Direct Measurements of Effect of Counterion Concentration
on Mechanical Properties of
Cationic Vesicles
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Abstract
Theoretical analyses of charged membranes
in aqueous solutions
have long predicted that the electric double layer surrounding them
contributes significantly to their mechanical properties. Here we
report the first, direct experimental measurements of the effect of
counterion concentration on the bending and area expansion modulus
of cationic surfactant vesicles. Using the classical technique of
micropipet aspiration coupled with a modified experimental protocol
that is better suited for cationic vesicles, we successfully measure
the mechanical properties of a double-tailed cationic surfactant,
diethylesterdimethyl ammonium chloride (diC18:1 DEEDMAC) in CaCl<sub>2</sub> solutions. It is observed that the area expansion modulus
of the charged membrane exhibits no measurable dependence on the counterion
concentration, in accordance with existing models of bilayer elasticity.
The measured bending modulus, however, is found to vary nonmonotonically
and exhibits a minimum in its variation with counterion concentration.
The experimental results are interpreted based on theoretical calculations
of charged and bare membrane mechanics. It is determined that the
initial decrease in bending modulus with increasing counterion concentration
may be attributed to a decreasing double layer thickness, while the
subsequent increase is likely due to an increasing membrane thickness.
These mechanical moduli measurements qualitatively confirm, for the
first time, theoretical predictions of a nonmonotonic behavior and
the opposing effects of ionic strength on the bending rigidity of
charged bilayers
