Micellar Films Research Articles

Water-induced morphology evolution of block copolymer micellar thin films

Morphology evolution of diblock copolymer polystyrene- block-poly(4-vinylpyridine) (PS- b-P4VP) micellar thin film in the presence of water was investigated. Surface holes with nanoscale cavities in hexagonal order could be induced by water treatment for certain periods. The nanoscale surface cavities could be transformed into isolated nanospheres in a dry environment or back to protruding densely packed spheres by toluene (a selective solvent for PS coronae) treatment. The morphology evolution of micellar thin film strongly depended on the slow evaporation of toluene solvent, the swelling of P4VP cores in the humid environment, and the subsequent movement of PS chains induced by air and toluene. The incompatibility between solvent and block, and that between the unlike blocks also played an important role in the morphology evolution.

High-Density Arrays of Titania Nanoparticles Using Monolayer Micellar Films of Diblock Copolymers as Templates

Highly dense arrays of titania nanoparticles were fabricated using surface micellar films of poly(styrene-block-2-vinylpyridine) diblock copolymers (PS-b-P2VP) as reaction scaffolds. Titania could be introduced selectively within P2VP nanodomains in PS-b-P2VP films through the binary reaction between water molecules trapped in the P2VP domains and the TiCl(4) vapor precursors. Subsequent UV exposure or oxygen plasma treatment removed the organic matrix, leading to titania nanoparticle arrays on the substrate surface. The diameter of the titania domains and the interparticle distance were defined by the lateral scale present in the microphase-separated morphology of the initial PS-b-P2VP films. The typical diameter of titania nanoparticles obtained by oxygen plasma treatment was of the order of approximately 23 nm. Photoluminescence (PL) properties were investigated for films before and after plasma treatment. Both samples showed PL properties with major physical origin due to self-trapped excitons, indicating that the local environment of the titanium atoms is similar.

Using Block Copolymer Micellar Thin Films as Templates for the Production of Catalysts for Carbon Nanotube Growth

We report a novel approach that uses block copolymer micelles as a means to create large area arrays of iron-containing nanoclusters capable of catalyzing the growth of carbon nanotubes (CNTs). The amphiphilic block copolymer poly(styrene-block-acrylic acid) (PS-b-PAA) forms micelles in solution which are capable of self-organizing into ordered structures on surfaces. By spin-coating these solutions onto a variety of substrates, we can create quasi-hexagonal arrays of PAA spheres within a PS matrix. The carboxylic acids groups in the PAA domains can be utilized in an ion-exchange protocol to selectively sequester iron ions, which results in iron-containing nanoclusters that are nearly monodisperse in size (diameter ∼8 nm) and patterned at a density of approximately 1011 particles per cm2. In principle, this route for synthesizing iron-containing nanoclusters offers the capability of controlling the cluster size and spacing by altering the molecular weight of the block copolymer. In this report, we demonst...

Computer Modeling of Ionic Micelle Structuring in Thin Films

The phenomenon of layering of like-charged particles (ionic surfactant micelles or other colloidal particles) between two plane-parallel film surfaces and the evolution of in-film particle structuring are studied using the canonical Monte Carlo method combined with a simulation cell containing both the bulk solution and the film region. The simulations are performed using an effective one-component fluid potential model that incorporates micelle hard-core repulsion and both the screened Coulomb electrostatic interaction and the entropic contribution due to the discrete nature of the solvent and finite size of electrolyte ions. The proposed potential model is applied to mimic the properties of surfactant micelle solutions in the bulk and under film confinement. The effects of surfactant concentration and film thickness on the in-film micellar structuring phenomenon are analyzed. The relation of obtained results to experiments on stratifying sodium dodecyl sulfate micellar films is discussed.

Cavitated Block Copolymer Micellar Thin Films: Lateral Arrays of Open Nanoreactors

We report the formation of novel self-assembled ultrathin (ca. 20 nm) films comprising in-plane arrays of nanoscale surface cavities. The original micellar thin films, formed by casting kinetically stable reverse micelles from a solution of a styrene (PS)−acrylic acid (PAA) diblock copolymer in toluene onto silicon nitride substrates, consist of lateral hexagonal arrays of spherical PAA microdomains in PS matrix. Upon aqueous treatment to load metal ions into the PAA domains, cavitation of the individual PAA micelle cores occurs, leading to the formation of cavities in the center of the micelles and exposing the PAA domains to the film's surface. Experimental evidence reveals that cavitation takes place as a result of hydration and swelling of the PAA micelle cores during sample treatment in an alkaline solution. The use of these cavitated, metal-loaded PAA domains as arrays of open nanoreactors for inorganic nanocluster synthesis is demonstrated.

Self-assembly of ordered microporous materials from rod-coil block copolymers

Rod-coil diblock copolymers in a selective solvent for the coil-like polymer self-organize into hollow spherical micelles having diameters of a few micrometers. Long-range, close-packed self-ordering of the micelles produced highly iridescent periodic microporous materials. Solution-cast micellar films consisted of multilayers of hexagonally ordered arrays of spherical holes whose diameter, periodicity, and wall thickness depended on copolymer molecular weight and composition. Addition of fullerenes into the copolymer solutions also regulated the microstructure and optical properties of the microporous films. These results demonstrate the potential of hierarchical self-assembly of macromolecular components for engineering complex two- and three-dimensional periodic and functional mesostructures.

A novel method for studying the dynamic behavior of both plane-parallel and curved thin liquid films

A new experimental technique was developed to study the thinning behavior of both flat and curved liquid films with fluid-fluid interfaces. The method uses a specially designed glass cell for forming flat or curved films having different sizes and, thereby, different capillary pressures. Our surface force balance equipped with this new cell in conjunction with differential interference microscopy was used to measure film thickness and disjoining pressure. This method is applicable to both symmetric foam and emulsion films, and asymmetric films such as those between approaching small oil droplets and an oil water surface, or water droplets and an air-oil surface. The results of the film thickness stability for the curved foam films were compared with those for the plane-parallel films. It was found that, for thicker films (i.e. greater than 100 nm), the curved film drained at a slower rate than the plane-parallel film but, for thinner films (i.e. less than 100 nm), the curved micellar films stratified with a higher rate of thinning and had decreased thickness stability.

Effect of surfactant concentration on dispersion stability as probed by the film thickness stability mechanism

From a practical point of view, we have identified the formation of microstructure inside a film leading to film stabilization and a new mechanism for the stepwise thinning of the film. The life time of foam with stratifying films has been observed to be much longer, especially if the size of the film is less than a critical size. Also, depending on the effective micellar concentration, monodispersity, film thickness, and area, a two-dimensional in-layer “hexagonal” structure (i.e. colloid crystal-like structure) was observed inside the film, which provides the film stability and improves dispersion stability. In addition, the localization of a lamellar liquid crystal at the interface between a gas phase and inverse micellar solution of a hydrocarbon has been explained by taking into account the relative contributions from methyl and methylene groups to the surface tension.

Effective viscosities in thin ionic micellar liquid films

AbstractThin liquid films stabilized by surfactants above the critical micelle concentration exhibit stratification or stepwise dynamic thinning. A continuum hydrodynamic model is outlined for stepwise film thinning that incorporates equilibrium micellar structuring through self‐consistent oscillatory disjoining pressures and effective viscosities. Effective viscosities as functions of thickness are evaluated with an extension of the local average density model, considering dilute colloidal suspension shear viscosities and solvent effects. To establish local shear viscosities, structured DFT micellar profiles, coarse‐grained densities, and disjoining pressure are used. Ionic micelles and other colloidal systems with repulsive interactions show structured effective viscosities that are generally less than the corresponding homogeneous solution shear viscosity, bounded by the pure solvent viscosity and that of the bulk micellar solution. For 0.1 and 0.2‐M sodium dodecylsulfate micellar solutions, the effective viscosities are less than 5 and 10%, respectively, below the homogeneous fluid viscosity, except at small thicknesses, indicating that the micellar film thins faster than a pure water film of the same thickness.Calculated thinning curves closely resemble experimental observations in the stepwise thinning behavior, displaying decreasing slopes and increased step durations at later times. Despite the micellar structuring within the film, the ionic micelles do not contribute appreciably to the viscous resistance of the thinning film. Rather, Reynolds' film thinning is obeyed, with the equilibrium oscillatory disjoining pressures driving the stepwise dynamics. The shear viscosity of the ionic micellar film is well approximated by that of the bulk solution.

Density-functional modeling of structure and forces in thin micellar liquid films

Recent equilibrium force measurements on aqueous films of surfactant above the critical micelle concentration show oscillations for film thicknesses up to 50 nm. To model this phenomenon we express the micellar contribution to the disjoining pressure in terms of thickness-dependent inhomogeneous micelle number density distributions through the film. Density functional theory is used to calculate micelle density profiles, presuming the micelles to behave as charged spheres interacting with each other, and with the film interfaces, through screened-Coulomb potentials. The background electrolyte permits dilute micellar solutions to act as concentrated systems exhibiting pronounced layering in the film. For a 0.1 M sodium dodecylsulfate (SDS) film we find up to five micellar layers for a film thickness equal to ten micelle diameters (d), the layer separation scaling with the effective diameter (deff/d=1.86) which includes the micelle Debye atmosphere. The peaks are largest near the interfaces and decay toward the bulk density at the film midplane. The corresponding disjoining pressures show oscillations with the same distance scaling between the branches as in the density profiles; these values are consistent with experiment. With decreasing film thickness, the (meta-)stable disjoining pressure regions represent micellar layers in the film being forced closer together, raising the pressure until the interior layer is expelled, allowing more space between the remaining micellar layers at that thickness. Repulsive (positive) disjoining pressures result from layer separations less than the corresponding bulk value whereas attractive (negative) regions represent more distance between layers than that in the bulk. The 0.2 M SDS disjoining pressure isotherm exhibits one additional layer than the 0.1 M case for thicknesses up to 50 nm. The pressure magnitudes of the former case are about twice that of the latter. Addition of ionic salts greatly inhibits the long-range micellar structuring. For SDS foam films, predicted disjoining pressures are much higher than measured values. Comparison with cetyltrimethyl-ammonium bromide (CTAB) micellar films in the surface forces apparatus, however, shows near quantitative agreement. The nature of the confining interfaces thus plays a key role in supporting the internal micellar structuring.

Interfacial behaviour of long-chain alkyl-pyrimidinium cations at the Hg/aqueous solution interface

Abstract The interfacial micellization of a homologous series of N-alkyl pyrimidinium salts and their transformations on a Hg electrode are studied by means of differential capacitance measurements. It is shown that below the cmc a monolayer film of surface micelles is formed at positive polarizations, around +0.15 V with respect to the saturated calomel electrode, and covers the electrode surface up to −1.2 V where this film is destroyed due to the onset of a reduction of N-alkyl pyrimidinium cations. The product of this reduction is strongly adsorbed on the electrode surface and undergoes a surface phase transition forming a compact layer in the region between −1.3 and −1.7 V. At concentrations above the cmc the picture is almost the same except the fact that in the polarization region from ca. +0.2 to +0.1 V the micellar film collapses to a compact layer.

Simple models for adsorption on electrodes: II. Aggregation processes and phase transitions

The aggregation of neutral organic molecules on electrode surfaces to form oligomers or surface micelles and the phase transformations of an adsorbed monolayer were studied by means of statistical mechanics. The model developed for aggregation phenomena predicts that the formation of oligomers is not depicted in the capacitance plots, which exhibit the same features as those for adsorption of monomers under all circumstances. The equilibrium between monomers and oligomers extends throughout the polarization range where these two states of the adsorbate coexist. As the number of monomer units in the aggregates increases, monomers and aggregates tend to separate, occupying different polarization regions. In addition, the equilibrium between monomers and large aggregates leads in general to complicated capacitance plots characterized by the appearance of very sharp capacitance peaks. The same features characterize the formation of mixed two-dimensional micellar films on electrode surfaces. Phase transformations of adsorbed monolayers on electrode surfaces, which may lead to the formation of either two immiscible concentrated surface solutions of adsorbate in solvent and vice versa or pure adsorbate precipitate, are examined within the framework of the molecular models developed in this series of papers. The separation of the adsorbed layer to immiscible surface solutions is related to the short-range particle-particle interactions, whereas the surface precipitation process is considered as an aggregation process where the aggregation number tends to infinity. It is shown that in general there is an acceptable agreement between theory and experiment.

Electrochemical investigation of cetylpyridinium cation micelle adsorption at the Hg | aqueous solution interface

Differential capacitance measurements at a mercury electrode in contact with 0.1 M Na 2SO 4 aqueous solutions containing cetylpyridinium cations (CP +) are carried out to examine the formation of micelles on the electrode surface and their phase transitions in the various polarization regions. Analysing the experimental data by means of theoretical treatments presented in J. Phys. Chem., 96 (1992) 8453 and J. Electroanal. Chem., 348 (1993) 59, it is shown that, below the critical micelle concentration (CMC), a monolayer film of surface micelles is formed at positive potentials, in the region from about +0.10 to 0 V with respect to the saturated calomel electrode. At concentrations above the CMC and up to 1 × 10 −3 M, the micellar film is formed in the region between +0.20 and +0.05 V, and extends to three dimensions, forming a bilayer on the mercury surface. Finally, at concentrations above 1 × 10 −3 M, the formation of a polylayer micellar film is observed. This film, either monolayer or polylayer, covers the electrode surface up to −0.95 V, where it is destroyed as a result of the onset of the electrochemical reduction of CP +. Thus, the micellar film of CP + cations is transformed into a film of uncharged monomer species of the reduction product. This film is not particularly stable and, in the region between −0.95 and −1.4 V, it collapses to a compact layer. At potentials more negative than about −1.4 V, the compact layer is destroyed and the reduction product is desorbed from the electrode. The substance, in the form of CP +, is also desorbed at potentials more positive than +0.2 V, except at concentrations above 1 × 10 −2 M, where a compact, possibly polylayer is formed in this polarization region.

Phase transformations at the Hg/aqueous solution interface in the presence of cetyltrimethylammonium cations

Differential capacity measurements at a mercury electrode in contact with aqueous solutions containing cetyltrimethylammonium cations, CTA + and supporting electrolytes of different types have been carried out. The results obtained are interpreted within the frames of the theoretical treatments presented in J. Phys. Chem. 96, 8453 (1992) and J. electroanal. Chem. 348, 59 (1993). It is shown that a surface film of aggregates starts to develop at positive polarizations. At pre-micellar concentrations this film is likely to be monolayer, whereas at concentrations around and above the cmc, the aggregation phenomena extend at least to two successive layers along the interface. The micellar film developed in this region covers the electrode surface throughout the negative polarization region. Depending on the potential this film undergoes a number of phase transformations. These transformations and more generally the properties of the film are affected by the presence of oxygen in the bulk solution and the nature of the supporting electrolyte. The desolve oxygen may affect the structure of bulk micelles and through them the structure of the adsorbed micellar film, whereas the possible role of counterions in the absorption process of CTA + is either direct, in the region where they are specifically adsorbed, or indirect, through the structure of the bulk micelles in the negative region where they are desorbed from the interface.

Corrosion inhibition study of a carbon steel in acidic media containing hydrogen sulphide and organic surfactants

Inhibition of the corrosion of a carbon steel (0.4% C) by 2-hexadecyl imidazoline and 2-hexadecyl imidazole has been evaluated by electrochemical techniques and correlated with surface tension measurements. Plots of inhibitor efficiency versus surfactant concentration produce S-shaped curves which are assumed to represent adsorption isotherms. A sharp increase in slope was observed at concentrations below the critical micellar concentration. This increase in slope is accounted for by changes in conformation of the adsorbed molecules: horizontal orientations (with respect to the surface) at lower concentrations reflecting cathodic behaviour and perpendicular orientations at the higher concentrations reflecting mixed cathodic and anodic behaviour. At increasing concentrations the inhibitory effect remained constant, suggesting complete saturation of the surface in a bilayered arrangement. Electrochemical impedance measurements, carried out for concentrations greater than the critical micellar concentration, corroborate this assumption: the inhibitors form a thick but adherent micellar film which acts as a diffusion barrier.

Adsorption of a mixture of sodium dodecyl sulphate and dodecyl alcohol on a mercury electrode and its effect on the electrochemical processes of cadmium(II) in sodium chloride solution

The adsorption behaviour of a mixture of the surfactants sodium dodecyl sulphate (SDS) and dodecyl alcohol (DOH) in 0.5 mol dm −3 NaCl has been studied on a hanging mercury drop electrode (HMDE) on the basis of capacity measurements using phase-sensitive ac voltammetry with prior accumulation of the surfactants at a potential E=−0.6 V (vs. SCE). Investigations were carried out in a wide concentration range of SDS (2.08×10 −7−5.20×10 −3 mol dm −3) and in the presence of trace quantities of DOH (0.5 and 1.4 wt%) in the mixture. The effect of surfactants on the electrode reaction of cadmium was studied by the same technique, based on faradaic current measurements. The standard rate constants of the electrode reaction of cadmium, k s, in the absence and presence of surfactants were estimated from the shape and height of ac voltammograms using the theoretical curves obtained by digital simulation. The results show that the adsorbed film on the mercury surface, which is formed by the mixture of sodium dodecyl sulphate and dodecyl alcohol, has different properties from those of films formed by individual components of the mixture. Two very interesting phenomena were observed: an increase of the electrode double-layer capacity of the mixture at the potential of maximum adsorption in comparison with the values of individual components, and an increased permeability of the mixed adsorbed layer for cadmium ions. Both phenomena were observed in a higher concentration range of surfactants (near the CMC value of SDS). It may be assumed that these phenomena are connected with the solubilization of dodecyl alcohol, which is sparingly soluble in water, and its penetration into the polylayer or micellar film of sodium dodecyl sulphate on the electrode surface.

Enthalpies of solution in sodium octanoate + water + alcohol mixtures

Enthalpies of solution of sodium octanoate in water, 1-propanol and aqueous mixtures of 1-propanol, 1-butanol, 1-pentanol and 1-hexanol, and of the alcohols in aqueous solutions of sodium octanoate at various concentrations were determined calorimetrically at 35 °C. MostΔH(soln) values are exothermic and strongly dependent on the solute concentration. The main energetic factor governing the process of dissolution of the surfactant is associated with changes in the water structure caused by the presence of alcohol. That governing the process of the alcohol dissolution in surfactant solutions is due to the effect alcohols have on the CMC of the octanoate. There is no indication of the alcohol being either solubilized in the interior of the aqueous micelle, or becoming part of the micellar film.

Effect of highly surface-active compounds on polaro-graphic electrode processes: Part I. Electrochemical adsorptlox and structure at the dropping mercury electrode-solution interface

Electrocapillary data were obtained by polarographic drop-time measurements to derive information concerning the properties of electrochemical adsorption and orientation of highly surface-active organic molecules at the electrode-solution interface. Two non-ionic iso-octylphenoxypolvethoxy ethanols (Tritons X-100 and X-305) were examined and the adsorption behaviour compared with that of other alkyi polyoxyethylenic compounds. Adsorption isotherms were determined for Triton X-100 for various electrode potentials. A model of “two-dimensional” micellar film formation was postulated for the structure of these adsorbed surfactants. Multilayer adsorption was found from solutions of concentration below that required to form micelles. The dependence of the potential of zero charge on surface coverage and on concentration of additive were determined and used to derive information relating to critical micelle concentration in the bulk solution phase.