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Micelles inverses d'AOT et de C12E4: Structure et évaluation de leurs compressibilités par simulation de
dynamique moléculaire

Abstract : In this Ph..D. dissertation, we have used constant pressure and temperature molecular dynamics simulations to study the structural and volumetric (Voronoi volume and isothermal compressibility) changes of the surfactant AOT and C12E4 reverse micelles (RM) as a function of hydration. The two systems were simulated in the apolar solvents isooctane and decane respectively. The concentrations of the two ternary systems were selected to fall within a L2 phase ranges. A key parameter of RM is the water-to-surfactant molar ratio Wo which determines their water content as well as the micellar size. In this work we focus on the simulated RM structural and volumetric changes in the Wo range 2 ≤ Wo ≤7
For the AOT/isooctane/water system, we obtain nonspherical aggregates of elliptical shape, with axial ratios between 1.24 and 1.41, the major axis being a, and the minor one, c. The hydrophilic inner core is also ellipsoidal with larger a/c ratios. Although experimental data indicate that the L2 phase of the AOT water/oil system is polydisperse, we have simulated only monodisperse RMs in our study. Nonetheless, we are able to reproduce the dimensions of the water-pool and its dependence on Wo as compared with scattering experiments carried out with SAXS or SANS. We have also investigated the static and dynamic properties of the RM's water inner core. Moving from smaller micelles to larger ones, we observe that the properties of confined water tend to near those of bulk water. In particular, we find that the solvation of the surfactant headgroups (hg) and counterions is more effective in larger micelles and that the diffusion of water is more retarded with respect to the bulk, in smaller RMs. For the volumetric properties, our calculations show that isothermal compressibilities of RMs increase linearly with Wo, in agreement with the experimental literature. In the case of the confined water volume, our calculations suggest small changes of the latter parameter (4% in difference) compared to bulk water. In contrast with literature, we found it remained constant with Wo. The compressibility of confined water varies between 24 and 60.10-5 MPa-1 according to the value of Wo.
To study the influence of the AOT headgroup hydration, we have confined an Α-helical alanine octapeptide in two RM with Wo=4.8 and 6.8. In the smallest RM, the preferential hydration of the AOT headgroup favors the stability of the Α-helix in contrast to the largest one, where the periodical conformation of the peptide was lost. Our results show that the major force affecting the stability of octaalanine in small-size RM of AOT is hydration, as previously observed with much larger peptides. Furthermore the volumetric properties of the “filled” micelles do not significantly change with the incorporation of the peptides into the water core.
For the surfactant C12E4 water system, we focused on the two possible imposed conformations (trans vs cis) of the surfactant headgroup, on the effect of RMs structures and water dynamics. Our results 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. These parameters are similar to experimental data when the surfactant headgroups are in the trans position much more than in the cis one. Finally we found that the volumetric properties of the C12E4 RM, are not significantly modified according to the headgroup conformation. The compressibility value is in the 54 and 57.10-5 MPa-1 range. The volume of the confined water molecules is not significantly affected by their state (bound or free) and are close to the volume of pure water. However, our calculations indicate that their isothermal compressibility presents differences according to their state. The compressibility of free water was found twice as small (25.10-5 MPa-1) as that of pure water and it increases with the fraction of micellar bound water.
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Submitted on : Sunday, September 23, 2007 - 5:54:57 PM
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Stéphane Abel. Micelles inverses d'AOT et de C12E4: Structure et évaluation de leurs compressibilités par simulation de
dynamique moléculaire. Autre. Université Pierre et Marie Curie - Paris VI, 2007. Français. ⟨tel-00174307⟩

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