Surface properties of thermally stable phosphonium-modified montmorillonite were investigated at both room temperature and 220 degrees C. These properties were compared with those of pristine and ammonium-modified montmorillonite. Surface properties at room temperature were calculated from contact angles measured using sessile drops. Several liquids with known polar and dispersion components of surface tension were used. Surface energy of nanofillers at 220 degrees C was calculated from contact angles, using sessile drops of polymer melts. Two commercial polystyrene (PS) resins, with different melt flow characteristics, and high-density polyethylene (HDPE) were used. Isothermal TGA experiments were used to determine the thermal stability of the resins and nanofillers. The dispersion behavior and mechanical properties of the nanocomposites are correlated with the values of the Hamaker constant and thermodynamic work of adhesion for these polymer-nanoclay systems. (C) Koninklijke Brill NV, Leiden, 2009