Locations of methanol in methanol-containing AOT reverse micelles revealed by photophysics of IR125

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.

Phase Stability and Miscibility in Ethanol/AOT/n-Heptane Systems: Evidence of Multilayered Cylindrical and Spherical Microemulsion Morphologies

We describe the effects of ethanol on the phase behavior of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in n-heptane. Using dynamic light scattering (DLS), molecular dynamics (MD) simulations, and nuclear magnetic resonance (¹H NMR) spectroscopy, we investigate the aggregation behavior of AOT across a wide range of ethanol/AOT/n-heptane compositions. We conclude that reverse micelles do not form at any of the investigated concentrations. Instead, we observe the formation of other surfactant aggregate morphologies unique to this system, namely, multilayered cylindrical structures and spherical AOT-in-ethanol structures, which vary significantly with changes in ethanol concentration. We also identify mixed-solvent polarity as a driving factor for the surfactant behavior in the system. When the concentration of ethanol is 20 wt % or below, the system is inhomogeneous with varying sizes of AOT, ethanol, and AOT + ethanol aggregates, with the ethanol primarily exhibiting a cosurfactant behavior, almost exclusively binding at the surface of AOT aggregates. With increased ethanol concentration, the ethanol in the system also exhibits solvent-like behaviors in addition to the cosurfactant behaviors. Most significantly, when the ethanol concentration is raised above 35 wt %, the transition to solvent-like behavior allows AOT Na⁺ counterions to dissociate from the headgroups and they are dissolved in the ethanol. We use these results to construct a preliminary phase diagram for the ethanol/AOT/n-heptane system.

Location of menaquinone and menaquinol headgroups in model membranes

Menaquinones are lipoquinones that consist of a headgroup (naphthoquinone, menadione) and an isoprenyl sidechain. They function as electron transporters in prokaryotes such as Mycobacterium tuberculosis. For these studies, we used Langmuir monolayers and microemulsions to investigate how the menaquinone headgroup (menadione) and the menahydroquinone headgroup (menadiol) interact with model membrane interfaces to determine if differences are observed in the location of these headgroups in a membrane. It has been suggested that the differences in the locations are mainly caused by the isoprenyl sidechain rather than the headgroup quinone-to-quinol reduction during electron transport. This study presents evidence that suggests the influence of the headgroup drives the movement of the oxidized quinone and the reduced hydroquinone to different locations within the interface. Utilizing the model membranes of microemulsions and Langmuir monolayers, it is determined whether or not there is a difference in the location of menadione and menadiol within the interface. Based on our findings, we conclude that the menadione and menadiol may reside in different locations within model membranes. It follows that if menaquinone moves within the cell membrane upon menaquinol formation, it is due at least in part, to the differences in the properties of headgroup interactions with the membrane in addition to the isoprenyl sidechain.

Anomalous Spectral Modulation of 4-Aminophthalimide inside Acetonitrile/AOT/ n-Heptane Microemulsion: New Insights on Reverse Micelle to Bicontinuous Microemulsion Transition

The behavior of acetonitrile/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/ n-heptane microemulsion, whether it remains as reverse micelle (RM) or bicontinuous microemulsion (BMC), has been controversial and even termed as a "problem system". Herein, we investigate the microemulsion using spectral and dynamical responses of a hydrophilic solvatochromic fluorophore 4-aminophthalimide (4-AP) at different w_s values (=[acetonitrile]/[AOT]). Interestingly, we found that emission parameters of 4-AP within the microemulsion vary differently at low and high w_s regimes. The quantum yield (ϕ_f) and lifetime (τ_f) of 4-AP first increase up to w_s = ∼1 and, thereafter, decrease upon a further increase in the w_s values. The emission maximum of 4-AP significantly shifts to a higher wavelength from 445 nm at w_s = 0 to 475 nm at w_s = 8. Interestingly, unlike aqueous RMs, the emission maximum at w_s = 1 matches with the emission maximum in neat acetonitrile and the emission maximum shifts to even longer wavelength at a higher w_s. Steady-state anisotropy also shows a break around w_s = 1; anisotropy decreases very sharply from w_s = 0 to 1 and, thereafter, remains nearly constant. Solvation dynamics becomes progressively faster with an increase in the acetonitrile content only in the low w_s regimes but remains almost independent of w_s after w_s > 2. All of the results collectively indicate that the morphology of the microemulsion may change at an intermediate w_s (∼1); below this, the system behaves like reverse micelles, and above this, the system may remain as BMC. The conjecture was further supported by dynamic light scattering measurements, where we observed a gradual increment of the average size at the low acetonitrile content (up to w_s = 1) but, thereafter, the size distribution becomes multimodal and sizes cannot be estimated correctly.

Photoisomerization and reorientational dynamics of DTDCI in AOT/alkane reverse micelles containing non-aqueous polar liquids

Molecular mobility of the symmetric carbocyanine fluorophore DTDCI was studied in AOT/alkane reverse micelles containing non-aqueous polar liquids DMF, formamide, ethylene glycol and glycerol by monitoring both the torsional photoisomerization and rotational reorientation, both of which were sensitive to microviscosity of the local environment. The DTDCI fluorophore resides completely within the AOT-polar liquid reverse micelle nano-droplets, where its dynamics were found to be significantly retarded irrespective of the polar liquid taken, due to a combination of electrostatic and hydrophobic forces that induce the guest DTDC(+) cation to attach to the AOT molecules of the host droplet. The addition of strong hydrogen-bond donating polar liquids like formamide, ethylene glycol and glycerol causes a systematic enlargement of the droplets. Rotational dynamics of the fluorophore inside the nano-droplets was characterized by a diffusion coefficient comparable to that in highly viscous solvents like ethylene glycol.

Steady-state and time-resolved spectroscopic investigations on the existence of stable methanol/AOT/n-heptane reverse micelles

In this work, we have reported our study on the controversial issue whether methanol molecules can be effectively encapsulated by surfactant AOT to form true reverse micelles. We compared the different photophysical properties of coumarin 153 (C153) in methanol/AOT/n-heptane reverse micelles and methanol/n-heptane binary mixture by means of steady-state absorption, fluorescence and time-resolved fluorescence spectroscopies. In the reverse micelles, the fluorescence emission spectra of C153 were dependent on the excitation wavelength, while in binary mixtures, the excitation wavelength dependence was not observed. The biexponential decay curves of C153 in reverse micelles give a further confirmation for the two different environments where C153 molecules reside in. In other words, C153 molecules can exist both inside the core of the reverse micelles and outside of it. These results proved that the methanol can be effectively encapsulated by AOT in n-heptane solvents to form stable methanol/AOT/n-heptane reverse micelles.