5 research outputs found
Influence of High pH on the Organization of Acetonitrile at the Silica/Water Interface Studied by Sum Frequency Generation Spectroscopy
The
acetonitrile–water mixture is one of the most commonly
used solvents in hydrophilic interaction chromatography, which contains
silica as the solid phase. As such, the silica/acetonitrile–water
interface plays a large role in the separation of compounds. Varying
the pH is one way to influence retention times, particularly of ionizable
solutes, yet the influence of high pH is often unpredictable. To determine
how the structure of this interface changes with pH, we utilized the
surface specific technique sum frequency generation (SFG). Previous
SFG studies at neutral pH have suggested the existence of acetonitrile
bilayers at the aqueous silica interface even at low acetonitrile
mole fractions. Here we find that the SFG signal from 2900 to 3040
cm<sup>–1</sup> at the silica/acetonitrile–water interface
increased as we adjusted the aqueous pH from near neutral to high
values. This increase in signal was attributed to a greater amount
of aligned water which is consistent with an increase in silica surface
charge at high pH. In contrast, complementary measurements of the
silica/acetonitrile–deuterium oxide interface revealed that
the acetonitrile methyl mode nearly vanished as the aqueous pH was
increased. This loss of methyl mode signal is indicative of a decrease
in the number density of acetonitrile molecules at the interface,
as orientation analysis indicates no significant change in the net
orientation of the outer leaflet of the acetonitrile bilayer over
the pH range studied
Probing Silica–Kaolinite Interactions with Sum Frequency Generation Spectroscopy
Treating the oil sands tailings ponds is a major challenge
because
of the vast amounts of tailings and the need for a reliable treatment
technique for releasing water and generating the highly consolidated
material required for land reclamation. Treatment with chemicals such
as lime (calcium (hydr)oxide) is a promising technology for tailings
dewatering and consolidation, particularly at higher pH. Given that
kaolinite and silica minerals are the main constituents of many oil
sands, we have investigated the influence of lime and NaOH addition
on the silica/aqueous kaolinite interface over the pH range 7.4–12.4
using vibrational sum frequency generation spectroscopy (SFG). With
lime addition, at pH 12.0 and above we observe a complete disappearance
of the vibrational features of the interfacial water molecules for
planar silica in contact with an aqueous dispersion of kaolinite particles.
A concurrent increase in the amount of adsorbed kaolinite on the silica
surface at pH 12.0 and above is observed, shown in the increased intensity
of the kaolinite SFG peak at 3694 cm–1. This suggests
that the absence of water features in the SFG spectra is associated
with conditions that facilitate dewatering. With NaOH addition, however,
the interfacial water SF intensity is still significant even under
highly alkaline conditions despite the increase in adsorbed kaolinite
at high pH. To better understand the SFG observations and get a deeper
insight into the chemistry of the silica/aqueous kaolinite interface,
we measure the ζ-potential on the planar silica/aqueous interface
and kaolinite aqueous dispersions under the same pH conditions with
NaOH and lime addition
Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities
The
electric double layer governs the processes of all charged
surfaces in aqueous solutions; however, elucidating the structure
of the water molecules is challenging for even the most advanced spectroscopic
techniques. Here, we present the individual Stern layer and diffuse
layer OH stretching spectra at the silica/water interface in the presence
of NaCl over a wide pH range using a combination of vibrational sum
frequency generation spectroscopy, heterodyned second harmonic generation,
and streaming potential measurements. We find that the Stern layer
water molecules and diffuse layer water molecules respond differently
to pH changes: unlike the diffuse layer, whose water molecules remain
net-oriented in one direction, water molecules in the Stern layer
flip their net orientation as the solution pH is reduced from basic
to acidic. We obtain an experimental estimate of the non-Gouy–Chapman
(Stern) potential contribution to the total potential drop across
the insulator/electrolyte interface and discuss it in the context
of dipolar, quadrupolar, and higher order potential contributions
that vary with the observed changes in the net orientation of water
in the Stern layer. Our findings show that a purely Gouy–Chapman
(Stern) view is insufficient to accurately describe the electrical
double layer of aqueous interfaces
pH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer
Specific ion effects
(SIEs) are known to influence the acid/base
behavior of silica and the interfacial structure of water, yet evidence
of the effect of pH on SIEs is lacking. Here broadband vibrational
sum frequency generation (SFG) spectroscopy was used to study SIEs
on the water structure at the electrical double layer (EDL) of silica
as a function of pH and monovalent cation identity from pH 2–12
at 0.5 M salt concentration. SFG results indicate a direct Hofmeister
series of cation adsorption at pH 8 (Li<sup>+</sup> < Na<sup>+</sup> < K<sup>+</sup> < Cs<sup>+</sup>), with an inversion in this
series occurring at pH > 10. In addition, an inversion in SFG intensity
trends also occurred at pH < 6, which was attributed to contributions
from asymmetric cation hydration and EDL overcharging. The highly
pH-dependent SIEs for silica/water have implications for EDL models
that often assume pH-independent parameters
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