Cd~2+-Induced Interfacial structural Changes of Lagnmuir-Blodegett Films of Stearic Acid on S9lid Substrates:A sum Frequency Generation Study

摘要

The molecular structures and their stabilities at the outmost-layer of the Langmuir-Blodgett(LB) films of stearic acid on solid substrates have been ivnestigatedby a highly surface-sensitive spectroscopic tehcnique,sum frequency genertion (SFG),in air and in aqeuosu solution,using the combination of both normal and deuterated stearic acid.Peaks observed in the SFG spectra are mainly attributed to the teminal and deuterated stearic acid.Peaks observed in the SFG spectra are mianly attributed to the therminal independent of the odd-even property and thickness (1-12) of the LB films,indicating that the evennumbered LB film changes its surface structure after passing through the interface between the water subphase and air,especially when the CD~2_ cation was included in the water subphase.Furthermore,we have demonstrated for the first time using in situ SFG measurement that the interfacial molecular structure at the LB bilayer of stearic acid on the hydrophilic substrates singificantly change with immersion in the watr subphase containining Cd~2+ cation while such a structural change has not been observed in the water subphase without Cd~2+.These results clearly indicate that a reorganization process takes place on the surface of the stearic acid bilayer induced by the Cd~2+ cation.The electrostatic interaction between the carboxylate headgroup of stearic acid via the Cd~2+ cation seems to play an important role in the surface reorganization process both in air and in solution.