General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms Effects of small ionic amphiphilic additives on reverse microemulsion morphology Research Highlights Cylindrical morphologies of reverse microemulsions induced by small molecule hydrotropes.  Ellipsoidal droplets of high aspect ratio in reverse AOT microemulsions are induced by short chain alkanoate additives.  A cylinder to sphere transition in reverse AOT microemulsions is seen with increasing water content. . Microemulsion domain structures were investigated by small-angle neutron scattering (SANS), and these data were fitted by structural models to yield information on the shapes (spheres, ellipsoids or cylinders) and sizes of the nanodroplets. Effects of small ionic amphiphilic additives on reverse microemulsion morphology FindingsUnder the conditions of study hydrotrope chemical structure has a significant effect on microemulsion structure: sodium cyclohexanoate does not induce the formation of cylindrical/ellipsoidal nanodroplets, whereas the aromatic analogue sodium benzoate does. Furthermore, the short chain sodium hexanoate does not cause anisotropic microemulsions, but the more hydrophobic longer chain heptanoate and octanoate analogues do induce sphere-to-ellipsoid transitions. This study shows that underlying microemulsion structures can be tuned by hydrotropes, and that the strength of the effect can be identified with hydrotrope hydrophobicity in terms of the bulk aqueous phase cac.