Grafts et al. 



8 



Water in Plants 



heats of vaporization. They tend to "squeeze out" non-polar or sHghtly 

 polar molecules resulting in strong deviations from Raoult's law and low 

 solvent power. 



This concept differs from that of the previous group in that it postu- 

 lates not definite polymers (dihydrol, trihydrol, etc.) but a liquid mass 

 that tends, by intermolecular attractive forces, to become one large 

 "group" molecule, as Langmuir has termed it. Such a mass resembles in 

 a way the crystal of sodium chloride in which the identity of any single 

 molecule of NaCl has completely disappeared. 



If the forces between molecules of one type are great, these molecules 

 will tend to associate, and solubility of compounds of the other type will be 

 low. This is illustrated by the low solubility of paraffins, benzene, etc., in 

 water. If the attractive forces between polar and non-polar molecules are 

 great, negative deviations from Raoult's law will occur and the molecules 

 will tend to unite to form compounds. 



Fig. 2.— The tetrahedral water molecule of Pennycuick 

 (1928). The oxygen nucleus with its two helium electrons is 

 omitted. 



This concept eliminates the distinction between chemical and physical 

 attractive forces. All intermolecular forces are electric in nature, as con- 

 ceived by Sutherland as early as 1902. 



Molecular Concentration: — In 1929, Longinescu attempted to reconcile the com- 

 plexities of liquid association under the concept of molecular concentration. Asso- 

 ciated liquids differ from normal ones in the degree of accumulation or crowding of 

 the molecules into a given volume. The molecular concentration C^ equals 1000 times 

 the density divided by the molecular weight. Thus the molar concentration of ether is 

 9.6; of ecu, 10.4; of toluene, 9.4; of benzene, 11.2; of water, the largest of all, 55.5. 

 For 350 compounds molar concentration values lie between 55.5 for water and 

 0.09 for tristearin. All organic liquids that are considered associated have values near 

 10. In 16 cases cited by Longinescu the association factors determined by various 



workers are compared with the value —^ and the agreement is very close. 



By this concept there is no molecular complexity ; only the number of simple mole- 

 cules in the unit volume varies; when this number exceeds a certain value, experi- 



mentally determined by the formula 



lOOd 



W 



here d = density, and m = molecular 



