Reply to Soper: Density measurement of confined water
with neutron scattering
The MIT Faculty has made this article openly available. Please share
how this access benefits you. Your story matters.
Citation
Zhang, Y. et al. “Reply to Soper: Density Measurement of
Confined Water with Neutron Scattering.” Proceedings of the
National Academy of Sciences 108.47 (2011): E1193–E1194.
Web. Copyright ©2011 by the National Academy of Sciences
As Published
http://dx.doi.org/10.1073/pnas.1113408108
Publisher
National Academy of Sciences
Version
Final published version
Accessed
Thu May 26 10:32:19 EDT 2016
Citable Link
http://hdl.handle.net/1721.1/73160
Terms of Use
Article is made available in accordance with the publisher's policy
and may be subject to US copyright law. Please refer to the
publisher's site for terms of use.
Detailed Terms
LETTER
Reply to Soper: Density measurement
of confined water with neutron
scattering
This is a response to Soper’s two comments (1) regarding our
papers (2, 3) in PNAS that (a) the distribution of water across
the pores is not uniform and (b) the majority of water may reside
outside the pores. Here, we show that we have given proper
consideration to both issues and have reconfirmed the validity of
our method and conclusion as elaborated in the following.
The possibility that layering effects across the pores may introduce errors in associating the (100) interchannel peak height
with density is not a new idea (reference 3 in ref. 1), and it
has already been addressed (2). The arguments of Soper (4)
mainly rest on the assumption that the average density of water
does not depend on temperature, which is drawn by linking the
temperature-independent incoherent background of his experimental data to the water density and not to the amount of
sample exposed to the neutron beam. As a result, he constructs
a density profile with a large spatial variation across the pores
in an effectively underfilled condition ðρwater ¼ 0:70 g=cm3 Þ (4),
reproduced as Fig. 1A. The existence of such voids is disproved
by the measurement on the contrast-matched sample with D2O/
H2O mixtures, because no evidence of the scattering from the
voids can be recognized. Although the water distributions suggested by Soper (4) may reproduce the experimentally measured
intensity change of the (100) interchannel peak at some selected
temperatures, they are inconsistent with numerous experimental and simulation studies (references 32 and 55–61 in ref. 2)
and introduce unnecessary complications to the model. On the
other hand, we have estimated the effect of a density profile
based on these published simulation results (Fig. 1 B–D). The
layering effect on the (100) peak intensity is found to be negligible compared with the experimentally observed intensity
change shown (figure 2 in ref. 2) as long as the spatial variations
do not exceed such a level. We therefore believe our average
density approach to be valid and accurate.
As for the second point raised by Soper (4), we have confirmed
that the amount of excess water is negligible compared with the
www.pnas.org/cgi/doi/10.1073/pnas.1113408108
interior water by measuring the water vapor adsorption-desorption isotherm. Also, both differential scanning calorimetry
scans and inelastic neutron scattering measurements of the
generalized librational density of state do not detect any signs of
freezing. In any case, a small amount of external water is unlikely
to affect the behavior of water within the pores. The (100) interchannel peak is sensitive only to the scattering contrast between the water in the channels and the surrounding silica and
has nothing to do with the excess water. We are aware that the
pore size, especially for those <20 Å, may be considerably underestimated by the standard Barrett–Joyner–Halenda (BJH)
method. The BJH pore size of 15 Å used in our paper is thus
a nominal value to allow meaningful comparisons with other
measurements. Using the Kruk–Jaroniec method (5), the pore
size of the MCM-41-S-15 sample may well be 25 Å, implying
a hydration level of 0.45 gD2O/gSiO2 using Soper’s formula (4),
which is close to our measured value.
Yang Zhanga, Antonio Faraoneb,c, William A. Kamitakaharab, KaoHsiang Liud, Chung-Yuan Moud, Juscelino B. Leãob, Sung Changb,
and Sow-Hsin Chene,1
a
Neutron Sciences Directorate and Joint Institute for Neutron Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831;
b
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899; cDepartment of
Materials Science and Engineering, University of Maryland, College
Park, MD 20742; dDepartment of Chemistry, National Taiwan
University, Taipei 106, Taiwan; and eDepartment of Nuclear Science and Engineering, Massachusetts Institute of Technology,
Cambridge, MA 02139
1. Soper AK (2011) Density minimum in supercooled confined water. Proc Natl Acad Sci
USA 108:E1192.
2. Zhang Y, et al. (2011) Density hysteresis of heavy water confined in a nanoporous silica
matrix. Proc Natl Acad Sci USA 108:12206–12211.
3. Liu D, et al. (2007) Observation of the density minimum in deeply supercooled confined
water. Proc Natl Acad Sci USA 104:9570–9574.
4. Soper AK (2011) Density profile of water confined in cylindrical pores in MCM-41 silica.
ArXiv e-prints. Available at http://arxiv.org/abs/1107.3492v2. Accessed July 20, 2011.
5. Kruk M, Jaroniec M, Sayari A (1999) Relations between pore structure parameters and
their implications for characterization of MCM-41 using gas adsorption and X-ray
diffraction. Chem Mater 11:492–500.
Author contributions: Y.Z., A.F., W.A.K., K.-H.L., C.-Y.M., and S.-H.C. designed research;
Y.Z., A.F., W.A.K., K.-H.L., C.-Y.M., J.B.L., S.C., and S.-H.C. performed research; and Y.Z.,
A.F., W.A.K., K.-H.L., C.-Y.M., and S.-H.C. wrote the paper.
The authors declare no conflict of interest.
1
To whom correspondence should be addressed. E-mail: sowhsin@mit.edu.
PNAS | November 22, 2011 | vol. 108 | no. 47 | E1193–E1194
A
B
C
D
Fig. 1. Comparison of the density profiles and their simulated intensities. (A) Density profiles constructed by Soper (4). Note that water is allowed to
penetrate into the MCM-41-S silica material. The simulated intensity can be found in figure 7 in ref. 4. (B) Density profile based on published simulation results
with a 3-Å surface water layer of 10% higher density, for which the computed intensity, I(Q), and particle structure factor, P(Q), are presented in C and D with
two different approaches: the average density approach (green) and the core-shell model (red). The simulated intensities with the two approaches show
negligible difference at the (100) interchannel peak compared with the experimentally observed intensity change shown in figure 2 in ref. 2.
E1194 | www.pnas.org/cgi/doi/10.1073/pnas.1113408108
Zhang et al.