THE HEAT OF WETTING OF SOIL COLLOIDS 1 
By M. S. Anderson 
Associate Chemist , Soil Chemical Investigations , Bureau of Soils, United States 
Department of Agriculture 
INTRODUCTION 
The property of liberating heat on being wetted is well known to be charac¬ 
teristic of most colloidal substances and is generally thought to be essentially a, 
function of surface. It was shown in a qualitative way over a century ago 
that certain powders liberate heat when wetted by liquids {12)? Soils and 
clays were included among the powders studied by some of the early investiga¬ 
tors, who noted marked increases in temperature when the dry material was 
wetted by water and other liquids ( 15 ). Mitscherlich ( 6) was the first investiga¬ 
tor to make a thorough study of the heat of wetting of soils. He used soils of 
different texture and varied the initial water content in a series of determina¬ 
tions. He was able to show a close relationship between heat of wetting and 
hygroscopicity, both of which he regarded as a measure of surface. He regarded 
the heat of wetting as a function of the colloidal material but considered the 
results as being of qualitative significance only, since the heat evolved probably 
depended upon the nature as well as upon the quantity of colloid present. 
Bouyoucos ( 2 ), however, has recently suggested that heats of wetting of soils 
should serve as a basis for the quantitative estimation of their colloid content. 
It is important in this connection to ascertain whether the colloidal material in 
different soils varies in heat of wetting. 
Heat of wetting has been assumed by different investigators to be essentially a 
function of surface. The older concepts assumed that the effective surface could 
be determined microscopically ( 9 ). However, it is now recognized that this is 
impossible because of the submicroscopic size of many of the primary particles in 
substances having high heats of wetting. In the case of silica gel, Patrick and 
Grimm {10) have recently suggested that the surface influencing the heat of 
wetting is that of a water film surrounding silica nuclei which are approximately 
5 fifi in diameter. The influence of surface of some kind upon colloidal properties 
is generally recognized. It is important, therefore, to know the heat of wetting 
of colloidal material from different soils, aside from any possibility that the 
determination could be used as a measure of the quantity of colloid in soils. 
EXPERIMENTAL PROCEDURE 
The method used for the determination of heat of wetting was very similar to 
that used by Patrick and Grimm {10) in their study of the heat of wetting of 
silica gel. The calorimeter consisted of a 150-cc. Dewar flask fitted with a fiber 
cap through which was inserted a small stirrer, a thermometer graduated to 
0.025° C., and a short piece of No. 36 therlo wire, with copper leads, to serve as a 
heating coil. The whole apparatus was placed in a large air thermostat regulated 
to 25°. 
The water equivalent of the calorimeter was determined by passing a measured 
electrical current through a heating coil of known resistance for 5 minutes and 
1 Received for publication July 2, 1924—issued Nov., 1924. 
8 Reference is made by number (italic) to “Literature cited,” p. 935. 
Journal of Agricultural Research, 
Washington, D. C. 
( 927 ) 
Vol. XXVIII, No.9 
May 31, 1924 
Key No. H-9 
