Colloid chemistry - Chapter 10: Emulsion - Ngo Thanh An

Nội dung text: Colloid chemistry - Chapter 10: Emulsion - Ngo Thanh An

1. 1. Introduction Emulsion – Suspension of liquid droplets (dispersed phase) of certain size within a second immiscible liquid (continuous phase).
2. 1. Introduction
3. 1. Introduction 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
4. 2. Emulsion agent
5. 2. Emulsion agent Surfactants Anionic – Sodium stearate, Potassium laurate, Sodium dodecyl sulfate, Sodium sulfosuccinate Nonionic – Polyglycol, Fatty acid esters, Lecithin Cationic – 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)
6. 2. Emulsion agent
7. 2. Emulsion agent Hydrophil area Lipophil area • Reduction of the surface (= lipophob) (= hydrophob) energy • Generation of steric and Emulsifier is solved in the outer phase electrostatic inhibitions (inhibition of coalescence of the drops)
8. 2. Emulsion agent
9. 2. Emulsion agent v CPP or P = l a0 v: Volume of hydrocarbon chain= 0.027(nc + nMethyl) l: hydrocarbon chain length= 0.15 + 0.127n  c Where nc = number of carbon atoms without the methyl group nMethyl = number of methyl groups ao: Effective head group area: difficult to calculate.
10. 2. Emulsion agent 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).
11. 2. Emulsion agent Surfactant Surfactant Packing Parameter = 1 Oil Wate r Oil Water Microemulsion Surfactant more soluble in Surfactant more soluble water (CPP 10) in oil (CPP > 1, HLB < 10) O/W emulsion W/O emulsion
12. 2. Emulsion agent
13. 2. Emulsion agent
14. 2. Emulsion agent
15. 3. STABILITY OF EMULSION
16. 3. STABILITY OF EMULSION (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.
17. 3. STABILITY OF EMULSION (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.
18. 3. STABILITY OF EMULSION
19. 3. STABILITY OF EMULSION • Creaming, in the laboratory sense, is the migration of the dispersed phase of an emulsion, under the influence of buoyancy. • The particles float upwards or sink, depending on how large they are and how much less dense or more dense they may be than the continuous phase, and also how viscous or how thixotropic the continuous phase might be. • For as long as the particles remain separated, the process is called creaming
20. 3. STABILITY OF EMULSION • the process of migration is called creaming while the particles of the substance remain separated. • In this it differs ideally from flocculation (where particles clump) or emulsion breaking (where particles coalesce). • One important difference between creaming and the other two processes; unlike flocculation and breaking, creaming of an emulsion is largely a simple process to reverse
21. 3. STABILITY OF EMULSION
22. 3. STABILITY OF EMULSION Acetic Acid Surfactant Tie line Benzene Water Oil Water Acetic acid & water are miscible in Surfactant and water are miscible all proportions in all proportions Benzene & water - partly miscible, Oil and water - partly miscible, acetic acid & water - partly miscible surfactant and oil - partly miscible Acetic acid added to a mixture of Surfactant added to a mixture of oil benzene & water, preferentially & water, preferentially partitions partitions into water (slope of tie line) into water (slope of tie line)
23. 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
24. 3. STABILITY OF EMULSION
25. 3. STABILITY OF EMULSION 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.
26. 4. 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 ~ 700C will rapidly break most emulsions.
27. 5. MANUFACTURING OF EMULSION
28. 6. TEST OF EMULSION TYPE • In this test the emulsion is diluted either with oil or water. If the emulsion is o/w type and it is diluted with water, it will remain stable as water is the dispersion medium" but if it is diluted with oil, the emulsion will break as oil and water are not miscible with each other. Oil in water emulsion can easily be diluted with an aqueous solvent whereas water in oil emulsion can be diluted with a oily liquid.
29. 6. TEST OF EMULSION TYPE • In this test an emulsion is mixed with a water soluble dye (amaranth) and observed under the microscope. If the continuous phase appears red, it means that the emulsion is o/w type as water is in the external phase and the dye will dissolve in it to give color. If the scattered globules appear red and continuous phase colorless, then it is w/o type. Similarly if an oil soluble dye (Scarlet red C or Sudan III) is added to an emulsion and the continuous phase appears red, then it is w/o emulsion.