Sorption of N₂ and EGME Vapors on Some Soils, Clays, and Mineral Oxides and Determination of Sample Surface Areas by Use of Sorption Data

Vapor sorption isotherms of ethylene glycol monoethyl ether (EGME) at room temperature and isotherms of N₂ gas at liquid nitrogen temperature were determined for various soils and minerals. The N₂ monolayer capacities [Q_m (N₂)] were calculated from the BET equation and used to determine the surface areas. To examine whether EGME is an appropriate adsorbate for determination of surface areas, the apparent EGME monolayer capacities [Q_m (EGME)_ap] were also obtained by use of the BET equation. For sand, aluminum oxide, kaolinite, hematite, and synthetic hydrous iron oxide, which are relatively free of organic impurity and expanding/solvating minerals, the Q_m (EGME)_ap values are in good conformity with the corresponding Q_m (N₂) values and would give surface areas consistent with BET (N₂) values. For other samples (Woodburn soil, a natural hydrous iron oxide, illite, and montmorillonite), the Q_m (EGME)_ap values overestimate the Q_m (N₂) values from a moderate to a large extent, depending on the sample. A high-organic-content peat shows a very small BET (N₂) surface area; the EGME/peat isotherm is linear and does not yield a calculation of the surface area. Large discrepancies between results of the two methods for some samples are attributed to the high solubility of polar EGME in soil organic matter and/or to the cation solvation of EGME with solvating clays. The agreement for other samples is illustrative of the consistency of the BET method when different adsorbates are used, so long as they do not exhibit bulk penetration and/or cation solvation.