Project Overview

Summary:

Dr. Daniel Savin at the University of Florida was awarded an RFP-VI grant at $970,553 to conduct the RFP-VI project titled, “Designing Nanoparticle-based Dispersants with Improved Efficiency and Biocompatibility”.  The project consisted of 1 other institution (Tulane University), 1 principal investigator (Savin), 3 co-PIs (Drs. Nancy Denslow, Scott Grayson, Wayne Reed); 3 research scientists (Curtis Jarand, Elif Oruc, Nurettin Sahiner); 11 PhD students (Brooke Barnes, Alban Charlier, Scarlett Godinez, Dominic Rucco, Chris Keller, Guillermo Kurita, Shuchen Li, Daniel Rees, Julia Siquira, Susan Walley, Ian Smith); 4 undergraduate students (Caroline Browne, Adriana Hernandez, Jason Hyman, Alex Shishlov); and 2 high school students (Delia Savin, Henry Wietfeldt).

The global demand for energy continues to increase at a rapid pace, and projections indicate that oil will remain a major fuel source for many decades. The accidental release of oil in bodies of water due to negligence or mishap represents a significant environmental threat because of oil’s insolubility in water. If left untreated, a film of oil remains concentrated at the air-water interface, resulting in significant damage to sensitive marine ecosystems. An obvious solution to minimizing the environmental damage is the use of dispersants to break up the spill. For this reason, over 7 million liters of dispersants (such as Corexit EC9500A) were applied to mitigate the environmental impact after the Deepwater Horizon disaster in 2010.

Dispersants are composed of small amphiphilic molecules bearing both a hydrophilic face and a hydrophobic face that stabilize oil microdroplets in an emulsion by acting as a compatibilizing interface between the two incompatible phases. Although such compounds have been successfully developed and applied to stabilizing emulsions in closed systems (e.g. bottles of salad dressing), they suffer from a critical weakness: oil remediation by surfactants such as Corexit occurs through non-equilibrium emulsification of oil droplets into unstable structures that require constant energy input, from unreliable natural sources such as wind, waves, and currents, to prevent re-coalescence of the oil (analogous to having to shake the bottle of salad dressing before use, but on the size scale of an ocean.) The surfactant merely slows down the coalescence; it does not prevent it. As such, the dispersant needs to be continually re-applied over time, at great financial expense. In addition, emulsified oil droplets become more bioavailable within the marine environment. Ideally the dispersed oil would exhibit minimal toxicity to aquatic life but be colloidally stable such that it can be bioremediated over a time scale longer than that of re-coalescence.

The objective of the proposed research is to develop a platform of next-generation oil dispersants with superior uptake and stability, but with reduced toxicity, compared to existing remediation technologies. Our general approach involves the study of unimolecular micelles (UMs), which are materials that can stably encapsulate oil under any concentration conditions without requiring energy input. UMs have the remarkable property of dispersing oil in an equilibrium state, regardless of the environment. Based on our previous studies, UMs can be produced using a nanoparticle template, and we hypothesize that this platform can be extended to additional templates to elucidate how structural factors in the dispersant design affect performance (payload capacity, stability, toxicity) and cost. Our comprehensive approach will involve the synthesis and physiochemical characterization of novel dispersants, while integrating studies that determine nanotoxicological effects in fish development and growth. The proposed research team is composed of experts in nanoparticle and polymer synthesis (Grayson, Savin), solution process characterization (Reed, Savin), and aquatic nanotoxicology (Denslow). From a fundamental standpoint, successful completion of the proposed research will establish a matrix of dispersant design parameters and corresponding structure/property/toxicity relationships. From a commercial standpoint, the proposed research will result in cost-effective materials that can be applied to a broad range of environments (e.g. salt or fresh water across a wide range of temperature).

Research Highlights

Dr. Savin’s research, which included 6 outreach products and activities, resulted in 1 peer-reviewed publications and 19 presentations to date and 10 datasets submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are available to the public.  Significant outcomes of their research (all related to GoMRI Research Themes 4 and 5) are highlighted below.

• Synthesis of functionalized nanoparticles. One class of amphiphilic grafted nanoparticle (AGN) exploits the polymerization of caprolactone (PCL) followed by oligoethylene glycol methacrylate (POEGMA) by the Grayson Group. The complete synthesis of the proposed nanoparticles has been successful.  Preliminary oil encapsulation has been visually observed in the case of the final AGN product, showing a clear dispersion of oil into the aqueous layer containing these prepared nanoparticles.  While samples containing both PCL and POEGMA were dispersible in water, increased ease of water dispersibility was observed when the percentage of attached POEGMA exceeded 30% by mass.  As POEGMA is a bottlebrush polymer, this skews the composition of the copolymers in favor of the hydrophilic block, accounting for the ease of water dispersibility. Comparing the molecular weights of these blocks to the percent polymer composition gives a clear idea of how to tailor our reactions to achieve the desired molecular weights.  Preliminary dynamic light scattering (DLS) studies reveal that in the case of the 10 kD PCL chain, overall size of the nanoparticles decreases by upwards of 40 nm when switching from THF (a good solvent for PCL) to water (a poor solvent for PCL). The ability of these AGNs to be dispersed under ocean salt conditions has also been investigated. While the particles can be redispersed in a solution of Instant Ocean after being completely dried out, the particles with longer PCL chains prefer deionized water.
  
  
• Variable grafting density nanoparticles.  Another class of amphiphilic grafted AGNs exploits the polymerization of caprolactone (PCL) followed by hyperbranched poly(glycidol) (HBPG) by the Savin Group. In addition, this group is focused on developing AGNs with varying grafting density in order to determine structural effect on oil uptake and stability. AGNs made from a 50 nm and 70 nm substrate have been fully synthesized and show water solubility.  While both systems display water dispersibility, the 70 nm system also shows higher emulsion stability and remains homogenous in solution for days after preparation. Oil uptake capacity has been tested using DLS, and it was found that the lowest grafting density is able to hold the most oil.
  
  
• Assessment of the impact of oil to surfactant ratio on the entrapment of oil.  Reed has found that significant shear stress must be applied to surfactant oil remediation agents, such as Corexit, in order for them to emulsify oil; if the shear rate is high enough, ‘furious’, then the emulsification is ‘fast’, within tens of seconds. Given the high shear needed, as applied by a high-speed colloid mill, shear flow in capillaries, and contact mixing, it seems doubtful that Corexit or other surfactants can emulsify oil simply by being sprayed onto an oil spill. Natural shear, applied by typical wave, wind, and water current action is not expected to produce the high shear rates needed for emulsification. It may be necessary to actively emulsify and process an oil spill using high volume, high speed colloid mills in order for an agent such as Corexit to be of any use in emulsifying oil. Once oil is emulsified, it is hoped that the emulsions will contain oil droplets, typically tens of microns in diameter, until sunlight, bacteria, and other agents degrade the oil.  In contrast, AGNs show encapsulation over ‘slow’ conditions; these particles are able to trap and hold oil via diffusion under low speed mixing, hence ‘gentle’ conditions. The project has hence demonstrated the stringent shear and processing requirements needed to use surfactant agents for oil remediation, such as Corexit, and the low energy mixing requirements of the hybrid nanoparticles.
  
  
• Nano exposure experiments to determine toxicity of nanoparticles.  The Denslow Group has developed a new, quicker assay to determine the toxicity effect of nanoparticles on fish.  They tested the ethoxyresorufin-O-deethylase (EROD) assay performance using fish exposed to different concentrations of oil.  Using fathead minnow early life stages, we can measure CYP1A activity using an EROD assay, rather than looking for mRNA induction of CYP1A.  This assay is very effective and relatively easy to perform, allowing testing of many more types of engineered nanoparticles.  Denslow tested two classes of AGNs containing HBPG as the hydrophilic corona block and found that generally the toxicity decreased in the presence of these AGNs; however, we are confirming the relationship of toxicity to oil uptake by the AGNs.

 Proposal Abstract - RFP-VI PI Daniel Savin