3 research outputs found
Simulated Structure and Nonlinear Vibrational Spectra of Water Next to Hydrophobic and Hydrophilic Solid Surfaces
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
Surface–Bulk Vibrational Correlation Spectroscopy
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
Separating the pH-Dependent Behavior of Water in the Stern and Diffuse Layers with Varying Salt Concentration
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