The intrinsic interfacial structure of ionic surfactant monolayers at water-oil and water-vapour interfaces

Abstract

We investigate using computer simulations the interfacial structure of Sodium Dodecyl Sulfate (SDS) monolayers adsorbed at the water surface and water-oil interfaces. Using an algorithm that removes the averaging effect of the capillary waves we obtain a detailed view of the solvation structure of water around the monolayer. We investigate surface concentrations between 45 and 33 Å per surfactant, which are near experimental conditions corresponding to the critical micellar concentration and the formation of Newton Black Films. The surfactants induce a layering structure in water, which disappears at ≈ 1 nm from the monolayer plane. The water molecules exhibit a preferred orientation with the dipoles pointing towards the monolayer. The orientational order decays slowly, but it does not influence the hydrogen bond structure of water, which is significantly disrupted in the interfacial region. These structural changes are qualitatively the same in SDS–water and oil–SDS–water interfaces. In the latter case we find a small degree of penetration of oil in the monolayer (between 0.2–0.25 molecules per SDS). This small penetration has a measurable effect on the monolayer, which increases its thickness by ≈ 10%. The bending modulus of the SDS monolayers is of the order of the thermal energy, kBT.

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Cite this paper

@inproceedings{Martnez2012TheII, title={The intrinsic interfacial structure of ionic surfactant monolayers at water-oil and water-vapour interfaces}, author={By Hector Mart́ınez and Enrique Chac{\'o}n and Pedro J. Trujillo Tarazona and Fernando Bresme and Nicol{\'a}s Cabrera}, year={2012} }