Effect of Hofmeister and
Alkylcarboxylate Anionic
Counterions on the Krafft Temperature and Melting Temperature of Cationic
Gemini Surfactants
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Abstract
The effect of counterions was investigated to probe the
principal
ionic effects on the solubility in water and melting behavior of cationic
gemini surfactants. We focused on two types of counterions: (1) small
inorganic counterions that are typically taken from the Hofmeister
series were studied to focus on the effect of ion type and (2) <i>n</i>-alkylcarboxylate counterions were studied to focus on
the effect of the hydrophobicity of counterions. The Krafft temperature
(<i>T</i><sub>k</sub>) and melting temperature (<i>T</i><sub>m</sub>) were obtained by conductivity measurements,
calorimetric measurements, and optical microscopy observation. The
results clearly indicate that <i>T</i><sub>k</sub>, which
represents the solubility of surfactants, is not determined by a single
parameter of ions such as the hydration free energy, as is too often
assumed, but rather by the combined effects between the hydrophobicity
of anions associated with other effects such as the polarizability,
dehydrated ion size, and ionic morphology. In parallel, our observation
demonstrated that all of the surfactants showed a transition from
a crystalline phase to a thermotropic liquid-crystalline phase at
around ca<i>.</i> 70 °C, which transformed to an isotropic
liquid phase at around ca<i>.</i> 150 °C, and that
the transition temperatures depended strongly on the counterion type.
The counterion effects on the solubilization and melting behaviors
were then compared with micellization properties that have been reported
previously. These results provide new insight into understanding the
effect of ions on the delicate balance of forces controlling the solution
properties and aggregate morphology of charged amphiphilic molecules.
Specifically, the solubilization properties of these cationic surfactants
with various counterions were determined mainly by the subtle interplay
between the hydration of counterions and the dissociation energies
(stability of crystallinity) of the ion pair