washed with 50 mL of water for 1 h and centrifuged at 1000g for 5 min. The slurries were washed two more times with water and centrifuged at 2000g for 30 min. The final supernatant was collected and measured for electrical conductivity using a LAQUAtwin AS-771 conductivity meter (Horiba, Kyoto, Japan), which resulted in 2.7-2.9 mS/m (~0.3 mM NaCl). The remaining slurries were frozen at -30°C and freeze-dried (FDU-1200, EYELA, Tokyo, Japan) to obtain dry powder of Na-kaolinite. For convenience, this Na-kaolinite will be referred to as kaolinite.