Showing papers in "Langmuir in 1989"

Structure and reactivity of alkylsiloxane monolayers formed by reaction of alkyltrichlorosilanes on silicon substrates

Abstract: : Long-chain alkyltrichlorosilanes, C13Si (CH2)nX, adsorb from solution onto silicon-silicon dioxide (Si/SiO2) substrates and form ordered alkylsiloxane monolayer films. These films were characterized by wettability, ellipsometry, and XPS. Except for very short chains (n = 0,1,2), the wetting of these monolayers was approximately independent of chain length. The presence of small amounts of water was necessary for the formation of these films. The alkylsiloxane monolayers were stable in common organic solvents, water, and acid, but were destroyed by prolonged exposure to base. Simple reactions on vinyl-terminated monolayers generated alcohol-, carboxylic acid-, and bromine- terminated films whose contact angles were lower than the starting monolayers, but whose lengths were largely unchanged. Measurements of the contact angle of acid-terminated interfaces as a function of pH indicated that ionization of soluble carboxylic acids. Monolayers containing mixtures of methyl- and carboxyl-functionalities exhibited wetting properties that mirrored the composition of the interface.

An isotherm equation for adsorption on fractal surfaces of heterogeneous porous materials

Abstract: Determination d'une equation pour le calcul des isothermes d'adsorption sur des surfaces fractales de materiaux poreux avec une certaine distribution de dimension des pores

Comparison of self-assembled monolayers on gold: coadsorption of thiols and disulfides

Abstract: : Ordered, organic monolayers were formed on gold slides by adsorption from ethanol of HS(CH2)10CH2OH, HS(CH2)10CH3, S(CH2)10CH2OH2, S(CH2)10Ch32, and of binary mixtures of these molecules in which one component was terminated by a hydrophobic methyl group and one by a hydrophilic alcohol group. The compositions of the monolayers were determined by X-ray photoelectron spectroscopy (XPS). Wettability was used as a probe of the chemical composition and structure of the surface of the monolayer. When monolayers were formed in solutions containing mixtures of a thiol and a disulfide, adsorption of the thiol was strongly preferred (approx. 75:1). Monolayer, Gold, Surface, Contact angles, Thiols, Disulfides.

A Study by Contact Angle of the Acid-Base Behavior of Monolayers Containing W-Mercaptocarboxylic Acids Absorbed on Gold: An Example of Reactive Spreading

Abstract: : Monolayers formed by the spontaneous assembly of organic thiols from solution onto gold provide a high degree of control over the chemistry and structure of an organic surface. In this paper we exploit the flexibility of these self-assembled monolayers to study the influence of the polarity of an interface on the acidity of carboxylic acids. Methyl-terminated thiols, such as HS(CH2)10CH3, adsorb from solution onto gold and form non-wettable, hydrophobic surfaces composed predominantly of nonpolar methyl groups. Thiols terminated by carboxylic acid groups form hydrophilic monolayers that are wetted by water. By coadsorbing mixtures of HS(CH2)10CH3 and HS(CH2)10CO2H in various mole fractions, we can synthesize surfaces in which the carboxylic acid groups are surrounded largely by nonpolar methyl groups, by other polar acid groups, or by mixtures of the two. This system allows us to examine the effect on the advancing contact angle of a chemical reaction occurring at the interface between a drop and a surface- in this case, neutralization of a carboxylic acid by aqueous base. Reactive spreading appears to be complex and may lead to different wetting behavior for the same chemical reaction on different substrates. A knowledge of the acid-base properties of carboxylic acids and other organic acids and bases at solid-liquid interfaces is important for understanding phenomena as diverse as the stability of colloids, the folding of proteins and the mechanisms of catalysis by enzymes. Keywords: Ethanols; Polarity; Polyethylene carboxylic acids; Contact angles; Reactive spreading; Titration.

Contact angle hysteresis in oxygen plasma treated poly(tetrafluoroethylene)

Abstract: Effets en fonction du temps de traitement, evalues par spectrometrie electronique RX et mesure de l'angle de contact. Introduction de groupements oxygenes pour des traitements courts; modifications profondes de surface sans modification chimique pour temps plus longs

Rheological properties of semidilute and concentrated aqueous solutions of cetyltrimethylammonium bromide in the presence of potassium bromide

Abstract: Les proprietes rheologiques des solutions aqueuses de bromure de cetyltrimethylammonium en presence de 0,25 M de KBr, sont bien en accord avec le modele de Cates qui prend en compte la variation de la dimension des micelles

Packing and Molecular Orientation of Alkanethiol Monolayers on Gold Surfaces

Abstract: Preliminary calculations based on a simple model give a good description of the molecular orientation and packing of akanethiol monolayers on gold surfaces. These calculations suggest that alkanethiol molecules on gold have a total alkyl chain axis tilt of approximately 38' in a plane that bisects the methylene H-C-H angles, followed by a rotation about the alkyl chain axis of -46'. The alkyl chain tilt is a function of the sulfur-sulfur spacing in a hexagonal crystal lattice layer and maximizes the attractive interactions between neighboring molecules. These results are in agreement with the molecular orientations obtained from grazing angle FTIR experiments on dodecanethiol monolayers on a (111) gold surface.

Temperature dependence of gas adsorption on a mesoporous solid: capillary criticality and hysteresis

Abstract: Etude de l'adsorption de Xe sur le Vycor afin de mesurer la dimension et la structure des pores

Formation of multilayers by self-assembly

Abstract: Monolayer and multilayer films were formed by self-assembly of methyl 23-(trichlorosilyl)tricosanoate (1) from organic solution. In agreement with published results, this compound was found to form good quality, close-packed monolayers on silicon surfaces. We have, however, found that films significantly thicker than the three monolayers previously obtained can be formed from continued chemisorption of monolayers of this compound followed by reduction of the surface ester with LiAlH4 in THF to form an “alcohol surface”. The quality of monolayer formation in the multilayer films was monitored in detail by ellipsometry, contact angle, and FTIR measurements, and, although generally increasing disorder can be detected, films of up to 25 discrete monolayers can be successfully made. These results indicate that self-assembly is a viable alternative to the Langmuir-Blodgett transfer technique for the construction of relatively thick (0.1-hm scale), ordered, multilayer films. In recent years organized molecular systems have attracted growing attention. The techniques which are presently available for the construction of such systems include both Langmuir-Blodgett (LB)’ and self-assembly methods, by which ordered, monomolecular layers can be formed on hydrophilic surfaces. These systems are believed to have technological potential in both optical and molecular e1ectronics.l They allow the chemist to potentially design new organic materials at a molecular level by incorporating useful functional groups into such systems and controlling such variables as the spacing of these groups within and between monomolecular layers. Although the LB method has been studied intermittently for many years and has been found successful for the formation of an extremely wide variety of monolayer and multilayer films (including relatively thick films of even several hundred layers), the SA method offers important advantages for future applications in such areas as molecular electronics and optical applications. The use of derivatives of alkyltrichlorosilanes (e.g., octadecyltrichlorosilane, OTS) results in monomolecular layers which are durable, thermally stable! and resistant to degradation by a variety of strong reagents.%l6 The trichlorosilyl head group forms covalent bonds to the hydrophilic surface as well as cross-links to adjacent molecules via Si-0 bonds created upon hydrolysis with trace water. This method can be adapted to the formation of multilayers by designing monolayers containing terminal functional groups that can be treated with various reagents to unmask a fresh, hydrophilic surface upon which a succeeding monolayer can be adsorbed.’J’

Friction and wear of self-assembled trichlorosilane monolayer films on silicon

Abstract: Etude par ellipsometrie, microscopie optique et mesure des angles de contact, des couches monomoleculaires d'alkyltrichlorosilane sur le silicium

Particle sizes and electrophoretic mobilities of poly(N-isopropylacrylamide) latex

Abstract: Les dimensions des particules et les mobilites electrophoretiques ont ete mesurees en fonction de la temperature. Les diametres des particules diminuent de 788 nm pour une temperature de 10°C a 380 nm pour une temperature de 50°C. Inversement, la mobilite electrophoretique augmente avec la temperature

Adsorption of benzene, toluene, and chlorobenzene on titanium dioxide

Abstract: Determination des isothermes d'adsorption et etude de la structure superficielle de TiO 2 par spectrometrie IR

Surface chemistry of zirconia polymorphs

Abstract: Etude de la structure superficielle de ZrO par adsorption de NH 3 et de pyridyne. Determination des sites acides

Photochemistry of Colloidal Semiconductors. 30. HPLC Investigation of Small CdS Particles

Abstract: HPLC is used to determine the size distribution of CdS colloids These distributions agree with those obtained from electron microscopy The method is applied to recognize “magic” agglomeration numbers and to follow thermal particle growth and photodissolution In the thermal growth a gradual increase in size is ascribed to Ostwald ripening However, in the beginning a rather abrupt increase in size takes place that is ascribed to particle combination In the photodissolution of a sample containing two size distributions, the larger particles disappear more rapidly than the smaller ones, this effect being particularly pronounced when light of longer wavelengths is used where the small particles have little absorption

Curvature elasticity of charged membranes

Abstract: Etude theorique sur l'energie libre des doubles couches diffuses dans les membranes chargees

Mechanism of electrochemical activation of carbon electrodes: role of graphite lattice defects

Abstract: By use of Raman spectroscopy as a probe, the relationship between carbon microstructure and increases in the heterogeneous electron-transfer rate for carbon electrodes was examined. A distinctive Raman band at 1360/cm is proportional to the density of graphitic edge planes and may be used to monitor changes in edge plane density induced by carbon pretreatment procedures. It was shown that electrochemical oxidation of highly ordered pyrolytic graphite (HOPG) caused fracturing of the graphite lattice, thus increasing edge plane density. This result is consistent with other reports from laser activation of HOPG, which correlate increased edge plane density with increased electron-transfer rate. Creation of edge plane is a phenomenon common to both oxidative and nonoxidative activation procedures and is responsible for HOPG activation. Arguments about the involvement of graphitic oxide or oxygen containing functional groups in electron-transfer activation are presented. After the present results are combined with those from the literature, it appears unlikely the oxygen functional groups are involved in electron-transfer activation of several benchmark redox systems on carbon electrodes.

Micelle formation of detergent molecules in aqueous media. 3. Viscoelastic properties of aqueous cetyltrimethylammonium bromide-salicylic acid solutions

Abstract: Etude de l'influence de l'addition de NaBr sur les proprietes viscoelastiques du melange eau/bromure de cetyltrimethylammonium

Measurement of film rigidity and interfacial tensions in several ionic surfactant-oil-water microemulsion systems

Abstract: Etude de la microstructure des emulsions eau/alcane/agent de surface anionique (dodecylsulfate de sodium, aerosol OT, hexadecylbenzenesulfonate de sodium) ou agent de surface cationique (bromure d'hexadecyltrimethylammonium et bromure de dodecyltrimethylammonium)

Surface characterization of calcium hydroxylapatite by Fourier transform infrared spectroscopy

Abstract: Etude de la structure superficielle de l'hydroxy-apatite apres traitement thermique et adsorption d'eau

The role of free surfactant in destabilizing oil-in-water emulsions

Abstract: Etude de la floculation d'une emulsion huile-eau par addition d'un exces d'agent de surface

UV absorption spectra of azobenzene-containing long-chain fatty acids and their barium salts in spread monolayers and Langmuir-Blodgett films

Abstract: L'influence de la pression superficielle sur la transition de phase (gel colloidal-cristal liquide) des couches monomoleculaires d'azobenzene contenant des acides gras a la surface de l'eau est etudiee

Collisions between point masses and fractal aggregates

Abstract: La simulation par ordinateur est utilisee pour etudier les collisions entre des agregats de particules

Magnetic water treatment: the effect of iron on calcium carbonate nucleation and growth

Abstract: Magnetic water treatment devices have been used for many years to prevent scale, primarily CaC03, from depositing in pipes and boilers. This treatment technique remains controversial, mostly b e c a u s e there is not an accepted scientific explanation for how the devices might work-if indeed they work at all. We have investigated several mechanisms via which magnetic water treatment devices could conceivably work. All involve iron as a key intermediate. Experiments were conducted to test the hypothesis that a particular crystal form of ferric hydroxide effectively induces the heterogeneous nucleation of CaC03, thus providing competitive sites for CaC03 precipitation and significantly reducing the amount of scale being deposited. We find that this mechanism is unlikely since none of the seven different synthetic ferric hydroxides tested was an effective heterogeneous nucleator of CaCOS. However, we found that trace concentrations of Fe2+ strongly inhibit calcite growth but not aragonite growth and that trace concentrations of Fe2+ also inhibit the transformation of aragonite into calcite. A similar effect was observed with Fe3+ but to a lesser degree. These experimental resulta suggest that m netic water treatment devices may be effective only to the

Infrared and fluorescence spectroscopic studies of self-assembled n-alkanoic acid monolayers

Abstract: : Infrared and fluorescence spectroscopic methods were used to study the kinetics and thermodynamics of the formation of self-assembled films of n- alkanoic acids by adsorption from solutions. The adsorption of stearic acid from hexadecane solutions onto glass and aluminum substrates was shown to lead to monolayer formation. A Langmuir-type transient adsorption model was shown to be applicable to these systems. The stationary fluorescence spectroscopy of a pyrene-labeled alkanoic acid probe was also used to determine the relative values of the kinetic constants of various fatty acids having different number of carbons in the chains. A linear increase of approximately 230 cal/mole in the negative free energy of adsorption with increasing chain length of the fatty acids was found. This attributed to the energetic contribution of the molecular organization of the aliphatic chains to the self-assembly process.

Binding constants of .beta.-cyclodextrin/surfactant inclusion by conductivity measurements

Abstract: Formation d'un complexe d'inclusion entre la β. cyclodextrine et differents agents de surface anioniques

Specific ion effects in electrical double layers: selective solubilization of cations in aerosol-OT reversed micelles

Abstract: Etude de la variation de la constante dielectrique lors de la solubilisation de cations de metaux alcalins ou alcalinoterreux dans des micelles inverses d'Aerosol OT