Molecular dynamics simulations of AOT-water/formamide reverse micelles: Structural and dynamical properties

Abstract

A molecular dynamics study of free energy of micelle formation for sodium dodecyl sulfate in water and its size distribution J. Chem. Phys. 124, 184901 (2006) We present results from molecular dynamics simulations performed on reverse micelles immersed in cyclohexane. Three different inner polar phases are considered: water ͑W͒, formamide ͑FM͒, and an equimolar mixture of the two solvents. In all cases, the surfactant was sodium bis͑2-ethylhexyl͒ sulfosuccinate ͑usually known as AOT͒. The initial radii of the micelles were R ϳ 15 Å, while the corresponding polar solvent-to-surfactant molar ratios were intermediate between w 0 = 4.3 for FM and w 0 = 7 for W. The resulting overall shapes of the micelles resemble distorted ellipsoids, with average eccentricities of the order of ϳ0.75. Moreover, the pattern of the surfactant layer separating the inner pool from the non-polar phase looks highly irregular, with a roughness characterized by length scales comparable to the micelle radii. Solvent dipole orientation polarization along radial directions exhibit steady growths as one moves from central positions toward head group locations. Local density correlations within the micelles indicate preferential solvation of sodium ionic species by water, in contrast to the behavior found in bulk equimolar mixtures. Still, a sizable fraction of ϳ90% of Na + remains associated with the head groups. Compared to bulk results, the translational and rotational modes of the confined solvents exhibit important retardations, most notably those operated in rotational motions where the characteristic time scales may be up to 50 times larger. Modifications of the intramolecular connectivity expressed in terms of the average number of hydrogen bonds and their lifetimes are also discussed.

The Size of AOT Reverse Micelles

“…18,22 The principal moments of inertia ( I 1 , I 2 , and I 3 ) were calculated by determining the eigenvalues of the inertia tensor. For ellipsoids, the relations between semi-axes and principal moments of inertia are given by equations 2 – 4.…”

“…The uncertainty about RM size is especially problematic when performing molecular dynamics (MD) simulations for insight into the structure and organization of RMs because the number of molecular components in most simulations is fixed. 14 The question of RM size remains unresolved despite numerous MD simulation studies with continuum solvent, 15–17 coarse-grained surfactant, 18–22 and all-atom models. 13,23 Indeed, RM simulations of W 0 = 7.5 have been recently described with as few as 70 AOT molecules per RM, 24 ( n AOT ) and as many as 106.…”

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

“…In particular, for W 0 =5 and AOT in isooctane, the experimental gyration and hydrodynamic radii of the reverse micelle ( and , respectively) and the estimates of its are quite disparate in literature. For instance, the of the water core is found 11.7 Å by Amararene et al 9 explained by 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 the experimental uncertainty on the water content of the micelles 17 and by the formation of oligomers illustrated in a series of papers by Hirai et al 6,7,13 . In the last few years, following from earlier molecular dynamics (MD) simulations of the inner polar core of reverse micelles 18,19 , more realistic molecular modeling of AOT reverse micelles have appeared in the literature [20][21][22][23][24] . All these latter investigations simulated a single reverse micelle in isooctane for given water contents.…”

Phosphatidylcholine reverse micelles on the wrong track in molecular dynamics simulations of phospholipids in an organic solvent

“…Although gaining popularity, so far the computational investigation of RM systems has been restricted to only a handful of systems. In particular, ternary systems composed of water, supercritical carbon dioxide, and acid functionalized perfluoroalkane or perfluoropolyether surfactants, [15][16][17][18][19][20] as well as, systems composed of water, apolar solvent (oil) and Aerosol OT (AOT) [21][22][23][24][25][26][27][28][29][30][31] have been characterized. Additionally, alkyl-PEG 32 and malonamide derived surfactant 33 systems have been studied by computer simulations.…”