서강대학교

초록/요약

The protonation status of amine headgroups of the fatty amine molecules in the Langmuir monolayer was calculated by Poisson-Boltzmann theory and the equation of chemical equilibrium. The theory predicted that the amine headgroup density at the surface changes the surface electric potential and can influence the protonation/deprotonation of the amine headgroup. Then Langmuir monolayers consisting of mixtures of 1-hexadecanol (HD) and octadecylamine (ODA) molecules at different molar ratios were prepared to investigate the effect of the amine headgroup density on the charge status of the monolayer. To compare with the above monolayer system, Langmuir monolayers consisting of mixtures of HD and 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) were also investigated at different molar ratios, because DPTAP is a surfactant molecule having quaternary amine headgroup (thus always positively charged). Here the fatty alcohol molecules worked as mere spacers to widen the distance between the amine headgroups. Sum-frequency vibrational spectroscopy (SFVS) in the O-H spectral range was used as an indicator of the surface charge and the resulting surface electric potential. As more HD occupied the HD:DPTAP mixtures in the monolayer, the spectral area decreased monotonically. By contrast, the OH signal from the HD:ODA mixtures were larger for monolayers having intermediate amine headgroup density, which indicated the surface is more charged even when there are fewer amine groups in the monolayer. The theory that also took into account the protonation/deprotonation of the amine group cannot explain the sum-frequency signal from the HD:ODA systems.