Malic acid

The PIL samples were synthesized based on a previously described procedure 14) (as presented in the Supporting Information) . For example, ［2TEtOHA］ ［ Malate］ was synthesized as follows: triethanolamine (Wako) and malic acid (Wako) were mixed with pure water and stirred at room temperature (approximately 20℃) for 3 h. A crude product was obtained by distilling the mixture under a reduced pressure. Subsequently, the product was washed with tetrahydrofuran and acetonitrile. The structure of the product was confirmed via FT-IR, 1 H-NMR, and 13 C-NMR spectroscopies, and the water content was measured using a Mitsubishi KF-200 Karl Fischer moisture meter. The freezing point was measured using a freezing point measuring instrument based on JIS K0065, and the viscosity was measured at 25℃ using a Brookfield DV-II＋Pro viscometer. These data are presented in the Supporting Information. A comparative nonionic material with hydroxy groups, 1,2,6-hexanetriol ［HT］ ( Tokyo Chemical Industry, TCI) , was used to evaluate whether the ionic nature of the PIL samples affected the adsorption and lubrication of the solid surface. The water used in this study was purified using a Millipore Direct-Q UV 3 water purification system.

Oxalic acid (purity＞98％) , malic acid (purity＞98.0％) , succinic acid (purity＞99.5％) , citric acid (purity＞99.5％) , metaphosphoric acid, acetone, methanol, acetonitrile, and triethylamine were purchased from Fujifilm Wako Pure Chemical Corporation (Osaka, Japan) . Phenacyl bromide, quinic acid (purity＞98％) , and butyric acid (purity＞ 99.0％) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) .

Sulfuric acid, 1-butanol, n-hexane, sodium carbonate, 2,2-dimethoxypropane, oxalic acid, malic acid, succinic acid, and citric acid were purchased from Fujifilm Wako Pure Chemical Corporation (Osaka, Japan) . Glutaric acid was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) .