58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Development of Surface Test Platforms. We are working to develop protocols with scanning probe microscopy that enable visualization of hydrocarbon-surfactant interactions. Our first year has focused on developing surface test platforms which present an upright configuration of hydrocarbon chains to enable 3D visualization of molecule-surfactant interactions. Nanofabrication using particle lithography and vapor deposition of organosilanes was used to reproducibly manufacture model test platforms of hydrocarbons with well-defined composition, orientation and dimensions. When hydrocarbon nanostructures are immersed in liquid media, the dimensions and surface properties of the interface are known to change, and atomic force microscopy (AFM) experiments enable us to visualize these changes at the nanoscale. We are developing experiments to study the influence of pH, solvents, and surfactants for producing surface changes with model surfaces of designed test structures of hydrocarbon chains.

Particle lithography with organosilanes provides capabilities for surface patterning to generate ultra-small test structures. To prepare ring patterns of organosilanes shown in Figure 1, close-packed arrays of silica or latex mesospheres were used as surface masks. Vapor-phase silanes attached to the surface where water residues are present to produce periodic surface structures. When the mesosphere masks were removed, ring patterns of organosilanes with upright hydrocarbons are revealed. The patterned silane SAMs will provide ideal test

Figure 1. Test structures of octadecyltrichlorosiloxane ring patterns imaged with contact-mode AFM in different ambient environments.

structures for in situ studies of hydrocarbon-surfactant interactions, in which the density of surface patterns is tuned by selecting different size latex masks.

Our strategies for studies of surfactants are based on using liquid imaging with scanning probe microscopy to study designed reactions between surfactants and model platforms of surface-bound hydrocarbons. The experiments will enable us to acquire information of the interactions between surfactants and designed patterns of long-chain hydrocarbons. The changes observed for test structures of octadecyltrichlorosiloxane (OTS) in different environments: air, ethanol and water are shown side-by-side in Figure 1 for the same sample and probe. Depending on the liquid media, the hydrocarbon chains of the multilayered areas of the OTS films were observed to have different configurations and orientations. In air, the hydrocarbons form compact structures, as shown in Figure 2. In liquid media, the molecular backbones stretch out and unfold, as solvent molecules permeate between hydrocarbon strands to cause the structures to swell in size.

Figure 2. Model of the changes in hydrocarbon backbones in air versus in liquid media (not to scale).

Visualization of Surfactant-Surface Binding. The next experimental tasks for this PRF award will be to use AFM experiments to directly visualize interactions and changes of surface-bound hydrocarbons that are immersed in dilute solutions of selected surfactants, as a model for petroleum-based formulations. Trace amounts of surfactants are known to dramatically change the properties of lubricants, petroleum-based fluids and oils. Surfactants are widely used in petroleum products as anti-foam agents, for detergency, and in oil reclamation processes. Fundamental understanding of the mechanisms of how surfactants interact with hydrocarbon chains or endgroups is mainly surmised by theoretical models and macroscopic measurements of properties such as viscosity or detergency. The questions to be addressed with scanning probe studies will address the specific nature of molecular associations between surfactants and the hydrocarbon chains of our test platforms of 3D hydrocarbon surface structures. Will surfactants intercalate between hydrocarbon chains, bind to the surface of the chains, or primarily associate with the methyl terminal groups of hydrocarbons? The effects of adding surfactants to lubricants are mainly understood from a macroscopic perspective through empirical approaches based on formulations. However, the interactions of designed surfactants with hydrocarbon-coated surfaces have not been well-studied at the molecular-level using time-lapse AFM imaging. Whether surfactants operate by encapsulation of molecules, lateral side-chain interactions, or primarily through binding with methyl headgroups of alkane chains will be visualized directly with high resolution AFM. We predict that the nature of the surfactant (anionic, cationic, nonionic, etc.) will dictate the mode of association (adsorption, intercalation, binding) and will mediate differences in interaction mechanisms.

Educational Impact - Mentoring and Career Development. Thus far, five female graduate students (including an underrepresented minority student) have received graduate fellowships from this PRF New Directions award. One of the students will complete her PhD defense for December 2013 graduation, one student graduated in August of 2013, and the other two senior students will graduate in 2014. Funds from this doctoral new investigator grant have been quite valuable for advancing the PI's research program at LSU, and for providing premiere opportunities for training graduate and undergraduate students who are learning scanning probe techniques and learning to design experiments to investigate the chemistry of surfaces. The funds appropriated for travel were used to sponsor student poster presentations at nearby regional and national conferences of the American Chemical Society. Currently, the PI's group includes six undergraduate researchers who contribute 3-6 hours each week to obtain research credit. Five of the undergraduates are from underrepresented groups, and the sixth is a Caucasian female. Fellowship support for graduate students from the ACS PRF New Directions program has bolstered the capacity of my laboratory for sponsoring undergraduate chemistry majors for research opportunities, which is critical to retention of students for the future pipeline of career chemists.