Critical Vesicle Concentration Research Articles

Protocell self-assembly driven by sodium trimetaphosphate.

The co-evolution of peptide formation and membrane self-assembly is considered an essential step in the origin of life. In this study, we systematically investigated the sodium trimetaphosphate (P3m)-activated peptide formation reaction of Phe in an alkaline decanoic acid-decanol vesicle system. The experimental results showed that the peptide formation could competitively occur with N-acyl amino acid (NAA) formation. NAA formation did not follow the traditional P3m-activated peptide formation reaction involving the intermediate cyclic acylphosphoramidate (CAPA), but a phosphoric acid- decanoic acid mixed anhydride intermediate. NAA can form vesicles independently and reduce the critical vesicle concentration of the fatty-acid vesicles. 11 other representative amino acids, namely Ala, Asp, Glu, Gly, Ile, Leu, Pro, Ser, Thr, Val, and Arg, were selected for investigation. All of them reacted with decanoic acid to form NAA via the activation effect of P3m. Our experimental results indicate that the P3m activation effect can provide diversified raw membrane materials to form and stabilize protocell membranes; moreover, the small peptides, such as Phe-Leu, formed in the same reaction system can induce the amplification of primitive cells. This implies that synergistic symbiosis between membrane and peptide can be realized via the P3m activation effect.

Primitive nucleobases @ sodium 2-Ketooctanoate vesicles with high salt resistance

• PNB@SKO is the strongest salt resistance P-SCA vesicle system to the best of our knowledge. • The rigid planar PNB molecules orientate via hydrogen bonding in the tilted bilayers of SKO. • The morphology, size, and membrane thickness of SKO vesicles are not affected by the PNB combination significantly. Proto-single chain amphiphiles (P-SCAs) vesicles are considered to be a plausible model of the protocell on early Earth due to their primordial relevancy and membrane structure. In this work, we selected P-SCAs sodium 2-ketooctanate (SKO) and proto-nucleobases (PNBs, including cytosine, uracil, adenine, and guanine) as the building constituents of the vesicles. The PNB molecules incorporate into SKO bilayers to form PNB@SKO vesicles with a binding rate of 7% − 12%. The interactions between them increase the solubility of PNB in water obviously while decreasing the critical vesicle concentration of SKO. The morphology, size, and membrane thickness of SKO PRB@SKO vesicles are not affected by the PNB combination significantly. A binding mechanism is proposed based on the results of Fourier transform infrared and small angle X-ray scattering. The rigid planar PNB molecules orientate via hydrogen bonding in the tilted bilayers of SKO, without excessive disruption to the membrane structure. Both ribozyme-catalyzed and non-enzymatic RNA copying reactions require high concentrations of divalent ions, fortunately, the salt resistance of these PNB@SKO vesicles is surprisingly remarkable, one or two orders of magnitude greater than the other typical P-SCAs vesicles systems. We expect that this primitively relevant vesicle system can exert its potential advantages in the proto-membrane mimicry of the origin of life.

Solvent effect ofD2Oon the thermochromic sensitivity of polydiacetylene vesicle systems

AbstractThe solvent effect of D2O on the thermochromic sensitivity of a polydiacetylene (PDA) vesicle system was characterized and estimated numerically in this study. Three types of diacetylene compounds (10,12‐pentacosadiynoic acid,N‐[2‐(2‐hydroxyethoxy)ethyl]pentacosa‐10,12‐diynamide, andN‐(4‐aminophenyl)pentacosa‐10,12‐diynamide) were used to form the vesicle solution by sonication in H2O and D2O. This was followed by polymerization using UV irradiation to form a PDA vesicle system. The solvent effect of D2O on the physical properties of the PDA vesicles was estimated. The physical properties investigated herein were PDA vesicle size, critical vesicle concentration, and thermochromic change rate. The thermochromic change rate was measured using the relative degree of blue color (RBC), which was obtained from the UV spectrum data. The rate constants (kHand kD) of the thermochromic change were measured by plotting RBC against temperature. The solvent effect of D2O was estimated via the ratio kD/kH. Finally, the solvent effect of D2O on three physical properties of the PDA vesicles (vesicle size, critical vesicle concentration, and thermochromic change rate) was characterized numerically.

Effect of Molecular Composition of Head Group and Temperature on Micellar Properties of Ionic Surfactants with C12 Alkyl Chain

The paper analyses influences of the temperature and hydrophilic groups on micellar properties of ionic surfactants with 12-carbonic hydrophobic chains. The aim is to assess the impact of hydrophilic groups and temperature on thermodynamic parameters and micellization. This knowledge is indispensable for the formulation of new dosage forms. The method uses conductometric measurements. The following hydrophilic groups are analyzed: trimethylammonium bromide, trimethylammonium chloride, ethyldimethylammonium bromide, didodecyldimethylammonium bromide, pyridinium chloride, benzyldimethyl-ammonium chloride, methylephedrinium bromide, cis and trans-[(2-benzyloxy)-cyclohexyl-methyl]-N, N-dimethylammonium bromide, sodium sulphate and lithium sulphate. Except for a few cases, there is a good agreement between values of critical micellar concentrations (CMC) and critical vesicle concentration (CVC) obtained here and those which were obtained by other authors and/or by other physicochemical methods. Values of the CMC are compared with respect to the molar masses of hydrophilic groups. It was found that CMC values increased non-linearly with increasing system temperature. The degrees of counterion binding and thermodynamic parameters, like the standard molar Gibbs energy, enthalpy and entropy of micellization are determined and discussed in detail. The results obtained will be incorporated into in silico processes of modeling and design of optimal dosage forms, a current interdisciplinary research focus of the team.

Cooperative Induction of Ordered Peptide and Fatty Acid Aggregates

Interactions between biological membranes and disease-associated amyloids are well documented, and their prevalence suggests that an inherent affinity exists between these molecular assemblies. Our interest in the molecular origins of life have led us to investigate the nature of such interactions in the context of their molecular predecessors (i.e., vesicle-forming amphiphiles and small peptides). Under certain conditions, amyloidogenic peptides or fatty acids are each able to form ordered structures on their own; however, we report here on their cooperative assembly into novel, to our knowledge, highly ordered structures. We first examined an amyloidogenic eight-residue peptide, which forms amyloids at pH 11, yet because of its positive electrostatic character remains soluble at a neutral pH. In mixtures with simple fatty acids, this peptide is also able to form novel, to our knowledge, coaggregates at a neutral pH whose structures are sensitive to both the fatty acid concentration and identity. Below the critical vesicle concentration, the mixtures of fatty acid and peptide yield a flocculent precipitate with an underlying β-structure. Above the critical vesicle concentration, the mixtures yield a translucent precipitate that consists of tube-like structures. Small-angle x-ray scattering and fiber diffraction data were used to model their structures as hollow-core two-shell cylinders in which the inner shell is a bilayer of fatty acid and the outer shell alternates between amyloid and bilayers of fatty acid. The further analysis of decanoic acid with a panel of 13 other basic amyloidogenic peptides confirmed the general nature of the observed interactions. The cooperativity within this heterogeneous system is attributed to the structurally repetitive natures of the fatty acid bilayer and the cross-β-sheet motif, providing compatible scaffolds for attractive electrostatic interactions. We show these interactions to be mutually beneficial, expanding the phase space of both peptides and fatty acids while providing a simple yet robust physical connection between two distinct entities relevant for life.

Physicochemistry of CTAB-SDS interacted catanionic micelle-vesicle forming system: An extended exploration

Interaction between cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS) has been known to form different types of assemblies. The mixed surfactant solutions may form clear, translucent, turbid, and precipitate comprising mixed micelles, mixed mono- and multi-lamellar vesicles and their insoluble products. These properties have been herein explored under three different concentrations of the surfactants (CTAB and SDS) (1) below, (2) near, and (3) above their critical micelle concentration (CMC) up to their equi-molar proportions and above. Their phase forming behaviors, and critical vesicle concentrations were determined from turbidimetry, conductometry and calorimetry measurements. Fluorescence anisotropy measurements were taken to confirm vesicle formation. Vesicle dimension was assessed from dynamic light scattering measurements. ζ- potential of the studied assemblies were found to be both positive and negative depending on their composition ratios. The formed vesicles at 1:1 M composition were very stable; they were extracted by isooctane by partition method from the dispersion state. Polar nonaqueous solvents were found to make the turbid vesicular solutions clear. Increased temperature favored transition of vesicles to mixed micelles. Field Emission Scanning Electron Microscopy and Confocal Fluorescence Microscopy evidenced formation of ion-pair products as well as both uni-, and multi-layer vesicles of moderate and large prolate type sizes. Antibacterial properties of the vesicles were also tested against E. coli and B. subtilis showing fair inhibition activities. The present multifarious detailed study contributes new and global inputs in the field of a model self-aggregated microheterogeneous amphiphilic vesicular systems with possible extension to future investigations with newer amphiphiles viz. bolaforms, pluronics, ionic liquids, geminis, etc.

Aggregation and pH Responsive Behavior of Thioester Surfactants and Formation of Disulfide Linkages in Aqueous Solutions.

pH responsive surfactants, [C12H25N(CH3)2(CH2)nSCOCH3]Br (C12nSAc, n = 4, 11, 12), were prepared, and their properties in aqueous solution were examined. The critical micelle concentration (cmc) and critical vesicle concentration (cvc) were determined based on changes in conductivity, as well as by fluorescence measurements, and light scattering methods. A significant increase in the light scattering intensities of the C12nSAc (n=11, 12) systems suggested that the growth of aggregates was accompanied by considerable counterion binding with increasing surfactant concentration. The diameter of C1211SAc, recorded by the dynamic light scattering measurements, was about 9.6 ±1.0 nm, which was slightly smaller than that for didodecyldimethylammonium bromide (DDAB) vesicles. The thioester group was easily hydrolyzed upon the addition of NaOH, while it was hardly hydrolyzed with the addition of HCl. The time course of alkaline hydrolysis was examined by the conductivity measurements and high-performance liquid chromatography analysis. [C12H25N(CH3)2(CH2)11SS(CH2)11N(CH3)2C12H25]2Br (2C1211SS) was generated in the C1211SAc alkaline solution because of air oxidation. The C1211SAc alkaline solution gradually became an opaque blue color with increasing light scattering at 346 nm, indicating the remarkable growth of vesicles. The chemical structure of 2C1211SS was consistent with that of a disulfide linked double tailed surfactant, similar to DDAB. The disulfide linkage between the double tailed surfactants will contribute to the stabilization and growth of vesicles.

Spontaneous vesicle formation and vesicle-to-micelle transition of sodium 2-ketooctanate in water

Single-tailed short-chain alkyl keto-acids/salts, a class of fatty acid/salt derivatives, such as sodium 2-ketooctanate (KOCOONa), are a kind of weakly acid/salt type amphiphiles and plausible prebiotic molecules, and the current understanding of their aggregation behavior in aqueous solutions is still limited. Herein, the aggregation behavior of KOCOONa in aqueous solution was studied by changing its concentration (C), using equilibrium surface tension, conductivity, and fluorescence measurements. The aggregates formed were characterized using freeze-fracture and cryogenic transmission electron microscopy, dynamic light scattering, atomic force microscopy, and confocal laser scanning microscopy. A concentration-driven stepwise aggregation was identified in the KOCOONa solution. Vesicles can spontaneously form from the single-component aqueous solution, with a critical vesicle concentration (CVC) of ∼15mM, which is obviously lower than that of octanoic acid/salt (120–200mM). With increasing C, a vesicle-to-micelle transition can occur, showing a critical micelle concentration (CMC) of ∼80mM. In addition, the membrane permeability of the KOCOONa vesicles was examined using small-size Calcein and large-size FITC-BSA as fluorescence probes, showing a size-selective permeability, similar to short-chain (C8-C11) fatty acid vesicles. For the first time, the aggregation behavior of single-tailed keto-acid salt surfactant is reported.

An investigation of kinetic and physicochemical properties of vesicular surfactants with oximate and hydroxamate ions: Hydrolytic reactions of organophosphorus pesticides

Rate constants have been determined spectrophotometrically for the reaction of paraoxon, parathion and fenitrothion (OP pesticides) with oximate (Ox−) and hydroxamate (HA−) ions in aqueous and cationic vesicular media at 300K. All reactions followed the pseudo-first-order kinetics. The surface properties i.e. the critical vesicle concentration (cvc), maximum surface access (Γmax), surface pressure at the cvc (Πcvc) and minimum surface area per molecule (Amin) of vesicular surfactants dioctadecyldimethylammonium chloride (DODAC) and didodecyldimethylammonium bromide (DDAB) with Ox− and HA− ions have been evaluated by conductivity and surface tension measurements at 300K. The standard Gibbs free energy of vesicle formation (ΔG0v), Standard Gibbs free energy of adsorption (ΔG0ad), and standard Gibbs free energy of vesicle formation per alkyl chain (ΔG0v, tail) of cationic vesicular surfactants with Ox− and HA− ions have also been evaluated. SEM (Scanning Electron Microscope) and TEM (Transmission Electron Microscope) were used to investigate the morphological behavior and size of vesicular surfactants. We observed a well-defined contour of vesicular surfactants that are mostly spherical and globular shape. The effects of vesicular morphology and chemical reactivity have been discussed.

Mineral Surface Chemistry and Nanoparticle-aggregation Control Membrane Self-Assembly

The self-assembly of lipid bilayer membranes to enclose functional biomolecules, thus defining a “protocell,” was a seminal moment in the emergence of life on Earth and likely occurred at the micro-environment of the mineral-water interface. Mineral-lipid interactions are also relevant in biomedical, industrial and technological processes. Yet, no structure-activity relationships (SARs) have been identified to predict lipid self-assembly at mineral surfaces. Here we examined the influence of minerals on the self-assembly and survival of vesicles composed of single chain amphiphiles as model protocell membranes. The apparent critical vesicle concentration (CVC) increased in the presence of positively-charged nanoparticulate minerals at high loadings (mg/mL) suggesting unfavorable membrane self-assembly in such situations. Above the CVC, initial vesicle formation rates were faster in the presence of minerals. Rates were correlated with the mineral’s isoelectric point (IEP) and reactive surface area. The IEP depends on the crystal structure, chemical composition and surface hydration. Thus, membrane self-assembly showed rational dependence on fundamental mineral properties. Once formed, membrane permeability (integrity) was unaffected by minerals. Suggesting that, protocells could have survived on rock surfaces. These SARs may help predict the formation and survival of protocell membranes on early Earth and other rocky planets, and amphiphile-mineral interactions in diverse other phenomena.

Aggregation Behavior of Disulfide Linked Gemini Surfactants Compared to that of Double Tailed Surfactants.

Disulfide linked gemini surfactant having a long spacer chain, [C10H21N(CH3)2(CH2)11SS(CH2)11N(CH3)2C10H21]2Br (2C1011SS), was prepared by the hydrolysis and oxidation process of thioester group for [C10H21N(CH3)2(CH2)11SCOCH3]Br. The critical vesicle concentrations of double tailed surfactants such as dialkyldimethylammonium bromide were observed by the conductivity and light scattering methods. The disulfide bonds of gemini surfactant, [C12H25N(CH3)2CH2CH2SSCH2CH2N(CH3)2C12H25]2Br (2C12SS), were rapidly cleaved by the addition of water-soluble dithiothreitol. However, it took long time to cleave the disulfide bonds of so-called double tailed surfactants 2C1011SS due to vesicle formation. The dynamic light scattering method showed that the diameters of 2C12SS micelles were increased with the cleavage of disulfide bonds, whereas those of 2C1011SS aggregates remained almost constant at 17.6 ±1.3 nm in similar size with dialkyldimethylammonium bromide vesicles. The time course of disulfide cleavage was examined by the conductivity and HPLC analysis. The produced thiol surfactants were returned to their original gemini surfactants by the addition of H2O2.

Development and Characterization of DOPEPEG2000 Coated Oleic Acid Liposomes Encapsulating Anticancer Drugs

AbstractLiposomes have been widely investigated as drug carriers for a broad range of anticancer drugs. However, their application as a drug delivery system may be challenging due to low encapsulation of active ingredient and poor stability which leads to uncontrolled delivery of bioactive ingredients. The present study aims at incorporating 1,2‐dioleoyl‐sn‐glycero‐3‐phosphoethanolamide‐N‐[methoxy(polyethylene glycol)‐2000 (DOPEPEG2000) into oleic acid (OA) liposomes in order to prolong the lifetime in blood circulation. The pH transition curve of oleic was obtained by titration of sodium oleate solution with 0.05 mol dm−3 HCl to determine the region where liposomes were abundantly present. The critical vesicle concentration (CVC) was then determined by surface tension measurements. Dry lipid liposomes were hydrated with phosphate buffer at just over twice the CVC value. Optical polarizing microscopy (OPM) showed the presence of liposomes while transmission electron microscopy (TEM) confirmed that the liposomes prepared were spherical in shape and less than 200 nm in size. The incorporation of DOPEPEG2000 into the oleic acid liposomes reduced the average particle size and zeta potential to mimic the red blood cells. The encapsulation efficiency of various anticancer drugs was more than 60% while more than 20% of the drugs were released after 24 h. The results suggest that DOPEPEG2000: oleic acid at a molar ratio of 1:50 fulfilled the requirements for intravenous drug delivery.

Reactivity of Hydroxamate Ions in Cationic Vesicular Media for the Cleavage of Carboxylate Esters

AbstractThe hydrolysis of carboxylate esters viz. p‐nitrophenyl acetate (PNPA), p‐nitrophenyl butyrate (PNPB) and p‐nitrophenyl octanoate (PNPO) in the presence of cationic vesicles of the surfactant dioctadecyldimethylammonium bromide (DODAC) by different hydroxamate ions i.e. acetohydroxamate (AHA−), benzohydroxamate (BHA−) and salicylhydroxamate (SHA−) were studied. The kinetic data was supported by spectrophotometric measurements. The effects of vesicular size on the reaction have been discussed. The differential reactivity under endo‐ and exovesicular conditions has also been evaluated. Critical vesicle concentrations (CVC) of cationic vesicular surfactants were determined from conductometric and fluorimetric measurements at 300 K. Fluorescence probe pyrene and pyrene‐1‐carboxaldehyde have been used for determination of CVC. Further, thermodynamic parameters viz. Standard Gibb's energy (∆G°), enthalpy (∆H°), and entropy (∆S°) of vesicles has also been evaluated within a temperature range of 303.15–328.15 K.

Synthesis and evaluation of novel lipopeptide as a vehicle for efficient gene delivery and gene silencing

Nucleic acid-based therapeutics have recently emerged as a new class of next generation agents for treatment and prevention of viral infection, cancer, and genetic disorders, but their wide use is limited by their relatively weak delivery into target cells. Usage of synthetic cationic amphiphiles with peptide hydrophilic domain as agents for non-viral gene delivery is an attractive approach. We developed the schemes for the synthesis of aliphatic peptides with different length of the hydrocarbon chains in hydrophobic domains and different amino acids in polar head. For the obtained derivatives we determined transfection efficiency, critical vesicle concentration, particle size, ζ-potential and aggregates stability. We have found that the transfection efficiency is increased if the ornithine is a part of polar head in an amphiphile. The most promising amphiphile for liposomal formation OrnOrnGlu(C16H33)2 was examined more carefully. It has been shown that the lipopeptide possesses low toxicity (in vitro and in vivo) and high transfection efficiency with pDNA and siRNA in different cell lines. In addition, the production of liposomes based on this lipopeptide is simple, quick and cheap. Thus OrnOrnGlu(C16H33)2 is a promising vehicle for gene delivery and gene silencing.

Reversible Chromatic Response of Polydiacetylene Derivative Vesicles in D2O Solvent.

The thermal chromatic sensitivity of polydiacetylenes (PDAs) with 10,12-pentacosadiynoic acid (PCDA) derivatives, which have a hydroxyl group (HEEPCDA) and an amine group (APPCDA), were investigated using D2O and H2O as solvents. The vesicle solution with polymerized HEEPCDA exhibited a reversible chromatic response during the heating and cooling cycle in D2O, but not in H2O. On the other hand, the vesicle solution with the polymerized APPCDA exhibited a reversible chromatic response in H2O during the heating and cooling cycle, but the color of the solution did not change much in D2O. The critical vesicle concentration of HEEPCDA was lower in D2O than in H2O, and the chromatic sensitivity of the polymerized vesicles to temperature was slower in D2O than in H2O. We think that it is due to D2O being a more highly structured solvent than H2O with the hydrogen bonding in D2O stronger than that in H2O.

Prebiotic Vesicle Formation and the Necessity of Salts.

Self-assembly is considered one of the driving forces behind abiogenesis and would have been affected by the environmental conditions of early Earth. The formation of membranes is a key step in this process, and unlike large dialkyl membranes of modern cells the first membranes were likely formed from small single-chain amphiphiles, which are environment-sensitive. Fatty acids and their derivatives have been previously characterized in this role without concern for the concentrations of ionic solutes in the suspension. We determined the critical vesicle concentration (CVC) for three single-chain amphiphiles with increasing concentrations of NaCl. All amphiphile species had decreasing CVCs correlated to increasing NaCl concentrations. Decanoic acid and oleic acid were impacted more strongly than monoacylglycerol, likely because of electric shielding of the negatively charged headgroups in the presence of salt. There was no impact on the salt species as 100mM NaBr, NaCl, and KCl all exhibited the same effect on CVC. This research shows the importance of salt in both the formation of life and in experimental design for aggregation experiments.

Thermal and phase behavior of didodecyldimethylammonium bromide aqueous dispersions

Abstract This work aims to delineate the single isotropic vesicle phase (Ves) in the binary didodecyldimethylammonium bromide (DDAB)/water system, limited by the critical vesicle concentrations, CVC 1 ≈ 0.05 mM (2.3 × 10 −3 wt%) and CVC 2 ≈ 0.7 mM (3.1 × 10 −2 wt%), as the onset of unilamellar vesicle formation and of two-phase separation into Ves + Lam (lamellar) phases, respectively. Isothermal titration calorimetry (ITC), turbidity, and crossed polaroids observations indicate that below CVC 1 the dispersion is dominated by free monomers or micelles, but rich in uni- and multilamellar vesicles between CVC 1 and CVC 2 , with CVC 2 = 0.21 mM (9.5 × 10 −3 wt%) being the onset of multilamellar vesicle formation. Above CVC 2 , the volume of the Lam phase increases, while the volume for the Ves phase decreases to vanish around CVC 3 ≈ 21 mM (0.95 wt%). Differential scanning calorimetry (DSC) data show that the gel-to-liquid crystalline transition at T m ≈ 16 °C is highly cooperative (Δ T 1/2 ≈ 0.3 °C), and the melting enthalpy (Δ H m ) increases with DDAB concentration. Because of the remarkably slow liquid crystalline-to-gel kinetics, a cooling transition around T m ′ ≈ 9.5°C is reported here for the first time, we ascribe to the liquid crystalline-to-gel transition (thermal hysteresis Δ T m ≈ 6.5 °C). Vesicle and lamellar structure formation is supported also by hydrodynamic diameter ( D H ) data, being 100–140 and 800–1200 nm, respectively, at 25 °C. The zeta potential ( ζ ) increases with DDAB concentration but does not change much with temperature, indicating no pronounced structural change when temperature varies around T m .

Micelle-Vesicle Transition by Cleavage of Disulfide Spacer Chain for Gemini Surfactant in Didodecyldimethylammonium Chloride Aqueous Solutions.

We examined the "micelle-vesicle transition" through the mixing effect of single-tailed thiol surfactants produced by the cleavage of gemini surfactants, [C12H25N(CH3)2CH2CH2SSCH2CH2N(CH3)2C12H25] 2Cl (C12SSC12), which have a disulfide bond in the spacer chain. Phase diagrams of C12H25N(CH3)2CH2CH2SHCl-didodecyldimethylammonium chloride (C12SH-DDAC) and C12SSC12-DDAC were determined by conductivity and pyrene fluorescence probe methods. The aggregate diameters were evaluated by dynamic light scattering (DLS). The critical vesicle concentration (CVC) was confirmed by the abrupt increase in the intensity of light scattering with excitation at 335 nm. Vesicle formation was confined to the DDAC-rich region of the C12SSC12-DDAC system, while the vesicle formation region for the C12SH-DDAC system spread out with the addition of dithiothreitol (DTT) to C12SSC12-DDAC. This implies that single-tailed surfactants can induce a more favorable environment for molecular packing of the vesicular surface. The micelle-vesicle transition occurs with disulfide spacer chain cleavage of gemini surfactants at a particular specific concentration range.

Vesicle formations at critical vesicle concentration in a polyoxyethylene type nonionic surfactant system

In a nonionic polyoxyethylene type surfactant CiEj aqueous solution below the Krafft temperature, multilamellar vesicles which have a hollow including excess water are formed (Kawabata et al., 2009). In this study, we surveyed the critical vesicle concentration, CVC, and the formation of the vesicles by means of fluorescence microscopy and light scattering. We have found that disk-like layers are formed below the CVC, and they are transformed to vesicles via a coexisting region of vesicles and disks with increasing surfactant concentration. Furthermore, by varying temperature, the transformations are induced reversibly. These phenomena could be interpreted by taking into account the elastic energy of unilamellar vesicle formation.

Self-assembly of new surface active ionic liquids based on Aerosol-OT in aqueous media

New anionic ionic liquid surfactants have been synthesized by replacing the sodium cation of Aerosol-OT (sodium dioctylsulfosuccinate, [Na]AOT) with various biocompatible moieties, such as 1-butyl-3-methyl imidazolium ([C4mim]), proliniumisopropylester ([ProC3]), cholinium ([Cho]), and guanidinium ([Gua]). The Aerosol-OT derived ionic liquids (AOT-ILs) were found fairly soluble in water and formed vesicles above a critical vesicle concentration (CVC) which depended upon the nature of cation, and followed the order: [ProC3]<[C4mim]<[Gua]<[Cho]<Na+. The self-assembly process was characterized using surface tension (ST), isothermal titration calorimetry (ITC), conductivity, dynamic light scattering (DLS), nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM). Unlike other AOT-ILs, a structural transformation has been observed for [C4mim]AOT above CVC, because of certain amphiphilic character in the cation [C4mim]. Thermodynamic parameters calculated from ITC and conductivity techniques revealed that the vesicle formation process is entropy driven for [C4mim]AOT, whereas the process is both enthalpy and entropy driven for other AOT-ILs. In order to check the versatility of synthesized AOT-ILs we have tested their dissolution behavior in a different class of ionic liquids. All the AOT-ILs were found fairly soluble in the hydrophilic IL, ethanolammonium formate (EOAF), whereas only [C4mim]AOT and [ProC3]AOT were found soluble in hydrophobic IL, [C4mim]Tf2N. Such combinations can have potential for construction of stable colloidal formulations or microemulsions in ionic liquid media.