Interfacial Water Structure of Binary Liquid Mixtures
Reflects Nonideal Behavior
The evaporation of molecules from water–organic solute binary mixtures is key for both atmospheric and industrial processes such as aerosol formation and distillation. Deviations from ideal evaporation energetics can be assigned to intermolecular interactions in solution, yet evaporation occurs from the interface, and the poorly understood interfacial, rather than the bulk, structure of binary mixtures affects evaporation kinetics. Here we determine the interfacial structure of nonideal binary mixtures of water with methanol, ethanol, and formic acid, by combining surface-specific vibrational spectroscopy with molecular dynamics simulations. We find that the free, dangling OH groups at the interfaces of these differently behaving nonideal mixtures are essentially indistinguishable. In contrast, the ordering of hydrogen-bonded interfacial water molecules differs substantially at these three interfaces. Specifically, the interfacial water molecules become more disordered (ordered) in mixtures with methanol and ethanol (formic acid), showing higher (lower) vapor pressure than that predicted by Raoult’s law.
Special Issue<!>Introduction<!><!>Introduction<!>Sample
Preparation<!>SFG
Measurements<!>MD Simulation Protocols<!>Variations of the C–H
and Free O–H
Stretching Modes at the Air–Binary Mixture Interfaces<!><!>Variations of the C–H
and Free O–H
Stretching Modes at the Air–Binary Mixture Interfaces<!>Orientation of the Interfacial
Water Molecules
Viewed through the Water Bending Mode<!><!>Orientation of the Interfacial
Water Molecules
Viewed through the Water Bending Mode<!><!>Orientation of the Interfacial
Water Molecules
Viewed through the Water Bending Mode<!>Conclusion<!><!>Author Contributions<!>
<p>Published as part
of The Journal of Physical Chemistry virtual special
issue "Dor Ben-Amotz Festschrift".</p><!><p>Binary liquid mixtures typically deviate from ideal mixing in a
thermodynamic context.1,2 In the bulk liquid phase, such
nonideal interactions can be trivially understood, as they only require,
in a mixture of A and B species, that the interactions between A and
B differ from A–A and B–B interactions. A textbook example
for nonideal mixtures where A–B interactions are stronger than
A–A and B–B interactions in water–formic acid
mixtures.3 As a result, the vapor pressure
of water–formic acid mixtures is markedly lower than that predicted
by Raoult's law. Conversely, water–alcohol mixtures
typically show excess vapor pressures.4 These excess interactions impact the energetics of the evaporation,
which is directly relevant to distillation and purification processes
in industrial processes and also to atmospheric processes.5−11</p><p>While these energetics have been traced to the intermolecular
interactions
in the bulk, evaporation takes place at the interface and molecular
interactions12 and the abundancy of the
molecules at the interface are decisive factors determining evaporation
rates. The activation barrier for the evaporation process and thus
evaporation rate of an ideal A–B liquid binary mixture vary
linearly with the concentration of A and B species, when A–A,
A–B, and B–B interactions are identical, as defined
in Raoult's law. However, also the evaporation kinetics of
real, nonideal mixtures show deviations from this simple rule.13−15 For example, the evaporation rate kH2O–Ethanol(xH2O) of the water–ethanol mixture at a water concentration of xH2O is higher than the rate predicted
from the linear combination of the rates of pure water (kH2O) and pure ethanol (kEthanol): kH2O–Ethanol(xH2O) > xH2OkH2O + (1 – xH2O)kEthanol, where xH2O denotes the mole fraction of water. In turn, the water–formic
acid mixture shows the opposite trend (Figure 1a, see the Supporting Information for more details). Although these differences in
the evaporation rates largely reflect the evaporation energetics (i.e.,
the respective vapor pressures as shown in Figure 1b),16 the impact
of the different intermolecular interactions in the bulk liquid on
the molecular conformation and ordering at the interface of these
binary mixtures—highly relevant to evaporation kinetics—has
been poorly understood. Figure 1 illustrates that the nonideal behavior is complex: not only
do the mixtures exhibit different deviations from ideality, but there
is also a strong asymmetry with respect to the molar composition.
For instance, for the water–ethanol mixture, the maximum deviation
of the vapor pressure occurs at xH2O = 0.85, while the excess evaporation rate peaks at lower
values of xH2O. The following
question presents itself: What is happening at the surface of the
liquid mixture?</p><!><p>(a) Relative evaporation rate obtained from aqueous binary
mixtures
of methanol, ethanol, and formic acid vs water mole fraction, xH2O. The relative evaporation rate
was calculated via Δk(xH2O) = kH2O–org(xH2O)/kpred(xH2O) –
1, where kH2O–org(xH2O) is the measured evaporation
rate of the binary mixture and kpred(xH2O) is the predicted evaporation
rate from the evaporation rate of pure water (kH2O) and pure organic species (korg): kpred(xH2O) = xH2OkH2O + (1 – xH2O)korg. The error bar indicates the 95% confidence interval of the measured
evaporation rate. (b) Relative vapor pressure for the binary mixtures
at 333 K. The data were calculated via Δp(xH2O) = pH2O–org(xH2O)/ppred(xH2O) – 1, where pH2O–org(xH2O) is
real vapor pressure3,4,29 and ppred(xH2O) is the predicted one from Raoult's law. See the Supporting Information for more details.</p><!><p>Here, we explore the molecular conformations of
aqueous binary
mixtures using surface-specific vibrational sum-frequency generation
(SFG) spectroscopy. This technique can uniquely probe molecules within
a few surface layers through their vibrational response.17,18 In fact, SFG spectroscopy has already been applied to probe the
interfacial structure of various binary liquid mixtures, including
water–methanol,19−23 water–ethanol,21,24,25 and water–formic acid26 mixtures
probing the O–H stretching mode. However, because these organic
species commonly contain O–H groups, SFG spectra in the O–H
stretching mode region cannot distinguish both molecular species individually.
This challenge can be circumvented by probing the water bending mode,
which allows disentangling the water contribution from other vibrational
modes such as the C–O–H bending mode of the organic
species.27,28 However, for aqueous alcohol or formic acid
mixtures, the water structure at the interface has not been isolated.</p><p>By combining SFG spectroscopy with molecular dynamics (MD) simulations,
we determine the interfacial structure and correlate the interfacial
structure with evaporation thermodynamics and kinetics for binary
mixtures of water with methanol/ethanol/formic acid. We found that
the variations of the C–H stretching mode of organic species
and the free O–H stretching mode of water upon the addition
of the organic species do not show a significant difference between
the aqueous binary mixtures of methanol, ethanol, and formic acid.
In contrast, the H–O–H bending mode of water in these
mixtures differs substantially. The spectra and MD simulation results
reveal that water's interfacial alignment becomes randomized
upon adding ethanol and methanol to water, while it becomes ordered
upon adding formic acid. Our results thus show that the nonideal interaction
in bulk is reflected in the structure of water at the interface, relevant
to the evaporation rates.</p><!><p>The mixtures
were prepared using methanol (HPLC, gradient grade, ≥99.8%
(GC), <0.01% water, VWR International), ethanol (Puriss., absolute,
≥99.8% (GC), Sigma-Aldrich), formic acid (Suprapur, 98–100%
(ACS), Merck KGaA), D2O (99.9 atom % D, Sigma-Aldrich),
and Milli-Q water (18.2 MÎ© cm). All substances were used without
further purification. To avoid the overlap between the C=O
stretching mode (∼1720 cm–1)26 and the H–O–H bending mode (∼1650–1750
cm–1),28 we measure the
H–O–D bending region for the water–formic acid
system. We prepared samples using H2O:D2O mixtures
and formic acid. We used 1:1 H2O:D2O mixtures
for samples with formic acid mole fractions of 0, 0.02, 0.08, 0.14,
and 0.32. 1:3 H2O:D2O mixtures and pure D2O were used for formic acid mole fractions of 0.53 and 0.66,
respectively. The determination of xHOD is given in the Supporting Information. The aqueous binary mixtures were contained in a trough with a diameter
of 5 cm for the SFG measurements.</p><!><p>The SFG measurements
for the free O–H stretching mode and H–O–H/H–O–D
bending mode of all of the aqueous binary mixtures as well as the
C–H stretching mode of water–methanol mixtures used
a femtosecond Ti:sapphire amplified laser system (Coherent Libra,
∼800 nm, ∼50 fs, 1 kHz) with 5 W output power. We used
2 W to pump an optical parametric amplifier (TOPAS, light conversion)
with a non-collinear difference frequency generation stage to generate
a broadband IR pulse. Another 1 W of the laser output was passed through
an etalon to generate a narrowband visible pulse (∼20 cm–1). The incident angles of the visible and IR beams
were 64 and 40° with respect to the surface normal, respectively.
The visible (20 Î¼J) and IR (1 Î¼J) pulses were overlapped
spatially and temporally at the sample position. Subsequently, the
generated SFG signal was dispersed in a spectrometer (Shamrock 303i,
Andor Technology) and detected by an EMCCD camera (Newton, Andor Technology).</p><p>For the C–H stretching mode of water–ethanol and
water–formic acid mixtures, the experiments were carried out
with a femtosecond Ti:sapphire amplified laser system (Spitfire Ace,
Spectra-Physics, ∼800 nm, ∼40 fs, 1 kHz) with 5 W output
power. The visible (13 Î¼J) and IR (5 Î¼J) incident angles
were 36 and 41°, respectively. The generated SFG pulse was subsequently
guided into a spectrograph (Acton SP 300i, Princeton Instruments)
and detected with an EMCCD (Newton, Andor Technology).</p><p>The C–H
stretching mode and the free O–H stretching
mode signals were recorded twice for 3 min at the air–aqueous
binary mixture interfaces. For water–alcohol mixtures, we measured
the H–O–H bending mode signals twice for 10–15
min at the air–solution interface. For water–formic
acid mixtures, the H–O–D bending mode was measured in
a similar manner to the H–O–H bending mode. To avoid
IR absorption of water vapor, we purged N2 for 10 min for
the measurements of the free O–H stretching mode and the H–O–H/H–O–D
bending mode. All spectra were collected in the ssp (denoting s-,
s-, and p-polarized SFG, visible, and IR, respectively) polarization
combination. The spectra for the air–binary mixture interfaces
were normalized to the non-resonant signal taken from z-cut quartz after subtracting a background spectrum.</p><!><p>We carried
out the MD simulation for the air–aqueous binary mixture interfaces
in the slab geometry. We used the GROMACS software for performing
the MD simulations.30 For the aqueous binary
mixture, we used the OPLS-AA force field model31 for methanol and ethanol, the modified OPLS-AA model for
formic acid, and the TIP4P/2005 model for water.32 The simulation cell was 26.6 Å × 26.6 Å
× 350 Å with the periodic boundary condition. We obtained
more than 100 ns trajectories, from which we computed the orientation
of water in aqueous binary mixtures. The details of the simulation
can be found in the Supporting Information.</p><p>We evaluated the orientations of the water molecules located
in the interfacial region. The interfacial region is defined as |z – zG| < a, where z is the z-coordinate
of the oxygen atom of a water molecule, zG is the position of the Gibbs dividing surface for the mixtures,
and a3 represents the average volume occupied
by a molecule in the liquid phase. Here, the z-axis
forms the surface normal.</p><!><p>We explore the interfacial organization of the organic components
at the air–aqueous binary mixture interfaces by probing the
C–H stretching mode ν(C–H) of methanol/ethanol/formic
acid with SFG spectroscopy. The measured SFG signal is determined
by the average molecular orientation and proportional to the number
of interfacial molecules contributing to the signal. Figure 2a displays the ν(C–H)
SFG spectra with various mixture concentrations. All of the data except
for pure H2O show clear C–H stretching mode peaks,
reflecting that the hydrophobic groups of the topmost methanol/ethanol/formic
acid molecules point out of the liquid phase. The SFG spectra at the
air–aqueous binary mixture of the methanol interface show peaks
at 2840 and 2960 cm–1 and a shoulder at 2930 cm–1, which are assigned to the symmetric C–H stretching
mode, the antisymmetric C–H stretching mode, and the Fermi
resonance of the C–H stretching and C–H bending modes,
respectively.33−35 Despite earlier assignments to the Fermi resonance,20 a recent study provides evidence for the ∼2960
cm–1 band to be due to the antisymmetric CH3 stretching mode.33 For the interface
of the water–ethanol mixtures, the 2876 cm–1 Fermi resonance and the 2924 cm–1 symmetric C–H
stretching mode of the CH3 group govern the SFG spectra
in the C–H range,36 while the 2970
cm–1 antisymmetric C–H stretching mode and
the C–H stretching mode of the CH2 groups are weak—in
line with a previous study.20 At the interface
of the aqueous binary mixture of formic acid with air, a single peak
is observed at 2930 cm–1, which arises from the
C–H stretching mode of formic acid.26 The frequency shift of the C–H stretching peak with varying
water concentration is attributed to cis-/trans-conformations of the
formic acid.37</p><!><p>(a) The C–H stretching
mode SFG spectra at the air–aqueous
binary mixture of methanol, ethanol, and formic acid interfaces. The
black lines represent the fits to the SFG spectra. (b) The variations
of the amplitudes for the C–H symmetric stretching mode peak
of methanol and ethanol CH3 group as well as for the C–H
stretching mode peak of formic acid together with the simulation data
of integrated ⟨N cos θ⟩ (solid lines). (c) The free O–H stretching mode SFG
spectra at the air–aqueous mixture interfaces. The black lines
represent the fits to the SFG spectra. (d) The variation of Aν(free O–H) with composition together with the simulation data of integrated
⟨N cos θ⟩ (solid
lines). Data for different mixtures in parts b and d are normalized
to the maximum amplitude and offset by increments of 1 for clarity.</p><!><p>To quantify the changes in the spectra, we fit
the data with a
Lorentzian line shape model, and the obtained fit curves are represented
by solid black lines in Figure 2a. We plot the inferred amplitude of the symmetric C–H
stretching mode of the CH3 group for methanol/ethanol and
the C–H stretching mode for formic acid (Aν(C–H)) in Figure 2b (see the Supporting Information). All mixtures have in
common that the C–H stretching mode contribution increases
steeply with decreasing xH2O from 1 to ∼0.7, while it starts to decrease when xH2O decreases below 0.7. The turnover
behavior of Aν(C–H) is consistent with previous studies,19,21,38−40 indicative
of randomization of the C–H groups,22 with decreasing water concentration in the binary mixtures.</p><p>We then investigated the structure of the topmost O–H groups.
We measure the free O–H stretching mode (v(free O–H)) SFG features at the air–aqueous binary
mixture interfaces, which allows quantifying the number of non-hydrogen-bonded,
free O–H groups at the interface. We note that these experiments
require purging with N2 to exclude water vapor, yet the
effect of purging on the C–H stretching mode is negligible
(Supporting Information) and our spectra
reflect the equilibrium interfacial composition. The measured spectra
displayed in Figure 2c show that the free O–H stretching peak is centered at 3700
cm–1 41 for all
mixtures and vanishes quickly upon the addition of methanol/ethanol/formic
acid. To quantify these trends, we determine the amplitudes of the
free O–H peak, Aν(free O–H) (Figure 2d and the Supporting Information). These amplitudes indicate that the free O–H
contribution decreases rapidly with decreasing xH2O and approaches zero already for xH2O = 0.85. Also, this observation is consistent
with previous studies.26,42</p><p>The variations of the C–H
and free O–H stretching
modes are consistent with MD simulation data. From the MD result,
we calculated the angle θ of the transition
dipole moment orientation of symmetric C–H stretching modes
of methanol and ethanol as well as the C–H stretching mode
of formic acid with respect to the surface normal. The MD simulations
also provide access to the number of (symmetric) C–H or O–H
stretching chromophores N. The SFG spectral amplitude
is approximately proportional to ⟨N cos θ⟩, allowing us to compare the simulation data
with the experimental data. Parts b and d of Figure 2 show ⟨N cos θ⟩ for, respectively, the symmetric C–H
stretching modes and the free O–H chromophores. The good agreement
between the simulation and experiment in Figure 2b and d supports the notion that the turnover
behavior of the variations of the C–H and steep decrease of
free O–H stretching contributions arise from the interfacial
structure change of the binary mixtures upon the addition of methanol/ethanol/formic
acid.22</p><p>The decrease in the free
O–H stretching mode contribution
together with the increase in the C–H stretching mode contribution
with decreasing xH2O suggests
that the addition of the organic solutes results in hydrogen-bond
formation of the organic solute to water's free O–H
groups already at very low solute concentrations in the aqueous binary
mixtures. Remarkably, the variations of the two molecular groups sticking
out of the interfaces, the C–H groups and the free O–H
group, behave very similarly for all three aqueous binary mixtures,
irrespective of their very different excess mixing properties and
the different excess evaporation rates for water–ethanol and
water–methanol mixtures compared to water–formic acid
mixtures. As such, based on these observations, the interfacial structures
as determined from the groups pointing toward air appear similar for
all three mixtures. However, the question of whether the different
mixing behavior is reflected in the hydrogen-bonded structure of water
at the interface remains.</p><!><p>To study interfacial
hydrogen-bonded water, one may consider using SFG spectroscopy to
probe the hydrogen-bonded O–H stretching mode region. However,
as noted in the Introduction, the SFG signal
from the O–H stretching modes of hydrogen-bonded water molecules
cannot be disentangled from the signal from the O–H groups
of methanol/ethanol/formic acid (see the Supporting Information). Moreover, vibrational couplings markedly alter
the line shape of the O–H stretching band.43 In contrast, the H–O–H bending mode signal
arises solely from water and contains no contributions from methanol/ethanol/formic
acid. What is more, the line shape of the water bending mode is rather
insensitive to intermolecular coupling,27,44 making it
ideal for probing interfacial water.45</p><p>In Figure 3a and b, we show the measured H–O–H
bending mode (δ(H–O–H)) spectra of H2O–methanol and H2O–ethanol mixtures, respectively.
The spectrum for the air–neat water interface (xH2O = 1) shows a dip at ∼1612 cm–1 and a broad bending mode band at 1650–1750
cm–1.28,46−48 These spectral features are assigned, respectively, to up-oriented
water molecules donating one hydrogen bond (the other O–H is
free) and to down-oriented water molecules donating two hydrogen bonds
(termed bend 1 and bend 2 in the Supporting Information). For mixtures with formic acid, we use the H–O–D
bending mode of H2O/D2O mixtures rather than
the H–O–H bending mode because the C=O stretching
mode (∼1720 cm–1)26 partly overlaps and interferes with the H–O–H bending
mode (∼1660 cm–1) (see the Supporting Information). Figure 3c displays the H–O–D bending
mode (δ(H–O–D)) spectra of the isotopically diluted
water–formic acid mixture. The H–O–D bending
mode frequency is red-shifted by ∼200 cm–1 compared to the H–O–H bending mode frequency.27 When formic acid is added to water, the C–H
bending mode (∼1350–1400 cm–1)49−52 and the C–O–H bending mode (∼1490 cm–1)53,54 appear as features in this frequency region
together with the H–O–D bending mode contribution (∼1460
cm–1).</p><!><p>(a–c) SFG spectra at the air–aqueous
binary mixture
of (a) methanol, (b) ethanol, and (c) formic acid interfaces. Spectra
are offset by 0.1 for clarity. The black lines represent the fits
to the SFG spectra. (d) Aδ(H–O–H) for the water–methanol and water–ethanol mixtures
together with Aδ(H–O–D) for the water–formic acid mixture. Aδ(H–O–H) and Aδ(H–O–D) were normalized to the values at xH2O of ∼0.85, where the free O–H stretching mode
contributions (Aν(free O–H)) become vanishingly small. For mixtures with formic acid, xwater corresponds to the fraction of all water
species (xH2O + xHOD + xD2O). The
error bars are obtained from the fits. The solid lines indicate the
simulation data of integrated ⟨N cos θ⟩;
the dashed lines connect the extrema of the measurements at xH2O = 0 and 0.85. Data for different
mixtures are offset by increments of 1 for clarity.</p><!><p>We extracted the composition-dependent amplitudes by fitting
a
Lorentzian model (see the Supporting Information) using two bands to the bending mode data. These two bands have
been attributed to water molecules that form one hydrogen bond and
the other O–H group being a free O–H group and to water
molecules whose O–H groups are both pointing into the liquid
and are both hydrogen-bonded. Figure 3d displays the variation of the sum of these amplitudes
for the H–O–H and H–O–D bending mode amplitudes
(Aδ(H–O–H) and Aδ(H–O–D)). As already apparent
from the raw data, the water response initially increases with increasing
concentration of the organic solute, showing a maximum at xH2O = 0.85, after which the band
decreases in intensity. The initial increase coincides with the disappearance
of the free O–H stretching contribution at xwater between 1.0 and ∼0.85 (Figure 2d). As such, the initial increase can be
ascribed to an increase in the number of doubly H-bond donating water
molecules at the interface: the water molecules with a free O–H
group reorient to have both O–H groups pointing into the bulk
liquid.</p><p>Unlike the data in Figure 2, which showed similar behavior for the three
mixtures, the
variation of Aδ(H–O–H) and Aδ(H–O–D) with
composition differs markedly for xwater < 0.85 for the three mixtures. To better illustrate these differences,
we normalize the bending mode amplitudes in Figure 3d to the maximum amplitude at xwater = 0.85 at which the free O–H contribution
has vanished and the associated rotation of water molecules hardly
affects the bending mode amplitudes and connect that to the zero signal
at xwater = 0 (dashed lines in Figure 3d). For the water–methanol
and water–ethanol mixtures, Aδ(H–O–H) is, respectively, moderately and significantly below this line;
for the water–formic acid mixture at xwater < 0.85, the results lie above the line (Figure 3d). As such, our results demonstrate
positive (negative) excess spectral amplitude for the water bending
vibration in mixtures with formic acid (methanol and ethanol) at xwater < 0.85. These excess spectral amplitudes
of water correlate with the evaporation energetics and kinetics: Mixtures
exhibiting a positive excess evaporation rate and vapor pressure exhibit
reduced spectral amplitudes.</p><p>The negative and positive excess
spectral amplitudes for the bending
mode are indicative of different degrees of ordering of interfacial
water for the different mixtures, with the larger signal for formic
acid implying more ordered water. We test this hypothesis by performing
MD simulations and calculate the angle θ of
the H–O–H angular bisector with respect to the surface
normal for hydrogen-bonded water molecules at the interface. By calculating
the integrated ⟨N cos θ⟩ along the surface normal, where N is the
number of water molecules, one can compare the simulation data of
⟨N cos θ⟩ with
the experimental data of Aδ(H–O–H) or Aδ(H–O–D). The
result is shown as solid lines in Figure 3d, demonstrating good agreement with the
experimental data. Clearly, the MD simulations capture the variation
of the interfacial water response.</p><p>The reduced (increased) ⟨N cos θ⟩ values (and spectral
amplitudes Aδ(H–O–H) or Aδ(H–O–D)) may
stem from a reduced
(increased) interfacial water density (N) or reduced
(enhanced) water alignment (⟨cos θ⟩).
Our MD simulation data show that the variations of the spectral amplitudes
are caused by reduced (enhanced) water alignment: At xH2O ∼ 0.85 (dotted line in Figure 4a–c), where
rotation due to hydrogen-bond formation of initially free O–H
groups is completed (see the discussion above), the widths of the
distributions of ethanol and formic acid are similar. However, for
ethanol, it is centered at larger angles (⟨cos θ⟩ = −0.5), as compared to formic acid (⟨cos θ⟩ = −0.3). The orientational distribution
in mixtures with methanol is much broader and centered at ⟨cos θ⟩ = −0.4. These trends are enhanced
upon increasing the solute concentration to xH2O ∼ 0.5: For the ethanol sample, the cos θ distribution of the angular H–O–H
bisector direction broadens upon the addition of ethanol, which means
that the orientation of water is further randomized. The distributions
for methanol are similar to ethanol, yet the variation of the distribution
upon increasing methanol content is smaller than that for ethanol.
In contrast to the methanol/ethanol samples, the distribution of water
orientations in the presence of formic acid becomes more narrow upon
increasing the formic acid concentration. As such, the simulation
results demonstrate an enhanced alignment of water upon mixing with
formic acid, slightly disordered water for methanol, and substantially
disordered water for ethanol.</p><!><p>(a–c) The cos θ distribution probability
of the angular H–O–H bisector of the interfacial water
for xH2O = 0.5 (solid lines)
and 0.85 (dotted lines) for the aqueous binary mixtures of (a) ethanol,
(b) methanol, and (c) formic acid. The horizontal black lines reflect
a totally randomized distribution; the arrows indicate the variations
of maximum (Max(PxH2O)) and minimum (Min(PxH2O)) ranges of the cos θ distribution. The percentages given in the figures indicate the
relative change of these ranges, [(Max(P0.5) – Min(P0.5)) – (Max(P0.85) – Min(P0.85))]/[Max(P0.85) – Min(P0.85)].</p><!><p>Our results, therefore, show that the different excess mixing properties
in the bulk liquids result in markedly different structuring of water
for these mixtures at the interface. The deviations from Raoult's
law point to stronger (weaker) solute–water interactions as
compared to water–water interactions for formic acid (methanol
and ethanol), leading to stronger (weaker) constraints on the orientation
of water. Interestingly, the stronger solute–water interactions
for formic acid lead to enhanced ordering of interfacial water, and
the sample shows reduced evaporation rates. Inversely, when the degree
of the surface organization is reduced, i.e., the surface structure
becomes more randomized, as is seen upon the addition of ethanol to
water, the sample shows an excessive evaporation rate.</p><!><p>We performed surface-specific vibrational
SFG spectroscopy and
MD simulations for the air–aqueous binary mixture interfaces
of ethanol, methanol, and formic acid to explore their surface molecular
conformations. Despite the largely differing excess mixing properties
of the three studied mixtures, the variation of the outermost arrangement
judged by the presence of spectral signatures of free O–H groups
and the C–H stretching mode is similar for all three mixtures.
On the other hand, we find different ordering of water molecules as
determined from water's bending mode vibrations: Our SFG and
MD simulation data show that the difference of excess mixing properties
also propagates to the interfacial structure of the one to two topmost
layers of the binary mixtures; reduced (enhanced) intermolecular interactions
and accelerated (reduced) evaporation rates relative to ideal evaporation
are linked with a lesser (greater) surface reorganization of the interfacial
water molecules. The bending mode provides us new information about
the complex interfacial structure, which paves a path for understanding
evaporation and condensation processes, and ultimately perhaps a pathway
toward controlling evaporation processes.</p><!><p>Evaporation measurement
details, all SFG spectra and
fitting procedures, estimation of HOD mole fraction, and MD simulation
details (PDF)</p><p>jp1c06001_si_001.pdf</p><!><p># X.Y. and T.S. contributed equally.</p><!><p>Open access funded by Max
Planck Society.</p><p>The authors
declare no competing financial interest.</p>