J. of Supercritical Fluids 48 (2009) 85–92 Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu Adsorption and thermal release of highly volatile compounds in silica aerogels Babu S.K. Gorle a , Irina Smirnova b,∗ , Mark A. McHugh c a Chair of Separation Science & Technology, Friedrich – Alexander University of Erlangen – Nuremberg, Egerlandstr. 3, 91058 Erlangen, Germany b Technical University of Hamburg - Harburg, Chair of Separation Science and Technology - V8 - Eißendorferstr. 38, 21073 Hamburg, Germany c Department of Chemical and Life Science Engineering, Virginia Commonwealth University, 601 West Main Street, Richmond, Virginia 23284, USA article info Article history: Received 14 May 2008 Received in revised form 10 September 2008 Accepted 10 September 2008 Keywords: Supercritical CO2 Menthol Pyrazine Adsorption TGA abstract This work reports the adsorption/desorption behaviour of two highly volatile compounds (1-menthol and 2-methoxy pyrazine) in hydrophobic and hydrophilic silica aerogels where CO 2 is used as an effective sol- vent to deliver these compounds to the inner surfaces of the aerogels. The thermal desorption behaviour of both substances depends intimately on the strength of the interactions of the absorbed substance with the type and availability of hydrophobic and hydrophilic surface groups in each aerogel. For example, as much as ∼50wt% of either compound remains absorbed on the hydrophilic aerogel surface at temper- atures of ∼250–300 ◦ C, whereas these same compounds are released from the hydrophobic aerogel at 100–150 ◦ C lower temperatures. The results reported here provide insight into the application of silica gels as delivery/storage devices with potential applications in the food, drug, flavors, and other allied industries. © 2008 Elsevier B.V. All rights reserved. 1. Introduction In the past few decades numerous reports have described the loading of aerogels with a variety of organic and inorganic com- pounds [1–5]. High loadings can occur as a result of the availability of high surface area and large pore volumes of aerogels [6] and aerogel-like materials. The target compound can adsorb on the internal surfaces of the aerogel or crystallize in the aerogel pores [7–9]. Both adsorption and crystallization are controlled not only by process parameters, such as pressure, temperature, bulk concentra- tion of the target compound, etc., but, at a more fundamental level, by the physico-chemical properties of the aerogel itself leading to intermolecular interactions that control the loading. For example, the adsorption of drugs on silica aerogels is a strong function of the nature and the number of functional groups available on the aerogel surface. We have previously studied the loading behaviour of com- pounds in hydrophilic aerogels [8,10] that possess surface-active (or free) –OH groups and bulk (or bound) –OH groups [11]. The observed loading behaviour was contrasted and compared to that observed with hydrophobic aerogels in which the free –OH groups are chemically converted to bulky, hydrophobic ester groups while leaving the majority of bound –OH groups intact. The adsorption on free –OH groups can occur quite readily, whereas the adsorption on bound –OH groups depends intimately on the geometrical config- ∗ Corresponding author. Tel.: +49 40 42878 3040; fax: +49 40 42878 4072. E-mail address: irina.smirnova@tu-harburg.de (I. Smirnova). uration of the adsorbate molecules suggesting that these two types of –OH groups are not energetically equal nor do they posses equal reactivities. A general conclusion from these early studies is that the strong, specific interactions between a target compound and the numerous surface –OH groups in a hydrophilic aerogel leads to significantly higher loading levels than those observed with a hydrophobic aerogel. Another general conclusion derived from silica aerogel adsorp- tion studies with several different organic drug molecules [8,9,14] is that the organic compounds exhibit much lower vapor (sub- limation) pressures when adsorbed onto the aerogel due to strong interactions with hydroxyl groups. Supercritical adsorp- tion behaviour (effects of concentration, temperature, pressure, and loading amounts based on solute-surface interactions) of organic substance/drug molecules on aerogels are reported com- prehensively in our previous work [8,9,14,20]. Thus, aerogels can be considered as effective carriers especially for highly volatile compounds. Another possible method for suppressing the vapor pressure of highly volatile substances, such as flavors, vitamins, colorants etc is the complexation with cyclodextrins [12,13]. How- ever, while it is possible to obtain exceptional stability of an organic compound encapsulated in -cyclodextrine (CD), the additive loadings typically are below ∼15 wt% since only a 1/1 (mol/mol) complex is formed between CD and the guest compound [13]. With an aerogel it is possible to load as much as 80 wt% of an organic compound while maintaining the stability of the compound [14]. Silica aerogels provide a chemically inert matrix, consisting of pharmaceutically-acceptable, amorphous silicon dioxide with 0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.09.010