Potential concerns related to using octadecyltrichlorosilane (OTS) in rendering soils and porous ceramics hydrophobic Mustafa E. Ahmed, Paul J. Van Geel ⁎ Department of Civil and Environmental Engineering, Carleton University, 3432 C.J. Mackenzie Building 1125 Colonel By Drive, Ottawa, Canada K1S 5B6 article info abstract Article history: Received 11 December 2007 Received in revised form 6 July 2009 Accepted 19 July 2009 Available online 3 August 2009 The treatment of hydrophilic porous ceramics to render them hydrophobic and wetting to non- aqueous phase liquids (NAPLs) is frequently needed in multiphase flow experiments to control the flow or to measure the pressure of the NAPL. In addition, research dealing with soil wettability implies a need for hydrophobic or NAPL-wet soils. The traditional procedure, which has been widely used in literature, to render hydrophilic porous ceramics and soils hydrophobic is achieved by placing the hydrophilic solid in a 5% (by volume) octadecyltrichlorosilane (OTS) solution in ethanol followed by rinsing in ethanol. This research assesses the use of this procedure as it was found that this treatment procedure resulted in excess OTS on the surface of treated hydrophobic solids which can dissolve in an organic phase and in turn alter the wettability condition of adjacent hydrophilic soils. A modified procedure, which results in hydrophobic solids free of excess OTS, is presented. © 2009 Elsevier B.V. All rights reserved. Keywords: Octadecyltrichlorosilane OTS Wettability Hydrophobic Silica sand Porous ceramics 1. Introduction The remediation of Brownfield sites impacted by non- aqueous phase liquids (NAPLs) remains a challenge. Many of the multiphase flow simulators and remediation technologies developed have assumed that the soils are water-wet. Yet several studies have suggested that the assumption of water- wet conditions may not be valid for a substantial proportion of field sites (e.g. Powers et al., 1996; Jackson and Dwar- akanath, 1999; Dwarakanath et al., 2002). Only in the past decade have significant research efforts been directed towards evaluating the movement and distribution of NAPL in intermediate-wet and NAPL-wet soils. To evaluate the impacts of wettability on the constitutive relationships that govern multiphase flow, a limited number of laboratory studies have been conducted (e.g. Bradford and Leij, 1995; O'Carroll et al., 2004). In all cases, the materials used to reflect intermediate-wet or NAPL-wet conditions were not natural soils but were originally water-wet soils that were rendered hydrophobic or NAPL-wet through the use of a treatment process, typically using OTS solution in ethanol followed by triplicate rinses in fresh ethanol and then air dried as proposed by Anderson et al. (1991). This treatment process is referred to as the traditional OTS treatment procedure and is the focus of this paper. 2. Background Bradford and Leij (1995) conducted a number of cap- illary pressure–saturation (P C –S) experiments with “frac- tional wettability” soils (i.e., soils that contained a fraction of water-wet soil and a fraction of NAPL-wet soil) and demonstrated that the NAPL-wet fraction shifted the P C –S relationship to negative capillary pressures (i.e., higher water pressure than NAPL pressure, contrary to the ‘ideal’ systems typically employed). Using air, Soltrol 220, and water, Bradford and Leij (1995) constructed primary drainage and wetting P C –S curves for air/water, and oil/ water in two-phase systems and capillary pressure–total liquid saturation curves in three-phase systems for 0%, 25%, 50%, 75%, and 100% OTS-treated fractions. The OTS-treated soils were shaken in a 5% (by volume) OTS solution in ethanol for 5 h after which the treated soil was air dried. The authors reference the work of Anderson et al. (1991) Journal of Contaminant Hydrology 110 (2009) 22–33 ⁎ Corresponding author. Tel.: +1 613 520 2600x1884; fax: +1 613 520 3951. E-mail addresses: mahmed4@connect.carleton.ca (M.E. Ahmed), paul_van_geel@carleton.ca (P.J. Van Geel). 0169-7722/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2009.07.007 Contents lists available at ScienceDirect Journal of Contaminant Hydrology journal homepage: www.elsevier.com/locate/jconhyd