Exchangeable cations on kaolinite were replaced by Na + using NaCl, as follows. Four 50-mL polypropylene centrifuge tubes (IWAKI, AGC Techno Glass, Yoshida, Japan) each received 2 g of kaolinite and 50 mL of 1 M NaCl aqueous solution. These tubes were roller-shaken overnight at 60 rpm using a Mix Rotor VMR-5R (AS ONE, Osaka, Japan) and centrifuged at 500g for 5 min using an S700T centrifuge (Kubota, Tokyo, Japan). The supernatant was removed, and the remaining slurries were washed four more times with 50 mL of 1 M NaCl for 3-4 h each time. The slurries were then