Electrical conduction was studied in AVadsorbed monolayer/AI junctions. The adsorbed monolayers were long chain saturated fatty acids (23 to 14 carbon atom chain), short chain perfluorinated fatty acids (10 to 7 carbon atom chain), and n -octadecyl-trichlorosilane. The current observed decreased rapidly with decreasing temperature from 295 K to approximately 77 K at which point the decrease in current with decreasing temperature became very small. At room temperature (295 K) there was no definite relation between the dc conductivity and the length of the fatty acids. At 77 K and lower temperatures, however, the dc conductivity was exponentially dependent on the fatty acid chain length. These results suggest that at 77 K and lower temperatures the conduction mechanism is tunneling through the monolayer. The tunneling barrier height was found to be of the order of 2.8 and 5.2 eV for fatty acids and perfluorinated fatty acids, respectively. By artificially producing molecular holes in monolayers it was shown that in the presence of such holes the current increases by at least one order of magnitude while the effective barrier height is lowered by 10%. Differences between the present results and those previously obtained with monolayers deposited from an air-water interface (Langmuir-Blodgett monolayers) are discussed.

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 pK i were obtained (pK i = 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 pK i 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.

In a series of three related papers we compare mechanical properties and stability, morphology and structure, and electrostatic potential and ellipsometric thickness of trifluoroethyl behenate (TFEB) and ethyl behenate (EB) Langmuir monolayers. The aim of these papers is to study the effect of fluorination of a methyl group in the hydrophilic head on monolayer properties and structure. In the present Part 1 we show that trifluoroethyl ester forms significantly more unstable films with higher compressibility (lower compressional modulus) than the unsubstituted ethyl ester. Both TFEB and EB surface pressure−area isotherms show compression−expansion hysteresis, but this hysteresis is larger for the fluorinated ester. The surface pressure−area loop of TFEB is shifted to larger molecular areas as compared to EB and gives larger limiting molecular areas at zero compression. This points to different volumes and/or conformations of the fluorinated and nonsubstituted hydrophilic heads. Maps of molecular lipophilicity and molecular electrostatic potential, based on semiempirical quantum mechanical models of the two molecules in vacuo, relate the observed differences in monolayer properties to decreased hydrophilicity of the trifluoroethyl group and a stronger electrostatic repulsion between the hydrocarbon chains of TFEB. Such a repulsion results from polarization of the CH2 groups adjacent to the heads that is more significant for the trifluoroethyl behenate molecule.

The photochromic properties of 1′‐octadecyl‐3′,3′‐dimethyl‐6‐nitrospiro[2H‐1‐benzopyran‐2,2′‐indoline] have been studied in monolayers at the air‐water interface by measuring the surface pressure‐area isotherms and the surface potential of several mixtures of this compound with tripalmitine. By using the Langmuir‐Blodgett technique we have transferred mixed monolayers of tripalmitine and spiropyran in a molar mixing ratio of 6:1 onto glass slides and have studied their photochromic properties in monolayer assemblies.

The exponential dependence of the tunneling conductivity on the thickness of the insulating material has been studied by varying the length of fatty-acid monolayers in metal/fatty-acid-monolayer/metal junctions. By varying the temperature between 20 and −100 °C it is shown that the tunneling current remains relatively constant, while the decrease in capacitance observed can be attributed to the influence of the Al2O3 layer in our junctions.