Physisorbed water layer formation on fully hydroxylated mesoporous silicas

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• 作者：Kenneth E. Collins ; Vanessa R. de Camargo ; Alessandra B. Dimiras ; Deborah T.C. Menezes ; Priscila A. da Silva and Carol H. Collins
• 关键词：Silica ; Adsorption ; Water ; Monolayer ; Kinetic isotherms
• 刊名：Journal of Colloid and Interface Science
• 出版年：2005
• 出版时间：2005
• 年：2005
• 卷：291
• 期：2
• 页码：353-360
• 全文大小：174 K

文摘

Kinetic adsorption isotherms were obtained by gravimetric determination of water adsorption into fully hydroxylated mesoporous silicas using samples exposed to controlled humidity air at 22±2 °C. Twenty kinetic isotherms at several relative humidities (11, 33, 43, 51, 75, and 85 % ) were obtained with 11 different batches of silica using this simple procedure to obtain quantitative information on the formation of H₂O adsorbates. The H₂O surface concentrations obtained from the plateau data of individual kinetic adsorption isotherms at 43 and 51 % RH, typically precise to about ±1 % , show that a complete monolayer is formed with negligible second-layer adsorption at these relative humidities. This monolayer has a surface concentration of 7.68±0.30 μmol H₂O/mp>2p>, which is lower than the quasi-equilibrium concentration at these relative humidities obtained by the conventional equilibrium-isotherm procedure. Comparison with the Kiselev–Zhuravlev concentration of silanol groups on fully hydroxylated silicas (7.6±0.8 μmol SiOH/mp>2p>) confirms 1:1 H₂O:SiOH stoichiometry of this monolayer. The presence of partial-layer structures at 2.85±0.1 and 5.7±0.1 μmol H₂O/mp>2p> is suggested by isotherms at 11 and 33 % RH, respectively, while a bilayer at l7"">l7&_user=10&_cdi=6857&_rdoc=8&_handle=V-WA-A-W-VU-MsSAYVW-UUA-U-AABZWYBCDW-AABBYZVBDW-CADVYWYCB-VU-U&_acct=C000050221&_version=1&_userid=10&md5=63561b33a35167277877fb578ec8d1ed"" title=""Click to view the MathML source"">p://www.sciencedirect.com/scidirimg/entities/223c.gif"" alt=""not, vert, similar"" border=0>14±1 μmol H₂O/mp>2p> is suggested by kinetic isotherms at 75 and 85 % RH.