Interfacial vs Bulk Phenomena Effects on the Surface Tensions of Aqueous Magnetic Surfactants in Uniform Magnetic Fields

The literature clearly reports that magnetic surfactant systems respond to magnetic fields. This manuscript investigates if the responses are because the magnetic fields directly alter the interfacial properties or if the surface-active properties are independent of the paramagnetic fluid responses. It uses uniform and gradient magnetic fields to determine the magnetically induced changes to the surface tensions independent of bulk paramagnetic fluid effects for ionic magnetic surfactants. The magnetically induced decrease in surface tensions is small compared to the bulk paramagnetic fluid effects. The reported decrease in surface tensions is significantly smaller than those previously found in the literature, which reported a combined interfacial and bulk paramagnetic effect. The magnetically induced surface tension changes are a function of the degree of association, α, of the magnetic moiety with the surfactant's amphiphilic structure. Therefore, the proposed answer to the question is that as α approaches zero, the magnetic properties of the magnetic surfactant system approaches the behavior of an ordinary paramagnetic fluid. For magnetic surfactants with α approaching one, there is a measurable interfacial response. For example in this study, a magnetic surfactant with α = 0.92 had a 2.5 times greater magnetically induced change in surface tension compared to a magnetic surfactant with α = undetectable, even thought they had similar magnetic moments.