COLLOIDS AND ADSORPTION 33 



Water is a polar molecule, the oxygen atom being 

 negatively and the hydrogen atoms positively charged. 

 When a compound containing a polar group, such as a 

 carboxyl group, is introduced into it, the polar group of 

 the water will be attracted to that of the compound and 

 water will associate itself with the compound : — 



oJ 



/ % carboxyl 

 + H 0- 



-0 H + 



I 

 + H water 



If the polar group of the compound is strong compared 

 with the rest of the molecule (the non-polar part) the 

 substance will be soluble in water, as is acetic acid ; but 

 if the non-polar " tail " is long the polar " heads " will 

 be attracted to and held in the water, leaving the tail 

 projecting out from the surface and making a film 

 arranged in an orderly manner on the surface of the 

 water. If a compound has only very weak polar groups, 

 or no polar groups at all, as is the case with the paraffins, 

 it will be insoluble in water, and its molecules will lie 

 higgledy-piggledy on the surface without forming an 

 orderly film . 



The proteins which make up bacterial protoplasm, 

 or which form the colloidal carriers of enzymes, or which 

 occur in serum and take part in the various immunological 

 reactions, possess considerable numbers of strong polar 

 groups, principally the peptide linkages, — CO.NH — , 

 the carboxyl group, — COOH, and the amino groups, 

 — NHg. In virtue of these strong polar groups they 

 have a strong affinity for water, and when they are in 

 solution they are surrounded by films of adsorbed water, 

 which lowers their critical surface potential and so renders 

 them much more stable and less liable to be coagulated 

 than the lyophobic colloids. The polysaccharides are 



