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

    Year: 2020

    Journal: Langmuir, Volume 36, SEP 1, page 10074–10081

    Authors: Reed, Derek M.; Fortenberry, Alex; Wolfe, Emily A.; Stanhope, Rachel A.; Daniel, Carla, I; Hem, Caleb M.; Smith, Adam E.; Scovazzo, Paul

    Organizations: National Science FoundationNational Science Foundation (NSF) [CBET 1605894]; Sally McDonnell Barksdale Honors College at the University of Mississippi

    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, alpha, of the magnetic moiety with the surfactant's amphiphilic structure. Therefore, the proposed answer to the question is that as alpha approaches zero, the magnetic properties of the magnetic surfactant system approaches the behavior of an ordinary paramagnetic fluid. For magnetic surfactants with alpha approaching one, there is a measurable interfacial response. For example in this study, alpha magnetic surfactant with alpha = 0.92 had a 2.5 times greater magnetically induced change in surface tension compared to a magnetic surfactant with alpha = undetectable, even thought they had similar magnetic moments.
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