13 research outputs found
Ethanol induces the formation of water-permeable defects in model bilayers of skin lipids
We show that ethanol can induce the formation of water-permeable defects in model membranes of skin, providing a fresh perspective on ethanol as a membrane modulator. We rationalise our findings in terms of the chemical nature of ethanol, i.e., a combination of its hydrogen bonding propensity and amphiphilic character
The Relationship Between Wormlike Micelle Scission Free Energy and Micellar Composition: The Case of Sodium Laurylethersulphate and Cocamidopropyl Betaine
The scission energy is the difference in energy between two hemispherical caps and the cylindrical region of a wormlike micelle. This energy difference is exponentially proportional to the average micelle length, which affects several macroscopic properties such as the viscosity of viscoelastic fluids. Here we use a recently published method by Wang et al (Langmuir 2018 34 1564-1573) to directly calculate the scission energy of micelles composed of monodisperse Sodium Laurylethersulphate (SLESnEO), an anionic surfactant. We perform a systematic study varying the number of ethoxyl groups (n) and salt concentration. The scission energy increases with increasing salt concentration, indicating that the formation of longer micelles is favoured. We attribute this to the increased charge screening that reduces the repulsion between head groups. However, the scission energy decreases with increasing number of ethoxyl groups as the flexibility of the head group increases and the sodium ion becomes less tightly bound to the head group. We then extend to look at the effect of a common co-surfactant, Cocamidopropyl Betaine (CAPB) and find that its addition increases the scission energy, stabilising wormlike micelles at a lower salt concentration
The behaviour of tributyl phosphate in an organic diluent
Tributyl phosphate (TBP) is used as a complexing agent in the Plutonium Uranium Extraction (PUREX) liquid-liquid phase extraction process for recovering uranium and plutonium from spent nuclear reactor fuel. Here, we address the molecular and microstructure of the organic phases involved in the extraction process, using molecular dynamics to show that when TBP is mixed with a paraffinic diluent, the TBP self-assembles into a bi-continuous phase. The underlying self-association of TBP is driven by intermolecular interaction between its polar groups, resulting in butyl moieties radiating out into the organic solvent. Simulation predicts a TBP diffusion constant that is anomalously low compared to what might normally be expected for its size; experimental nuclear magnetic resonance (NMR) studies also indicate an extremely low diffusion constant, consistent with a molecular aggregation model. Simulation of TBP at an oil/water interface shows the formation of a bilayer system at low TBP concentrations. At higher concentrations, a bulk bi-continuous structure is observed linking to this surface bilayer. We suggest that this structure may be intimately connected with the surprisingly rapid kinetics of the interfacial mass transport of uranium and plutonium from the aqueous to the organic phase in the PUREX process
Micelle Formation in Alkyl Sulfate Surfactants Using Dissipative Particle Dynamics
We
use dissipative particle dynamics (DPD) to study micelle formation
in alkyl sulfate surfactants, with alkyl chain lengths ranging from
6 to 12 carbon atoms. We extend our recent DPD force field [J. Chem. Phys. 2017, 147, 094503] to include
a charged sulfate chemical group and aqueous sodium ions. With this
model, we achieve good agreement with the experimentally reported
critical micelle concentrations (CMCs) and can match the trend in
mean aggregation numbers versus alkyl chain length. We determine the
CMC by fitting a charged pseudophase model to the dependence of the
free surfactant on the total surfactant concentration above the CMC
and compare it with a direct operational definition of the CMC as
the point at which half of the surfactant is classed as micellar and
half as monomers and submicellar aggregates. We find that the latter
provides the best agreement with experimental results. Finally, with
the same model, we are able to observe the sphere-to-rod morphological
transition for sodium dodecyl sulfate (SDS) micelles and determine
that it corresponds to SDS concentrations in the region of 300–500
mM
Polymers of Intrinsic Microporosity Containing Tröger Base for CO<sub>2</sub> Capture
Properties of four polymers of intrinsic
microporosity containing
Tröger’s base units were assessed for CO<sub>2</sub> capture experimentally and computationally. Structural properties
included average pore size, pore size distribution, surface area,
and accessible pore volume, whereas thermodynamic properties focused
on density, CO<sub>2</sub> sorption isotherms, and enthalpies of adsorption.
It was found that the shape of the contortion site plays a more important
role than the polymer density when assessing the capacity of the material,
and that the presence of a Tröger base unit only slightly affects
the amount adsorbed at low pressures, but it does not have any significant
influence on the enthalpy of adsorption fingerprint. A comparison
of the materials studied with those reported in the literature allowed
us to propose a set of guidelines for the design of polymers for CO<sub>2</sub> capture applications