Author's personal copy Effects of aging and drying conditions on the structural and textural properties of silica gels Juncal Estella a , Jesu ´ s C. Echeverrı ´a a , Mariano Laguna b , Julia ´n J. Garrido a, * a Departamento de Quı ´mica Aplicada, Universidad Pu ´ blica de Navarra, Campus Arrosadı ´a, 31006 Pamplona, Spain b Instituto de Ciencia de Materiales de Arago ´n, CSIC, Universidad de Zaragoza, Campus de Plaza San Francisco, 50009 Zaragoza, Spain Received 3 October 2006; received in revised form 4 January 2007; accepted 5 January 2007 Available online 17 January 2007 Abstract Sol–gel derived materials are widely used as porous matrices for preparing optical chemical sensors or biosensors. The porosity of the support matrix is a key variable that affects the sensitivity and response time of the sensors. The aim of this research is to study the effect of different aging and drying conditions on the structure and porosity of silica gels to obtain materials of tailored porosity. The gels were aged in ethanol or NH 3(aq) (0.5 M and 2.0 M), and dried under atmospheric (xerogels) or supercritical conditions for ethanol (aerogels). Aging and drying conditions strongly affected the porous texture of silica gels. The surface area and micropore volume was higher in xerogels than in aerogels. Xerogel aged in ethanol was mainly microporous, while xerogels aged in ammonia were mesoporous due to cross-linking reactions in alkaline media. The maximum from the BJH distribution appeared at 4.0 nm for the xerogel aged in 0.5 M NH 3(aq) and at 5.4 nm for the xerogel aged in 2 M NH 3(aq) . Aerogels were macroporous materials, and macropore volume con- stituted over 92% of the total pore volume, independently of the aging media. Esterification reactions inside the reactor promoted cross- linking, which resulted in a higher skeletal density and an increase in the absorbance of Si–O related IR bands. Cross-linking strengthens the gel network and reduces the amount of shrinkage under atmospheric conditions. SEM and TEM micrographs confirmed the textural properties of xero- and aerogels deduced by gas adsorption, Hg porosimetry and density measurements. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Xerogels; Aerogels; Aging; Supercritical drying; Porous texture 1. Introduction The use of the sol–gel process to produce materials for optical chemical sensors and biosensors is attracting con- siderable research interest. The main advantages of porous silica are its transparency and its chemical and thermal sta- bility [1]. In most applications, the sol–gel derived material provides a porous support matrix in which analyte-sensi- tive species are entrapped and into which analyte molecules may diffuse and interact. The porosity of the matrix plays a key role in determining the sensitivity and response time of such optical sensors [2] because the diffusion coefficient for the analyte through the matrix increases with film porosity [3]. Several parameters such as the chemical precursor, sol- vent composition, pH, and temperature affect hydrolysis and condensation reactions, and therefore the porous tex- ture of silica gels [4]. In general, acid-catalyzed hydrolysis and condensation lead to weakly branched and micropo- rous structures, whereas basic conditions or two step acid–base processes increase cross-linking, leading to decreased microporosity and a broader distribution of lar- ger pores in silica gels [5–7]. Aging and drying also affect the pore structures of silica gels. Vega and Scherer [8] demonstrated that condensation in silica gels continues long after gelation due to the large concentration of labile hydroxyl groups. By creating new bonds bridging separate chains, polymerisation reactions 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.01.007 * Corresponding author. Tel.: +34 948 169601; fax: +34 948 169606. E-mail address: j.garrido@si.unavarra.es (J.J. Garrido). www.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 102 (2007) 274–282