(750e) Molecular Dynamic Simulations of Tween 80 Surfactant with Squalane/Water Mixtures, As a Model for the Deepwater Horizon Oil Spill

In 2010, the explosion of Deepwater Horizon resulted in the release of about 4.9 million barrels of crude oil into the Gulf of Mexico¹. To protect the coastline and marine environments, 1.84 million barrels of dispersant (Corexit) were sprayed into the gulf, either on the sea surface or deep under the water into the oil plume. Chemical dispersants accelerate the dispersal processes by breaking up oil slick and forming water-soluble micelles into the water column. One of the main active ingredients of Corexit is Tween-80², which is a nonionic long chain surfactant (contour length about 6 nm). The micellization properties of Tween-80 have never been studied at atomistic level. In this study, we carried out molecular dynamic simulations on systems composed of Tween-80, squalane (as a model oil), and sea salts at atomic united atom levels³. Some of the partial charges of Tween 80, as available, were adopted from existing force field parameters^4,5. The rest of the partial charges were estimated using Gaussian density function method Bly3p at 6-31g(d,p) basis⁶ followed by Boltzmann energy average method over the conformers. We then carried out molecular dynamics simulations of systems composed of Tween 80, squalane in the presence or absence of sea salt. In particular, we studied the stabilities of Tween 80 micelles by molecular dynamics simulations at varying aggregation numbers. 60 randomly distributed Tween 80 formed a spherical micelle within 10 ns. This is consistent with Anwar’s study from a Coarse grained model⁷. Radii of gyration, solvent accessible surface area, probability distributions of different atoms of Tween 80 around the micelle center of mass, and radial densities of atoms around the micelle center of mass were investigated. We observed the formation of a surfactant-laden oil-water interface occurred within 20 ns after exposing squalane oil to a surfactant-laden aqueous phase in a simulation box of size 12×12×15 nm³. Initially randomly distributed Tween 80 and oil in water formed a suspended oil-filled micellar aggregate coated with hydrophilic head groups of Tween 80 within 20 ns.  Such results might be used to help determine mass transport coefficients required for engineering dispersant systems for control of oil releases.

References:

1. http://en.wikipedia.org/wiki/Deepwater_Horizon_oil_spill
2. http://en.wikipedia.org/wiki/Corexit
3. D. R. Kester, I. W. Duedall, D. N. Connors, and R. M. Pytkowicz, Limnol. and Oceano., 1967, 12(1), 176-179
4. W. F. V. Gunsteren et. al . J. Peptide Sci., 2005, 11, 74–84.
5. W. F. V. Gunsteren et. al. J. Chem. Theory Comput., 2011, 7 (5), 1237–1243
6. Gaussian 09, Revision A.1, M. J. Frisch, et. al. Gaussian, Inc., Wallingford CT, 2009.
7. A. Amani, P. York, H. de Waard, and J Anwar, Soft Matter, 2011, 7, 2900-2908.