We report a new class of molecules, linactants, that partition at phase boundaries and reduce the line tension between coexisting two-dimensional phases in molecular monolayers. The line tension between hydrocarbon-rich and fluorocarbon-rich phases was determined by monitoring the relaxation kinetics of deformed domains. Two partially fluorinated linactant molecules (with one and two tails, respectively) were synthesized and tested; the more efficient single-tail variant reduced the line tension by more than 20% at a mole fraction of only 8 x 10(-4).

The thermal stability of SAMs generated from the adsorption of n-octadecanethiol (n-C18), 2-hexadecylpropane-1,3-dithiol (C18C2), 2-hexadecyl-2-methylpropane-1,3-dithiol (C18C3), and 1,1,1-tris(mercaptomethyl)heptadecane (t-C18) on colloidal gold and evaporated "flat" gold was investigated. The optical extinction of the monolayer-protected nanoparticles (MPCs) was monitored as a function of thermal stress by using ultraviolet-visible (UV-vis) spectroscopy, which revealed that the evolution of the surface plasmon resonance varied with the nature of the adsorbate. Specifically, MPCs functionalized with monodentate n-C18 showed the fastest red shift of the surface plasmon resonance while those functionalized with tridentate t-C18 showed the slowest red shift, with those derived from the bidentates C18C2 and C18C3 falling in between, suggesting a correlation between film stability and the degree of chelation. In separate studies, X-ray photoelectron spectroscopy (XPS) was used to evaluate the desorption of the monolayers on both colloidal gold and flat gold as a function of thermal stress. In these studies, SAMs generated from monodentate n-C18 showed the fastest desorption while SAMs generated from tridentate t-C18 showed the slowest desorption, with those derived from the bidentates C18C2 and C18C3 falling in between, again suggesting a correlation between film stability and the degree of chelation. As a whole, the following trend in thermal stability was observed: t-C18 > C18C2 approximately C18C3 > n-C18.

The interfacial electrochemical properties of self-assembled monolayers (SAMs) on gold derived from a structurally tailored series of monodentate, bidentate, and tridentate chelating alkanethiols were investigated. Specific adsorbates included 1-hexadecanethiol (C16), 2-tetradecylpropane-1,3-dithiol (C16C2), 2-tetradecyl-2-methylpropane-1,3-dithiol (C16C3), 2,2-ditetradecylpropane-1,3-dithiol (C16C16), and 1,1,1-tris(mercaptomethyl)pentadecane (t-C16). Reductive desorption of the SAMs as a function of potential was probed by voltammetric measurements, which indicated the following relative order of electric potential stability: t-C16 > C16C2 ≈ C16C3 ≈ C16C16 > C16. The ionic permeability was investigated under various applied cathodic potentials by electrochemical impedance spectroscopy (EIS). An examination of SAMs prepared at room temperature and accessed by EIS at open-circuit potential showed that the ionic permeability increased in the order C16C2 < C16 < C16C3 < C16C16 < t-C16. The ionic permeability of films was further influenced by the electric potential of the metal substrate and the temperature at which the monolayers were assembled. The potential dependence of the ionic permeability was qualitatively rationalized by considering both the initial ionic permeability and the electric potential stability of the SAMs. Similarly, the ionic permeability of the SAMs prepared at elevated temperature showed contributions from both their thermal stability and their insulating properties at room temperature.

A systematically varying series of monolayer-protected clusters (MPCs) was prepared by exposing small gold nanoparticles ( approximately 2 nm in diameter) to the following four adsorbates: n-octadecanethiol ( n - C18), 2-hexadecylpropane-1,3-dithiol ( C18C2), 2-hexadecyl-2-methylpropane-1,3-dithiol ( C18C3), and 1,1,1-tris(mercaptomethyl)heptadecane ( t - C18). The resultant MPCs were characterized by solubility studies, UV-vis spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FT-IR). All of the MPCs were soluble in common organic solvents; moreover, analysis by TEM showed that the core dimensions were unaffected by exposure to any of the adsorbates. Separate studies by XPS revealed that the sulfur atoms in all MPCs were predominantly bound to the surface of gold (i.e., approximately 85% or better). Analysis by FT-IR showed that MPCs functionalized with n - C18 possessed alkyl chains having the greatest conformational order in both the solid-state and dispersed in solution; in contrast, those generated from the other three adsorbates were more liquid-like with reduced conformational order (or crystallinity). The rate of nanoparticle decomposition induced by cyanide ions was monitored by UV-vis spectroscopy. While MPCs functionalized with n - C18 showed the fastest rate of decomposition, those functionalized with C18C3 were the most resistant to decomposition. Overall, the following trend in chemical stability was observed, C18C3 >> C18C2 > t - C18 >> n - C18.

This letter describes the preparation of monolayer-protected nanoparticle clusters (MPCs) from the adsorption of n-tetradecanethioacetate onto colloidal gold nanoparticles using the Brust-Schiffrin two-phase synthesis method. The MPCs were characterized by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) spectroscopy, (1)H nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. These studies found that the monolayer coatings on the gold nanoparticles were nearly indistinct with regard to chemical composition, monolayer structure, and Au-S ligation when compared to those prepared from the analogous adsorption of n-tetradecanethiol (i.e., the thioacetate headgroup adsorbs to gold as a thiolate, with concurrent loss of the acetyl group). Under equivalent conditions of formation, however, the size of the gold nanoparticles formed was larger when using the alkanethioacetate adsorbate (e.g., 4.9 +/- 1.2 nm) compared to the alkanethiol adsorbate (e.g., 1.6 +/- 0.3 nm). The observed difference in size is rationalized on the basis of the stronger ligating ability of the thiol compared to that of the thioacetate during gold nanoparticle nucleation and/or growth. The use of alkanethioacetates affords significant control of particle size and allows the formation of MPCs with thiol-sensitive omega-functional groups.