Reduction of structural Fe in Na-exchanged dioctahedral smectites decreases swellability in water, but because clay interlayers also collapse in the process the concomitant effect on surface hydration energy is uncertain. This study examined the hydration behavior of oxidized and reduced dioctahedral smectite clays exchanged with polar (Na) and weakly-polar (organic) cations to determine the nature of the surface before and after Fe reduction, and to determine if clay surfaces are hydrophilic or hydrophobic. The H2O content in various dioctahedral smectites decreased if Na was replaced by tetramethylammonium (TMA), trimethylphenylammonium (TMPA), or hexadecyltrimethylammonium (HDTMA). Among the organo-clays, H2O adsorption decreased with increasing complexity of the cation. For oxidized smectites, those exchanged with TMPA retained less H2O than those exchanged with Na at all pressures. The extent of this difference depended on the clay and decreased with increasing applied pressure. Reduction of Fe(III) to Fe(II) in the octahedral sheets decreased the swelling of Na-saturated smectites, apparently causing some previously swelling interlayers to collapse. If the Na interlayer cation was exchanged to alkylammonium after reduction, but prior to swelling-pressure measurements, the swelling increased or remained near constant, suggesting that the organo-cation disrupted the collapse process of the interlayers associated with the reduced smectite layers. Reduced TMPA-saturated smectite surfaces are more strongly hydrated if the octahedral sheet is reduced than if oxidized. Thus, reduction of structural Fe increases the hydration energy of smectite basal surfaces, but swellability could decrease or increase depending on the extent of interlayer collapse occurring with different exchangeable cations.

The reactivity of colloidal particles is regulated by their surface properties. These properties affect the wettability, flocculation-dispersion characteristics, ion exchange, sorption capacities and transport of inorganic colloids. Most studies have focused on hydrophilic, charged-particle surfaces, often ignoring the alterations in surface properties produced by the adsorption of natural organic matter, surfactants and other compounds. Adsorption of these substances can potentially render a surface substantially more hydrophobic. Nevertheless, comparatively little is known about changes in surface properties and reactivity of minerals upon sorption of hydrophobic organic compounds. In this study, the properties of four minerals (kaolinite, pyrophyllite, montmorillonite and Min-U-Sil®) and two inorganic materials (X-ray amorphous Al hydroxide and X-ray amorphous Si oxide) were compared before and after treatment with the common silylating agent, trimethylchlorosilane (TMCS). The samples were characterized by measurements of total carbon, cation exchange capacity (CEC), particle size, specific surface area (SSA), electrophoretic mobility, contact angle, particle aggregation, and by X-ray diffraction and diffuse reflectance infrared spectroscopy. For the layer silicates, surface coverage was limited to ∼2% trimethyl silane (TMSi). TMSi covered 7.5% of the Min-U-Sil® surface and 33% of the X-ray amorphous Si oxide. Treatment did not affect the structure of the minerals but reduced the CEC, SSA and electrophoretic mobilities. Water contact angles increased to between 18 and 114° with treatment. While the apolar characteristic of the surfaces decreased minimally with treatment, the Lewis acid/base properties were substantially reduced and interfacial free energy shifted from positive to negative values indicating a more hydrophobic surface character. For all the samples except kaolinite, these changes affected the stability of the colloids in suspension depending upon solution pH. Although the grafting of TMSi altered colloidal mineral surface properties and increased their hydrophobicity, these changes were not sufficient to predict colloid aggregation behavior.

Pasteuria penetrans is a bacterium with the potential to control plant-parasitic nematode populations; the mechanism of spore adhesion, however, is poorly understood. Attachment assays were performed in media supplemented with various concentrations of fibronectin and in the presence and absence of KSCN which modulates hydrophobic interactions. A reduction in the strength of the hydrophobic effect prevented spores from binding to the cuticle as did 20 μg/ml fibronectin. It was also shown directly utilizing a newly-developed technique which utilizes 3-hexadecanoyl-7-hydro-coumarin as an indicator of the fibronectin binding to the spore surface that the presence of KSCN prohibited binding. This effect was interpreted to indicate that the reduction of binding was the direct result of the influence of hydrophobic interactions between the fibronectin and the spore surface. Western blot analysis of cuticle extracts of Meloidogyne incognita and Caenorhabditis elegans revealed small amounts of fibronectin to be present. Fibronectin, or a similar receptor, present in the cuticle could be responsible for the adhesion of P. penetrans by hydrophobic interactions.