814 L. L. Jansen 



tained growth promotion from Tween 20, a polyoxyethylene sorbitan 

 monolaurate. He has also employed surfactants in application pro- 

 cedures to emulsify and stabilize fatty materials. Classic uses of sur- 

 face active agents in growth-regulator and herbicide research have 

 presumably been associated with improved wetting, sticking, or 

 spreading qualities. They have also been used to emulsify or disperse 

 poorly soluble or nonsoluble compounds in aqueous and/or oil 

 systems. 



The majority of surfactants have a characteristic type of surface 

 activity with respect to the energy relationships of the interfaces 

 formed. The definite free energy relationships of these interfaces are 

 measured in two different ways. They are referred to as surface ten- 

 sion for the liquid-gas interface and as interfacial tension for the 

 liquid-liquid and liquid-solid interfaces. In Figure lA are illustrated 

 the generalized ranges of values for these free energies found for solu- 

 tions of most sinfactants. From a value of 72 dynes/cm for pure 

 water, surface tension falls rapidly as surfactant concentration is in- 

 creased to approximately 0.1 per cent. At higher sinfactant concen- 

 trations the surface tension remains more or less constant. Interfacial 

 tension relative to surfactant concentration behaves similarly. The 

 latter is usually measured between an aqueous and a Nujol or min- 

 eral oil phase. Since the effective biological concentrations of sur- 

 factants in general are greater than 0.1 per cent, one must assume that 

 biological effectiveness is better correlated with other properties than 

 surface energies. 



One point which may be significant in explaining biological effec- 

 tiveness is the fact that actual measurements of surface and inter- 

 facial tensions do not always coincide with theoretical values. A great 

 many surfactants show the initial rapid drop in tension but at higher 

 concentrations exhibit a slight increase in tension. The lowest point 

 on the tension curve appears to correlate with the formation of col- 

 loidal micelles and is referred to as the critical micellar concentration. 

 Since the end of the steep drop in tension is usually found in the 

 concentration range from 0.01 to 0.1 per cent, it seems probable that 

 at biologically effective levels surfactants are behaving as strong col- 

 loidal systems. 



Several other properties of surfactants undergo marked changes 

 in regions corresponding to the critical micellar concentration. One 

 of these is the relative conductivity of the solution. In Figure IB the 

 conductivities of an homologous series of alcohol sulfonates are illus- 

 trated. As concentration is increased, very little change in electrical 

 conductivity is found for the C-2 to C-7 homologues. Higher members 

 of ihc series, however, exjiibit a rapid drop in relative conductivity 



