Classical molecular dynamics simulations are employed to monitor the aggregation behavior of six bile salts (nonconjugated and glycine- and taurine-conjugated sodium cholate and sodium deoxycholate) with concentration of 10 mM in aqueous solution in the presence of 120 mM NaCl. There are 150 ns trajectories generated to characterize the systems. The largest stable aggregates are analyzed to determine their shape, size, and stabilizing forces. It is found that the aggregation is a hierarchical process and that its kinetics depends both on the number of hydroxyl groups in the steroid part of the molecules and on the type of conjugation. The micelles of all salts are similar in shape-deformed spheres or ellipsoids, which are stabilized by hydrophobic forces, acting between the steroid rings. The differences in the aggregation kinetics of the various conjugates are rationalized by the affinity for hydrogen bond formation for the glycine-modified salts or by the longer time needed to achieve optimum packing for the tauro derivatives. Evidence is provided for the hypothesis from the literature that the entirely hydrophobic core of all aggregates and the enhanced dynamics of the molecules therein should be among the prerequisites for their pronounced solubilization capacity for hydrophobic substances in vivo.
The
studied anionic surfactants linear alkyl benzene sulfonate
(LAS) and sodium lauryl ether sulfate (SLES) are widely used key ingredients
in many home and personal care products. These two surfactants are
known to react very differently with multivalent counterions, including
Ca2+. This is explained by a stronger interaction of the
calcium cation with the LAS molecules, compared to SLES. The molecular
origin of this difference in the interactions remains unclear. In
the current study, we conduct classical atomistic molecular dynamics
simulations to compare the ion interactions with the adsorption layers
of these two surfactants, formed at the vacuum–water interface.
Trajectories of 150 ns are generated to characterize the adsorption
layer structure and the binding of Na+ and Ca2+ ions. We found that both surfactants behave similarly in the presence
of Na+ ions. However, when Ca2+ is added, Na+ ions are completely displaced from the surface with adsorbed
LAS molecules, while this displacement occurs only partially for SLES.
The simulations show that the preference of Ca2+ to the
LAS molecules is due to a strong specific attraction with the sulfonate
head-group, besides the electrostatic one. This specific attraction
involves significant reduction of the hydration shells of the interacting
calcium cation and sulfonate group, which couple directly and form
surface clusters of LAS molecules, coordinated around the adsorbed
Ca2+ ions. In contrast, SLES molecules do not exhibit such
specific interaction because the hydration shell around the sulfate
anion is more stable, due to the extra oxygen atom in the sulfate
group, thus precluding substantial dehydration and direct coupling
with any of the cations studied.
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