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.     

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.     

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.     

Confined space effects driving to heterogenization of solutions at the interfaces

Water and concentrated solutions of acids (HCl, H₃PO₄, and H₃PO₃) or H₂O₂ are clustered at the interfaces of nanosilicas and multi-layer graphene oxide (MLGO) as shown by low-temperature ¹H NMR spectroscopy. The acid solutions in contact with silica or MLGO powders placed in nonpolar (CCl₄) or weakly polar (CDCl₃) media form nanodomains and clusters of several types and sizes with different contents of water and solutes. This differentiation of the solutions is stronger at a surface of more compacted nanosilicas because of enhanced confinement effects in narrower pores (voids) between adjacent nanoparticles. For MLGO, the confined space effects are relatively weak but the formation of ice crystallites during freezing of the suspension changes the material texture. Obtained results are of importance for deeper insight into the mechanism of the interfacial behavior of complex solutions under different confined space effects and due to the influence of organic co-solvents.     

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.     

Relaxation of surfactants adsorption layers at liquid interfaces

The relaxation behaviour of surfactant layers provides a deep insight into the composition and structure of adsorbed layers at liquid interfaces. The development of professional experimental tools created a helpful basis for an increasing interest in these studies. In addition, the theoretical basis has been improved in many aspects such that for several surfactant systems a quantitative understanding is already possible. In particular the consideration of the changes in molar area of adsorbed molecules, introduced into the thermodynamics of adsorbed layers first by Fainerman in 1995, due to changes in the surface coverage allowed a considerably better, in many cases even quantitative understanding of the surface relaxation. Another important additional property, introduced into the thermodynamics and consequently also into the mechanisms of relaxation processes in interfacial layers, is the two-dimensional compressibility, important for the response to deformations of rather packed interfacial layers. The experimentally observed negative dilational viscosity is discussed only briefly and considered essentially in terms of experimental and theoretical shortcomings. The relaxation behaviour of nano- and microparticles, in literature often addressed is compounds able to act “instead of surfactants” are also addressed and some peculiarities discussed, while the obvious interrelation between the dilational rheology and stability of foams and emulsions is not analysed in detail.     

DNA and surfactants in bulk and at interfaces

Recent investigations of the DNA interactions with cationic surfactants and catanionic mixtures are reviewed. Several techniques have been used such as fluorescence microscopy, dynamic light scattering, electron microscopy, and Monte Carlo simulations.

The conformational behaviour of large DNA molecules in the presence of cationic surfactant was followed by fluorescence microscopy and also by dynamic light scattering. These techniques were in good agreement and it was possible to observe a discrete transition from extended coils to collapsed globules and their coexistence for intermediate amphiphile concentrations. The dependence on the surfactant alkyl chain was also monitored by fluorescence microscopy and, as expected, lower concentrations of the more hydrophobic surfactant were required to induce DNA compaction, although an excess of positive charges was still required.

Monte Carlo simulations on the compaction of a medium size polyanion with shorter polycations were performed. The polyanion chain suffers a sudden collapse as a function of the concentration of condensing agent, and of the number of charges on the polycation molecules. Further increase in the concentration increases the degree of compaction. The compaction was found to be associated with the polycations promoting bridging between different sites of the polyanion. When the total charge of the polycations was lower than that of the polyanion, a significant translational motion of the compacting agent along the polyanion was observed, producing only a small-degree of intrachain segregation, which can explain the excess of positive charges necessary to compact DNA.

Dissociation of the DNA–cationic surfactant complexes and a concomitant release of DNA was achieved by addition of anionic surfactants. The unfolding of DNA molecules, previously compacted with cationic surfactant, was shown to be strongly dependent on the anionic surfactant chain length; lower amounts of a longer chain surfactant were needed to release DNA into solution. On the other hand, no dependence on the hydrophobicity of the compacting agent was observed. The structures of the aggregates formed by the two surfactants, after the interaction with DNA, were imaged by cryogenic transmission electron microscopy. It is possible to predict the structure of the aggregates formed by the surfactants, like vesicles, from the phase behaviour of the mixed surfactant systems.

Studies on the interactions between DNA and catanionic mixtures were also performed. It was observed that DNA does not interact with negatively charged vesicles, even though they carry positive amphiphiles; however, in the presence of positively charged vesicles, DNA molecules compact and adsorb on their surface.

Finally Monte Carlo simulations were performed on the adsorption of a polyelectrolyte on catanionic surfaces. It was observed that the mobile charges in the surface react to the presence of the polyelectrolyte enabling a strong degree of adsorption even though the membrane was globally neutral. Our observations indicate that the adsorption behaviour of the polyelectrolyte is influenced by the response given by the membrane to its presence and that the number of adsorbed beads increases drastically with the increase of flexibility of the polymer. Calculations involving polymers with three different intrinsic stiffnesses showed that the variation is non-monotonic. It was observed also that a smaller polyanion typically adsorbs more completely than the larger one, which indicates that the polarisation of the membrane becomes less facilitated as the degree of disruption increases.     

Competitive adsorption of surfactants at air/water interfaces

The zeta potential of an air bubble suspended in an aqueous solution of mixed fluorocarbon and hydrocarbon anionic surfactants is studied over a wide range of concentrations and mixture compositions. The zeta potential is related to surfactant ion adsorption. The two surfactants, which exhibit an antipathy manifested by micellar demixing, compete for surface sites. The total surfactant adsorption is reduced when both surfactants are present. Adsorption phenomena are closely correlated to the micellar phase diagram.     

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.     

Impact of regularity in structure-property relationships

The regular evolution of properties with structural modification is quantitatively formulated. It is defined in a structural space which is exhaustive, ordered, flexible and explicit. It is detected along the ordered pathways of structural filiations by inference tools, which take into account the experimental precision and proceed by a heuristic modulation of the initial representation space. The introduction of nuances in the quantitative formulation of regularity leads to diverse tools for setting up and exploiting the relationships. These latter constitute a prediction law, which is both simple and as precise as the measurements. The detected regularities make for a generalization of certain structural effects and suggest a fruitful interpretation. Zeroth- and first-order regularities, characteristic of strongly linear variation, are used to safely extrapolate the prediction range and to orient experimental planning.     

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.     

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.     

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.