The hydrophobicity of fine particles is important for their behavior at interfaces, for example, in stabilizing emulsions. In this study, contact angles were evaluated for silanized fumed silica nanospheres with mean primary diameter of about 12 nm, using heat flow microcalorimetry. Three systems were investigated: water-air-nanospheres, toluene-air-nanospheres, and toluene-water-nanospheres. For the water-air-nanospheres system, n-propanol at various concentrations in water was used to aid in dispersing the nanospheres, and the enthalpy of immersion between water, air, and nanospheres was obtained by extraploting to zero n-propanol concentration. Measurements of enthalpy of immersion for toluene-air-nanospheres system were straightforward, as all the nanospheres samples were dispersible in toluene. The enthalpy of immersion for toluene-water-nanospheres system was calculated from the data for the aforementioned first and the second systems. For water-air-nanosphere systems, contact angles were in the range of 14 to 118 degrees, corresponding to enthalpy of immersion from -0.0905 to 0.0041 J/m(2). For the case of toluene-air-nanospheres systems, the contact angles varied from 72 to 94 degrees with corresponding enthalpy of immersion from -0.0295 to -0.0189 J/m(2). For toluene-water-nanospheres systems, however, contact angles were in the range of 0 to 96 degrees, corresponding to enthalpy of immersion from -0.0717 to -0.0175 J/m(2). Copyright 2000 Academic Press.