48 



LIFE: ITS BEGINNINGS AND NATURE 



Fig. 2-18. Stable and unstable emulsions of water and oil (with a red dye) as seen under the microscope. A is the 



oiUin-water, C the water-in-oil, and B the unstable emulsion. 



two phases of suspensions and emulsions, 

 that is, to differentiate between the dis- 

 persed particles and the dispersing medium. 

 The continuous phase refers to the latter, 

 whereas the discontinuous phase identifies 

 the former. Using milk again as an illustra- 

 tion, the fat globules constitute the discon- 

 tinuous phase while the fluid portion identi- 

 fies the continuous phase. 



The nature of an emulsion can best be 

 understood from a very simple experiment. 

 If olive oil, a non-polar compound, is shaken 

 up with water, a polar compound, an emul- 

 sion forms in which the tiny oil droplets are 

 dispersed throughout the water. If, how- 

 ever, the emulsion is allowed to stand a few 

 minutes, the oil will collect on the top of the 

 water and there will no longer be two inter- 

 mingling phases, merely two homogeneous 

 fluids completely separated from one an- 

 other. Such an emulsion is said to be unsta- 

 ble. Now, if a small amount of soap or sol- 

 uble protein is added to the two and shaken 

 vigorously an emulsion will form, but this 

 time it will remain for an indefinite time; 

 tliis is a stable emulsion. The soap or pro- 

 tein is known as a stabilizer. The reason 

 why the stabilizer produces a stable emul- 

 sion is that it is both a polar and a non-polar 

 compound, and therefore tends to accumu- 

 late at the surfaces between the oil and 

 water, that is, it tries to find a place where 

 the non-polar end of the molecules can rest 

 in the oil (which is also non-polar), while 

 the polar end can lie in the water (which 



is also polar). The stabilizer, when so ar- 

 ranged, forms a thin protective film at the 

 surfaces between the oil and water, pre- 

 venting the oil droplets from coalescing. 

 Thus they remain permanently separated. 



Phase reversal 



Watching an amoeba crawl leads one to 

 believe that its protoplasm does not always 

 have the same viscosity or fluidity, and care- 

 ful experiments with a micro-dissection ap- 

 paratus ( an instrument that makes cellular 

 surgery possible) verifies this fact. The na- 

 ture of the emulsion has some bearing on 

 these constantly changing conditions within 

 the protoplasm. 



Returning again to the oil-in-water emvil- 

 sion experiment, we find that when soap is 

 added the typical oil-in-water emulsion ap- 

 pears ( Fig. 2-18A). If, however, a few drops 

 of a calcium salt are added and the con- 

 tainer shaken vigorously, an emulsion will 

 again be established, but this time tiny wa- 

 ter droplets will be surrounded by oil ( Fig. 

 2-18C). In other words, water becomes the 

 discontinuous or dispersed phase and oil 

 the continuous phase. This makes a beauti- 

 ful experiment, particularly if a fat-soluble 

 red dye is added to the mixture. In the first 

 case, red spheres appear in a clear back- 

 ground of water; in the second, clear watery 

 spheres shine out in a brilliant red back- 

 ground. Just what has gone on to bring 

 about these striking changes? 



Obviously, the stabilizer is responsible 



