Molecular Dynamics Simulation Studies of Polyether and Perfluoropolyether Surfactant Based Reverse Micelles in Supercritical Carbon Dioxide

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

Chemical reactions in confined environments are important in areas as diverse as heterogenous catalysis, environmental chemistry and biochemistry, yet they are much less well understood than the equivalent reactions in either the gas phase or in free solution. The understanding of chemical reactions in solution was greatly enhanced by real time studies of model reactions, through ultrafast spectroscopy (especially when supported by molecular dynamics simulation). Here we review some of the efforts that have been made to adapt this approach to the investigation of reactions in confined media. Specifically, we review the application of ultrafast fluorescence spectroscopy to measure reaction dynamics in the nanoconfined water phase of reverse micelles, as a function of the droplet radius and the charge on the interface. Methods of measurement and modelling of the reactions are outlined. In all of the cases studied (which are focused on ultrafast intramolecular reactions) the effect of confinement was to suppress the reaction. Even in the largest micelles the result in the bulk aqueous phase was not usually recovered, suggesting an important role for specific interactions between reactant and environment, for example at the interface. There was no simple one-to-one correspondence with direct measures of the dynamics of the confined phase. Thus, understanding the effect of confinement on reaction rate appears to require not only knowledge of the dynamics of the reaction in solutions and the effect of confinement on the medium, but also of the interaction between reactant and confining medium.

Computer simulation study of fluorocarbon phosphate surfactant based aqueous reverse micelle in supercritical CO2: roles of surfactant functional groups, ionic strength, and phase changes in CO2

Structural and dynamic properties of an aqueous micelle organized from fluorocarbon phosphate surfactant molecules in supercritical carbon dioxide (CO₂) are investigated via molecular dynamics computer simulations. The roles of the functional groups and ionic strength of the surfactants on the formation of reverse micelles in supercritical CO₂, and related water dynamics characterized as translational and reorientational dynamics, are systematically demonstrated by employing three different phosphate-based surfactants paired with sodium cations. The strong electrostatic interactions between the phosphate head groups and sodium cations result in formation of an aqueous core inside the surfactant aggregates, where water molecules are bonded together with loss of the tetrahedral hydrogen bonded network found in bulk water. It is found that all the three surfactants with CO₂-philic fluorocarbon double tails build up well-stabilized reverse micelles in supercritical CO₂, avoiding direct contacts between CO₂ and water molecules. Despite this, the surfactant with a carboxylic ester linkage between the phosphate head and fluorocarbon tail group tends to coordinate water molecules toward sustaining the inter-water hydrogen bonds, indicating better efficiency at covering the aqueous core with hydrophobic groups compared to one without a carboxylic ester group. As for water molecules confined in the reverse micelle, their translational and reorientational motions, and fluctuating dynamics of the inter-water hydrogen bonds, significantly slow down compared to bulk water at ambient temperature. The water dynamics become more restricted with an increase in ionic strength of the anionic surfactant; this is attributed to divalent surfactant heads and sodium cations being more tightly bound together with bonding to water compared to monovalent ones. Lastly, the structural and dynamic changes of the reverse micelle caused by a phase change in CO₂ are monitored with gradually decreasing temperature and pressure from the supercritical to gaseous state for CO₂. The average reverse micelle structure equilibrated in supercritical CO₂ is found to remain stable over a time period of 0.2 ms through a depressurization process to gaseous CO₂. We note that the diverse pathways of surfactant self-aggregation in gaseous CO₂ could be controlled by the preceding solvation procedure in the supercritical regime which governs the final aggregated structures in gaseous CO₂.

Modeling Solvation in Supercritical CO2

In recent decades, a microscopic understanding of solute-solvent intermolecular interactions has been key to advances in technologies based on supercritical carbon dioxide. In many cases, computational work has provided the impetus for new discoveries, shedding new light on important concepts such as the local structure around the solute in the supercritical medium, the influence of the peculiar properties of the latter on the molecular behavior of dissolved substances and, importantly, CO₂ -philicity. In this Review, the theoretical work that has been relevant to these developments is surveyed and, by presenting some crucial open questions, the possible routes to achieving further progress based on the interplay between theory and experiments is discussed.

Perfluoropolyethers: Development of an All-Atom Force Field for Molecular Simulations and Validation with New Experimental Vapor Pressures and Liquid Densities

A force field for perfluoropolyethers (PFPEs) based on the general optimized potentials for liquid simulations all-atom (OPLS-AA) force field has been derived in conjunction with experiments and ab initio quantum mechanical calculations. Vapor pressures and densities of two liquid PFPEs, perfluorodiglyme (CF₃-O-(CF₂-CF₂-O)₂-CF₃) and perfluorotriglyme (CF₃-O-(CF₂-CF₂-O)₃-CF₃), have been measured experimentally to validate the force field and increase our understanding of the physical properties of PFPEs. Force field parameters build upon those for related molecules (e.g., ethers and perfluoroalkanes) in the OPLS-AA force field, with new parameters introduced for interactions specific to PFPEs. Molecular dynamics simulations using the new force field demonstrate excellent agreement with ab initio calculations at the RHF/6-31G* level for gas-phase torsional energies (<0.5 kcal mol^-1 error) and molecular structures for several PFPEs, and also accurately reproduce experimentally determined densities (<0.02 g cm^-3 error) and enthalpies of vaporization derived from experimental vapor pressures (<0.3 kcal mol^-1). Additional comparisons between experiment and simulation show that polyethers demonstrate a significant decrease in enthalpy of vaporization upon fluorination unlike related molecules (e.g., alkanes and alcohols). Simulation suggests this phenomenon is a result of reduced cohesion in liquid PFPEs due to a reduction in localized associations between backbone oxygen atoms and neighboring molecules.

Structural Plasticity of Cholesteryl Ester Transfer Protein Assists the Lipid Transfer Activity

Cholesteryl ester transfer protein (CETP) mediates the transfer of cholesteryl esters (CEs) and triglycerides between different lipoproteins. Recent studies have shown that blocking the function of CETP can increase the level of HDL cholesterol in blood plasma and suppress the risk of cardiovascular disease. Hence, understanding the structure, dynamics, and mechanism by which CETP transfers the neutral lipids has received tremendous attention in last decade. Although the recent crystal structure has provided direct evidence of the existence of strongly bound CEs in the CETP core, very little is known about the mechanism of CE/triglyceride transfer by CETP. In this study, we explore the large scale dynamics of CETP by means of multimicrosecond molecular dynamics simulations and normal mode analysis, which provided a wealth of detailed information about the lipid transfer mechanism of CETP. Results show that the bound CEs intraconvert between bent and linear conformations in the CETP core tunnel as a consequence of the high degree of conformational flexibility of the protein. During the conformational switching, there occurred a significant reduction in hydrophobic contacts between the CEs and CETP, and a continuous tunnel traversing across the CETP long axis appeared spontaneously. Thus, our results support the recently proposed "tunnel mechanism" of CETP from cryo-EM studies for the transfer of neutral lipids between different lipoproteins. The detailed understanding obtained here could help in devising methods to prevent CETP function as a cardiovascular disease therapeutic.

Molecular simulation study of water mobility in aerosol-OT reverse micelles

In this work, we present results from molecular dynamics simulations on the single-molecule relaxation of water within reverse micelles (RMs) of different sizes formed by the surfactant aerosol-OT (AOT, sodium bis(2-ethylhexyl)sulfosuccinate) in isooctane. Results are presented for RM water content w(0) = [H(2)O]/[AOT] in the range from 2.0 to 7.5. We show that translational diffusion of water within the RM can, to a good approximation, be decoupled from the translation of the RM through the isooctane solvent. Water translational mobility within the RM is restricted by the water pool dimensions, and thus, the water mean-squared displacements (MSDs) level off in time. Comparison with models of diffusion in confined geometries shows that a version of the Gaussian confinement model with a biexponential decay of correlations provides a good fit to the MSDs, while a model of free diffusion within a sphere agrees less well with simulation results. We find that the local diffusivity is considerably reduced in the interfacial region, especially as w(0) decreases. Molecular orientational relaxation is monitored by examining the behavior of OH and dipole vectors. For both vectors, orientational relaxation slows down close to the interface and as w(0) decreases. For the OH vector, reorientation is strongly affected by the presence of charged species at the RM interface and these effects are especially pronounced for water molecules hydrogen-bonded to surfactant sites that serve as hydrogen-bond acceptors. For the dipole vector, orientational relaxation near the interface slows down more than that for the OH vector due mainly to the influence of ion-dipole interactions with the sodium counterions. We investigate water OH and dipole reorientation mechanisms by studying the w(0) and interfacial shell dependence of orientational time correlations for different Legendre polynomial orders.

Molecular dynamics simulations of cytochrome c unfolding in AOT reverse micelles: The first steps

This paper explores the reduced form of horse cytochrome c confined in reverse micelles (RM) of sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) in isooctane by molecular dynamics simulation. RMs of two sizes were constructed at a water content of W (o) = [ H₂O ]/[AOT] = 5.5 and 9.1. Our results show that the protein secondary structure and the heme conformation both depend on micellar hydration. At low hydration, the protein structure and the heme moiety remain stable, whereas at high water content the protein becomes unstable and starts to unfold. At W (o) = 9.1 , according to the X-ray structure, conformational changes are mainly localized on protein loops and around the heme moiety, where we observe a partial opening of the heme crevice. These findings suggest that within our time window (10ns), the structural changes observed at the heme level are the first steps of the protein denaturation process, previously described experimentally in micellar solutions. In addition, a specific binding of AOT molecules to a few lysine residues of the protein was found only in the small-sized RM.

Effect of compressed CO2 on the properties of lecithin reverse micelles

Lecithin is a very useful biosurfactant. In this work, the effects of compressed CO 2 on the critical micelle concentration (cmc) of lecithin in cyclohexane and solubilization of water, lysozyme, and PdCl 2 in the lecithin reverse micelles were studied. The micropolarity and pH value of the polar cores of the reverse micelles with and without CO 2 were also investigated. It was found that CO 2 could reduce the cmc of the micellar solution and enhance the capacity of the reverse micelles to solubilize water, the biomolecule, and the inorganic salt significantly. Moreover, the water pools could not be formed in the reverse micelles in the absence of CO 2 because of the limited amount of water solubilized. However, the water pools could be formed in the presence of CO 2 because large amounts of water could be solubilized. All of these provide more opportunity for effective utilization of this green surfactant. The possible mechanism for tuning the properties of the reverse micelles by CO 2 is discussed.

Model of drug-loaded fluorocarbon-based micelles studied by electron-spin induced (19)f relaxation NMR and molecular dynamics simulation

Rf-IPDU-PEGs belong to a class of fluoroalkyl-ended poly(ethylene glycol) polymers (Rf-PEGs), where the IPDU (isophorone diurethane) functions as a linker to connect each end of the PEG chain to a fluoroalkyl group. The Rf-IPDU-PEGs form hydrogels in water with favorable sol-gel coexistence properties. Thus, they are promising for use as drug delivery agents. In this study, we introduce an electron-spin induced 19F relaxation NMR technique to probe the location and drug-loading capacity for an electron-spin labeled hydrophobic drug, CT (chlorambucil-tempol adduct), enclosed in the Rf-IPDU-PEG micelle. With the assistance of molecular dynamics simulations, a clear idea regarding the structures of the Rf-IPDU-PEG micelle and its CT-loaded micelle was revealed. The significance of this research lies in the finding that the hydrophobic drug molecules were loaded within the intermediate IPDU shells of the Rf-IPDU-PEG micelles. The molecular structures of IPDU and that of CT are favorably comparable. Consequently, it appears that this study opens a window to modify the linker between the Rf group and the PEG chain for achieving customized structure-based drug-loading capabilities for these hydrogels, while the advantage of the strong affinity among the Rf groups to hold individual micelles together and to interconnect the micellar network is still retained in hopes of maintaining the sol-gel coexistence of the Rf-PEGs.

Self-assembled reverse micelles in supercritical CO2 entrap protein in native state

Molecular dynamics simulations of random quaternary mixtures of protein-water-CO2-fluorosurfactants show the self-assembly of reverse micelles in supercritical carbon dioxide where the protein becomes entrapped inside the aqueous pool. Analyses show that the protein native state remains intact in the water pool. This is because of the bulk nature of the enclosed water that provides a suitable environment for the extracted protein. Results from ab initio calculations imply that the existing fluorosurfactants can be made more effective in stabilizing water-in-CO2 microemulsions by a partial hydrogenation in their tails. A Lewis acid-Lewis base interaction among CO2 and the surfactant tails enhances the stability of the aqueous droplets substantially. The study can help accelerate the search for surfactant process for environmentally benign applications in dense CO2.

Adsorption and self-assembly of surfactant/supercritical CO2 systems in confined pores: a molecular dynamics simulation

A coarse-grained molecular dynamics simulation has been carried out to study the adsorption and self-organization for a model surfactant/supercritical CO2 system confined in the slit-shape nanopores with amorphous silica-like surfaces. The solid surfaces were designed to be CO2-philic and CO2-phobic, respectively. For the CO2-philic surface, obviously surface adsorption is observed for the surfactant molecules. The various energy profiles were used to monitor the lengthy dynamics process of the adsorption and self-assembly for surfactant micelles or monomers in the confined spaces. The equilibrium properties, including the morphologies and micelle-size distributions of absorbed surfactants, were evaluated based on the equilibrium trajectory data. The interaction between the surfactant and the surface produces an obvious effect on the dynamics rate of surfactant adsorption and aggregation, as well as the final self-assembly equilibrium structures of the adsorbed surfactants. However, for the CO2-phobic surfaces, there are scarcely adsorption layers of surfactant molecules, meaning that the CO2-phobic surface repels the surfactant molecules. It seems to conclude that the CO2 solvent depletion near the interfaces determines the surface repellence to the surfactant molecules. The effect of the CO2-phobic surface confinement on the surfactant micelle structure in the supercritical CO2 has also been discussed. In summary, this study on the microscopic behaviors of surfactant/Sc-CO2 in confined pores will help to shed light on the surfactant self-assembly from the Sc-CO2 fluid phase onto solid surfaces and nanoporous media.

How strongly can calcium ion influence the hydrogen-bond dynamics at complex aqueous interfaces?

The author has performed three independent molecular dynamics computer simulations to examine the effects of counterion identity on hydrogen-bond dynamics in the enclosed water pool of anionic surfactant-based reverse micelles. The water-water hydrogen-bond lifetime in the reverse micelle (RM) with calcium ions is found to be longer than that in the RM with sodium or ammonium ions. The hydrogen bond between a polar head group and a water molecule, on the other hand, breaks but reforms most rapidly in the RM with calcium ions, indicating that there exists a strong competition between head group-counterion and head group-water interactions at such complex interfaces.

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.

Effect of surfactant conformation on the structures of small size nonionic reverse micelles: a molecular dynamics simulation study

We used constant pressure (P=0.1 MPa) and temperature (T=298 K) molecular dynamics simulations to study the structures and dynamics of small size reverse micelles (RMs) with poly(ethylene glycol) alkyl ether (CmEn) surfactants. The water-to-surfactant molar ratio was 3, with decane as the apolar solvent. We focused on the effect of the two possible imposed conformations (trans vs gauche) for the surfactant headgroups on RMs structures and water dynamics. For this purpose, we built up two RMs, which only differ by their surfactant headgroup conformations. The results obtained for the two RMs were compared to what is known in the literature. Here, we show that the surfactant headgroup conformation affects mainly the water-related properties such as the water core size, the area per surfactant headgroup, the headgroup hydration, and the water core translational diffusion. The properties computed for the RM with the surfactant in trans conformation fit better with the experimental data than the gauche conformation. We further show that the surfactant hydrophilic headgroup plays a crucial role in the micellar structures, favors the entrapment of the micellar water, and reduces strongly their diffusion compared to the bulk water.

In-situ Synthesis of a Tacrine-Triazole-Based Inhibitor of Acetylcholinesterase: Configurational Selection Imposed by Steric Interactions

Recently, researchers have used acetylcholinesterase (AChE) as a reaction vessel to synthesize its own inhibitors. Thus, 1 (syn-TZ2PA6), a femtomolar AChE inhibitor, which is formed in a 1:1 mixture with its anti-isomer by solution phase reaction from 3 (TZ2) and 4 (PA6), can be synthesized exclusively inside the AChE gorge. Our computational approach based on quantum mechanical/molecular mechanical (QM/MM) calculations, molecular dynamics (MD), and targeted molecular dynamics (TMD) studies answers why 1 is the sole product in the AChE environment. Ab initio QM/MM results show that the reaction in the AChE gorge occurs when 3/azide and 4/acetylene are extended in a parallel orientation. An MD simulation started from the final structure of QM/MM calculations keeps the azide's and acetylene's parallel orientations intact for 10 ns of simulation time. A TMD simulation applied on an antiparallel azide-acetylene conformation flips the acetylene easily to bring it to a position that is parallel to azide. A second set of QM/MM calculations performed on this flipped structure generates a similar minimum-energy path as obtained previously. Even a TMD simulation carried out on a parallel azide-acetylene conformation could not deform their parallel arrangement. All of these results, thus, imply that inside the AChE gorge, the azide group of 3 and the acetylene group of 4 always remain parallel, with the consequence that 1 is the only product. The architecture of the gorge plays an important role in this selective formation of 1.

The Effect of the Rigidity of Perfluoropolyether Surfactant on Its Behavior at the Water/Supercritical Carbon Dioxide Interface

We performed a series of molecular dynamics simulations to study the PFPE (perfluoropolyether) and PE (polyether) surfactant monolayers at the water/supercritical carbon dioxide interface. Molecular differences between fluorocarbon surfactant PFPE and its hydrocarbon analogue PE were analyzed. We observed that values of intramolecular bonded interaction parameters which are related to chain rigidity determine the monolayer surface pressure. We show that "good" and "bad" properties of PFPE/PE surfactants are connected to conformational entropy. These results are consistent with our previous micellar simulations.

From homogeneous dispersion to micelles-a molecular dynamics simulation on the compromise of the hydrophilic and hydrophobic effects of sodium dodecyl sulfate in aqueous solution

The structural and functional diversity of surfactant systems has attracted simulation works in atomistic, coarse grain, and mesoscopic models (Bandyopadhyay, S.; et al. Langmuir 2000, 16, 942; Senapati, S.; et al. J. Phys. Chem. B 2003, 107, 12906; Maiti, P. K.; et al. Langmuir 2002, 18, 1908; Srinivas, G.; et al. J. Phys. Chem. B 2004, 108, 8153; Groot, R. D.; et al. J. Chem. Phys. 1999, 110, 9739; Rekvig, L.; et al. Langmuir 2003, 19, 8195). However, atomistic models have suffered from their tremendous computational cost and are, so far, not able to simulate the structural behaviors in sufficient spatio-temporal scales (Shelley, J. C.; Shelley, M. Y. Curr. Opin. Colloid Interface Sci. 2000, 5, 101). The other two approaches are not microscopic enough to describe the configurations of the surfactants that determine their behaviors (Shelley and Shelley). In this study, we propose to simplify atomistic models based on the observation that the compromise of the hydrophilic and hydrophobic effects (Li, J.; Kwauk, M. Chem. Eng. Sci. 2003, 58, 521-535) and molecular structures of surfactants are the dominant factors shaping their structures in the systems. With this simplification, we are able to simulate with moderate computing cost the whole process of micelle formation from an initially uniform dispersion of sodium dodecyl sulfate (SDS) in aqueous solution. The resulting micelle structures are different from those predicted by atomistic simulations that started with a predefined micelle configuration at the same surfactant concentrations. However, if we use their initial micelle configuration, micelle structures the same as theirs are obtained. Analyses show that our results are more realistic and that the results of the atomistic simulations suffer from artificial initial conditions. Therefore, our model may serve as a reasonable simplification of atomistic models in terms of the general structure of micelles.

Solvation Dynamics in Reverse Micelles: The Role of Headgroup−Solute Interactions

We present molecular dynamics simulation results for solvation dynamics in the water pool of anionic-surfactant reverse micelles (RMs) of varying water content, w(0). The model RMs are designed to represent water/aerosol-OT/oil systems, where aerosol-OT is the common name for sodium bis(2-ethylhexyl)sulfosuccinate. To determine the effects of chromophore-headgroup interactions on solvation dynamics, we compare the results for charge localization in model ionic diatomic chromophores that differ only in charge sign. Electronic excitation in both cases is modeled as charge localization on one of the solute sites. We find dramatic differences in the solvation responses for anionic and cationic chromophores. Solvation dynamics for the cationic chromophore are considerably slower and more strongly w(0)-dependent than those for the anionic chromophore. Further analysis indicates that the difference in the responses can be ascribed in part to the different initial locations of the two chromophores relative to the surfactant interface. In addition, slow motion of the cationic chromophore relative to the interface is the main contributor to the longer-time decay of the solvation response to charge localization in this case.

Molecular Dynamics Simulation of a Reverse Micelle Self Assembly in Supercritical CO2

In this communication we report on molecular dynamics computer simulations of self-assembly of reverse micelles in supercritical carbon dioxide. The reverse micelles contain perfluoropolyether ammonium carboxylate surfactants and an aqueous core. We observed a quick self-assembly of these micelles over time periods of approximately 5 ns, irrespective of initial conditions. In most cases, the self-assembled perfluorinated reverse micelles have a nice spherical shape and properties consistent with experiments. When the fluorinated surfactant is replaced by its hydrogenated analogue, the assembled aggregate contains a region of direct contact between water and carbon dioxide, indicating that hydrogenated surfactant is not a good agent for creation of microemulsions in water/carbon dioxide mixtures.