Observation of a dynamic crossover in water confined in double-wall carbon nanotubes
X.-Q. Chu,
1
A. I. Kolesnikov,
2
A. P. Moravsky,
3
V. Garcia-Sakai,
4
and S.-H. Chen
1,
*
1
Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2
Intense Pulsed Neutron Source Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
3
MER Corporation, Tucson, Arizona 85706, USA
4
NIST Center for Neutron Research, Gaithersburg, Maryland 20899-8562, USA
and Department of Material Science and Engineering, University of Maryland, College Park, Maryland 20742, USA
Received 19 June 2007; published 22 August 2007
High-resolution quasielastic neutron scattering spectroscopy was used to measure H
2
O hydrated double-wall
carbon nanotubes DWNT. The measurements were made at a series of temperatures from 250 K down to
150 K. The relaxing-cage model was used to analyze the quasielastic spectra. We observed clear evidence of
a fragile-to-strong dynamic crossover FSC at T
L
=190 K in the confined water. We further show that the
mean-square atomic displacement of the hydrogen atoms in water exhibits a sharp change in slope at approxi-
mately the same temperature 190 K. Comparing the result with that obtained from the confined water in
hydrophilic porous silica material MCM-41, we demonstrate experimentally that water confined in a hydro-
phobic substrate exhibits a lower dynamic crossover temperature by T
L
35 K.
DOI: 10.1103/PhysRevE.76.021505 PACS numbers: 64.70.Pf, 61.12.Ex, 61.20.Lc
I. INTRODUCTION
Bulk water shows many anomalous behaviors, especially
in the supercooled temperature range below 0 °C: An
anomalous increase of thermodynamic response functions
and an apparent divergent behavior of transport coefficients
toward a singular temperature T
s
=228 K 1. To explain
these anomalies it was proposed that supercooled water at
ambient pressure consists of a mixture of a high HDL and
a low LDL density water. At sufficiently low temperatures
and higher pressures there is a first order phase transition line
in the P-T plane separating the HDL from the LDL, the end
point of which is the second liquid-liquid critical point of
water conjectured to be around T
C
'
220 K and P
1000 bar 2. With an increasing supercooling the struc-
tural relaxation time of water shows a steeper temperature
dependence compared to that of the Arrhenius law. This be-
havior is known as a characteristics of a fragile liquid. It is
known that many glass forming liquids exhibit the fragile
behavior at moderately supercooled temperatures and then at
sufficiently low temperature make a transition to a strong
Arrhenius liquid. Water is supposed to have a glass transi-
tion temperature at T
g
=165 K 3. Unfortunately for bulk
water, the observation of the fragile to strong dynamic tran-
sition, as well as the experimental study of the water behav-
ior around the second critical point are impossible due to
intervention of the homogeneous nucleation phenomenon. It
starts at T
H
= 235 K, resulting in crystallization to form a
hexagonal ice before it reaches the supercooled range of in-
terest. It was predicted 4 that water should also show the
transition to a strong liquid in this inaccessible temperature
range, around T
L
=228 K.
Many theoretical simulations and experimental studies
showed that the nucleation of water in nanometer size con-
finement can be strongly suppressed down to about T
g
, thus
opening an opportunity to study water behavior in this “no
man’s land” range. A series of quasielastic neutron scattering
QENS experiments were performed recently on water in
porous Vycor glass 5 and MCM-41-S materials, both hav-
ing hydrophilic surfaces, with well calibrated pore size 50 Å
in the case of Vycor glass and 10 to 18 Å in the case of
MCM-41-S, where the freezing process of water was
strongly inhibited down to 160 K 6–8. It was clearly
shown that water in these confinements exhibits a dynamic
crossover from fragile to strong liquids at T
L
225 K 8.
Recently, a series of neutron diffraction and NMR relaxation
measurements on water/ice at a silica surface also demon-
strate some kind of transition occurring at around 220 K 9.
MD simulations predict 10,11 that the porous materials
with hydrophobic surface can further reduce T
L
compared to
water in hydrophilic confinement. One of these materials can
be carbon nanotubes CN.
Carbon nanotubes of nanometer diameter and micrometer
length, besides many other interesting properties, can serve
as a quasi-one-dimensional confinement for other materials.
Due to hydrophobic interaction of water with carbon atoms
CN can play a very important role in studying the properties
of confined water. Owing to very weak van der Waals type
interaction of water molecules with carbon 12 compared to
a hydrogen bond interaction between water molecules, water
confined in small diameter CN can be considered as quasi-
one-dimensional water cluster.
Many MD simulations 13–24 and recently a few experi-
mental studies 18,25–29 were dedicated to understand the
structure and dynamics of water in single wall carbon nano-
tubes SWNT. The behavior of water in small diameter
SWNT cannot be continuously scaled by nanotube diameter.
Water cannot enter SWNT at all for nanotubes of diameter
smaller than 8 Å. Water in 6,6 SWNT 8 Å diameter can
enter the nanotube as a small file or chain of water mol-
ecules and fast transfer through the nanotubes, as was shown
by MD simulations 13. Neutron scattering study of water in
10,10 SWNT 14 Å diameter revealed an anomalously
*Author to whom correspondence should be addressed.
sowhsin@mit.edu
PHYSICAL REVIEW E 76, 021505 2007
1539-3755/2007/762/0215056 ©2007 The American Physical Society 021505-1