Glycerol-treated montmorillonite gradually loses one layer of the normal two-layer glycerol complex upon moderate heating at 110°C, forming a monolayer complex with a sharp x-ray diffraction peak at about 14A. In the presence of free glycerol vapor, this complex is quantitatively stable on further heating. A simple gravimetric procedure has been devised to determine the weight of glycerol retained. The procedure involves addition of a dilute glycerol solution to a dried and weighed sample of clay in an aluminum foil dish, and subsequent oven drying over a glycerol surface until constant weight is attained. The portion of the glycerol retained on external surfaces is assessed by measuring the retention of a replicate sample previously collapsed irreversibly by heating at 600°C or higher.

The retention of monolayer glycerol complexes of vermiculite and endellite can be similarly determined. Ethylene glycol forms an analogous series of complexes, and its retention against heating can be measured in exactly the same way. Differential thermal analysis of glycerol- and glycol-treated montmorillonite suggests that both complexes decompose with endothermic reactions at about 300°C.

Glycerol retention values were shown to be independent of sample size and to be almost independent of equilibration temperature between 70°C and 150°C. As expected, they are inversely proportional to particle size. Cation saturation affects the values of both internal and external retention for montmorillonites. It is shown that retention is a function of montmorillonite content in a series of artificial mixtures. Glycerol retention values of a number of clay materials are presented.