EFFECTS OF MOLECULAR DISSYMMETRY 283 



dynes per cm. for hexadecane. This latter value does not differ greatly from 

 that calculated from Adam's data on the myristic acid films {I'o= — 13)- 

 The difference is not greater than should be expected because of the un- 

 certain part played by thermal agitation in the spreading of surfaces. 



According to Equation (12) with negative values of Fq, the expanded 

 film should not expand indefinitely even if the compressive force F is made 

 negligibly small. Placing F = o we see that the maximum area covered per 

 molecule will be 



kT 

 Om^ oo--^. (14) 



For the case we have considered where ao = 18 and Fo = —13, we 

 obtain Om = 18 + 32.3 = 50.3A- as the maximum area for an "expanded 

 film" of myristic acid on water. This equation shows that the maximum 

 area Om depends principally upon Fo. By Equation (13) we see that Fq 

 is the difference between two relatively large quantities which are nearly 

 equal. A very small percentage change in y^ causes a relatively large change 

 in Fo and therefore in Om- As a matter of fact Adam finds that Qm varies 

 only moderately for different substances. As the length of the chain in- 

 creases he finds a slight decrease in am and considers this a strong argument 

 against the view that the molecules tend to lie flat if they have sufficient 

 room to do so. We have seen, however, by our comparison of octane with 

 tetradecane that y^ and therefore — Fq increases with the length of the 

 chain, and therefore by Equation (14) Qm should actually increase as the 

 chain is made shorter. The changes in Qm observed by Adam may be ac- 

 counted for by very small percentage changes in y- 



Another factor which may need to be taken into account is that the 

 value of yjft for the CH3 groups at the ends of hydrocarbon chains 

 (particularly noticeable in iso-compounds) is about 10 per cent lower than 

 for the CH2 groups constituting the central portions of the chains. Thus as 

 a film becomes more expanded the value of y« increases, tending to make 

 the equilibrium of such a film more stable. Of course an expanded film can 

 be dealt with by equations of the type we are now considering only as long 

 as the upper surface fomis a continuous hydrocarbon surface. The area 

 of such a film is thus limited to that given by the molecules when they lie 

 flat in the surface. 



This theory of "expanded films" requires that the molecules remain in 

 contact while Adam concluded that in such films the "molecules become 

 separated and move about independently on the surface" and that "there 

 are spaces not covered by molecules." Adam considered that the molecules 

 of the film behave as a two-dimensional gas while we conclude that it is 

 only the heads of the molecules that behave in this way. 



It is hoped that the illustrations given in this paper have helped to prove 



