Phosphatidylcholine-based nonaqueous photorheological fluids: effect of geometry and solvent

We report a detail study on photoinduced rheological changes in nonaqueous photorheological (PR) fluids obtained with 1,2-diacyl-sn-glycero-3-phosphocholine-based reverse wormlike micelles, 1-palmitoyl-2-oleophosphatidylcholine (POPC)/cyclohexane/H₂O, POPC/isooctane/H₂O, and L-α-dioleophosphatidylcholine (DOPC)/isooctane/H₂O systems. Initially, the mixtures form highly viscoelastic fluids of long, reverse wormlike micelles as confirmed by the rheological measurements. When photosensitive molecule, trans-CA, is added to these mixtures, they exhibit photosensitivity, and the viscoelasticity increases that decreases on UV irradiation. The nature of the substituent on the benzene ring of trans-CA influences the rheology. When hydroxycinnamic acid (HCA) (hydrophilic, ?OH group attached to the benzene ring of CA) is added to DOPC/isooctane/H₂O phase, separation occurs; while with the methoxycinnamic acid (MOCA) and methylcinnamic acid (MCA) (hydrophobic groups, ?OCH₃, and –CH₃ attached to the benzene ring of CA respectively) led to higher viscoelasticity. The study on the effects of position of the substitution on CA reveals the viscosity enhancement is in the order of p-?>?m-?>?o-isomers. The different geometries obtained because of substitution and photoinduced trans-cis isomerisation is responsible for the different rheology as confirmed by the dynamic rheology, the UV absorption, and the ¹H NMR spectrums. The ¹H NMR spectra revealed a change in the solubilization site of CA with irradiation. cis-CA (high solubility in water) is solubilized in the vicinity of water (at the hydrophilic end of DOPC) as compared with trans-CA. This disrupts the 3D networks of reverse wormlike micelles, decreasing the η ₀, plateau modulus, G _0, and relaxation time, τ _R of solution.     

Ion exchange in reverse micelles

The distribution and dynamics of alkali cations inside Na-AOT reverse micelles have been investigated using Monte Carlo and molecular dynamics simulations. Water is modeled using the extended simple point charge (SPC/E) model. Simulations were carried out for alkali salts of Li+, Na+, K+, and Cs+ placed into the aqueous core of the reverse micelle, for situations corresponding to one and three molecules of added salt. In all cases, we observe that the larger K+ and Cs+ ions exchange with the Na+ counterion; however, the smaller Li+ ion prefers to remains solvated within the core of the reverse micelle. Our study reveals that the oil-water interface of the Na-AOT reverse micelle has the greatest selectivity toward Cs+ followed by K+ and Li+. A model based on enthalpic contributions illustrates that the solvation energies of the different cations in water control the ion-exchange process. The hydration number of the first water shell for Li+ situated in the aqueous core of the reverse micelle with radius R = 14.1 A was similar to that observed at infinite dilution in bulk water.     

A simple class of photorheological fluids: surfactant solutions with viscosity tunable by light

Photorheological (PR) fluids, i.e., those with light-tunable rheological properties, may be useful in a variety of applications, such as in sensors and microfluidic devices. Currently, the need to synthesize complex photosensitive molecules hampers the applicability of these fluids. Here, we report a simple class of PR fluids that require no special synthesis and can be easily replicated in any lab from inexpensive chemicals. The fluids consist of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), and the photoresponsive organic derivative, trans-ortho-methoxycinnamic acid (OMCA). Aqueous mixtures of CTAB and OMCA in basic solution self-assemble into long, wormlike micelles. Upon irradiation by UV light (<400 nm), OMCA undergoes a photoisomerization from its trans to its cis form, which alters the molecular packing at the micellar interface. The result is to transform the long micelles into much shorter entities and, in turn, the solution viscosity decreases by more than 4 orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic reduction in micellar length. The extent of viscosity reduction in these PR fluids can be tuned based on the composition of the mixture as well as the duration of the irradiation.     

Stability of invertase in reverse micelles

The stability of invertase was studied under various conditions, including at 75°C, in presence of stabilizers (sorbitol and glycerol) at 75°C, and in the presence of denaturants (urea and trichloroacetic acid) at 37°C in reverse micelles. Stability of the invertase in reverse micelles was found to be improved over that of the enzyme in bulk aqueous solution. Sorbitol could enhance enzyme stability as it does in the bulk aqueous system. The stabilizing effect of glycerol was reduced in reverse micelles. The denaturation pattern of urea remains unaltered. However, the denaturation effect of trichloroacetic acid has been reduced in reverse micelles.     

Protons in non-ionic aqueous reverse micelles

Using molecular dynamics techniques, we investigate the solvation of an excess proton within an aqueous reverse micelle in vacuo, with the neutral surfactant diethylene glycol monodecyl ether [CH3(CH2)11(OC2H4)2OH]. The simulation experiments were performed using a multistate empirical valence bond Hamiltonian model. Our results show that the stable solvation environments for the excess proton are located in the water-surfactant interface and that its first solvation shell is composed exclusively by water molecules. The relative prevalence of Eigen- versus Zundel-like solvation structures is investigated; compared to bulk results, Zundel-like structures in micelles become somewhat more stable. Characteristic times for the proton translocation jumps have been computed using population relaxation time correlation functions. The micellar rate for proton transfer is approximately 40x smaller than that found in bulk water at ambient conditions. Differences in the computed rates are examined in terms of the hydrogen-bond connectivity involving the first solvation shell of the excess charge with the rest of the micellar environment. Simulation results would indicate that proton transfers are correlated with rare episodes during which the HB connectivity between the first and second solvation shells suffers profound modifications.     

Amide-ligand hydrogen bonding in reverse micelles

One approach to modeling the second coordination shell of metalloproteins is to pair amide-containing counterions with metal complexes to form hydrogen bonds in the solid state. In a more general approach, we have designed a surfactant counterion that can sustain hydrogen bonding interactions with metal complexes in solution. The surfactant is cationic and incorporates an amide as part of its headgroup to form hydrogen. The surfactant forms hydrogen bonding reverse micelles that accommodate anionic metal complexes in their polar core. In reverse micelles containing an iron(III) hexacyanide complex, spectroscopic evidence suggests that the anion is confined to the polar core region in solution. Single-crystal X-ray diffraction data on the surfactant ferricyanide system reveals a layered structure with interdigitated alkyl chains and an extensive network of hydrogen bonds that link amide groups to the cyanide ligands and to neighboring headgroups.     

Photodetachment of ferrocyanide in reverse micelles

Solvated electrons have been generated in reverse micelles (RMs) through photodetachment of ferrocyanide (Fe(CN)(6)(4-)) in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) RMs. We have measured both bleach recovery of the parent ferrocyanide CN stretch in the infrared and the decay of the solvated electron absorption at 800 nm. The bleach recovery has been fit to a diffusion model for the geminate recombination process. The fit parameters suggest a narrowing of the spatial distribution of ejected electrons due to confinement in the RMs when compared to bulk water. The diffusion coefficient of the solvated electron does not appear to be significantly affected by RM confinement. The decay of the solvated electron absorption exhibits an additional decay component that is not observed in bulk water and is smaller for larger RMs. No corresponding additional component is seen in the parent ferrocyanide IR bleach recovery, which supports our interpretation that the confinement-induced new decay process in RMs is due to electrons reacting with AOT headgroups.     

Structure of AOT reverse micelles under shear

Reverse micelles in the AOT/water/isooctane system, at various water contents (W(0)), were studied using rheometry and small angle X-ray scattering (SAXS) experiments under static conditions and under shear. The SAXS analysis confirmed the spherical shape of the micelles at low water content and revealed a transition into elongated micelles at higher water content. A population of spherical micelles was found to coexist with the cylindrical ones, even above the percolation threshold. The shape transformation was correlated with a viscosity leap observed in the rheometry measurements. Reverse micelles at low water content under shear act as a Newtonian fluid, without any detectable shape changes. In contrast, reverse micelles at high water content behave as a shear thinning fluid. SAXS measurements at high water content under shear force have shown that the shear forces induced alignment of the cylindrical micelles in the flow direction, without any other changes in the micelle dimensions. The anisotropy parameter, a measure of the degree of the spatial order, was found to increase with increasing water content and shear rate.     

Vibrational dynamics of ice in reverse micelles

The ultrafast vibrational dynamics of HDO:D(2)O ice at 180 K in anionic reverse micelles is studied by midinfrared femtosecond pump-probe spectroscopy. Solutions containing reverse micelles are cooled to low temperatures by a fast-freezing procedure. The heating dynamics of the micellar solutions is studied to characterize the micellar structure. Small reverse micelles with a water content up to approximately 150 water molecules contain an amorphous form of ice that shows remarkably different vibrational dynamics compared to bulk hexagonal ice. The micellar amorphous ice has a much longer vibrational lifetime than bulk hexagonal ice and micellar liquid water. The vibrational lifetime is observed to increase linearly from 0.7 to 4 ps with the resonance frequency ranging from 3100 to 3500 cm(-1). From the pump dependence of the vibrational relaxation the homogeneous linewidth of the amorphous ice is determined (55+/-5 cm(-1)).     

Ultrafast energy transfer in water-AOT reverse micelles

A spectroscopic investigation of the vibrational dynamics of water in a geometrically confined environment is presented. Reverse micelles of the ternary microemulsion H2O/AOT/n-octane (AOT = bis-2-ethylhexyl sulfosuccinate or aerosol-OT) with diameters ranging from 1 to 10 nm are used as a model system for nanoscopic water droplets surrounded by a soft-matter boundary. Femtosecond nonlinear infrared spectroscopy in the OH-stretching region of H2O fully confirms the core/shell model, in which the entrapped water molecules partition onto two molecular subensembles: a bulk-like water core and a hydration layer near the ionic surfactant headgroups. These two distinct water species display different relaxation kinetics, as they do not exchange vibrational energy. The observed spectrotemporal ultrafast response exhibits a local character, indicating that the spatial confinement influences approximately one molecular layer located near the water-amphiphile boundary. The core of the encapsulated water droplet is similar in its spectroscopic properties to the bulk phase of liquid water, i.e., it does not display any true confinement effects such as droplet-size-dependent vibrational lifetimes or rotational correlation times. Unlike in bulk water, no intermolecular transfer of OH-stretching quanta occurs among the interfacial water molecules or from the hydration shell to the bulk-like core, indicating that the hydrogen bond network near the H2O/AOT interface is strongly disrupted.     

Synthesis and characterization of cross-linked reverse micelles

The design and synthesis of a new cross-linkable amphiphile is reported. Solutions of the amphiphile in a toluene/water mixture form reverse micelles as indicated by dynamic light scattering and NMR spectroscopy. As indicated by dynamic light scattering, TEM, and NMR spectroscopy data, these reverse micelles can be cross-linked without drastically changing the radius of the reverse micelles. Mixed reverse micelles are also characterized and cross-linked. The cross-linked reverse micelles are demonstrated to facilitate phase transfer and can be used to site isolate a catalyst.     

Dynamics of water confined within reverse micelles

We report structural and dynamical properties of water confined within reverse micelles (RMs) ranging in size from R = 10 A to R = 23 A as determined from molecular dynamics simulations. The low-frequency infrared spectra have been calculated using linear response theory and depend linearly on the fraction of bulklike water within the RMs. Furthermore, these spectra show nearly isosbestic behavior in the region near 660 cm(-1). Both of these characteristics are present in previously measured experimental spectra. The single dipole spectra for interfacial trapped, bound, and bulklike water within the RMs have also been calculated and show region-dependent frequency shifts. Specifically, the bound and trapped water spectra have a peak at lower frequencies than that for the inner core water. We therefore assign the low-frequency band in the IR spectra to bound and trapped interfacial water. Finally, region-dependent hydrogen bonding profiles and spatial distribution functions are also presented.     

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.     

Excited state proton transfer in reverse micelles

The aqueous phase of water/AOT reversed micelles having varying diameters was probed by a single free diffusing proton that was released form a hydrophilic photoacid molecule (2-naphthol-6,8-disulfonate). The fluorescence decay signals were reconstructed through the geminate recombination algorithm, accounting for the reversible nature of the proton-transfer reactions at the surface of the excited molecule and at the water/detergent interface. The radial diffusion of the proton inside the aqueous phase was calculated accounting for both the entropy of dilution and the total electrostatic energy of the ion pair, consisting of the pair-energy and self-energy of the ions. The analysis implied that micellar surface must be modeled with atomic resolution, assuming that the sulfono residue protrudes above the water/hydrocarbon interface by approximately 2 A. The analysis of the fluorescence decay curves implies that the molecule is located in a solvent with physical-chemical properties very similar to bulk water, except for the dielectric constant. For reversed micelles with r(max) > or = 16 A, the dielectric constant of the aqueous phase was approximately 70 and for smaller micelles, where approximately 60% of the water molecule is in contact with the van der Waals surface of the micelle, it is as low as 60. This reduction is a reflection of the increased fraction of water molecule that is in close interaction with the micelle surface.     

Selective transition metal extraction by reverse micelles

In this report we have studied the extraction of a series of heavy metals ions (Cu2+, Ni2+, Fe3+, Cr3+, CrO4(2-)) from water bulk solutions by means of reverse micelles. The parameters explored are the nature and concentration of the accompanying electrolyte, as well as the surfactant nature and its concentration. The extracted metals can be recovered and eventually concentrated in a new water solution carrying out a back extraction. The extracted amount of metal is strongly dependent on the charge of the metal to be extracted. Therefore the extracted water solution is enriched in higher charge metal. Anions of amphoteric metals, like the chromate ions, can be quantitatively separated from their positive cations, like Cr3+ by properly choosing the cationic or the anionic surfactants. The transfer of the metal is essentially controlled by electrostatic forces. A model based on the Poisson-Boltzmann distribution allow us to get the potential profile inside the water pool by determining the concentrations of the surfactant counter ions. From the potential profile and mass balance it is possible calculate the extraction percentage.     

反胶束系统及萃取蛋白质的分子热力学模型1: 反胶束系统的分 子热力学模型

以反胶束系统稳定性的热力学分析为基础,综合分析了反胶束系统的三大效应,即低界面张力效应、界面弯矩效应、混合熵效应,提出了一个分子热力学模型,模型所预言的反胶束水分含量随无机盐种类、浓度、表面活性剂浓度以及助表面活性剂含量的变化与所获实验规律定量相符,还能预言反胶束内表面处电势值、表面活性剂解离度。     

Stabilizing effect of low concentrations of urea on reverse micelles

Urea is a well-known destabilizing agent for biopolymers like proteins and molecular aggregates like micelles and reverse micelles. Several theories have been proposed to explain the destabilizing/denaturing effect of urea. In this work, we present evidence for a stabilizing effect of a low concentration (<1 M) of urea incorporated in the central pool of AOT/n-heptane/water reverse micelles. Static light-scattering experiments were performed to measure (w0)cr--the molar ratio of water to AOT beyond which the micelles become unstable--as a function of the concentration of urea in the central water pool. The stabilizing effect of urea is reflected in an increase in the value of (w0)cr at low urea concentrations over that in the absence of urea. Dynamic light-scattering experiments show that the hydrodynamic radius of the micelles is smaller at low urea concentrations (<1 M) than in the absence of urea. Size-distribution analysis shows that for w0=20 the microemulsion containing 0.5 M urea in its pool is significantly more monodisperse than that containing no urea. Temperature-dependent studies in the range 15-65 degrees C indicate that the magnitude of this stabilizing effect decreases with increasing temperature, vanishing at temperatures higher than 65 degrees C. A model is proposed to explain the above results.     

Endogenous growth of the population of reverse micelles

The coupling reaction between cetylbromide (CB) and trimethylamine (TMA) to yield the surfactant cetyltrimethylammonium bromide (CTAB) is studied in the system chloroform/isooctane (2/1,v/v)/water in which CTAB forms reverse micelles. This system affords an endogenous micelle population growth, i.e., an increase of the concentration of the micelles due to appearance of the surfactant in situ. The reaction is studied in the presence of preexisting CTAB reverse micelles. The rate of CTAB formation is measured by NMR spectroscopy, and the endogenous micelle population growth is directly monitored by time-resolved fluorescence quenching analysis. Under our experimental conditions, a 100% yield of the chemical reaction brings about a fourfold increase in the population of the reverse micelles. Since the water concentration is constant during chemical reaction, the newly formed water pools are formed at the expense of the initial ones, which brings about a decrease of the average water pool radius during micellar growth. The implication of the endogenous micelle population growth as a model for biological systems is briefly discussed.     

Nanometer metallic copper particle synthesis in reverse micelles

We describe the synthesis in situ of copper nanoparticles in reverse micelles. It is possible to form metallic particles either surrounded or not surrounded by an oxide layer.