation type stability optical properties stabilizing agents interfacial tension size microemulsions spontaneous, no energy input requied O/W; W/O; bicontinuous structure thermodynamically stable transparent; translucent surfactants; co-surfactants ( nm emulsions energy input required O/W; W/O; + complex: O/W/O; W/O/W thermodynamically unstable; kinetically stable turbid; milky surfactants; polymers; solid particles (Θ.90) ($1 mj/m µm

54 Winsor-microemulsions phase inversion may be generated by the variations of temperature/salinity nonionic surfactants: temperature increases ionic surfactants: electrolyte (NaCl) concentration increases

55 Winsor-microemulsions pure oil pure water O/W bicontinuous W/O Winsor-I Winsor-III Winsor-II