TY - JOUR
T1 - Effects of small ionic amphiphilic additives on reverse microemulsion morphology
AU - Hopkins Hatzopoulos, Marios T
AU - James, Craig
AU - Rogers, Sarah E
AU - Grillo, Isabelle
AU - Dowding, Peter J
AU - Eastoe, Julian
PY - 2014/5/1
Y1 - 2014/5/1
N2 - HypothesisInitial 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.ExperimentsWater-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] = 0.10 M 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.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.
AB - HypothesisInitial 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.ExperimentsWater-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] = 0.10 M 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.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.
KW - Hydrotropes
KW - Microemulsions
KW - Nanostructures
KW - Phase behaviour
KW - Small-angle neutron scattering
UR - http://www.scopus.com/inward/record.url?scp=84893980073&partnerID=8YFLogxK
U2 - 10.1016/j.jcis.2014.01.024
DO - 10.1016/j.jcis.2014.01.024
M3 - Article (Academic Journal)
C2 - 24594032
SN - 0021-9797
VL - 421
SP - 56
EP - 63
JO - Journal of Colloid and Interface Science
JF - Journal of Colloid and Interface Science
ER -