128 



PROTOPLASM 



Soap 

 Membrane 



The solubility theory of Bancroft is based on the difference in 

 the surface tension of the two sides of the stabilization membrane. 

 If the emulsifier is soap, and we imagine it to be a flat membrane 

 with water on the one (left) side and oil on the other (right) 

 (Fig. 84), then the way it will bend, and therefore whether it 

 will enclose water or oil, is determined by the difference in the 

 tensions of its two surfaces. That side with the higher tension 

 will force the membrane to bend its way; which 

 side this will be is determined by the degree of 

 solubility of the membrane substance in oil and 

 in water. For example, alcohol and water mix 

 fully and freely; consequently, there can be no 

 membrane between them; oil and water do not 

 mix at all, and the membrane separating them is 

 at high tension. Imagine that the particular 

 kind of soap forming the membrane around an 

 emulsion droplet mixes more readily with water 

 than with oil; then the water side of the mem- 

 brane will be at a lesser tension than the oil side. 



Wafer 



y / 



/ 



Oil 

 \ 



\ 



b 

 a c 



Fig. 84.— Dia- 

 gram of a soap 



membrane {h) with This being the case, the soap-oil interface, having 



its two interfacial ,i i • i , • -n u j xu j. u-i- a- 



films (a and c) the ^"^ higher tcnsion. Will bend the stabilization 

 relative tensions of membrane toward the oil in opposition to the 

 tivr^solubiSr Tn weaker pull of the soap-water interface (Fig. 84) ; 

 oil and water, will an oil-in-water emulsion is the result. If we 

 direction" o" \end know that a sodium soap is more soluble in water 

 of the membrane than in oil, then we can predict that it will give 

 type *oremuision^ ^^ oil-in-water emulsion, which it does; and if a 

 calcium soap is more soluble in oil than in water, 

 then it should give a water-in-oil emulsion, as it does. This ex- 

 planation tells also why a calcium salt added to an oil-in-water 

 emulsion stabilized with a sodium soap will reverse the emulsion 

 to the water-in-oil type. 



The molecular-wedge hypothesis of Harkins and Hildebrand 

 had its origin in the work of the physical chemist Irving 

 Langmuir. In a most ingenious manner Langmuir was able to 

 make oil molecules stand up, lie down, or lean at an angle when 

 on water and to know which position they were in. He used a 

 very shallow, rectangular vessel containing water on which was 

 spread a thin (monomolecular) film of oil. A bar, just touching 

 the water surface lightly, was freely suspended above so that 



