4 research outputs found
New Information on the Hydrophobic Interaction Revealed by Frequency Modulation AFM
Using
ultrahigh resolution atomic force microscopy (AFM) operated
in frequency modulation mode, we extend existing measurements of the
force acting between hydrophobic surfaces immersed in water in three
essential ways. (1) The measurement range, which was previously limited
to distances longer than 2–3 nm, is extended to cover all distances,
down to contact. The measurements disclose that the long-range attraction
observed also by conventional techniques, turns at distances shorter
than 1–2 nm into pronounced repulsion. (2) Simultaneous measurements
of the dissipative component of the tip–surface interaction
reveal an anomalously large dissipation commencing abruptly at the
point where attraction begins. The dissipation is more than 2 orders
of magnitude larger than expected from bulk water viscosity or from
similar measurements between hydrophilic surfaces. (3) The short-range
repulsion is oscillatory, indicating molecular ordering of the medium
as the hydrophobic surfaces approach each other. The oscillation period,
∼0.5 nm, is larger than the ∼0.3 nm period observed
with hydrophilic surfaces. Their range, ∼1.5 nm, is longer
as well. These observations are consistent with a conspicuous change
in the properties of the surrounding medium, taking place simultaneously
with the onset of attraction as the two surfaces approach each other
Three-Dimensional Characterization of Layers of Condensed Gas Molecules Forming Universally on Hydrophobic Surfaces
Understanding the
solvation layer of hydrophobic surfaces is essential
for elucidating the interaction between hydrophobic surfaces in aqueous
solutions. Despite their importance, little is known on these layers
due to the lack of lateral resolution in spectroscopic or scattering
experiments and probe instability in the static scanning probe methods
used in most experiments. Using a high-resolution FM-AFM with stiff
cantilevers and hydrophilic tips, we overcome this instability to
provide the first detailed 3d maps of the solvation/hydration layer
of two archetypal hydrophobic surfaces: graphite (HOPG) and self-assembled
fluoro-alkane monolayer (FDTS). In degassed solutions we find different
tip–surface interactions for the two surfaces; hydration oscillations
superimposed on van der Waals attraction with HOPG and electrostatic
repulsion with FDTS. Both are similar to interactions observed with
hydrophilic surfaces. In solutions equilibrated with atmospheric air
or high-pressure nitrogen, the tip–surface interaction changes
dramatically, disclosing the formation of a 2–5 nm thick layer
of condensed gas molecules adsorbed to the hydrophobic surfaces. This
layer leads to strikingly similar tip–surface interactions
for HOPG and FDTS with only weak dependence upon the concentration
of dissolved gas molecules, indicating universality in the way hydrophobic
surfaces present themselves to nondegassed aqueous solutions. Measurements
at low cantilever oscillation amplitudes reveal the inner structure
of the layer of condensed gas molecules with an average distance between
its constituents, 0.5–0.8 nm, agreeing with recent molecular
dynamics calculations. In addition to the uniform condensed layers,
we probe sparse nanobubbles found on the surface. Those show distinct
interaction with the tip, different from that with the flat layer
Regulation of Surface Charge by Biological Osmolytes
Osmolytes, small
molecules synthesized by all organisms, play a
crucial role in tuning protein stability and function under variable
external conditions. Despite their electrical neutrality, osmolyte
action is entwined with that of cellular salts and protons in a mechanism
only partially understood. To elucidate this mechanism, we utilize
an ultrahigh-resolution frequency modulation-AFM for measuring the
effect of two biological osmolytes, urea and glycerol, on the surface
charge of silica, an archetype protic surface with a p<i>K</i> value similar to that of acidic amino acids. We find that addition
of urea, a known protein destabilizer, enhances silica’s surface
charge by more than 50%, an effect equivalent to a 4-unit increase
of pH. Conversely, addition of glycerol, a protein stabilizer, practically
neutralizes the silica surface, an effect equivalent to 2-units’
reduction of pH. Simultaneous measurements of the interfacial liquid
viscosity indicate that urea accumulates extensively near the silica
surface, while glycerol depletes there. Comparison between the measured
surface charge and Gouy–Chapman–Stern model for the
silica surface shows that the modification of surface charge is 4
times too large to be explained by the change in dielectric constant
upon addition of urea or glycerol. The model hence leads to the conclusion
that surface charge is chiefly governed by the effect of osmolytes
on the surface reaction constants, namely, on silanol deprotonation
and on cation binding. These findings highlight the unexpectedly large
effect that neutral osmolytes may have on surface charging and Coulomb
interactions
Mechanism of Ultrafast Triplet Exciton Formation in Single Cocrystals of π‑Stacked Electron Donors and Acceptors
Ultrafast
triplet formation in donor–acceptor (D–A)
systems typically occurs by spin–orbit charge-transfer intersystem
crossing (SOCT-ISC), which requires a significant orbital angular
momentum change and is thus usually observed when the adjacent π
systems of D and A are orthogonal; however, the results presented
here show that subnanosecond triplet formation occurs in a series
of D–A cocrystals that form one-dimensional cofacial π
stacks. Using ultrafast transient absorption microscopy, photoexcitation
of D–A single cocrystals, where D is coronene (Cor) or pyrene
(Pyr) and A is N,N-bis(3′-pentyl)-perylene-3,4:9,10-bis(dicarboximide)
(C5PDI) or naphthalene-1,4:5,8-tetracarboxydianhydride
(NDA), results in formation of the charge transfer (CT) excitons Cor•+-C5PDI•–, Pyr•+-C5PDI•–, Cor•+-NDA•–, and Pyr•+-NDA•– in <300 fs, while triplet exciton
formation occurs in Ï„ = 125, 106, 484, and 958 ps, respectively.
TDDFT calculations show that the SOCT-ISC rates correlate with charge
delocalization in the CT exciton state. In addition, time-resolved
EPR spectroscopy shows that Cor•+-C5PDI•– and Pyr•+-C5PDI•– recombine to form localized 3*C5PDI excitons with zero-field splittings of |D| = 1170 and 1250 MHz, respectively. In contrast, Cor•+-NDA•– and Pyr•+-NDA•– give triplet excitons in which |D| is only 1240 and 690 MHz, respectively, compared to that of NDA
(2091 MHz), which is the lowest energy localized triplet exciton,
indicating that the Cor-NDA and Pyr-NDA triplet excitons have significant
CT character. These results show that charge delocalization in CT
excitons impacts both ultrafast triplet formation as well as the CT
character of the resultant triplet states