Bernd 



extensive cavitation, thus implying that an insoluble protein film was being pro- 

 duced that subsequently could not redissolve to continue the film forming action. 

 A hydrocarbon film upon compression behaves similarly, but should ultimately 

 redissolve. 



Opposed to this is that little information is available about the resistance of 

 protein films to gas diffusion. It is in fact occasionally stated in the literature 

 that proteins do not offer a barrier to diffusion. This belief must be overcome 

 if one wishes to justify that protein films play a role in retarding dissolving. 



A barrier to gas diffusion should be selected on the basis that it have few 

 holes in it, and/or be thick. The molecules comprising the film should be 

 tightly packed. If the surface film of a protein could be highly compressed, as 

 may occur when a nucleus dissolves, examination of the molecular structure 

 leads one to conclude that some proteins should have a resistance to diffusion of 

 about the same order of magnitude as a hydrocarbon. A few tests in the litera- 

 ture on compressed protein films (on gases other than air) do show appreciable 

 resistance to diffusion (21), tending to support this supposition. 



Unfortunately, intimate details of molecular structure and composition are 

 not well known for many proteins. Furthermore, the change in protein struc- 

 ture as it becomes insoluble reorients the molecule (12,13). This lack of knowl- 

 edge poses a difficulty in understanding protein action about a nucleus, and 

 selecting suitable proteins for forming a film. 



Structure of Surface Films 



The general form of various linear, chain hydrocarbons capable of film for- 

 mation, and known to offer a barrier to gas diffusion, are given in Fig. 11a. The 

 molecules have no side branches, and so pack well together. The alcohol, fatv 

 acid, and amine shown each have different polar ends, i.e., OH, COOH, CH2NH2. 

 Otherwise the composition and form are similar. The length of the molecule is 

 varied by changing the number of repeating hydrocarbon units. To give an idea 



of the length we are concerned 



with, cetyl alcohol is 16 carbon 



units long. At a surface, these 



molecules align side by side to 



form a film as shown in Fig. lib. 



In studies of the diffusion of water 



POLAR END 



REPEATED 



HYDROCARBON 



UNIT 



HYDROCARBON 

 END 



GAS 



(hc) @ (hc) @' 



® 



®pj(bp 



.hydrocarbon ends 



SURFACE 



-CROSS SECTIONAL 

 AREA = 21 [aJ^ 



-POLAR ENDS 



® 



® 



Fig. 11a - Structure of long-chain, 

 surfactant hydrocarbons 



Fig. lib - Monomolecu- 

 lar film at the surface 

 of water 



98 



