Structural and dynamical properties of guest molecules confined in mesoporous silica materials revealed by NMR

Abstract

In the last fifteen years several novel porous silica materials, which are periodically structured on the mesoscopic length scale, have been synthesized. They are of broad interest for fundamental studies of surface-substrate interactions, for studies of the dynamics of guest molecules in confinement and for studies of the effect of confinement on the structural and thermophysical properties of fluids. Examples of such confinement effects include the change of the freezing and melting points or glass transitions of the confined liquids. These effects are studied by combinations of several NMR techniques, such as (15)N- and (2)H-solid-state NMR line shape analysis, MAS NMR and NMR diffusometry with physico-chemical characterization techniques such as nitrogen adsorption and small angle diffraction of neutrons or X-rays. This combination does not require crystalline samples or special clean and well defined surfaces such as conventional surface science techniques, but can work with typical ill-defined real world systems. The review discusses, after a short introduction, the salient features of these materials and the applied NMR experiments to give the reader a basic knowledge of the systems and the experiments. The rest of the review then focuses on the structural and dynamical properties of guest molecules confined in the mesoporous silica. It is shown that the confinement into the pores leads to fascinating new features of the guests, which are often not known for their bulk phases. These features depend strongly on the interplay of the their interactions with the silica surface and their mutual interactions.

Full text

Structural and dynamical properties of guest molecules confined in
mesoporous silica materials revealed by NMR
Gerd Buntkowsky,*
a
Hergen Breitzke,
a
Anna Adamczyk,
a
Frank Roelofs,
a
Thomas Emmler,w
b
Egbert Gedat,
b
Bob Gru
¨nberg,
b
Yeping Xu,
b
Hans-Heinrich Limbach,
b
Ilja Shenderovich,
b
Anastasia Vyalikhz
b
and
Gerhard Findenegg
c
Received 15th May 2007, Accepted 14th June 2007
First published as an Advance Article on the web 9th July 2007
DOI: 10.1039/b707322d
In the last fifteen years several novel porous silica materials, which are periodically structured on
the mesoscopic length scale, have been synthesized. They are of broad interest for fundamental
studies of surface–substrate interactions, for studies of the dynamics of guest molecules in
confinement and for studies of the effect of confinement on the structural and thermophysical
properties of fluids. Examples of such confinement effects include the change of the freezing and
melting points or glass transitions of the confined liquids. These effects are studied by
combinations of several NMR techniques, such as
15
N- and
2
H-solid-state NMR line shape
analysis, MAS NMR and NMR diffusometry with physico-chemical characterization techniques
such as nitrogen adsorption and small angle diffraction of neutrons or X-rays. This combination
does not require crystalline samples or special clean and well defined surfaces such as
conventional surface science techniques, but can work with typical ill-defined real world systems.
The review discusses, after a short introduction, the salient features of these materials and the
applied NMR experiments to give the reader a basic knowledge of the systems and the
experiments. The rest of the review then focuses on the structural and dynamical properties of
guest molecules confined in the mesoporous silica. It is shown that the confinement into the pores
leads to fascinating new features of the guests, which are often not known for their bulk phases.
These features depend strongly on the interplay of the their interactions with the silica surface
and their mutual interactions.
Introduction
15 years ago a new class of mesoporous silica materials has
become available which is characterized by a periodic pore
structure although the silica matrix is disordered on an atomic
length scale.
1–6
Typical representatives of such periodic
mesoporous silica (PMS) materials are MCM-41 and SBA-15,
which constitute two-dimensionally hexagonal arrays of
cylindrical pores. Owing to the presence of surface silanol
groups at the pore walls, these materials can be chemically
tailored to various functions.
7
PMS materials opened up
fascinating possibilities for new applications in many
fields, including catalysis,
8–11
drug delivery or size selective
molecular separation.
12
At the same time, these materials are
of broad interest for fundamental studies of surface–substrate
interactions and of the dynamics of guest molecules in con-
finement.
13–21
PMS materials are also very well-suited for fundamental
studies of the effect of confinement on the structural and
thermophysical properties of fluids. This interest is motivated
by the desire to better understand the influence of surface
forces and finite-size effects on the behavior of the substrate in
the pores.
22
Examples of such confinement effects include the
change of the freezing and melting points or glass transitions
of the confined liquids.
20,22–25
The change of the melting point
is often discussed by the Gibbs–Thomson equation,
26
which
predicts a linear dependence of the melting point depression
DT
m
on the inverse pore radius 1/R:
DTm¼TmðbulkÞTmðporeÞ¼2VmTmðbulkÞðgcw glwÞ
DHmRð1Þ
where T
m(bulk)
is the bulk melting temperature, V
m
is the molar
volume of the liquid phase, DH
m
the molar enthalpy of
melting, and (g
cw
g
lw
) the difference of the surface free
energies crystal-wall and liquid-wall, which for the case of
complete wetting of the pore walls by the liquid is given by the
crystal/liquid interfacial free energy g
cl
.
22
A detailed survey for
the current state of the theory of melting points is given in the
review by Alcoutlabi and McKenna.
27
a
FSU Jena, Institut fu
¨r Physikalische Chemie, Helmholtzweg 4,
07743 Jena, Germany. E-mail: gerd.buntkowsky@uni-jena.de;
Fax: +49 3641 948302
b
Freie Universita
¨t Berlin, Institut fu
¨r Chemie, Takustraße 3, 14195
Berlin, Germany
c
Technische Universita
¨t Berlin, Stranski-Laboratorium fu
¨r
Physikalische und Theoretische Chemie, Straße des 17. Juni 124,
10623 Berlin, Germany
wPresent address: GKSS-FZ Geesthacht, D-21502, Geesthacht,
Germany.
zPresent address: IFW Dresden, D-01171 Dresden, Germany.
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A melting behavior conforming with eqn (1) or with a
modified Gibbs–Thomson equation was indeed found for
water
5,28
and other liquids
29
in a series of MCM-41 and
SBA-15 materials of 2 to 10 nm pore width.
However, as discussed in detail by the review of McKenna
et al.,
27
the Gibbs–Thomson equation is not the end of the
story. Until now the depression of the melting point in
confined media or in particular the change of the glass transi-
tion temperature is theoretically not completely understood,
despite the fact that even technical systems nowadays are
approaching nanometer dimensions. It is therefore necessary
to improve our knowledge of how geometrical confinement on
the nanometer scale alters the behavior of disordered con-
densed matter relative to bulk materials. A recent survey about
the theoretical modelling and description of fluids confined in
nanoporous and mesoporous material is given in the textbook
by Schoen and Klapp.
30
Part of the difficulty in the description of these systems is the
complex interplay of interactions between the molecules them-
selves on the one hand and the molecules and the surface on
the other hand. Since the pore diameter is typically a factor 10
to 100 greater than the size of the guest molecules, a sub-
stantial fraction of the guest molecules is in direct vicinity of
the surface. As a result the two types of interactions are of
comparable magnitude. Moreover both types of interactions
might have the simple geometry dependence of an electrostatic
or dispersion interaction or the complex angular behavior of a
strong hydrogen bond.
A deeper understanding of the structure–property relation
in these systems, which goes beyond the thermodynamic level,
necessitates a detailed characterization of the surface and guest
structures inside these pores and their changes. Much progress
has been made in the characterization of the pore structure of
PMS materials at a mesoscopic length scale by small-angle
X-ray diffraction.
31,32
Recently, this method has also been
applied to in situ studies of the condensation of vapors in the
porous matrix,
33,34
and adsorbed surfactant layers in the pore
space.
35
For studying the behaviour at a molecular level, the
large inner surface area and inner volume of these materials
provides the necessary sensitivity to allow a detailed analysis
using a variety of NMR technique. A great advantage of
NMR techniques is that they do not necessitate special clean
and well defined surfaces such as conventional surface science
techniques, but can work with the typical ill-defined real world
systems, as for example technical silica materials. In particular
since NMR is an integral method it is not strongly affected by
small concentrations of impurities. NMR techniques allow
also the study of modified silica materials, which are prepared
for example under low surfactant concentration
36
or functio-
nalized by organic groups.
37
Combining several of these NMR
techniques (see Fig. 1) it is possible to reveal the binding
situation, the translational and the rotational dynamics of the
guest molecules on or near the surface, respectively, inside the
pores and the adsorption/desorption kinetics of the
guests.
20,38–49
Additional information about dynamic pro-
cesses inside the pores are obtainable by other NMR methods,
as for example the stimulated echo or 2D-exchange NMR are
discussed in detail in the textbook
50
and the recent review by
Boehmer et al.
51
The purpose of this short review is to give the reader an
overview about the current state of the art in NMR studies of
these systems and the underlying NMR methodology for the
characterization of these new materials, employing mainly
research examples from our group. A deeper discussion of
the formation of glasses is beyond the scope of this review.
Here the reader is referred to the recent papers by Paul and
Smith
52
and Berthier et al.
53
and references cited therein.
Materials and methods
Mesoporous silica materials
PMS materials consist of pseudo-crystalline powders, where
each crystallite constitutes a large number of more or less
parallel cylindrical pores. The silica formation takes place
in aqueous media using surfactants or amphiphilic block
copolymers as structure-directing agents (template). The pore
diameters can be adjusted by the templating agents. Typically,
they are between 2 and 4 nm in the case of MCM-41 silicas,
and between 5 and 12 nm for SBA-15 silicas.
5
Materials with
wider pores (up to 50 nm and more) but nonperiodic structure
can be synthesized by similar procedures. After silica forma-
tion the templating agents are removed by calcination. The
high porosity causes a large inner surface of these materials.
Owing to their wide range of pore sizes, they are very versatile
molecular sieves. Since the physical properties of their
inner surfaces, such as the surface acidity, can be chemically
modified,
54,55
mesoporous silica materials are very promising
candidates for catalytic applications.
NMR spectroscopy
Nuclear magnetic resonance
56,57
(NMR) is a well known
spectroscopic technique which covers a very large range of
application areas, due to the multitude of different NMR
techniques which are available today. Roughly three principal
NMR domains can be distinguished, namely liquid-state
NMR spectroscopy,
58
spatially resolved NMR techniques
59–61
and solid-state NMR spectroscopy.
50,62
While the main appli-
cation of liquid-state NMR spectroscopy refers to the area
of chemical and biochemical analysis of liquid or soluble
compounds, spatially resolved NMR techniques are primarily
employed for medical and technical applications. Solid-state
NMR spectroscopy is devoted to the chemical analysis of
Fig. 1 Overview of the different NMR techniques which are used in
this review for the study of the molecular processes in a confined
geometry.
4844 |Phys.Chem.Chem.Phys., 2007, 9, 4843–4853 This journal is cthe Owner Societies 2007

insoluble compounds, to the study of electronic structures in
conducting systems, and generally to the characterization and
investigation of structural and dynamic properties of solid
systems.
Rotational dynamics
The rotational dynamics of organic guest molecules is most
favorably studied by
2
H solid-state NMR spectroscopy,
employing deuterated –CD groups as sensors.
50
Solid-state deuterium NMR (
2
H NMR) has been shown to
be a powerful technique to probe the dynamics of organic
molecules adsorbed in porous materials.
46,63–65
The
2
HNMR
line shape is mainly determined by intramolecular quadru-
polar interactions and is highly sensitive to the mode of
molecular motion and its rate. Information about molecular
motions is commonly extracted from the line shapes of poly-
crystalline powders by comparing the experimental spectra with
a series of simulated spectra. In these simulations characteristic
parameters such as the quadrupolar coupling constant, the
asymmetry parameter, and the correlation rate, are varied.
The leading interaction in
2
H solid-state NMR is the
quadrupolar interaction.
50,57
In the usual high field approx-
imation, the first order quadrupolar interaction is character-
ized by two orientation dependent resonance frequencies n
Q
of
the two spin transitions of the deuteron, given as:
nQðW;jÞ¼Qzz
1
2ð3 cos2W1Zsin2Wcos 2jÞð2Þ
Q
zz
is a measure for the strength of the quadrupolar interac-
tion. It corresponds to the largest frequency and the spectral
width of possible frequencies is twice this value. The asym-
metry parameter Zis a measure of the deviation of quadru-
polar interaction from axial symmetry. An alternative
description of the strength of the quadrupolar interaction is
the quadrupolar coupling constant, which for deuterons is
given as Q
cc
= 4/3Q
zz
.
While in single crystals only two lines at n
Q
are observable,
the average over all possible orientations has to be calculated
in a non-oriented powder sample. This integration gives the
typical NMR powder line shapes, which are called the Pake
pattern. For most organic CD groups a practically axial
symmetric quadrupolar tensor with Zo0.05 and a value of
Q
zz
D120–140 kHz is found. Because of the large spectral
width, the
2
H-NMR spectra are measured with the solid echo
sequence (Fig. 2(a)).
If the molecule undergoes fast re-orientations, the value of
the quadrupolar tensor and thus also the quadrupolar
coupling in general is changed, depending on the type and
speed of the motion.
66
If the motions are fast on the NMR
time scale, a relatively simple scenario is found: fast isotropic
re-orientations of the benzene molecule, as for example in
liquid benzene, cause a complete averaging of the quadrupolar
tensor (Q
iso
zz
= 0). Anisotropic rotations or rotational jump
diffusions around the C
6
-axis reduce the value of Q
zz
to Q
rot
zz
=
1
2Q
zz
. In these situations the normalized line shape of the solid
echo spectrum is identical to the FID spectrum. In addition to
these rotational motions there are often also librational mo-
tions which cause a partial reduction of the value of Q
zz
at
room temperature.
The situation is more complicated if the motions are not fast
on the NMR time scale. In this case the line shape of the solid
echo spectra depends very strongly on the echo delay time t,
and the distribution of rotational correlation times
67,68
Two
limiting cases have to be considered, namely (a) the distribu-
tion function is very narrow (crystal-like behavior) and (b) the
distribution function is very broad (glass-like behavior). In the
crystal-like case the line shape of the solid echo spectra has to
be calculated by standard NMR methods.
50
In the glass-like
case however, the spectra are always a simple superposition of
the slow and the fast limit spectra with varying weights.
68
Translational dynamics
The effect of confinement and interaction with the large
surfaces of these materials does not only affect the rotational
but also the translational dynamics of guest molecules inter-
acting with the pores. In particular, hydrogen bonding does
not only influence molecular and intermolecular structures but
can have also drastic effects on diffusion rates, as revealed by
gradient NMR spectroscopy.
69–71
The main reason why these
effects were discovered so relatively late is the fact that for most
diffusing systems the surface to volume ratio of the pores is so
small that molecules in direct contact to the surface are only a
minor percentage of the overall molecules inside the pores. Thus
the diffusion restrictions imposed on these molecules by hydro-
gen bonding are only of minor importance. However, in the
mesoscopically structured porous silica this is no longer true
and consequently the translational dynamics of guest molecules
are strongly influenced by the porous environment.
The geometry of the pores is highly anisotropic and the
pores’ cylinder axis is the preferred direction of the diffusion.
As a result of this the diffusion of guest liquids in the pores
exhibits deviations from ordinary diffusion behavior.
72,73
In
general, diffusion processes in porous media depend strongly
on the type of the host and of the guest molecules. As a result
of this dependence, different diffusion models for liquids and
liquid guests in porous materials are discussed in the literature
for various systems: ordinary diffusion,
74,75
restricted diffu-
sion,
61,76
anisotropic diffusion,
60,77
and diffusion of different
phases with individual diffusion coefficients.
78
All these models
Fig. 2 Timing scheme of the (a) solid echo sequence for
2
H solid-state
NMR spectroscopy and (b) stimulated echo sequence for the PFG
experiments.
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differ more or less strongly from ordinary diffusion in isotropic
systems such as bulk liquids. Anisotropic diffusion has been
studied by NMR for various systems, as for example water
between lamellar layers of a liquid crystal,
60
for salt-water
ice,
79
and water in MCM-41.
77
Another possibility in the pores
is a multi-phase scenario for the diffusion of guest molecules.
Here the diffusion coefficients depend on the distance of the
guest molecules from the surface, similar to the velocity profile
of a laminar flow. Indications for such a behavior are found
for example in water molecules interacting with protein
surfaces.
61,80–85
Diffusion processes are measured with NMR diffusometry,
employing gradient techniques where a constant or pulsed
magnetic field gradient in the direction of the external
magnetic field is employed. The basics of diffusion in con-
densed matter are described for example in refs. 61 and 86 and
are not repeated here. In the case of a constant gradient most
experiments are performed in the stray field of a standard
NMR magnet (stray field gradient, STRAFI NMR
87–93
) un-
less specialized anti-Helmholtz magnet systems with ultra high
gradients are employed.
93
In both cases the experiment con-
sists of three time periods. In the first period the initial spatial
position is encoded via the position dependent Larmor fre-
quency of the molecule; in the second period the molecule
diffuses and changes its position; in the third period this
change is revealed by determining the attenuation of the
NMR signal caused by the diffusion (see Fig. 2(b)).
Chemical exchange
Finally also chemical exchange processes, for example between
hydrogen donors on the surface and hydrogen acceptors on
the guest molecules, are of importance for understanding the
dynamical processes of the guest molecules in the vicinity of
the surface. A detailed description of the influence of chemical
exchange on NMR spectra is beyond the scope of this short
review and can be found in the recent review articles by Bain
94
and some of us.
95
Here only the salient facts are given and we
restrict here to first-order kinetics. The basic idea of this
formalism is that the time scale of a chemical reaction or an
exchange process is fast compared to the NMR time scale.
Under this assumption the chemical reaction leads to a
fluctuating time dependence of the chemical shift, i.e. the
NMR frequency. It is permissible to treat the reaction as an
instantaneous exchange between different static configura-
tions, characterized by their static chemical shift, without
having to take into account the actual reaction coordinates
or pathway. In particular in the case of a fast exchange, where
the exchange rates are much larger than the frequency differ-
ences, the result of the exchange is the weighted average of
these static chemical shifts. As a simple example, let’s assume
that a proton is exchanging with rate constants k
12
and k
21
for
the forward and backward reaction between two positions
characterized by their Larmor frequencies o
1
and o
2
. In the
case of fast exchange k
21
c|o
1
o
2
| and/or k
21
c|o
1
o
2
|
the average chemical shift is given as:

o¼k12
k12 þk21
o1þk21
k12 þk21
o2:ð3Þ
Results and discussions
Guest molecules with weak surface interactions
Benzene is an example of a molecule with weak surface
interactions. As a result of this the effects of the surface will
be mainly caused by the confinement and steric interactions
with the surface. These effects are monitored by the rotational
dynamics of the benzene molecule. In bulk benzene three
states with different kinds of rotational motion can be distin-
guished by
2
H-NMR, namely a liquid state and two solid
states: The first (liquid-like) state corresponds to fast isotropic
rotational motions of the benzene molecule. The second
(solid I) state is a fast anisotropic rotational jump diffusion
around the six-fold axis. The third (solid II) state is the
situation where all rotational motion appears frozen on the
time scale of the
2
H solid-state NMR spectra, i.e. where
the rotational correlation times are longer than ca. 1 ms. In
the region of the solid I–solid II transition, where the rate of
the 601jump is comparable to quadrupolar frequencies,
typical motion induced deviations from the common Pake
pattern line shape are observed. These line shapes were
reported by Vold and co-workers
67
for bulk benzene-d
6
and
for benzene-d
6
confined in the cages of the cyclamer 1,3-
cyclohexanedione.
From the properties of bulk benzene it is evident that also in
the silica the frozen phase (solid II) is expected at temperatures
well below 100 K. For glass-like systems of benzene however, a
different behavior has been found.
96
Here a broad distribution
of correlation times of the six-fold jump motion renders the
spectra corresponding to the intermediate (solid I–solid II)
invisible. This leads to spectra, which are a weighted super-
position of spectra from molecules rotating slowly on the NMR
time scale (solid II-like spectra) and spectra from molecules
rotating fast on the NMR time scale (solid I-like spectra).
Fig. 3 compares the superposition of the experimental and
simulated
2
H-NMR spectra of bulk benzene-d
6
, benzene-d
6
inside the relatively narrow pores of SBA-15 (8 nm) and
benzene-d
6
inside the mesoporous glass MCF with wide pores
(30 nm) as a function of temperature. Comparing these spectra
striking differences are visible. In the case of the bulk benzene
(Fig. 3(a)) the temperature dependence reveals the typical line
shapes of a crystal with well defined activation energies for the
six-fold rotation as reported by Vold and co-workers.
67
This
set of spectra serves as a landmark for the interpretation of the
spectra of benzene inside the pores. The high temperature
spectrum at 206.6 K shows the solid I spectrum of bulk
benzene-d
6
with a strength of the quadrupolar interaction of
67 kHz. It represents the fast jump limit (high-temperature
limit). In the temperature region between 130.2 and 107.0 K
the jump rate is comparable to the NMR time scale and the
typical effects on the line shape of the solid echo spectra
97
are
visible. At 88.4 K the slow jump limit (low-temperature limit)
is reached with the solid II spectrum with Q
zz
= 133 kHz.
From the simulation of the data the rate constants of the 601
jumps around the molecular axis as a function of the inverse
temperature and from this the activation energy E
a
= 16.8 kJ
mol
1
and the pre-exponential factor k
0
= 2.0 10
13
s
1
of
the six-fold jump were determined.
20,67
4846 |Phys.Chem.Chem.Phys., 2007, 9, 4843–4853 This journal is cthe Owner Societies 2007

The
2
H-NMR spectra of benzene-d
6
in SBA-15 (Fig. 3(b))
are a superposition of two different components. In both
components we note the absence of any signals characteristic
for an intermediate rotational exchange regime in the
2
H-NMR spectra, i.e. spectra with the line shape of bulk
benzene at temperatures in the range of 100–130 K. Here,
the benzene molecules are found in two separate Pake sub-
spectra of solid I and solid II type with temperature dependent
relative concentrations. At higher temperatures most mole-
cules are in the solid I-like state. Upon lowering of the
temperature more and more of the benzene molecules are
found in the solid II-like line. In particular there is no longer
a well defined phase transition temperature for the freezing of
the rotational motion. Similar behavior is found for the
temperature range from 154 to 204 K. Here the spectra are a
superposition of the solid I spectrum and the liquid-like
narrow line. This is the typical behavior of a glass-like
amorphous phase with a broad distribution of activation
energies. This result shows that an amorphous benzene phase
is formed inside the pores of SBA-15.
Fig. 3(c) displays the experimental
2
H-NMR spectra of
benzene-d
6
confined in the pores of the mesoporous silica
MCF in the temperature range between 90.1 and 180.3 K.
At all temperatures the spectra exhibit a fairly complex line
shape. Each spectrum represents a superposition of several
sub-spectra with varying intensity ratios. A simulation of these
spectra as a superposition of an inner crystalline and an
amorphous surface benzene phase, characterized by super-
position of solid I and solid II components with crystalline
components from the intermediate regime, leads to an excel-
lent reproduction of the experimental line shape, in particular
for the spectra measured at 110 and 120 K.
From the intensities of the spectral components the relative
numbers of molecules in the amorphous and crystalline phases
can be determined. Employing the spectra at 110, 120 and
180 K the determined ratios are:
46
I
Core
:I
Glass
= 0.82 : 0.18 at
110 K, I
Core
:I
Glass
= 0.79 : 0.21 at 120 K and I
Core
:I
Glass
=
0.77 : 0.23 at 180 K, which coincide within the experimental
error to an average value of I
Core
:I
Glass
= 0.8 : 0.2. Knowing
the pore diameter it is possible to determine the thickness of
the amorphous surface phase to 2–3 molecules of benzene
from this value.
Guest molecules with strong surface interactions
The situation changes strongly when the guest molecules can
exhibit strong surface interactions, as for example hydrogen
bonds to the silanol groups. Here two limiting cases are of
interest, namely molecules which interact only strongly with
the surface, as for example pyridine, and molecules where
interactions among themselves compete with the surface
interaction, as for example in water.
Pyridine
In the case of pyridine the ring nitrogen is a strong hydrogen
bond acceptor. As a result of this the pyridine interacts
Fig. 3
2
H-Solid-state NMR spectra of benzene-d
6
. (a) Experimental and simulated crystalline bulk benzene in the temperature region where the
transition from the pure solid II type (88.4 K) to the pure solid I type (206.6 K) spectrum occurs; all molecules have the same rotational correlation
time, which reflects the crystalline order of the system. (b) Amorphous benzene inside mesoporous silica SBA 15 (pore diameter 8.0 nm); below
120 K the spectra are always the weighted superposition of the solid II and the solid I spectrum; starting at 154.3 K and above a narrow central line
appears which is characteristic for liquid-like benzene. (c) Benzene inside mesoscopically organized controlled porous glass MCF (pore diameter
30 nm). Note that the spectra in (c) are decomposable into a crystalline component with spectra like (a) and an amorphous component like (b).
(Figures adapted from data from refs. 20 and 46). From the intensity ratio of these two components the thickness of the amorphous surface layer is
determinable.
46
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strongly with the surface. The results of these interactions
are most pronounced for low filling factors, where mainly a
monolayer equivalent of pyridine molecules is inside the
pores. In the case of pyridine a combination of several
experimental techniques reveals the dynamic properties. From
static
15
N-CP-NMR and
2
H-solid echo NMR of labeled
pyridine (Fig. 4(a)–(e)) the rotational dynamics of the pyridine
is revealed. Calculations of possible rotations reveal that
the rotational motion of the adsorbed pyridine molecules
(see Fig. 5(c)) is describable as a combined rotation around
the Si–O bond (R
2
) and an overall rotation (R
1
) of the pyridine
molecule, which is connected to a surface hopping process.
A rotation around the OHN hydrogen bond is not
observed, presumably due to steric hindrance.
42
From the
residual anisotropy of the NMR interaction tensors at high
temperatures it is possible to elucidate a semi-quantitative
model of the silica surface (Fig. 5(d)). While the smaller
MCM-41 has relatively smooth surfaces, the large diameter
SBA-15 is characterized by a rough surface with silica
islands.
42
The second rotation (R
1
) is connected to the breaking and
reformation of the hydrogen bond to the surface. Thus the
motion of the pyridine molecules is a surface hopping, which
results in a complicated interplay of rotational and transla-
tional motions (see Fig. 5(a)).
The translational diffusion of pyridine as a guest molecule
inside mesoporous MCM-41 is studied with stray field gradient
NMR for different filling factors of the pyridine (see Fig. 4(h)),
ranging from a mono-molecular layer of pyridine on the inner
surfaces of the pores to nearly completely filled pores.
20
The
analysis of the resulting diffusion data reveals as expected a
strongly anisotropic diffusion tensor. At a low filling factor of
nominal 25%, which corresponds to a monolayer coverage, the
principal components of the diffusion tensor are D
||
=1.0
10
9
m
2
s
1
for the parallel and D
>
=3.72.0 10
11
m
2
s
1
for the perpendicular component. While the perpendicular com-
ponent is independent of the filling factor, the parallel component
grows with the pore filling. This is an indication that at higher
filling factors the diffusion is exchange mediated (see Fig. 5(b)).
From
15
N-CP-MAS NMR (see Fig. 4(g) and (f)) it is evident that
these exchange processes are fast on the NMR time scale.
Water
The situation is even more complicated if the guest molecule is
water. The special physical and solvent properties of the water
Fig. 4 Pyridine as a guest in mesoporous silica. (adapted from refs. 39 and 42): (a) The experimental room temperature
15
N-NMR spectrum
obtained with cross polarization and high power proton decoupling reveals a residual anisotropy. The simulation (b) of the spectrum is employing
the motional model described in Fig. 5(c) using an –SiOHN angle of a= (49 2)1and the
15
N CSA data from ref. 98 shown in (c). (d, e)
Superposition of experimental and simulated
2
H-NMR solid-echo spectra of pyridine-4-d
1
as guest in MCM-41. The low-temperature (175 K)
spectrum (e) exhibits the full size and anisotropy of a quadrupolar-CD tensor; the room-temperature spectrum (d) exhibits a reduced quadrupolar
line width due to the molecular motions of the pyridine on the silica surface. The simulations are done as in (b). (f, g) Room-temperature and low-
temperature CP-MAS NMR spectra of pyridine-
15
N in silica. In the low-temperature spectrum (g) two lines are visible which correspond to free
and hydrogen bound pyridine. In the room-temperature spectrum (f) only a single line is visible, which shows that all pyridine molecules are in fast
exchange with each other.
4848 |Phys.Chem.Chem.Phys., 2007, 9, 4843–4853 This journal is cthe Owner Societies 2007

stem largely from its extraordinary internal cohesiveness,
compared to most other low molecular weight liquids. This
cohesiveness is mainly the result of the water molecules high
polarity and their ability to form hydrogen bonded networks
among themselves, as for example in the frozen phase or in the
bulk liquid phase. In restricted geometries the water molecules
can also interact with the surfaces through hydrophobic and
hydrophilic interactions and hydrogen bond interactions;
hence there is a competition between the surface-liquid and
liquid–liquid interactions. This competition leads to interest-
ing new structures of water, as for example partial ordering of
water molecules in the vicinity of the confining surface.
Important examples of such systems are water molecules
enclosed in porous media like zeolites
99
or cements,
100
or
water molecules in hydration shells of proteins.
61,8083,101,102
or bound water near the pore surface
26,103113
or water
clusters in narrow pores
114
or wells on nanoparticles.
115
Theoretical description of the behavior of water on silica
surfaces is possible by the use of CPMD calculations.
116
In the case of silica surfaces the water molecules are
characterizable by virtue of their apparent chemical shifts.
These chemical shifts are the result of an exchange of protons
bound to the silica or water molecules. Fig. 6 displays the
different possible scenarios of water molecules hydrogen
bonded to the silica surface or among each other or free. Each
scenario is characterized by an individual
1
H-chemical shift.
While in principle these
1
H-chemical shifts are unique for a
defined structure of the water interacting with the surface and
other water molecules, in practice it must be taken into
account that dynamic exchange effects such as molecular
reorientations of the water molecules, rotations of the surface
–SiOH groups and proton transfer, can and in general will,
cause changes of these chemical shifts that lead to complete or
full averaging of the line positions. Nevertheless it is still
possible to distinguish between different environments and
thus determine the relative amounts of the individual species
by virtue of the
1
H-chemical shift, as discussed by Gru
¨nberg
et al.
44
Fig. 5 (a) Surface hopping of pyridine adsorbed via hydrogen bonding on the surface of mesoporous silica, developed from Fig. 4. The motion is a
superposition of translational diffusion (b), which is analyzed by NMR diffusometry
20
and rotational motion (c), which is analyzed by
15
N and
2
H
line shape analysis.
42
From the rotational data a detailed model of the surface morphology is deduced.
Fig. 6 Overview of possible –OH groups in the water/silica samples
and the corresponding chemical shifts in ppm (TMS). Upper row:
chemical shifts of the constituents of monomeric water, water clusters
and silanol groups. Lower row: chemical shifts (adapted from refs. 44,
54, 117 and 118) observed in various hydrogen bonding scenarios.
This journal is cthe Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 4843–4853 |4849

Fig. 7 displays the
1
H-MAS-NMR spectra of water in
MCM-41, SBA-15 and CPG 10-75 for different water contents
recorded at a MAS frequency of 10 kHz. All presented spectra
are normalized to their maximal intensity. The spectra, in
general, exhibit several resolved lines with typical line-widths
of (0.2–0.7) ppm. The
1
H-MAS-NMR spectrum which was
measured after drying on the vacuum line exhibits a single line
at d= 1.74 ppm in all samples. This single line is characteristic
for surface silanol groups. However, even in the nominally
completely dried sample some spectral intensity between two
and three ppm in the SBA-15 and CPG 10–75 samples is
observed.
This spectral intensity is evidence for strongly bound resi-
dual water molecules, which are probably stuck in the pore
defects and hydrogen bonded to silica surface. This result is in
contrast to the well ordered surface of MCM-41, where all
water molecules are easily removable. Comparing the ob-
served chemical shifts of the water signals at low filling levels
(2.0–3.2 ppm) with the values given for the possible chemical
shifts of –OH groups (see Fig. 6), it is evident that none of
these shifts matches the observed shift. From this it follows
that the observed shift is the result of a weighted averaging
between different water species, which are in fast chemical
exchange among themselves. Consequently all water molecules
contribute to hydrogen bonds. Upon further increase of the
water content two processes start: on the one hand the average
of the line is low-field shifted towards the chemical shift values
of water clusters and on the other hand the number of free
surface –SiOH groups is reduced, which is visible as a decline
in the intensity of the 1.74 ppm line. This process continues
until the whole accessible surface is covered by a nominally
monomolecular water layer. In the case of SBA-15, this
happens at a water content of 8%. This corresponds to
ca. 3.5 water molecules nm
2
. This shows that now all –SiOH
groups are part of a hydrogen bonding network of exchanging
hydrogen bonded protons (see Fig. 6).
Upon further increase of the water content, the silica
systems behave differently. In the case of the narrow pore
MCM-41, a bifurcation in the spectra (see 23% sample) occurs
with two lines, one at the position of the monomolecular
surface layer and the second at the position of a pore
completely filled with water. In the case of the larger pores
of CPG 10-75 and SBA-15 the line close to 3 ppm increases in
intensity and shifts low-field towards the chemical shift values
of water clusters. This indicates different pore filling mechan-
isms in the pores. The wide pores are filled layer wise starting
at the pore wall and the narrow pores are filled axial via water
droplets.
Binary mixtures
As was shown in the previous examples, already simple liquids
inside mesoporous silica exhibit new structures and complex
dynamics, which may differ strongly from the bulk behavior.
Thus one can expect an even more complicated behavior if
mixtures of several liquids are filled into the pores, in parti-
cular if the bulk mixtures of these liquids already exhibit a
complicated phase behavior themselves. These effects were
investigated by a binary mixture of water and isobutyric acid
(iBA), a system with well-known bulk phase behavior. This
has frequently been used as a model system in experimental
studies of critical phenomena and phase separation in binary
liquid mixtures. It is a representative example of ‘‘simple’’
water-containing systems, which have a lower miscibility gap,
where the phase separation occurs at low temperatures (see
Fig. 8(b)). Moreover it is a model for the large family of
water-containing binary mixtures, and for their behavior in a
hydrophilic porous matrix.
Fig. 7 Experimental
1
H-MAS solid-state NMR spectra (10 kHz) of water in (a) MCM-41, (b) SBA-15 and (c) CPG 10-75. Note the bimodal
filling mechanism in the narrow pores of MCM-41 and the smooth variation of the CS in SBA-15 and CPG 10-75 (adapted from refs. 44 and 118).
4850 |Phys.Chem.Chem.Phys., 2007, 9, 4843–4853 This journal is cthe Owner Societies 2007

Before studying the binary mixture we investigate as a
starting point the behavior of the pure iBA inside the pores.
49
Fig. 8(a) compares the temperatures of the melting of the
translational degrees of freedom obtained by DSC with the
melting temperatures of the rotational degrees of freedom
obtained by
2
H solid-state NMR as a function of the inverse
pore diameter. The full line indicates the Gibbs–Thomson
equation with a constant C
GT
= 80 K nm. Similar to the case
of the benzene there are pronounced deviations of the data for
iBA in MCM-41 from this relation. The rotational melting
temperatures of d
6
-iBA are 20 to 45 K lower than the
respective thermodynamic melting temperatures T
m
. These
reductions of the melting points and the deviations from the
Gibbs–Thomson equation indicate a strong interaction
between the iBA molecules and the silica surface. In ref. 49
this deviation from the Gibbs–Thomson equation in MCM-41
is tentatively attributed to the strong binding of a monolayer
of iBA molecules to the silica surface via hydrogen bonds.
Owing to the amphipathic nature of the iBA-molecule, this
renders the pore walls more hydrophobic than in the pristine
state, such that the interaction of the second layer of molecules
with the modified pore wall is grossly different. The conse-
quences of this effect are clearly more pronounced for narrow
pores (MCM-41) than in wider pores (CPG-10 and SBA-15).
This strong interaction between the iBA and the silica pores
is also present in the case of the binary mixture.
119
The
temperature dependence of the self-diffusion data (Fig. 8(c))
reveals a strong deviation from the Einstein behavior above
39 1C, where a slowdown of the diffusion is observed. This
slowdown is the result of the binding of iBA molecules to the
silica surface,
118
which is blocked for the iBA molecules at
lower temperatures by water molecules. This interpretation is
corroborated by the spin/spin relaxation data (Fig. 8(d)),
which reveal the presence of immobilized iBA molecules with
a very short T
2
at temperatures above 39 1C.
Discussion
As the previous examples from our group and many other
research results worldwide have shown, the combination of
various NMR techniques indeed provide the means for a full
characterization of the behavior of guest molecules inside the
pores of porous silica and related materials. These techniques
reveal not only the basic physico-chemical properties of these
molecules, but show also that the guest molecules exhibit new
features which are not known for the bulk systems. These new
features now naturally raise the question, what are typical
effects of the confinement or the mesoscopic structure of the
Fig. 8 Properties of pure isobutyric acid and water–isobutyric acid mixtures inside mesoporous silica. (a) Decoupling of rotational and
translational melting point. (b) Phase diagram of the system isobutyric acid plus deuterated water (iBA/D
2
O) (adapted from ref. 119). The arrow
marks the composition studied in (Fig. 8c, 8d) as a guest inside the mesoporous silica. (c) Stimulated echo decay curves of iBA inside CPG 10-75
(experiment and calculation). Note that the fastest decay is observed for the 39 1C curve. This shows that the diffusion coefficient gets smaller above
this temperature. (d) Temperature dependence of the spin–spin relaxation time T
2
in the confined mixture iBA/D
2
O. The squares show the second
component of spin–spin relaxation at high temperatures, which are attributed to immobilized iBA molecules, which are bound to the surface.
This journal is cthe Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 4843–4853 |4851

system. From the experimental results on benzene and iBA
inside the mesoporous silica two effects are evident, namely the
reduction of the thermodynamic melting point and the decou-
pling of the rotational and translational degrees of freedom.
For the first effect the Gibbs–Thomson equation is the com-
monly accepted explanation. However, as discussed above
there are deviations from the Gibbs–Thomson equation,
which show that even in relatively simple systems the thermo-
dynamic behavior is not yet completely understood. Even
more complicated is the behavior of the rotational melting
temperature on the pore size. The size dependence on the
temperature shows that new theoretical models are necessary
for its understanding. A similar situation is found for the glass
transition temperature in confined systems. Quoting the review
of McKenna:
27
‘‘We survey the observations that show that the
glass transition temperature decreases, increases, remains the
same or even disappears depending upon details of the experi-
mental (or molecular simulation) conditions. Indeed, different
behaviors have been observed for the same material depending on
the experimental methods used.’’
Taking into account that in the case of guest molecules
inside the pores in general amorphous surface phases are
formed, which exhibit glass-like dynamical behavior, one can
at least speculate that both problems are related to similar
physical phenomena and might be soluble by the same
approach.
Conclusions
Employing solid-state NMR techniques it is nowadays easily
feasible to study confined molecules, their mutual interactions
and their interactions with the confining surface on a
molecular level. From these studies it is possible to reveal
the structure of the confined molecules inside the pores, their
rotational and translational dynamics. These data now permit
the development of a description of these confined systems
which goes beyond the thermodynamic level.
Acknowledgements
Financial support by the DFG Sonderforschungsbereich
SFB-448 is gratefully acknowledged.
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