Adsorption of microstructured particles at liquid-liquid interfaces

The solid particles are adsorbed at interfaces and form self-assembled structures when the particles have suitable wettability to both liquids. Here, we show theoretically how the microstructure on the particle surface affects their adsorption properties. The physical properties of the interface adsorbing a particle will be described by taking into account the surface roughness due to the microstructure. The microstructure on the surface changes drastically the wettability and the equilibrium position of the adsorbed particle. Therefore, the contact angle of the particle at the three-phase contact line shifts with the particle surface area, because the surface roughness enhances the interfacial properties of the particle surface. Moreover, the range of the interfacial tensions at which the particle is adsorbed becomes narrower with the increase of the surface roughness. The effect of the particle shape on the adsorption properties is also studied. In the case of disk-shaped particles, the energy changes discontinuously when the plane surface of the particle contacts the liquid-liquid interface. The adsorbing position does not change with the surface roughness. The orientation of a parallelepiped particle at the liquid-liquid interface is governed by the aspect ratio and the surface area of the particle. On the other hand, the particle which is partially covered with the microstructured surface is adsorbed firmly at the interface in an oriented state. We should consider not only the interfacial tensions but also the surface structure and the particle shape to control the adsorption behavior of the particle.     

Oscillatory phenomena at liquid-liquid interfaces

The mechanism of (i) Yoshikawa and Matsubara liquid-liquid interface oscillators and (ii) density/salt-water oscillators has been investigated. A modified simple mathematical formalism of both has been presented. Qualitative support for the model is provided by the available experimental results.     

Janus particles at liquid-liquid interfaces

Following recent theoretical predictions, we report on the first experiments on the interfacial activity of so-called Janus nanoparticles (i.e., bifacial particles consisting of a gold and an iron oxide moiety). Using pendant drop tensiometry, we show that the amphiphilicity derived from the Janus character of the particles leads to a significantly higher interfacial activity compared to that of the respective homogeneous particles of the same size. The self-assembly of Janus particles at the hexane-water interface results in a significant decrease in the interfacial tension. Furthermore, we demonstrate control over the interfacial activity by tuning the particles' amphiphilicity via ligand-exchange reactions.     

Janus cylinders at liquid-liquid interfaces

We describe the first study on the self-assembly behavior of Janus cylinders at liquid/liquid interfaces. The Janus cylinders are characterized by a phase separation along the major axis into two hemicylinders of different wettability. The pendant drop technique and microscopic imaging were used to characterize the adsorption behavior and self-assembly of Janus cylinders at perfluorinated oil/dioxane and perfluorinated oil/dimethyl sulfoxide interfaces. According to the evolution of the interfacial tension and a series of TEM images taken during the cylinder adsorption, we will specify the characteristics of early to late stages of the Janus cylinder adsorption at a liquid-liquid interface and discuss the effect of Janus cylinder length and their concentration. We also establish that the broken symmetry of the corona leads to significantly higher interfacial activity as compared to homogeneous core-shell cylinders. The adsorption is characterized by three different adsorption stages: first, free diffusion to the interface, followed by continuous adsorption of cylinders including ordering and domain formation and, finally, additional packing with a rearrangement of domains and formation of a loose multilayer system.     

Melittin adsorption and lipid monolayer disruption at liquid-liquid interfaces

Melittin, a membrane-active peptide with antimicrobial activity, was investigated at the interface formed between two immiscible electrolyte solutions (ITIES) supported on a metallic electrode. Ion-transfer voltammetry showed well-defined semi-reversible transfer peaks along with adsorptive peaks. The reversible adsorption of melittin at the liquid-liquid interface is qualitatively discussed from voltammetric data and experimentally confirmed by real-time image analysis of video snapshots. It is also demonstrated that polarization of the water/1,2-DCE interface results in drastic drop shape variations caused by large variations of the interfacial tension. The experimental data also confirmed that maximum adsorption occurs near the ion transfer potential. Finally, the interaction of melittin with a monolayer of L-α-dipalmitoyl phosphatidylcholine (DPPC) was also investigated showing that melittin destabilizes the lipidic monolayer facilitating its desorption. The non-covalent complex formation between melittin and DPPC was confirmed by mass spectrometry.     

Adsorption of polymers at the solution-solid interfaces

Summary Adsorption of ethyl cellulose, so-called rigid polymer, on the glass surface from various solvents having no groups to make the hydrogen bonds with solid surface was carried out. And the solvent dependence was found. The dependence is, however, small as compared with polyvinyl acetate-glass system (1). A main factor to control the solvent dependence is not competitive between solvent and polymer, but the effect of solvent to extension of polymer in solution, that is, segment density per solid surface area in poor solvent is greater than in better solvent. Maximum adsorption is independent of molecular weight of ethyl cellulose used (Mv=2×10⁴6×10⁴). It is expected that ethyl cellulose lies flat on the glass surface considering adsorption energy per segment (₀>5kT).

---

Zusammenfassung Adsorptionen von Äthylzellulose als sozusagen steifes Polymeres an Glasoberfläche aus verschiedenen Lösungsmitteln heraus, die keine Gruppen für Wasserstoff bindungen an die Festoberflächen besitzen, wurden ausgeführt. Es ergab sich in der Tat Lösungsmittelabhängigkeit. Diese ist jedoch klein im Vergleich zu der beim System Polyvinylacetat/Glas. Eine Ursache für die Lösungsmittelabhängigkeit ist nicht direkt die unterschiedliche Solvatation zwischen Lösung und Polymer, sondern der Effekt des Lösungsmittels auf die räumliche Aufweitung des Polymers in Lösungen. Letzteres bedeutet nämlich, die Segmentdichte ist in schlechten Lösungsmitteln größer als in guten. Die maximale Adsorption ist unabhängig vom Molekulargewicht im Bereich vonMv=2×10⁴ bis 6×10⁴. Es ist zu erwarten, daß die Äthylzellulosc flach auf der Glasoberfläche aufliegt, wenn man die Adsorptionsenergie pro Segment betrachtet.

---

---

With 5 figures and 4 tables     

Electrochemical determination of lead and lead ion transfer at liquid-liquid interfaces

The transfer behavior of lead (II) ion at a liquid-liquid interface, facilitated by neutral carriers (polyethylene glycols), is studied by an electrochemical method. The transfer process is discussed in terms of the formation of complexes in two phases and attributed to the transfer of a complex ion across the interface. The apparent standard transfer potential, ΔwoφPbL2+o, apparent standard Gibbs energy of transfer, ΔwoGPbL2+o, and the dissociation constant of the complex, KPbL2+, in the aqueous phase are obtained from the experimental data. The results suggest a new electrochemical method for the determination of lead.     

Molecular dynamics and phase transitions in phospholipid monolayers at liquid-liquid interfaces

Stablen-hexadecane/water andn-tetradecane/water macroemulsions containing monolayers of natural (egg yolk lecithin, EY) and synthetic (dimyristoylphosphatidylcholine, DMPC) phospholipids at liquid-liquid interfaces were prepared. The existence of the monolayers was proved by studying the reduction kinetics of a surface-active spin probe with ascorbate anions. Spin labeled derivatives of stearic acid in which the nitroxide group is locared at different distances from the polar head (5-, 12-, and 16-doxylstearic acids) were used to study the temperature dependences of the molecular ordering, rotational mobility, and local polarity in the monolayers in emulsions and also in bilayers in liposomes obtained from the same lipids. In the EY monolayers, the degree of spin probe solubilization is higher, while the order parameters (S) and rotational correlation times (τ) are lower than those in EY bilayers. The differences between these parameters for mono- and bilayers increase with an increase in the distance of the reporter group from the aqueous phase. In the DMPC monolayers, a first-order phase transition was detected by measuring the temperature dependences ofS and τ. The temperature region of the phase transition in monolayers is shifted to lower temperatures with respect to that for bilayers and depends on the nature of the oil phase. It was concluded that the phospholipid monolayers in emulsions incorporate hydrocarbon molecules, whose concentration in the DMPC monolayers increases on going from the low-temperature (gel) to the high-temperature (liquid crystal) phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 418–425, March, 1998.     

Adsorption of frothers at coal/water interfaces

The adsorption of phenol and three commonly used frothers, namely, terpineol, methylisobutylcarbinol (MIBC) and cresol, on coal surfaces has been studied through UV spectrophotometric and gas chromatographic techniques. The rate of attaining adsorption equilibrium of these alcohols onto coal is very slow, possibly due to pore diffusion. The adsorption isotherms are of typical Langmuir-type except for MIBC, where the adsorption density rises linearly with equilibrium concentration in solution in the range of concentration under study. Free energies of adsorption were calculated from the adsorption isotherms. The results indicate that adsorption occurs through hydrophobic interactions between the frother molecules and the coal surface. The effect of oxidation and pH on adsorption behavior was also studied. Oxidized coal is more hydrophilic and hence the adsorption of these nonionic surface-active agents is reduced after oxidation.     

Self-assembly and cross-linking of FePt nanoparticles at planar and colloidal liquid-liquid interfaces

Terpyridine thiol functionalized FePt and Au NPs were self-assembled and cross-linked at the liquid-liquid interfaces using Fe(II) metal ion. Complexation of terpyridine with Fe(II) metal ion leads to NP network and affords stable membranes and colloidal shells at the liquid-liquid interfaces.     

Exploring parameter space effects on structure-property relationships of surfactants at liquid-liquid interfaces

The ubiquitous use of surfactants in commercial and industrial applications has led to many experimental, theoretical, and simulation based studies. These efforts seek to provide a molecular level understanding of the effects on structuring behavior and the corresponding impacts on observable properties (e.g., interfacial tension). With such physical detail, targeted system design can be improved over typical techniques of observational trends and phenomenological correlations by taking advantage of predictive system response. This research provides a systematic study of part of the broad parameter space effects on equilibrium microstructure and interfacial properties of amphiphiles at a liquid-liquid interface using the interfacial statistical associating fluid theory density functional theory as a molecular model for the system from the bulk to the interface. Insights into the molecular level physics and thermodynamics governing the system behavior are discussed as they relate to both predictions qualitatively consistent with experimental observations and extensions beyond currently available studies.     

Control of surface tension at liquid-liquid interfaces using nanoparticles and nanoparticle-protein complexes

Subtle changes in the monolayer structure of nanoparticles (NPs) influence the interfacial behavior of both NPs and NP-protein conjugates. In this study, we use a series of monolayer-protected gold NPs to explore the role of particle hydrophobicity on their dynamic behavior at the toluene-water interface. Using dynamic surface tension measurements, we observed a linear decrease in the meso-equilibrium surface tension (γ) and faster dynamics as the hydrophobicity of the ligands increases. Further modulation of γ is observed for the corresponding NP-protein complexes at the charge-neutralization point.     

Adsorption of hydrophobin proteins at hydrophobic and hydrophilic interfaces

The surface activity of two hydrophobin proteins, HFBII and SC3, at the solid–liquid, liquid–liquid and liquid–vapor interface has been investigated. Hydrophobins are fungal proteins that are known to adsorb and affect the physico-chemical properties of an interface. In this study, the surface activity was determined by measuring the interaction of hydrophobin molecules with various liquids, solid particles and films that are commonly used or produced in industrial processes. We found that a very low concentration of hydrophobin is required to facilitate the wet-in of hydrophobic solid particles, such as Teflon^®, into aqueous solutions. It is also capable of stabilizing aqueous dispersions of Kevlar^® nanopulp, reversing the wettability of hydrophobic films and stabilizing polyunsaturated fatty acid (PUFA) oil-in-water emulsions.     

Proteins at liquid interfaces: II. Adsorption isotherms

Adsorption isotherms for the three proteins β-casein, bovine serum albumin, and lysozyme at the air-water and oil-water interfaces have been determined independently using ellipsometry and surface radioactivity methods; the surface pressure and surface potential were also monitored. Saturated monolayer coverage occurs via irreversible adsorption of 2–3 mg M^?2 of protein; the resultant films generate surface pressures of about 20 mN m^?1 and are 50–60 Å thick. Molecules adsorbed in the first layer dominate the film pressures so that further adsorption causes no change in the pressure although the film thickness can increase to more than 100 Å. The molecules which give rise to this increase in film thickness are reversibly adsorbed with respect to aqueous substrate exchange. The experimental isotherm data and the Langmuir adsorption isotherm are in close agreement at low protein concentrations. However, comparison with the Gibbs adsorption equation is not valid, although reasonable agreement can be achieved if some account is taken of the fact that the protein molecules in the first layer are irreversibly adsorbed.     

Adsorption and adhesion processes in microbial growth at interfaces

By considering bacteria as living colloidal particles it is possible to define, at least in part, the physicochemical and biological factors involved in the adhesion of bacteria to apparently inert surfaces. Adhesion is considered in terms of the attraction and repulsion forces operative as the organism approaches a substratum surface, as well as in terms of the surface free energies of the bacterium, the substratum and the liquid phase. Bacterial adhesion at a surface is preceded by molecular adsorption, a process that may alter the substratum surface properties and may provide a concentrated source of nutrients allowing limited growth of the bacteria at the surface.     

Ion transfer of bromophenol blue across liquid-liquid interfaces

The ion transfer of the acidic dye bromophenol blue (BPB) at the interfaces of water/nitrobenzene (W/NB), water/1,2-dichloroethane (W/1,2-DCE) and water/(nitrobenzene+chlorobenzene) (W/(NB +CB)) was studied in detail by cyclic voltammetry (CV), chronopotentiometry with linear current scanning (CLC), controlled potential electrolysis and UV spectroscopic methods. Using controlled potential electrolysis, we observed successfully the transfer process of BPB across the W/NB interface from the colour changes of BPB in two different phases. The proposed transfer mechanism for BPB is proved to be reasonable using UV spectroscopy of the product of the electrolysis. The standard potential differences Δ^o_w^o and the standard Gibbs energies of the BPB transfer from water to some organic solvents were calculated. The dissociation constants of BPB obtained were quite close to the literature values.     

Adsorption behavior of phospholipid vesicles at oil/water interfaces

By measuring the change in interfacial tension after adding phospholipid vesicles to an aqueous solution of electrolyte, we studied the adsorption behavior of phospholipid vesicles at oil/water interfaces. The effects of concentration of three kinds of electrolyte (NaCl, MgCl₂, LaCl₃) and of the mixing ratio of two kinds of phospholipids (phosphatidylcholine and phosphatidylserine), on the adsorption behavior at an oil/water interface were examined. The results were interpreted using the DLVO theory.