丙酮和水两种液体混合介质吉布斯表面吸附层结构和能级的确认

The orientation, structure, and energetics of the vapor/acetone-water interface are studied with sum frequency generation vibrational spectroscopy (SFG-VS). We used the polarization null angle (PNA) method in SFGVS to accurately determine the interfacial acetone molecule orientation, and we found that the acetone molecule has its CdO group pointing into bulk phase, one CH3 group pointing up from the bulk, and the other CH3 group pointing into the bulk phase. This well-ordered interface layer induces an antiparallel structure in the second layer through dimer formation from either dipolar or hydrogen bond interactions. With a double-layer adsorption model (DAM) and Langmuir isotherm, the adsorption free energies for the first and second layer are determined as ¢G°ads,1 ) - 1.9 ( 0.2 kcal /mol and ¢G°ads,2 ) - 0.9 ( 0.2 kcal /mol, respectively. Since ¢G°ads,1 is much larger than the thermal energy kT ) 0.59 kcal /mol, and ¢G°ads,2 is close to kT, the second layer has to be less ordered. Without either strong dipolar or hydrogen bonding interactions between the second and the third layer, the third layer should be randomly thermalized as in the bulk liquid. Therefore, the thickness of the interface is not more than two layers thick. These results are consistent with previous MD simulations for the vapor/pure acetone interface, and undoubtedly provide direct microscopic structural evidences and new insight for the understanding of liquid and liquid mixture interfaces. The experimental techniques and quantitative analysis methodology used for detailed measurement of the liquid mixture interfaces in this report can also be applied to liquid interfaces, as well as other molecular interfaces in general.