Jacob Sagiv

On the formation and structure of self-assembling monolayers. I. A comparative atr-wettability study of Langmuir—Blodgett and adsorbed films on flat substrates and glass microbeads

Abstract Organized oleophobic monolayers of several long chain compounds and steroid derivatives produced on flat solid substrates by spontaneous adsorption from organic solutions are compared with Langmuir—Blodgett (LB) monolayers transferred on identical substrates from the water-air interface. Quantitative infrared ATR and polarized ATR spectroscopy, and wettability measurements are used to correlate the various films and to determine their molecular density and orientation, mode of film-to-surface binding, and other structural characteristics. Formation of oleophobic adsorbed monolayers on a model powder substrate—smooth glass microbeads—is also investigated. It is concluded that, irrespective of the mode of film-to-surface binding (ionic, covalent, or hydrogen bonding), and the nature of the substrate (Ge, Si, ZnSe, glass slides, glass microbeads), saturation of the adsorption leads in all studied systems to the formation of tightly packed and highly oriented monolayers, structurally equivalent to LB monolayers of same or similar compounds deposited on the bare surfaces of the respective substrates. These findings are interpreted in terms of a cooperative surface process leading to aggregation of molecules into a characteristic “monolayer phase.” Significant structural differences may develop in LB built-up films thicker than one monolayer. A mechanism for the formation of covalently bonded silane monolayers is proposed.

On the formation and structure of self-assembling monolayers. II: A comparative study of Langmuir-Blodgett and adsorbed films using ellipsometry and IR reflection-absorption spectroscopy

Abstract Ellipsometry and infrared reflection-absorption (RA) spectroscopy on aluminium mirrors provide independent evidence confirming the basic structural similarity of Langmuir-Blodgett (LB) and adsorbed monolayers. The systems presently analyzed comprise a homologous series of saturated fatty acids, a saturated long chain trichlorosilane, a vinylic long chain acid, and a vinylic long chain trichlorosilane. Conclusive evidence is obtained from RA spectra of adsorbed monolayers of a long chain acid and a long chain silane for the independence of the chain orientation in such monolayers on the specific mode of film-to-surface binding. The orientation of the chains is found to be essentially perpendicular, but not perfect. The unique value of RA spectroscopy in the detection of order-disorder transformations accompanying interactions of a highly oriented monolayer with physical or chemical external agents is demonstrated. Covalently bonded silane monolayers are shown to be perfectly stable under conditions causing a major deterioration of the structure of adsorbed fatty acid films. The adsorption of fatty acids on aluminium is found to involve physical forces only. Under the conditions of the present experiments ellipsometry is found to be of limited value as a routine analytical tool in the characterization of organic monolayer films. This has to do with the hypersensitivity of this technique to a large number of factors, part of which are difficult to control and reproduce.

Adsorbed monolayers versus Langmuir-Blodgett monolayers—Why and how? I: From monolayer to multilayer, by adsorption☆

A new approach to the formation of oriented monolayers and the assembling of built-up multilayer films is presented. The new method is fundamentally different from the Langmuir-Blodgett (LB) procedure, being based on the spontaneous occurrence of self-organization of amphiphiles at liquid-solid interfaces. Under suitable conditions monolayers may be obtained by direct adsorption (or chemisorption) on polar solid substrates contacting an amphiphile solution. The formation of compact oriented monolayers by this method is a spontaneous self-controlled process; there are no mechanical manipulation steps equivalent to the compression and transfer inherent in the LB procedure. Multilayer films can be built up monolayer by monolayer via a two-step sequence involving monolayer formation at the liquid-solid interface followed by chemical activation of the exposed monolayer outer surface. The new method is demonstrated with a monolayer system based on a bifunctional silane-terminal ethylenic double-bond surfactant. Although the present approach and the classical LB method may be regarded as complementary rather than mutually exclusive, the new approach is in a way superior to the LB method as it demonstrates a more advanced principle, namely utilization of molecular self-organization for the purpose of constructing planned supermolecular structures. On the practical level the new method offers certain unique advantages that might prove of particular importance in a number of special applications, such as device fabrication.

Coverage of Si substrates by self-assembling monolayers and multilayers as measured by IR, wettability and X-ray diffraction☆

Abstract A study is presented of the amount of coverage of Si substrates by monolayers and multilayers of molecules deposited by the self-assembling technique. Self-assembly was achieved by chemisorption of silane compounds from solutions, on to smooth n-Si substrates. The coverage was examined by IR absorption, wettability and X-ray diffraction. For n-octadecyltrichlorosilane (OTS), prepared as a single layer, the coverage appears to be close to 100%. For a monolayer of a silane-methyl ester, containing 24 carbons (C24SME), the coverage is at least 90%. A film comprising three layers of C24SME molecules could be modeled by a mixture of two- and three-layer regions.

On the formation and structure of self-assembling monolayers: III. Time of formation, solvent retention, and release

Abstract FTIR spectroscopy (in the ATR mode) and contact angle measurements are used to determine the characteristic time scales of formation of some representative oleophobic monolayers produced on polar solids by spontaneous adsorption from organic solutions, and to investigate the mechanis of solvent incorporation into such organized films during their formation and its subsequent release. The more general question of adsorption of low-energy surfaces, not wetted by the bulk liquid adsorbate, is also addressed, in connection with the present observations of solvent retention on surfaces coated with nonwettable oleophobic monolayers. Two amphiphile/solid systems and two saturated hydrocarbon solvents were selected for this study, with the purpose of providing examples for different modes of binding to the surface and different geometrical relationships between solvent and amphiphile molecules. Ionic binding (arachidic acid/ZnSe) is found to yield practically complete monolayers within immersoin times not longer than 15 s, whereas several minutes are required for the formation of complete monolayers involving covalent binding to the surface ( n -octadecyltrichlorosilane/Si). Solvent incorporation into the films during their formation is facilitated by geometrical matching between the solvent and the non polar portion of the amphiphile, as indicated by the observed differences between n -hexadecane and bicyclohexyl in this respect. Most of the solvent is usually squeezed out of the inner core of the film and replaced by amphiphile, with the completion of a compact and continuous monolayer of the latter. However, in some of the studied systems, variable amounts of solvent were found to be retained on top of the outer monolayer surface after its separation from the bulk liquid phase. Liquid retention on such nonwettable surfaces appears to involve adhesion at microscopic defect sites in the film. It is, finally, demonstrated that high qualtiy, solvent-free monolayers may be obtained by adsorption from organic solutions, provided geometrical matching between solvent and amphiphile is avoided and amphiphiles capable of irreversible binding to the surface are employed.

Hydrogen-bonded multilayers of self-assembling silanes: structure elucidation by combined Fourier transform infra-red spectroscopy and X-ray scattering techniques

Abstract Hydrogen-bonded multilayer stacks of laterally interconnected long-chain silanes are a new class of synthetic self-assembling thin film organizates endowed with a somewhat unusual combination of structural and dynamic characteristics. In this paper we present experimental results obtained from a combined Fourier transform infra-red (FTIR) spectroscopic and X-ray scattering study, on the basis of which it is possible to derive a rather detailed picture of some of the main features of the microstructure of these novel multilayer films and their monolayer precursors. The films are shown to be composed of discrete monolayers, coupled to each other in a flexible, non-epitaxial manner, via interlayer multiple hydrogen bonds. The hydrocarbon tails assume a perpendicular average orientation on the layer planes and form a ‘rotator phase’-like hexagonal lattice with a lateral packing density of ca. 21 A 2 per molecule and a positional coherence length of ca.70 A. Extensive lateral coupling of the silane head groups appears to be responsible for the high structural robustness and defect self-healing capability of these films, while the interlayer hydrogen bonding accounts for the facile post-assembly intercalation of various polar guest species into their vertically expandable interlayer polar regions. As the SiO bond is too short to permit extensive intralayer polymerization under the steric constraints imposed by the compact packing of the perpendicularly oriented hydrocarbon tails, the observed high interconnectivity of the silane head groups is rationalized in terms of a dynamic equilibrium model involving continuous redistribution of the SiO bonds within a two-dimensional network of oligomeric siloxane and silanol species. This model of dynamic equilibriation of siloxane linkages can also help to explain other intriguing properties of such silane monolayers.

Adsorbed monolayers versus Langmuir-Blodgett monolayers—why and how? II: Characterization of built-up films constructed by stepwise adsorption of individual monolayers☆

Abstract The formation of oriented monolayer and multilayer films prepared by the two- step chemical method described in Part I of this series of papers is studied using contact angle measurements, ellipsometry, and IR internal and external multiple-reflection spectroscopy. The film growing process and the quality of the resulting built-up films are shown to be critically dependent on the degree of structural perfection attainable in the first adsorbed monolayer, which is, in turn, strongly dependent on the hydrocarbon chain length of the film-forming surfactant. Trends observed within a series of four bifunctional silane surfactants with hydrocarbon chain lengths in the range from 11 to 16 carbon atoms suggest that surfactants longer by an additional 2–4 carbon atoms may be expected to yield high quality multilayer structures regardless of the total number of superimposed monolayers.

Postassembly Chemical Modification of a Highly Ordered Organosilane Multilayer: New Insights into the Structure, Bonding, and Dynamics of Self-Assembling Silane Monolayers

Experimental evidence derived from a comprehensive study of a self-assembled organosilane multilayer film system undergoing a process of postassembly chemical modification that affects interlayer-located polar groups of the constituent molecules while preserving its overall molecular architecture allows a quantitative evaluation of both the degree of intralayer polymerization and that of interlayer covalent bonding of the silane headgroups in a highly ordered layer assembly of this type. The investigated system consists of a layer-by-layer assembled multilayer of a bifunctional n-alkyl silane with terminal alcohol group that is in situ converted, via a wet chemical oxidation process conducted on the entire multilayer, to the corresponding carboxylic acid function. A combined chemical-structural analysis of data furnished by four different techniques, Fourier transform infrared spectroscopy (FTIR), synchrotron X-ray scattering, X-ray photoelectron spectroscopy (XPS), and contact angle measurements, demonstrates that the highly ordered 3D molecular arrangement of the initial alcohol-silane multilayer stack is well preserved upon virtually quantitative conversion of the alcohol to carboxylic acid and the concomitant irreversible cleavage of interlayer covalent bonds. Thus, the correlation of quantitative chemical and structural data obtained from such unreacted and fully reacted film samples offers an unprecedented experimental framework within which it becomes possible to differentiate between intralayer and interlayer covalent bonding. In addition, the use of a sufficiently thick multilayer effectively eliminates the interfering contributions of the underlying silicon oxide substrate to both the X-ray scattering and XPS data. The present findings contribute a firm experimental basis to the elucidation of the self-assembly mechanism, the molecular organization, and the modes and dynamics of intra- and interlayer bonding prevailing in highly ordered organosilane films; with further implications for the rational exploitation of some of the unique options such supramolecular surface entities can offer in the advancement of a chemical nanofabrication methodology.

Langmuir Monolayers Designed for the Oriented Growth of Glycine and Sodium Chloride Crystals at Air/Water Interfaces

Abstract Oriented growth of crystals of α-glycine and sodium chloride under compressed Langmuir monolayers at air-water interfaces was achieved. For α-glycine, a variety of monolayers containing resolved glycyl head groups and different hydrophobic moieties were used, thus establishing the structural requirements necessary for oriented crystallization. For sodium chloride, monolayers of positive, negative and zwitterionic charge induced crystal nucleation from faces of the type {100}, {110} and {111}, the latter two faces not being naturally occurring.

Rotational and state‐resolved translational distributions of NO scattered from organized amphiphilic monolayers

Two‐photon ionization has been used to probe NO scattered from two different long chain organic amphiphiles. Rotational and state‐resolved translational distributions were obtained. The results show that there is a large difference in the dynamics of scattering from an unsubstituted aliphatic chain as compared to a monolayer in which the exposed end has been perfluorinated. NO scattered from the latter is more energetic both rotationally, and translationally. This effect becomes particularly noticeable as the incident energy of the NO is raised. The results can be explained by a mechanism which ignores the weak NO–surface potential and treats only the differences in rigidity and phonon modes in the two monolayers.