Effect of Headgroup Size, Charge, and Solvent Structure on Polymer−Micelle Interactions, Studied by Molecular Dynamics Simulations

Abstract

We performed atomistic molecular dynamics simulations of anionic and cationic micelles in the presence of poly(ethylene oxide) (PEO) to understand why nonionic water-soluble polymers such as PEO interact strongly with anionic micelles but only weakly with cationic micelles. Our micelles include sodium n-dodecyl sulfate (SDS), n-dodecyl trimethylammonium chloride (DTAC), n-dodecyl ammonium chloride (DAC), and micelles in which we artificially reverse the sign of partial charges in SDS and DTAC. We observe that the polymer interacts hydrophobically with anionic SDS but only weakly with cationic DTAC and DAC, in agreement with experiment. However, the polymer also interacts with the artificial anionic DTAC but fails to interact hydrophobically with the artificial cationic SDS, illustrating that large headgroup size does not explain the weak polymer interaction with cationic micelles. In addition, we observe through simulation that this preference for interaction with anionic micelles still exists in a dipolar "dumbbell" solvent, indicating that water structure and hydrogen bonding alone cannot explain this preferential interaction. Our simulations suggest that direct electrostatic interactions between the micelle and polymer explain the preference for interaction with anionic micelles, even though the polymer overall carries no net charge. This is possible given the asymmetric distribution of negative charges on smaller atoms and positive charges on larger units in the polymer chain.

Disassembly of an Interconjugated Polyelectrolyte Complex Using Ionic Surfactants

“…We used two model surfactants with oppositely charged headgroups and identical aliphatic tails, as there is prior work showing that there is reason to suspect that the sign of the headgroup charge may be important to complexation. 42 We find evidence that at symmetric polyion compositions, both the anionic and cationic surfactants were effective at disassembling the CPEC in salt-free aqueous solution, thereby largely shutting off EET. This took place both below and above the critical surfactant micelle concentration.…”

“…We examined the photophysics of the complexes at varying CPE and surfactant stoichiometries to systematically interrogate the ternary macromolecular system. We used two model surfactants with oppositely charged headgroups and identical aliphatic tails, as there is prior work showing that there is reason to suspect that the sign of the headgroup charge may be important to complexation . We find evidence that at symmetric polyion compositions, both the anionic and cationic surfactants were effective at disassembling the CPEC in salt-free aqueous solution, thereby largely shutting off EET.…”

Atomistic Structure of a Micelle in Solution Determined by Wide Q-Range Neutron Diffraction

“…It is our intention to now use this method to: study the origin of the changes in micelle size and shape with different counterions and temperature; investigate the interaction of small inorganic species with the micelle (representing part of the templating process for meso porous materials); examine polymer-micelle complexes to understand if the polymer binds to the micelle surface, or if it thread through the centre of the structure? 3,53,54 Advances in the instrumentation for wide Q-range diffraction experiments will provide an even greater Q-range, allowing larger self-assembling systems to be examined at atomic resolution, and reduced counting times will make time-resolved interrogation of processes in solution possible. 55 ; the size of the largest TAB cluster (i = L); the number of monomers (n i=1 ); the total number of clusters (∑ L i=1 n i ); the average cluster size (�i�); and finally, the R-factor.…”

Structural Characterization and in Vivo Evaluation of β-Hairpin Peptidomimetics as Specific CXCR4 Imaging Agents

“…Previous studies of PEG interactions with different types of micelles showed that it exhibits a preference for interaction with anionic surfactants such as SDS, whereas interactions with cationic or zwitterionic micelles are rather limited . The Gadolinium–DOTA complex (Gd(DOTA)), which is generally used as a paramagnetic probe in NMR studies, preferably localizes at the water–micelle interface and enables the detection of the solvent-accessible fragments of the peptide .…”