Maria Sammalkorpi

Ionic surfactant aggregates in saline solutions: Sodium dodecyl sulfate (SDS) in the presence of excess sodium chloride (NaCl) or calcium chloride (CaCl2)

The properties of sodium dodecyl sulfate (SDS) aggregates in saline solutions of excess sodium chloride (NaCl) or calcium chloride (CaCl(2)) ions were studied through extensive molecular dynamics simulations with explicit solvent. We find that the ionic strength of the solution affects not only the aggregate size of the resulting anionic micelles but also their structure. Specifically, the presence of CaCl(2) induces more compact and densely packed micelles with a significant reduction in gauche defects in the SDS hydrocarbon chains in comparison with NaCl. Furthermore, we observe significantly more stable salt bridges between the charged SDS head groups mediated by Ca(2+) than Na(+). The presence of these salt bridges helps stabilize the more densely packed micelles.

Formation and Regulation of Lipid Microdomains in Cell Membranes: Theory, Modeling, and Speculation

Compositional lipid microdomains (“lipid rafts”) in plasma membranes are believed to be important components of many cellular processes. The biophysical mechanisms by which cells regulate the size, lifetime, and spatial localization of these domains are rather poorly understood at the moment. Over the years, experimental studies of raft formation have inspired several phenomenological theories and speculations incorporating a wide variety of thermodynamic assumptions regarding lipid–lipid and lipid–protein interactions, and the potential role of active cellular processes on membrane structure. Here we critically review and discuss these theories, models, and speculations, and present our view on future directions.

Atomistic Simulations of Micellization of Sodium Hexyl, Heptyl, Octyl, and Nonyl Sulfates

Molecular dynamics simulations have been used to study the micellization behavior of atomistic models for sodium alkyl sulfates in explicit water. A major finding of the present work is the observation of a strong dependence of free surfactant concentration on overall surfactant concentration, that has not been reported previously and that is key to comparing simulation results for the critical micelle concentration (CMC) to experimental data. The CMC and aggregate size distributions were obtained for alkyl tail lengths from six to nine at temperatures from 268 to 363 K, from 400 ns simulations covering a number of surfactant and water model combinations. The free surfactant concentration is much lower than the critical micelle concentration for strongly micellizing systems at the relatively high concentrations accessible by simulations. Thus, counterion association must be accounted for in determining the CMC from the raw simulation data. Simulation results are in qualitative agreement with experimental trends for aggregate size and CMC as functions of alkyl tail length and temperature.

Simulations of micellization of sodium hexyl sulfate

Micellization of the ionic surfactant sodium hexyl sulfate has been studied using atomistic explicit-solvent molecular dynamics simulations with and without excess NaCl or CaCl(2). Simulations were performed at surfactant loadings near the critical micellization concentration. Equilibrium micelle size distributions and estimates of the critical micellization concentration obtained from the simulations are in agreement with experimental data. In comparison to the sodium dodecyl sulfate surfactant, the shorter alkyl chain of sodium hexyl sulfate results in increased disorder of the micellar core and water exposure of the hydrocarbon tail groups. However, water and ions do not penetrate into the micellar core even for these weakly micellizing surfactants. Excess NaCl is observed to have a minor influence on the micelle structure but excess CaCl(2) induces drastic changes both in the structure and the dynamics of the micellar system. Furthermore, in the absence of excess salt, sodium hexyl sulfate forms predominantly spherical, disorganized aggregates but an increase in ionic strength drives an increase in aggregate size and leads to prolate aggregates.

Micelle fission through surface instability and formation of an interdigitating stalk

We report on the first detailed atomic-scale studies of micelle fission in micellar systems consisting of anionic sodium dodecyl sulfate with explicit solvent. We demonstrate a new micelle fission pathway for ionic surfactants and show how micelle fission can be induced by varying the ionic concentration. We argue that this fission pathway proceeds through an initial Rayleigh instability driven by Coulombic interactions and show how the intermediate stages proceed through the formation of a highly interdigitated stalk. This pathway may facilitate easier compartmentalization and functionalization of micelles.

Structure and Dynamics of Surfactant and Hydrocarbon Aggregates on Graphite: A Molecular Dynamics Simulation Study

We have examined the structure and dynamics of sodium dodecyl sulfate (SDS) and dodecane (C12) molecular aggregates at varying surface coverages on the basal plane of graphite via classical molecular dynamics simulations. Our results suggest that graphite-hydrocarbon chain interactions favor specific molecular orientations at the single-molecule level via alignment of the tail along the crystallographic directions. This orientational bias is reduced greatly upon increasing the surface coverage for both molecules due to intermolecular interactions, leading to very weak bias at intermediate surface coverages. Interestingly, for complete monolayers, we find a re-emergent orientational bias. Furthermore, by comparing the SDS behavior with C12, we demonstrate that the charged head group plays a key role in the aggregate structures: SDS molecules display a tendency to form linear file-like aggregates while C12 forms tightly bound planar ones. The observed orientational bias for SDS molecules is in agreement with experimental observations of hemimicelle orientation and provides support for the belief that an initial oriented layer governs the orientation of hemimicellar aggregates.

Surfactant and Hydrocarbon Aggregates on Defective Graphite Surface: Structure and Dynamics

We have simulated the structure and aggregation kinetics of sodium dodecyl sulfate (SDS) and dodecane (C 12) on a graphite surface in the presence of point and line defects. We find that while vacancies do not affect the orientational bias of the molecules, they interfere with aggregate formation. Specifically, they disrupt the formation of extended aggregates. Line defects in the form of surface steps, on the other hand, tend to localize the aggregates in their vicinity and induce specific orientations along the step edges. We demonstrate that this orientational bias can be tuned by manipulating the terrace widths. These results suggest that extended defects could be employed to localize and orient surfactant aggregates on the basal plane of graphite, thus providing a means to create patterned aggregate domains.

The influence of ionic strength and mixing ratio on the colloidal stability of PDAC/PSS polyelectrolyte complexes

Polyelectrolyte complexes (PECs) form by mixing polycation and polyanion solutions together, and have been explored for a variety of applications. One challenge for PEC processing and application is that under certain conditions the as-formed PECs aggregate and precipitate out of suspension over the course of minutes to days. This aggregation is governed by several factors such as electrostatic repulsion, van der Waals attractions, and hydrophobic interactions. In this work, we explore the boundary between colloidally stable and unstable complexes as it is influenced by polycation/polyanion mixing ratio and ionic strength. The polymers examined are poly(diallyldimethylammonium chloride) (PDAC) and poly(sodium 4-styrenesulfonate) (PSS). Physical properties such as turbidity, hydrodynamic size, and zeta potential are investigated upon complex formation. We also perform detailed molecular dynamics simulations to examine the structure and effective charge distribution of the PECs at varying mixing ratios and salt concentrations to support the experimental findings. The results suggest that the colloidally stable/unstable boundary possibly marks the screening effects from added salt, resulting in weakly charged complexes that aggregate. At higher salt concentrations, the complexes initially form and then gradually dissolve into solution.

Polyelectrolyte Decomplexation via Addition of Salt: Charge Correlation Driven Zipper

We report the first atomic scale studies of polyelectrolyte decomplexation. The complex between DNA and polylysine is shown to destabilize and spontaneously open in a gradual, reversible zipper-like mechanism driven by an increase in solution salt concentration. Divalent CaCl2 is significantly more effective than monovalent NaCl in destabilizing the complex due to charge correlations and water binding capability. The dissociation occurs accompanied by charge reversal in which charge correlations and ion binding chemistry play a key role. Our results are in agreement with experimental work on complex dissociation but in addition show the underlying microstructural correlations driving the behavior. Comparison of our full atomic level detail and dynamics results with theoretical works describing the PEs as charged, rigid rods reveals that although charge correlation involved theories provide qualitatively similar responses, considering also specific molecular chemistry and molecular level water contributions provides a more complete understanding of PE complex stability and dynamics. The findings may facilitate controlled release in gene delivery and more in general tuning of PE membrane permeability and mechanical characteristics through ionic strength.

Carbon nanotube bundling: influence on layer-by-layer assembly and antimicrobial activity

Antimicrobial surfaces are needed for many health care applications. Single walled carbon nanotubes (SWNT) have shown promise as antimicrobial agents, but important questions persist concerning the effects of tube bundling, a common phenomenon owing to strong hydrophobicity. We investigate here the influence of bundling on the layer-by-layer (LbL) assembly of SWNT with charged polymers, and on the antimicrobial properties of the resultant films. We employ a poly(ethylene glycol) functionalized phospholipid (PL-PEG) to disperse SWNT in aqueous solution, and consider cases where SWNT are dispersed (i) as essentially isolated objects and (ii) as small bundles. Quartz crystal microgravimetry with dissipation (QCMD) and ellipsometry measurements show the bundled SWNT system to adsorb in an unusually strong fashion – with layers twice (when hydrated) and three times (when dried) as thick as those of isolated SWNT. Molecular dynamics simulation reveals a lower PL-PEG density and degree of solution extension on bundled versus isolated SWNT, suggesting thicker adsorbed layers may result from suppressed steric repulsion between bundled nanotubes. Enhanced van der Waals attraction in the bundled system may also play a role. Scanning electron micrographs reveal Escherichia coli on films with bundled SWNT to be essentially engulfed by the nanotubes, whereas the bacteria rest upon films with isolated SWNT. While both systems inactivate 90% of bacteria in 24 h, the bundled SWNT system is “fast-acting,” reaching this inactivation rate in 1 h. This study demonstrates the significant impact of SWNT bundling on LbL assembly and antimicrobial activity, explores the molecular basis of nanotube–nanotube interactions, and demonstrates the possibility of bacteria-engulfing, fast-acting, SWNT-based antimicrobial coatings.