title: Effects of small ionic amphiphilic additives on reverse microemulsion morphology.
abstract: OBJECTIVE
  Initial studies (Hopkins Hatzopoulos et al. (2013)) have shown that ionic hydrotropic additives can drive a sphere-to-cylinder (ellipsoid) transition in water-in-oil (w/o) microemulsions stabilized by the anionic surfactant Aerosol-OT; however the origins of this behaviour remained unclear. Here systematic effects of chemical structure are explored with a new set of hydrotropes, in terms of an aromatic versus a saturated cyclic hydrophobic group, and linear chain length of alkyl carboxylates. It is proposed that hydrotrope-induced microemulsion sphere-to-cylinder (ellipsoid) transitions are linked to additive hydrophobicity, and so a correlation between the bulk aqueous phase critical aggregation concentration (cac) and perturbation of microemulsion structure is expected.
  
  
  METHODS
  Water-in-oil microemulsions were formulated as a function of water content w (= [water]/[AOT]) and concentration of different hydrotropes, being either cyclic (sodium benzoate or sodium cyclohexanoate), or linear chain systems (sodium hexanoate, sodium heptanoate and sodium octanoate). Phase behaviour studies were performed as a function of w, additive type and temperature at total surfactant concentration [ST]=0.10M and constant mole fraction x=0.10 (x=[hydrotrope]/[ST]). 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.
  
  
  RESULTS
  Under 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.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
authors.forename: Marios Hopkins
authors.initials: MH
authors.lastname: Hatzopoulos
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
authors.forename: Craig
authors.initials: C
authors.lastname: James
authors.affiliation: ISIS-STFC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, UK.
authors.forename: Sarah
authors.initials: S
authors.lastname: Rogers
authors.affiliation: ILL, BP 156-X, F-38042 Grenoble Cedex, France.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Grillo
authors.affiliation: Infineum UK Ltd., Milton Hill Business & Technology Centre, Abingdon, Oxfordshire OX13 6BB, UK.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Dowding
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK. Electronic address: julian.eastoe@bristol.ac.uk.
authors.forename: Julian
authors.initials: J
authors.lastname: Eastoe
mesh_terms: D004655:Emulsions; D007477:Ions; D053838:Scattering, Small Angle
chemical_list: D004655:Emulsions; D007477:Ions
keywords: Hydrotropes; Microemulsions; Nanostructures; Phase behaviour; Small-angle neutron scattering