There are two possible explanations for these results. not be able to optimize both types of interactions simultaneously. The heterogeneous distribution of charged functional groups and hydrophobic siloxane segments, as discussed above, could explain such a misfit for short-chain BACs. Wang et al. reported that the length of the shorter alkyl chain of double-chain cationic surfactants affects sorption much less than the length of the alkyl chain of a single-chain cationic surfactant, also suggesting the influence of molecular shape on kaolinitecationic surfactant interactions. The second possibility is that organic impurities in the kaolinite sample increased the sorption of long-chain BACs. The carbon content of the kaolinite sample in this study was measured using an elemental analyzer (Flash EA1112, Thermo Fisher Scientific) and was found to be 0.03 wt%, which was below the method limit of quantification (0.1 wt%). Nevertheless, reported organic carbon/water partition coefficients (Koc, L/kg) are high, particularly for long-chain BACs (>C12), e.g., 4.5-6.8 log units for different natural organic matter. If the log Koc of C12-BAC is say 6, even 0.03 wt% organic carbon could contribute significantly to the total sorption capacity of the current kaolinite sample. Thermal or chemical pretreatment of the kaolinite sample to remove organic residues prior to the sorption experiment would provide additional insight into the role of impurities in sorption, but such treatment will inevitably modify the surface of the kaolinite.