Abstract: Current knowledge of clay mineralogy and changing concepts of clay behavior have suggested a re-examination of the experimental data concerning adsorbed water. Data published between 1935 and 1959 have been studied and evaluated. In some instances this requires a reinterpretation of data that may lead to inferences or conclusions not intended by the original author. The intention of the re-evalution of data is to clarify understanding of the nature of adsorbed water and to suggest fruitful avenues for future research. While all literature on the subject has not been included, a concerted effort has been made to include representative data from all viewpoints and experimental methods. Data reviewed come under the following headings: X-ray and electron diffraction, density, dielectric and magnetic, thermodynamic, diffusion and fluid flow, freezing, and rigid water films.
Utilizing present knowledge of crystal and surface chemistry of clay leads to the following conclusions: (a) Positions of the oxygen atoms of the adsorbed water molecules have been established by X-ray diffraction of vermiculite. These positions preclude both the ice structure and the Hendricks-Jefferson net structure even after modification to accommodate the exchangeable ions, (b) Density of water sorbed on Na montmorillonite has a minimum value of about 0.97 g/cm3 at a water content of 0.7 g H2O/g clay (approximately the plastic limit). For water contents less than 0.7 the density rapidly rises to about 1.4, and for water contents greater than 0.7 the density gradually rises until at about 6.5 g H2O/g clay the density of the adsorbed water equals that of normal liquid water. (c) The differential entropy of water adsorbed on kaolinite has a minimum value approximately that of ice; however, (1) this minimum occurs at about 0.7 of a monolayer, and (2) the integral entropy is greater than that for normal liquid water up to at least two molecular layers. The apparently contradictory entropy of sorbed water on montmorillonitic clay has not been resolved but is believed to be at least partially associated with clay swelling. (d) Diffusion and fluid flow phenomena are shown to be extremely sensitive to clay fabric; therefore, it is the writer's opinion that diffusion and fluid flow data on loosely compacted clay are of little help in ascertaining the structure of the adsorbed water phase. (e) Adsorbed water is easily supercooled and an appreciable fraction of the adsorbed water remains unfrozen after ice has once formed.
The two major hypotheses indicate that the nature of the adsorbed water is: (1) a solid-like substance, or (2) a two-dimensional fluid. In the writer's opinion the only data that cannot be adequately explained by both of the hypotheses are the integral entropy data on kaolinite. The integral entropy data favor the two-dimensional fluid hypothesis; however, the paucity of data requires that this be a very tentative conclusion.
The major difficulty encountered in the re-examination of data for this review was that a rather poorly defined clay surface was employed by various investigators. If progress is to be made in unraveling the water-clay complex, it is deemed absolutely essential that experiments be carried out on very carefully defined and controlled clay surfaces. The nature of adsorbed water as interpreted from physico-chemical data on water-clay systems is no better than the purity of the clay surface regardless of the accuracy and precision of the measurements.