34 GENERAL BIOCHEMISTRY 



nents in the system. It is customary to call the dispersion medium 

 the solvent (usually the major component on the basis of quantity). 

 The material dispersed is known as the solute. Occasionally when 

 liquids are combined in equal amounts, either might be regarded as 

 solvent or solute, and the two terms do not really differ in meaning. 

 Certain properties of liqviid solutions are of special interest in the 

 present context and will be considered briefly in the following sections. 

 When dissolved in liquids, certain solutes are dispersed as charged 

 particles called ions, and the solutions thus formed conduct electricity. 

 As a result these materials are known as electrolytes. If solutes are 

 dispersed as uncharged molecules, the solutions are non-conductors 

 and the solutes are non-electrolytes. 



Surface Tension 



The forces leading to characteristic properties for the surfaces of 

 liquids may be modified when solutes are dissolved. These added 

 particles possess force fields of their own interacting with those of the 

 solvent molecules. These interactions in the surface layers may 

 markedly change the strength of surface films. Many electrolytes, like 

 inorganic salts, acids, or bases, have little effect on the surface tension 

 of water or perhaps increase it very slightly. 



A number of non-electrolytes have dipole moments making them 

 somewhat polar, and these compounds are more or less water soluble. 

 Moreover they also lower the surface tension of water, often markedly. 

 Common groups of substances in this category include monohydroxy 

 alcohols, ketones, aldehydes, some esters, organic acids, and the like. 



Some electrolytes possess one large ion with the charge at one end, 

 the remainder of the ion being non-polar. Such ions tend to dissolve 

 in water because the charges interact with the dipoles of water, but 

 the large non-polar groups resist contact with the water. As a result, 

 the polar portions tend to disperse in the water, the non-polar portions 

 tend to remain out, and the ion as a whole is concentrated in the 

 surface. In turn this concentration partially supplants the polar 

 water molecules. In general, non-polar substances have low surface 

 tensions so the surface tensions of aqueous solutions coated with the 

 non-polar groups are low. Soaps, bile salts, lecithins, and household 

 detergents function in this way and are useful because they lower the 

 surface tension and assist in spreading aqueous systems over oily sur- 

 faces not ordinarily covered by pure water. Since many biological 

 solutes lower the surface tension of water, biological fluids in general 

 possess surface tensions distinctly below that of water itself. Low sur- 

 face tension increases the wetting of non-polar or greasy surfaces and 



