Magnetic control over liquid surface properties with responsive surfactants.

Surfactants responsive to pH, temperature, CO2, [3] and light are well known. Here we report for the first time ionic liquid surfactants that are magneto-responsive, thus offering the potential to perturb liquid emulsions simply by the application of an external magnetic field. Although ionic liquids (ILs) containing transition metal complexes have been known for some time, it had always been assumed that the metallic centers were isolated, lacking long-range interactions and communication necessary to be magnetically active. Only recently have ionic liquids containing magneto-active metal complex anions, such as 1-methyl-3-butylimidazolium tetrachloroferrate ([bmim]FeCl4), [7] been reported. These magnetic ionic liquids (mag-ILs) are especially interesting as they are molecular liquids, rather than typical magnetic fluids (ferrofluids) which comprise magnetic colloidal particles ( 10 nm) dispersed in a carrier fluid. The nanoparticle-free mag-ILs are themselves paramagnetic. As such they contain high effective concentrations of metal centers and allow physico-chemical properties (hydrophobicity, electrical conductivity, melting point, etc.) to be controlled by external magnetic fields. Furthermore, because mag-ILs and magnetic ionic liquid surfactants (MILSs) are non-volatile they offer advantages over conventional ferrofluids which often employ flammable organic solvents. Previous work has shown ionic liquids exhibiting magnetic responses. Further examples are introduced here (Figure 1, left), but more significantly these new materials are surface active, which is a fundamental property of colloidal systems. Now, magneto-responsive emulsions become accessible, which to date have only been realized with Pickering emulsions stabilized bymagnetic nanoparticles but not for molecular liquids. Synthesis of the MILSs is readily achieved by mixing an iron trihalide with the appropriate cationic surfactant (see Supporting Information for details). Electrical conductivity measurements of dilute aqueous solutions show that the critical micelle concentrations (cmcs, Table 1) are not greatly affected by the changes in anion alone. At first sight this is surprising as the larger anions (FeCl4 , FeCl3Br ) should be less effective at screening cation–cation headgroup repulsions, thus increasing the cmc (surfactants become more hydrophilic). However, the FeCl4 and FeCl3Br anions may interact with the hydrophobic moieties, and it is seen that the degree of dissociation, b, increases when exchanging halide for the tetrahalogenferrate(III) anion. The MILSs investigated here show no saturation magnetization, but do exhibit paramagnetic behavior (see Supporting Information) and the values for magnetic susceptibility, c, are similar to those reported in the literature (Table 1). Effective magnetic moments, meff, have also been estimated and are similar to literature values, lying close to the values expected for high-spin d Fe ions (spin-only value: 5.92 mB). [15] The susceptibility c is significantly higher for MILS1 than for MILS2 or MILS3, perhaps as the increased size and polarizability of the added bromide distorts the perfect tetrahedral geometry resulting in a smaller contact angle of the spins in the complex. It may also be explained in terms of the lower ligand field strength of Br with respect Figure 1. Left: Inert (SURFs) and magnetic surfactants (MILSs) studied. Right: Response of liquid droplets to the field from a 0.4 T NdFeB magnet. [SURF1]=20 wt%, [MILS1]=20 wt%.