Dynamics of nm to μm size ordered structure formation of adsorbed monolayers at solid surfaces and spread monolayers on water surface studied by probe microscope is reviewed. The most important requirement for forming ordered molecular films on the solid surfaces is the constrained movement of the adsorbed molecules at the solid surface potentials. Two-dimensional cluster formation of nm size is a rather general phenomenon of the spread monolayers of the materials forming condensed films. However, they generally fuse into large domains spontaneously for simple amphiphiles such as long-chain acids. Partially fluorinated long-chain acids form very stable monodisperse nm size clusters in the spread monolayers by the mismatch of cross sections of hydrocarbon and fluorocarbon parts. Some other examples of forming such structures are shown.
X-ray reflectometry (XR) is a powerful tool for the evaluation of the nanostructure of the adsorbed polymer layers at the air-water interface. We newly established the Air/Water Interface X-ray Reflectometer for laboratory use and have performed in-situ XR measurements for various kinds of spread monolayers. In this review, we report the measurements for electrolytic amphiphilic diblock copolymer, “command surface type” photochromic polymer/liquid crystal complex, and lipid/protein complex on a water surface. From XR data, the layer thickness and density profile were determined precisely. The XR results will not only reveal the structure of the adsorbed layers at the surface and interface, but also provide the new knowledge about the relationship between the structure, assembling mechanism, and the functionality. In addition, we have examined the application to detect the capillary wave on the water surface using the XR technique.
Ripplon light scattering technique is introduced together with its application to the study on physical properties of molecular films formed on liquid surfaces. Ripplon is high frequency capillary waves generated by the thermal fluctuation of the liquid displacement, of which propagation constants yields information on the molecular dynamics in and near the surface. The ripplon frequency and damping constant is determined accurately by the light scattering system equipped by the optical beating spectroscopy techniques. The technique was successfully applied to the investigation of two dimensional gas-liquid phase transition of the Langmuir films. It reveals the critical behavior of the film, which is observed as the critical softening of the condensed phase with increasing temperature. Several other applications of ripplon light scattering for the observation of dynamic process of molecular adsorption/desorption process on to the surface are also presented.
This paper introduces our recent results on surface plasmon resonance (SPR) measurements of organic ultrathin films such as monomolecular layers. Two examples are addressed: one is the exchange kinetics measurement of alkanethiol self-assembled monolayers (SAMs) on an Au surface by immersion of the sample in a solution of a different alkyl chain length, and the other is the investigation of an adsorption process of liquid crystal molecules by vapor deposition. These studies well demonstrate the advantage of the SPR measurement. A new optical technique for investigation of such thin films is also proposed. This technique is complementary to the SPR measurement, and is a promising method for application of small biosensors.
Surface structure and surface mechanical properties of organosilane monolayers prepared by the Langmuir-Blodgett (LB) and chemisorption methods were investigated. The organosilane monolayers with longer alkyl and fluoroalkyl chains were in crystalline state at 293 K. The lateral force of the organosilane monolayers increased with an increase in their chain length. The magnitude of lateral force of the fluoroalkylsilane monolayer was higher than that of the alkylsilane one. This can be attributed to the rigid characteristics of fluoroalkyl chain. The lateral force of the n-octadecyltrichlorosilane (OTS) monolayer prepared by the LB method was higher than that of the chemisorbed one because of the higher packing density of alkyl chain for the LB monolayer. Molecular aggregation state of the LB OTS monolayer changed from the rectangular phase to the amorphous one via the hexagonal phase with an increase of temperature. The distinct lateral force decrease for the OTS monolayer was observed at the corresponding temperature range.
In this paper, we review the dynamic contact angle measurements by Wilhelmy method as one of the powerful tools for surface characterization. The dynamic contact angle measurements enable us to characterize not only simple wettability but also various surface-related phenomena, such as surface stability, topochemical structures, dynamic movement of adsorbed molecules on surfaces, etc. Especially, combination with scanning probe microscope (SPM) techniques for microscopic survey of surfaces provides clear and reliable understandings of features of monolayer surfaces. As examples of the performance, we introduce three typical studies with monomolecular films; 1) dimethyldioctadecyl ammonium bromide Langmuir-Blodgett (LB) monolayers on cleaved mica, 2) dioctadecyl sulfide self-assembled monolayer (SAM) on gold, and 3) unsymmetrical azobenzene disulfide SAMs designed for surface photo reactions. Different responses in the contact angle measurements on the surfaces revealed unique film properties. New ideas for the experiment or data analysis are also introduced.
Gallium ion TOF-SIMS fragment patterns from some metal-chlorides and -oxides can be qualitatively inferred. Considering the electron affinities of Cl and O, comparatively higher than those of other metal elements, rules of nx ≥ (y + 1) for positive ions and nx ≤ (y + 1) for negative ions were introduced and applied to infer the fragment patterns of metal-chlorides, MxCly, where the oxidation number of metal M is n. For oxides, MxOy (oxidation number of metal M is n), the rules used to infer the fragment patterns nx ≥ (2y + 1) for positive ions and nx ≤ (2y + 1) for negative ions. Further, the effect of water on the surfaces of chlorides and oxides, and the fragment patterns of some metal-nitrates are discussed.
Partial oxidation of methane into oxygenates, such as methanol and formaldehyde, over MoO3/SiO2 catalyst was studied at 873 K in excess amounts of water vapor. The selectivity of oxygenates was found to increase as the fraction of water vapor increased. This was ascribed to the formation of silicomolybdic acids (SMA) on the catalyst surface during the reaction: SiO2 + 12 MoO3 + 2H2O→H4SiMo12O40. Then, SiO2 supported SMA catalysts were prepared for the partial oxidation of methane at 873 K in the presence of excess water vapor. It was found that SMA still remained on the catalysts and worked well as active species for partial oxidation of methane to formaldehyde. It was concluded that water vapor depressed the decomposition of SMA into SiO2 and MoO3 and enhanced the activity for partial oxidation of methane. At the fraction of water vapor higher than 50%, the degree of methane conversion increased up to 20% with the formaldehyde selectivity of 90%, corresponding to 18% yield of formaldehyde.