Adhesion of amphipathic molecules to solid surfaces

Summary Oil/water contact angles, coefficient of friction and electron diffraction have been used to study the adsorption of three long chain surface-active substances (a fatty acid, a sulphate and a substituted amine, from aqueous solution on to electropolished copper, aluminium and iron. According to the p_H of the solution, the adsorption occurs by one of the following processes (a) physical adsorption, (b) chemisorption, (c) adherence of a precipitate, (d) sensitised adsorption to form a mixed film. Certain applications e.g. lubrication and the prevention of fretting corrosion, are discussed in relation to the structure and rate of adsorption of the four types of film. The first requirement for an efficient lubricant film is the chemisorption of the long chain compound as a basic metal soap. The film is considerably strengthened by subsequent adsorption of a less polar compound, such as cholesterol, by process (d). A suitable vehicle for the two compounds is an oil/water emulsion.Electron diffraction has been used to determine the physical and chemical nature of the electropolished metal surfaces. Adsorbed monolayers can only be detected on extremely smooth surfaces. A new technique is described for depositing insoluble monolayers on reactive metals, which avoids the roughening of the metal surface experienced in the normal Langmuir-Blodgett method.

---

Zusammenfassung Randwinkel öl-Wasser, Reibungskoeffizient und Elektronenstrahlbeugung wurden benutzt zum Studium der Adsorption dreier langkettiger Substanzen (eine Fettsäure, ein Sulfat und ein substituiertes Amin) aus wäßriger Lösung heraus auf elektropoliertes Kupfer, Aluminium und Eisen. Entsprechend dem p_H-Wert der Lösung geschieht die Adsorption durch einen der folgenden Prozesse:a) Physikalische Adsorption, b) Chemisorption, c) Haftung eines Niederschlags und d) sensibilisierte Adsorption unter Bildung eines Mischfilms.Einige Anwendungen, z. B. Schmierung und die Verhinderung von Reibungskorrosion, wurden unter BerÜcksichtigung der Struktur und der Adsorptionsgeschwindigkeit der vier genannten Typen von Adsorptionsschichten diskutiert. Die erste Bedingung fÜr wirksame Schmierung ist die Chemisorption der langkettigen Substanz als basische, metallorganische VerdÜnnung (Seife). Der erhaltene Adsorptionsfilm erfährt eine beachtliche Verfestigung durch nachfolgende Adsorption (nach dem Prozeß d) einer schwächer polaren Substanz, wie des Cholesterols. Ein brauchbarer Träger fÜr beide Komponenten ist eine öl-Wasser-Emulsion.Zur Aufklärung der chemischen und physikalischen Eigenschaften der elektropolierten Metalloberflächen wurde die Elektronenstrahlbeugung eingesetzt. Adsorbierte Monoschichten können nur auf extrem ebenen Oberflächen beobachtet werden. Zur Verhinderung der Aufrauhung von Metalloberflächen, wie sie von der Langmuir-Blodgett-Methode her bekannt ist, wurde eine neue Auftragungstechnik fÜr unlösliche Monoschichten auf reaktionsfähige Metalloberflächen entwickelt.

---

---

This work was carried out during the tenure of Consolidated Zinc Studentships at Trinity Hall, Cambridge, on the part of two of the authors (R. B. W. & J. A. S.). Further financial support was provided by Almin Limited and C. S. I. R. O. (Australia).     

Packing of molecules on solid surfaces in physical adsorption

The packing of molecules in a monolayer under physical adsorption conditions has been analyzed.

---

.

     

Chiral close-packing of achiral star-shaped molecules on solid surfaces

From the interplay of scanning tunneling microscopy and theoretical calculations, we study the chiral self-assembly of achiral HtB-HBC molecules upon adsorption on the Cu(110) surface. We find that chirality is expressed at two different levels: a +/-5 degrees rotation of the molecular axis with respect to the close-packed direction of the Cu(110) substrate and a chiral close-packed arrangement expected for star-shaped molecules in 2D. Out of the four possible chiral expressions, only two are found to exist due the effect of van der Waals (vdW) interactions forcing the molecules to simultaneously adjust to the atomic template of the substrate geometry and self-assemble in a close-packed geometry.     

From smart polymer molecules to responsive nanostructured surfaces

A heteroarm star block copolymer made from seven polystyrene and seven poly(2-vinylpyridine) arms was grafted onto a solid substrate to fabricate a responsive polymer surface consisting of a densely packed monolayer of copolymer molecules. The grafted layer demonstrates a two-level hierarchical response upon external stimuli combining core-shell transitions of single stars with cooperative transitions of the interacting arms between "dimple" and "ripple" morphologies of the monolayer. The response allows for the switching of the surface properties upon changing solvent selectivity or pH of the aqueous environment.     

Neutral and charged polymer brushes at solid surfaces

Neutral and charged polymer brushes covalently attached to planar solid surfaces were generated by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. The brushes were characterized by FTIR-spectroscopy, optical waveguide-spectroscopy and Ellipsometry. Especially the film thicknesses of surface bound polyelectrolyte (PEL) monolayers were measured by optical waveguide spectroscopy (OWS) as a function of the humidity of the environment. The PEL brushes show strong increases in thickness as well as strong decrease of the refractive index of the surface attached layer due to water incorporation caused by the exposure to the humid environment. Additionally the behavior of neutral as well as charged brushes in contact with solvent was investigated by using multiple-angular-scans of ellipsometry in a total internal reflectance setup. The scaling behavior of the brush height as a function of the graft density of the attached polymer molecules was investigated for the neutral brush as well as for the PEL brush system.     

Equilibrium interaction of solid surfaces across a polymer melt

Forces across polymer melts are poorly understood despite their importance for adhesion and fabricating composite materials. Using an atomic force microscope (AFM), this interaction was measured for poly(dimethyl siloxane) (PDMS). The structure of the polymer at the surface changed during the first approximately 10 h. Afterward, short-range attractive forces were observed with short-chain PDMS (M(w) = 4200 g/mol). Using PDMS with a molecular weight (M(w) = 18 000 g/mol) above the entanglement limit, we measured a monotonically decaying repulsive force, which indicates that a quasi-immobilized layer had formed at the solid surface. Due to the small radius of curvature of the tip, forces could be measured in equilibrium.     

Novel self-organization of polymer particles on hydrophobic solid surfaces through hydrophobic interaction

A novel technique of particle monolayer fabrication based on hydrophobic interactions in aqueous systems is described in this paper. When alkylated glass plates modified with various silane coupling agents were immersed in aqueous dispersions of submicron-sized polystyrene particles of cationic or anionic surface charges, cationic particle monolayers containing active ester groups were effectively formed at the plate surfaces, whereas no anionic particles were self-organized on the plate surfaces. The coverage of the plates with cationic particles and the morphology of the monolayers varied with the hydrophobicities of the particles and plates as well as with the ionic strength of the medium and temperature. For less hydrophobic methylated glass surfaces modified with methyltriethoxysilane, cationic particles were self-organized at relatively regular intervals, whereas they were self-organized in the form of aggregates for the more hydrophobic octadecylated glass plates treated with n-octadecyltriethoxysilane. Closely packed monolayers were fabricated by adjusting ionic strength and temperature. Fluorescence labelling of cationic particle monolayers was successfully accomplished by the reaction of remaining active ester groups on the monolayers with a fluorescence probe containing amino groups. Cationic particle monolayers were physically stabilized by heating above the glass transition temperature (T _g) of the particles.     

High-yield activation of scaffold polymer surfaces to attach cell adhesion molecules

Zirconium tetra(tert-butoxide) reacts with surface amide groups of polyamide nylon 6/6 to give (eta(2)-amidate)zirconium complexes in high yield. These surface complexes react to bond the cell-adhesive peptide arginine-glycine-aspartic acid (RGD) to the polymer surface. A surface loading of 0.18 nmol/cm(2) of RGD is achieved, which is 20-1000 times higher than previously reported attainable on natural or synthetic polymers by other strategies. Approximately 40% of the nylon surface is covered by the RGD, which gives a surface that is both stable to hydrolysis and highly active for cell adhesion and spreading in vitro.     

Immobilization of cationic polymer particles having active ester groups onto solid surfaces

Monodispersed cationic polymer particles with sulfonium groups and active ester groups at their surfaces were prepared by emulsifier-free emulsion copolymerization of styrene (ST) with a water-soluble active ester monomer, methacryloyloxyphenyldimethylsulfonium methylsulfate (MAPDS). The cationic polymer particle monolayers were fabricated on unmodified and aminated glass plates by electrostatic interactions and chemical reactions, respectively. The polymer particles were immobilized onto unmodified glass plates at relatively regular intervals in the absence of electrolytes, and the morphology of particle monolayers on the glass plates was changed with solid content of latex, electrolyte and cationic surfactant concentration. The polymer particles were immobilized onto aminated glass plate as aggregates by controlling the pH of latex and electrolyte concentration. Remaining active ester groups of the particle monolayers were confirmed to react easily with primary amino compounds.     

Phase transition of short linear molecules adsorbed on solid surfaces from a density functional approach

A microscopic density functional theory is used to investigate the adsorption of short chains on strongly attractive solid surfaces. We analyze the structure of the adsorbed fluid and investigate how the layering transitions change with the change of the chain length and with relative strength of the fluid-solid interaction. The critical temperature of the first layering transition, rescaled by the bulk critical temperature, increases slightly with an increase of the chain length. We have found that for longer chains the layering transitions within consecutive layers are shifted toward very low temperatures and that their sequence is finally replaced by a single transition.     

On the interplay between chemical reactions and phase transitions for molecules adsorbed on solid surfaces

We use Monte Carlo simulations to examine the manner in which reagent aggregation affects the reaction rate between molecules adsorbed on a solid surface. We discuss the temperature and concentration dependence of the rate of product formation.     

Effect of orientation and mobility of polymer molecules at surfaces on contact angle and its hysteresis

The contact angle of a water droplet on the surface of a solid polymer or hydrogel (water-swollen three-dimensional network) depends on whether a hydrophilic moiety of the polymer molecule is oriented towards the air interface or towards the bulk of the solid, but not on the hydrophilicity of the molecule. Therefore, the short-range rotational mobility of a polymer molecule has a major influence on the apparent hydrophilicity of a polymer surface as measured by the contact angle of water. By the came principle, the abnormally large hysteresis effect observed in advancing and receding contact angles of water on some polymer surfaces can be attributed to the reorientation of hydrophilic moieties of polymer molecules at the surface. These factors are demonstrated by selected polymer surfaces with different degrees of mobility at the polymer-air interface.     

MMX polymer chains on surfaces

Fibres of [Ru(2)Br(micro-O(2)CEt)4]n polymer have been isolated on different surfaces under specific conditions, and morphologically characterised by AFM and STM, showing an unexpected helical internal structure.     

Sorption of organic molecules on surfaces of a microporous polymer adsorbent modified with different quantities of uracil

The sorption of organic molecules on the surfaces of a number of adsorbents based on a microporous copolymer of styrene and divinylbenzene modified with different quantities of uracil is studied by means of inverse gas chromatography at infinite dilution. Samples containing 10^–6, 10^–5, 10^–4, 10^–3, 10^–2, and 0.5 × 10^?1 weight parts of uracil (the рС of uracil ranges from 1.3 to 6) are studied. The contributions from different intermolecular interactions to the Helmholtz energy of sorption are calculated via the linear free energy relationship. It is found that as the concentration of uracil on the surface of the polymer adsorbent grows, the contributions from different intermolecular interactions and the conventional polarity of the surface have a bend at рС = 3, due probably to the formation of a supramolecular structure of uracil. Based on the obtained results, it is concluded that the formation of the supramolecular structure of uracil on the surface of the polymer adsorbent starts when рС < 3.     

Spreading of liquid drops over dry surfaces

Spreading of thin, axisymmetric, non-volatile, Newtonian liquid drops over a dry, smooth, flat solid surface is considered both theoretically and experimentally in the case of complete wetting. The drop profile is solved analytically by matching the “outer” solution for large film thicknesses, where only the capillary effects are important, with the “inner” solution for small film thicknesses, where the viscous and disjoining pressure effects are comparable to capillary effects. It is shown that the apparent radius of the wetted spot, the apex height of the drop, and the apparent advancing dynamic contact angle follow different power laws in time and the advancing dynamic contact angle follows a power law in capillary number. Both the prefactor and the exponent of each power law are derived theoretically. Good agreement between the theory predictions and experimental measurements is shown for both the prefactor and exponent of each power law. It is necessary to emphasize that the theory suggested does not include any fitting parameters.     

Interaction between solid surfaces in a polymer melt studied by atomic force microscopy

Using an atomic force microscope (AFM) the interaction between an AFM tip and a planar silicon oxide surface has been measured across poly(dimethylsiloxane) (PDMS, M_W = 18 000). Due to the small radius of curvature of the AFM tip the hydrodynamic repulsion of the tip was negligible and forces could be measured in equilibrium. This is confirmed by the fact that force-versus-distance curves measured at different approaching velocities were indistinguishable. In equilibrium a repulsive force was observed which could best be described by a power law, F ∝ 1/d^2.5 where d is the distance.     

Silicon‐modified surfactants and wetting: IV. Spreading behaviour of trisiloxane surfactants on energetically different solid surfaces

The spreading behaviour of defined trisiloxane surfactants of general formula [(CH₃)₃SiO]₂ CH₃Si(CH₂)₃(OCH₂CH₂) _nOCH₃ (n = 3–9) on five different solid surfaces has been investigated. Maximum spreading areas and rates are found on non‐polar or slightly polar surfaces of 30 to 40 mN m⁻¹ surface energy. Extremely low or high surface energies substantially reduce the spreading rates. On non‐polar surfaces rapid spreading is observed for 1 wt % solutions of the relatively short‐chained penta‐ and hexa‐ethylene glycol derivatives. On slightly polar surfaces dilute 0.1 wt % solutions of longer‐chained derivatives spread faster. This spreading pattern shift coincides with a change of the phase behaviour. Solutions of Silwet L77 do not prefer one specific surface, since 1 wt % solutions abruptly stop spreading after a few seconds and the maximum spreading rates are found for 0.1 wt % solutions. Therefore, Silwet L77 essentially belongs among the long‐chained derivatives. Copyright © 2000 John Wiley & Sons, Ltd.     

Spreading of non-Newtonian liquids over solid substrates

The spreading of drops of a non-Newtonian liquid (Ostwald-de Waele liquid) over horizontal solid substrates is theoretically investigated in the case of complete wetting and small dynamic contact angles. Both gravitational and capillary regimes of spreading are considered. The evolution equation deduced for the shape of the spreading drops has self-similar solutions, which allows obtaining spreading laws for both gravitational and capillary regimes of spreading. In the gravitational regime case of spreading the profile of the spreading drop is provided.