Journal of Colloid and Interface Science 317 (2008) 206–213 www.elsevier.com/locate/jcis Characterization of the hydrophobicity of mesoporous silicas and clays with silica pillars by water adsorption and DRIFT João Pires a,∗ , Moisés Pinto a , Juncal Estella b , Jesús C. Echeverría b a Departamento de Química e Bioquímica da Faculdade de Ciências de Lisboa, Centro de Química e Bioquímica, Edifício C8, Campo Grande, Lisboa, Portugal b Departamento de Química Aplicada, Universidad Pública de Navarra, Campus Arrosadía, 31006 Pamplona, Spain Received 21 June 2007; accepted 14 September 2007 Available online 18 September 2007 Abstract The hydrophobic–hydrophilic properties of a solid are related to the material chemistry and, often, these properties are relevant to the applica- tions of a particular material. Contrarily to what happens with other properties, such as specific surface areas or pore volumes, the methodologies to ascertain on the hydrophilicity of a porous material are not well defined. In this work, we discuss and relate the information on the hydrophobic- ity degree obtained from water adsorption isotherms and from diffuse reflectance infrared Fourier transform (DRIFT), in a set of porous materials. The studied materials were mainly mesoporous solids, namely of MCM-41 and SBA-15 types, two xerogels and also different porous clays het- erostructures. Both techniques were informative on the hydrophobic–hydrophilic properties of the studied samples, but the correlation between the information obtained by each technique was not straightforward. Water adsorption isotherms are much more sensitive to the differences of the studied materials than the DRIFT spectra. For silica-based mesoporous materials with similar surface chemistry, the water adsorption process and hence, the hydrophobic–hydrophilic properties, is mainly dependent on the pore diameters. However, water adsorption is much more sensitive to changes in the nature of the adsorbent surface than to changes in the pore diameter. © 2007 Elsevier Inc. All rights reserved. Keywords: Water adsorption; DRIFT; Mesoporous materials; Silicas; Hydrophilicity; Hydrophobicity 1. Introduction Mesostructured silica and silica-like materials, either un- modified or surface modified, have a wide range of applications in adsorption and catalysis [1–3]. Textural properties such as specific surface area, or pore size distributions and pore vol- umes are the most relevant properties of these materials in re- lation with their potential uses. However, for some applications as, for instance, in the abatement of volatile organic compounds (VOCs) [4], in the controlled drug release [5], or even for the stability of some silica structures [6,7], the hydrophobic nature of the material is also a relevant property. There are in the literature various proposal to access the hy- drophobicity degree of porous materials. Some of these propos- als are based on the water loss at different temperatures using * Corresponding author. Fax: +351 217 5000 088. E-mail address: jpsilva@fc.ul.pt (J. Pires). thermogravimetry [8], and the use of data from thermogravime- try and nitrogen adsorption [9] or heats of immersion [10] were also suggested. In the case of zeolites, the use of competitive ad- sorption of water and hydrocarbons was also considered [11]. The degree of hydrophobicity of the surface reflects the ma- terial chemistry and, by its very nature, can be related with the interaction with water molecules and, therefore, it is expected that water adsorption isotherms can be highly informative in this context. A considerable number of studies exist in the lit- erature related to the adsorption of water in mesoporous silica or related materials [7,12–15]. However, these studies normally concern more about the characterization of a given surface, or the structural stability toward water, and less the hydrophobicity degree of a given sequence of materials. One of the few studies that proposed the assessment of the hydrophobic–hydrophilic properties of a series porous solids [16], with different mate- rial chemistry, from the water adsorption isotherms used the energetic parameter of the Dubinin–Asthakov equation [17]. More recently, the use of the diffuse reflectance infrared Fourier 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.035