jp5b03208_si_001.pdf (533.11 kB)
Adsorption and Aggregation at Silica/Methanol Interfaces: The Role of Solute Structure
journal contribution
posted on 2015-06-25, 00:00 authored by B. Lauren Woods, Jenna K. George, Alex M. Sherman, Patrik
R. Callis, Robert A. WalkerSecond harmonic generation (SHG)
and time-resolved, total internal
reflection fluorescence (TR-TIRF) spectroscopy were used to examine
the adsorption, solvation, and aggregation of a coumarin solute, Coumarin
152 (C152), at the silica/methanol interface. Experiments were performed
as a function of bulk C152 concentration with SHG data providing information
about relative surface coverage and ground state solvation environment.
TR-TIRF data measured emission lifetimes of C152 adsorbed to the silica/methanol
interface. SHG spectra show strong resonance enhancement at 365 nm,
a result that is blue-shifted considerably from C152’s electronic
excitation of ∼400 nm in bulk methanol. Given C152’s
solvatochromic behavior, this observation implies that C152 adsorbed
to the silica/methanol interface experiences a local dielectric environment
that is significantly less polar than in bulk methanol. TR-TIRF decays
at sub-micromolar bulk concentrations were fit to two lifetimes: one
assigned to C152 emission in bulk methanol (0.9 ns) and a longer lifetime
assigned to contributions from adsorbed C152 (∼4 ns). The longer
lifetime is similar to C152 in alkanes, a result that is consistent
with SHG data. Isothermal data from SHG experiments show unusual behavior
as bulk C152 concentration increases. Instead of approaching an asymptotic
limit signifying monolayer coverage, the SHG response rises at the
lowest C152 concentrations and then decreases dramatically, suggesting
the onset of aggregate formation. Changes in the TR-TIRF emission
behavior of C152 at higher C152 bulk concentrations support this hypothesis.
These findings are interpreted in terms of C152’s ability to
self-associate, and the energetics of dimer formation are explored
using ab initio calculations and polarizable continuum
models.