Although many alternatives to fossil fuels are being developed, worldwide demand for oil as a fuel will remain high for many decades, sustaining both drilling and shipping of petroleum throughout the world’s waterways and making future oil spills inevitable. The active ingredients of traditional dispersants used to remediate oil spills are surfactants that break up the oil into microdroplets by stabilizing the oil-water interface. Once dispersed, the oil biodegrades more rapidly and is less toxic. However, while these small molecule surfactants work well in closed systems, they are rapidly diluted by the vastness of an open body of water, and as a result suffer from a loss of function at these lower concentrations. For this reason, most commercial dispersants exhibit emulsion instabilities within a few days of their initial application.
This emulsion instability upon dilution is an inescapable consequence of the fact that the selfassembled micelle structures are in dynamic equilibrium with free, unaggregated surfactant molecules. In open water, this means that the surfactants are steadily stripped from the oil-water interface of the microdroplets until reaching a critical point, at which the droplets are destabilized, coalesce, and eventually resurface. Considering the near infinite diluting power of the open ocean, the most promising approach to accessing long-term stability in emulsions is the use of unimolecular micelles (UMs). UMs are unique amphiphilic macromolecules whose covalently linked structure mimics the entire spherical surface of traditional self-assembled micelles. Because their structure is held together by covalent bonds, they will not disaggregate even under extreme dilution and yield stabile emulsions until the covalent bonds that hold them together chemically degrade (typically months to years).
Two complimentary macromolecular materials will be investigated in this proposal: highly branched amphiphilic “star” polymers (ASPs) (~2-20nm) and amphiphilic grafted nanoparticles (AGNs) (~10- 100nm). Initial studies will focus on very well-defined structures in order to develop a fundamental understanding of how each of their critical structural parameters (polymer chain length, hydrophilic/hydrophobic block ratio, size of dendritic or nanoparticle core, etc.) affects their performance as dispersants for remediation. In the case of the ASPs, a synthetic route has already been published by Grayson which will enable the preparation of a library of ASPs with varying numbers of arms and differing degrees of polymerization for the polar and non-polar blocks. This approach involves the conjugation of pre-formed block copolymers bearing a single azido group to a series of monodisperse dendrimers bearing differing numbers (e.g. 4, 8, 16, etc.) of alkyne groups. The versatility of this approach enables the incorporation of polymers that are either biocompatible or biodegradable. A similar modular approach can be employed for the grafting of amphiphilic polymers to well-defined, functionalized silica nanoparticles in order to prepare a library of AGNs. The collaboration has experience with both the “graft to” (Savin) and “graft from” (Grayson) approach and will investigate both routes to determine the most efficient route towards preparing AGNs with superior dispersant properties.
The effectiveness of both classes of amphiphiles in encapsulated petroleum and stabilizing their emulsions can be measured via traditional “static” means (e.g. UV-Vis absorption of encapsulated model dyes) but is best measured by dynamic means. Using unique instrumentation developed by Reed, the kinetics and long term stability of oil absorption/emulsion formation can be probed. More importantly, the breadth of detection capabilities of these instruments enables such measurements to be made under actual remediation conditions, for example using crude oil in sea water.
Finally, and most importantly, while the well-defined architectures of the ASPs and AGNs are essential for rapidly isolating and understanding the critical structural parameters for optimal use as dispersants, their synthetic costs must be negligible to be commercially viable. Both of these platforms were selected because exceptionally low cost analogs (though with increased size dispersity) are readily available. Alternative cores include hyperbranched polymers, which can be prepared in one synthetic step, and silica clays which are available in bulk from natural and synthetic sources. Using the knowledge gained from the ASP/AGN studies (e.g. optimal cores sizes, polymer block ratios, etc.) a focused library of inexpensive amphiphilic materials will be prepared. Successful completion of the proposed research will identify a set of novel, commercially viable surfactants that exhibit both concentration-independence and exceptional biocompatibility.