Capillary effects of surfactants at curved interfaces

The influence of surfactant on the solvent vapour pressure and equilibrium states in open capillary systems has been considered both in the presence and absence of gravity. The relationships have been derived describing a change in vapour pressure in the course of surfactant adsorption in closed one-phase and two-phase capillary systems. The stability conditions for such systems are discussed as related to surface tension, surface elasticity, surfactant adsorption and concentration, and the interfacial curvature.     

Direct visualization of mesh structures at solid/solution interfaces by atomic force microscopy

The formation of adsorbed surfactant layers consisting of a mesh or network of branched cylindrical aggregates on muscovite mica by several surfactant systems is described. The curvature of the adsorbed aggregates is varied by a variety of mechanisms that all generate morphologies between adsorbed cylinders and bilayers, and the resulting lateral structure is imaged by "soft contact" atomic force microscopy. We compare the direct images and Fourier transforms of the adsorbed layer structures, and relate them to those formed in bulk solution.     

Assembly and electrochemical characterization of nanometer-scale electrode|solid electrolyte interfaces

A technique is herein described for the assembly and characterization of nanometer-scale metal electrode|solid electrolyte interfaces of variable dimensions. The specific system examined in this work involves a sharp Pt tip attached to the piezo-driven head of a scanning tunneling microscope (STM) allowing the tip to be inserted into (or retrieved from) a Nafion membrane placed normal to the direction of tip travel. The actual Pt|Nafion area of contact was determined by coulometric analysis of the characteristic voltammetric features of Pt, using the tip as the working electrode and a much larger Pt gauze attached to the other side of the Nafion as a counter-reference electrode, yielding for some of the interfaces examined values equivalent to as low as 35 000 Pt surface atoms. This rather versatile arrangement allows experiments to be performed in both inert (Ar) and reactive atmospheres, such as oxygen or hydrogen on either or both sides of the membrane, under controlled humidity conditions, and thus sheds light into such phenomena as changes in the overall faradaic currents induced by plastic deformations of the Nafion as well as fundamental aspects of mass transport at reactant gas|Pt|Nafion three-boundary interfaces of relevance to polymer electrolyte fuel cells (PEFCs).     

Salting out of hydrophobic ions at immisicble electrolyte interfaces

The increase in the polarisation window of electrified electrolyte interfaces due to salting out effects of the organic electrolyte has been studied. It is shown that the salting out theory of Kirkwood can be applied to these systems and that the changes in the transfer potentials of large hydrophobic ions with the concentration of aqueous electrolytes is in reasonable agreement with the known molecular dimensions of the ions studied.     

DNA adsorption at liquid/solid interfaces

DNA adsorption on solid or liquid surfaces is a topic of broad fundamental and applied interest. Here, we study by X-ray reflectivity the adsorption of monodisperse double-stranded DNA molecules on a positively charged surface, obtained through chemical grafting of a homogeneous organic monomolecular layer of N-(2-aminoethyl) dodecanamide on an oxide-free monocrystalline Si(111) wafer. The adsorbed dsDNA is found to embed into the soft monolayer, which is deformed in the process. The surface coverage is very high, and this adsorbed layer is expected to display 2D nematic ordering.     

Globular proteins at solid/liquid interfaces

Seven years have passed since one of us (W.N.) published the last comprehensive review on the mechanism of globular protein adsorption to solid/water interfaces. Since that time, annual contributions to the field have steadily increased and substantial progress has been made in a number of important areas. This review takes a fresh look at the driving force for protein adsorption by combining recent advances with key results from the past. The analysis indicates that four effects, namely structural rearrangements in the protein molecule, dehydration of (parts of) the sorbent surface, redistribution of charged groups in the interfacial layer, and protein surface polarity usually make the primary contributions to the overall adsorption behavior.     

Direct observation of layered structures at ionic liquid/solid interfaces by using frequency-modulation atomic force microscopy

Presence of inhomogeneous layered structures of ionic liquid (IL) molecules at IL/HOPG and IL/mica interfaces was directly detected and imaged by using frequency-modulation atomic force microscopy. High stability of the layered structures may disturb their interface applications to catalysis and electrochemistry.     

Modelling the electric double layer at electrode/electrolyte interfaces

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Theory of electrochemistry at miniaturised interfaces between two immiscible electrolyte solutions

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Direct measurement of acid-base interaction energy at solid interfaces

We have studied acid-base interactions at solid-liquid and solid-solid interfaces using interface-sensitive sum frequency generation (SFG) spectroscopy. The shift of the sapphire hydroxyl peak in contact with several polar and nonpolar liquids and polymers was used to determine the interaction energy. The trend in the interaction energies cannot be explained by measuring only water contact angles. Molecular rearrangements at the sapphire interface, to maximize the interaction of the acid-base groups, play a dominant role, and these effects are not accounted for in the current theoretical models. These results provide important insights into understanding adhesion, friction, and wetting on solid interfaces.     

Molecular modeling of adsorption and ordering at solid interfaces

Interfaces between liquids and solid surfaces are of considerable scientific as well as technological interest, in particular in the context of the adsorption and organization of molecular films. In recent years the direct observation of the molecular structure and often even the dynamics of ordered monolayers at such hidden interfaces has been made possible by the rapid development in scanning probe microscopy. Nevertheless, there is still a lack of understanding with respect to the formation and organization of such films and their interaction with the experimental apparatus. Here computer modeling plays an increasing role as both the complexity of the interfaces and the available computer power increase. This article addresses the application of phenomenological molecular modeling to physisorption at solid surfaces with a special emphasis on the liquid-solid interface. The paper presents an overview over different modeling approaches and illustrates their application in a series of examples ranging from the simulation of adsorption isotherms of small molecules to the prediction of the structure of physisorbed layers for larger molecules.     

Plasmonic response of electrified metal-liquid interfaces during faradaic and non-faradaic reactions by enhanced optical transmission

Thin Au films, patterned by focused ion beam (FIB) milling to contain an array of subwavelength nanopores, exhibit enhanced optical transmission (EOT) via front-back resonance coupling. The films also serve as working electrodes capable of controlling the local potential, allowing electrochemical processes to be monitored using wavevector-resolved spectral mapping. The precise value of the surface plasmon resonance (SPR) wavevector can be extracted from the enhanced optical transmission signal and correlated with several distinct classes of electrochemical processes: double layer reorganization, faradaic adsorption/desorption, heterogeneous electron transfer, and anion adsorption. Specifically, the protonation/deprotonation reaction of an adsorbed monolayer of 4-mercaptobenzoic acid, the adsorption/desorption reaction of dodecanethiol to Au, the solution-phase reaction of ferri-ferrocyanide, and sulfate adsorption/desorption are investigated. A simple model is presented that encompasses both the EOT signal and electrochemical processes and produces semi-quantitative agreement with the SPR spectral wavevector mapping observed experimentally.     

Polymers at solid interfaces: A spin labelling approach

Polymers physically adsorbed or chemically grafted on a solid surface can be studied at a microscopic molecular level by Electron Paramagnetic Resonance (EPR) spectroscopy. The signal of nitroxide free radicals used as spin labels is sensitive to the Brownian motion of the segments and generally shows lines characteristic of two different environments, the loops and tails in solution, and the trains on the surface. The previsions of simple models for what can be expected from the measurements are recalled. The relevance of the method is confirmed on linear neutral chains in well defined situations. The technique is then applied to a number of new systems giving valuable information on the behaviour of the macromolecules in these more complex conditions.     

Adsorption of Janus particles to curved interfaces

We investigate the adsorption of a spherical Janus particle to a spherically curved liquid-liquid interface. We show that the equilibrium contact angle is determined by the geometry of the particle, its wettability, and also the interfacial curvature. In contrast with a homogeneous particle, there is a preferred interfacial curvature (spontaneous curvature) due to the Janus particle when the particle satisfies certain conditions.     

Adsorption of colloidal particles to curved interfaces

As a simple model for a Pickering emulsion droplet, we consider the adsorption of spherical particles to a spherical liquid-liquid interface in order to investigate the curvature effect on the particle adsorption. By taking into account both the surface and the volume energies due to the presence of a particle, we show that the equilibrium contact angle is determined by the classical Young's equation although the adsorption energy depends on the curvature. We also calculate the partitioning of the colloidal particles among the two liquids and the interface. The distribution of colloidal particles is expressed in terms of the interfacial curvature as well as the relative wettability of the particle.     

Adsorption of bovine serum albumin at solid/aqueous interfaces

Adsorption of soluble serum proteins on hydrophilic and hydrophobic solid surfaces is important for biomaterials and chromatographic separations of proteins. The adsorption of bovine serum albumin (BSA) from aqueous solutions was studied with in situ ATR-IR spectroscopy, and with ex situ ATR-IR, ellipsometry, and water wettablity measurements. The results were used to quantitatively determine the adsorbed film thickness and surface density of BSA on hydrophilic silicon oxide/silicon surfaces, and on these surfaces covered with a hydrophobic lipid monolayer of dipalmitoylphosphatidylcholine (DPPC). The water contact angles were 5° for silicon oxide, 47° ± 1° for the DDPC monolayer, and 53° ± 1° for the BSA monolayers. At 25 °C, and with 0.01–1 wt% BSA in water, the surface densities range from Γ = 2.6–5.0 mg/m², and the film thicknesses range from d = 2.0–3.8 nm, on the assumption that the film is as dense as bulk protein. These results, and certain changes in the IR amide I and II bands of the protein, indicate that the protein adsorbs as a side-on monolayer, with some flattening due to unfolding or denaturation. The estimated -helical content for protein in buffer solutions is 15% higher than for solutions in water. The adsorption density reaches a steady-state value within 10 min for the lowest concentration, but does not appear to reach a steady-state value after 3 h f‘or the higher concentrations. Adsorption of BSA on a silicon oxide surface covered with a monolayer of DPPC leads to an adsorbed protein film of about half the thickness and surface density than on silicon oxide, but the same contact angle, indicating more protein unfolding on the hydrophobic than on the hydrophilic surface.     

Potential dependent organization of water at the electrified metal-liquid interface

In situ infrared visible sum frequency generation spectroscopy (SFG) is used to examine the structure of water at the Ag-water interface in NaF and KF electrolyte solutions. Water is observed in environments associated with both the electrode surface and the diffuse double layer. Peaks are observed that are correlated with low-order water, water interacting with electrolyte ions, specifically adsorbed water to the electrode surface, and hydronium. Spectra obtained from a thiol-modified Ag surface enabled discrimination between surface-bound water and that in the double layer. The water organization is dependent on applied potential, with the observed intensities for specifically adsorbed and ion solvating water diminishing near the pzc.     

Experimental aspects of polymer adsorption at solid/solution interfaces

Several recent review articles have been concerned with the topic of polymers at interfaces from the theoretical standpoint. This reflects the extensive effort made in this area over the last 10 – 15 years. However, new experimental techniques for studying polymers at interfaces have also begun to appear in recent years; so have better defined model systems. This article is therefore directed more to a survey of these experimental aspects of the subject. However, a short review of the current state of the theory is given first as background and to define concepts. In the following chapter, details of the modern experimental methods are given. The last chapter comprises an extensive comparative review of results obtained using these techniques with model systems, covering homopolymers, copolymers and polyelectrolytes.     

Evidence that phosphatidylinositol promotes curved membrane interfaces

We have identified the phase behavior of phosphoinositol (PI) lipid extracts from bovine liver and wheat in dioleoylphosphatidylcholine (DOPC) model membranes under physiological conditions (pH 7.4) and show, for the first time, that the physicochemical properties of phosphatidylinositol lipids are capable of driving changes in membrane curvature. Ten mole percent phosphoinositol (PI) extract in DOPC is sufficient to induce the formation of the inverse hexagonal (H II) and inverse micellar cubic ( Fd3 m) phases at 37 degrees C. The phase behavior of several hydrated lipid samples was analyzed using small-angle X-ray scattering, and their lattice parameters were calculated.     

Thermodynamic forces in highly curved fluid interfaces

The identification of the force distribution in curved interfaces as a thermodynamic force [Baus and Lovett, J. Chem. Phys. 101, 377 (1995)] can be interpreted as a relation between the force distribution and the grand canonical free energy difference between two distinct systems. Using this interpretation, molecular expressions are developed for the force distribution in cylindrical and spherical interfaces that remain valid for very highly curved interfaces.