Spontaneously organized molecular assemblies. 2. Quantitative infrared spectroscopic determination of equilibrium structures of solution-adsorbed n-alkanoic acids on an oxidized aluminum surface

Abstract

Taken together, the data show that closest packed, oriented monolayers of n-alkanoic acids can be formed by adsorption from dilute solution. The formation of these quasi-two-dimensional organic surface phases is characterized by complicated kinetics in which surface and/or monolayer defects, as well as impurities, seemingly play important roles. The structures so obtained are dynamic in nature in that they undergo rapid exchange with comparable ligands in solution. These results, and those presented in the following paper, further suggest an important limitation to closest-packed, self-assembly on this substrate which is related directly to the length of the alkyl chain. The data further show that oleophobicity is an insufficiently sensitive probe to determine the formation of equilibrium closest packed structures. In a companion paper we describe these structures and their relationship to many of the above observations in detail. Registry No. n-Infrared reflection spectroscopy has been applied to the determination of the structures of adsorbed monolayer films of n-alkanoic acids on oxidized aluminum substrates. Acids of 16-22 carbons, terminated by methyl, vinyl, or propargyl groups, were absorbed from hexadecane solution at 25 O C , using immersion times up to several days. These results, together with those for perdeuterated acids, indicate that close-packed assemblies are formed with extended alkyl tails oriented with their chain axes tilted away from normal to the surface with a value of -10" for the longer chains. Frequency shifts to higher values are observed for CH stretching modes of the vinyl and propargyl terminal groups measured in monolayer films compared to the bulk acids. These shifts are of similar magnitude to gas-liquid phase shifts for these groups and imply the environment of the ambient-monolayer interface exhibits very diminished intermolecular interactions for these terminal groups compared to the bulk. The spectra clearly show that chemisorption occurs by proton dissociation to form carboxylate species. Examination of both line width and peak positions of the carboxylate stretching modes suggests a variety of binding geometries of the carboxylate groups to the surface exists. These results lend support to the general concept of forming stable, oriented and ordered two-dimensional organic films by solution adsorption and show the utility of applying surface vibrational spectroscopy to structural determination in these films.

Reactive uptake of ozone by proxies for organic aerosols: Surface versus bulk processes

“…The greater 3` value for ATS than OcEnTS indicates a difference in their packing structure. The longer alkyl chains tend to orient more normal to the surface compared to shorter chains [Allara and Nuzzon, 1985b]. Therefore it is likely that the shorter allyl chain lies more tangential to the surface, allowing bonding of additional allyltrichlorosilane molecules to itself (rather than the substrate) and forming a bilayer, as has been detected for the short-chain methyltrichlorosilane by X-ray photoelectron spectroscopy (XPS) measurements [Wasserman et al, 1989].…”

Application of Polarization Modulation Infrared Reflection Absorption Spectrosocpy Under Electrochemical Control for Structural Studies of Biomimetic Assemblies

“…Due to the surface selection rule, the integral intensity of a given absorption band depends on two factors: ( ) the surface concentration ( ) of a given molecule in the film Δ ≈ 2.3 and (ii) the orientation of the transition dipole vector of a given IR absorption in this anisotropic film [29,51]. The integral intensity of a given IR absorption band is proportional to the square of the absolute value of the dot product of the transition dipole vector ( ⃗ ) and the electric field vector of the p-polarized light ( ⃗ ) according to the following equation [84,85]:…”

Experimental Optimization of p-Polarized MAIR Spectrometry Performed on a Fourier Transform Infrared Spectrometer

“…When a molecule of metallic stearate is involved in an ordered monolayer, the νa(CH3) mode is known to split into the inskeleton and out-of-skeleton modes. 12,26 In this situation, the two modes would give different components on the IP and OP spectra, which depend on the molecular tilt angle. Therefore, there is always a band shift between the band locations of the νa(CH3) mode in the two spectra.…”