Effects of electrostatic interaction on the properties of ionic liquids correlated with the change of free volume.

In this study, the amount of free volume in ionic liquids (ILs) was calculated and found to be almost half to that of their isoelectronic neutral analogues. MD simulations revealed that the significantly compressed free volume in the ILs was dominantly attributed to the strong inter-ion electrostatic interactions, which are comparable to the application of an external pressure of ∼250 MPa to the neutral analogues. Furthermore, the change in the free volume shows an interconnection with other properties of ILs, especially viscosity. The inherent high viscosity of ILs was quantitatively correlated to a low free volume available for mass transfer, and the sharp decrease in the viscosity of ILs with the addition of organic solvents was essentially caused by the introduction of free volume.

First fluorinated zwitterionic micelle with unusually slow exchange in an ionic liquid.

The micellization of a fluorinated zwitterionic surfactant in ethylammonium nitrate (EAN) was investigated. The freeze-fracture transmission electron microscope (FF-TEM) observations confirm the formation of spherical micelles with the average diameter 25.45 ± 3.74 nm. The micellization is an entropy-driven process at low temperature but an enthalpy-driven process at high temperature. Two sets of (19)F NMR signals above the critical micelle concentration (cmc) indicate that the unusually slow exchange between micelles and monomers exists in ionic liquid; meanwhile, surfactant molecules are more inclined to stay in micelle states instead of monomer states at higher concentration. Through the analysis of the half line width (Δν1/2), we can obtain the kinetic information of fluorinated zwitterionic micellization in an ionic liquid.

Spontaneous transformation of lamellar structures from simple to more complex states.

Spontaneous transformation of lamellar structures, such as multilamellar vesicles from micelles or unilamellar vesicles, is an important challenge in the field of amphiphile molecules, which may serve as models to understand biologically relevant bilayer membranes. Herein, we report a progressive self-assembly progress of N-tetradecyllactobionamide (C14G2) and tetraethylene glycol monododecyl ether (C12EO4) mixtures in aqueous solution. Increasing temperature or surfactant compositions causes spontaneous transformation from simple to high-level aggregates, i.e., from unilamellar vesicles, to coexisting multilamellar vesicles, terraced planar bilayers, and finally terraced planar bilayers. Deuterium nuclear magnetic resonance ((2)H NMR), freeze-fracture transmission electron microscopy (FF-TEM), and small-angle X-ray scattering (SAXS) measurements clearly demonstrate the spontaneously progressive self-assembly process. The interlamellar spacing (d) of the bilayers decreases from unilamellar vesicles to the terraced planar bilayers with an increase of the temperature or surfactant compositions. Lamellar samples consisting of terraced planar bilayers at higher temperature still show viscoelastic properties, being Bingham fluids, and both the viscoelasticity and yield stress increase with the composition and decrease with the temperature. The spontaneous transformation of the progressive self-assembly progress of C14G2 and C12EO4 aqueous mixtures is due to a balance of three driving forces, hydrophobic interactions, hydrogen bonding, and steric effects.

Investigations into the bending constant and edge energy of bilayers of salt-free catanionic vesicles.

Using molecular dynamics simulation, we performed theoretical calculations on the curvature constant and edge energy of bilayers of salt-free, zero-charged, cationic and anionic (catanionic) surfactant vesicles composed of alkylammonium cations (C(m)(+)) and fatty acid anions (C(n)(-)). Both the minimum size and edge energy of vesicles were calculated to examine the relation between the length of the surfactant molecules and the mechanical properties of the catanionic bilayers. Our simulation results clearly demonstrate that, when the chain lengths of the cationic and anionic surfactants are equal, both the edge energy and the rigidity of the catanionic bilayers increase dramatically, changing from around 0.36 to 2.77 kBT·nm(-1) and around 0.86 to 6.51 kBT·nm(-1), respectively. For the smallest catanionic vesicles, the curvature is not uniform and the surfactant molecules adopt a multicurvature arrangement in the vesicle bilayers. We suspect that the multicurvature bending of bilayers of catanionic vesicles is a common phenomenon in rigid bilayer systems, which could aid understanding of ion transport through bilayer membranes.

Self-assembly in the mixtures of surfactant and dye molecule controlled via temperature and ß-cyclodextrin recognition.

A new ternary system of tetradecyldimethylamine oxide (C(14)DMAO)/4-phenylazo benzoic acid (AzoH)/H(2)O was first investigated, and it was found that the self-assembly can be regulated via temperature and ß-cyclodextrin (ß-CD) recognition. In the temperature regulated self-assembly, the self-assembled phase structural transition between wormlike micelles and multilamellar vesicles (onions) were determined by cryogenic-transmission electron microscopy (cryo-TEM) images and (2)H nuclear magnetic resonance ((2)H NMR) spectra. The phase structural transition temperatures (PSTT) controlled by changing the amount of AzoH were measured by differential scanning calorimetry analysis. The self-assembled phase structural transition mechanism was discussed. It is argued that the self-assembled phase structural transition is the synergetic balance among the hydrophilic headgroup, steric structures of the hydrophobic chain, and membrane charge. ß-CD molecules were used as controlling hands to modulate the phase structural transition of self-assembly of the C(14)DMAO/AzoH/H(2)O system in solution via snatching C(14)DMAO molecules. The phase structural transitions from the threadlike micellar phase to the lamellar phase and from the lamellar phase to the vesicular phase can each be controlled because of the ß-CD molecular recognition. The phase structural transitions were confirmed by cryo-TEM observations and (2)H NMR measurements. The rheological properties were also investigated to display the importance in the phase structural transition. It was found that the dye molecule, AzoH, is harder to enclose by ß-CD than by C(14)DMAO because of the lower complex stability constant (i.e., K(C(14)DMAO@ß-CD) ≫ K(AzoH@ß-CD). Therefore, the phase structural transition is mainly controlled by the inclusion of C(14)DMAO into the hydrophobic cavity of ß-CD molecules. The phase structural transition controlled via temperature and ß-CD may find potential applications such as in actuators, shape memories, drug delivery systems, and drag-reducing fluids, etc.

Phase behavior and self-assembly aggregation of hydrocarbon and fluorocarbon surfactant mixtures in aqueous solution.

Similar to hydrogenated surfactant mixtures, the ones of hydrocarbon and fluorocarbon surfactants can also self-assemble into various aggregates, including mixed micelles and vesicles, however, completely different phase behavior and self-assembly of CH/CF surfactant mixtures in solution can be observed and exhibit novel features because of the repellence between the two hydrophobic chains. These systems of hydrocarbon and fluorocarbon surfactant mixtures can provide important considerations for both theoretical and applied interest. Several advantaged techniques, including small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), (19)F- and (1)H-NMR, cryo-TEM, and Freeze-fracture TEM (FF-TEM) have been widely employed to characterize these mixture systems. In this review, the aggregation behavior, self-assembly aggregation, and interaction of hydrocarbon and fluorocarbon surfactant mixtures in solution are described and focused three aspects, (i) immiscibility and nonideal mixing in hydrocarbon and fluorocarbon surfactant mixed micelles systems; (ii) spontaneous vesicles of hydrocarbon and fluorocarbon surfactant mixtures in aqueous solution; and (iii) self-assembled aggregates of hybrid fluorocarbon/hydrocarbon surfactants in aqueous solutions.

Influence of counterions on micellization of tetramethylammonium perfluorononanoic carboxylate in 1-butyl-3-methylimidazolium ionic liquid.

The influence of counterions on micellization of perfluorononanoic carboxylate ammonium salts in water and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF(4)) solutions was investigated by surface tension and (19)F NMR measurements and freeze-fracture transmission electron microscopy (FF-TEM) observations. Changes in the counterions of the fluorocarbon surfactants have different effects on the two solvents. With the increase of counterion volume, the critical micelle concentration (cmc) value of relevant fluorinated surfactant decreases in aqueous solutions. This is because the counterions with larger size, such as (+)N(CH(3))(4), can be little hydrated, which can screen the electrostatic repulsion of the headgroups of the fluorocarbon surfactant and thus facilitate micelle formation. However, the fluorocarbon surfactants can dissolve and form micelles in [bmim]BF(4) only when they provide with largest counterion such as (+)N(CH(3))(4). This is because the counterion, (+)N(CH(3))(4), disperses the charge of the cations, which could weaken the electrostatic interaction between the ion pair of the surfactant, leading to a higher degree of counterion binding. The thermodynamic parameters estimated from the temperature dependence of the cmc values tell us that the micelle formation for tetramethylammonium perfluorononanoic carboxylate (C(8)F(17)COON(CH(3))(4), PFNT) in ionic liquids (ILs) is an entropy-driven process at low temperature but an enthalpy-driven process at high temperature. The driving force of the micellization for fluorocarbon surfactants in [bmim]BF(4) is the solvophobic effect, due to the hydrophobic and oleophobic properties of fluorocarbon chains.

pH-Sensitive vesicles and rheological properties of PFLA/NaOH/H2O and PFLA/LiOH/H2O systems.

The formation of pH-sensitive vesicles and the rheological properties of the mixtures of perfluorolauric acid (PFLA) and its salts (PFL-Na and PFL-Li) neutralized via NaOH or LiOH were investigated in aqueous solution. When the right mixing ratios of the ionized to nonionzed PFLA molecules with a very high Krafft point are established, vesicles can spontaneously form at room temperature. The vesicles spontaneously formed in the PFLA/PFL-Na/H(2)O system with the rigid fluorocarbon chains were determined by atomic force microscopy images. Compared to those of hydrocarbon amphiphiles, these vesicle samples, which can be kept for 2 years at room temperature, are more stable. The phase transition from the vesicle phase to the lamellar lyotropic liquid crystal phase with the increase of pH was determined by freeze-fracture transmission electron microscopy images in the PFLA/PFL-Li/H(2)O system. The system of perfluoro fatty acid vesicles exhibits much more interesting rheological properties, compared to hydrocarbon fatty acid vesicles. The perfluoro fatty acid vesicle solutions display a much higher yield stress and viscoelastic properties, which depend on two factors: (i) the fluorinated alkyl chains of PFL(-), which are in the crystalline state at room temperature because of their rigid chains compared to analogous hydrocarbon chains, and (ii) the packing of the vesicles, which is very dense. This is the first time that pH-sensitive vesicles exhibiting birefringence were constructed through ionizing perfluoro fatty acid, which may direct primarily toward acquiring an understanding of the mechanism of vesicles depending on the right mixing ratios of the ionized to the nonionzed perfluoro fatty acid molecules with a very high Krafft point and secondarily to expand the development of fluorinated surfactants in both fundamental research and practical applications.