From vesicles to micelles

Nano-demixing as a novel strategy for magnetic field responsive systems: the case of dibutyl phosphate/bis(2-ethylhexyl)amine systems

Pure surfactant liquids and their binary mixtures, owing to the amphiphilic nature of the molecules involved, can exhibit nano-segregation and peculiar transport properties with the formation of magnetic field-responsive supramolecular structures.

Advanced molecular self-assemblies facilitated by simple molecules

Advanced materials are often based on smart molecular self-assemblies that either respond to external stimuli or have hierarchical structures. Approaches to this goal usually stem from complicated molecular design and difficult organic synthesis. In this invited feature article, we demonstrate that desired molecular self-assemblies can be made conveniently by introducing simple functional molecules into amphiphilic systems. We show that upon introducing specific small molecules which serve as responders, modulators, or even building blocks, smart supramolecular architectures can be achieved which avoid complicated organic synthesis. We expect that this could be a general and economical way to produce advanced materials in the near future.

Decoupling of dynamic processes in surfactant-based liquid mixtures: the case of lithium-containing bis(2-ethylhexyl)phosphoric acid/bis(2-ethylhexyl)amine systems

Pure surfactant liquids and their binary mixtures, because of the amphiphilic nature of the molecules involved, can exhibit nanosegregation and peculiar transport properties. The idea that inspired this work is that the possibility of including in such media salts currently used for technological applications should lead to a synergy between the properties of the salt and those of the medium. Therefore, the dynamic features of bis(2-ethylhexyl)amine (BEEA) and bis(2-ethylhexyl)phosphoric acid (HDEHP) liquid mixtures were investigated as a function of composition and temperature by (1)H nuclear magnetic resonance (NMR) spectroscopy and rheometry. Inclusion of litium trifluoromethanesulfonate (LiT) has been investigated by infrared spectroscopy, pulsed field gradient NMR, and conductimetry methods to highlight the solubilizing and confining properties of these mixtures as well as the lithium conductivity. It was found that BEEA/HDEHP binary liquid mixtures show zero-threshold percolating self-assembly with a maximum in viscosity and a minimum in molecular diffusion at a 1:1 composition. Dissolution of LiT in such system can occur via confinement in the locally self-assembled polar domains. Despite this confinement, Li(+) conduction is scarcely dependent on the medium composition because of the possibility of a field-induced hopping decoupled by the structural and dynamical features of the medium.

Kinetics of the micelle-to-vesicle transition: aqueous lecithin-bile salt mixtures

Important routes to lipid vesicles (liposomes) are detergent removal techniques, such as dialysis or dilution. Although they are widely applied, there has been only limited understanding about the structural evolution during the formation of vesicles and the parameters that determine their properties. We use time-resolved static and dynamic light scattering to study vesicle formation in aqueous lecithin-bile salt mixtures. The kinetic rates and vesicle sizes are found to strongly depend on total amphiphile concentration and, even more pronounced, on ionic strength. The observed trends contradict equilibrium calculations, but are in agreement with a kinetic model that we present. This model identifies the key kinetic steps during vesicle formation: rapid formation of disk-like intermediate micelles, growth of these metastable micelles, and their closure to form vesicles once line tension dominates bending energy. A comparison of the rates of growth and closure provides a kinetic criterion for the critical size at which disks close and thus for the vesicle size. The model suggests that liposomes are nonequilibrium, kinetically trapped structures of very long lifetime. Their properties are hence controlled by kinetics rather than thermodynamics.

Formation of vesicles in diluted aqueous solutions of surfactant investigated by direct analysis of light scattering

Formation of aggregates in the binary systems of double-tailed surfactant, sodium 4-(1-pentylheptyl)benzenesulfonate, and water in the dilute regime was supposed to occur within 5.0-8.5% of surfactant concentration. The size of particles was determined by light scattering. In addition, the samples were observed at room temperature using an Axiovert 35 Zeiss polarized light microscope operated with differential interference contrast optics. The observed aggregates could, in theory, belong a vesicle phase. All the histograms obtained by light scattering showed a bimodal distribution of particles. Weight factors including intensity, volume and number distribution indicate 97-100% of small aggregate sizes, since the peaks for the big sizes indicate only a small number of the aggregate population. Small aggregates have shown monodispersity with diameters of the aqueous core amounting to 38.94 and 54.94 nm relating to the surfactant concentration of 6.0 and 8.0%, respectively. Hydrodynamic radii determined by the Cumulant method, by the inverse Laplace transformation, and using a plot 1/tau vs. q2, showed values within the usual precision limits.

Effects of protonation on the thermodynamic properties of alkyl dimethylamine oxides

The effects of protonation on alkyldimethyl amine oxide micelles are reviewed, mainly with regard to dodecyl and tetradecyl homologs. The topics discussed are hydrogen ion titration properties, critical micelle concentration (CMC), area per surfactant and micelle aggregation number. A hydrogen bond hypothesis is proposed to interpret the several characteristic results associated with protonation: between two cationic species as well as between the non-ionic-cationic pair. The dipole-dipole interaction of the non-ionic micelle is discussed in relation to both: (a) the unusually high CMC values of the non-ionic micelles compared with other non-ionic surfactants with the same hydrocarbon chain; and (b) the reversal of the stability of the non-ionic and the cationic micelles at high ionic strengths. Two different approaches of the salting out effect on the ionic micelles are compared, the Chan-Mukerjee approach and ours, in relation to the non-linear Corrin-Harkins relation. The obtained salting out constants of the surfactants carrying a dodecyl chain decreased as the head group becomes more polar. Infrared and 13C-NMR spectra data are examined from the point of the specific interaction claimed by the hydrogen bond model. Mixed surfactant systems including amine oxides and the solid state phase behavior of amine oxides are both briefly reviewed.

Phase boundaries in mixtures of membrane-forming amphiphiles and micelle-forming amphiphiles

The phase behavior of mixtures of phospholipids and detergents in aqueous solutions is an issue of basic importance for understanding the solubilization and reconstitution of biological membranes. We review the existing knowledge on the compositionally induced reversible transformation of phospholipid bilayers into lipid-detergent mixed micelles. First, we describe the experimental protocols used for preparation of such mixtures and emphasize the scope and limitations of the various techniques used for evaluation of the microstructures of the self-assembled amphiphiles in the mixture. Subsequently, we interpret the existing data in terms of the spontaneous curvature of the amphiphiles and the finite size of the mixed micelles. These considerations lead to a general description of the phase behavior, which forms the basis for a rational approach to solubilization and reconstitution experiments.