Synthesis of hydrophilic and hydrophobic xerogels with superior properties using sodium silicate

Abstract

Highly porous hydrophilic and hydrophobic silica xerogels were synthesized by surface modification of silica hydrogels at ambient pressure drying. The silica hydrogels were prepared by a sol–gel polymerization of an inexpensive silica precursor (sodium silicate) under atmospheric conditions. In order to minimize shrinkage due to drying, the hydrogel surface was modified using trimethylchlorosilane (TMCS) in the presence of ethanol/n-hexane solution before ambient pressure drying (APD). Properties of the final product were investigated using Field-Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric and Differential Analysis (TG–DTA), and nitrogen physisorption studies. The final product was observed to have an extremely high specific surface area (783 m²/g) and a large cumulative pore volume (2.74 cm³/g). Highly porous hydrophilic xerogels were obtained after heat-treating the modified xerogels. At temperatures above 450 °C the surface alkyl groups (CH₃) were significantly oxidized and, consequently, the properties of the resulting xerogels were altered. Products obtained via the proposed inexpensive approach have superior properties and the method exploits an inexpensive silica source (sodium silicate). Thus it is feasible for large-scale economic industrial production.

Introduction

Aerogels are the most highly porous nanostructured materials. They exhibit large surface area (∼1200 m²/g), high porosity (80–98%), low bulk density (∼0.03 g/cm³), extremely low thermal conductivities (0.005 W/mk), and unique acoustic properties (sound velocities as low as 100 m/s) [1], [2]. Because of these properties, aerogels are utilized as thermal super-insulators in solar energy systems, refrigerators, and thermal flasks [3]. Despite these applications, the high production costs have thus far prevented their commercial use. Meanwhile, applications for porous silica xerogels continuously expand as their production costs decrease and their properties improve. Hydrophobic and hydrophilic silica xerogels with superior physical properties such as high surface area and large pore volume have potential applications in fields such as adsorbents, separations, biomedicine, sensors, drug delivery systems, catalyst carriers, thermal insulation, glazing, paints, and oil spill clean-up [4], [5], [6], [7], [8].

Conventional silica xerogels have relatively high density, low surface area, and small pore volume, restricting their applications. Recent observations suggest that the properties of porous materials improve following modification with silica gels (alcogel or hydrogel) during synthesis before the ambient pressure drying (APD) [9], [10], [11], [12], [13]. Moreover, silylating hydrogels and drying at ambient pressure can give less-dense silica xerogels. During the drying process, non-polar alky groups (which repel each other) replace surface OH groups, resulting in the “spring back-effect”, which preserves the silica gel network and, hence, the porosity [14]. Surface modification of silica hydrogels by alkyl groups has been reported to preserve the porous network even after drying at ambient pressure [15]. Prakash et al. [16] have synthesized silica aerogel films at ambient pressure via solvent exchange and surface modification processes. Solvent exchange is a lengthy and tedious process because it simply depends on diffusion of the solution within the gel. Hence, its take several days to produce silica aerogels at ambient pressure. Schwertfeger et al. [17] developed a new synthesis for sodium silicate-based silica aerogel at ambient pressure. Since then, many researchers have focused on synthesizing sodium silicate-based silica aerogels at ambient pressure. Nevertheless, the solvent exchange process, which is required for silica aerogel synthesis at ambient pressure, makes it a tedious process. Recently, Shi et al. [18] reported a new method, called one-step solvent exchange and surface modification process. This method is based on combining different solvents (trimethylchlorosilane (TMCS), n-hexane, and ethanol) for surface modification. Though Shi et al. used ethanol to reduce the reaction between the silylating agent and water, the ethanol consumes large amount of the silylating agent in the surface modification process. We found that this process may be more suitable for surface modification of high content silica hydrogels, as there is less water present in the pore compared to low content silica hydrogels, leaving less silylating agent consumed. Here we observed that very little silylating reagent is required for surface modification of water silica gels with low pore content than those with higher pore content. The amount of silylating agent consumed for surface modification of silica hydrogels depends upon the amount of pore water, since the silylating reagent can directly react with pore water. Also, modifying silica hydrogels (high pore water content) can take a long time to replace the pore fluids. Therefore, a one-step solvent exchange and surface modification process is more suitable to produce hydrophobic silica xerogel, reducing both the time and the amount of silylating agent required. This study is not intended to compare silica aerogel and xerogel properties. Our intention was to produce high porous silica xerogels (hydrophobic and hydrophilic) with extreme physical properties, such as high surface area and large pore volume, using one-step solvent exchange and surface modification of silica hydrogels and drying at ambient pressure.

In the present study, we synthesized low density, relatively transparent, very high specific surface area, and high pore volume hydrophobic silica xerogels by surface modification of hydrogel followed by APD. The silica precursor utilized in this study is relatively inexpensive (sodium silicate) and the reaction procedures employed are considerably versatile. We employed a simultaneous solvent exchange and surface modification process (one-step solvent exchange and surface modification) to reduce the synthesis duration and the drying shrinkage of silica xerogels (at an ambient pressure) from 7 to ∼2 days. The results for the modified silica xerogel were compared with those for the unmodified silica xerogel. Moreover, hydrophilic silica xerogels resulted from heat-treating modified silica xerogel. The specific surface area, pore volume and pore diameter of TMCS modified hydrophobic silica gel increase slightly with increase in heating temperature from 150 to 500 °C. This paper reports, in detail, the results obtained.