An aqueous solution of 15 mM NaCl containing a predetermined amount of NaOH (~0.04 mmol per L) was aerated and used. The amount of NaOH was adjusted to give a pH of 7 when the solution was equilibrated with atmospheric CO2. A stock of kaolinite suspension at pH 7 was prepared in the same way with 15 mM NaCl solution and a predetermined amount of NaOH (~0.15 mmol per L). The pH was checked regularly with a pH electrode (LAQUA 9615S-10D, Horiba, Kyoto, Japan) and was confirmed to be 7.0 ± 0.2. An aqueous stock solution of a single cationic surfactant was prepared by dissolving the pure substance in the 15 mM NaCl solution, which was diluted in one or two steps to 6 lower concentration stocks. No solvent carrier was used to avoid a possible influence on sorption.