CHROMATOGRAPHY 



151 



(0) (b) 



Fig. 11-4. Two-dimensional paper chromatography. A spot of the mix- 

 ture was placed at M, and solvent was allowed to ascend to point S, 

 separating the components of the mixture (a). In step b, the paper 

 was rotated and another solvent was allowed to ascend to level S', 

 bringing about a further separation of the components. 



between a liquid and a solid or between a gas and a liquid. A certain 

 amount of energy will be associated with this interface, as can be seen 

 from the unequal energy distribution which causes the surface tension 

 of water in contact with air. Some dissolved materials tend to reduce the 

 amount of interfacial energy, in which case, they collect at the interface. 

 Charcoal is useful for decolorizing solutions, because the colored ma- 

 terials are adsorbed on the charcoal, or for purifying air, because certain 

 gases are adsorbed. 



Adsorption certainly is involved in at least some forms of chromatog- 

 raphy. In the separation of chloroplast pigments on the column, the vari- 

 ous pigments are bound (adsorbed) on the surfaces of the sugar par- 

 ticles, but different pigments are held with different degrees of tenacity. 

 Each of the pigments has its own characteristic solubility in the solvent 

 also. When the mixture of pigments is placed on the top of the column, 

 the molecules are bound by the adsorbing material (sugar). A fraction 

 of the molecules escape from this binding, however, and are carried 

 downward with the solvent, only to be bound again in another location. 

 The binding strength and the solubility in the solvent determine how 

 far the average molecule travels before being bound again. This combina- 

 tion of properties would be different for each component of the mixture. 

 When the solvent is modified by adding acetone, these relationships are 

 changed slightly by providing a solvent mixture in which certain pig- 

 ments are more soluble. 



Liquid-liquid Partition: Whenever a material is placed in a mixture 

 of two solvents which are insoluble in each other, the dissolved material 



