Vibrational sum frequency
generation (SFG) spectroscopy was utilized
to distinguish different populations of water molecules within the
electric double layer (EDL) at the silica/water interface. By systematically
varying the electrolyte concentration, surface deprotonation, and
SFG polarization combinations, we provide evidence of two regions
of water molecules that have distinct pH-dependent behavior when the
Stern layer is present (with onset between 10 and 100 mM NaCl). For
example, water molecules near the surface in the Stern layer can be
probed by the pss polarization combination, while other polarization
combinations (ssp and ppp) predominantly probe water molecules further
from the surface in the diffuse part of the electrical double layer.
For the water molecules adjacent to the surface within the Stern layer,
upon increasing the pH from the point-of-zero charge of silica (pH
∼2) to higher values (pH ∼12), we observe an increase
in alignment consistent with a more negative surface with increasing
pH. In contrast, water molecules further from the surface appear to
exhibit a net flip in orientation upon increasing the pH over the
same range, which we attribute to the presence of the Stern layer
and possible overcharging of the EDL at lower pH. The opposing pH-dependent
behavior of water in these two regions sheds new light on our understanding
of the water structure within the EDL at high salt concentrations
when the Stern layer is present.
Photoacoustic spectroscopy is a standout technique widely used for absorption measurements of atmospheric aerosols. Here we investigate the relative humidity dependence of photoacoustics and its implication for evaporation kinetics.
Molecular dynamics simulations have been used to study
the structure
of water molecules adjacent to solid hydrophobic and hydrophilic surfaces.
The hydrophobic surfaces resemble self-assembled monolayers with methyl
termination, whereas the hydrophilic surfaces are terminated with
hydroxyl groups. The resulting water structure is characterized by
its density profile, order parameters, and molecular tilt-twist distribution
as a function of distance from the surface. In both cases, results
are compared to those obtained in bulk water and also to the vapor–water
interface. To make a deeper connection to experimental studies, we
have applied a frequency-domain approach to calculate the nonlinear
vibrational spectra of the O–H stretching response. We have
observed that, despite the sharp atomic discontinuity imposed by the
surface, water next to a hydrophobic surface is similar in structure
and spectral response to what is observed for the more diffuse vapor–water
interface. At the hydrophilic surface, water ordering persists for
a greater distance from the surface, and therefore the spectral response
accumulates over a greater depth. In the strongly hydrogen bonded
side of the spectrum, this is seen as an increased nonlinear susceptibility.
However, in the energy region of the uncoupled O–H oscillators
we demonstrate that the low experimental signal is likely not due
to an absence of those species but instead a net cancellation of the
microscopic response due to opposing water orientations over a distance
well within the experimental coherence length.
Homo- and heterospectral correlation analysis are powerful methods for investigating the effects of external influences on the spectra acquired using distinct and complementary techniques. Nonlinear vibrational spectroscopy is a selective and sensitive probe of surface structure changes, as bulk molecules are excluded on the basis of symmetry. However, as a result of this exquisite specificity, it is blind to changes that may be occurring in the solution. We demonstrate that correlation analysis between surface-specific techniques and bulk probes such as infrared absorption or Raman scattering may be used to reveal additional details of the adsorption process. Using the adsorption of water and ethanol binary mixtures as an example, we illustrate that this provides support for a competitive binding model and adds new insight into a dimer-to-bilayer transition proposed from previous experiments and simulations.
The mass accommodation
coefficient, α
M, describes evaporation
and condensation kinetics at the liquid–vapor
interface. In spite of numerous experimental efforts, reliable values
of α
M are still not available for
many substances. Here, we present a novel experimental technique,
photothermal single-particle spectroscopy (PSPS), that allows for
a robust retrieval of mass accommodation coefficients from three simultaneous
independent measurements. PSPS combines resonant photoacoustic absorption
spectroscopy with modulated Mie scattering measurements on single
particles. We study the mass transport of water on organic aerosol
droplets that are optically trapped using counter-propagating tweezers.
We find the mass accommodation coefficient of water on a pure model
organic that is fully miscible with water to be 0.021 at 296 K and
to decrease by more than an order of magnitude when the temperature
increases to 309 K. The experimentally observed temperature dependence
of α
M shows an Arrhenius behavior.
Furthermore, the water content of the droplets is found to have a
profound effect on α
M. No concentration
dependence of α
M is observed at
low water concentrations, while at elevated water concentrations,
we observe a 5-fold increase in α
M. The technique presented in this work has the potential to become
a reliable method for the retrieval of α
M values at liquid–vapor interfaces, which are essential
for accurate global climate and pharmaceutical aerosol inhalation
modeling, to mention but a few.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.