4 research outputs found
Direct Measurements of Effect of Counterion Concentration on Mechanical Properties of Cationic Vesicles
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
Origins of Microstructural Transformations in Charged Vesicle Suspensions: The Crowding Hypothesis
It is observed that
charged unilamellar vesicles in a suspension
can spontaneously deflate and subsequently transition to form bilamellar
vesicles, even in the absence of externally applied triggers such
as salt or temperature gradients. We provide strong evidence that
the driving force for this deflation-induced transition is the repulsive
electrostatic pressure between charged vesicles in concentrated suspensions,
above a critical effective volume fraction. We use volume fraction
measurements and cryogenic transmission electron microscopy imaging
to quantitatively follow both the macroscopic and microstructural
time-evolution of cationic diC18:1 DEEDMAC vesicle suspensions at
different surfactant and salt concentrations. A simple model is developed
to estimate the extent of deflation of unilamellar vesicles caused
by electrostatic interactions with neighboring vesicles. It is determined
that when the effective volume fraction of the suspension exceeds
a critical value, charged vesicles in a suspension can experience
âcrowdingâ due to overlap of their electrical double
layers, which can result in deflation and subsequent microstructural
transformations to reduce the effective volume fraction of the suspension.
Ordinarily in polydisperse colloidal suspensions, particles interacting
via a repulsive potential transform into a glassy state above a critical
volume fraction. The behavior of charged vesicle suspensions reported
in this paper thus represents a new mechanism for the relaxation of
repulsive interactions in crowded situations
Adsorption Energies of Poly(ethylene oxide)-Based Surfactants and Nanoparticles on an AirâWater Surface
The self-assembly of polymer-based
surfactants and nanoparticles
on fluidâfluid interfaces is central to many applications,
including dispersion stabilization, creation of novel 2D materials,
and surface patterning. Very often these processes involve compressing
interfacial monolayers of particles or polymers to obtain a desired
material microstructure. At high surface pressures, however, even
highly interfacially active objects can desorb from the interface.
Methods of directly measuring the energy which keeps the polymer or
particles bound to the interface (adsorption/desorption energies)
are therefore of high interest for these processes. Moreover, though
a geometric description linking adsorption energy and wetting properties
through the definition of a contact angle can be established for rigid
nano- or microparticles, such a description breaks down for deformable
or aggregating objects. Here, we demonstrate a technique to quantify
desorption energies directly, by comparing surface pressureâdensity
compression measurements using a Wilhelmy plate and a custom-microfabricated
deflection tensiometer. We focus on polyÂ(ethylene oxide)-based polymers
and nanoparticles. For PEO-based homo- and copolymers, the adsorption
energy of PEO chains scales linearly with molecular weight and can
be tuned by changing the subphase composition. Moreover, the desorption
surface pressure of PEO-stabilized nanoparticles corresponds to the
saturation surface pressure for spontaneously adsorbed monolayers,
yielding trapping energies of âŒ10<sup>3</sup> <i>k</i><sub>B</sub><i>T</i>
Adsorption Energies of Poly(ethylene oxide)-Based Surfactants and Nanoparticles on an AirâWater Surface
The self-assembly of polymer-based
surfactants and nanoparticles
on fluidâfluid interfaces is central to many applications,
including dispersion stabilization, creation of novel 2D materials,
and surface patterning. Very often these processes involve compressing
interfacial monolayers of particles or polymers to obtain a desired
material microstructure. At high surface pressures, however, even
highly interfacially active objects can desorb from the interface.
Methods of directly measuring the energy which keeps the polymer or
particles bound to the interface (adsorption/desorption energies)
are therefore of high interest for these processes. Moreover, though
a geometric description linking adsorption energy and wetting properties
through the definition of a contact angle can be established for rigid
nano- or microparticles, such a description breaks down for deformable
or aggregating objects. Here, we demonstrate a technique to quantify
desorption energies directly, by comparing surface pressureâdensity
compression measurements using a Wilhelmy plate and a custom-microfabricated
deflection tensiometer. We focus on polyÂ(ethylene oxide)-based polymers
and nanoparticles. For PEO-based homo- and copolymers, the adsorption
energy of PEO chains scales linearly with molecular weight and can
be tuned by changing the subphase composition. Moreover, the desorption
surface pressure of PEO-stabilized nanoparticles corresponds to the
saturation surface pressure for spontaneously adsorbed monolayers,
yielding trapping energies of âŒ10<sup>3</sup> <i>k</i><sub>B</sub><i>T</i>