Probing liquid-liquid interfaces with spatially resolved NMR spectroscopy.

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Interface Analysis
DOI: 10.1002/anie.200901389
Probing Liquid–Liquid Interfaces with Spatially Resolved NMR
Spectroscopy
J?rg Lambert, Roland Hergenr?der, Dieter Suter, and Volker Deckert*
Phenomena occurring at the interface between two immis-
cible liquids have a deep impact on many processes of
everyday life.[1]The stability of emulsions depends on the
interaction of proteins or surfactants at the oil–water inter-
face.[2]Solvent extraction and phase-transfer catalysis rely on
optimizing reactions at the boundary of two liquids. More-
over, the liquid–liquid interface between an organic solvent
and water represents a simple model of a biological mem-
brane. Historically, the knowledge of the structure[3]and
dynamics[4]of liquid–liquid interfaces mainly stemmed from
surface-tension measurements and thermodynamic analysis.[1]
Over the last couple of decades probing of liquid–liquid
interfaces using nonlinear optical methods has developed.[5]
Many traditional bulk techniques have been adapted for
studying the interface. Second-harmonic generation and
vibrational sum-frequency (VSF) spectroscopy both provide
information that is inherently surface-specific.[6]The latter,
coupled with molecular dynamics,[7]helped to unravel the
structure of many interfaces. X-ray[8]and neutron[9]scattering
have also been applied to study liquid–liquid interfaces and
can provide useful and reliable information on the interfacial
widths formed. Surface second-harmonic generation mainly
uses molecular probes (such as push–pull molecules) to
evaluate surface effects.
Probing liquid–liquid interfaces with scanning probe
techniques still remains a challenge, though first results
from atomic force microscopy (AFM)[10]and scanning
electron microscopy (SEM)[11]have been obtained. Transient
phase grating experiments with evanescent fields resulting
from total internal reflection at an interface between a polar
absorbing and a nonpolar transparent phase were used to
measure the dimension of liquid–liquid interfaces.[12]Vibra-
tional sum frequency spectroscopy (VSF)[13]
probes the molecular structure at hydrocarbon–water inter-
faces and shows that the hydrogen bonding between adjacent
water molecules at the interface is weak and results in a
selectively
substantial orientation of the water molecules in the inter-
facialregion.[14]
Scanning
(SECM) can be used to study localized processes occurring
at liquid–liquid interfaces.[15]
In general, investigations of liquid–liquid interfaces
impose a significant technical challenge: the discrimination
between the information contained in the miniscule volume
of the interface and that from the abundant bulk liquid. Apart
from the above-mentioned nonlinear optical techniques, only
one approach exists to obtain structural information without
modifying the interface. The combination of near-field
microscopy and Raman spectroscopy[16,17]in order to obtain
information of gradual changes correlated with distance
towards the interface has been also presented. Illumination
of a sample surface with a near-field probe provides high
spatial resolution beyond the diffraction limit, and theoret-
ically the depth resolution for a 100 nm aperture lies at
approximately 10 nm. In combination with Raman spectros-
copy this results in highly resolved information on the
molecular structure of the surface. The drawback of the
technique is the use of a probe that must be very close to the
surface. As soon as the interface contacts the tip, a meniscus is
formed and the entire experiment must be restarted. In
addition this probe can already influence the results by adding
a further component into the system.
Here, we present a technique based on volume-selective
nuclear magnetic resonance (NMR) spectroscopy,[18]which
induces no mechanical perturbation of the interface while
also providing a high chemical contrast. Volume-selective
NMR spectroscopy restricts the detection of magnetic
resonance data to a detection volume element of definable
size and position. This mode is well known, for instance, from
medical applications. The volume of the voxel (a volume
element that represents a property on a regular grid in three-
dimensional space) is determined by the desired spatial
resolution as well as by the required detection limit and
sensitivity of the NMR experiment. The crucial point is to use
a cuboid voxel geometry with a small width in the direction
orthogonal to the interface of interest and larger dimensions
elsewhere: in obtaining information on a liquid–liquid inter-
face, not all three spatial dimensions are equally important.
Fluctuations parallel to the interfacial surface will be slow on
NMR timescales and hence can be neglected.
The voxel geometry used in our experiments is shown
schematically in Figure 1. Basically the number of spins
required for the NMR signal is achieved by extending the
voxel size parallel to the surface while at the same time
reducing the size orthogonal to the surface. Thus in this voxel
geometry the number of spins contributing to a signal is the
same as that for a cubic voxel. The resolution in the dimension
electrochemicalmicroscopy
[*] Priv.-Doz. Dr. V. Deckert
Department of Proteomics, ISAS—Institute for Analytical Sciences
Bunsen-Kirchhoff-Strasse 11, 44139 Dortmund (Germany)
and
IPHT – Institut f?r Photonische Technologien
Albert-Einstein-Straße 9, 07745 Jena (Deutschland)
Fax: (+49)3641-206-139
E-mail: volker.deckert@ipht-jena.de
Homepage: http://www.ipht-jena.de
Dr. J. Lambert, Dr. R. Hergenr?der
Department of Material Analysis, ISAS, Dortmund (Germany)
Prof. Dr. D. Suter
Lehrstuhl Experimentelle Physik 3
Technische Universit?t Dortmund (Germany)
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of interest is improved by sacrificing resolution in the other
two dimensions of less importance.
The detection voxel can be shifted within the liquid
sample to any position by changing the carrier frequency of
one of the selective NMR pulses, that is, without the use of
any mechanical equipment interfering with the sample or
generating vibrations. By approaching the interface in (sub)-
micrometer steps starting from one of the bulk phases,
structural and compositional changes can be detected from
the measurement of the chemical shifts and signal intensities,
respectively. Figure 2 shows a pseudo three-dimensional
representation and an image plot of a series of
spectra as obtained with the volume-selective NMR tech-
nique for the system water–benzene.
A volume element of the size 250?250?1 mm3is shifted
in steps of 50 nm from the bulk benzene phase across the
1H NMR
interface into the water phase. In the present experiment
approximately twenty subsequent traces contain signals of
both benzene and water, which is well in agreement with a
detection volume thickness of 1 mm and the shift increment of
50 nm, both measured orthogonal to the liquid–liquid inter-
face. The thickness of the water–benzene interface for the
applied detection dimensions can be safely neglected. The
voxel dimensions are limited by the present setup in terms of
the strength of the field gradient in the z direction, Gz=
1 Tm?1. In principle, however, field gradients of 100 Tm?1
corresponding to resolutions down to 10 nm can be achieved.
Possible limitations of the spatial resolution, such as dis-
turbances at the water–benzene interface (surface tension:
35 mNm?1) by thermally excited capillary waves, are of the
order of a few nm and are far below the length scale of the
measurements.[19]
The impact of diffusion on the limitation of the spatial
resolution is usually overestimated. Under conditions of free
translational diffusion a mean free path hx2i=2Dtseq, (where
tseq is the duration of the pulse sequence) of 10 mm is
calculated, assuming a typical diffusion coefficient D=
10?9m2s and tseq=60 ms. Molecular dynamics studies, how-
ever, suggest, that the self-diffusion near liquid–liquid inter-
faces is anisotropic. If the self-diffusion is described by
diffusion coefficients tangential (Ds,T) and normal (Ds,N) to
the interfacial plane, theoretically it is predicted that Ds,T>
Ds,Nnear the interface.[20]However, in the bulk the diffusion is
isotropic. To get experimental evidence, we performed NMR
gradient echo diffusion measurements[21]on the benzene–
water system studied here. An excitation sequence employing
a selective RF pulse in the presence of a Gzgradient was used
to select a slice of 10 mm comprising the benzene–water
interface. For benzene and water, a reduction of Ds,Nby a
factor of 5.4 and 4.8, respectively, was found relative to Ds,T.
The roughly fivefold reduction of Ds,Nfor benzene and water
a few micrometers from the surface relative to the bulk value
weakens the diffusion limitation usually claimed for volume-
selective NMR experiments and helps to enhance the spatial
resolution by one order of magnitude into the micrometer
range. As an explanation for this, it is assumed that the
occurrence of a translational barrier impermeable for the
translating spins results in the reduction of the translational
mean free path perpendicular to the surface.[20]Higher spatial
resolution in the nanometer range can be established follow-
ing twostrategies:both extension ofthe selective pulsefor the
z direction and the application of larger z-gradient strengths
allow smaller slices to be excited. While the former strategy
prolongs the sequence duration tseqinterfering even with the
“weakened” translational diffusion limitation, the latter
strategy keeps the sequence duration constant and is clearly
the method of choice.
In conclusion, we have demonstrated a novel approach for
the investigation of liquid–liquid interfaces at high spatial
resolution. The key issue is the use of a flat cuboid as the
detection volume element. This shape preserves a sufficient
number of spins that contribute to the NMR signal while
maintaining high spatial resolution in the direction of interest.
We found that the diffusion coefficient at the interface is five
times smaller than that in the bulk. As a result measurements
Figure 1. The voxel used for spatial selection has a cuboid shape,
which assures sufficient resolution in the direction of interest as well
as sufficient spins for a good signal-to-noise ratio.
Figure 2. A series of volume-selective1H NMR spectra recorded for
the system benzene–water in a pseudo three-dimensional representa-
tion as well as an image plot. For details see the text and the
Experimental Section.
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with increased resolution in the low nanometer range should
be feasible and potentially allow the observation of molecular
changes at the interface. Future investigations comprise
studies of surfactants at the interface in order to model
biomembranes, as well as the improvement of the spatial
resolution perpendicular to the surface by the application of
higher gradient strengths.
Experimental Section
Measurements were performed at a field strength of 14.1 T
(600 MHz) in a 15 mm sample tube in order to reduce the effects of
the capillary curvature on the interface. Doubly distilled water and
analytical-grade benzene were used to prepare the sample. The
STEAM[18]technique was used for volume selection. The size of the
detection volume element was chosen 250?250?1 mm3in the x, y,
and z direction, with the z direction being orthogonal to the liquid–
liquid interface. The extended dimensions in the x and y direction
were selectedfor sensitivity reasons. Subsequent, overlapping slices in
the z direction are 50 nm apart, corresponding to a carrier frequency
increment of 2.15 Hz. Gradient strengths in the x, y, and z direction
were Gx=95.8 mTm?1, Gy=96.9 mTm?1, and Gz=1010 mTm?1,
respectively. Bandwidths of the selective 908 8 pulses in the x, y, and
z direction were 1.03 kHz, 1.02 kHz, and 43 Hz, respectively. 112
scans were recorded for each increment. For the diffusion measure-
ments, a selective sinc-shaped 908 8 pulse of bandwidth 100 Hz and a
field gradient of Gz=234 mTm?1were used for excitation.
Received: March 12, 2009
Published online: July 13, 2009
.
spatially resolved NMR spectroscopy
Keywords: liquid–liquid interfaces · NMR spectroscopy ·
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