Self-Assembly of Charged Oligomeric Bilayers at the Aqueous/Organic Interface Regulated by Ion-Pair Associations

<div> <p>Phospholipid bilayer membranes show promise as biomolecular soft materials that mimic the ability of living systems to sense, respond and learn but are fragile. Amphiphilic charged oligomers (oligodimethylsiloxane-methylimidazolium cation, ODMS-MIM⁽⁺⁾), assembled into bilayers at the oil-aqueous interfaces of droplet interface bilayers (DIBs), possessed similar size and functionality as phospholipid bilayers, but were stable. The ionic liquid headgroups (MIM⁽⁺⁾) of the oligomers were covalently bound to short-chain hydrophobic tails (ODMS). Bilayer self-assembly was influenced both by the charged headgroups, constrained to two-dimensional diffusion at the liquid-liquid interface, which formed electric double layers in the aqueous phase, and the tails in the organic phase. Bilayers formed spontaneously at low ionic strength but required an external voltage to form at higher ionicities. This switch in assembly behavior was due to ion-pairing of the MIM⁽⁺⁾ headgroups with chloride ions, resulting in an increase in the density of the charged headgroups at the interface and the ODMS hydrophobic tails in the oil phase as they were covalently grafted to the headgroups. <a>Chain overlap led to repulsive disjoining pressures between droplets due to osmotic stress</a>. The applied voltage caused an attractive electrocompressive stress that overcame the repulsion, enabling bilayer formation. <a>Bilayer assembly at high ionic strength, while requiring a voltage to initiate, was irreversible, and the resulting membrane was considerably more stable than those formed at lower values of the ionic strength</a>. This switching of assembly behavior can be exploited as an additional mechanism for short-term synaptic plasticity in neuromorphic device applications using soft materials.</p> </div>

Self-Assembly of Charged Oligomeric Bilayers at the Aqueous/Organic Interface Regulated by Ion-Pair Associations

<div> <p>Phospholipid bilayer membranes show promise as biomolecular soft materials that mimic the ability of living systems to sense, respond and learn but are fragile. Amphiphilic charged oligomers (oligodimethylsiloxane-methylimidazolium cation, ODMS-MIM⁽⁺⁾), assembled into bilayers at the oil-aqueous interfaces of droplet interface bilayers (DIBs), possessed similar size and functionality as phospholipid bilayers, but were stable. The ionic liquid headgroups (MIM⁽⁺⁾) of the oligomers were covalently bound to short-chain hydrophobic tails (ODMS). Bilayer self-assembly was influenced both by the charged headgroups, constrained to two-dimensional diffusion at the liquid-liquid interface, which formed electric double layers in the aqueous phase, and the tails in the organic phase. Bilayers formed spontaneously at low ionic strength but required an external voltage to form at higher ionicities. This switch in assembly behavior was due to ion-pairing of the MIM⁽⁺⁾ headgroups with chloride ions, resulting in an increase in the density of the charged headgroups at the interface and the ODMS hydrophobic tails in the oil phase as they were covalently grafted to the headgroups. <a>Chain overlap led to repulsive disjoining pressures between droplets due to osmotic stress</a>. The applied voltage caused an attractive electrocompressive stress that overcame the repulsion, enabling bilayer formation. <a>Bilayer assembly at high ionic strength, while requiring a voltage to initiate, was irreversible, and the resulting membrane was considerably more stable than those formed at lower values of the ionic strength</a>. This switching of assembly behavior can be exploited as an additional mechanism for short-term synaptic plasticity in neuromorphic device applications using soft materials.</p> </div>

Self-Assembly of Charged Oligomeric Bilayers at the Aque-ous/Organic Interface Regulated by Ion-Pair Associations

<div> <p>Phospholipid bilayer membranes show promise as biomolecular soft materials that mimic the ability of living systems to sense, respond and learn but are fragile. Amphiphilic charged oligomers (oligodimethylsiloxane-methylimidazolium cation, ODMS-MIM⁽⁺⁾), assembled into bilayers at the oil-aqueous interfaces of droplet interface bilayers (DIBs), possessed similar size and functionality as phospholipid bilayers, but were stable. The ionic liquid headgroups (MIM⁽⁺⁾) of the oligomers were covalently bound to short-chain hydrophobic tails (ODMS). Bilayer self-assembly was influenced both by the charged headgroups, constrained to two-dimensional diffusion at the liquid-liquid interface, which formed electric double layers in the aqueous phase, and the tails in the organic phase. Bilayers formed spontaneously at low ionic strength but required an external voltage to form at higher ionicities. This switch in assembly behavior was due to ion-pairing of the MIM⁽⁺⁾ headgroups with chloride ions, resulting in an increase in the density of the charged headgroups at the interface and the ODMS hydrophobic tails in the oil phase as they were covalently grafted to the headgroups. <a>Chain overlap led to repulsive disjoining pressures between droplets due to osmotic stress</a>. The applied voltage caused an attractive electrocompressive stress that overcame the repulsion, enabling bilayer formation. <a>Bilayer assembly at high ionic strength, while requiring a voltage to initiate, was irreversible, and the resulting membrane was considerably more stable than those formed at lower values of the ionic strength</a>. This switching of assembly behavior can be exploited as an additional mechanism for short-term synaptic plasticity in neuromorphic device applications using soft materials.</p> </div>

Mechanisms of Surface Charge Modification of Carbonates in Aqueous Electrolyte Solutions

The influence of different types of salts (NaCl, CaCl 2 , MgCl 2 , NaHCO 3 , and Na 2 SO 4 ) on the surface characteristics of unconditioned calcite and dolomite particles, and conditioned with stearic acid, was investigated. This study used zeta potential measurements to gain fundamental understanding of physico-chemical mechanisms involved in surface charge modification of carbonate minerals in the presence of diluted salt solutions. By increasing the salt concentration of divalent cationic salt solution (CaCl 2 and MgCl 2 ), the zeta potential of calcite particles was altered, resulting in charge reversal from negative to positive, while dolomite particles maintained positive zeta potential. This is due to the adsorption of potential-determining cations (Ca 2 + and Mg 2 + ), and consequent changes in the structure of the diffuse layer, predominantly driven by coulombic interactions. On the other hand, chemical adsorption of potential-determining anions (HCO 3 - and SO 4 2 - ) maintained the negative zeta potential of carbonate surfaces and increased its magnitude up to 10 mM, before decreasing at higher salt concentrations. Physisorption of stearic acid molecules on the calcite and dolomite surfaces changed the zeta potential to more negative values in all solutions. It is argued that divalent cations (Ca 2 + and Mg 2 + ) would result in positive and neutral complexes with stearic acid molecules, which may result in strongly bound stearic acid films, whereas ions resulting in negative mineral surface charges (SO 4 2 - and HCO 3 - ) will cause stearic acid films to be loosely bound to the carbonate mineral surfaces. The suggested mechanism for surface charge modification of carbonates, in the presence of different ions, is changes in both distribution of ions in the diffuse layer and its structure as a result of ion adsorption to the crystal lattice by having a positive contribution to the disjoining pressures when changing electrolyte concentration. This work extends the current knowledge base for dynamic water injection design by determining the effect of salt concentration on surface electrostatics.

Lubrication in Tight Spots

This chapter discusses the interesting phenomena that happen when the thickness of a lubricant film is reduced to nanoscale dimensions. For liquid lubricants sandwiched between two solid surfaces, the interesting phenomena associated with confined liquids include: molecules forming a layered structure, enhanced viscosity, and solidification. In boundary lubrication, an adsorbed monolayer resists penetration of contacting asperities and sliding takes place over the low shear strength surface of the boundary lubricant. The absence of boundary lubrication can lead to cold welding where adhesion at the interface leads to ultra-high friction and seizure. The last part of this chapter discusses how capillary and disjoining pressures lead to the formation of lubricant menisci around contacting asperities from a thin lubricant film on one of the surfaces and how these menisci influence adhesion and friction. The kinetics of meniscus formation from capillary condensation and its impact on friction are also discussed.

Two models based on local microscopic relaxations to explain long-term basic creep of concrete

In this study, we propose an exhaustion model and an adapted work-hardening model to explain the long-term basic creep of concrete. In both models, the macroscopic creep strain originates from local microscopic relaxations. The two models differ in how the activation energies of those relaxations are distributed and evolve during the creep process. With those models, at least up to a few dozen MPa, the applied stress must not modify the rate at which those relaxations occur, but only enables the manifestation of each local microscopic relaxation into an infinitesimal increment of basic creep strain. The two models capture equally well several phenomenological features of the basic creep of concrete. They also make it possible to explain why the indentation technique enables the quantitative characterization of the long-term kinetics of logarithmic creep of cement-based materials orders of magnitude faster than by macroscopic testing. The models hint at a physical origin for the relaxations that is related to disjoining pressures.

Surface force at the nano-scale: observation of non-monotonic surface tension and disjoining pressure

The disjoining pressures of thin aqueous salt films at different salt concentrations and temperatures were calculated using MD simulations.

Estimating vertical and lateral pressures in periodically structured montmorillonite clay particles

Given a montmorillonitic clay soil at high porosity and saturated by monovalent counterions, we investigate the particle level responses of the clay to different external loadings. As analytical solutions are not possible for complex arrangements of particles, we employ computational micromechanical models (based on the solution of the Poisson-Nernst-Planck equations) using the finite element method, to estimate counterion and electrical potential distributions for particles at various angles and distances from one another. We then calculate the disjoining pressures using the Van't Hoff relation and Maxwell stress tensor. As the distance between the clay particles decreases and double-layers overlap, the concentration of counterions in the micropores among clay particles increases. This increase lowers the chemical potential of the pore fluid and creates a chemical potential gradient in the solvent that generates the socalled 'disjoining' or 'osmotic' pressure. Because of this disjoining pressure, particles do not need to contact one another in order to carry an 'effective stress'. This work may lead towards theoretical predictions of the macroscopic load deformation response of montmorillonitic soils based on micromechanical modelling of particles.