Aqueous dual-tailed surfactants simulated on the alumina surface

Atomistic molecular dynamics (MD) simulations were used to compare the morphology of aqueous surfactant self-assembled aggregates on a flat alumina substrate. The substrate was modeled using the CLAYFF force field, and it was considered fully protonated. Three ionic surfactants were considered, all with a sulfate headgroup. The first surfactant was the single-tailed, widely studied sodium dodecyl sulfate (SDS), for which previous simulation results are available on several substrates. The results obtained for this surfactant were used for benchmarking the behavior of two dual-tailed surfactants. These latter surfactants have equal structure, except that in one case both linear tails are composed by seven fully protonated carbon atoms [CH3(CH2)6CHOSO3(CH2)6CH3(-), 2H7], whereas in the other, one tail is composed by seven fully protonated carbon atoms and the other tail is composed by seven fully fluorinated carbon atoms [CF3(CF2)6CHOSO3(CH2)6CH3(-), H7F7]. Our results suggest that preferential interactions lead to surfactant aggregates for H7F7 that differ compared to both those obtained for SDS and 2H7. Although molecular-level geometric structural differences can be invoked to explain differences between H7F7 and SDS aggregates, those between H7F7 and 2H7 aggregates can only be ascribed to atomic-scale interactions. Because as the adsorbed amount of surfactant increases, the self-assembled surfactant aggregates change, suggesting that the substrate on which adsorption occurs effectively evolves as adsorption progresses, compared to bare alumina. The morphological differences observed in our simulations coupled with molecular-level microphase separation might explain, in part, the unusual retrograde adsorption isotherm that has been observed experimentally for H7F7 surfactants on alumina.