Structure and properties of AOT reversed micelles as studied by the fluorescence probe technique

Freezing of Water Solvation Dynamics in Nanoconfinement by Reverse Micelles at Room Temperature

Much attention has recently been paid to anomalously low dielectric constants of nanoconfined water between two slabs at room temperature (Fumagalli et al. Science, 2018, 360, 1339). These low values imply that the dipole rotation of the interfacial water on the slab is completely suppressed. Such freezing has so far been observed for water confined between solids. In contrast, it remains unclear whether this holds for water in soft confinement, which is omnipresent naturally and artificially. Here, we address this question using encapsulated reverse micelles with a dye molecule, allowing us to study water sandwiched between the surfactant and dye molecules in solution. Moreover, we examine the solvation related to the dielectric property of water, which is reorientational motion in the hydration layer of the dye molecule, by persistent hole-burning spectroscopy. We first show that the dye molecule is surrounded by water without contact with the surfactant and that the dye molecule has two or three hydration layers on average. We next demonstrate that the solvation dynamics is frozen below the water droplet size of ∼4 nm, whereas they become liquid-like when the RM size is further increased. The average gap distance (∼1.5 nm) for freezing the solvation agrees with the gap distance with no rotational water motions between slabs. Our findings may have biological relevance, providing a new aspect for understanding biological function in cells.

Dioctyl sodium sulfosuccinate surfactant self-assembly dependency of solvent hydrophilicity: a modelling study

The self-assembly of dioctyl sodium sulfosuccinate (AOT) model surfactant in solvent environments of differing polarity is examined by means of dissipative particle dynamics (DPD) bead model parametrized against Hildebrand solubility parameters from atomistic molecular dynamics (MD) simulations. The model predicts that in hydrophobic solvents (e.g. dodecane) the surfactant forms small (Nagg ∼ 8) reverse micellar aggregates, while in a solvent corresponding to water lamellar assembly takes place, in good agreement with literature structural parameters. Interestingly, solvents of intermediate polarity lead to formation of large, internally structured aggregates. In these, the surfactant headgroups cluster within the aggregate, surrounded by a continuous phase formed by the hydrocarbon tails. We show that the partitioning of the headgroups between the aggregate surface layer and the inner clustered phase depends primarily on solvent polarity, and can be controlled by the solvent, but also system composition. Finally, we compare the DPD assembly response to simplified effective interaction potentials derived at dilute concentration limit for the interactions. The comparison reveals that the simplified effective potential descriptions provide good level of insight on the assembly morphologies, despite drastic, isotropic interactions simplification involved.

Enhanced Hydrodynamic Radius of AOT/n-heptane/Water Reverse Micellar System Through Altered Electrostatic Interactions and Molecular Self-Assemblies

We have demonstrated a unique approach to alter the aqueous pool size of an AOT/n-heptane/water reverse micellar system. A positively charged dye Rhodamine B (RhB) and negatively charged Rose Bengal (RB) were incorporated in the reverse micellar pool to investigate the effect of electrostatic interactions and stacking effects among the dye molecules on the AOT/n-heptane/water interface. Dynamic light scattering revealed increase in reverse micellar pool size in presence of positively charged dye aggregates at the oil-water interface. However, less expansion was observed in presence of negatively charged dye aggregates (RB). This confirms the role of electrostatic interaction in modulating the hydrodynamic radius. A head-to-tail type of stacking of RhB molecules at the interface favors this expansion. The differences in stacking of the two dyes inside the reverse micelles and their torsional mobility indicated the role of the reverse micellar interface and H-bonding ability of the microenvironment on dye aggregation. Conductivity measurements demonstrated a significant drop in percolation temperature of the reverse micellar system in presence of dye aggregates. This confirms the effect of dye aggregation and electrostatic interaction on such expansion. This strategy can be exploited for solubilizing greater amounts and a wider variety of drug molecules in microemulsions.

Persistent optical hole-burning spectroscopy of nano-confined dye molecules in liquid at room temperature: Spectral narrowing due to a glassy state and extraordinary relaxation in a nano-cage

Persistent optical hole-burning spectroscopy has been conducted for a dye molecule within a very small (∼1 nm) reverse micelle at room temperature. The spectra show a spectral narrowing due to site-selective excitation. This definitely demonstrates that the surroundings of the dye molecule are in a glassy state regardless of a solution at room temperature. On the other hand, the hole-burning spectra exhibit large shifts from excitation frequencies, and their positions are almost independent of excitation frequencies. The hole-burning spectra have been theoretically calculated by taking account of a vibronic absorption band of the dye molecule under the assumption that the surroundings of the dye molecule are in a glassy state. The calculated results agree with the experimental ones that were obtained for the dye molecule in a polymer glass for comparison, where it has been found that the ratio of hole-burning efficiencies of vibronic- to electronic-band excitations is quite high. On the other hand, the theoretical results do not explain the large spectral shift from the excitation frequency and small spectral narrowing observed in the hole-burning spectra measured for the dye-containing reverse micelle. It is thought that the spectral shift and broadening occur within the measurement time owing to the relaxation process of the surroundings that are hot with the thermal energy deposited by the dye molecule optically excited. Furthermore, the relaxation should be temporary because the cooling of the inside of the reverse micelle takes place with the dissipation of the excess thermal energy to the outer oil solvent, and so the surroundings of the dye molecule return to the glassy state and do not attain the thermal equilibrium. These results suggest that a very small reverse micelle provides a unique reaction field in which the diffusional motion can be controlled by light in a glassy state.

Formose reaction accelerated in aerosol-OT reverse micelles

The formose reaction in reverse micelles of aerosol-OT (AOT), triton X-100 (TX), and hexadecyltrimethylammonium bromide (CTAB) was investigated. Time–conversion data have indicated that the interfacial water layer of AOT reverse micelles is a medium that accelerates formation of glycolaldehyde in the formose reaction. The ¹³C NMR spectra for the products of the formose reaction using formaldehyde-¹³C as starting material are indicative of the formation of ethylene glycol as a major product.

Characterization of dimethylsulfoxide/glycerol mixtures: a binary solvent system for the study of “friction-dependent” chemical reactivity

The properties of binary mixtures of dimethylsulfoxide and glycerol, measured using several techniques, are reported.

Structural effects of the β-vinyl linker in pyridinium porphyrins: spectroscopic studies in organic solvents and AOT reverse micelles

Two isomeric β-vinylpyridinium porphyrins, 2-[2-(2-methylpyridinium)vinyl]-5,10,15,20-tetraphenylporphyrin (1, ortho isomer) and 2-[2-(4-methylpyridinium)vinyl]-5,10,15,20-tetraphenylporphyrin (2, para isomer), which have shown different photodynamic behavior were investigated in organic solvents and sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. In organic systems, the absorption spectra present a red-shifted band that is more intense in the para isomer, in addition to the usual Soret band. This new band presents interesting solvatochromic effects which obey the multiparametric Kamlet-Taft equation. In AOT reverse micelles, the ortho isomer exhibits a strong dependence with the parameter ω0 = [H2O]/[AOT] which indicates that the molecule resides at the interface toward the organic phase. By contrast, no evidence was detected for the encapsulation of para isomer 2 in AOT reverse micelles. The hypothesis of two ground state isomers with different contributions of trans and quinoid structures is advanced on the basis of the overall data collected from electronic absorption, steady-state, and transient-state fluorescence emission. A charge transfer state in which an electron is fully transferred from the porphyrin to the pyridinium moiety is associated to a quinoid structure in isomer 2. The trans/quinoid relative proportions may be accounted for by the orientation of the ortho-/para-pyridinium isomers relatively to the porphyrin core.

Nanometer-scale water droplet free from the constraint of reverse micelles at low temperatures

Temperature dependence of the configurational fluctuation of water confined in a reverse micellar solution has been studied by absorption spectroscopy of a probe molecule. We have found that the configurational fluctuation is liquidlike below the homogeneous nucleation temperature. This is proposed to be due to a large reduction in the confinement of water, and is explained in terms of water shedding from the reverse micelle. Further, the configurational fluctuation is frozen at ~210 K. A reverse micellar solution is considered to be a promising candidate for studies of supercooled water.

Fluorescence lifetime probe for solvent microviscosity utilizing anilinonaphthalene sulfonate

The correlation between the fluorescent dynamics of excited anilinonaphthalene sulfonate (ANS) and the microviscosity of solvent molecules surrounding ANS is investigated by time-resolved fluorescence spectroscopy. ANS has been widely used to probe the local hydrophobicity due to the drastic change in its intensity. It is revealed that the fluorescence lifetime from the charge transfer (CT) state of ANS sensitively reflects the microviscosity. The higher sensitivity of 2,6-ANS than of 1,8-ANS demonstrates that the spatial freedom of the rotating phenylamino group in the photoexcited ANS is an important factor that determines the sensitivity. As an application, the measurements of the microviscosity of water in biologically important systems, such as hyaluronan, gellan gum, and gelatin aqueous solutions are also presented. The present results suggest that the fluorescence lifetime of ANS enables the estimation of the solvent microviscosity and provide a useful probe molecule for fluorescence lifetime imaging microscopy.

The preparation of size-controlled functionalized polymeric nanoparticles in micelles

The reverse micellar system of dioctyl-sulfosuccinate (AOT)/octane and toluene have been used as a template for polymerization of acrylamide (AA)/bisacrylamide (BAA)-based functionalized polymeric nanoparticles. Such nanoparticles are typically sized between 20 and 90 nm. They can be synthesized with different functional groups according to the monomers added to the polymerization mixture. In our experiments the nanoparticles carried amino and carboxyl groups following incorporation of allylamine (AAm) or methacrylic acid (MAA) monomers, respectively. The available amine or carboxyl groups can then be used for immobilization of enzymes or other biomolecules. These enzymes, subtilisin, laccase and lipase, were immobilized onto polyAA/BAA/MAA nanoparticles covalently after activating the MAA carboxylic groups with Woodward's K reagent. Non-covalent immobilization via electrostatic interaction was also performed.

What is inside a nonionic reverse micelle? Probing the interior of Igepal reverse micelles using decavanadate

The interiors of reverse micelles formed using nonionic surfactants to sequester water droplets in a nonpolar environment have been investigated using the decavanadate molecule as a probe. Chemical shifts and line widths of the three characteristic signals in the 51V NMR spectrum of decavanadate, corresponding to vanadium atoms in equatorial peripheral, equatorial interior, and axial locations, measure the local proton concentration and characteristics of the reverse micellar interior near the decavandate probe. All samples investigated indicate deprotonation of the vanadate probe in the reverse micelle environment. However, the relative mobility of the decavanadate molecule depends on the reverse micellar components. Specifically, the 51V NMR signals of the decavandate in reverse micelles formed using only the Igepal CO-520 surfactant display sharp signals indicating that the decavandate molecule tumbles relatively freely while reverse micelles formed from a mixture of Igepal CO-610 and -430 present a more viscous environment for the decavanadate molecule; the nature of the interior of the nonionic reverse water pool varies significantly depending on the specific Igepal. The 51V NMR spectra also indicate that the interior core water pool of the reverse micelles is less acidic than the bulk aqueous solution from which the samples were created. Together, these data provide a description that allows for a comparison of the water pools in these different nonionic reverse micelles.

Reactive dynamics in confined liquids: ultrafast torsional dynamics of auramine O in nanoconfined water in aerosol OT reverse micelles

The effects of confinement on the ultrafast torsional reaction of auramine O in aqueous solution are investigated through ultrafast fluorescence up-conversion with 50 fs time resolution. The aqueous solution is confined in nanoscale water droplets by an ionic surfactant. The torsional motion is orders of magnitude slower in the confined droplets than in bulk aqueous solution. The dynamics become faster with increasing radius of the nanodroplet but never reach the bulk value, even when the radius is as large as 10 nm. Time-dependent fluorescence spectra were constructed and subsequently analyzed using a one-dimensional generalized Smoluchowski equation. An accurate description of the data was achieved using a time-dependent diffusion coefficient. This is suggested to arise because the medium friction reflects dynamics on a broad range of time scales spanning the reaction dynamics. The friction recovered suggests strongly hindered motion in the confined droplet and can be qualitatively related to solvation dynamics measured in AOT, consistent with auramine O torsional dynamics being accompanied by intramolecular charge redistribution.

Calorimetric study of the alpha-tocopherol solubility in reversed AOT micelles

The experimental data of heat of mixing (Q) for heterogeneous system alpha-tocopherol/AOT/n-heptane with and without water at 25 degrees C are presented. The Q dependence on AOT (sodium bis (2-ethylhexyl) sulfosuccinate) concentration, and R parameter defined as R=[H2O]/[AOT] with flow calorimetric method were investigated. Using the D'Aprano model (which is formally identical to that used earlier by Magid et al.) the binding constant (K), the distribution constant of alpha-tocopherol (K distr) between hydrocarbon and the micellar phase, and the standard enthalpy of transfer (DeltaH tr 0) of alpha-tocopherol from the hydrocarbon to AOT reversed micelles were calculated. The solubility of alpha-tocopherol in AOT reversed micelles explored with the calorimetric technique was compared to the literature data obtained respectively with UV spectrophotometry for reversed micelles and by other techniques for the phospholipid bilayer.

Observation of Three Behaviors in Confined Liquid Water within a Nanopool Hosting Proton-Transfer Reactions

In this contribution, we report on studies of rotational and diffusional dynamics of 7-hydroxyquinoline (7HQ) within a reverse micelle (RM) containing different amounts of water. Analyzed in terms of the wobbling-in-a-cone model, the data reveal structural and dynamical properties of the nanopool. We clearly observed three regions in the behavior of confined water molecules within the RM hosting a double proton-transfer reaction between the probe and water. This observation remarkably reproduces the change of calculated water density within this life-mimicking medium. The number of water molecules per AOT head in the transition regions changes from 2 to 5, the latter being very near to the full solvation number (6) of the RM heads. Moreover, the H-bonds breaking and making within the RM to give new structures of the probe strongly affect the environment fluidization in different extents, reflected in different relaxation times of these structures; however, they are of similar sizes. We discuss the role of RM confinement and the proton-transfer dynamics on the behavior of water and their relationships to the packing of water molecules in the studied range of concentrations.

Probing the Behavior of Confined Water by Proton-Transfer Reactions

The picosecond dynamics of a bifunctional and H-bonding molecule, 7-hydroxyquinoline (7HQ), has been studied in a reverse micelle with increasing water content. The fluorescence kinetics has a complex behavior as the water content is changed. All reactions are irreversible, and a two-step mechanism is invoked to explain the observations. H2O/D2O exchange and excitation energy effects show that the second step has a higher barrier and that the corresponding reaction occurs through tunneling. The results clearly indicate two regimes of water nanopool behavior switching at W0 approximately 5 (W0 = [water]/[surfactant]). Water collective dynamics explains these observations. The lower fluidity of confined water within the reverse micelle with respect to normal bulk water alters the related H-bond network dynamics and therefore is responsible for the slower proton-transfer processes.

New Approaches to Liquid Interfaces through Changes in the Refractive Index and Nonlinear Susceptibility Utilizing Ultrashort Laser Pulses

Molecules in inhomogeneous liquid environments, such as air/liquid, liquid/liquid, solid/liquid interfaces interact with each other specifically, and sometimes form characteristic structures and emerge unique properties. Here, we introduce two newly developed spectroscopic techniques, the total-internal-reflection ultrafast transient lens method (TIR-UTL) and second harmonic generation-coherent vibrational spectroscopy (SHG-CVS), to investigate the characteristic behaviors of molecules in such inhomogeneous environments. TIR-UTL probes the refractive-index change with sub-picosecond resolution and provides information on ultrafast changes in the population, density, and thermal properties, such as temperature increase and energy transfer from the solute molecules to the surrounding solvent molecules. On the other hand, SHG-CVS probes nonlinear susceptibility changes at the interfacial areas, and is expected to provide spectroscopic information on the low-frequency vibrational modes that reflect the corrective motion of the molecules in such an inhomogeneous environment. These new approaches are based on pump-probe techniques utilizing (ultra) short laser pulses. They are expected to provide further information on inhomogeneous environments from the viewpoints of solute-solvent interactions, changes in the molecular orientation, and the corrective motion of molecules at liquid interfaces.

Spectroscopic probing of the effect of alkanols on the properties of the head group region in reverse micelles of AOT-heptane-water

The effects of addition of alkanols (ethanol, n-hexanol, and 3-ethyl-3-pentanol) on the micropolarity and microviscosity of the head group region in reverse micelles of AOT-heptane-water have been investigated by fluorescence probing methods (ANS fluorescence yield and TMADPH fluorescence anisotropy), complemented by the use of the solvatochromic probe E(T)(30) in absorption spectroscopy. For all the alkanols considered, ANS fluorescence in AOT reverse micelles (at W=3) is quenched by additive incorporation, being the effect elicited almost independent of the alkanol chain length and topology. As sensed by the E(T)(30) parameter, the micropolarity of the micelle surface increases, remains unmodified, and decreases upon addition of ethanol, 3-ethyl-3-pentanol, and hexanol, respectively. While ethanol barely modifies the fluorescence anisotropy of TMADPH, 3-ethyl-3-pentanol and n-hexanol addition strongly decrease it. The similarity of the tendencies of ANS data to TMADPH anisotropies and the differences between ANS data and E(T)(30) values would indicate that, at least for 3-ethyl-3-pentanol and n-hexanol, microviscosity, rather than micropolarity, must be considered to interpret the effect of the alkanols upon the fluorescent behavior of ANS.

Substituent Effects on Binding Constants of Carotenoids to n-Heptane/AOT Reverse Micelles

The absorption spectra of 6'-apo-beta-caroten-6'-ol (1), 6'-apo-beta-caroten-6'-oic acid (2), and ethyl 6'-apo-beta-caroten-6'-oate (3) were analyzed in homogeneous media and in reversed micelles of AOT (sodium 1,4-bis(2-ethylhexyl) sulfosuccinate) in n-heptane. The possible solute-solvent interactions of these compounds were analyzed in pure solvents by Taft and Kamlet's solvatochromic comparison method. These carotenoids show sensitivity similar to that of medium polarity-polarizability as measured by pi*. Moreover, the absorption spectra of carotenoid 3 and to much less extent carotenoid 2 display broadening of the visible bands induced by polar solvents characteristic of carotenoids that contain a carbonyl functional group in conjugation with the carbon-carbon pi-electron system. They are also sensitive to the ability of the solvent to accept protons in a hydrogen bond interaction measured by beta. This sensitivity follows the expected order: 2>1>3. In the reverse micellar system, while the spectra for 3 remain unchanged, the intensity of the absorption band characteristic of n-heptane for 1 and 2 decreases as the AOT concentration increases, and a new band develops. This new band is attributed to the solute bound to the micelle interface. These changes allowed us to determine the binding constant (K(b)) between these compounds and AOT. At W(0)=[H(2)O]/[AOT]=0 the values of K(b) of 326+/-5 and 6.2+/-0.3 were found for the acid 2 and the alcohol 1, respectively. The strength of binding is interpreted considering their hydrogen-bond donor ability and the solubility in the organic pseudophase. For 1 K(b) decreases as W(0) is increased, while for 2 no variation was observed. These effects are discussed in terms of carotenoid-water competition for interfacial binding sites. Copyright 2001 Academic Press.