Formation and Structure of Self-Assembled Monolayers

Monolayer Adsorption for the Subcritical Lattice Gas and Partially Miscible Binary Mixtures

Lattice theories have been used extensively to predict and correlate liquid-liquid equilibria in mixtures. Lattice theories also have been used to predict the behavior of gases adsorbing onto solid surfaces. Here, we use a lattice model based on the ideas of Ono and Kondo to predict the phase behavior in adsorbed monolayers for systems that are below their bulk-phase critical points. For such an analysis, it is important that molecular behavior in the bulk and the adsorbed layer is based on consistent assumptions. Here, this is accomplished by treating the fluid in the bulk as well as in the adsorbed layer as a strictly regular solution. Interesting new adsorption isotherms and phase diagrams are generated that provide useful insights into adsorption of both lattice gases (i.e., "mixtures" of molecules and holes) and dense lattice liquids (i.e., "mixtures" of molecules without holes), illustrating the similarities between adsorption of gases and liquids. Copyright 1999 Academic Press.

Gold and Silver Nanoparticles Functionalized by the Adsorption of Dialkyl Disulfides

The formation of three-dimensional self-assembled monolayers (3-D SAMs) generated by the adsorption of n-octadecyl disulfide onto colloidal gold and silver nanoparticles is described. The functionalized nanoparticles were characterized by solubility, transmission electron microscopy, ultraviolet-visible spectroscopy, ¹H nuclear magnetic resonance spectroscopy, surface-enhanced Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. On gold nanoparticles, this new functionalization method affords crystalline 3-D SAMs that are indistinct from those prepared by the analogous adsorption of n-octadecanethiol. On silver nanoparticles, however, the films derived from n-octadecyl disulfide appear to be somewhat less crystalline than those prepared similarly from n-octadecanethiol. The origin of this difference is briefly explored and discussed.

Measuring the Contact Angle of Individual Colloidal Particles

The aim of this study was to measure the contact angles of individual colloidal spheres ( solidus in circle 4.4 µm) and compare it to contact angles obtained on similarly prepared planar surfaces. For this purpose the particles were attached to atomic force microscope cantilevers. Then the force between the particle in aqueous medium and an air bubble was measured versus the distance. From the resulting force curves we obtained contact angles and detachment forces of single particles. Contact angles of gold coated silica particles were adjusted between 20 degrees and 100 degrees by self-assembling monolayers from different mixtures of undecanethiols and omega-hydroxy undecanethiols from solution. In parallel, contact angles on flat gold surfaces prepared in the same way were determined by the sessile drop method. A systematic difference between contact angles measured with particles and on planar surfaces was observed. Results are discussed in terms of line tension of the three-phase contact line. In addition, detachment forces were measured. Detachment forces were slightly higher than predicted from flotation theory. This might be caused by a pinning of the three phase contact line. Copyright 1998 Academic Press.

Effect of Co-Adsorbed Surfactant on the Structure of Self-Assembled Monolayer of Thiol on Polycrystalline Gold

The structure and formation kinetics of a self-assembled monolayer (SAM) of long-chain alkane thiol in the presence and absence of an anionic surfactant like sodium dodecyl sulfate (SDS) has been studied using impedance, cyclic voltammetic, and quartz crystal microbalance (QCM) measurements. The comparison of these results, especially the ionic permeability of the monolayer, suggests a composite monolayer where both the surfactant and thiol molecules are cooperatively adsorbed to form distinct surfactant patches. This is further substantiated by the voltammetric experiments with redox probe in solution. A gradual change in the double layer capacitance value for the thiol monolayer alone (1.4 µF/cm2) to the composite monolayer (6.7 µF/cm2) and to the surfactant alone (11 µF/cm2) further supports the nature of the mixed monolayer. The kinetics of monolayer formation also shows interesting changes as revealed by the QCM studies, where a phase transition from simple to composite monolayer has been observed around an approximate coverage of 0.44. The same growth scheme is also observed for 1-pentanethiol, naphthalene disulfide, and diphenyl disulfide, suggesting that the mechanism appears to be general. Copyright 1998 Academic Press.