48 FUNDAMENTALS OF SUBMIC ROSCOPIC MORPHOLOGY 



only to those cases where the adjacent phase itself is not hydrophilic. 

 This will almost always hold good at the liquid/gas phase boundary. 

 Fig. 43a shows the arrangement of alcohol molecules at the surface. 

 As their molar cohesion is less than that of water with its OH-groups 

 (see Table IV, p. 32), the surface tension will decrease. The molecules 



fo ^° o 1)1 '0% 



o o ^^0/ 

 o o 



o o 



o o 



o 

 o 





o 



o 



o o 

 o 



o 



o o 



V 



''0^0 o^o 



o=c 



^OH 



o 





0^0 ogooOo 



o°o 



o 



o o 



d) 



Fig. 43. Molecular surface structure of aqueous solutions. Accumulation at the 

 surface of a) ethanol, h) ethyl ether. Monomolecular films of c) fatty acids, 

 (i) di-basic acids, o water; hydrophilic groups white; lipophilic groups black; 



oxygen encircled. 



of ether or amyl alcohol, in which the lipophilic groups are predom- 

 inant, will have still less affinity for water and will lower the surface 

 tension to a greater extent. This explains the first rule in the theory of 

 Gibbs-Thomson, and also explains why very small amounts are suffi- 

 cient to lower the surface tension appreciably, since the majority of the 

 molecules dissolved accumulate at the surface. 



For a substance to raise the surface tension it must, so to speak, be 

 more hydrophilic than water. This applies, for example, to sugars: 

 because, with their numerous OH-groups, they are able to attract 

 the water strongly. For this reason they do not enter the surface, but 

 remain in the bulk of the phase. Their action on the surface tension 

 is due to the fact that the density at the surface is somewhat increased 

 by the attractive forces acting on the water molecules. Clearly, this 

 will only be possible if the concentration of the sugar is very high; in 

 a 0.25 molar solution of cane sugar (the only substance in Table V 



