CH3(CH2)nCOOH/Cd2+ system on the aqueous cadmium acetate solution investigated in situ by polarization modulation infrared spectroscopy

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate the classical CH3(CH2)(n)COOH/Cd2+ system (abbreviated as CdSt, CdA, and CdB for n = 16, 18, and 20, respectively) in situ on the aqueous cadmium acetate subphase of pH = 6.7 in a Langmuir trough. The alkanoic acids were deprotonated and spontaneously formed crystalline domains after spreading to the subphase. The spectra were recorded at a resolution of 4 cm(-1) and the crystal lattice information contained in the CH2 scissoring mode was unambiguously concluded. It is found that the crystal lattice has a systematic dependency on the monolayer temperature and chain length. The CdSt monolayer possesses a hexagonal packing at 293-283 K and pseudohexagonal packing at 274 K. The CdA monolayer possesses a hexagonal unit cell at 293 K and an orthorhombic one at 283-274 K. The CdB monolayer always has an orthorhombic unit cell at 293-274 K. Both temperature lowering and chain lengthening can increase the interchain interaction and tend to render an orthorhombic unit cell. After being transferred from the aqueous subphase onto solid substrates at 283 K, the orthorhombic monolayer of either CdA or CdB relaxes into a hexagonal structure. Finally, barrier compression has no effect on the crystal lattice of the uniform monolayer but does affect that of the collapsed films.