Gradual hydrophobization of silica aerogel for controlled drug release.

Nir Ganonyan,Raz Gvishi,Galit Bar,David Avnir

doi:10.1039/d1ra00671a

Abstract

We report on the successful fine-tuning of silica aerogel hydrophobicity, through a gas-phase surface modification process. Aerogel hydrophobicity is a widely discussed matter, as it contributes to the aerogel's preservation and determines its functionality. Still, a general procedure for tuning the hydrophobicity, without affecting other aerogel properties was missing. In the developed procedure, silica aerogel was modified with trimethylchlorosilane vapor for varying durations, resulting in gradual hydrophobicity, determined by solid-state NMR and contact angle measurements. The generality of this post-synthesis treatment allows its application on a variety of aerogel materials, while having minimum effect on their porosity and transparency. We demonstrate the applicability of the gradual hydrophobization by tuning drug release rates from the silica aerogel. Two chlorhexidine salts – widely employed as antiseptic agents – were used as model drugs, one representing a soluble drug, and the other an insoluble drug; they were entrapped in silica aerogel, following hydrophobization to varying degrees. The drug release patterns showed that depending on the degree, hydrophobization can increase or decrease release kinetics, compared to the unmodified aerogel. This arises from the effect of the hydrophobic degree on pore structure, diffusional rates and wetting of the aerogel carrier. We suggest the use of the gradual hydrophobization process for other drug-aerogel systems, as well as for other aerogel applications, such as transparent insulation panels, contaminate sorbents or catalysis supports.

Highlights

Aerogels were introduced into our lives almost 90 years ago,[1] and still, their extraordinary properties that include high porosity (>95%) and immense surface area (>800 m2 gÀ1) remain unmatchable.[2,3] The aerogels' highly versatile synthesis procedures allow tailoring of the material's properties, such as the bulk density,[4] pore size,[5] and surface chemistry.[6]
Cross polarization magic angle spinning (CPMAS) measurements of 1H–29Si nuclei resulted in spectra with the following bands: trimethyl silyl at 12 ppm (TMS), Si coordinated with two bridging oxygens (BO) and two nonbridging oxygens (NBO) at À93 ppm (Q2), Si coordinated with three BOs and one NBO at À103 ppm (Q3) and Si coordinated with four BOs at À112 ppm (Q4).[38]
The rst reaction is the silylation itself, introducing hydrophobic trimethyl silyl at ppm (TMS) groups on the aerogel surface. This accounts for part of the increase in Q4 content, as the NBO of Q3 reacts with TMCS and turns into a BO

Summary

Introduction

Aerogels were introduced into our lives almost 90 years ago,[1] and still, their extraordinary properties that include high porosity (>95%) and immense surface area (>800 m2 gÀ1) remain unmatchable.[2,3] The aerogels' highly versatile synthesis procedures allow tailoring of the material's properties, such as the bulk density,[4] pore size,[5] and surface chemistry.[6]. The surface chemistry is essential for determining the aerogels' absorptive properties, and for regulating diffusion rates through their

Methods

Results

Conclusion