206 SURFACES AND MEMBRANES 



ing outward. If, however, a drop of oil is suspended in water as in (b), 

 the oil molecules orient themselves in the opposite direction. 



The fundamental principles involved in the structure of the interfaces 

 are : The molecules in the surfaces of liquids seem to be arranged in such 

 a way that the least polar groups are oriented towards the vapor phase. 

 At the water-air interface the hydrogen atoms turn towards the vapor 

 phase and the oxygen atoms toward the liquid phase. At the liquid 

 surfaces of organic paraffin derivatives the CH 3 groups turn outward, 

 and the more active groups, such as N0 2 , CN, COOH, CHO, OH, or 

 groups which contain double bonds, turn toward the interior of the 

 liquid. If the organic compounds are soluble in water, their orientation 

 is such as to place the active groups inward. 



The stability of emulsoid particles seems to be brought about by the 

 orientation of molecules at the interface. In order to have the emulsoid 

 particles stable, the molecules which make the transition from the inte- 

 rior of the drop to the dispersion medium must form what may be termed 

 a bounding membrane. 



If the thermal agitation is taken into consideration, it may be assumed 

 that the molecules in a bounding surface, composed of flexible hydro- 

 carbon chains, will orient themselves at any angle. The only restriction 

 on their motion is imposed by the condition that the lower end of each 

 molecule must remain in contact with the underlying water. Langmuir 

 found that palmitic acid could be spread to form an expanded structure 

 13.6 A thick, and that the film could also be compressed to form a struc- 

 ture 22.5 A thick. Thus an expanded film may be pictured in contra- 

 distinction to a compressed film, in which the oriented molecules, though 

 still anchored to the underlying water, are set at any angle and hence 

 loosely packed. 



If for the sake of simplicity such a monomolecular structure is used to 

 represent the superficial wall of a cell, then, to a good approximation, 

 the cell wall may be represented as an expanded organic molecular film 

 with a transition thickness of less than one hundred molecules. Such a 

 structure could possess semi-permeable properties and even change its 

 surface energy by the reorientation of its structure. Under the action 

 of the temperature kinetic agitation it would be possible to find, on the 

 average, areas not covered with molecules. Such a membrane would 

 allow for a shifting lattice structure and the adoption of a modified sieve 

 theory for the basic pattern of a semi-permeable living membrane. 



Some data may now be examined in support of the view that a cell has 

 an outer boundary or cell wall (cell membrane) which fundamentally 

 may have the structure of an expanded molecular film. It has been 

 shown that the walls of cells are made of substances which are colloidal in 



