THE PHYSICAL CHEMISTRY OF PROTOPLASM 231 



SURFACE BOUNDARY PHENOMENA 



In the preceding chapter, a brief consideration of the phenom- 

 ena arising at surface boundaries was presented. It was pointed 

 out that when any substance exists in the colloidal, or dispersed, 

 condition, it has relatively enormous surface area and that, con- 

 sequently, enormous surface boundaries between the dispersed 

 phase and the dispersion medium exist in all colloidal mixtures. 

 Since protoplasm is conceived to exist in the form of a colloidal 

 gel, having a foam-like structure, it is apparent that it has these 

 enormous surface boundaries between the different phases of the 

 system, and that the phenomena arising from this condition are of 

 great importance in its biological activities. The following neces- 

 sarily brief discussion will serve to give some indication of the 

 physiological importance of the surface boundaries in such a 

 system. 



It is easy to see that the molecules which are in the surface 

 layers at the interface, where two phases of a colloidal system are in 

 contact, are under the influence of forces quite different from those 

 which are acting upon the molecules in the interior of either phase. 

 It is apparent that the molecules in the surface layer are exposed 

 on the inner side to the attraction and influence of similar mole- 

 cules, while on the opposite, or outer, side they are exposed to the 

 influence of molecules of an entirely different kind. This results 

 in a state of tension, known as " surface tension," with the devel- 

 opment of resultant forces and energy which profoundly affect 

 the chemical reactivity of the molecules which are present in this 

 surface layer. The so-called " surface energy," which results 

 from this surface tension, produces marked increases in the pos- 

 sibility of chemical reaction between the materials which are 

 present at the surface boundaries. In colloidal gels, this effect 

 is so pronounced, in many cases, as to completely overshadow 

 other types of influences upon reaction velocities. Also, the sur- 

 face layer of a liquid is compressed by its surface tension, to such 

 an extent that the solubility of substances in this surface layer is 

 greatly increased over that of the same substances in the interior 

 of the liquid, which results in greatly increased concentration of 

 dissolved substances in the surface layer, and so increases the rate 

 of chemical changes which take place there, as contrasted with the 

 rate of the same reactions going on in the interior of the solution. 



