io8 IMBIBITION 



trariwise, a piece of rubber does not imbibe water, but does imbibe appreciable 

 amounts of ether and other organic compounds when in contact with them 

 in either the liquid or the vapor state. Obviously a difference in diffusion 

 pressures between the liquid in an imbibant and in its surrounding medium 

 is not the only requisite for the occurrence of imbibition. Certain specific 

 attractive forces between the molecules of the imbibant and the imbibed 

 liquid must also be present. In default of such specific attractions imbibition 

 fails to occur, even if all other necessary conditions for the process are ful- 

 filled. While perhaps not an invariable principle it appears to be generally 

 true that only strongly polar substances (Chap. X) imbibe strongly polar 

 liquids such as water, while non-polar substances such as rubber imbibe only 

 non-polar or very weakly polar organic liquids. 



Since in living organisms water is the only liquid imbibed, the further 

 discussion will disregard other types of imbibitional phenomena. The physical 

 relationship between the imbibed water and the imbibant is undoubtedly a 

 complex one. The bulk of the imbibed water in any system is probably 

 adsorbed on the molecules or micelles which constitute the structural units 

 of the imbibing material. The possible relations of adsorbed water to the ad- 

 sorbing particles in colloidal systems have already been considered (Chap. V). 

 There is good reason for believing that similar ph3'sical relations exist between 

 the imbibed water and unit particles in an imbibant. Such water may be 

 pictured as either actually in solution in the adsorbing particles, or as adsorbed 

 as a "shell" of from one to many molecular layers in thickness on their sur- 

 face. It is possible that either or both of these conditions obtain in many sys- 

 tems containing imbibed water, but the second of these pictures appears to 

 be more likely. It is also probable that a portion of the imbibed water enters 

 and occupies minute submicroscopic capillaries which ramify between the 

 component molecules or micelles of the imbibant. The result of imbibition 

 is often the production of a system which in its essential properties may be 

 regarded as a gel. The physical status of the water in a system into which 

 it has been imbibed and in an elastic gel is undoubtedly very similar. In fact 

 the imbibition of a dispersion medium by a dry substance is generally regarded 

 as a method of gel formation. 



Volume Changes in Imbibition. — The volume of the imbibant always 

 increases during imbibition. The final volume of the entire system (liquid 

 + imbibant) is always less, however, than the sum of the initial volume of 

 the liquid and the imbibing substance. In other words a contraction in the 

 volume of the system, liquid + imbibant, occurs during the process of imbibi- 

 tion. This volume shrinkage is not principally due to the occupancy of minute 

 spaces within the substance of the imbibing material by the liquid, as might 



