Cylindrical Reverse Micelles Research Articles

Robust sugar alcohol with lecithin functionalities for sustainable gelation of edible oils

Edible oleogels are of interest to many researchers in the food industry. However, the applications of oleogels in foods are limited owing to the demand for food-grade and low-cost gelators. In this study, we investigated a facile and reliable route for the formation of oleogels using lecithin and D-sorbitol as gelators, which are food-grade and commercially available. Grapeseed, olive, rice bran, and sesame oils are used as edible oils and all of which gel at a certain molar ratio of lecithin (40 mM) and D-sorbitol (42–54 mM), wherein the samples do not flow downward in the inverted vials and the formed oleogels are analyzed. The rheological properties of the samples, such as zero-shear viscosity (∼20 Pa.s to infinite viscosity), viscous modulus, and elastic modulus (70–110 Pa) under diverse conditions (i.e., shear-stress and temperature), were explored in detail by steady-shear and dynamic rheology. The mechanism of gel formation was also studied using small-angle X-ray scattering (SAXS) technique, which revealed that the gel was formed by the entanglement of the reverse cylindrical micelles (slop of I vs q: ∼1) induced by the hydrogen bonds between lecithin and D-sorbitol. Furthermore, the hydrogen bonds were confirmed with the Fourier transform infrared spectroscopy (FTIR), in which the absorbance bands of phosphate (red-shift) and carbonyl groups (blue-shift) of lecithin are shifted upon addition of D-sorbitol. Finally, this method to create oleogels of diverse edible oils using lecithin/D-sorbitol mixtures is simple and reproducible; thus, mass production of oleogels for marketable consumer products is expected to be achieved.

Structural transition of reverse cylindrical micelles to reverse vesicles by mixtures of lecithin and inorganic salts

HypothesisThe transformation from reverse micelles to reverse vesicles is influenced by electrostatic interactions between lecithin headgroups and inorganic salts. The electrostatic interactions are expected to influence molecular geometry of lecithin, resulting in a reduction in critical packing parameter (p). Hence, it should be possible to drive structural transitions of reverse self-assembled structures by addition of inorganic salts to lecithin solutions. ExperimentsStructural transitions of reverse micelles and reverse vesicles were formulated including lecithin and inorganic salts as a function of concentration in cyclohexane. A systematic study was performed using inorganic salts with the different valences of the cations such as Li+, Ca2+, and La3+. To probe the nanodomain structures from the lecithin/salt mixtures, small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used. FindingsAdding salts to lecithin solutions induced the systematic transformation of reverse self-assembled structures from reverse spherical micelles to reverse cylindrical micelles and finally to reverse vesicles. The transformation was also correlated with interactions between lecithin headgroups and salts, that is, Li+ < Ca2+ < La3+. In addition, a water-soluble dye such as rhodamine B (RB) can be readily encapsulated into reverse micelles and vesicles, indicating that they are potentially useful for controlled solute delivery.

Effect of temperature on the reverse self-assembly of lecithin and sugar alcohol mixtures in a nonpolar solvent

It has been shown that the addition of sugar alcohols to lecithin solutions induces a transformation from reverse spherical micelles into reverse cylindrical micelles, resulting in the formation of viscoelastic Maxwell fluids. Herein, we systematically investigated the effects of temperature on the rheological parameters, such as the plateau modulus (Gp), relaxation time (tR), and zero-shear viscosity (η0), of lecithin and sugar alcohol mixtures in decane. The rheological properties upon heating of mixtures of lecithin and sugar alcohols with different numbers (3–6) of hydroxyl (–OH) groups and concentrations were studied in detail. Gp, tR, and η0 for all reverse cylindrical micelles decreased with temperature. This decrease can be attributed to a reduction in the lengths of the reverse cylindrical micelles, as evidenced by the small-angle X-ray scattering results at different temperatures. More importantly, the decay rates of Gp, tR, and η0 upon heating decreased more rapidly when the hydrogen bonds between the lecithin and sugar alcohol molecules were weaker as the hydrogen bonds were closely associated with the formation of the reverse self-assembled structure. This indicates that the driving force for the formation of reverse cylindrical micelles can be weakened by increasing the temperature.

Effect of hydrocarbon chain length of aliphatic solvents on the reverse self-assembly of lecithin and monovalent ion mixtures

Surfactants are used as organogelators, which form surfactant-based gels in nonpolar solvents. One of the well-known surfactants that form organogels is lecithin, which self-assembles into reverse spherical micelles in nonpolar solvents. When inorganic salts are added to lecithin solutions, the reverse spherical micelles transform into long reverse cylindrical micelles, giving rise to organogels. Although previous studies have shown that the addition of inorganic salts to lecithin solutions can induce the gel formation, the solvent effects on the gel formation have not been mapped out in detail. In this study, we systematically investigated the effects of hydrocarbon chain length of alkanes on the self-assembly of lecithin and monovalent ion mixtures. Our findings showed that LiCl, LiBr, LiI, NaBr, NaI, and KI salts formed organogels in the presence of lecithin, whereas NaCl and KBr formed viscoelastic solutions with lecithin. More importantly, with an increase in the hydrocarbon chain length of alkanes, the lecithin/salt mixtures formed gels more efficiently. The gel formation is closely related to the length of the reverse cylindrical micelles, as confirmed by small-angle X-ray scattering analysis. In addition, the length of the cylinders is affected by the interactions between the lecithin head groups and ions, as evidenced by Fourier transform infrared spectroscopy.

Effect of aliphatic solvents on the reverse self-assembly of lecithin and calcium chloride mixtures

Reverse spherical micelles are formed by self-assembly of lecithin molecules in nonpolar solvents. They are transformed into reverse cylindrical micelles when inorganic salts such as CaCl2 are added to lecithin solutions. When the reverse cylindrical micelles are long enough to be entangled, the solution viscosity increases significantly, forming organogels. In this study, we investigate the effects of aliphatic solvents on the reverse self-assembly of lecithin and CaCl2 mixtures. When the hydrocarbon chain length of the aliphatic solvents increases, the viscosities of lecithin and CaCl2 mixtures increase. In addition, for the same hydrocarbon chain length, mixtures in alkanes have higher viscosities than those in alkenes. Small-angle X-ray scattering (SAXS) results suggest that the viscosities of the mixtures are correlated with the length of the reverse cylindrical micelles. Herein, Fourier transform infrared spectroscopy was used to investigate the interactions between lecithin headgroups and CaCl2, which play a significant role in forming the reverse cylindrical micelles.

Mechanism for Transition of Reverse Cylindrical Micelles to Spherical Micelles Induced by Diverse Alcohols.

Herein, the effects of various alcohols on lecithin/CaCl2 organogels are investigated. Mixtures of lecithin and CaCl2 form reverse cylindrical micelles, resulting in optically transparent organogels. The addition of various alcohols to a mixture of lecithin and CaCl2 induces a decrease in viscosity through which reverse cylindrical micelles are transformed into spherical micelles (or short cylindrical micelles). Long-hydrocarbon-chain alcohols decrease the viscosity of lecithin/CaCl2 mixtures more efficiently. Hydrogen bonding and hydrocarbon chain interactions between lecithin and alcohol play important roles in the morphological transition. More importantly, isothermal titration calorimetry was conducted to obtain thermodynamic variables such as the enthalpy, equilibrium constant, Gibbs free energy, entropy, and stoichiometry of the associated molecules observed in the transition. It was found that the transition is an entropically driven process, in which the endothermic and exothermic behaviors were observed depending on the hydrocarbon chain length in the alcohol. In addition, the enthalpy for the association of the alcohol with lecithin showed a linear relationship depending on the hydrocarbon chain length, in which the magnitude of hydrogen bonding and hydrocarbon chain interactions was obtained quantitatively. To the best of our knowledge, this is the first study reporting the thermodynamic properties of the morphological transition observed in a reverse self-assembly process.

Formation of Two-Dimensional Network of Organic Charge-Transfer Complexes at the Air-Water Interface.

The air-water interface is an ideal platform to produce two-dimensional (2D) structures involving anything from simple organic molecules to supramolecular moieties by exploiting hydrophobic-hydrophilic interactions. Here, we show, using grazing incidence X-ray scattering, the formation of a 2D ordered structure of a charge-transfer (C-T) complex, namely, dodecyl methyl viologen (DMV) as acceptor and coronene tetracarboxylate potassium salt (CS) as donor, at the air-water interface. We have observed a phase transition in the 2D ordered structure as the area per molecule is decreased with increasing surface pressure in a Langmuir trough. The high-pressure ordering of the hydrocarbon chains associated with DMV destroys long-range C-T conjugation of DMV and CS at the air-water interface. Our results also explain the formation of DMV-CS cylindrical reverse micelles and eventually long nanowires that get formed in the self-assembly process in the bulk medium to preserve both the C-T conjugation and the organic tail-tail organization.

Turbulence and Cavitation Suppression by Quaternary Ammonium Salt Additives

We identify the physical mechanism through which newly developed quaternary ammonium salt (QAS) deposit control additives (DCAs) affect the rheological properties of cavitating turbulent flows, resulting in an increase in the volumetric efficiency of clean injectors fuelled with diesel or biodiesel fuels. Quaternary ammonium surfactants with appropriate counterions can be very effective in reducing the turbulent drag in aqueous solutions, however, less is known about the effect of such surfactants in oil-based solvents or in cavitating flow conditions. Small-angle neutron scattering (SANS) investigations show that in traditional DCA fuel compositions only reverse spherical micelles form, whereas reverse cylindrical micelles are detected by blending the fuel with the QAS additive. Moreover, experiments utilising X-ray micro computed tomography (micro-CT) in nozzle replicas, quantify that in cavitation regions the liquid fraction is increased in the presence of the QAS additive. Furthermore, high-flux X-ray phase contrast imaging (XPCI) measurements identify a flow stabilization effect in the region of vortex cavitation by the QAS additive. The effect of the formation of cylindrical micelles is reproduced with computational fluid dynamics (CFD) simulations by including viscoelastic characteristics for the flow. It is demonstrated that viscoelasticity can reduce turbulence and suppress cavitation, and subsequently increase the injector’s volumetric efficiency.

Gelation of commercially available mineral oils by lecithin and CaCl2 mixture

We describe the gelation of commercially available mineral oils used in cell cultures, food preparations and cosmetics. The addition of lecithin and CaCl2 mixtures (<5wt%) to mineral oils induced the gelation of the mineral oils by controlling the molar ratio of lecithin and CaCl2. The formation of the gels is believed to reflect the entanglement of reverse cylindrical micelles created by interactions between the lecithin and CaCl2. The rheological properties of the gels were studied by analyzing dynamic and steady rheology data, and FTIR techniques were used to clarify the interactions between the headgroups of lecithin and CaCl2. In addition, we showed that mineral oil gels were readily ungelled (liquefied) by the addition of an ungelling agent, such as a small amount of ethanol, or by heating.

Gelified reverse micellar dispersions as percutaneous formulations

This study describes the use of lecithin organogels (LO) as versatile vehicles for percutaneous administration of methyl nicotinate (MN), fenretinide (4HPR) and curcumin (CUR). LO are able to increase the solubility of poor water-soluble molecules probably due to the formation of a short-range hexagonal organization of the lecithin cylindrical reverse micelles. After six months from production, no phase separation and an almost absence of aggregates was observed. LO exhibited similar viscosities and rheological behaviour both in the absence and in the presence of drug. The in vitro drug diffusion from LO studied by Franz cell showed that higher viscous vehicles result in lower diffusion coefficient. The microscopic investigation of human skin displayed no significant alterations after treatment with LO and with PBS. In vivo topical activity on erythema after cutaneous application of LO showed that LO-MN induce a strong erythema, while LO-CUR inhibit skin erythema due to CUR anti-inflammatory activity. Tape-stripping experiments performed on skin after topical administration of LO showed a decrease in the amount of CUR in the stratum corneum after 1 h from the occlusion. Shelf life stability studies demonstrated the higher stability of LO-HPR as compared to the others drugs.

Structure and dynamics of water in nanoscopic spheres and tubes.

We study the reorientation dynamics of liquid water confined in nanometer-sized reverse micelles of spherical and cylindrical shape. The size and shape of the micelles are characterized in detail using small-angle x-ray scattering, and the reorientation dynamics of the water within the micelles is investigated using GHz dielectric relaxation spectroscopy and polarization-resolved infrared pump-probe spectroscopy on the OD-stretch mode of dilute HDO:H2O mixtures. We find that the GHz dielectric response of both the spherical and cylindrical reverse micelles can be well described as a sum of contributions from the surfactant, the water at the inner surface of the reversed micelles, and the water in the core of the micelles. The Debye relaxation time of the core water increases from the bulk value τ(H2O) of 8.2 ± 0.1 ps for the largest reverse micelles with a radius of 3.2 nm to 16.0 ± 0.4 ps for the smallest micelles with a radius of 0.7 nm. For the nano-spheres the dielectric response of the water is approximately ∼6 times smaller than expected from the water volume fraction and the bulk dielectric relaxation of water. We find that the dielectric response of nano-spheres is more attenuated than that of nano-tubes of identical composition (water-surfactant ratio), whereas the reorientation dynamics of the water hydroxyl groups is identical for the two geometries. We attribute the attenuation of the dielectric response compared to bulk water to a local anti-parallel ordering of the molecular dipole moments. The difference in attenuation between nano-spheres and nano-cylinders indicates that the anti-parallel ordering of the water dipoles is more pronounced upon spherical than upon cylindrical nanoconfinement.

DNA with Double-Chained Amphiphilic Counterions and Its Interaction with Lecithin

Complex salts of double-stranded DNA with amphiphilic counterions offer novel opportunities for studies of DNA-lipid interactions. Here the effect of the hydrophobicity of the amphiphilic counterion is in focus. For this purpose, double stranded DNA with didodecyldimethylammonium ions as counterions, DDADNA, is prepared and investigated with respect to microstructure. In particular, in order to monitor the interactions with phospholipids, the phase diagram of the DDADNA/lecithin/water system is determined and compared with the previously determined phase diagram with single alkyl chain counterions, dodecyltrimethylammonium, DTA. In both systems, there is a formation of lamellar and reverse hexagonal phases, where hydrated DNA is sandwiched between bilayers or forms the core of reverse cylindrical micelles, respectively. However, whereas the lecithin lamellar phase can incorporate large amounts of DDADNA, there is in the case of the single chain surfactant, DTADNA, a transition to a bicontinuous cubic phase at higher DTADNA concentrations. The general appearance of the phase diagrams, and in particular the role of counterion hydrophobicity, can be rationalized in a simple geometric model.

Cylindrical-to-Spherical Shape Transformation of Lecithin Reverse Micelles Induced by CO2

The effect of CO(2) on the microstructure of L-alpha-phosphatidylcholine (lecithin) reverse micelles was studied. The small-angle X-ray scattering (SAXS) results show that CO(2) could induce a cylindrical-to-spherical micellar shape transformation. Fourier transform infrared (FT-IR) and UV-vis techniques were also utilized to investigate intermolecular interactions and micropolarity in the reverse micelles at different CO(2) pressures. The reduction of the degree of hydrogen bonding between surfactant headgroups and water with added CO(2) was found to be the main reason for the micellar shape transformation. In the absence of CO(2), the hydrogen bonding between water and P=O of lecithin forms a linking bridge in the interfacial layer. Therefore, the free movement of the polar head of lecithin is limited and the cylindrical reverse micelles are formed. Upon adding CO(2) to the reverse micelles, the hydrogen bonds between lecithin and water in reverse micelles are destroyed, which is favorable to forming spherical micelles. Moreover, the CO(2)-combined reverse micelles were utilized in the synthesis of silica particles. Rodlike silica nanoparticles were obtained in the absence of CO(2), and ellipsoidal and spherical mesoporous silica particles were formed in the presence of CO(2). This method of tuning micellar shape has many advantages compared to traditional methods.

Synthesis and Self-Assembly of fcc Phase FePt Nanorods

We report a synthesis of FePt nanorods by confining decomposition of Fe(CO)5 and reduction of Pt(caca)2 in surfactant reverse cylindrical micelles. The controlled nucleation and growth kinetics in confined environment allows easy control over Fe/Pt composition, nanorod uniformity, and nanorod aspect ratio. The FePt nanorods tend to self-assemble into ordered arrays along three-dimensions. Directed assembly under external magnetic field leads to two-dimensional ordered arrays, parallel to the substrate magnetic field. We expect that with optimized external magnetic fields, we should be able to assemble these nanorods into orientated one or two-dimensional arrays, providing a uniform anisotropic magnetic platform for varied applications in enhanced data storage, magneto-electron transport, etc.

Formation of 1D Poly(3,4‐ethylenedioxythiophene) Nanomaterials in Reverse Microemulsions and Their Application to Chemical Sensors

Abstract1D poly(3,4‐ethylenedioxythiophene) (PEDOT) nanomaterials, including ellipsoidal nanoparticles, nanorods, and nanotubes, are fabricated via chemical oxidation polymerization in reverse (water‐in‐oil) microemulsions. The reverse cylindrical micelles are prepared with sodium bis(2‐ethylhexyl) sulfosuccinate (AOT) and aqueous FeCl3 solution in hexane. The morphology of the final products is determined by carefully tuning the degree of oxidation potential at the micelle surface. Notably, the fabrication of gram‐scale amounts of products is possible under optimized synthetic conditions, suggesting that this methodology is appropriate for the large‐scale production of the corresponding nanomaterials. The as‐prepared PEDOT nanomaterials are applied to the precise detection of alcohol vapors. The chemical sensors based on the PEDOT nanomaterials present excellent reversibility and reproducibility in response.

The SIV Tilted Peptide Induces Cylindrical Reverse Micelles in Supported Lipid Bilayers

Elucidation of the molecular mechanism leading to biomembrane fusion is a challenging issue in current biomedical research in view of its involvement in controlling cellular functions and in mediating various important diseases. According to the generally admitted stalk mechanism described for membrane fusion, negatively curved lipids may play a central role during the early steps of the process. In this study, we used atomic force microscopy (AFM) to address the crucial question of whether negatively curved lipids influence the interaction of the simian immunodeficiency virus (SIV) fusion peptide with model membranes. To this end, dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC) bilayers containing 0.5 mol % dioleoylphosphatidic acid (DOPA) were incubated with the SIV peptide and imaged in real time using AFM. After a short incubation time, we observed a 1.9 nm reduction in the thickness of the DPPC domains, reflecting either interdigitation or fluidization of lipids. After longer incubation times, these depressed DPPC domains evolved into elevated domains, composed of nanorod structures protruding several nanometers above the bilayer surface and attributed to cylindrical reverse micelles. Such DOPC/DPPC/DOPA bilayer modifications were never observed with nontilted peptides. Accordingly, this is the first time that AFM reveals the formation of cylindrical reverse micelles in lipid bilayers promoted by fusogenic peptides.

Formation Mechanism of Conducting Polypyrrole Nanotubes in Reverse Micelle Systems

Polypyrrole (PPy) nanotubes were readily fabricated through chemical oxidation polymerization in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse (water-in-oil) emulsions. The reverse cylindrical micelle phase was characterized, and the key factors affecting the formation of PPy nanotubes were systematically inspected. AOT reverse cylindrical micelles were prepared via a cooperative interaction between an aqueous FeCl3 solution and AOT in an apolar solvent. In the H2O/FeCl3/AOT/apolar solvent system, the aqueous FeCl3 solution played a role in increasing the ionic strength and decreasing the second critical micelle concentration of AOT. As a result, AOT reverse cylindrical micelles could be spontaneously formed in an apolar solvent. In addition, iron cations were adsorbed to the anionic AOT headgroups that were capable of extracting metal cations from the aqueous core. Under these conditions, the addition of pyrrole monomer resulted in the chemical oxidation polymerization of the corresponding monomer at the surface of AOT reverse cylindrical micelles, followed by the formation of tubular PPy nanostructures. In a typical composition (74.0 wt % hexane, 22.4 wt % AOT, and 3.6 wt % aqueous FeCl3 solution at 15 degrees C), the average diameter of PPy nanotubes was approximately 94 nm and their length was more than 2 mum. The PPy nanotube dimensions were affected by synthetic variables such as the weight ratio of aqueous FeCl3 solution/AOT, type of apolar solvent, and reaction temperature. Moreover, the relationship between the diameter and the conductivity of the nanotubes was investigated.

Periodic Arrays of Interfacial Cylindrical Reverse Micelles

We report an approach for the fabrication of periodic molecular nanostructures on surfaces. The approach involves biomimetic self-organization of synthetic wedge-shaped amphiphilic molecules into multilayer arrays of cylindrical reverse micelles. The films were characterized by atomic force microscopy and X-ray reflectivity. These nanostructured films self-assemble in solution but remain stable upon removal and exposure to ambient conditions, making them potentially suitable for a variety of dry pattern transfer methods. We illustrate the generality of this approach by using two distinct molecular systems that vary in size by a factor of 2.

Biocompatible Lecithin Organogels: Structure and Phase Equilibria

The microstructure of organogels formed upon the addition of tiny amounts of water to a solution of lecithin in fatty acid esters (viz. isopropylpalmitate and ethyloleate) was investigated by means of molecular self-diffusion measurements. In both systems lecithin and water form disconnected cylindrical reverse micelles. The ternary phase map for the lecithin/water/isopropylpalmitate has been investigated in detail. The organogel exists in a narrow region close to the lecithin-oil binary axis; for higher water content equilibrium between lamellae and reverse micelles is found. Lamellar phase occupies the lecithin-rich region, close to the lecithin corner (with the exception of a small island of hexagonal phase) and coexists with neat water close to the water-lecithin axis. The remaining part of the phase map shows the three-phase coexistence of water, oil, and lamellar phase.

Phase Behavior of the Lecithin/Water/Isooctane and Lecithin/Water/Decane Systems

The isothermal pseudo-ternary-phase diagram was determined at 25 degrees C for systems composed oflecithin, water, and, as oil, either isooctane or decane. This was accomplished by a combination of polarizing microscopy, small-angle X-ray scattering, and NMR techniques. The lecithin-rich region of the phase diagram is dominated by a lamellar liquid-crystalline phase (Lalpha). For lecithin contents less than 60% and low hydration (mole ratio water/lecithin = W0 < 5.5), the system forms a viscous gel of branched cylindrical reverse micelles. With increase in the water content, the system phase separates into two phases, which is either gel in equilibrium with essentially pure isooctane (for lecithin < 25%) or a gel in equilibrium with Lalpha (for lecithin > 25%). These two-phase regions are very thin with respect to water dilution. For 8 < W0 < 54 very stable water-in-oil emulsions form. It is only after ripening for more than 1 year that the large region occupied by the emulsion reveals a complex pattern of stable phases. Moving along water dilution lines, one finds (i) the coexistence of gel, isooctane and Lalpha, (ii) equilibrium between reverse micelles and spherulites, and, finally, (iii) disconnected reverse micelles that fail to solubilize water for W0 > 54. This results in a Winsor II phase equilibrium at low lecithin content, while for lecithin > 20% the neat water is in equilibrium with a reverse hexagonal phase and an isotropic liquid-crystalline phase. The use of the decane as oil does not change the main features of the phase behavior.