Controlling Factors in Gemini Micelle Morphology
A R T I C L E S
ing aqueous domain. The resulting increase in interfacial
counterion and reduction in interfacial water concentrations,
which are quantified by our chemical trapping method, permit
tighter packing into cylindrical morphologies. Spherical micelles,
on the other hand, are favored when the hydration free energies
of headgroups and counterions are stronger and the ions are
less polarized, such that fully hydrated ions are more abundant.
In summary, we demonstrate for the first time that aggregate
morphology and specific ion pairing by gemini amphiphiles are
intimately related.
lamellar, cubic, etc.) depend on the structure and hydrophobicity
of the amphiphilic tail(s), as is well-known, but also on specific
ion hydration, ion-pairing, and the release of water into the
aqueous domain. To our knowledge, the concept embedded in
our model, namely that morphological transitions occur because
interfacial ion-pair formation and release of interfacial water
are driven by the hydrophobic effect, has not been previously
considered and is one of the main subjects of this paper.
Gemini amphiphiles are receiving vast academic and indus-
trial attention.33,34 They often form viscoelastic solutions, a form
of “soft matter,”35 because they readily form rodlike, threadlike,
or wormlike micelles at very low concentrations and because
their solution viscosities are “tunable” by changing the head-
group spacer length.36 Potential applications, embodied in scores
of patents and publications, include thickeners, drag reducers,
oil well service fluids, heat and solid transfer agents, detergents,
compounds for controlling aerosol droplet size,37-39 and gene
transvection.40
Many other micellar properties also depend on headgroup
structure and counterion type such as the cmc, aggregation
number, and Krafft temperature,6 and catalysis of chemical
reactions.15,16 Properties of ion specific electrodes,17 bio-
membranes and proteins,18-21 and ion-exchange resins,22 and
polyelectrolytes and DNA23 also depend on the nature of the
surface charge group and counterion type and concentra-
tions. Most comparisons of specific ion effects focus on
counterion type, and counterion effectiveness generally follows
a Hofmeister series;24 i.e., the larger, more polarizable, less
strongly hydrated counterions have greater effects on a particular
property, the same factors that enhance ion pair formation.
Traditionally, ionic effects on colloidal structure are attributed
to Coulombic interactions, electrostatic repulsions between the
charged headgroups of the amphiphiles that are mediated by
water and screened by counterions in an electrical double
layer.25,26 The shortcomings of this perspective have been
identified repeatedly and center around specific ion effects that
have not been successfully incorporated.27-29 Molecular dynamic
simulations of micelles and bilayers show that a significant
fraction of the counterions in the interfacial region are in direct
contact with the headgroups without intervening water and that
the number of these contacts depends on counterion type.30-32
Thus, our interfacial specific ion-pairing/hydration model has
substantial literature precedence. Indeed, the varied structural
motifs of micelles and other association colloids (spheres, rods,
The three gemini surfactants used in these experiments differ
only in spacer length and have about the same charge-to-
hydrocarbon ratio or hydrophilic-lipophilic balance (HLB) as
their single-chained analogue, C12H25N(CH3)3+Br- (DTABr),
but the properties of geminis and DTAB differ dramatically.
The cmc values of the three geminis are about 1 mM,41,42 more
than an order of magnitude smaller than the cmc of 14 mM for
DTABr.43 DTABr forms rodlike micelles only when the solution
contains huge quantities of NaBr (1.8 M),43 but 12-2-12 forms
threadlike micelles in salt-free solutions at 4.2 mM.44 Curiously,
12-3-12 and 12-4-12 micelles remain spheroidal up to much
higher concentrations,34,45 yet reported values of the degree of
counterion binding to the micelles (R) are not too different for
all three surfactants: DTABr (0.20); 12-2-12Br (0.16); 12-3-
12Br (0.21); 12-4-12Br (0.16, 0.26).41,44 Camesano and
Nagarajan developed a thermodynamic model for the low cmc
values of gemini surfactants, including spacer length effects,46
but they did not consider the possibility ion-pairing and low
hydration in the interfacial region.
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