The effect of micelle (capsule) size with the presence of nanoparticles was investigated and it has been tried to produce magnetic surfactant by replacing magnetic metals Co, Ni and Mn with Na in NaAOT surfactant. The structure and potential energy of magnetic surfactant-Capsules containing the Fe3O4 nanoparticles (MCap-NPs) were investigated by dynamic light scattering (DLS), optical microscopy and zeta potential. In the studied molar ratio range (ω = 14–59), NiCap-NPs and CoCap-NPs samples were stable, while MnCap-NPs samples were unstable. The Fe3O4 nanoparticles density of stable samples was obtained CNiCap = 86 mM and CCoCap = 110 mM at ω = 22 and ω = 26, respectively. The Fe3O4 nanoparticles density was described by the Gouy-Chapman theory according to the electrical charge interactions in the core and shell double-layer. In the MCap-NPs (M = Co, Ni) structure: the Fe3O4 nanoparticles density is related to the distance and the surface electric charge of the doublelayer. The results of Zeta potential also confrm the stability of MCap-NPs (M = Co, Ni) structure. Considering the direct relationship between electric charge and potential energy, surface dielectric constant values were obtained using the Holovko model. The Holovko model showed that the surface dielectric constant proportional to the size of MCap-NPs (M = Co,Ni). The Gouy-Chapman theory and Holovko model describe the phase change of NiCapNPs structure at ω = 26.

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