Mixed saturated-unsaturated alkyl-chain assemblies: Solid solutions of zinc stearate and zinc oleate

Article abstract of DOI:10.1021/jp068675x

The linear saturated stearic acid and the bent mono-unsaturated oleic acid do not mix and form solid solutions. However, the zinc salts of these acids can. From X-ray diffraction and DSC measurements we show that the layered zinc stearate and zinc oleate salts form a homogeneous solid solution at all composition ratios. The solid solutions exhibit a single melting endotherm, with the melting temperature varying linearly with composition but with the enthalpy change showing a minimum. By monitoring features in the infrared spectra that are characteristic of the global conformation of the hydrocarbon chain, and hence can distinguish between stearate and oleate chains, it is shown that solid solution formation is realized by the introduction of gauche defects in a fraction of the stearate chains that are then no longer linear. This fraction increases with oleate concentration. It has also been possible from the spectroscopic measurements to establish a quantitative relation between molecular conformational order and the thermodynamic enthalpy of melting of the solid solutions.

Full text of DOI:10.1021/jp068675x

5212
J. Phys. Chem. B 2007, 111, 5212-5217
Mixed Saturated-Unsaturated Alkyl-Chain Assemblies: Solid Solutions of Zinc Stearate
and Zinc Oleate
S. Barman† and S. Vasudevan*,‡
Solid State and Structural Chemistry Unit, and Department of Inorganic and Physical Chemistry, Indian
Institute of Science, Bangalore-566012, India
ReceiVed: December 18, 2006; In Final Form: February 25, 2007
The linear saturated stearic acid and the bent mono-unsaturated oleic acid do not mix and form solid solutions.
However, the zinc salts of these acids can. From X-ray diffraction and DSC measurements we show that the
layered zinc stearate and zinc oleate salts form a homogeneous solid solution at all composition ratios. The
solid solutions exhibit a single melting endotherm, with the melting temperature varying linearly with
composition but with the enthalpy change showing a minimum. By monitoring features in the infrared spectra
that are characteristic of the global conformation of the hydrocarbon chain, and hence can distinguish between
stearate and oleate chains, it is shown that solid solution formation is realized by the introduction of gauche
defects in a fraction of the stearate chains that are then no longer linear. This fraction increases with oleate
concentration. It has also been possible from the spectroscopic measurements to establish a quantitative relation
between molecular conformational order and the thermodynamic enthalpy of melting of the solid solutions.
Introduction
of the chain effectively decouples the thermal evolution of
conformational disorder in the chain segments on either side of
the double bond, and the melting of each of these segments in
the assembly occurs as an independent event.10 Although crystal
structures of ZnSt and ZnOl have not been reported, on the basis
of X-ray absorption,11 diffraction,7,8,12 and infrared spectroscopic
studies,7,10,12 it has been established that these compounds have
a layer structure with tetrahedrally co-coordinated Zn cations
connected by carboxylate bridges to four stearate/oleate chains.
We had shown previously that in ZnSt the alkyl chains of the
bilayer adopt an all-trans conformation with all 16 methylene
units of the stearate chain in trans registry while in ZnOl the 7
methylene units on either side of the double bond are organized
in all-trans segments.7,10 On melting, the coordination of the
carboxylate to zinc changes from bridging bidenate to one that
has considerable monodentate character, resulting in the breakup
of the inorganic sheets.7 Metal coordination plays an important
role in the thermal stability of these compounds. Both ZnSt and
ZnOl are solids at room temperature, each with a melting point
considerably higher than that of the corresponding fatty acid.10
Coordination also defines the closest interchain distance in the
soap, 4.69 Å, which is the Zn-Zn distance in the layer.11 In
the corresponding fatty acids the interchain distance is consider-
ably smaller, typically 3.7 Å.13 Here we have used DSC and
X-ray diffraction measurements to establish that ZnSt and ZnOl
form a solid solution over the entire composition range and
vibrational spectroscopy to understand how small changes in
conformation allow stearate and oleate chains to pack together
to form ZnStxOl(1-x) solid solutions.
Linear and bent rodlike molecules do not mix. The linear
saturated fatty acid, stearic acid, and the bent mono-unsaturated
fatty acid, oleic acid (cis-9-octadecanoic acid), for example, do
not form a solid solution at any composition ratio.1 This is
understandable; it is difficult to pack the linear stearic acid with
the bent oleic acid. Even elaidic acid (trans-9-octadecanoic) in
which the segments on the either side of the double bond are
trans and hence the molecule “nearly” linear does not form a
solid solution with stearic acid except over an extremely narrow
composition range.1 These molecules also do not mix in
Langmuir-Blodgett films. Stearic and elaidic acids, for ex-
ample, are immiscible in monolayers irrespective of the surface
pressure.2 There are, however, natural situations like lipid “rafts”
in cell plasma membranes in which submicron domains of
sphingolipids that contain mostly long-chain saturated acyl
chains coexist with glycerophospholipids that consist of unsatur-
ated acyl chains that have a “kinked” or bent structure.3-5 The
sphingolipids pack effectively and hence their melting temper-
atures are higher than those of the corresponding bent glycero-
phospholipids. It has been suggested that it is the differential
packing ability that leads to differential miscibility and hence
to phase segregation in the membrane.6 A variety of biological
functions have been attributed to these structures.3,4
Here we report that zinc salts of the saturated stearic acid
and the unsaturated oleic acid can form homogeneous solid
solutions over the entire composition range. These compounds
were chosen for this study because zinc stearate (ZnSt), unlike
most other polyvalent metal soaps, is known to pass directly
from the solid to liquid phase at 403 K,7 without the complica-
tion of any intermediate mesophases.8,9 In zinc oleate (ZnOl),
on the other hand, the presence of the double bond in the middle
Experimental
ZnSt and ZnOl were prepared by addition of aqueous
solutions of the sodium salt of the corresponding fatty acid to
ZnCl2 solution. The resulting precipitate was washed with etha-
* Author to whom correspondence should be addressed. Tel: +91-80-
2293-2661. Fax: +91-80-2360-1552/0683. E-mail: svipc@ipc.iisc.ernet.in.
† Solid State and Structural Chemistry Unit.
nol and water. Solid solutions of ZnSt and ZnOl, (ZnStxOl(1-x)
)
‡ Department of Inorganic and Physical Chemistry.
were prepared by mixing the required quantities of ZnSt and
10.1021/jp068675x CCC: $37.00 © 2007 American Chemical Society
Published on Web 04/19/2007

Mixed Saturated-Unsaturated Alkyl-Chain Assemblies
J. Phys. Chem. B, Vol. 111, No. 19, 2007 5213
Figure 1. (a) DSC traces of ZnStxOl(1-x) for different compositions. Variation of (b) the melt transition temperature, Tm, and (c) enthalpy change,
∆H, with composition.
ZnOl, heating to above the melt temperature (410 K), and then
cooling to ambient temperature.
All points lie on a straight line, indicating the validity of the
indexing and the fact that this material is characterized by a
unique interlayer spacing.
Differential scanning calorimetric (DSC) measurements were
recorded on a Perkin-Elmer DSC-2C instrument operated at a
scanning rate of 5 K/min under N2 atmosphere. The temperature
scale and enthalpies were calibrated using an indium standard.
Powder X-ray diffraction patterns were recorded on a Shimadzu
XD-D1 diffractometer using Cu KR radiation. Variable-tem-
perature infrared spectra were recorded on a Perkin-Elmer
spectrum 2000 FT-IR spectrometer in the diffuse reflectance
mode using a DRIFT (P/N 19900 series) accessory with a cooled
MCT detector.
There are two possible ways in which the layered ZnSt and
ZnOl can organize to form a single-phase compoundsthey could
form an interstratified structure, either regular or random, or a
homogeneous solid solution. If the structure of ZnStxOl(1-x) was
interstratified, a layer would have either all ZnSt molecules or
all ZnOl molecules. The ZnSt and ZnOl layers could then be
stacked either in a periodic regular fashion, for example,
alternating ZnSt and ZnOl layers for the ZnSt0.5Ol0.5 composi-
tion, or stacked randomly. The former corresponds to regular
interstratification and the latter to random interstratification.
Regular interstratification would give rise to super lattice lines
in the X-ray diffraction pattern.15 Such an arrangement can be
ruled out for the ZnStxOl(1-x) compounds; for no compositions
are super structure reflections observed in the diffraction pattern
(Figure 2a). Random interstratification, too, can be ruled out.
The diagnostic criterion of a random interstratified structure is
that it exhibits a nonintegral series of basal reflections,15,16 that
is, all spacings are not sub-multiples of a fundamental repeat
distance, d(00l). As illustrated in Figure 2b, the observed 00l
reflections in the ZnStxOl(1-x) compounds are sub-multiples of
a unique interlayer spacing. Interstratification having been ruled
out, the remaining arrangement is that where ZnSt and ZnOl
components form a homogeneous solid solution. In such an
arrangement, each layer would contain both ZnSt and ZnOl
molecules in a molar ratio identical to that of the bulk. Support
for such an arrangement is the fact that the interlayer spacing
for ZnStxOl(1-x) compositions obey a one-dimensional form of
Vegard’s law, d ) xd(ZnSt) + (1-x)d(ZnOl), where d is the
Results and Discussions
Thermal Studies. The DSC traces for different compositions
of ZnStxOl(1-x) are shown in Figure 1a. All compositions show
a single reversible endotherm with the melting transition
temperature lying between that of the end members, ZnSt (403
K) and ZnOl (373 K).14 The melt transition temperature shows
a linear variation with composition (Figure 1b). The enthalpy
change associated with the endothermic melting transitions is,
however, not linear and has a minimum at a composition with
x ) 0.4 (see also Supporting Information). This behavior of
ZnStxOl(1-x) may be contrasted with that of the corresponding
mixtures of fatty acids, stearic and oleic acids, that do not form
solid solutions and whose DSC shows two transitions corre-
sponding to those of stearic and oleic acids.1 In ZnStxOl(1-x) it
is clear from the DSC traces that a homogeneous solid solution
is formed at all compositions.
X-ray Diffraction. The room-temperature powder X-ray
diffraction patterns of ZnStxOl(1-x) (0 e x e1) are shown in
Figure 2a. For all compositions only 00l reflections are seen;
this is due to the extremely high degree of preferred orientation
arising as a consequence of the layered morphology of these
compounds. In all compounds the 00l reflections could be
indexed to a unique interlayer basal spacing. This is illustrated
for ZnStxOl(1-x) (x ) 0.5); the d spacing associated with each
of the 00l reflections has been plotted against 1/l in Figure 2b.
experimentally determined interlayer spacing of ZnStxOl(1-x)
,
and d(St) and d(Ol) are the interlayer spacing of ZnSt and ZnOl.
The values of the interlayer spacing for ZnSt and ZnOl are 42.7
and 41.8 Å, respectively. The linear variation of the interlayer
spacing as a function of composition (Figure 2c) is clear
evidence that ZnStxOl(1-x) forms a true solid solution over the
entire composition range.

5214 J. Phys. Chem. B, Vol. 111, No. 19, 2007
Barman and Vasudevan
Figure 2. (a) Powder X-ray diffraction patterns for ZnStxOl(1-x) at different compositions. (b) d00l versus 1/l for ZnStxOl(1-x), (x ) 0.5) and (c)
interlayer basal spacing of ZnStxOl(1-x) at different compositions.
The DSC and diffraction measurements clearly establish that
ZnSt and ZnOl can mix and form a homogeneous solid solution
over the entire composition range with the interlayer repeat
distance obeying a one-dimensional form of Vegard’s law. In
layered compounds, situations where Vegard’s law, or deviations
from it, are observed have been analyzed in terms of the rigidity
of the layer.17-20 It is in situations where the layers are flexible
or “floppy” that the law is obeyed. In ZnStxOl(1-x) the layers
are flexible and could form a corrugated or pillbox-like
arrangement to account for the presence of stearate and oleate
molecules that are of different length. The average layer
thickness would yield a normalized basal spacing that obeys
Vegard’s law.17,18 However, the question of how the apparently
linear stearate and bent oleate rodlike molecules are packed
within a layer remains unanswered. To understand how the
molecules are arranged and the conformational changes that
occur on solid solution formation we have used infrared
vibrational spectroscopy.
Infrared Spectroscopy. Vibrational spectroscopy, both in-
frared and Raman, has been used extensively to characterize
organization and conformation in alkyl-chain assemblies.21-23
In particular, the position of the symmetric and antisymmetric
methylene stretching modes that appear between 2918 and 2848
cm-1 are sensitive to the conformation of the alkyl chain shifting
to higher frequencies with increased conformational disorder.24,25
This region is, however, not so informative for this system. The
spectra of ZnStxOl(1-x), in the methylene stretching region, for
all compositions are almost identical to that of the end members.
The methylene antisymmetric stretching mode appears at 2919
cm-1 and the symmetric stretching mode at 2848 cm-1. These
values indicate a fair degree of trans conformational order in
the alkyl chains of solid solution.
antisymmetric stretching and symmetric stretching modes are
observed as intense bands at 1400 and 1542 cm-1 in ZnStxOl(1-x)
,
a difference of 142 cm-1, for all compositions (see also
Supporting Information). The observed difference of 142 cm-1
in ZnStxOl(1-x) is characteristic of bridging bidentate coordina-
tion,26 and the spectra indicate that there is no change in the
nature of zinc-carboxylate coordination with composition.
In order to identify the changes in conformation of the stearate
and oleate chains on solid solution formation, it is necessary to
be able to distinguish features in the infrared spectra that are
characteristic of each type of chain. This can be achieved, for
example, if one of the chains, either stearate or oleate, is
deutrated. An alternate, and perhaps easier, method in these
compounds is to examine the progression bands in the infrared
that are characteristic of the length of the all-trans chain/segment.
Infrared Progression Bands. The progression bands in the
infrared arise from the coupling of vibrational modes of
methylene units in trans registry and provide a quantitative
measure of chain conformation. The coupling of CH2 wagging
(ν3), twisting-rocking (ν7), rocking-twisting (ν8), and C-C
skeletal stretch (ν4) modes of all-trans segments in alkyl chains
gives rise to a series of bands that are de-localized over the
length of the all-trans segment and whose spacing and position
depend on the number of coupled trans CH2 units. Progression
bands are analyzed by assigning to each band a wave-vector, k,
which represents the phase angle or difference, φk, between
adjacent oscillators, and which is related to the number, N, of
contiguous all-trans methylene units.27,28 It had been shown
previously, from an assignment of the progression bands in the
infrared (see also Supporting Information), that in ZnSt all 16
methylene units are in trans registry (N ) 16), whereas in ZnOl
the double bond effectively decouples these modes into two
identical series, delocalized over two all-trans 7-CH2 (N ) 7)
segments.7,10
The infrared spectra can also provide information on how
the carboxylate group of the fatty acid is coordinated to zinc.7
The difference in the positions of the carboxylate antisymmetric
stretching and the symmetric stretching modes is indicative of
the nature of metal-carboxylate coordination.26 The carboxylate
The progression band region, 1100-1400 cm-1, of ZnStxOl(1-x)
(Figure 3a) shows a larger number of bands as compared to
that of either ZnSt (N ) 16) or ZnOl (N ) 7), indicating that

Mixed Saturated-Unsaturated Alkyl-Chain Assemblies
J. Phys. Chem. B, Vol. 111, No. 19, 2007 5215
Figure 3. (a) Infrared spectra of ZnStxOl(1-x) in the wagging band progression region at different compositions. The spectrum of ZnSt is in blue
and that of ZnOl in red. Stearate bands are labeled as St and oleate bands as Ol (b) the intensity ratio ∑ I(St)i/I(Ol) for ZnStxOl(1-x) and physical
mixtures, xZnSt + (1-x) ZnOl, at different compositions. (c) The fraction of stearate chains in ZnStxOl(1-x) that are all-trans for different compositions.
there could be two sets of progression bands with differing
spacings. Two different progression series could be identified
by assigning features in ZnStxOl(1-x) common to ZnSt with k
values appropriate for a set of N ) 16 coupled oscillators
(labeled by the subscript St in Figure 3a.). The remaining bands
appear at frequencies very similar to the progression bands of
ZnOl and could therefore be assigned k values appropriate for
a set of N ) 7 coupled oscillators (see also Supporting
Information). The features corresponding to the progression
series for N ) 7 are labeled with the subscript Ol in Figure 3a.
Since features characteristic of ordered stearate (N ) 16) and
ordered oleate (N ) 7) chains can be identified in the spectra
of ZnStxOl(1-x), it is possible to obtain quantitative information
regarding the concentration of each type of ordered chain.
The intensities of the progression bands assigned to St and
Ol chains in the spectra of ZnStxOl(1-x) (Figure 3a) may be
related to the molar ratio of all-trans stearate and oleate chains
present in the solid solution.29,30 This can be done from the ratio
of the integrated intensities of selected bands corresponding to
the different progression series of the St and Ol chains:
bands with k ) 1, 2, 3, and 4 for the stearate chains and k ) 1,
2, and 3 for the oleate chains.
The intensity ratio I(St)i/I(Ol)i for ZnStxOl(1-x) and the physical
mixtures are plotted as a function of composition in Figure 3b.
It may be seen that for the physical mixture the ratio shows, as
expected, a linear variation with composition. For the solid
solution, however, the ratio is less than that of the corresponding
physical mixture and is nonlinear. Figure 3b indicates there is
increased disorder of the stearate chains in the solid solution as
compared to the physical mixture. In principle, the ratio reflects
changes in concentration of both ordered stearate and oleate
chains in the solid solutions. However, when the intensities,
∑ I(Ol)i, for different compositions of ZnStxOl(1-x) were deter-
mined after normalizing the spectra with respect to the intensity
of the C-H stretching mode, associated with the oleate double
bond, that appears at 3008 cm-1, it was found to be a constant.
The C-H stretching mode associated with the double bond was
chosen for normalization since it shows no change either in
position or intensity with change in conformation. This was
established by monitoring the band through the melting transi-
tion of ZnOl. The difference in the intensity ratio, I(St)i/I(Ol)i, of
the solid solution and the physical mixture is, therefore, a
measure of the decrease in concentration of all-trans stearate
chains on formation of the solid solution.
X(St)/X(Ol) ) κ Σ I(St)i/I(Ol)i
(1)
where X(St) and X(Ol) are the molar concentrations of all-trans
St and Ol chains, I(St)i and I(Ol)i are the integrated intensities of
the wagging progression bands with k ) i of the stearate and
oleate chains. The summation of the intensities is over a set of
selected k values. The proportionality constant, κ, relates the
intensity ratio to the molar concentration ratio. The proportion-
ality constant was determined from the intensities of the
progression band spectra of physical mixtures of ZnSt and ZnOl
of known X(St)/X(Ol) (see Supporting Information). In both ZnSt
and ZnOl, it had been established that all chains are in all-trans
registry at room temperature.7,10 The wagging progression bands
selected for determining the proportionality constant are the
In pure ZnSt, all stearate chains adopt an all-trans planar
conformation at room temperature, while oleate chains in both
ZnOl and ZnStxOl(1-x) are ordered at room temperature. It is,
therefore, possible to determine the fraction, fxSt, of stearate
chains that are all-trans in ZnStxOl(1-x) at any composition from
the ratio of the progression band intensity ratio of the solid
solution with that of the physical mixture of the same composi-
tion, xZnSt + (1-x)ZnOl.
fxSt ) [Σ I(St)i/I(Ol)i ZnStxOl(1-x)
/[Σ I(St)i/I(Ol)i](xZnSt+(1-x)ZnOl)
]

5216 J. Phys. Chem. B, Vol. 111, No. 19, 2007
Barman and Vasudevan
Figure 4. Schematic of single layer of ZnStxOl(1-x) of nominal
composition x ) 0.6. The oleate chains are shown in green, and the
stearate chains are in gray. Stearate chains having gauche disorder are
shaded darker.
This fraction, that is, the fraction of stearate chains in
ZnStxOl(1-x) that are all-trans, has been plotted as a function of
composition in Figure 3c. It can be seen that as the composition
changes the fraction of stearate chains that are all-trans decreases
with increasing oleate concentration, the variation being almost
linear. This implies that that although the oleate chains retain
their rigid bent rodlike structure, all stearate chains in ZnStxOl(1-x)
do not retain a rigid linear geometry. The presence of a gauche
defect would allow a stearate chain to adopt a bent structure.
The formation of ZnStxOl(1-x) is, therefore, no longer a problem
of how to accommodate rigid linear and bent rodlike molecules
in a two-dimensional layer. It may be seen from Figure 3c that
not all stearate chains in ZnStxOl(1-x) are disordered; some retain
a rigid planar conformation. A single sheet of ZnStxOl(1-x)
would, therefore, have oleate chains that are ordered, a fraction
of the stearate chains, fxSt, that are all-trans ordered, with the
remaining stearate chains having at least one gauche defect. A
schematic of a possible in-layer structure of ZnStxOl(1-x) is
shown in Figure 4. The average layer thickness for such an
arrangement would show a linear variation with composition
and would explain why the interlayer basal spacing obeys
Vegard’s law.
Figure 5. Wagging progression bands in the infrared spectra of
ZnStxOl(1-x), x ) 0.5, at different temperatures. The spectrum at melt
(390 K) is shown in red.
Relating Conformational Disorder with Enthalpy of Melt-
ing. Increase in temperature induces conformational disorder
in the chains. The presence of a single gauche bond in an all-
trans ordered chain/segment is sufficient to decouple the
vibrational modes, and such chains/segments no longer con-
tribute to the intensity of the progression bands. The intensity
of the progression bands is, therefore, directly proportional to
the concentration of all-trans chains in the ensemble. It has been
shown previously that melting in ZnSt and ZnOl is associated
with the total disappearance of all-trans chains/segments, as
inferred from the absence of progression bands at melt.7,10
A
similar situation exists in the solid solutions. The wagging
progression bands of ZnStxOl(1-x) (x ) 0.5) at different
temperatures are shown in Figure 5. It may be seen that melting
is characterized by the complete disappearance of progression
bands due to all-trans chains of stearate and the trans 7-CH2
segments of the oleate.
If it is assumed that only all-trans stearate chains and trans
7-CH2 oleate segments contribute significantly to the enthalpy
of melting and that disordered chains/segments do not (a disor-
dered chain/segment is defined as having one or more gauche
bonds), it is possible to estimate the enthalpy of the melting
transition from the fraction of stearate chains in ZnStxOl(1-x)
that are all-trans ordered at room temperature, as estimated from
the infrared progression band intensities (Figure 3c):
Figure 6. Comparison of the enthalpy of melting of ZnStxOl(1-x)
calculated from the infrared data with the experimental DSC values
(the line is a guide to the eye).
enthalpy of melting of pure ZnSt and ZnOl, and their values
are 115 and 80 KJ mol-1, respectively. (The oleate chains in
both ZnOl and ZnStxOl(1-x) are completely ordered at room
temperature, i.e., fxOl ) 1.) The ∆H values calculated using the
above equation have been compared with DSC-determined
values for different compositions of ZnStxOl(1-x) in Figure 6. It
may be seen that the values are close and that the calculated
∆H values show the same variation with composition as the
experimentally determined DSC values. The close coincidence
∆H ) x∆H(ZnSt) x
f
St + (1 - x)∆H(ZnOl)
(2)
st
where fx is the fraction of stearate chains in ZnStxOl(1-x) that
are all-trans at room temperature; ∆H(ZnSt) and ∆H(ZnOl) are the

Mixed Saturated-Unsaturated Alkyl-Chain Assemblies
J. Phys. Chem. B, Vol. 111, No. 19, 2007 5217
indicates that the estimates of the concentration of ordered
stearate chains in ZnStxOl(1-x) (Figure 3c) are reasonable and
that the assumption that only all-trans stearate chains and trans
7-CH2 oleate segments contribute to the enthalpy of melting is
not too drastic.
ZnStxOl(1-x) at different compositions. (2) Infrared spectra of
the solid solution ZnStxOl(1-x) in the carboxylate stretching
region for different compositions. (3) Progression bands in the
infrared spectra of ZnSt and ZnOl and the dispersion of the
band frequencies. (4) Table of observed peak positions and
assignments of the infrared progression bands of ZnSt and ZnOl.
(5) Table of observed peak positions of the methylene wagging
band progression series in the infrared spectra of ZnStxOl(1-x)
and their assignment. (6) Infrared spectrum in the wagging band
progression region, 1100-1400 cm-1, of physical mixtures of
ZnSt and ZnOl, xZnSt + (1-x)ZnOl, at different molar ratios.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Conclusion
The zinc salts of the linear saturated stearic acid and the bent
mono-unsaturated oleaic acid form solid solutions over the entire
composition range. This was established by X-ray diffraction
as well as DSC measurements. The melting temperature of
ZnStxOl(1-x) shows a linear variation with composition. The
enthalpy of melting, however, does not show this trend and
has a minimum at a composition with x ) 0.4. X-ray diffraction
rules out a structural organization of the layered ZnStxOl(1-x)
in which ZnSt and ZnOl layers are interstratified in either a
regular or random fashion. Instead, it indicates that both stearate
and oleate chains are present in the same layer with molar ratio
identical to the bulk and with the interlayer spacing, d, obeying
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a one-dimensional form of Vegard’s law, d ) xd(ZnSt)
(1-x)d(ZnOl)
+
.
Infrared spectroscopy tells us how the ZnSt and ZnOl are
packed in a layer. Part of the ZnSt molecules present in
ZnStxOl(1-x) are conformationally disordered. The presence of
a gauche bond would allow these stearate chains to adopt bent
structures; the formation of ZnStxOl(1-x) is, therefore, no longer
a problem of how to mix rigid linear and bent rodlike molecules.
The concentration of conformationally disordered stearate chains
increases with oleate concentration in ZnStxOl(1-x). Quantitative
estimates of this concentration were obtained from a comparison
of the intensities of the progression bands in the infrared
spectrum of ZnStxOl(1-x) with that of physical mixtures of ZnSt
and ZnOl wherein all chains adopt an all-trans conformation.
The progression bands arise from a coupling of vibrational
modes of methylene units in trans registry and provide a
characteristic signature to distinguish between all-trans ordered
stearate and oleate chains in the infrared. The enthalpy of the
melting transition of ZnStxOl(1-x) was estimated from the fraction
of stearate chains in ZnStxOl(1-x) that are all-trans ordered and
the assumption that only all-trans stearate and oleate chains
contribute to the enthalpy. The estimated values were close to
the DSC-determined enthalpy values and were able to reproduce
the observed experimental variation with composition. In the
ZnStxOl(1-x) system we have thus been able to establish a
quantitative relation between molecular conformational order
and the thermodynamic enthalpy of melting of the ensemble.
The question then arises as to why stearic acid and oleic acid
do not form a solid solutions or why a situation similar to that
described here is not observed in LB films of stearic and oleic
acids or even stearic and elaidic acids. The answer is in the
crucial role played by zinc coordination. Coordination to zinc
keeps the layer intact while allowing a greater distance between
chains as compared to the corresponding fatty acids. In zinc
stearate, the interchain distance is 4.69 Å, which is the Zn-Zn
distance in the layer, while in stearic acid it is 3.7 Å. The larger
volume available to the alkyl chains in these zinc soaps permits
them a much greater degree of conformational freedom. As a
consequence, stearate and oleate chains can mix in a random
homogeneous fashion to form ZnStxOl(1-x) solid solutions.
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in the DSC heating run associated with the disordering of the methylene
segments on either side of the double bond (Barman, S.; Vasudevan, S. J.
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hysteresis, and consequently in the cooling run and subsequent heating cycle
only a single transition is observed with an enthalpy change equal to the
sum of the enthalpy change of the two endotherms of the first heating cycle.
The ZnStxOl(1-x), solid solutions were prepared by repeated cycling through
the melting transition, and consequently only a single endotherm is observed
in the DSC heating run.
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Supporting Information Available: (1) Table of the melting
temperature (Tm), and enthalpy change, ∆H (KJ mol-1), of