The degree of hydrophobicity or water-repellency at water/mineral interfaces can be quantified by water contact angle (WCA). Clabel et al. , in their study on Amazonian Spodosols, found a positive correlation between measured soil WCA values and the ratio of C-H (aliphatic) to C=O (carbonyl) IR absorption band intensity. WCA values decreased down the soil profile where the presence of organo-mineral associations is highly reduced, further suggesting higher hydrophobicity in the presence of organic matter. Increases in hydrophobicity of organomineral associations have been reported to occur after increases in surface loading of hydrophilic DOM (i.e., Leonardite humic acid, LHA) sorbed on goethite (α-FeOOH), with adsorption of LHA occurring via ligand exchange and/or hydrogen bonding . Additionally, the authors observed that with higher LHA loadings the sorption isotherms transitioned from representing monolayer coverage to multilayer adsorption, suggesting organic-organic interactions and conformational changes occur in sorbed LHA. These organic-organic interactions may be driven by van der Waals and hydrophobic interactions among LHA molecules. Increases in WCA have also been observed after exchange of n-alkylammonium (6 ≤ n ≤ 15) cations on montmorillonite and laponite clays that corresponded to decreased surface tension of the formed organo-clays.