Grafts et al. 



142 



Water in Plants 



brane from a concentrated to a more dilute salt solution may be demon- 

 strated. 



Electroosmosis may be defined as the movement of liquid along a 

 .barged solid surface (e.g., pore walls in a membrane) due to a potential 

 difference across a membrane exerted tangentially to the interface (that is, 

 the pore walls). The difference in potential, termed membrane potential 

 or liquid junction potential, is dependent on the presence of electrolytes 

 in the solution ; non-electrolytes are inefifective. Differential ion migration 

 velocities and a heterogeneous pore system contribute to the production of 

 the potential difference. 



+ + + + + + 



- H + - + 



+ - H + 



■^f: 



OoHCL _ 4. _ ^. _ ^ 



+ -t- H^ + "- 



C0//CI 



av/CL 



Co> C/ 



Co> C/ 



CiHCL 



Fig. 41. — Diagrammatic representation of two membranes, 

 (A) permeable to cations only, the other, (B) anion permeable. 

 Each separates two HCl solutions of concentrations Co and Ci 

 where Co > Ci. A diagrams positive anomalous osmosis and B, 

 negative anomalous osmosis. The mechanisms are based on 

 electroosmosis. From Hober (1945). 



The electroosmotic flow is dependent on the production of an electrical 

 double layer at the solid-liquid interface. On immersing the membrane in 

 a solution containing electrolyte the surface assumes a charge (usually 

 negative for natural membranes) which may result from ionization of the 

 membrane or adsorption of ions from the solution. The liquid at the inter- 

 face carries the opposite charge and becomes, at least in part, a mobile 

 layer. The electrokinetic potential across the interface (double layer) is 

 determined by the concentration and kind of electrolyte, the nature of the 

 membrane, and other factors. Figure 42C diagrams the nature of this 

 double layer within the pore of a membrane. 



In order that electroosmosis may proceed, there must be a flow of cur- 

 rent across the membrane ; that is, there must be a closed circuit. Bartell 

 (1923) suggested that the return circuit occurs through the medium of the 

 double layer. However, Sollner (1930) and Sollner and Grollman 

 (1932) pointed out that a heterogeneous system of different sized pores 

 might account for return flow. Figure 41 helps to explain this mechanism. 



Figures 41 A and B represent membranes having pores of unequal size. 

 Each separates two HCl solutions, those on the left being the more con- 

 centrated. In A the smaller pore is permeable to cations only ; the wall is 



