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Colloids and Surfaces A: Physicochem. Eng. Aspects 334 (2009) 112–115

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Colloids and Surfaces A: Physicochemical and Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa

Investigation of the states of water and OH groups on the surface of silica
Liu Peng ∗ , Wang Qisui, Li Xi, Zhang Chaocan
School of Sciences, Wuhan Universityof Technology, Wuhan 430070, China

a r t i c l e

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a b s t r a c t
The states of water and OH groups on the surface of amorphous nanosilica with heat treatment at different temperature for hours were studied by thermogravimetry (TG), differential scanning calorimetry (DSC) and Fourier-transform infrared (FT-IR) spectroscopy. The physically adsorbed water, OH groups on surface andOH groups below surface were removed mainly at 343, 709, and 1210 K, respectively. With the increasing temperature in DSC curves, the physically adsorbed water was removed firstly; secondly OH groups on surface were dehydrated; lastly were OH groups below the surface. In a wide temperature range, the dehydration of the three kinds of water took place at the same time. According to the surface areaand TG results, the H2 O density and OH group density on surface were obtained. For the sample without heat treatment, the physically adsorbed water and OH groups on surface were 9.80 and 8.20 nm−2 , respectively. And the physically adsorbed water and OH groups of the sample with heat treatment at 1273 K were about 0 and 2.50 nm−2 , respectively. © 2008 Elsevier B.V. All rights reserved.

Articlehistory: Received 3 June 2008 Received in revised form 27 September 2008 Accepted 9 October 2008 Available online 30 October 2008 Keywords: Silica Dehydration Adsorbed water Silanol groups Surface area

1. Introduction Silica powders are widely used in industry such as fillers, catalysts, adsorbents, chromatographic, agents, and so on [1–4]. Because SiO2 powders have high free energy, they areeasily adsorbed by water under room temperature which forms hydroxylation. Moreover, silica powders may be made of siloxane group ( Si–O–Si ) and several forms of silanol groups ( Si–OH) [5]. Both the water and silanol groups on surface may affect the reactions used to modify the surface with alkyl silanes. Surface water and OH groups would thus play a role in the selectivity and thermodynamics inthe process of surface modification or self-assembly on silica surface. Therefore, the investigation of water and OH groups would be of interest in understanding modification of silica with affinants. It has been reported that both “chemisorption” and “physical adsorption” water exist on the surface of silica [6–9]. Analysis of chemisorption and physical adsorption requires that a difference be madebetween SiOH groups and physically bound or hydrogenbonded water on the surface. It is difficult, however, to distinguish between adsorbed water and actual surface hydroxyl groups. To determine the surface structure (especially OH groups) of powders, infrared (IR) spectroscopy, Raman spectroscopy, and thermogravimetric analysis (TGA), as well as chemical methods such as titration

and nuclearmagnetic resonance (NMR) are used [10,11]. Fouriertransform infrared (FT-IR) spectroscopy is most commonly applied for monitoring the surface hydroxylation of silica [12]. Furthermore, the thermal analysis coupled with other methods is often used to study dehydroxylation and to estimate the number of OH groups on the surface because it does not requires samples that are hard to prepare [7,13]. Thegoal of this work was to evaluate the properties of water and OH groups associated with the surface of amorphous silica. After the amorphous silica was heated to 873 K to dehydrate, it was dehydrated to different degrees with heat treatment at 323, 473, 673, 873, 1073 and 1273 K. Thermal analysis including thermogravimetry (TG) and differential scanning calorimetry (DSC) combined with FT-IR...
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