Surface Electrostatic Potential and Water Orientation in the presence of Sodium Octanoate Dilute Monolayers Studied by Means of Molecular Dynamics Simulations

Abstract

A 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.

Molecular Arrangement and Surface Tension of Alcohol Solutions

“…It is noteworthy that changes in the inner zone were not included in the calculation. This can be explained by the gradual contribution between two forces: enthalpic force acting on the asymmetric orientation and the entropic forces which are responsible for the ideal distribution . The peak in dipole moment corresponds to a critical threshold between these two competing forces: the outer zone is dominated by enthalpic force and inner zone is dominated by entropic one.…”

“…This can be explained by the gradual contribution between two forces: enthalpic force acting on the asymmetric orientation and the entropic forces which are responsible for the ideal distribution. 40 The peak in dipole moment corresponds to a critical threshold between these two competing forces: the outer zone is dominated by enthalpic force and inner zone is dominated by entropic one. While the enthalpic force can contribute to molecular arrangement in the inner zone, this should be compensated for by the entropic contribution in the outer zone.…”

Insights into the Assembly of the Pseudogemini Surfactant at the Oil/Water Interface: A Molecular Simulation Study

“…The value and distribution of the MEP greatly affect the exact combined sites in the surfactant assembly. 47 The ionic surfactant can be bound with another in the form of the complementary electrostatic potential, that is, the region with the positive MEP value tends to combine with that of the negative value to largely reduce the system energy. The larger MEP value greatly enhances the combination intensity.…”

Contribution of Different Molecules and Moieties to the Surface Tension in Aqueous Surfactant Solutions

“…These questions seem to be particularly interesting in the light of our recent results, namely (i) that in neat molecular liquids, the largest part (i.e., at least 80%) of the surface tension comes from the first molecular layer, whereas the rest comes from the second layer , and (ii) that the immersion depth of the surfactant molecules into the aqueous phase, in terms of molecular layers, depends sensitively on the type of the headgroup and can be as large as 6–8 molecular layers, at least for ionic surfactants . In spite of the large number of both experimental − and computer simulation studies ,− targeting a number of properties of aqueous surfactant solutions, this problem has, to the best of our knowledge, not been addressed yet.…”