Comment on ''Physicochemical controls on adsorbed water film thickness in unsaturated geological media'' by Tetsu K. Tokunaga

[1] In the last few years, adsorbed water films on soil particles have been the focus of sustained attention in the scientific literature. A number of articles have attempted to improve our understanding and predictions of the transport of water in unsaturated geological media by taking into account the flow that may be occurring in these films [e.g., Tokunaga, 2009; Lebeau and Konrad, 2010]. In a different arena, the significance of straining by adsorbed water films to the retardation of (bio)colloids in soils has been an object of debate among researchers [Crist et al., 2004; Bradford and Torkzaban, 2008]. In particular, it is still uncertain at this point to what extent water films in unsaturated soils or sediments are thick enough to strain and immobilize engineered nanoparticles, given the latter’s tendency under field conditions to rapidly form aggregates with diameters in the hundreds of nanometers or even microns [e.g., Baveye and Laba, 2008; Chen et al., 2008; French et al., 2009]. [2] In this context, the recent article by Tokunaga [2011] presents undeniable interest in that it proposes a detailed description of a theoretical method to evaluate the thickness of adsorbed water films. If its predictions were somehow confirmed experimentally, Tokunaga’s [2011] model could be very useful to deal with situations where water films might have a significant influence on water and chemicals movement. However, at this juncture, it is largely unclear whether the model proposed by Tokunaga [2011] provides a reasonable depiction of reality. [3] Indeed, one of the key limitations of Tokunaga’s [2011] modeling effort is that there are extremely few data that can be used to assess whether or not the calculations that are presented are in the right ballpark for common geological media. Many articles present data related to systems that are not directly relevant to unsaturated soils, such as wetting films containing surfactants [Hanni-Ciunel et al., 2009], adsorbed to polyelectrolyte-coated surfaces [Ciunel et al., 2005], or on highly hydroxylated silicon [Anderson and Ashurst, 2009]. The experimental data that are relevant to soils, on the other hand, are few and far between, and they are not convergent. The 43-year-old article of Read and Kitchener [1969], which Tokunaga [2011] cites, describes measurements of the thickness of ‘‘equilibrium wetting films’’ of dilute solutions of KCl, LiCl, BaCl2, and LaCl2 on a polished vitreous silica surface. These films are not the result of a traditional adsorption process. Rather, they are produced ingeniously by trapping some solution between a gas bubble and the silica surface. For an internal pressure in the bubbles set by interpolation at 145 Pa, these authors found wetting film thicknesses ranging from 30 to >120 nm. These numbers are large compared to those found in a number of more recent articles, which Tokunaga [2011] does not cite, even though they seem very relevant. In the first of these articles by Beaglehole et al. [1991], adsorption of water on molecularly smooth mica was monitored by using an angle-averaging, refractive-index-matching ellipsometric technique. Near saturation in the vapor phase, the water film on the mica tended to a finite thickness of 2 nm, equivalent to roughly seven water layers. In the article by Asay and Kim [2005], attenuated total reflection (ATR)-infrared spectroscopy was used to determine the molecular configuration of water adsorbed on a hydrophilic silicon oxide surface at room temperature, as a function of the amount of water vapor in the air. At a relative humidity as close to 100% as these authors could get, the adsorbed water film had a thickness near 2.8 nm, equivalent to about 10 water layers. In a more recent article using synchrotron X-ray reflectivity (SXR), Bohr et al. [2010] measured the thickness of the water film that adsorbs on a cleavage surface of calcite (CaCO3) in a sample chamber where relative humidity could be controlled within the range from <4% to 90%. The water films that developed in this case were remarkably constant in thickness at 1.55 nm (60.1 nm) and independent of humidity. The fact that techniques used by Beaglehole et al. [1991], Asay and Kim [2005], and Bohr et al. [2010] are much more sophisticated than those implemented by Read and Kitchener [1969], but especially the fact that more recent measurements were made under conditions similar to what one would expect to find in unsaturated subsurface materials, would appear to lend strong credence to the notion that adsorbed water films on subsurface materials are likely to be very thin. Circumstantial evidence pointing in the same direction is also provided in a very interesting article by Mokady and Bresler [1968]. These authors pointed out that to explain the apparent sharp decrease in cation exchange capacity, observed as a result of a decrease in moisture Laboratory of Soil and Water Engineering, Department of Civil and Environmental Engineering, Rensselaer Polytechnic Institute, Troy, New York, USA.