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Surface Electrostatic Potential and Water Orientation in the presence of Sodium Octanoate Dilute Monolayers Studied by Means of Molecular Dynamics Simulations
journal contribution
posted on 2015-10-13, 00:00 authored by Kalil Bernardino, André F. de MouraA series of atomistic molecular dynamics
simulations were performed
in the present investigation to assess the spontaneous formation of
surfactant monolayers of sodium octanoate at the water–vacuum
interface. The surfactant surface coverage increased until a saturation
threshold was achieved, after which any further surfactant addition
led to the formation of micellar aggregates within the solution. The
saturated films were not densely packed, as might be expected for
short-chained surfactants, and all films regardless of the surface
coverage presented surfactant molecules with the same ordering pattern,
namely, with the ionic heads toward the aqueous solution and the tails
lying nearly parallel to the interface. The major contributions to
the electrostatic surface potential came from the charged heads and
the counterion distribution, which nearly canceled out each other.
The balance between the oppositely charged ions rendered the electrostatic
contributions from water meaningful, amounting to ca. 10% of the contributions
arising from the ionic species. And even the aliphatic tails, whose
atoms bear relatively small partial atomic charges as compared to
the polar molecules and molecular fragments, contributed with ca.
20% of the total electrostatic surface potential of the systems under
investigation. Although the aliphatic tails were not so orderly arranged
as in a compact film, the C–H bonds assumed a preferential
orientation, leading to an increased contribution to the electrostatic
properties of the interface. The most prominent feature arising from
the partitioning of the electrostatic potential into individual contributions
was the long-range ordering of the water molecules. This ordering
of the water molecules produced a repulsive dipole–dipole interaction
between the two interfaces, which increased with the surface coverage.
Only for a water layer wider than 10 nm was true bulk behavior observed,
and the repulsive dipole–dipole interaction faded away.