Formation, structure, and properties of alkanethiolate monolayers on micrometrically driven hanging mercury drop electrodes were investigated electrochemically. Alkanethiols with the chain length from C8 to C18 were shown to form densely packed (ca. 20.3 Å2/molecule for C12SH), perpendicularly oriented monolayers on mercury in a process involving two electron oxidation of Hg to form mercuric thiolate, in agreement with earlier literature reports for a number of thiols. Electron tunneling rates across these films (due to Ru(NH3)6 3+ electro-reduction in aqueous 0.50 M KCl) exhibit characteristic exponential increase with the electrode potential (with transfer coefficient α = 0.25), and an exponential decay with the monolayer thickness (with a through-bond decay constant, βtb = 1.14 per methylene group or 0.91 Å-1). Slow stepwise expansion of the mercury drop electrodes coated with alkanethiolates (C9−C14 only) results in an only small increase of the tunneling current maintaining the pin-hole free structure of the monolayers. Capacitance measurements showed that the film thickness changes inversely proportionally with the electrode surface area. The increase of the tunneling current recorded in the drop expansion experiments was accounted for by postulating existence of an additional tunneling pathway involving chain-to-chain coupling. Data analysis in view of this parallel pathways model yielded a through-space decay constant, βts = 1.31 Å-1. Ab initio computations of the electronic coupling matrix element (based on Koopmans' theorem approximation) and its distance dependence across a number of perpendicularly orientated n-alkanes yielded a decay constant of 1.25 Å-1 in excellent agreement with the measurements.

In the l80-exchanged reaction, the 02 atoms in the internal SiOH groups or in the nonintact Si-O-Si groups in the nest type defect sites attained the equilibrium state at ca. 15 h. Moreover, the bands at 3505 cm"1 2were observed less in the IR spectra for the HZSM-5 having fewer defect sites. From these results, it is suggested that the broad band at 3505 cm"1 can be identified with the SiOH groups of the hydroxyl nests.If aluminum atoms are inserted into hydroxyl nests in the HZSM-5 by alumination with A1C13, the band at 3505 cm"1 will disappear. The IR spectrum for aluminated HZSM-5 prepared from the parent HZSM-5 (sample 9 in Table I) is depicted in Figure 8E. The aluminated HZSM-5 exhibited a new band at 3610 cm"1 attributed to the framework Si(OH)Al groups,23 while the band at 3505 cm"' disappeared. These results indicate that (23) Jacobs, P.

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The importance of a solvent in regulating the adhesion forces between surfaces is studied quantitatively with scanning force microscopy. Both samples and tips are coated with alkyl thiolate monolayers of type HS(CH2)10Y and force measurements are conducted as a function of terminal group Y (Y = CH2CH3, CH2OCH3, CO2CH3, CO(NH2), CO2H, and CH2OH) and solvent (water, ethanol, and n-hexadecane). Adhesive forces in water span the greatest range (0.30−12.5 nN), with hydrophobic surfaces adhering most strongly and hydrophilic surfaces most weakly. In ethanol the adhesive forces are substantially smaller and in n-hexadecane they are negligible. In water, these adhesive forces are consistent with the work required to exclude solvent from the tip−sample interface, indicating that solvent exclusion dominates adhesion. Such macroscopic solvent exclusion cannot fully explain the adhesive forces in ethanol. This force data is used to evaluate the tip−sample interfacial energies (γts) of like CH3- and CH2OCH3-terminated surfaces and the surface−vacuum interfacial energies (γsv) of the hydrophilic surfaces. An effective tip radius of ∼30 nm and contact area of ∼10 nm2 (or ∼50 contacting molecules) is estimated from the adhesion between methyl groups in water. Since solvent exclusion regulates adhesion between these model organic surfaces, it provides a source of chemical contrast in force imaging. We explore this chemical contrast with friction force measurements of co-block polyethylene glycol−polyamide polymer surfaces.