Experimental studies on the applicability of the Kelvin equation to highly curved concave menisci

The thermodynamic properties of liquids trapped in microscopic pores are described in theory by the Kelvin equation, which relates the equilibrium meniscus curvature to the relative vapor pressure. We report here two series of experiments designed to test the validity of the Kelvin equation by direct measurement of the mean radius of curvature of the surface of cyclohexane condensed between crossed mica cylinders. In one series of experiments, the relative vapor pressure of the volatile cyclohexane was controlled by mixing it with a relatively involatile solute (n-dodecane or n-hexadecane). We found that the mean radius of curvature rapidly reached that predicted by the Kelvin equation at each relative vapor pressure of the volatile liquid, but that there was also a slow, but continuous, accumulation of the “involatile” solute at the point of condensation as the system approached true equilibrium. Such accumulation of very low vapor pressure materials may be one factor responsible for the discordant results reported by earlier workers. We find that the process of impurity buildup is complex, and suggest that studies of real porous systems may be affected by accumulation of “involatile” impurities through the vapor phase and by surface diffusion. The other series of experiments was designed to eliminate the impurity problem by maintaining the vapor pressure by temperature control of the pure liquid. The results from this series of experiments were not time dependent, and no evidence of contamination was found. The measured radii were within ±6% of those predicted by the Kelvin equation, for radii in the range 4–20 nm. We conclude that the thermodynamic basis of the Kelvin equation is valid in principle for menisci with radii as low as 4 nm.     

Alpha,omega-bis(thioacetyl)oligophenylenevinylene chromophores from thioanisol precursors

The selective cleavage of arylmethyl thioethers provides a convenient protocol for the synthesis of all-E isomers of alpha, omega-bis(thioacetyl)oligophenyenevinylene molecules (OPVs). The S-methyl group is tolerant of Wittig-type and Heck-type reactions for forming OPV structures and can be converted to the S-acetyl group by treatment with sodium thiomethoxide and acetyl chloride. The thermal conditions of the deprotection/reprotection step concurrently isomerize the conjugated chromophore to the all-E isomer, regardless of the stereochemistry of the starting olefins. This approach is demonstrated for a variety of linear and [2.2]paracyclophane containing OPVs, which have been characterized by electrochemical and spectroscopic techniques. Additionally, these S-acetyl-terminated OPVs self-assemble on gold surfaces. Monolayers containing these molecules were characterized by water contact angle measurements, ellipsometry, and X-ray photoelectron spectroscopy.     

Direct observation of shear-induced orientational phase coexistence in a lyotropic system using a modified x-ray surface forces apparatus

The second generation x-ray surface forces apparatus (XSFA-II) allows for the first time simultaneous in situ small-angle x-ray scattering and surface force measurements. We have used the XSFA-II to monitor shear-induced orientational transitions in a lyotropic model lubricant system. Upon applying small shear amplitudes (approximately 20 micrometer) to a relatively thick (approximately 800 micrometer) film, we observed evidence for the formation of an orientational boundary layer at the shearing surface. Time-resolved x-ray diffraction revealed the gradual transition to shear-favored orientation by growth of the boundary layer.     

Determination of the Capillary pressure in menisci of molecular dimensions

The measured adhesion force arising from the Lapse presure in capillary condensed liquid between two curved molercular smooth surface is well described by the Gibbs-Kevin equation of classical themodynamics for liquid hydrocarbons with meniscus radii down to 0.5-1.0. The corresponding limit for liqiud water is much higher, about 5 nm.     

Forces between surfaces across nanoparticle solutions: role of size, shape, and concentration

Using a surface force apparatus, we have measured the normal forces between mica surfaces across various types of nanoparticles consisting of ZnS cores coated with a monolayer of physisorbed surfactant, dispersed in organic solvents. We focused on the effects of nanoparticle size, shape, and concentration on the force-distance profiles. Forces were exponentially repulsive when the surfactant layers were strongly bound to the nanoparticles and were roughly linear when there was adhesion between the nanoparticle cores, i.e., when the surfactant layers detached from the nanoparticles. In both cases, the range and magnitude of the forces were dependent upon the particle size, shape, and solution concentration. Fine details in the otherwise smooth force-distance profiles indicate specific effects due to particle chemistry and geometry and the existence of first-order disorder-order phase transitions upon confinement. Small amounts of water in the (hydrophobic) organic solvents had dramatic effects on the measured forces. Understanding and controlling the effects of particle shape, size, and concentration and the presence of water (or other surface-active solutes) on particle-particle and particle-surface interactions are important for the processing of nanoparticles into ordered superstructured materials.