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 CaCl₂ 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 “crowding” 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 Air–Water Surface

The self-assembly of polymer-based surfactants and nanoparticles on fluid–fluid 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 pressure–density compression measurements using a Wilhelmy plate and a custom-microfabricated deflection tensiometer. We focus on poly­(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 ∼10³ <i>k</i>_B<i>T</i>

Adsorption Energies of Poly(ethylene oxide)-Based Surfactants and Nanoparticles on an Air–Water Surface

The self-assembly of polymer-based surfactants and nanoparticles on fluid–fluid 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 pressure–density compression measurements using a Wilhelmy plate and a custom-microfabricated deflection tensiometer. We focus on poly­(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 ∼10³ <i>k</i>_B<i>T</i>