Modeling the Self-Aggregation of Small AOT Reverse Micelles from First-Principles

Shape of AOT Reverse Micelles: The Mesoscopic Assembly Is More Than the Sum of the Parts

AOT reverse micelles are a common and convenient model system for studying the effects of nanoconfinement on aqueous solutions. The reverse micelle shape is important to understanding how the constituent components come together to form the coherent whole and the unique properties observed there. The shape of reverse micelles impacts the amount of interface present and the distance of the solute from the interface and is therefore vital to understanding interfacial properties and the behavior of solutes in the polar core. In this work, we use previously introduced measures of shape, the coordinate-pair eccentricity (CPE) and convexity, and apply them to a series of simulations of AOT reverse micelles. We simulate the most commonly used force field for AOT reverse micelles, the CHARMM force field, but we also adapt the OPLS force field for use with AOT, the first work to do so, in addition to using both 3- and 4-site water models. Altogether, these simulations are designed to examine the impact of the force field on the shape of the reverse micelles in detail. We also study the time autocorrelation of shape, the water rotational anisotropy decay, and how the CPE changes between the water pool and AOT tail groups. We find that although the force field changes the shape noticeably, AOT reverse micelles are always amorphous particles. The shape of the micelles changes on the order of 10 ns. The water rotational dynamics observed match the experiment and demonstrate slower dynamics relative to bulk water, suggesting a two-population model that fits a core/shell hypothesis. Taken together, our results indicate that it is likely not possible to create a perfect force field that can reproduce every aspect of the AOT reverse micelle accurately. However, the magnitude of the differences between simulations appears relatively small, suggesting that any reasonably derived force field should provide an acceptable model for most work on AOT reverse micelles.

Ultrafast Excited-State Proton Transfer of a Cationic Superphotoacid in a Nanoscopic Water Pool

The excited-state proton transfer (ESPT) of a cationic superphotoacid, N-methyl-7-hydroxyquinolium, was studied within the water pool of an anionic aerosol-OT (AOT), bis(2-ethylhexyl) sulfosuccinate, reverse micelle (RM). Previously, we had found that the cationic photoacid residing at the anionic AOT interface was conducive to ESPT to the bound water having concentric heterogeneity on the time scale of hundreds of picoseconds to nanoseconds. In our present study, on the time scale of hundreds of femtoseconds to a few tens of picoseconds, the photoacid underwent an ultrafast ESPT influenced by mobile water constituting the core of the RM. The two subpopulations of the core water molecules that determine the ultrafast biphasic deprotonation of the photoacid on time scales differing by an order of magnitude were identified. The core water molecules solvating the counteranion of the photoacid showed a higher basicity than typical water clusters in bulk resulting in ESPT on a subpicosecond time scale. Bare water clusters sensed by the photoacid showed a slower ESPT, over several picoseconds, as typically limited by the rotational motion of water molecules for similar types of the photoacid.

How to Characterize Amorphous Shapes: The Tale of a Reverse Micelle

Aerosol-OT reverse micelles represent a chemical construct where surfactant molecules self-assemble to stabilize water nanodroplets 1-10 nm in diameter. Although commonly assumed to adopt a spherical shape, all-atom molecular dynamics simulations and some experimental studies predict a nonspherical shape. If these aggregates are not spherical, then what shape do they take? Because the tools needed to evaluate the shape of something that lacks regular structure, order, or symmetry are not well developed, we present a set of three intuitive metrics─coordinate-pair eccentricity, convexity, and the curvature distribution─that estimate the shape of an amorphous object, and we demonstrate their use on a simulated aerosol-OT reverse micelle. These metrics are all well-established methods and principles in mathematics, and each provides unique information about the shape. Together, these metrics provide intuitive descriptions of amorphous shapes, facilitate ways to quantify those shapes, and follow their changes over time.

Structural Insights into Self-Assembled Aerosol-OT Aggregates in Aqueous Media Using Atomistic Molecular Dynamics

In water, the surfactant dioctyl sulfosuccinate (Aerosol-OT or AOT) exhibits diverse aggregate structures, ranging from micelles to lamella. An atomic-level understanding, however, of the formation and structure of these aggregates is lacking. Herein, using atomistic molecular dynamics (MD) with microsecond-long simulations, self-assembly of AOT in water is studied for concentrations of 1, 7.2, and 20 wt % at 293 K and for 7.2 wt % at 353 K. Assembly proceeds through stepwise association and dissociation of single AOT molecules, and the fusion and fission of AOT clusters. At 293 K, AOT self-assembles into either (i) spherical micelles (1 wt %), (ii) biphasic systems consisting of rod-like and prolate spheroidal micelles (7.2 wt %), or (iii) bilayers (20 wt %). We hypothesize that the observed rod-like structure is a precursor to lamellar microdomains found experimentally in biphasic dispersions. Increasing temperature to 353 K at 7.2 wt % results in a system consisting of prolate micelles but no rod-like micelles. Simulated phase behavior agrees with previously published experimental observations. Individual aggregates formed during self-assembly are identified using graph theory. Structural metrics of these aggregates like the radius of gyration, shape anisotropy, and prolateness are presented. Trends in structural metrics quantitatively reflect how shapes and sizes of AOT aggregates vary with surfactant concentration and temperature. These simulations provide deeper insight into open questions in the scientific community and demonstrate a method to generate physics-based micelle structures that can be used to rationalize experimental observations.

Aqueous phase behavior of the PEO-containing non-ionic surfactant C12E6: A molecular dynamics simulation study

HYPOTHESIS

Non-ionic surfactants containing polyethylene oxide (PEO) chains are widely used in drug formulations, cosmetics, paints, textiles and detergents. High quality molecular dynamics models for PEO surfactants can give us detailed, atomic-scale information about the behavior of surfactant/water mixtures.

SIMULATIONS

We used two molecular dynamics force fields (FFs), 2016H66 and 53A6_DBW, to model the simple non-ionic PEO surfactant, hexaoxyethylene dodecyl ether (C₁₂E₆). We investigated surfactant/water mixtures that span the phase diagram of starting from randomly distributed arrangements. In some cases, we also started with prebuilt, approximate models. The simulations results were compared with the experimentally observed phase behavior.

FINDINGS

Overall, this study shows that the spontaneous self-assembly of PEO non-ionic surfactants into different colloidal structures can be accurately modeled with MD simulations using the 2016H66 FF although transitions to well-formed hexagonal phase are slow. Of the two FFs investigated, the 2016H66 FF better reproduces the experimental phase behavior across all regions of the C₁₂E_6/water phase diagram.

Deciphering the Structure of the Gramicidin A Channel in the Presence of AOT Reverse Micelles in Pentane Using Molecular Dynamics Simulations

Structural studies of proteins and, in particular, integral membrane proteins (IMPs) using solution NMR spectroscopy approaches are challenging due to not only their inherent structural complexities but also the fact that they need to be solubilized in biomimetic environments (such as micelles), which enhances the slow molecular reorientation. To deal with these difficulties and increase the effective rate of molecular reorientation, the encapsulation of IMPs in the aqueous core of the reverse micelle (RM) dissolved in a low-viscosity solvent has been proven to be a viable approach. However, the effect of the reverse micelle (RM) environment on the IMP structure and function is little known. To gain insight into these aspects, this article presents a series of atomistic unconstrained molecular dynamics (MD) of a model ion channel (gramicidin A, gA) with RMs formed with anionic surfactant diacyl chain bis(2-ethylhexyl) sodium succinate (AOT) in pentane at a water-to-surfactant molar ratio (W₀) of 6. The simulations were carried out with different protocols and starting conditions for a total of 2.4 μs and were compared with other MDs used with the gA channel inserted in models of the SDS micelle or the DMPC membrane. We show here that in the presence of AOT RMs the gA dimer did not look like the "dumbbell-like" model anticipated by experiments, where the C-terminal parts of the gA are capped with two RMs and the rest of the dimer is protected from the oil solvent by the AOT acyl chains. In contrast, the MD simulations reveal that the AOT, Na⁺, and water formed two well-defined and elongated RMs attached to the C-terminal ends of the gA dimer, while the rest is in direct contact with the pentane. The initial β^6.3 secondary structure of the gA is well conserved and filled with 6-9 waters, as in SDS micelles or the DMPC membrane. Finally, the water movement inside the gA is strongly affected by the presence of RMs at each extremity, and no passage of water molecules through the gA channel is observed even after a long simulation period, whereas the opposite was found for gA in SDS and DMPC.

Optimal aggregation number of reverse micelles in supercritical carbon dioxide: a theoretical perspective

The aggregation number is one of the most fundamental and important structural parameters for the micelle or reverse micelle (RM) system. In this work, a simple, reliable method for the determination of the aggregation number of RMs in supercritical CO2 (scCO2) was presented through a molecular dynamics simulation. The process of pulling surfactants out of the RMs one by one was performed to calculate the aggregation number. The free energies of RMs with different numbers of surfactants were calculated through this process. We found an RM with the lowest free energy, which was considered to have the optimal number of surfactants. Therefore, the optimal aggregation number of RMs was acquired. In order to explain the existence of an optimal aggregation number, detailed analyses of surfactant accumulation were conducted by combining molecular dynamics with quantum chemistry methods. The results indicated that in the RMs with the lowest free energy, the head-group and tail-terminal of the surfactants accumulated on an equipotential surface. In this case, the surfactant film could effectively separate water and CO2; thus, the lowest free energy was expected. This method determined the aggregation number of RMs by theoretical calculations that did not depend on experimental measurements. This presented approach facilitates the evaluation of the characteristics of RMs in scCO2 and can be further applied in the RM system of organic solvents or even in the micellar system.

Water loading driven size, shape, and composition of cetyltrimethylammonium/hexanol/pentane reverse micelles

Cetyltrimethylammonium bromide (CTAB)/hexanol reverse micelles have found a variety of applications that demand control over physical parameters. Water content or loading is among the most basic tunable components and is the major driver of the physical properties of these systems. This study uses small-angle scattering with contrast variation to characterize these systems as a function of water loading. The scattering data were analyzed with a variety of approaches, resulting in converging physical specifications. Equations that describe basic physical parameters were determined that allow for characterization and manipulation of the CTAB/hexanol reverse micelle surfactant system. The shape of the reverse micelles was revealed to be slightly ellipsoidal and varies slightly through the water loading range. The surfactant shell is shown to contain a higher fraction of hexanol upon addition of water. Analysis reveals that the size, shape, and surfactant/cosurfactant composition are directly tunable by variation of the water content and that these properties are consequences of the balance of forces present in the reverse micelles.

Towards capturing cellular complexity: combining encapsulation and macromolecular crowding in a reverse micelle

This paper studies the orientational structure and dynamics of multi-protein systems under confinement and discusses the implications on biological cells.

Mass Exchange and Equilibration Processes in AOT Reverse Micelles

Reverse micelles (RMs) made with sodium bis(2-ethylhexyl)sulfosuccinate suspended in isooctane are commonly used experimental models of aqueous microenvironments. However, there are important unanswered questions about the very characteristic that makes them of interest, namely their size. To explore the factors that determine the size of RMs, all-atom molecular dynamics simulations of RMs with different sizes but the same water-loading ratio were performed. An Anton 2 machine was used so that systems of the necessary size could be extended into the microsecond timescale, and mass exchange processes could be observed. Contrary to hypothesis, there were no net gains or losses of water by diffusion between RMs of different size. However, gains and losses did occur following fusion events. RM fusion followed RM contact only when waters were present among the hydrophobic surfactant chains at the point of contact. The presence of an encapsulated 40-residue amyloid beta peptide did not directly promote RM fusion, but it quickly and efficiently terminated each fusion event. Before fusion terminated, however, the size of the peptide-containing RM increased without a corresponding change in its water-loading ratio. We conclude that the mass transfer between RMs is most likely accomplished through transient fusion events, rather than through the diffusion of component molecules through the organic phase. The behavior of the amyloid beta peptide in this system underscores its propensity to embed in, and fold in response to, multiple interactions with the surfactant layer.

Revival of collective water structure and dynamics in reverse micelles brought about by protein encapsulation

A novel mechanism of depolarization in reverse micelles with zwitterionic surfactants and containing polar species but lacking ions is reported.

Aggregation response of triglyceride hydrolysis products in cyclohexane and triolein

Aggregation mechanism and the existence of cmc depend on apolar solvent quality and surfactant head group polarity.

Reverse Micelles As Antioxidant Carriers: An Experimental and Molecular Dynamics Study

Water-in-oil microemulsions with biocompatible components were formulated to be used as carriers of natural antioxidants, such as hydroxytyrosol (HT) and gallic acid (GA). The system was composed of a mixture of natural surfactants, lecithin and monoglycerides, medium chain triglycerides, and aqueous phase. A dual approach was undertaken to study the structure and dynamics of these complicated systems. First, experimental data were collected by using adequate techniques, such as dynamic light scattering (DLS) and electron paramagnetic resonance (EPR) spectroscopy. Following this, a coarse-grained molecular dynamics (CGMD) study based on the experimental composition using the MARTINI force field was conducted. The simulations revealed the spontaneous formation of reverse micelles (RMs) starting from completely random initial conformations, underlying their enhanced thermodynamic stability. The location of the bioactive molecules, as well as the structure of the RM, were in accordance with the experimental findings. Furthermore, GA molecules were found to be located inside the water core, in contrast to the HT ones, which seem to lie at the surfactant interfacial layer. The difference in the antioxidants' molecular location was only revealed in detail from the computational analysis and explains the RM's swelling observed by GA in DLS measurements.

The Size of AOT Reverse Micelles

Reverse micelles (RMs) made from water and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) are commonly studied experimentally as models of aqueous microenvironments. They are small enough for individual RMs to also be studied by molecular dynamics (MD) simulation, which yields detailed insight into their structure and properties. Although RM size is determined by the water loading ratio (i.e., the molar ratio of water to AOT), experimental measurements of RM size are imprecise and inconsistent, which is problematic when seeking to understand the relationship between water loading ratio and RM size, and when designing models for study by MD simulation. Therefore, a systematic study of RM size was performed by MD simulation with the aims of determining the size of an RM for a given water loading ratio, and of reconciling the results with experimental measurements. Results for a water loading ratio of 7.5 indicate that the interaction energy between AOT anions and other system components is at a minimum when there are 62 AOT anions in each RM. The minimum is due to a combination of attractive and repulsive electrostatic interactions that vary with RM size and the dielectric effect of available water. Overall, the results agree with a detailed analysis of previously published experimental data over a wide range of water loading ratios, and help reconcile seemingly discrepant experimental results. In addition, water loss and gain from an RM is observed and the mechanism of water exchange is outlined. This kind of RM model, which faithfully reproduces experimental results, is essential for reliable insights into the properties of RM-encapsulated materials.

Dielectric depolarisation and concerted collective dynamics in AOT reverse micelles with and without ubiquitin

In this computational study we present molecular dynamics (MD) simulations of reverse micelles, i.e. nano-scale water pools encapsulated by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and dissolved in isooctane. Although consisting of highly polar components, such micro-emulsions exhibit surprisingly low dielectric permittivity, both static and frequency-dependent. This finding is well supported by experimental dielectric measurements. Furthermore, the computational dielectric spectra of reverse micelles with and without the polar protein ubiquitin are almost identical. A detailed component analysis of our simulated systems reveals the underlying mechanism of the observed dielectric depolarisation. While each component by itself would make a remarkable contribution to the static dielectric permittivity, mutual compensation leads to the observed marginal net result. This compensatory behavior is maintained for all but the highest frequencies. Dielectric model theory adapted to the peculiarities of reverse micelles provides an explanation: embedding a system in a cavity engulfed by a low dielectric medium automatically leads to depolarization. In this sense experiment, simulation and theory are in accordance.

On the Structural and Dynamical Properties of DOPC Reverse Micelles

The structure and dynamics of phospholipid reverse micelles are studied by molecular dynamics. We report all-atom unconstrained simulations of 1,2-dioleoyl-sn-phosphatidylcholine (DOPC) reverse micelles in benzene of increasing sizes, with water-to-surfactant number ratios ranging from W₀ = 1 to 16. The aggregation number, i.e., the number of DOPC molecules per reverse micelle, is determined to fit experimental light-scattering measurements of the reverse micelle diameter. The simulated reverse micelles are found to be approximately spherical. Larger reverse micelles (W₀ > 4) exhibit a layered structure with a water core and the hydration structure of DOPC phosphate head groups is similar to that found in phospholipid membranes. In contrast, the structure of smaller reverse micelles (W₀ ≤ 4) cannot be described as a series of concentric layers successively containing water, surfactant head groups, and surfactant tails, and the head groups are only partly hydrated and frequently present in the core. The dynamics of water molecules within the phospholipid reverse micelles slow down as the reverse micelle size decreases, in agreement with prior studies on AOT and Igepal reverse micelles. However, the average water reorientation dynamics in DOPC reverse micelles is found to be much slower than in AOT and Igepal reverse micelles with the same W₀ ratio. This is explained by the smaller water pool and by the stronger interactions between water and the charged head groups, as confirmed by the red-shift of the computed infrared line shape with decreasing W₀.

Rotational dynamics of water molecules near biological surfaces with implications for nuclear quadrupole relaxation

Based on Molecular Dynamics simulations of two different systems, the protein ubiquitin dissolved in water and an AOT reverse micelle, we present a broad analysis of the single particle rotational dynamics of water. A comprehensive connection to NQR, which is a prominent experimental method in this field, is developed, based on a reformulation of its theoretical framework. Interpretation of experimental NQR results requires a model which usually assumes that the NQR experiences retardation only in the first hydration shell. Indeed, the present study shows that this first-shell model is correct. Moreover, previous experimental retardation factors are quantitatively reproduced. All of this is seemingly contradicted by results of other methods, e.g., dielectric spectroscopy, responsible for a long-standing debate in this field. Our detailed analysis shows that NQR omits important information contained in overall water dynamics, most notably, the retardation of the water dipole axis in the electric field exerted by a biological surface.

Molecular mechanisms responsible for hydrate anti-agglomerant performance

Steered and equilibrium molecular dynamics simulations were employed to study the coalescence of a sI hydrate particle and a water droplet within a hydrocarbon mixture.

Probing Nanoscale Thermal Transport in Surfactant Solutions

Surfactant solutions typically feature tunable nanoscale, internal structures. Although rarely utilized, they can be a powerful platform for probing thermal transport in nanoscale domains and across interfaces with nanometer-size radius. Here, we examine the structure and thermal transport in solution of AOT (Dioctyl sodium sulfosuccinate) in n-octane liquids using small-angle neutron scattering, thermal conductivity measurements, and molecular dynamics simulations. We report the first experimental observation of a minimum thermal conductivity occurring at the critical micelle concentration (CMC): the thermal conductivity of the surfactant solution decreases as AOT is added till the onset of micellization but increases as more AOT is added. The decrease of thermal conductivity with AOT loading in solutions in which AOT molecules are dispersed as monomers suggests that even the interfaces between individual oleophobic headgroup of AOT molecules and their surrounding non-polar octane molecules can hinder heat transfer. The increase of thermal conductivity with AOT loading after the onset of micellization indicates that the thermal transport in the core of AOT micelles and across the surfactant-oil interfaces, both of which span only a few nanometers, are efficient.