back to results for ""
Below is a cache of http://www.solids.bnl.gov/home/ben/publications/OCKO.PDF.PAPERS/OCKO.PDF.PAPERS/sloutskin_2002b.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Surface and bulk phase behavior of dry and hydrated tetradecanol:octadecanol alcohol mixtures Surface and bulk phase behavior of dry and hydrated
tetradecanol:octadecanol alcohol mixtures
E. Sloutskin
Physics Department, Bar Ilan University, Ramat Gan 52900, Israel
E. B. Sirota
ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801
H. Kraack, O. Gang,
a)
and A. Doerr
b)
Physics Department, Bar Ilan University, Ramat Gan 52900, Israel
B. M. Ocko
Physics Department, Brookhaven National Laboratory, Upton, New York 11973
M. Deutsch
c)
Physics Department, Bar Ilan University, Ramat Gan 52900, Israel
Received 18 January 2002; accepted 6 February 2002
Surface freezing was studied in dry and hydrated octadecanol:tetradecanol (C
18
OH:C
14
OH)
mixtures, using surface tension and synchrotron x-ray surface diffraction techniques. Even small
amounts of admixed C
18
OH were found to induce surface freezing in C
14
OH, which does not
exhibit this effect when pure. The phase diagram of the bulk was measured by calorimetry and bulk
x-ray diffraction. Upon increasing the bulk mole fraction of C
18
OH
a sharp increase in the bulk
supercooling occurs at
0.4 in dry mixtures, while no supercooling was observed for the hydrated
mixtures. A simple thermodynamical model based on the theory of s-regular mixtures is shown to
account well for the dependence of the surface freezing onset temperature of both dry and hydrated
mixtures, and the hydrated bulks freezing temperature on
. Only a phenomenological description
exists for the dry bulks phase diagram. This study is expected to provide a baseline for the general
surface and bulk behavior of long-chain alcohol mixtures. © 2002 American Institute of Physics.
DOI: 10.1063/1.1465401
I. INTRODUCTION
Normal alcohols are the simplest of the substituted
hydrocarbons.
Having
the
molecular
structure
CH
3
(CH
2
)
n
1
OH denoted C
n
OH in the following they are
almost identical with normal-alkanes CH
3
(CH
2
)
n
2
CH
3
,
the only difference being the exchange of a H on one termi-
nal methyl group by a hydroxyl OH group. However, this
small change breaks the inversion symmetry between the
two ends of the molecule and renders it slightly polar. More
importantly, it modies the molecular interactions by allow-
ing the formation of hydrogen bonds between adjacent mol-
ecules through their hydroxyl groups, while alkanes interact
solely by vdW forces.
1
Both monocomponent alkane and al-
cohol melts were found to exhibit surface freezing SF , i.e.,
the formation, by a rst order transition, of an hexagonally
packed, quasi-2D solid layer at the liquidvapor interface at
a temperature T
s
several degrees above their bulk freezing,
T
f
.
25
The surface-frozen layer in alkanes is a single mol-
ecule thick, while in alcohols it is a bilayer, with the OH
groups residing at the center of the bilayer and forming hy-
drogen bonds between the adjacent molecules from the upper
and lower monolayers comprising the bilayer.
5,6
SF occurs in
alcohols of lengths 16
n
28, but, unlike in alkanes, only
molecules having an even number of carbons show this ef-
fect. This oddeven effect may be related to differences in
the orientation of the terminal OH group relative to the mo-
lecular axis, which renders the formation of hydrogen bonds
unfavorable in odd alcohols.
5
One attempt to explain the oc-
currence of surface freezing in pure alkanes employed the
entropic stabilization of the surface layer by thermal uctua-
tions of the surface molecules along their long axes, which
for bulk molecules are suppressed by neighboring layers.
7,8
Another explanation assigned it to a wetting effect of the
liquid layer by a solid monolayer due to a favorable surface
energy balance established, over a certain temperature range,
among the temperature-dependent entropic terms of the vari-
ous surface energies involved.
9
For alcohols, a small but
non-negligible contribution is also obtained from the hydro-
gen bonding.
10
For long molecules it has been argued that the
entropic reduction of the free energy, due to positional uc-
tuations along the axis and rotational disorder of the molecu-
lar planes, is large enough to stabilize the surface frozen
layer. However, as n goes below 16, the reduction in the free
energy becomes too small to stabilize the surface frozen
layer with respect to the liquid surface phase, and the SF
vanishes. The present study shows, among other things, that
a
Present address: Gordon McKay Lab, Division of Applied Sciences, Har-
vard University, Cambridge, Massachusetts 02138.
b
Present address: Continental AG, Jaedekamp 30, D-30419 Hannover,
Germany.
c
Author to whom correspondence should be addressed. Electronic mail:
deutsch@mail.biu.ac.il
JOURNAL OF CHEMICAL PHYSICS
VOLUME 116, NUMBER 18
8 MAY 2002
8056
0021-9606/2002/116(18)/8056/11/$19.00
© 2002 American Institute of Physics
Downloaded 04 Nov 2003 to 130.199.3.22. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp adding even a small amount of C
18
OH to the bulk phase of
C
14
OH induces SF in the short C
14
OH molecules. As previ-
ous studies show,
11
it is reasonable to assume that, in general,
by mixing molecules of two different lengths it is possible to
reach regions in phase space where the temperature ranges of
existence
T
T
s
T
f
are nonzero, and larger than those of
the pure components. Moreover, new phases and transitions
between them can be discovered by varying the molar con-
centration
of the mixtures and the temperature T .
12,13
The importance of understanding the behavior of alkane and
alcohol mixtures is further enhanced by the fact that in most
real-world situations the alkanes and alcohols do not exist
as pure materials, but as mixtures. Thus, the investigation of
mixtures and their surface behavior has important implica-
tions not only for basic science, but also for applied science
and industry.
We have chosen to study the C
14
OH and C
18
OH alcohols
because their chain lengths are close enough to form a solu-
tion in the solid state, yet different enough to be clearly
distinguishable by x-ray reectivity XR measurements and
by thermodynamical techniques. The surface behavior of
mixtures is almost always more complicated than that of the
bulk, because of phenomena like prewetting, surface enrich-
ment and surface layering.
14
In this study, the theoretical
description of the surface behavior turned out to be simpler
than that of the bulk, since in the bulk the existence of ki-
netic barriers for phase formation renders the dry bulk be-
havior very complicated from a theoretical point of view.
Because of the reduced dimensionality at the surface and the
possibility of molecular exchange between the surface and
the bulk phases, no kinetic barriers exist for the formation of
the surface-frozen phase in alkanes and alcohols.
3,11,15
A sig-
nicant increase in the temperature range of SF was ob-
served for alcohol surfaces residing in a saturated water va-
por atmosphere denoted hereafter as hydrated or wet
mixtures , as compared to those residing in a dry atmosphere
denoted dry mixtures .
5,16
Water intercalation into the
center of the SF bilayer varies the hydrogen bonding, and
imparts an increased stability to the SF bilayer. Due to an
anomalous increase in water solubility in the solid rotator
phase as compared to the liquid phase, the hydrated alcohols
solidify at higher temperatures than the dry ones, both at the
surface
5
and in the bulk.
17
However, the temperature shifts
relative to the dry case are not equal for the bulk and the
surface, rendering the existence ranges
T signicantly
larger than those in the dry samples. We also nd that hydra-
tion causes bulk supercooling to disappear. The theory of
s-regular binary mixtures
18,19
is used here to account for the
behavior of T
s
( ) in hydrated and dry samples, with only a
single tting parameter: the repulsion energy between C
14
OH
and C
18
OH. The results are further supported by the
-variation of the thickness of the surface frozen layer, d,
derived from the XR measurements. The same theory ac-
counts for the behavior of the freezing temperature T
f
( ) of
the hydrated bulk. The behavior of the quasi-2D lattice con-
stant for different molar concentrations and temperatures was
investigated by grazing-incidence x-ray diffraction methods,
both in dry and hydrated samples. In two of the samples the
temperature range of SF was large enough to permit the mea-
surement of the linear expansion coefcient of the surface
frozen layer. Calorimetric studies were performed to study
the phase diagram of the bulk in thermodynamical equilib-
rium and under kinetic constraints. The bulk phase diagram
was also studied by x-ray powder diffractio