Atomic force microscopy measurements of topography and friction on dotriacontane films adsorbed on a SiO2 surface

We report comprehensive atomic force microscopy (AFM) measurements at room temperature of the nanoscale topography and lateral friction on the surface of thin solid films of an intermediate-length normal alkane, dotriacontane (n-C32H66), adsorbed onto a SiO2 surface. Our topographic and frictional images, recorded simultaneously in the contact mode, reveal a multilayer structure in which one to two layers of molecules adsorb adjacent to the SiO2 surface oriented with their long axis parallel to the interface followed by partial layers of molecules oriented perpendicular to the surface. The thicknesses of the parallel and perpendicular layers that we measured with the AFM agree with those inferred from previous x-ray specular reflectivity measurements on similarly prepared samples. We also observe bulk dotriacontane particles and, in contrast with our previous measurements, are able to determine their location. Above a minimum size, the bulk particles are separated from islands of perpendicularly oriented molecules by regions of exposed parallel layers that most likely extend underneath the particles. We find that the lateral friction is sensitive to the molecular orientation in the underlying crystalline film and can be used effectively with topographic measurements to resolve uncertainties in the film structure. We measure the same lateral friction on top of the bulk particles as on the perpendicular layers, a value that is about 2.5 times smaller than on a parallel layer. Scans on top of parallel layers indicate a constant height but reveal domains having different sublevels of friction. We explain this by the domains having different azimuthal orientations of the molecules.

Surface free energies of silica fillers and their relation to the adsorption of poly(ethylene terephthalate)

The surface free energy of modified silica as well as of PET oligomers was evaluated through measurements of specific retention volumes of several probe molecules by use of the adsorption and adhesion principles in inverse gas chromatography. The nondispersive component of surface free energy of most silica fillers was larger than the dispersive component and the acidic component was much larger than the basic one, which indicated that the surfaces of most silica fillers were rather acidic. These methods were also applied to PET oligomer and it was found that the surface free energy of PET oligomer, regardless of preparation method, consisted of an almost dispersive component, suggesting that the surface of PET was neutral. The amount of PET oligomer adsorbed for the heat-treated silica fillers in acidic solvent increased linearly with increased acidic component of the surface free energy, which indicates that the acidic component of the surface free energy may be responsible for the adsorption. However, the adsorption amount on modified silica is much smaller than that for the heat-treated silica fillers because of steric hindrance caused by the attached organic chain, suggesting that the adsorption cannot be determined only by the surface free energy.

Adsorbed Surfactants as Templates for the Synthesis of Morphologically Controlled Polyaniline and Polypyrrole Nanostructures on Flat Surfaces: From Spheres to Wires to Flat Films

Nanostructures of polyaniline (PAni) and polypyrrole (PPy) with controlled morphologies have been synthesized on atomically flat surfaces using adsorbed surfactant molecules as templates. Atomic force microscopy (AFM) has been used to investigate polymer film formation on highly oriented pyrolytic graphite (HOPG) and chemically modified HOPG. Morphological control over the resulting polymer film is possible by the addition of coadsorbing molecules, manipulation of the length of the surfactant hydrophobe, or by changing the surface chemistry of the adsorbing substrate. Phase transitions between spheres, cylinders/wires, and featureless films have been observed which exactly parallel transitions between spheres, cylinders, and flat layers in the adsorbed surfactant. Parallel arrays of PAni nanowires can be synthesized with alignment evident over large areas in a simple self-assembly technique in which fabrication and arrangement take place simultaneously. Such a technique in which one can engineer sub-100-nm-ordered nanoscale pi-conjugated polymer structures of a desired shape by a simple self-assembly process presents potential as templates, sensors, and microelectronic devices.

NMR study of n-dodecane adsorbed on graphite

In this brief contribution we demonstrate that 1H and 2H NMR spectroscopy can be an effective method of investigating adsorption from liquids at the solid-liquid interface. The method is illustrated here with the adsorption of a simple alkane adsorbed on graphite, in particular the system n-dodecane and graphite at coverages of 1 and 5 monolayers. Static single-pulse proton nuclear magnetic resonance and static quadrupolar echo deuterium nuclear magnetic resonance spectra were recorded for both coverages. The experimental NMR results presented here show features clearly consistent with earlier calorimetric and neutron scattering work and demonstrate the formation of solid adsorbed layers that coexist with the bulk adsorbate with both isotopes. This ability to probe both deuterated and protonated materials simultaneously illustrates that this experimental approach can be readily extended to investigate the adsorption behaviour of multicomponent mixtures.

On the Applicability of the Langmuir Equation to Estimation of Adsorption Equilibrium Constants on a Powdered Solid from Aqueous Solution

The Langmuir adsorption equation has become popular for summarizing solution-solid adsorption data and estimating adsorption equilibrium constants. The applicability of the Langmuir equation to estimation of adsorption equilibrium constants at solid-aqueous solution interfaces was theoretically examined in terms of (1) comparisons of solid-solution adsorption with solid-gas adsorption, (2) deductions of the Langmuir mathematical expression for the nonelectrolyte/solvent exchange, the phase exchange, the ion exchange, and the simple association reactions, and (3) the fitting procedure. The restrictive assumptions underlying the Langmuir equation for solid-gas adsorption may not be valid for adsorption of solute of small molecules or ions by a powdered solid surface from the aqueous solution phase. The fit of adsorption data to the Langmuir mathematical expression cannot be ascertained solely by the linearity of the commonly used equilibrium concentration/amount adsorbed versus the equilibrium concentration plot and does not provide the actual mechanism of the loss of small molecules or ions from aqueous solution. Therefore, the parameters of fitting to the Langmuir mathematical expression may not provide an unambiguous chemical meaning. In addition, we attempt to bring attention to the importance of the standard states of surface phase and solution phase. Copyright 2000 Academic Press.

Binary and Ternary Adsorption of n-Alkane Mixtures on Activated Carbon

The adsorption isotherms of the binary n-alkane mixtures n-hexane/n-octane, n-octane/n-tetradecane, and n-hexane/n-tetradecane on the activated carbon TA 95 are measured at 298 K and described with mathematical functions. About 40 experimental values of the adsorption excess of the ternary mixture n-hexane/n-octane/n-tetradecane on activated carbon TA 95 at 298 K are gas chromatographically measured inside the ternary triangle. The ternary data are represented in the three-dimensional space with the help of transformation of coordinates and by utilization of the conception of the quasi-two-component representation of the mole fractions. A consistency test for the specific wetting Gibbs energies calculated from the binary data is carried out. The possibilities for a mathematical prediction of ternary data from adsorption data for the constituent binary mixtures are proved. Copyright 1999 Academic Press.

Biomimetic engineering of non-adhesive glycocalyx-like surfaces using oligosaccharide surfactant polymers

The external region of a cell membrane, known as the glycocalyx, is dominated by glycosylated molecules, which direct specific interactions such as cell-cell recognition and contribute to the steric repulsion that prevents undesirable non-specific adhesion of other molecules and cells. Mimicking the non-adhesive properties of a glycocalyx provides a potential solution to the clinical problems, such as thrombosis, that are associated with implantable devices owing to non-specific adsorption of plasma proteins. Here we describe a biomimetic surface modification of graphite using oligosaccharide surfactant polymers, which, like a glycocalyx, provides a dense and confluent layer of oligosaccharides. The surfactant polymers consist of a flexible poly(vinyl amine) with dextran and alkanoyl side chains. We show that alkanoyl side chains assemble on graphite through hydrophobic interaction and epitaxial adsorption. This constrains the polymer backbone to lie parallel to the substrate, with solvated dextran side chains protruding into the aqueous phase, creating a glycocalyx-like coating. The resulting biomimetic surface is effective in suppressing protein adsorption from human plasma protein solution.