Single-Point and Multi-Point Water-Sorption Methods for Specific Surface Areas of Clay

A single-point water-vapor-sorption method for measuring specific surface area (SSA) from simplified Brunauer-Emmett-Teller (BET) analysis is evaluated for application to natural clayey soils. The approach is predicated on the constraint that the BET C parameter related to surface hydration energy be a relatively large value. Water-vapor-sorption isotherms are obtained for 20 clayey soils along adsorption and subsequent desorption paths between 3 % relative humidity (R-H) and 95 % R-H. Surface areas calculated using the single-point method at 30 % R-H are compared with values from conventional multi-point BET analysis. The average difference between single-point and multi-point SSA values is within 10 %. Correction factors are proposed based on measured BET C values for the suite of soils, and are shown to reduce differences between single-point and multi-point SSA to within 3 %. A simple and inexpensive method involving equilibration of specimens in closed chambers containing saturated MgCl2 solution is proposed and demonstrated.