Effect of aging time and concentration of aging solution on the porosity characteristics of subcritically dried silica aerogels

Abstract

Subcritical drying technique has emerged as effective and convenient method for the synthesis of silica aerogels compared to the conventional supercritical route. One of the important steps in this method is aging of the wet gel which leads to dissolution and reprecipitation of silica monomers to the gel structure and results in stronger gels with desired pore characteristics. This paper deals with effect of concentration of aging solution and duration of aging on the development of pore characteristics of silica aerogels. The specific surface area, pore size and pore volume increase with an increase in concentration of TEOS in the aging solution. With an 80% tetraethoxysilane in isopropanol at 50 °C, a surface area of 1098 m²/g, pore volume 1.3 cm³ g⁻¹ and an average pore size of 47.7 Å were obtained. Time of aging also has a similar effect on bulk density, surface area and pore volume as the concentration of aging solution. Under the reported experimental conditions, a duration of 48 h for aging has been most favourable, after which there appears a negative effect. The result presented in this paper provides data for designing the mesoporous silica gels through subcritical drying technique.

Introduction

Aerogels are sol–gel derived materials which possess unique properties such as large inner surface area, low density, small index of refraction, low sound velocity, low thermal conductivity and low dielectric constant. Aerogels find applications as thermal insulators, fillers, particle detectors, catalysts, gas sensors, optics and light guides, capacitors, cosmic dust capture, waste water treatment, aerocapacitors, absorbing media for dessication, heat storage device for automobiles, encapsulation media, hydrogen fuel storage material, ideal dielectric for ultra-fast integrated circuits [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Hydrolysis and polycondensation of a multifunctional silicon alkoxide followed by supercritical drying is the well known method to prepare silica aerogels [11]. However the supercritical drying process involves the heating and evacuation of highly flammable solvents such as alcohols, which are hazardous at high temperature and high pressures and are also not very ecofriendly. In addition, the supercritical drying results in aerogels which are fragile having very low strength making it difficult for easy handling [12]. As an alternative route, Smith et al. synthesised mesoporous aerogel like materials at ambient pressures by introducing hydrophobic groups in the gel net work [13]. During drying, the gel initially shrinks to accommodate loss of pore fluid maintaining the liquid–vapour interface at the exterior surface of the gel. At the final stage of drying liquid–vapour menisci recede into the gel interior. The magnitude of the capillary pressure, P_c exerted on the network depends on the surface tension of the liquid γ, the contact angle θ and the pore size r,

$$P_{\mathrm{c}}=2\gamma \cos \theta /r$$

Because the pore size can be very small, the capillary pressure will be enormous and hence a collapse of the gel structure can take place [14]. The increase in stiffness and large pore size bring down the capillary pressure during drying, resulting in low density xerogels.

Aging the precursor gel in silane solution is another approach to obtain high strength [15], [16], [17], [18], [19], [20]. Earlier reports indicate that the mechanical properties of the wet gel are improved by aging in mother liquid at different temperatures [21], [22]. During the aging step, the strength and stiffness of the wet gel increase due to an increased degree of condensation reactions and siloxane cross linking within the gel network, dissolution and reprecipitation of silica and attachment of unreacted oligomers from the gelation process or monomers added from an aging solution [23]. Titulaer et al. studied the fluid composition effects on silica gel aging by surface area analysis [24]. Solvent effect on aging of silica gels was studied by Chou and Lee [25]. They related surface area to the polarity parameter of the aging solvent. Effect of aging and pH on the modulus of aerogels was studied by Hdach et al. [26]. Pore structure evolution in silica gel during aging and drying by varying pore fluid pH was investigated by Davis et al. [27]. The strength and stiffness of the wet gels as a function of aging time, temperature and pH in the TEOS/ethanol solution were also described [17], [18]. Einarsrud and Nilsen in a detailed study on subcritical drying showed that gels synthesised by water glass and colloidal sol can be strengthened by aging in TEOS [15]. Physical properties of water glass based aerogels with respect to aging conditions were reported [20]. Earlier we reported the synthesis of high surface area silica by solvent exchange in alkoxy derived silica aerogel under subcritical conditions [28]. A detailed investigation on aging time and the concentration of aging solution on the development of pore structure of subcritically dried aerogels has not been reported and such an experimental data will be most essential in designing the mesoporous silica aerogels. Therefore, we have prepared a number of mesoporous silica starting from TEOS through a subcritical drying technique and the influence of aging time and concentration of aging solution on the porosity features are reported.