Self-assembly of fatty acid-alkylboladiamine salts

The carboxy terminus causes interfacial assembly of oleate hydratase on a membrane bilayer

The soluble flavoprotein oleate hydratase (OhyA) hydrates the 9-cis double bond of unsaturated fatty acids. OhyA substrates are embedded in membrane bilayers; OhyA must remove the fatty acid from the bilayer and enclose it in the active site. Here, we show that the positively charged helix-turn-helix motif in the carboxy terminus (CTD) is responsible for interacting with the negatively charged phosphatidylglycerol (PG) bilayer. Super-resolution microscopy of Staphylococcus aureus cells expressing green fluorescent protein fused to OhyA or the CTD sequence shows subcellular localization along the cellular boundary, indicating OhyA is membrane-associated and the CTD sequence is sufficient for membrane recruitment. Using cryo-electron microscopy, we solved the OhyA dimer structure and conducted 3D variability analysis of the reconstructions to assess CTD flexibility. Our surface plasmon resonance experiments corroborated that OhyA binds the PG bilayer with nanomolar affinity and we found the CTD sequence has intrinsic PG binding properties. We determined that the nuclear magnetic resonance structure of a peptide containing the CTD sequence resembles the OhyA crystal structure. We observed intermolecular NOE from PG liposome protons next to the phosphate group to the CTD peptide. The addition of paramagnetic MnCl2 indicated the CTD peptide binds the PG surface but does not insert into the bilayer. Molecular dynamics simulations, supported by site-directed mutagenesis experiments, identify key residues in the helix-turn-helix that drive membrane association. The data show that the OhyA CTD binds the phosphate layer of the PG surface to obtain bilayer-embedded unsaturated fatty acids.

Self-Assembly of Palmitic Acid in the Presence of Choline Hydroxide

To disperse fatty acids in aqueous solution, choline, a quaternary ammonium ion, has been used recently. So far, only the self-assembly of myristic acid (MA) in the presence of choline hydroxide as a function of the molar ratio has been investigated, and, thus, the current understanding of these fatty acid systems is still limited. We investigated the self-assembly of palmitic acid (PA) in the presence of choline hydroxide (ChOH) as a function of the molar ratio (R) between ChOH and PA. The self-assemblies were characterized by phase contrast microscopy, cryo-TEM, small-angle X-ray scattering, and 2H NMR. The ionization state of PA was determined by pH, conductivity, and FT-IR measurements. With increase in R, various self-assembled structures, including vesicles, lamellar phase, rigid membranes (large sheets, tubules, cones, and polyhedrals), and micelles, form in the PA/ChOH system, different from those of the MA/ChOH system. The change in R induces pH variation and, consequently, a change in the PA ionization state, which, in turn, regulates the molecular interactions, including hydrogen bonding and electrostatic interaction, leading to various self-assemblies. Temperature is an important factor used to tune the self-assembly transitions. The fatty acid choline systems studied here potentially may be applicable in medicine, chemical engineering, and biotechnology.

Aqueous Binary Mixtures of Stearic Acid and Its Hydroxylated Counterpart 12-Hydroxystearic Acid: Fine Tuning of the Lamellar/Micelle Threshold Temperature Transition and of the Micelle Shape

This study examines the structures of soft surfactant-based biomaterials which can be tuned by temperature. More precisely, investigated here is the behavior of stearic acid (SA) and 12-hydroxystearic acid (12-HSA) aqueous mixtures as a function of temperature and the 12-HSA/SA molar ratio (R). Whatever R is, the system exhibits a morphological transition at a given threshold temperature, from multilamellar self-assemblies at low temperature to small micelles at high temperature, as shown by a combination of transmittance measurements, Wide Angle X-ray diffraction (WAXS), small angle neutron scattering (SANS), and differential scanning calorimetry (DSC) experiments. The precise determination of the threshold temperature, which ranges between 20 °C and 50 °C depending on R, allows for the construction of the whole phase diagram of the system as a function of R. At high temperature, the micelles that are formed are oblate for pure SA solutions (R = 0) and prolate for pure 12-HSA solutions (R = 1). In the case of mixtures, there is a progressive continuous transition from oblate to prolate shapes when increasing R, with micelles that are almost purely spherical for R = 0.33.

Aqueous Binary Mixtures of Stearic Acid and Its Hydroxylated Counterpart 12-Hydroxystearic Acid: Cascade of Morphological Transitions at Room Temperature

Here, we describe the behavior of mixtures of stearic acid (SA) and its hydroxylated counterpart 12-hydroxystearic acid (12-HSA) in aqueous mixtures at room temperature as a function of the 12-HSA/SA mole ratio R. The morphologies of the self-assembled aggregates are obtained through a multi-structural approach that combines confocal and cryo-TEM microscopies with small-angle neutron scattering (SANS) and wide-angle X-ray scattering (WAXS) measurements, coupled with rheology measurements. Fatty acids are solubilized by an excess of ethanolamine counterions, so that their heads are negatively charged. A clear trend towards partitioning between the two types of fatty acids is observed, presumably driven by the favorable formation of a H-bond network between hydroxyl OH function on the 12th carbon. For all R, the self-assembled structures are locally lamellar, with bilayers composed of crystallized and strongly interdigitated fatty acids. At high R, multilamellar tubes are formed. The doping via a low amount of SA molecules slightly modifies the dimensions of the tubes and decreases the bilayer rigidity. The solutions have a gel-like behavior. At intermediate R, tubes coexist in solution with helical ribbons. At low R, local partitioning also occurs, and the architecture of the self-assemblies associates the two morphologies of the pure fatty acids systems: they are faceted objects with planar domains enriched in SA molecules, capped with curved domains enriched in 12-HSA molecules. The rigidity of the bilayers is strongly increased, as well their storage modulus. The solutions remain, however, viscous fluids in this regime.

Fatty Acid Photodecarboxylase Is an Interfacial Enzyme That Binds to Lipid-Water Interfaces to Access Its Insoluble Substrate

Fatty acid photodecarboxylase (FAP), one of the few natural photoenzymes characterized so far, is a promising biocatalyst for lipid-to-hydrocarbon conversion using light. However, the optimum supramolecular organization under which the fatty acid (FA) substrate should be presented to FAP has not been addressed. Using palmitic acid embedded in phospholipid liposomes, phospholipid-stabilized microemulsions, and mixed micelles, we show that FAP displays a preference for FAs present in liposomes and at the surface of microemulsions. The kinetics of adsorption onto phospholipid and galactolipid monomolecular films further suggests the ability of FAP to bind to and penetrate into membranes, with a higher affinity in the presence of FAs. The FAP structure reveals a potential interfacial recognition site with clusters of hydrophobic and basic residues surrounding the active site entrance. The resulting dipolar moment suggests the orientation of FAP at negatively charged interfaces. These findings provide important clues about the mode of action of FAP and the development of FAP-based bioconversion processes.

Dynamics of the vesicles composed of fatty acids and other amphiphile mixtures: unveiling the role of fatty acids as a model protocell membrane

Fundamental research at the interface of chemistry and biology has the potential to shine light on the question of how living cells can be synthesized from inanimate matter thereby providing plausible pathways for the emergence of cellular life. Compartmentalization of different biochemical reactions within a membrane bound water environment is considered an essential first step in any origin of life pathway. It has been suggested that fatty acid-based vesicles can be considered a model protocell having the potential for change via Darwinian evolution. As such, protocell models have the potential to assist in furthering our understanding of the origin of life in the laboratory. Fatty acids, both by themselves and in mixtures with other amphiphiles, can form different self-assembled structures depending on their surroundings. Recent studies of fatty acid-based membranes have suggested likely pathways of protocell growth, division and membrane permeabilisation for the transport of different nutrients, such as nucleotides across the membrane. In this review, different dynamic processes related to the growth and division of the protocell membrane are discussed and possible pathways for transition of the protocell to the modern cell are explored. These areas of research may lead to a better understanding of the synthesis of artificial cell-like entities and thus herald the possibility of creating new form of life distinct from existing biology. Graphical Abstract Table of Content (TOC) only.

Bile acid derivative-based catanionic mixtures: versatile tools for superficial charge modulation of supramolecular lamellae and nanotubes

Self-assembled structures formed by mixtures of cationic and anionic surfactants are interesting tools for applications requiring interactions with charged particles and molecules. Nevertheless, they present instability close to the equimolar composition and poor morphological versatility, which is generally restricted to vesicles and micelles. Against this general trend, we report on bile salt derivative based catanionic mixtures assembling in tubules and lamellae depending on the mixture composition. Electrophoretic mobility measurements prove that the composition also dictates their superficial charge, which can be tuned from negative to positive by increasing the positively charged surfactant fraction in the mixtures. The study of the catanionic aggregates was conducted by means of microscopy and spectroscopy techniques and compared to the self-assembly behaviors of the individual building blocks. This study broadens the so far small array of bile salt derivative catanionic systems, confirming their distinctive behavior in the spectrum of catanionic mixtures.

Rheological behavior and mechanism of pH-responsive wormlike micelle variations induced by isomers of phthalic acid

Responsive wormlike micelles (WLMs) constructed by different carboxylic acids are fascinating. However, it is unknown how the position of the carboxylic groups alters the stimuli-response of wormlike micellar systems. Herein, three pH-responsive WLMs based on Gemini-like surfactants (named o-EAPA, m-EAPA, and p-EAPA) were formed and studied through the complexation of N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and o-phthalic acid (o-PA), m-phthalic acid (m-PA), or p-phthalic acid (p-PA) at the molar ratio of 2 : 1. The viscoelasticity, phase behavior and aggregate microstructure were separately explored by rheological, appearance observation and cryo-TEM methods. The results show that all phthalic acids can protonate UC22AMPM, thereby forming WLMs. However, with the shorter spacer distance between two carboxyl groups in phthalic acid, o-EAPA exhibits the longer length scale of aggregates and a more efficient thickening ability compared to the other two systems. Similar results in the N,N-dimethyl oleoaminde-propylamine (DOAPA) and o-PA, m-PA, and p-PA systems further verify the applicability of this mechanism. Furthermore, the phthalic acid based WLMs are found to exhibit intriguing reversible pH-responsive behaviors, which include promptly switching between a high elastic system and a low viscosity fluid by pH control. The o-EAPA system possesses a larger viscosity maximum, which produces more precipitous viscosity changes as the pH varies. This study is beneficial for the formation of pH-responsive WLMs and to determine their advantages for applications.

Aggregation of Oligomeric Surfactant Constructed by Four-Armed Carboxylic Acid Sodium and Cationic Surfactant

A star-shaped oligomeric-like surfactant with variable oligomeric degrees has been formed with a four-arm carboxylate salt (4EOCOONa) and cationic single chain surfactant dodecyl trimethylammonium bromide (DTAB). The aggregation of the 4EOCOONa/(DTAB)_n complexes has been investigated by surface tension, electrical conductivity, isothermal titration microcalorimetry, ζ potential, dynamic light scattering, ¹H NMR spectroscopy, and steady-state fluorescence measurements. The calorimetric result shows that 4EOCOONa interacts strongly with DTAB and each 4EOCOONa molecule binds with six DTAB molecules, wherein four DTAB molecules electrostatically bind to one 4EOCOONa molecule and additional two DTAB molecules further bind to the 4EOCOONa/(DTAB)_n complex by hydrophobic interaction. The critical micelle concentration (CMC) of the 4EOCOONa/(DTAB)_n complexes is remarkably lower than the CMC of DTAB, similar to synthesized star-shaped oligomeric surfactants. The micelle properties of the DTAB/4EOCOONa mixtures depend on the component changes of the 4EOCOONa/(DTAB)_n complexes. By increasing the DTAB/4EOCOONa molar ratio and/or concentration, the DTAB/4EOCOONa mixtures gradually form the complexes of 4EOCOO(DTA)₁^3-, 4EOCOO(DTA)₂^2-, 4EOCOO(DTA)₃^-, 4EOCOO(DTA)₄, and 4EOCOO(DTA)₆²⁺, and the corresponding aggregates are small anionic micelles with loose molecular packing, and nearly nonionic or positively charged small micelles with more compact packing. Moreover, the positive charge of the small micelles increases with the increase of the concentration and the DTAB/4EOCOONa molar ratio. Therefore, constructing oligomeric-like surfactants by adding appropriate organic salts into conventional surfactants is a convenient method to achieve desired properties of surfactant aggregates.

A renaissance of soaps? - How to make clear and stable solutions at neutral pH and room temperature

Soaps are the oldest and perhaps most natural surfactants. However, they lost much of their importance since "technical surfactants", usually based on sulfates or sulfonates, have been developed over the last fifty years. Indeed, soaps are pH- and salt-sensitive and they are irritant, especially to the eyes. In food emulsions, although authorized, they have a bad taste, and long-chain saturated soaps have a high Krafft temperature. We believe that most or perhaps all of these problems can be solved with modern formulation approaches. We start this paper with a short overview of our present knowledge of soaps and soap formulations. Then we focus on the problem of the lacking soap solubility at neutral pH values. For example, it is well known that with the food emulsifier sodium oleate (NaOl), clear and stable aqueous solutions can only be obtained at pH values higher than 10. A decrease in the pH value leads to turbid and unstable solutions. This effect is not compatible with the formulation of aqueous stable and drinkable formulations with neutral or even acidic pH values. However, the pH value/phase behavior of aqueous soap solutions can be altered by the addition of other surfactants. Such a surfactant can be Rebaudioside A (RebA), a steviol glycoside from the plant Stevia rebaudiana which is used as a natural food sweetener. In a recent paper, we showed the influence of RebA on the apKa value of sodium oleate in a beverage microemulsion and on its clearing temperature. In the present paper, we report on the effect of the edible bio-surfactant RebA, on the macroscopic and microscopic phase behavior of simple aqueous sodium oleate solutions at varying pH values. The macroscopic phase behavior is investigated by visual observation and turbidity measurements. The microscopic phase behavior is analyzed by acid-base titration curves, phase-contrast and electron microscopy. It turned out that even at neutral pH, aqueous NaOl/RebA solutions can be completely clear and stable for more than 50days at room temperature. This is for the first time that a long chain soap could be really solubilized in water at neutral pH at room temperature. At last, these findings were applied to prepare stable, highly translucent and drinkable aqueous solutions of omega-3-fatty acids at a pH value of 7.5.

Self-Assembly and Rheological Properties of a Pseudogemini Surfactant Formed in a Salt-Free Catanionic Surfactant Mixture in Water

The surface and bulk properties of bola-type dicarboxylic acid (sebacic acid, SA) and zwitterionic surfactant tetradecyldimethylamine oxide (C14DMAO) mixtures in aqueous solutions were studied. Surface tension measurements indicate a pronounced synergistic effect between SA and C14DMAO. In bulk aqueous solutions, rich phase behavior was observed with a varied SA-to-C14DMAO ratio (ρ) and a total surfactant concentration. Typically at ρ = 0.5, a novel pseudogemini surfactant (C14-S-C14) forms, driven by electrostatic interaction and hydrogen bonding. The C14-S-C14/H2O system exhibits rich phase behavior induced by the transition of aggregates. With increasing concentration of C14-S-C14, one can observe a viscous L1 phase, an L1/Lα two-phase region where a birefringent Lα phase is on the top of an L1 phase, a single Lα phase, and finally a mixture of an Lα phase and a precipitate. Microstructures formed in the Lα phases were determined by freeze-fracture transmission electron microscopy (FF-TEM) and cryogenic-transmission electron microscopy (cryo-TEM) observations. Polymorphic aggregation behavior was observed with the formation of a variety of bilayer structures including unilamellar vesicles, onions, and open and hyperbranched bilayers. Rheological measurements showed that the Lα phases are viscoelastic and sensitive to temperature where a quick loss of viscoelasticity was observed at elevated temperature.

CO2-Controllable Foaming and Emulsification Properties of the Stearic Acid Soap Systems

Fatty acids, as a typical example of stearic acid, are a kind of cheap surfactant and have important applications. The challenging problem of industrial applications is their solubility. Herein, three organic amines-ethanolamine (EA), diethanolamine (DEA), and triethanolamine (TEA)-were used as counterions to increase the solubility of stearic acid, and the phase behaviors were investigated systematically. The phase diagrams were delineated at 25 and 50 °C, respectively. The phase-transition temperature was measured by differential scanning calorimetry (DSC) measurements, and the microstructures were vesicles and planar sheets observed by cryogenic transmission electron microscopy (cryo-TEM) observations. The apparent viscosity of the samples was determined by rheological characterizations. The values, rcmc, for the three systems were less than 30 mN·m(-1). Typical samples of bilayers used as foaming agents and emulsifiers were investigated for the foaming and emulsification assays. CO2 was introduced to change the solubility of stearic acid, inducing the transition of their surface activity and further achieving the goal of defoaming and demulsification.

Fully deuterated magnetically oriented system based on fatty acid direct hexagonal phases

There is strong demand in the field of NMR for simple oriented lipid supramolecular assemblies, the constituents of which can be fully deuterated, for specifically studying the structure of host protonated molecules (e.g., peptides, proteins...) in a lipid environment. Also, small-angle neutron scattering (SANS) in fully deuterated oriented systems is powerful for gaining information on protonated host molecules in a lipid environment by using the contrast proton/deuterium method. Here we report on a very simple system made of fatty acids (dodecanoic and tetradecanoic) and ethanolamine in water. All components of this system can be obtained commercially as perdeuterated. Depending on the molar ratio and the concentration, the system self-assembles at room temperature into a direct hexagonal phase that is oriented by moderate magnetic fields of a few tesla. The orientation occurs within the magnetic field upon cooling the system from its higher-temperature isotropic phase: the lipid cylinders of the hexagonal phase become oriented parallel to the field. This is shown by solid-state NMR using either perdeuterated fatty acids or ethanolamine. This system bears strong interest for studying host protonated molecules but also in materials chemistry for building oriented solid materials.

Self-assembly of fatty acids in the presence of amines and cationic components

Fatty acids can self-assemble under various shapes in the presence of amines or cationic components. We assemble and compare these types of self-assembly leading toward a catanionic system either with a cationic surfactant or with an amine component playing the role of counter-ion. First, we focus on the molar ratio as a key driving parameter. Known and yet un-known values from other quantities governing the colloidal properties of these systems such as structural surface charge, osmotic pressure, molecular segregation, rigidity, in plane colloidal interactions and melting transition are discussed. We include also recent results obtained on the interfacial and foaming properties of these systems. We will highlight the specificity of these self-assemblies leading to unusual macroscopic properties rich of robust applications.

Self-assembly of fatty acids: from foams to protocell vesicles

Stable aqueous dispersions of fatty acids can now be obtained and yield multiple applications.

Polymorphism of natural fatty acid liquid crystalline phases

We study the phase behavior in water of a mixture of natural long chain fatty acids (FAM) in association with ethylenediamine (EDA) and report a rich polymorphism depending on the composition. At a fixed EDA/FAM molar ratio, we observe upon dilution a succession of organized phases going from a lamellar phase to a hexagonal phase and, finally, to cylindrical micelles. The phase structure is established using polarizing microscopy, SAXS, and SANS. Interestingly, in the lamellar phase domain, we observe the presence of defects upon dilution, which SAXS shows to correspond to intrabilayer correlations. NMR and FF-TEM techniques suggest that these defects are related to an increase in the spontaneous curvature of the molecule monolayers in the lamellae. ATR-FTIR spectroscopy was also used to investigate the degree of ionization within these assemblies. The successive morphological transitions are discussed with regards to possible molecular mechanisms, in which the interaction between the acid surfactant and the amine counterion plays the leading role.

Self-assembly, foaming, and emulsifying properties of sodium alkyl carboxylate/guanidine hydrochloride aqueous mixtures

Unsaturated fatty acids may be extracted from various agricultural resources and are widely used as soaps in the industry. However, there also exist a large variety of saturated and hydroxy fatty acids in nature, but their metal salts crystallize at room temperature in water, hampering their use in biological and chemical studies or for industrial applications. Addition of guanidine hydrochloride (GuHCl) to sodium salt of myristic acid has been shown to prevent its crystallization in water, forming stable flat bilayers at room temperature. Herein, we extend this finding to two other saturated fatty acids (palmitic and stearic acids) and two hydroxyl fatty acids (juniperic and 12 hydroxy stearic acids) and study more deeply (by using small angle neutron scattering) the supramolecular assemblies formed in both saturated and hydroxyl fatty acid systems. In addition, we take the advantage that crystallization no longer occurs at room temperature in the presence of GuHCl to study the foaming and emulsifying properties of those fatty acid dispersions. Briefly, our results show that all fatty acids, even juniperic acid, which is a bola lipid, are arranged in a bilayer structure that may be interdigitated. Depending on the nature of the fatty acid, the systems exhibit good foamability and foam stability (except for juniperic acid), and emulsion stability was good. Those findings should be of interest for using saturated long chain (and hydroxyl) fatty acids as surfactants for detergency or even materials chemistry.

Magnetic self-orientation of lyotropic hexagonal phases based on long chain alkanoic (fatty) acids

It is presently shown that long chain (C14, C16, and C18) alkanoic (saturated fatty) acids can form magnetically oriented hexagonal phases in aqueous concentrated solutions in mixtures with tetrabutylammonium (TBAOH) as the counterion. The hexagonal phase occurred for a molar ratio, alkanoic acid/TBAOH, higher than 1, i.e., for an excess of fatty acid. The hexagonal phase melted to an isotropic phase (micelles) upon heating at a given temperature depending on the alkyl chain length. The self-orientation of the hexagonal phase occurred upon cooling from the "high-temperature" isotropic phase within the magnetic field. The long axis of the hexagonal phase was shown to self-orient parallel to the magnetic field as evidenced by deuterium solid-state NMR. This finding is expected to be of interest in the field of structural biology and materials chemistry for the synthesis of oriented materials.

Self-assembly and foaming properties of fatty acid-lysine aqueous dispersions

We report on dispersions of fatty acid-lysine salts in aqueous solutions which are further used to produce foams. The alkyl chain length is varied from dodecyl to stearic. In aqueous solutions, the lysine salt of the dodecyl chain yields an isotropic solution, probably micelles, whereas for longer alkyl chains, vesicles formed but crystallized upon resting at room temperature or when kept at 4 degrees C. Solid-state NMR showed that in vesicles fatty acids are embedded in a lamellar arrangement passing from a gel to a fluid state upon heating; the transition temperature at which it occurs was determined by DSC. Those results are confirmed by small-angle neutron scattering which also give additional information on the bilayer structure. Incredibly stable foams are obtained using the palmitic acid/Lys salt whereas for other alkyl chain length, poor or no foam is formed. We conclude that the foamability is related to the phase behavior in aqueous solution.

Headgroup effects on the unusual lamellar-lamellar coexistence and vesicle-to-micelle transition of salt-free catanionic amphiphiles

Salt-free ion-paired catanionic amphiphiles of the C(m)(+)C(n)(-) type, with a high solubility mismatch (n >> m or m >> n) display a remarkable phase behavior in water. A temperature-driven vesicle-to-micelle transition in the dilute side together with a coexistence of two lamellar phases on the concentrated side is one of the peculiar effects that have been reported for the hexadecyltrimethylammonium octylsulfonate surfactant, C(16)C(8) or TA(16)So(8) (extensive to C(14)C(8) and C(12)C(8)). In this work, with TA(16)So(8) as a reference, the cationic trimethylammonium (TA(+)) and pyridinium (P(+)) headgroups are combined with the anionic sulfate (S(-)) and sulfonate (So(-)) headgroups to yield other C(16)C(8) compounds: hexadecyltrimethylammonium octylsulfate (TA(16)S(8)), 1-hexadecylpyridinium octylsulfonate (P(16)So(8)), and 1-hexadecylpyridinium octylsulfate (P(16)S(8)). We show that, if the asymmetry of the chain lengths is kept constant at C(16)C(8) and the headgroup chemistry is changed, most of the unusual self-assembly properties are still observed, indicating that they are not system-specific but extensive to other combinations of headgroups and mainly dictated by the ion-pair solubility mismatch. Thus, all the compounds in water quite remarkably show a lamellar-lamellar phase coexistence and spontaneously form vesicles upon solubilization. Moreover, P(16)So(8) undergoes a temperature-driven vesicle-to-micelle transition that involves an intermediate planar lamellar state, similar to TA(16)So(8). Some interesting effects on the global phase behavior, however, do arise when the headgroups are changed. Geometric packing effects are shown to be important, but the differences in phase behavior seem to be mainly dictated by (i) the charge density of the headgroups, which tunes the solubility mismatch of the ion-pair, and (ii) specific interactions between headgroups, which affect the short-range repulsive force that controls the swelling of the concentrated lamellar phase.

Chiral cones and vesicles from Gemini-type fatty acid-heteroditopic amine mixtures

A twin-chiral and twin-tailed Gemini-type fatty acid with a rigid spacer, 2,3-bis(decyloxy) succinic acid, formed chiral self-assembled structures in aqueous solution with heteroditopic amines, such as 6-aminohexanoic acid and 4-aminobutyric acid. The morphology of such structures was governed by the stoichiometry between the fatty acid and the amine. Mixtures with 1 : 2 molar ratio formed exclusively spherical vesicles, while the 1 : 1 mixtures resulted in entropically driven chiral cones in addition to spherical vesicles. Surface tension measurements, light scattering, and TEM studies proved the existence of vesicles in aqueous solution. Cones were formed through disclinations introduced in the hexatic chiral membrane with specific angles, in accordance with θ = arcsin (1 - (n/6)), [0 < θ < π/2, 1 < n < 5]. Mixtures prepared with 4-aminobutyric acid formed only spherical vesicles and no conical self-assemblies, emphasizing the geometry of the Gemini-heteroditopic amine pair, which directs the morphology of the final self-assembled structures.

Temperature-dependent self-assembly and mixing behavior of symmetrical single-chain bolaamphiphiles

The temperature-dependent self-assembly and the mixing behavior of symmetrical single-chain bolaamphiphiles with different polymethylene chain lengths and different headgroup structures were investigated in water by differential scanning calorimetry (DSC), cryo transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). The even-numbered polymethylene-1,omega-bis(phosphocholines) (PC-C n-PC) are known to form nanofibers composed of stretched molecules with an all- trans alkyl chain conformation (Drescher, S.; Meister, A.; Blume, A.; Karlsson, G.; Almgren, M.; Dobner, B. Chem.Eur. J. 2007, 13, 5300-5307). The odd-numbered analogues were synthesized to study a possible even-odd effect of these bolaamphiphiles during their aggregation in water. In addition to these bolaamphiphiles with phosphocholine headgroups, a new series of polymethylene-1,omega-bis(phosphodimethylethanolamines) (Me2PE-Cn-Me2PE) with smaller headgroup sizes was synthesized. These bolaamphiphiles show an additional fiber-fiber transition when the alkyl chain length exceeds 26 carbon atoms. The mixing behavior of both types of bolaamphiphiles indicates that differences in the alkyl chain length up to six carbon atoms are tolerated within the fiber structure. The mixing of two Me2PE-Cn-Me2PE or PC-Cn-PC type bolaamphiphiles with different alkyl chain lengths offers the possibility to adjust the temperature, where the cross-linking of the fibers is disrupted and where the fibers break apart. As a consequence, temperature switchable hydrogels are obtained that can be fine-tuned for drug delivery applications. The comparison with dotriacontane-1,32-diyl-bis[2-(methylammonio)-ethylphosphate] (MePE-C32-MePE), a new bolaamphiphile with even smaller phosphomonomethylammonio headgroups, illustrates the importance of the headgroup size for the aggregation behavior. This bolaamphiphile self-assembles exclusively into lamellar structures, and this aggregate type persists in mixtures with the fiber forming Me2PE-C32-Me2PE.

Self-assembly of fatty acids and hydroxyl derivative salts

We report the dispersions of a fatty acid and hydroxyl derivative salts in aqueous solutions that were further used to produce foams and emulsions. The tetrabutyl-ammonium salts of palmitic acid, 12-hydroxy stearic acid, and omega-hydroxy palmitic acid formed isotropic solutions of micelles, whereas the ethanolamine salts of the same acids formed turbid birefringent lamellar solutions. The structure and dimension of those phases were confirmed by small-angle neutron scattering and NMR. Micelles exhibited a surprisingly small radius of about 20 A, even for hydroxyl fatty acids, suggesting the formation of hydrogen bonds between lipids in the core of the micelles. In the case of ethanolamine salts of palmitic and 12-hydroxy stearic acids, the lipids were arranged in bilayers, with a phase transition from gel to fluid upon heating, whereas for omega-hydroxy palmitic acid, monolayers formed in accordance with the bola shape of this lipid. Foams and emulsions produced from ethanolamine salt solutions were more stable than those obtained from tetrabutyl-ammonium salt solutions. We discuss these results in terms of counterion size, lipid molecular shape, and membrane curvature.

Use of degradable and nondegradable nanomaterials for controlled release

Drug-delivery devices are fundamentally important in improving the pharmacological profiles of therapeutic molecules. Nanocontrolled-release systems are attracting a lot of attention currently owing to their large surface area and their ability to target delivery to specific sites in the human body. In addition, they can penetrate the cell membrane for gene, nucleic acid and bioactive peptide/protein delivery. Representative applications of nanodrug-delivery systems include controlled-release wound dressings, controlled-release scaffolds for tissue regeneration and implantable biodegradable nanomaterial-based medical devices integrated with drug-delivery functions. We review the present status and future perspectives of various types of nanocontrolled-release systems. Although many of the well-established degradable and nondegradable controlled-release vehicles are being investigated for their processing into nanocarriers, several new emerging nanomaterials are being studied for their controlled-release properties. The release of multiple bioactive agents, each with its own kinetic profile, is becoming possible. In addition, integration of the nanocontrolled-release systems with other desirable functions to create new, cross-discipline applications can also be realized.

Wormlike micelles: where do we stand? Recent developments, linear rheology and scattering techniques

Wormlike micelles are elongated flexible self-assembly structures formed by the aggregation of amphiphiles. Above a threshold concentration, they entangle into a dynamic network, reminiscent of polymer solutions, and display remarkable visco-elastic properties, which have been exploited in numerous industrial and technological fields. Relating the microstructure of these intricate structures with their bulk properties is still an ongoing quest. In this review, we present a classification of wormlike micelles, with a focus on novel systems and applications. We describe the current state of understanding of their linear rheology and give a detailed account of recent progress in small-angle neutron scattering, a particularly powerful technique to elucidate their microstructure on a wide range of length-scales.

Ion exchange in catanionic mixtures: from ion pair amphiphiles to surfactant mixtures

We have studied concentrated equimolar mixtures of tetradecanoic acid (myristic acid, C13COOH) and hexadecyltrimethylammonium hydroxide (CTAOH) in which the OH- counterions are gradually exchanged by other anions (Cl-, Br-, CH3COO-, CH3-(C6H4)-SO3-). We demonstrate that the stability of a Lbeta phase can be achieved at equimolarity between both surfactants, provided that the phase contains also a sufficient number of anions exchanged with OH-. In the absence of exchange (equimolar mixture of C13COOH and CTAOH), a three-dimensional crystalline Lc phase is produced. As the OH- ions are replaced by other ions, a swollen Lbeta lamellar phase appears, first in coexistence with the Lc (D* = 400 A) and then in coexistence with a dilute phase only (D* = 215 A). In the latter regime, the repeating distance depends very little on the exchange ratio, but rather on the nature of the counterion. If too many OH- ions are exchanged, the Lbeta phase becomes unstable again. A Poisson-Boltzmann model with charge regulation computed for a closed system predicts qualitatively the existence of this narrow domain of stability for the Lbeta phase.