Chiral Surfactant Research Articles

The aggregation of amphiphilic (L)-proline-porphyrin derivatives in ethanol-water mixtures promoted by chiral anionic surfactants

The use of chiral surfactants results in a powerful tool to form chiral assemblies of (L)proline-functionalized porphyrin derivatives in EtOH/H[Formula: see text]O exploiting hydrophobic effect. The formation of chiral heteroaggregated species strictly depends both on the configuration of the chiral center of the surfactant and on its concentration, opening the possibility to tune the supramolecular asymmetry of the final structures.

Fluorescence emission originated from the H-aggregated cyanine dye with chiral gemini surfactant assemblies having a narrow absorption band and a remarkably large Stokes shift.

We demonstrate that the fluorescence emission that originated from a cyanine dye forming H-aggregates with a narrow absorption band and a remarkably large Stokes shift can be induced by symmetry breaking. The H-aggregate formation was successfully induced using chirally assembled cationic gemini surfactants with enantiomeric tartrate counterions as templates in water.

Concentration Dependent Specific Rotations of Chiral Surfactants: Experimental and Computational Studies.

Recent experimental studies have shown unexpected chiroptical response from some chiral surfactant molecules, where the specific rotations changed significantly as a function of concentration. To establish a theoretical understanding of this experimentally observed phenomena, a novel methodology for studying chiral surfactants via combined molecular dynamics (MD) and quantum mechanical (QM) calculations is presented. MD simulations on the +10 000 atom surfactant systems have been performed using MD and QM/molecular mechanics (MM) approaches. QM calculations performed on MD snapshots coupled with extensive analysis on lauryl ester of phenylalanine (LEP) surfactant system indicate that the experimentally observed variation of specific rotation with concentration may be due to the conformational differences of the surfactant monomers in the aggregates. Though traditional MM simulations did not show significant differences in the conformer populations, QM/MM simulations using the forces derived from the PM6 method did predict conformational differences between aggregated and nonaggregated LEP molecules, which is consistent with experimental data. Additionally the electrostatic environment of charged surfactants may also be important, since dramatic changes in the Boltzmann populations of surfactant monomers can be noted in the presence of an electric field generated by the chiral ionic aggregates.

Amino-Acid-Based Polymerizable Surfactants for the Synthesis of Chiral Nanoparticles.

Amino-acid-based chiral surfactants with polymerizable moieties are synthesized, and a versatile approach to prepare particles thereof with a chiral surface functionality is presented. As an example of an application, the synthesized particles are tested for their ability as nucleating agents in the enantioselective crystallization of amino acid conglomerate systems, taking rac-asparagine as a model system. Particles resulting from chiral surfactants with different tail groups are compared and the results demonstrate that only the chiral nanoparticles made of the polymerizable surfactant are able to act efficiently as nucleation agent in enantioselective crystallization.

Cationic chiral surfactant based micelle-guided asymmetric Morita-Baylis-Hillman reaction

Cationic chiral surfactant (1R, 2S)-(−)-N-dodecyl-N-methylephedrinium bromide (DMEB) was utilized for the first time in inducing asymmetry to Morita-Baylis-Hillman reaction in aqueous medium. Proton NMR studies carried out to determine the locus of the reaction in micro-heterogeneous micellar environment, were found useful in proposing a plausible model for asymmetric induction. This work demonstrates that under such mild and non-hazardous reactions conditions, the reaction rates increase, good yields are favored and above all reasonable enantiomeric excesses are obtained.

Non-covalent modification of reduced graphene oxide by a chiral liquid crystalline surfactant.

In order to effectively disperse reduced graphene oxide (RGO) in functional materials and take full advantage of its exceptional physical and chemical properties, a novel and effective approach for non-covalent modification of RGO by a chiral liquid crystalline surfactant (CLCS) consisting of chiral mesogenic units, nematic mesogenic units with carboxyl groups and non-mesogenic units with a polycyclic conjugated structure is firstly established. The polycyclic conjugated structure can anchor onto the RGO surface via π-π interactions, the chiral mesogenic units possess affinity for chiral materials by joining the helical matrix of chiral material and the carboxyl groups in nematic mesogenic units are supposed to form coordination bonds with nano zinc oxide (ZnO) to fabricate functional nano hybrids. The transmittances of CLCS-RGO hybrids exhibit S-shaped nonlinear increase with the increase of wavelength, but the total transmittances from 220 nm to 800 nm show a linear decreasing trend with the increase of RGO content in the CLCS-RGO hybrid. Due to the superior thermal properties of RGO and the interactions between RGO and CLCS, the dispersed RGO can improve the glass transition and increase the thermal stability and decomposition activation energy of CLCS. The intercalation of RGO can decrease the thermochromism temperature and improve the pitch uniformity of CLCS. Furthermore, CLCS can promote the dispersion of RGO in chiral nematic liquid crystals (CNLCs), and the CNLC-RGO-CLCS hybrids present decreased driving voltage and accelerated electro-optical response. The CLCS non-covalently modified RGO can strengthen the photocatalytic degradation of ZnO by suppressing the aggregation of ZnO and RGO.

Electrospray ion mobility mass spectrometry of positively and negatively charged (1R,2S)-dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide aggregates.

Self-assembling processes of surfactants in the gas phase constitute a developing research field of interest since they allow information to be gained on the peculiar structural organization of these aggregates, on their ability to incorporate from small molecules up to proteins and on their possible use as carriers of drugs in the gas phase or as cleaning agents and exotic reaction media. The mass spectra of charged aggregates of the chiral surfactant (1R,2S)-dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB) in the gas phase have been recorded using a Synapt G2-Si mass spectrometer in the positive and negative ion mode. For comparison purposes, the mass spectra of sodium bis(2-ethylhexyl)sulfosuccinate and sodium octane sulfonate aggregates have also been recorded under the same experimental conditions. The collisional cross sections of positively and negatively charged DMEB aggregates were obtained through an appropriate calibration of the measured drift times. For all the surfactants investigated, it has been found that there is a lowest and a highest limit of the aggregation number at each charge state: no aggregates are found outside this range. Moreover, the occurrence at each aggregation number and extra charge of a unique value of drift time points toward aggregates whose conformations do not show discernible shape change in the experiment time scale. The analysis of the collisional cross sections emphasizes that the DMEB aggregates are nearly spherical clusters somewhat affected by the charge state and constituted by interlaced polar and apolar domains. The analysis of all the experimental findings indicates that in the gas phase DMEB forms supramolecular aggregates characterized by an internal organization whose stability is triggered by the charge state. The comparison of the behavior of DMEB aggregates with that of sodium bis(2-ethylhexyl)sulfosuccinate and sodium octane sulfonate aggregates allows us to highlight the effects on the aggregate organization in gas phase due to nature of the head group and alkyl chain steric hindrance.

Development of an enantioselective assay for simultaneous separation of venlafaxine and O-desmethylvenlafaxine by micellar electrokinetic chromatography-tandem mass spectrometry: Application to the analysis of drug–drug interaction

To-date, there has been no effective chiral capillary electrophoresis-mass spectrometry (CE-MS) method reported for the simultaneous enantioseparation of the antidepressant drug, venlafaxine (VX) and its structurally-similar major metabolite, O-desmethylvenlafaxine (O-DVX). This is mainly due to the difficulty of identifying MS compatible chiral selector, which could provide both high enantioselectivity and sensitive MS detection. In this work, poly-sodium N-undecenoyl-L,L-leucylalaninate (poly-L,L-SULA) was employed as a chiral selector after screening several dipeptide polymeric chiral surfactants. Baseline separation of both O-DVX and VX enantiomers was achieved in 15min after optimizing the buffer pH, poly-L,L-SULA concentration, nebulizer pressure and separation voltage. Calibration curves in spiked plasma (recoveries higher than 80%) were linear over the concentration range 150–5000ng/mL for both VX and O-DVX. The limit of detection (LOD) was found to be as low as 30ng/mL and 21ng/mL for O-DVX and VX, respectively. This method was successfully applied to measure the plasma concentrations of human volunteers receiving VX or O-DVX orally when co-administered without and with indinivar therapy. The results suggest that micellar electrokinetic chromatography electrospray ionization-tandem mass spectrometry (MEKC-ESI-MS/MS) is an effective low cost alternative technique for the pharmacokinetics and pharmacodynamics studies of both O-DVX and VX enantiomers. The technique has potential to identify drug-drug interaction involving VX and O-DVX enantiomers while administering indinivar therapy.

Dispersing carbon nanotubes by chiral network surfactants.

Chiral network surfactants (CNSs) possessing miscibility with carbon nanotubes (CNTs) and chiral materials are applied to disperse CNTs. Ultraviolet-visible absorption spectroscopy is used to quantitatively determine the CNT concentration in homogeneous CNT-CNS dispersions, results indicate that CNSs with more mole fraction of polycyclic conjugated structure have better ability to load and disperse CNTs and the maximal concentration reaches 0.79 mg mL(-1). Fourier transform infrared imaging system is utilized to analyze the dispersibility of CNTs in CNT-CNS composites, and CNS with 6 mol % nonmesogens (S6) induces the best dispersibility. The CNT doped CNSs exhibit lower glass transition temperature, strengthened thermal stability, decreased the thermochromic temperature and enriched reflected colors of CNSs. Furthermore, S6 are used as a promoter to disperse CNTs in chiral host, here, a left-handed chiral liquid crystal (CLC) is selected, the miscibility between CNTs and CLCs is studied by polarized optical microscope, and CNTs can be effectively dispersed in CLCs by S6. The CNT dispersed CLCs can exhibit a faster electro-optical response process than neat CLCs.

Chiral surfactants for dispersing carbon nanotubes

Chiral surfactants induce excellent dispersion of CNTs. The dispersed CNTs improve the thermal properties and chiral stability of the dispersion medium.

Detailed kinetic observation revealing the formation mechanism of chiral mesoporous silica (CMS) synthesized by cooperative self-assembly of anionic chiral surfactant

The synthesis of chiral mesoporous silica (CMS) is desirable for emerging applications, particularly in enantioselective catalysis. However, the detailed evolution during the CMS formation templated by self-assembly of chiral surfactants have not been clearly explained. In this study, we reported the evolutional formation of CMS which was understood by complementary analyses involving advanced techniques such as spectroscopy, microscopy and thermogravimetry. The samples were kinetically studied starting from the time intervals during precursors mixing until fully well-ordered CMS was obtained. Insignificant interactions between C14AlaA and APS were observed at the initiate state and the interaction was becoming crucial for the CMS formation. Additionally, we also speculated that the electrostatic interaction between NH2 group and COO− anionic head group on the micelle surface is the driving force for the CMS formation.

Spectroscopic characterization of supramolecular chiral porphyrin homoassociates at the air-water interface.

The water-soluble 4-sulfonatophenyl meso-substituted porphyrin (TPPS) dye exhibits a transformation to a chiral self-aggregate from the non-aggregated species (diprotonated H4TPPS(2-)) at low concentration (no more than 1 × 10(-5) M). Immobilization of supramolecular chiral porphyrin homoassociates was mediated by the electrostatic interaction between the anionic TPPS molecule and cationic surfactant monolayer at the air-water interface. With the immobilization, a reversible transformation from monomeric TPPS to J-aggregate (M↔J) could be changed into an irreversible (M→J), which is desirable for stabilization of aggregation structure for a long period. The novel finding was achieved using a fine-tuned specialized solid-state circular dichroism (CD) spectrophotometer and derived analytical procedure to obtain artifact-free CD signals. To our knowledge, this is the first report achieving the chiral control of a homoassociate induced by a chiral surfactant at the air-water interface, indicating that the handedness of the formed homoassociate could be determined.

Combined use of chiral ionic liquid surfactants and neutral cyclodextrins: Evaluation of ionic liquid head groups for enantioseparation of neutral compounds in capillary electrophoresis

Cyclodextrins (CDs) are most commonly used chiral selectors in capillary electrophoresis (CE). Although the use of neutral CDs and its derivatives have shown to resolve plethora of charged enantiomers, they cannot resolve neutral enantiomers. The use of ionic liquids (ILs) surfactants forming successful complex with CDs present itself an opportunity to resolve neutral enantiomers. In this work, the effect of IL head groups and their complexation ability with heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD) was studied for the separation of neutral enantiomers by CE. First, cationic IL type surfactants with different chiral head groups were synthesized. Physicochemical properties such as critical micelle concentration were determined by surface tension, whereas aggregation and polarity were determined by fluorescence spectroscopy. The complexation ability of ILs with TM-β-CD was characterized in the gas phase by CE-mass spectrometry. The influence of the type of ILs head group and its concentration on chiral resolution, resolution per unit time and selectivity were investigated for four structurally diverse neutral compounds. The binding constants of the neutral analytes to the IL–CD complex were estimated by y-reciprocal method. The hydrophobicity of the side chain of the IL head group displayed significant effect on the binding constants and enantioseparations.

Effects of different chain lengths and chiral branched amino acids on the synthesis of mesoporous silica.

Ordered mesoporous silica was successfully synthesized by using different chain-length chiral anionic surfactants, N-acylalanine and N-acylvaline as templates, and N-trimethoxy silylpropyl-N, N, N-trimethyl ammonium (TMAPS) as a co-structure-directing agent (CSDA) under weakly acidic conditions. The synthesized products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen sorption analysis. In the TMAPS/N-acylalanine system, the mesophase changed from a 3D disordered stucture into a 3D hexagonally structure for the first, and finally into a 3D cubic structure with increased hydrophobic chain lengths of the anionic surfactants. But in the TMAPS/N-acylvaline system, the mesophase changed into a 2D hexagonally structure for the first, and finally into a 3D hexagonally structured mesoporous silica with the helical pore channels. The changes of the pore diameter, the surface area and the pore volume with the chain lengths were also discussed.

Synthesis and Physicochemical Characterization of Chiral Pyrrolidinium-Based Surfactants

Chiral surfactants, (2S)-2-(hydroxymethyl)-1-methyl-1-alkyl pyrrolidinium bromides (L-CnPB, n = 12, 14, 16), were synthesized by the reaction of N-methyl-L-prolinol with long-chain alkyl bromides. The structures were characterized by IR spectra, 1H NMR, ESI-MS, and elemental analysis. Their surface activity, thermodynamic properties, and aggregation behavior were investigated by surface tension, electrical conductivity, and steady-state fluorescence. It was found that L-CnPB have lower critical micelle concentration (CMC) and surface tension at CMC (γcmc) in comparison with N,N,N-trimethyl-1-alkyl ammonium bromide (CnTAB) and N-alkyl-N-methyl pyrrolidinium bromide (CnMP). The CMC and the minimum average area per surfactant molecule (Amin) values of L-CnPB decreased with the increase in the alkyl chain length from 12 to 16. A series of thermodynamic parameters (, and ) of micellization obtained from electrical conductivity indicated that micellization is a spontaneous and entropy-driven process. Furthermore, the micelle aggregation number (Nagg) of L-CnPB obtained from steady-state fluorescence quenching was found to be 42, 48, and 53 for L-C12PB, L-C14PB, and L-C16PB, respectively.

Spontaneously Formed Robust Steroidal Vesicles: Physicochemical Characterization and Interaction with HSA

Self-assembled multimolecular aggregates, such as vesicles, have earned tremendous attention for their applications as model membranes and drug delivery systems. Over the past decades, enormous efforts have been dedicated to the development of stable and biocompatible vesicles that form spontaneously in aqueous solution. With the aim of preparing highly stable vesicles, we herein report the physicochemical characterization of a novel cholesterol-based chiral surfactant with l-alanine headgroup. Different techniques, such as surface tensiometry, fluorescence spectroscopy, dynamic light scattering, UV-vis spectroscopy, transmission electron microscopy, and confocal fluorescence microscopy were employed to investigate the self-assembly properties of the aforementioned single-tailed steroidal surfactant in aqueous solution. The surfactant molecule is weakly surface-active, but self-assembles to form unilamellar vesicles facilitated by the strong hydrophobic association of the cholesterol moieties, above a very low critical aggregation concentration. The vesicles are fairly stable with respect to aging, temperature, and pH of the aqueous medium. Additionally, the vesicles were found to fuse together, leading to large unilamellar vesicles. The intervesicular fusion pertaining to high stability of the vesicles could be ascribed to large hydrophobic interactions among steroidal skeletons. Furthermore, the interaction of the vesicles with human serum albumin is also investigated.

Surfactant-induced chirality on reluctant aggregates of a chiral amphiphilic cationic (l)-proline–Zn(ii)porphyrin conjugate in water

The aggregation of an amphiphilic chiral Zn(ii)porphyrin derivative occurs in aqueous solutions of chiral surfactants with highly specific molecular recognition.

Supramolecular Organization and Heterochiral Recognition in Langmuir Monolayers of Chiral Azobenzene Surfactants

We study the self-assembly of novel azobenzene-based chiral surfactants at the air/water interface, and find that while the pure enantiomers lack the ability to organize in ordered mesophases, the racemic mixture spontaneously forms a hexatic phase at low lateral pressures, which we detect by means of Brewster angle microscopy. This work provides a unique example of heterochiral recognition in which the racemic monolayer is not only condensed with respect to the pure enantiomers, but causes an ordered mesophase to form. Although hexatic order vanishes at high surface pressures, long-range orientational order is regained for all compositions upon monolayer collapse, which proceeds through the formation of birefringent trilayers with a well-defined lateral microstructure, as revealed by atomic force microscopy.

Anionic surfactant templated chiral nanospheres and their enantioselective adsorption

Monodispersed chiral mesoporous spheres of nanometer size with high specific area and good thermal stability were synthesized using the chiral anionic surfactant (N-palmitoyl-L-phenylalanine) as a template according to the co-structure directing agent (CSDA) method. The resulting mesoporous nanospheres were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), small angle X-ray powder diffraction (XRD) and nitrogen sorption experiments. The particle distribution was 190–210 nm with pore diameters of 3–4 nm. The enantioselective ability of this chiral mesoporous silica, after extraction of the chiral surfactant, was investigated by the selective adsorption of enantiomers and racemic solutions of alanine. The chiral selective adsorbability of this material type was shown by the asymmetric preferential adsorption of alanine as observed using circular dichroism (CD) spectroscopy and was further verified in a racemic mixture of valine. It was found that the adsorption of L-alanine was higher than that of D-alanine on as-prepared chiral mesoporous materials with a chiral selectivity factor of 3.15, which generates new possibilities for enantiomeric separation and other enantioselective applications.

Aggregation behavior of imidazolium-based chiral surfactant in aqueous solution

A novel imidazolium-based chiral surfactant with a Y-type hydrophobic chain, (S)-(+)-1-(2,3-bis(octanoyloxy)propyl)-3-methylimidazolium chloride ([Bopmim]Cl), was synthesized. The aggregation behavior of [Bopmim]Cl in aqueous solution was then investigated by surface tension, electrical conductivity, 1H NMR, and fluorescence measurements. Compared with [C12mim]Cl, the critical micelle concentration for [Bopmim]Cl is lower, indicating that the novel chiral surfactant has superior capacity to form micelles. A larger value of pC 20, a greater minimum area per surfactant molecule (A min), a smaller degree of counterion binding (β), and a looser aggregate are caused by the relatively larger Y-type hydrophobic chain of [Bopmim]Cl. Furthermore, analysis of the 1H NMR spectra revealed that the introduced Y-type hydrophobic chain may prevent the hydrophobic group from forming an extended chain configuration and cause a changeover from trans to gauche conformations upon micellization. The micelles of the novel chiral surfactant may provide some potential applications in the stereochemical recognition of surfaces or of biological structures.