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Emulsions

An emulsion represents a disperse system in which the phases are immiscible or partly immiscible liquids.

In nearly all emulsions, one of the phases is aqueous and the other is an oil. If the oil is disperse phase, the emulsion is termed an oil in water (o/w) one. If the aqueous medium is the disperse phase, the emulsion is termed a water in oil (w/o) one.

If one shakes vigorously a vessel containing two immiscible liquids, both liquids are broken up into the droplets whose size depends upon the viscosity of the liquid, surface, interfacial tensions and the vigor of the shaking. As soon as the mechanical dispersive action ceases, the droplets begin to coalesce in order that the total surface free energy may be reduced. Most often, particularly in the case of two pure liquids, the coalescence process is rapid, and within a very few minutes the system consists only of two liquids layers. In the presence of the small amounts of additional components, termed emulsifiers the rate of coalescence of the droplets may be greatly reduced. Emulsions are intrinsically unstable, thus resembling lyophobic colloids. Three distinct kinds of instability are found to exist, each may be of great importance in industrial products.

Emulsions may " cream" , i. e. separate into layers of aqueous phase with a concentrated layer of oil droplets floating on the top, the rate depending primarily on the viscosity of the aqueous phase, the size of the droplets, and the density difference between aqueous phase and the droplets. They may also flocculate as do other lyophobic colloids. The flocs, being larger than individual drops, have a higher creaming rate.

Finally, the drops may coalesce giving a separated bulk layer of the once emulsified liquid, in which case re-emulsification can be affected only by drastic mechanical action.

No satisfactory quantitative theory of the emulsion stability has yet been developed. It is nevertheless becoming apparent in the case of coalescence that it is the structure of the interfacial film, which is controlling the behaviour of the system. For flocculated o/w emulsions the rate of coalescence is linearly dependent on the concentration of the adsorbed emulsifier in the interfacial film, and appears to extrapolate to zero at complete coverage of the surface of droplets.

It might be thought that dilute emulsions would be ideal system on which to test theories of the flocculation of lyophobic colloids.

Many industrially important emulsions are " stabilized " (given long life) by the use of solids as emulsifiers.

Exercises

I. Answer the following questions:

1. What is an emulsion?

2. What emulsions are widely known?

3. What emulsions phases do you know?

4. What happen if one shakes vigorously a vessel containing two immiscible liquids?

5. When is the coalescence process rapid?

6. In what way can industrially important emulsions be "stabilized "

II. Translate the following derivatives:

1. disperse, dispersed, disperser, dispersing, dispersion, dispersity, dispersive.

2. emulsification, emulsifier, emulsify, emulsifying, emulsion.

3. miscibility, miscible, mix, mixer, mixed, mixing, mixture.

4. polymer, polymeric, polymerism, polymerization, polymerize, polymerized, polymerizing.

III. Translate the following sentences paying attention to the words in bold type:

1. Water dispersible dye is a good example of an emulsion in which the pigment helps to control the emulsion stability.

2. Polyformaldehyde is used for the manufacture of many consumer goods.

3. Latex foam from Government Rubber Styrene (GRS) of the proper type is good in colour, pore structure, and ageing but rather lacking in strength.

4. Even with a high alkali reserve, the storage life of neoprene latex is not as good as many other latices.

5. Neoprene is very desirable for dipped goods where special service conditions are required.

6. Gum arabic is a fairly goods dispersing agent and has one desirable feature - good ultraviolet resistance.

7. The extensive data obtained in emulsion polymerization of metyl methacrylate are in good agreement with the asumption that the rate of the reaction at high conversion is governed only by the diffusion rate of the monomer and the radical end of the polymeric chains.

IV. Translate the following sentences paying attention to the negative pronoun "no":

1. No satisfactory quantitative theory of the emulsion stability has yet been developed.

2. The difference between suspension and emulsion polymerization points to a process of formation of particle in the latter, whereas no such process occurs in suspension reaction.

3. No equipment embodying brass or bronze should be allowed to come into contact with latex or its compounding ingredients at any stages of the manufacture.

4. No synthetic high polymers are chemically pure substances in the strict sense.

V. Translate the following sentences and states the function of the

1. In nearly all emulsions, one of the phases is aqueous and the other is an oil.

2. The colloidal state for a substance is one in which it exhibits colloidal properties.

3. If one shakes vigorously a vessel containing two immiscible liquids, both liquids are broken up into droplets.

4. If the oil is the disperse phase, the emulsion is termed an oil in water one, if the aqueous medium is the disperse phase, the emulsion is termed a water in oil one.

VI. Translate the following sentences into Vietnamese paying attention to the use of verb "to do" :

1. Emulsions may flocculate, as do other lyophobic colloids.

2. When polymerization does take place, average molecular weights in the thousands are not obtained.

3. Reinforcing agents did not generally have the affect on latex rubber that they did on milled rubber.

4. The experiments conducted show that adhesion does increase for a time after curing, but that it comes to a stable value during the first 24 hours.

VII. Translate into Vietnamese paying attention to the rule of Sequence of Tences.

1. Berzelius stated that rubber could be redispersed.

2. It was known that ultraviolet - light had a harmful influence on a rubber surface and caused photochemical effects on antioxidants.

3. Dent assumed that in the alkali halides the positive and negative ions in the surface remained coplanar.

4. Willbourne assumed that the methyl absorptivity was constant regardless of the position of the methyl group in the molecule.

5. It might be thought that dilute emulsions would be ideal systems.

6. It was considered that stabilizers belonged to a class of substances, which had been known as " surface active agents ".

7. In the early 1920's it was reasoned in England that individual particles of latex would be vulcanized without breaking the emulsion.

VIII. Translate into English

1. Nh­ ®• biÕt, ®Æc tÝnh riªng biÖt cña nhò t­¬ng ®iÓn h×nh lµ d¹ng h×nh cÇu cña h¹t keo.

2. Ng­êi ta ®• biÕt r»ng, ®­êng kÝnh cña h¹t nhò t­¬ng phô thuéc vµo ®é nhít vµ søc c¨ng bÒ mÆt còng nh­ vµo c­êng ®é l¾c.

3. Ng­êi ta cho r»ng, khi thªm vµo mét l­îng nhá chÊt nhò ho¸ th× cã thÓ t¨ng tÝnh æn ®Þnh cña nhò t­¬ng.

4. Nh­ ®• biÕt , cho ®Õn nay ch­a cã mét lý thuyÕt ®Þnh l­îng phï hîp nµo vÒ ®é bÒn cña nhò t­¬ng ®­îc ®­a ra.

5. ý nghÜa thùc tiÔn cña nhò t­¬ng trong thùc tÕ còng nh­ trong kü thuËt lµ rÊt lín.

6. Nh÷ng nhò t­¬ng tæng hîp, ®­îc ®iÒu chÕ tõ nh÷ng chÊt láng kh¸c nhau cã mÆt chÊt nhò ho¸ ®Æc biÖt, cã ý nghÜa lín.

IX. Write a short summary of the text.