Ken-ichi Iimura

A study on structures and formation mechanisms of self-assembled monolayers of n-alkyltrichlorosilanes using infrared spectroscopy and atomic force microscopy

Abstract The structures and formation mechanisms of self-assembled monolayers of n-alkyltrichlorosilanes on glass substrates have been examined systematically using Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). The adsorption temperature dependency of FTIR frequencies and intensities of CH2 stretching vibration bands indicates the existence of a characteristic temperature, Tc, for each monolayer. The hydrocarbon chains observe an ordered and closely packed state when the monolayers are prepared below Tc, whilst disordered monolayers with low molecular density are formed above Tc. AFM observation clearly demonstrates an island-type growth of monolayers prepared below Tc. The results obtained contribute to an understanding of the growth behavior of the monolayers and the physical meaning of the Tc.

Model for Analysis of XPS Electron Take‐off Angle Experiments in Layer‐structured Samples: Determination of Attenuation Lengths in a Well‐characterized Langmuir–Blodgett Film

A series of equations have been proposed to calculate take-off angle dependency of the elemental composition of layer-structured models from the experimentally obtained peak intensities of XPS. We have measured the take-off angle dependency of XPS peaks of O 1s, C 1s, Si 2p and Cd 3d from a well-characterized one-layer Langmuir–Blodgett (LB) film of polymerized cadmium 10,12-pentacosadiynoate on a silicon wafer substrate and have applied the newly proposed equations to deduce attenuation lengths of photoelectrons for every element in the layer-structured organic materials. From the best fitting between the theoretically calculated and the experimentally obtained data by the least-squares method, attenuation lengths for a monolayer film of polymerized cadmium 10,12-pentacosadiynoate have been evaluated to be 3.4, 3.7, 4.0 and 4.5 nm for the respective photoelectrons of O 1s (955 eV), Cd 3d5/2 (1082 eV), C 1s (1202 eV) and Si 2p (1388 eV) with the kinetic energy dependency ofE0.75. It is essential to use polymerized LB films and to confirm the surface structure of the film directly by suitable techniques such as atomic force microscopy, scanning electron microscopy or transmission electron microscopy.© 1997 by John Wiley & Sons, Ltd.

Phase transition in adsorbed monolayers of 2-hydroxyethyl laurate at the air-water interface

Abstract Kinetics of adsorption of a sparingly water soluble amphiphile, 2-hydroxyethyl laurate (2-HEL), at the air-water interface is followed by the measurement of surface pressure ( π ) with time ( t ). Simultaneous change in the surface morphology of the adsorbed monolayers is investigated with Brewster angle microscopy (BAM). A cusp point in each of the π – t curves indicates the start of a phase transition. Highly ordered condensed phase domains having stripes in their structures are observed to grow during the phase transition. One end of the separate lines of stripes gather to a point on the circular peripheral line of the domain like a vertex of the boojum. However, another end of the separate lines disappear at the peripheral line keeping the stripe width almost constant. After the completion of the phase transition, surface pressure starts to increase again and reaches a final equilibrium value after a long time. Spread monolayers of the same amphiphile can be obtained by the usual solvent spreading technique and compared with the adsorbed monolayers. The critical surface pressures necessary to cause the phase transition are almost the same for both adsorbed and spread monolayers. Although the shapes of the domains are circular for both cases, the size of those in the spread monolayers is considerably smaller exhibiting no internal stripes than those in adsorbed monolayers.

Orthorhombic structure of cadmium behenate monolayers on the water surface of a Langmuir trough detected by polarization modulation infrared spectroscopy

Abstract Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate the structure of behenic acid monolayers on the surface of aqueous Cd 2+ sub-phase in a Langmuir trough at the room temperature of 293 K. The PM-IRRAS spectra were recorded at representative points throughout 0.401∼0.100 nm 2 and the surface-pressure area isotherms were recorded during compression to these points. It is found that compressed cadmium behenate (CdB) monolayers possess an orthorhombic unit cell. On the other hand, single monolayers of CdB deposited on CaF 2 substrates at 20 mN m −1 possessed a triclinic parallel packing.

Polarized infrared study on the structure of two-dimensional nanoclusters of partially fluorinated long-chain fatty acid salts at ambient and elevated temperatures

Atomic force microscopy (AFM) images of the single monolayer of five partially fluorinated fatty acid salts [CF3(CF2)m(CH2)nCOO−]2Cd2+ [(m,n)=(7,10), (7,16), (7,22), (5,22), and (3,22)] transferred from aqueous Cd2+ subphase to solid substrate revealed that the molecules had assembled into monodispersed two-dimensional clusters of tens of nanometers, whose morphology developed systematically with (m,n). Polarized infrared measurements detected a corresponding change with (m,n) in the –(CF2)–m and –(CH2)–n orientation angles. It is found that the van der Waals interaction between the C–C–C trans zig–zag planes of adjacent hydrocarbon segments is the driving force for the cluster formation, while the overlapping interaction between the fluorocarbon tails of neighbor salt molecules is responsible for cluster compactness. Grazing incidence reflection absorption spectra of the (m,n)=(7,10) single monolayer recorded during temperature elevation from 25 to 150 °C show that heating has caused the hydrocarbon chai...

Robustness of monolayers on the solids; comparative studies on thermal, solvent, pH, and mechanical resistance among 1-layer LB films of cadmium arachidate and alkylchlorosilanes

Abstract The resistance of Langmuir–Blodgett (LB) monolayers of n -alkylchlorosilane compounds against the external disturbances has been investigated. The silane compounds used as film materials are trichloro(octadecyl)silane (TCOS), dichloro(methyl)(octadecyl)silane (DCMOS), and chloro(dimethyl)(octadecyl)silane (CDMOS). Cadmium arachidate (CdA) monolayers are also employed for comparison. FT-IR transmission and reflection absorption spectroscopic analysis of 5-layer LB films of the silanes indicates that the film molecules have residual silanol groups. The results reveal that the silane molecules can be fixed on the hydroxylic solid surfaces with covalent bonds after the monolayers are transferred on such substrates. Thermal stability and dissolution resistance of the LB monolayers on glass slides are investigated by the change of wavenumbers and intensities of CH 2 stretching bands in FT-IR transmission spectra. It becomes clear that the TCOS monolayers are very stable against heating, organic solvents and acid, whereas the DCMOS and CDMOS monolayers are very weak. The mechanical durability of the monolayers is investigated by following friction coefficient profiles. The results reveal that the friction coefficients for the silane monolayers begin to increase after a few transits under high normal loads although those for the CdA monolayers maintain lower values. These results can be interpreted from the view point of monolayer structures.

Substrate and chain length dependencies of the thermal behavior of [CF3(CF2)m(CH2)nCOO]2Cd single monolayers investigated by infrared reflection absorption spectroscopy

Temperature-variable grazing incidence reflection absorption (GIR) spectra were recorded for the single monolayer of [CF3(CF2)m(CH2)nCOO)]2Cd [(m,n)=(7,10), (7,16), (7,22), (5,22), and (3,22)], transferred from aqueous Cd2+ subphase to gold- and aluminum-evaporated glass substrates. The spectra reveal that these monolayers have better thermal stability on Al substrates than on Au. An “interaction band” is identified at 1484∼1480 cm−1, due to the νs(COO−) mode of carboxylate headgroups in ionic bonding with the Al surface. It is found that both the van der Waals interaction between the trans zig-zag hydrocarbon chains and the overlapping interaction between the fluorocarbon helixes are responsible for the systematic variation of the monolayer thermal behavior with (m,n). The thermal behavior of a single monolayer of cadmium stearate, serving as a model system, has been investigated to further confirm the spectral interpretation about the partially fluorinated monolayer. In addition, temperature-dependent f...

Temperature dependent dendritic domain shapes in Langmuir monolayers of tetradecanoyl N-ethanolamide at the air-water interface

The effect of temperature on the surface phase behavior of tetradecanoyl N-ethanolamide (NHEA-14) in Langmuir monolayers at the air-water interface has been investigated by film balance and Brewster angle microscopy (BAM). It has been observed that dendritic domains are formed in the coexistence region between liquid-expanded (LE) and liquid-condensed (LC) phases at different temperatures. At 10 and 15°C, the domains are four-armed dendrites having wide arms which have a tendency to be fractal while growing in size. At 20°C, five-armed dendritic domains are formed. At a temperature higher than 20°C, the domains are mainly six-armed dendrites having very narrow and sharp arms. The formation of dendritic domains should be due to the presence of interfacial hydrogen bonding among the head groups of the amphiphile. Increased dehydration of the head groups with an increase in the temperature should be responsible for the temperature dependency of the dendritic domain shapes in the monolayers of NHEA-14.

Effect of head groups on the phase transitions in Gibbs adsorption layers at the air–water interface

The adsorption kinetics and the surface phase behavior of four different amphiphiles, which are 2-hydroxyethyl laurate (2-HEL), dodecanoyl N-ethanolamide (NHEA-12), dodecanoyl N-methylethanolamide (NMEA-12) and tetradecanoyl N-methylethanolamide (NMEA-14), have been investigated at the air-water interface by film balance, surface tensiometer and Brewster angle microscopy (BAM). The former two amphiphiles show a first-order phase transition from a lower density liquid like phase to a higher density condensed phase in Gibbs adsorption layers. On the other hand, the latter two amphiphiles are unable to show such characteristics under any experimental conditions. The presence of a methyl group in the head group of NMEA-12 sterically hinders the molecules and resists the formation of any condensed phases. This steric hindrance is so high that even an increase in the chain length by two CH(2) groups in NMEA-14 does not allow the formation of condensed domains. Although, both 2-HEL and NHEA-12 are able to form the condensed phase, the domain morphology formed in these monolayers is different from each other. The domains of 2-HEL at lower temperatures are circular having a stripe texture, while those at higher temperatures show fingering patterns having uniform brightness. On the other hand, the domains of NHEA-12 are dendritic in shape. The presence of hydrogen bonding sites close to the interface should be responsible for the formation of such domains in NHEA-12.

Crystal lattice of the cadmium alkanoate monolayer at the air/water interface investigated by polarization modulation infrared spectroscopy

The crystal lattice of an arachidic acid/Cd2+ (CdA) monolayer on the surface of aqueous CdCl2 subphase in a Langmuir trough was investigated in situ by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The spectra reveal a hexagonal packing of the hydrocarbon chains on the subphase of pH=6.2 and 293–283 K. Cooling from 283 to 280 K induces the hexagonal unit cell to transform to an orthorhombic one, as evidenced by the splitting of the CH2 scissoring mode. This transition is observed to be reversible with temperature variation within 293–274 K. On the subphase of pH=8.4, the crystal lattice of CdA is orthorhombic even at 293 K. The temperature-dependent crystal lattice transition of CdA is compared with the monotonic hexagonal subcell of a cadmium stearate monolayer within 293–274 K and the orthorhombic subcell of a cadmium behenate monolayer within 293–274 K.