(750c) Simulations-Based Design of a Biocompatible Oil Dispersant Additive

Oil spills have caused major environmental incidents over the past 50 years, and similar occurrences are likely to happen in the future.  Dispersants are commonly used to clean up oil spills, however they show mild to moderate toxicity to aquatic life.  There is currently a need for oil dispersant additives that are biocompatible and effective at stabilizing oil droplets in water, thereby reducing the amount of dispersant required.  We are using discontinuous molecular dynamics (DMD) simulations to design a biocompatible oil dispersant additive based on hydrophobically-modified chitosan (HMC).  DMD is a fast alternative to traditional molecular dynamics, that allows simulations of larger systems over longer time scales than traditional molecular dynamics   Our simulations are being used to supplement experiments by Dr. Vijay John and coworkers at Tulane University who have shown that  HMCs are able to prevent oil aggregation.  We model HMCs as comb copolymers with a hydrophilic chitosan backbone and hydrophobic modification chains and oil molecules as short linear chains.  Two simulation scenarios are considered: a bulk oil scenario (implicit water), and an interfacial oil scenario (explicit water modeled as single spheres).  Bulk oil simulations begin with a random initial configuration of oil and HMCs throughout the simulation box while interfacial simulations begin with a pre-formed oil droplet and a pre-formed water/air interface.     The length of the chitosan backbone (50 – 300 spheres), length of the modification chains (5-15 spheres), and the modification density (0 – 20%) are varied to determine their role in stabilizing oil in both scenarios.  Preliminary results show that increasing modification chain length and modification density leads to increased oil surface area in bulk simulations, indicating that the HMCs prevent oil aggregation.  HMCs with short modification chains stabilize oil by forming a chitosan backbone network and anchoring oil droplets to the chitosan network, while HMCs with long modification chains penetrate deeply into the oil droplets and deform the shape of the droplets.  However, there appears to be a saturation concentration of modification spheres above which increasing modification chain length does not improve oil dispersion.  Preliminary results also show that as oil droplets are exposed to an air/water interface, they spread on the water surface.  HMCs applied to the air/water interface prevent the spreading of oil droplets and  promote the formation of an oil gel on the water surface.  The ability of HMCs to prevent oil spreading at an air/water interface has significant application in chemical herding, where a special surfactant is applied to the perimeter of an oil slick to “herd” the oil towards the center of the slick allowing in-situ burning.  HMCs can be used to stabilize the already herded oil slick allowing a longer time window for the in-situ burning.  Our results will be compared to experiments by John and coworkers.