Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Together these techniques are expected to have the required surface sensitivity and molecular
specificity to explore, in fundamentally new ways, the heterogeneous chemistry of hydrocarbons at
aqueous interfaces.
Future Plans
Quantifying the link between molecular structure and surface reactivity
Experiments will be designed to isolate how molecular structure of a hydrocarbon radical controls its
spatial distribution and overall reactivity at an aqueous interface. The general approach can be
illustrated by a simple example of pentanol (C5H12O), of which there are eight structural isomers. The
–OH group in each isomer is located at different points along the carbon backbone yielding a
collection of molecules (straight vs. branched) with a distinct distribution of reactive sites (i.e. the
number of primary, secondary, and tertiary carbon atoms in each isomer). Furthermore, the location of
the hydrophilic -OH group (terminal vs. branched) in each isomer may play a critical role in
controlling the orientation and distribution of each isomer at the aqueous interface. These factors
might enhance surface reactivity, stabilize unique transition states and lead to novel product formation
pathways for certain isomeric structures that would not be observed in the gas phase. This approach
will be used for a variety of hydrocarbon structures to isolate how the interfacial reactivity of
hydrocarbon radicals depend upon the kinds and number of functional groups (e.g. aliphatic vs.
aromatic), carbon number, molecular weight, etc.
Interfacial molecular weight growth and oxidative degradation reaction pathways
Determining the rate at which a surface active hydrocarbon molecule is chemically transformed to
either become solvated in the bulk solution or broken down into small gas phase species will be a key
objective of future work. This will be done through careful measurements of how the rate of
heterogeneous oxygen addition (molecular weight growth) competes with surface reaction channels
that lead to carbon and/or oxygen loss (decomposition) from the interface. Such measurements would
be key steps in understanding and predicting interfacial reaction pathways to identify the atmospheric
and terrestrial sinks of hydrocarbons emitted by energy production and use.
Surface solvation structure and the interfacial reactivity of hydrocarbon radicals
The addition of solute molecules (e.g. NaI) has been previously shown to modify the solvation
structure of the liquid water surface, through the preferential adsorption of large polarizable anions to
the interface. A key question to be addressed is: how does the presence of inorganic solutes modify
the reactivity of a hydrocarbon radical or ion at an interface? One might expect that anions or cations
at or near the liquid water interface might lead to changes in the interfacial distribution and orientation
of a hydrocarbon at the surface. Such changes might play important roles in altering the kinds and
availability of hydrocarbon reactive sites at the interface.
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