Three-Capacitor Model for Surface Potential of Insoluble Monolayers

Abstract

Three three-capacitor model of Demchak and Fort (Demchak, R. J.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191) that is widely applied to interpret surface potentials of monolayers at water−air interfaces postulates independent contributions of the hydrocarbon chains, polar head groups, and hydration water. We present an experimental verification of this a priori assumption for condensed monolayers of n-heptadecanol and 16-bromohexadecanol. These substances have different terminal CH3CH2 and BrCH2 groups and exhibit as monolayers ΔV potentials and vertical dipole moments that are opposite in sign. Demchak and Fort's analysis of the head group's conformation leads to controversial conclusions which question the basic assumptions of the three-capacitor model. Three-dimensional maps of the molecular electrostatic potential (MEP) and the molecular lipophilic potential (MLP) show that the ω-dipoles do not influence the electrostatic potential and hydrophilicity of the head groups of single n-heptadecanol and 16-bromohexadecanol molecules. The models also show that molecular dipole moments are not parallel to the hydrocarbon chains and may, therefore, cause collective (inductive and orientational) polarization leading to different dipole moments and conformations of the head groups of the condensed monolayers under study. These possibilities were examined by means of the interfacial polarity probe 4-heptadecyl-7-hydroxycoumarin embedded in heptadecanol and 16-bromohexadecanol monolayers. Practically the same values of pKi were obtained (pKi = 7.8 ± 0.1 and 7.9 ± 0.1) in the two matrices, thus indicating the same dipole moment and conformation of the OH-head group and, thus, negligible head group polarization. By relating pKi to the interfacial dielectric constant, values of εi = 65 ± 5 for heptadecanol and 60 ± 5 for 16-bromohexadecanol were obtained, thus showing that ω-dipoles have an almost negligible effect on the dielectric constant of the monolayer−water boundary. The above values for εi are in good agreement with other spectroscopic data for monolayers or micelles of neutral surfactants having mono- or polyhydroxy groups.