Bernd 



Characteristics of Surface Films 



One can conclude that certain types of hydrocarbons and proteins should be 

 suitable for forming effective surface films about gas nuclei, were it not for two 

 important uncertainties: Can a hydrocarbon dissolve and form an adequate film 

 from solution? Can a protein film prevent dissolving? These gaps in the pic- 

 ture were subsequently investigated experimentally. 



It seems reasonable that a surface film about a nucleus can oppose dis- 

 solving in various ways, i.e., by: 



1. Reducing surface tension. Surface tension is reduced as soon as cer- 

 tain materials reach the nuclei surface because of the lower inherent "surface 

 energy" of the material. Water has a high surface tension of 73 dynes/cm. An 

 oil, hydrocarbon, or protein film — when completely formed — reduces surface 

 tension to about 20 to 30 dynes/cm. 



2. Acting as a harrier to gas diffusion. Certain hydrocarbon, monomolec- 

 ular films are known to be relatively impermeable and act as a barrier to dif- 

 fusion. Thus, cetyl alcohol has been used with success to retard evaporation 

 from water storage reservoirs. 



3. Structurally opposing surface tension with a solidlike film. Certain 

 surface films when compressed acquire characteristics similar to solids. The 

 film possesses considerable strength in compression, is elastic, and progres- 

 sively opposes surface tension as the surface is contracted. Thus Fox and 

 Herzfeld put forth the premise that fatty acid films could form a rigid shell 

 about a nucleus, and that the life of the nucleus would end when the film was 

 crushed in compression (9). 



In addition to these primary actions, certain secondary characteristics 

 should also be important. They are: 



1. Solubility. The material must dissolve in water in at least trace quan- 

 tities so that it is available to form the surface film. Materials normally con- 

 sidered relatively insoluble can form surface films. For instance, egg albumin 

 (a protein) forms excellent surface films when coming out of solution; its solu- 

 bility is somewhat less than 1 part in 1000. For a given type of chain molecule, 

 solubility decreases with increasing molecular weight. One therefore expects 

 to find a maximum molecular weight beyond which a material cannot dissolve in 

 adequate quantities to supply a surface film. 



2. Being Surfactant. The material must be "surfactant." That is, it must 

 be capable of migrating to the surface of the water and forming a film. By this 

 process, an extremely low concentration of material in solution is brought to a 

 high concentration at the surface; i.e., a little has a large effect. For a chain 

 hydrocarbon, this sets the requirement that the molecules comprising the film 

 be at least about 12 atoms long, and that each end of the molecule be different. 

 One end must be highly attractive to water, i.e., polar, such as an OH or car- 

 boxyl group (COOH). The other end must be less attractive, such as a hydro- 

 carbon group. Being surfactant thus sets the general type of molecule and 



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