Before discussing the results of the passive sampling method, we explore the sorption characteristics of the PA fiber for cationic surfactants. The PA-water sorption isotherm for C6-BAC was nearly linear, whereas the isotherms for the other BACs became increasingly nonlinear with increasing alkyl chain length (Figure ). The influence of alkyl chain length on sorption was generally greater for PA than for kaolinite. The isotherms were curved in the log-log plot and thus slightly deviated from the Freundlich equation (see all fitted parameters in Tables ). The isotherm nonlinearity was particularly high (nFr,PA < 0.5) above CPA = 1,000 μM for any compound. The higher nonlinearity observed for long-chain compounds may be due, at least in part, to the higher CPA levels investigated in this study. The observed nonlinearity of PA sorption was similar to that reported for C12-BAC in the previous study. As with the kaolinite shown above, the PA-water isotherms for C10-BAC and C8-C8-DADMAC overlapped, as did the isotherms for C14-BAC and C10-C10-DADMAC. These two pairs of cationic surfactants appear to have similar sorption properties.