FACTORS INFLUENCING IMBIBITION 



109 



be surmised, although in some systems a part of the contraction in volume 

 possibly may be accounted for in this way. The explanation is undoubtedly 

 to be ascribed to the fact that the adsorbed water molecules are definitely 

 oriented in relation to the adsorbing surfaces, and hence occupy less space 

 than when in the free state. This is equivalent to a compression of the 

 adsorbed water so that its density is greater than that of free water. 



Energy Relations of Imbibition. — The process of imbibition always re- 

 sults in the release of heat. Liberation of heat during imbibition may be 

 easily detected by allowing dry starch or some other material with a high 

 imbibitional capacity to imbibe water while contained in a calorimeter, and 

 noting the change in temperature (Table 16). 



TABLE 16 HEAT OF IMBIBITION OF DRY STARCH (dATA OF RODEWALD, 1 897) 



The adsorption of water molecules which occurs when they are imbibed 

 results in a loss of a large part of their kinetic energ>^ which reappears in 

 the system as heat energ}^ This increase in the temperature of the system 

 corresponds to an increase in the kinetic activity of all except the adsorbed 

 molecules. The essential energ>' change in the process of imbibition is a loss 

 of kinetic energy by the adsorbed molecules, and its transference to the other 

 molecules of the system. Their increased kinetic activity is the cause of the 

 observed rise in temperature. As shown in Table i6, the greatest evolution of 

 heat accompanies the initial stages of imbibition. This is to be expected, as 

 a larger proportion of the first molecules of water imbibed would be firmly 

 adsorbed than of those which enter the imbibant later. 



Factors Influencing Imbibition. — In discussions of the factors influenc- 

 ing imbibition it is often necessary to distinguish bet\veen the influence of a 

 factor upon the rate of imbibition, and upon the total amount of water which 

 will be held in the system when an equilibrium has been attained. The prin- 

 cipal factors which influence the entrance of water into an imbibant are as 

 follows : 



