2. EXPERIMENTAL AND THEORETICAL METHODS
2.1 Materials .
Ethylene glycol (99.8%) and 1,2-butanediol (98%) were obtained from Sigma-Aldrich. Both products were used without any further treatment. 1,2-butanediol was racemic. The samples for SFG-VS experiment were filled in a round Teflon beaker (diameter ∼4.5 cm). The smaller Teflon beaker (inner diameter ∼2.4 cm) whose bottom thickness is thinner (~ 0.2 cm) was used in thermal sum-frequency generation study. Each mixture in this work was prepared by mass in 20 ml glass screw-cap bottle. The balance precision was ±1ⅹ10-4 g. Although there are a few versions of the azeotropic composition for this mixture, the composition is fixed at 0.457 mole fraction (0.55 mass fraction) in our study.36-38