60th Annual Report on Research 2015

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Reports: DNI554763-DNI5: Direct Structural Characterization of Individual Asphaltenes Using Scanning Probe Microscopy and Interface-Sensitive Techniques

Scanning probe imaging of asphaltenes: In this project period, we began to establish sample preparation methods for structural analysis of individual asphaltenes utilizing scanning probe methods and surface-sensitive techniques. A key consideration in the sample preparation is the goal of creating monolayers of well-separated asphaltenes for analysis by multi-modal scanning tunneling microscopy. In other studies of amphiphiles that can form large aggregates at interfaces, we have previously observed that drop-casting dilute samples (typically ~0.015 mg/mL from organic solvents) onto hot substrates (typically ~100 °C, above the boiling point of the solvent) provides adequate kinetic energy to distribute the molecules across the substrate with areas of monolayer thickness in addition to some areas of multilayers. In AFM studies of asphaltenes from a Maya (Mexico) source provided by ConocoPhillips through collaborator Hilkka Kenttamaa, we observed that this strategy did distribute asphaltenes across the substrate (Figure 1), but at molecular concentrations (0.01 mg/mL) adequate to produce large areas of monolayer coverage for other large amphiphiles, large aggregates were still observed for samples drop cast from pyridine onto HOPG substrates with an initial temperature of 120 °C. While spaces between large aggregates (>100 nm in diameter, >10 nm high) were adequate for STM imaging, all areas between large aggregates still contained smaller aggregates (apparent diameter > 20 nm, > 5 nm high), which are still large enough to cause tip fouling during STM imaging.  
Reducing sample concentration to 0.005 mg/mL and 0.001 mg/mL produced surfaces free of the very large aggregates observed at higher concentrations, with some 500 nm x 500 nm areas containing surface topography variations less than 3 nm. (Figure 2). While in principle these are adequately small variations, STM imaging of these sample still proved challenging, presumably due to the low ordering of the interface and relatively weak interactions with the substrate.  
Therefore, we investigated methods for increasing ordering and the strength of molecule–substrate interactions. Initially, we examined the possibility of etching single-atom-deep pits in the HOPG surface using oxygen plasma, a strategy referred to as creating ‘molecular corrals’. In practice, this strategy created unacceptably large variations in surface chemistry and topography that would have been indistinguishable from the asphaltene chemistry we were investigating, so we instead created surfaces templated with ordered patterns of hydrophilic and hydrophobic chemistry with controllable pitch between the lines of hydrophilic (or ionic) groups. AFM images (Figure 3) show the ordered structures; in the image shown, the pitch between lines of hydrophilic groups is 6.2 ± 0.1 nm, and the topographic protrusions created by the hydrophilic ridges are ~0.1 nm. Such arrays can be created in a straightforward manner from long-chain diynoic acids, which are commercially available with chain lengths between 20 and 30 carbons. Diynoic acids undergo surface-templated photopolymerization when exposed to UV radiation, creating a conjugated ene-yne polymer that stabilizes the template.  
The carboxylic acid head groups and ene-yne then appear as features in STM images (Figure 4), providing an internal standard for such films after deposition of asphaltenes. In the second project period, we will use these surfaces to order asphaltene samples for additional structural characterization.  
Impact on the PI and supported student careers: The PI is developing a research program that seeks both to understand and to control amphiphilic chemistries at interfaces. Questions provoked by the aggregation of asphaltenes at concentrations well below the critical micelle concentration, and the need to control interactions with an interface for structural characterization have led to development of new amphiphilic interface chemistries that will take the group in a variety of new directions. Through the proposed work, the PIs group has begun a collaboration with prominent asphaltene researcher Hilkka Kenttamaa, which has also been growing to address multiple classes of amphiphiles. In the past year, the PI led a successful NSF Major Research Instrumentation proposal for a multi-modal atomic force microscope that will provide complementary structural characterization information. In particular, the new AFM will provide the capability to perform high-speed imaging in liquids with sub-nanometer resolution, while controlling temperature. This will be helpful in establishing interfacial binding characteristics of asphaltenes leading up to structural studies. In the past year, the PI and the supported student Jae Jin Bang authored an invited review on multi-modal scanning probe methods for an Emerging Investigators issue of the journal Analytical Methods. For the student supported in the past year, Jae Jin Bang, protected research time provided by the grant support has enabled her to make three key contributions. First, she has established extensive expertise in both AFM and STM imaging in her work with asphaltene and model asphaltene samples. While the AFM we use is a commercial instrument, Ms. Bang has also participated in the optimization of a microwave-modulated STM scanning head, to maximize signal transmission to the tunneling junction. This was important in her second contribution: early this year she was first author on the invited review on multi-modal scanning probe techniques published in Analytical Methods. Ms. Bang has a remarkable grasp of the field for an early-career graduate student, and the ability to make an early contribution to the literature will be helpful to her career trajectory. Finally, due to the work she performed in amphiphilic surface templating to stabilize asphaltenes, she will also be co-first author on amphiphilic interface templating to be submitted in the next week.  Support through this grant is enabling Ms. Bang to maximize her early research productivity, which will increase her competitiveness in establishing a STEM research career.