Immediate water contact angle (WCA) measurements indicate wetting transitions occur on the hydrophobic (i.e., diamond) and hydrophilic (i.e., goethite) surfaces as a function of PS surface loading (Figure ). Goethite's surface becomes increasingly hydrophobic with increasing PS loading (WCA = 27° at PS = 0 μM, 70° at PS = 40 μM, 99° at PS = 196 μM). In contrast, low concentrations of PS make the PS-diamond surface less hydrophobic, but higher PS loadings revert this trend (WCA = 105° at PS = 0 μM, 78° at PS = 3 μM, 103° at PS = 196 μM). Results thus indicate a polar hydrophilic surface (i.e., goethite) becomes hydrophobic upon organic matter adsorption whereas low levels of organic matter adsorption renders a hydrophobic surface (i.e., diamond) relatively more hydrophilic, although the hydrophobic surface remains hydrophobic at higher levels of organic matter adsorption. Regardless of the initial polarity of the mineral surface, relatively high concentrations of surface-associated PS results in increased surface hydrophobicity of the organo-mineral association (i.e., results in a hydrophobic particle).