Adsorption of water vapor and hydrophobicity of ordered mesoporous silica, FSM-16

Surface hydrophobicity of FSM-16 was evaluated by repeated adsorption-desorption isotherm measurements for water on the same sample. The influence of the pore size of FSM-16 on an adsorption hysteresis was also discussed. Although the calcined FSM-16 sample showed hydrophobic features, the hydrophobicity was decreased by contact with water vapor even at 25 degrees C. The contact angle between liquid water and FSM-16 surface, which was calculated from the isotherm by using the corrected Kelvin equation, decreased from 42 to 19 degrees during the first, second and third measurements of the isotherm. An irreversible adsorption of water on the FSM-16 surface would change the hydrophobic property; this is suggested by infrared spectroscopy and benzene adsorption isotherms of FSM-16 before and after contact with water vapor. Large hysteresis observed for the FSM-16 sample with pore diameter of 2.7 nm was decreased with decreasing the pore size and disappeared at 1.4 nm. Although similar behavior of disappearing hysteresis is also observed for the other adsorbates such as nitrogen, argon and oxygen, the pore size at which the hysteresis disappears for water is smaller than that for the other adsorbates. (C) 1998 Elsevier Science B.V. All rights reserved.