Structure and diffuse-boundary in hydrophobic and sodium dodecyl sulfate-modified silica aerogels

https://doi.org/10.1016/j.micromeso.2015.11.017Get rights and content

Highlights

  • Hydrophobic ambient pressure drying aerogels were prepared from surfactant-modified silica gels.

  • A mixture of hexamethyldisiloxane and trimethylchlorosilane was used as silylating agent.

  • Silytation and ambient pressure drying modified the mass-fractal structure of the wet gels.

  • The silica particles in the aerogels exhibited interfacial diffuse-boundary and internal inhomogeneity.

Abstract

Small-angle X-ray scattering (SAXS) and nitrogen adsorption were used to study ambient pressure drying (APD) silica aerogels prepared from hydrolysis of tetraethoxysilane (TEOS) with additions of sodium dodecyl sulfate (SDS). The surfactant-extracted precursor wet gels have presented mass-fractal structure with typical fractal dimension 2.25 in a SAXS characteristic length scale from ∼10 nm to ∼0.35 nm. Hydrophobic APD aerogels with typical specific surface of 800 m2/g and bulk density of 0.20 g/cm3 were obtained after silylation of the precursor wet gels with a mixture of hexamethyldisiloxane (HMDSO) and trimethylchlorosilane (TMCS). The pore volume and the mean pore size of the APD aerogels increased with increasing the SDS quantity. APD aerogels presented most of the mass-fractal characteristics of the precursor wet gels at large length scales. The radius of gyration of the clusters of the APD aerogels (typically 17 nm) increased with increasing the SDS quantity, while the radius of the silica primary particles (typically 2.0 nm) increased at first with the addition of SDS (with respect to the sample without SDS) and decreased regularly afterward with increasing the SDS quantity. The primary particles presented yet some internal inhomogeneity and a diffuse-boundary interface with thickness of about 0.7 nm, according to a linear-gradient model for the diffuse boundary.

Introduction

Silica aerogels exhibit interesting structural properties often associated with low density and high specific surface [1], which make them be considered largely for scientific and technological applications in several areas of the knowledge as catalysis [2], adsorption [3], separation [4], sensing [5], thermal isolation [6], drug delivery [7], enzyme immobilization [8], and nanotechnology [9].

Drying is the most critical step in the obtaining of aerogels from the sol–gel process [10], [11]. Conventional drying often causes collapse of the silica network due to capillary forces associated with the liquid surface tension. Supercritical extraction of the liquid phase of the wet gels (supercritical drying) often yields aerogels with structure not so far from that of the original wet gels, at least at large length scales. Supercritical drying (SCD) aerogels may be limited for application in some areas because they were hydrophilic (as they have OH end groups) and the structure of the aerogels could collapse even in moderate humid atmosphere with time [12], [13]. Ambient pressure drying (APD), after a proper silylation pre-treatment on the silica surface, is an alternative method to prepare high-porosity hydrophobic aerogels [12], [13], [14], which diversifies the applicability of aerogels in several areas, since the hydrophobic surface of the APD aerogels prevents the structure deterioration in humid environments.

The process of obtaining hydrophobic APD aerogels requires a pre-treatment on the silica surface, named silylation, modifying the characteristic of the surface from hydrophilic to hydrophobic. Hexamethyldisiloxane (HMDSO) and trimethylchlorosilane (TMCS) are typical chemical silylating agents often used for this purpose [13]. Silylation replaces –H from hydrophilic Si–OH groups on the silica surface for stable hydrophobic –SiR3 groups. Silylation also prevents the collapse of the silica network on drying provoked by capillary forces [11], [13]. It would not be possible to dry the wet gel before the silylation if the aim is to obtain a sparse silica network as that of the aerogels, because of the collapsing of the silica network on drying occurring in the production of xerogels.

Surfactants can form micelles and structural organization in a reactant silicate medium so they have been used as structure modifiers for a variety of mesoporous silica [15], [16], [17], [18]. Particularly, sodium dodecyl sulfate (SDS) is an anionic surfactant which has been used in a few cases for this purpose [19], [20], [21], [22], [23], [24]. In a previous work [23], a fixed quantity of SDS was used with varied quantities of an oil phase to produce hydrophobic APD silica aerogels. In a more recent work [24], we have obtained SCD aerogels in an autoclave, starting from a set of precursor wet gels prepared using varied quantities of SDS in the hydrolysis step of the process. It was concluded that the surface of the silica particles develops a surface-fractal characteristic with the supercritical process. In this work, the same set of precursor wet gels prepared with varied SDS quantities was used to produce hydrophobic APD aerogels, using a mixture of HMDSO and TMCS as silylating agent. The surface of the silica particles in the present APD aerogels showed completely different characteristics from that of the previous SCD aerogels, with the developing of a diffuse-boundary in the interface silica-pore, likely due to the attaching of hydrophobic groups on the silica surface. An original modified analytical approach was employed by incorporating the diffuse-boundary effect in the mass-fractal model characterizing the precursor wet gels, which was able to describe the SAXS curves in the whole q-domain (q being the modulus of the scattering vector), yielding complete characterization of the mass-fractal structure and the thickness of the diffuse-boundary of the present APD aerogels. The procedure certainly will be of interest for several researchers dealing with surface science. Interesting structural properties of the hydrophobic APD aerogels were yet inferred by combining small-angle X-ray scattering (SAXS) and nitrogen adsorption.

Section snippets

Material and methods

The APD aerogels were prepared from a set of silica wet gels prepared from acid hydrolysis of tetraethoxysilane (TEOS) in several water solutions of SDS. The water solutions of SDS were prepared with relative concentrations CR (with respect to the SDS critical micelle concentration ∼8.2 × 10−3 M) varying to CR = 0, 1, 25, 50, 75 and 100. The hydrolysis of TEOS was carried out into the SDS solutions (after additions of ethanol and HCl) at 45 °C, so the TEOS/water/ethanol/HCl molar ratio was

SAXS

Fig. 1 shows the curves of the SAXS intensity I(q) for the hydrophobic APD aerogels in direct comparison with those of the original precursor wet gels. A little-modified but basically the same model used in the SAXS structural characterization of the precursor wet gels [24] was used here to study the present APD aerogels.

The structure of the precursor wet gels was described in terms of a mass-fractal system for which the SAXS intensity could be decomposed as [25], [26].I(q)=AP(q)S(q),where A is

Conclusions

Hydrophobic APD silica aerogels, with typical specific surface of 800 m2/g and bulk density of 0.20 g/cm3, were obtained after silylation (with a mixture of HMDSO and TMCS) from a set of wet gels prepared from TEOS hydrolysis with additions of SDS. The surfactant-extracted precursor wet gels presented a mass-fractal structure with typical fractal dimension 2.25 in a SAXS characteristic length scale from ∼10 nm to ∼0.35 nm.

The pore volume and the mean pore size of the APD aerogels increased with

Acknowledgment

This work was supported by the Brazilian Synchrotron Light Laboratory (LNLS), FAPESP, and CNPq, Brazil.

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