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12-D-Hydroxyoctadecanoic acid organogels : a small angle neutron scattering study
P. Terech
To cite this version:
P. Terech. 12-D-Hydroxyoctadecanoic acid organogels : a small angle neutron scattering study. Jour- nal de Physique II, EDP Sciences, 1992, 2 (12), pp.2181-2195. �10.1051/jp2:1992259�. �jpa-00247797�
Classification
Physics Abstracts
82.70 61.12 61.14
12-D-Hydroxyoctadecanoic acid organogels : a small angle
neutron scattering study
P. Terech
Institut Laue Langevin, 156X, 38042 Grenoble Cedex, France (Received 24 June 1992, accepted in final form 16 September 1992)
Rdsumk. Un acide gras optiquement actif, l'acide D-hydroxy-12 octad6canoique, donne des
gels thermiquement r6versibles et plastiques dans une vari6t6 de solvants organiques. Les
paramdtres structuraux des agr6gats fibrillaires qui constituent le r£seau du gel sont ddduits h partir d'exp6riences de diffusion de neutrons aux petits angles. La forme de la section-droite des fibres peut dtre soit cams et relativement monodisperse, soit tr~s rectangulaire selon le type de solvant et la concentration. Pour des gels dans le benzdne, la section-droite est un coma de 214 A de c&td avec
environ 40 moldcules par angstrom de longueur de fibre rigide. La symdtrie monoclinique des
agrdgats cristallins de D-HOA conduit d'une part, aux arrangements mol£culaires d'une s6quence
infInie de liaisons H le long de l'axe de la fibre et d'autre part, h la capacit£ de ddvelopper des structures en rubans.
Abstract. An optically active fatty acid derivative, 12-D-hydroxyoctadecanoic acid, gives thermally reversible and plastic gels in a variety of organic solvents. The structural parameters of the fibrous aggregates constituting the gel network are obtained from small angle neutron
scattering experiments. The cross-sectional shape can be either a rather monodisperse square or a
very elongated rectangle dependent upon the solvent type and concentration. With gels in benzene, the cross-section is a square of 214 A side with
ca. 40 molecules per angstrom length of rigid fibre.
The monoclinic symmetry of the crystalline aggregates induces, on the one hand, the appropriate
molecular arrangements for an infinite H-bonding sequence along the fibre axis and, on the other hand, the ability to develop ribbon-like structures.
1. Introduction.
Gels formed with low molecular weight organic derivatives in organic solvents constitute a
special class of physical gels. Their viscoelastic properties indicate the presence of a three- dimensional thermoreversible network built through an aggregation process of the amphiphilic
molecules. «Weak» and «strong» physical gels are distinguished. On the one hand,
« weak » physical organo gels are viscoelastic solutions involving a transient network of worm- like species : up to now, examples in organic media of such surfactant-made systems are very
rare [I]. On the other hand, « strong » physical gels are self-supporting plastic materials
(viscoelastic solids) and are the subject of the present study. Above a temperature-dependent
critical concentration C*, a solution of such surfactants evolves on cooling into a ther-
moreversible gel phase. Mechanisms of the surfactant aggregation kinetic steps, local
structures of the aggregates and of their junction zones in the gel network are topics of interest in the field.
In view of the limited number of such surfactant-made binary systems [2], the aggregates are
usually fibrillar-like. Structures can be dependent upon minor changes of the solvent type. For instance, with a chiral steroid derivative in hydrocarbons [3], significant changes of the diameter of the fibres have been observed between cyclohexane and ethyl-cyclohexane. The
rigidity of the steroid fibres is increased from trans to cis-decalin gels and oriented helical fibres are much more easily obtainable in cis-decalin [4]. In addition, structures also depend
upon the polarity of the amphiphilic steroid compound itself [5]. In that case, limited dipolar
interactions between small groups (hydroxyl, amine or nitroxide) confine the gelling solvents to apolar hydrocarbons.
To enlarge the range of organic solvents which can be gelled, this study deals with a more
polarizable compound : the 12-D-hydroxyoctadecanoic acid (referred as 12-D-HOA). The solvents can have a wide range of dielectric constant (from cyclohexane to nitrobenzene).
Previous studies of the organogels by infra~red absorption spectroscopy [6], optical measure-
ments, wide angle X-ray scattering (WAXS) [7] and electron microscopy [8] have shown that the fatty acid molecules are connected by hydrogen bonds within fibrillar structures. A
supramolecular helicity associated with a lamellar organization has also been correlated with the chirality of the D-HOA molecule. The mechanical properties were shown to be dependent
upon the thermal history of the 3d network growth [9]. However, direct structural
investigations of the gel phase are rare. A preliminary small angle X-ray and neutron scattering study [10] has confirmed that the aggregates are very long fibres with a square cross-section.
The connections between the fibres lye in microcrystalline domains which strengthen and stiffen the gel network. These results support the description of D-HOA/organic solvent systems as « strong » physical organogels.
A thorough study of the structural features of these systems is presented here. The local and
long-range structures of the HOA aggregates are investigated by small angle neutron scattering (SANS) while the intemal organization is complemented with WAXS experiments on gels and solids. Special attention is paid for the variations induced by the solvent type (stericity, polarity) in a large range of concentrations.
2. Experimental.
D-HOA (ca. 98 9b, F
=
81 °C) was obtained from the natural raw material (castor oil I-e- a mixture of fatty triglycerides) as previously described [9]. The optical activity was due to the molecular asymmetry of the 12th carbon bearing the hydroxyl. All solvents (deuterated or fluorinated) were reagent grade and used as received from Aldrich. The water content was low
enough to consider the gelling samples as binary systems. The dissolution occurred at a temperature near the melting point of the crystalline surfactant. Gels appeared on cooling the hot homogeneous solutions. According to the solvent and concentration, the gels were more or
less transparent (benzene) or turbid (octane, hexafluorobenzene) and linear birefringence was
observed as spherulitic domains with extinction crosses or Schlieren textures. On heating, the fluid solutions were recovered, which allowed a convenient initialization of the system by removing any prior sample history. The concentration range was ca 1-25 9bwt.
SANS experiments used the Dll, D17 and D16 spectrometers [I I] of the ILL high flux
reactor (Grenoble, France) at respective wavelengths of 7 h, 12 h and 4.5 h. D16 (two-circle
mode) used a pyrolitic graphite crystal monochromator while the cold neutron beam was
passing through velocity selectors with helical slots for Dll and D17. The wavelength
resolution was AA/A
=
99b FWHM. A planar square (BF3) multidetector with 64 x 64
elements (I cm linear resolution in both dimensions) was used. The momentum transfer Q
(h~ )
was defined as Q
= (4 «IA ) sin 8 where A was the radiation wavelength and 8 half the
scattering angle. The complete Q-range investigated was 0.0008 ~ Q ~ l.0h~'. Samples
were held in Hellma quartz cells of I mm thick (I x lo x 30 mm). The irradiated volume was
ca, loo mm3 which allowed counting times of ca. 500 s to give satisfactory signal/noise ratios.
The incoherent scattering of a protonated water sample was used to calibrate the intensities.
ILL programs were used for regrouping, correction for background scatterings and trans- mission ratios.
Complementary WAXS experiments have been performed with an Elliot GX13 rotating
anode generator at CuKa radiation (A = 1.5405 h ). A double focussing system consisting of two Frenk's mirrors gave a point-focussed beam. Thin (0.01mm) circular glass capillaries
were used as sample holders. Relative intensities were measured with a Photoscan P1000 densitometer (Optronics Inc.) from photographic films used as detectors.
3. Neutron scattering formalism.
The D-HOA aggregates being fibrillar [6-10], it is convenient to discuss the cross-sectional scattered intensity I~. In a dilute situation of non-interacting long and rigid fibres, the scattered
intensity I(Q) is reduced to a form factor separating the axial and cross-sectional [12, 13]
terms :
1(Q)= ~Ab2M~,i QZ
Q ~ 2
(1)
where &5 is a maximum dimension within the cross-section, C is the rod-like scatterer
concentration (g.cm~~) and M~ the molecular weight per unit length of fibre (g.h~~).
Ab (cm/g) is the specific contrast calculated from the neutron scattering lengths of the nuclei involved in the surfactant aggregate/solvent system. Curves Q. I vs. Q reduce to the cross- sectional scattering profiles where I is the absolute differential scattering cross-section to which the experimental intensity is proportional. A step profile of the neutron scattering length density describing the interface between the HOA fibres and solvent is assumed. The neutron
contrast reads :
Ab=b~-p~v~ (2)
where b~ is the mean specific scattering length of the aggregate (cm/g), v~ the specific volume of the aggregates (cm~/g) and p~ the scattering length per unit volume of solvent
(cm/cm~).
In the intermediary Q-range (I)
~ Q ~ &5 ~, where (I) is the persistence length of the fibrillar aggregate, I,e, the length below which it can be considered as straight and rigid, the cross-section intensity decrease is approximated with a Gaussian exponential decay. The cross-sectional gyration radius R~ is determined from the slope of Guinier plots In (Q I)
f(Q~), while the ordinate at origin leads to M~ values according to : =
(Q.I)~~o = (Q.i)oexp(-)Q~) (3)
At larger Q-values, the cross-sectional form factor I~ exhibits oscillations characterizing the
structure of the cross-section (shape and homogeneity). Because they are superimposed to a
Q~~ asymptotic behavior (Porod's law), they are usually better visualized in a Q~ I vs. Q plot.
For a rectangular shape of the cross-section, the relevant equations read [14] :
l~j~
ka sin v2 ~ d~~ 2" sin (Qa C°~ ~' Ii
sin v2
~~~~a, k)
= p
~ Qa cos V~
R(
=
~~
l + (4)
3 k~
where a is half the length and b half the width of the rectangular cross-section of anisotropy
k
= bla. The influence of the polydispersity e of the cross-sectional geometry upon the scattered intensity is simulated [3] by convoluting expressions (4, 5) with a centered Gaussian
distribution. e is defined as the relative variation of dimensions at half maximum of the distribution (e includes the instrumental resolution).
When k
~ 0, the overall shape of the ribbon tends towards a platelet whose apparent thickness and characteristic dimension of lateral extension are respectively t and T. In the range
T~ ~ Q
~ t~ ', the scattering is described by
:
I(Q)
= ~$~~
b~M~)
exp (- ~)
~ j ~
~ 2 e
~
e~ ~~~
fi fi 2 In 2
M~ being the mass per unit area of the polydisperse lamellar aggregate and R~ the transverse radius of gyration.
4. Results.
In the following, typical SANS behaviors are detailed for a selection of solvents.
Figure I shows the cross-sectional scattering curves for HOA/benzene gels. Within 0.0018
~ Q ~ 0.011 h~ ~, the scattering exhibits a nearly pure Q~ behavior and is followed
by a sharp decay in the range 0.011 ~ Q ~ 0.03 h~ At larger Q-values (Q > 0.035 h~ ~),
Q.1
Fig. I. -D-HOA/benzene gels : cross-section scattering curves (Q .I) vs. Q for rod-like scatterers
(Log-Log plot). Q is expressed in A~' and I is the absolute intensity in cm~ '. Arrows indicate two of the cross-sectional form factor oscillations. From bottom to top : I c
=
2.5 fbwt (C = 2.35 g.cm~~),
2 : c 6fbwt, 3 c 21.4fbwt. Bragg peak position 0.149A~~ (bold arrow). Full line is
an
adjustment of expression (4) for a square cross-section : k I, a b lo? A,
e= 0.065, R(~
=
91.2 A.
oscillations are superimposed to the intensity decrease and finally a Bragg peak is clearly seen
at Q
=
0,149h~~. When the HOA concentration is increased (see curves1, 2, 3), the
following observations are made: (I) the flatness of the low-Q asymptotic behavior is improved, (it) the form factor oscillations of the cross-section are less resolved, (iii) the Bragg peak is broadened but the position remains unchanged.
These features are preserved for HOA/cyclohexane gels (Fig. 2). The shape of the Porod oscillations is somewhat different from those observed for HOA/benzene gels and suggests that slight modifications of the cross-section geometry have occurred.
Q.1
Q
~3 ~2
Fig. 2. D-HOA/cyclohexane gels cross-section scattering curves (Q I vs. Q for rod-like scatterers
(Log-Log plot). I c
=
I.4 fbwt (C
= 1.25 g. cm~~); 2 c
=
4.2 fbwt 3 : c
= 21.4 fbwt. Bragg peak position (bold arrow) : 0.149 A~ Full line is
an adjustment of expression (4) for a rectangular cross-
section : k
= 0.36, b
= 50 A,
e = 0.075, R(~
= 90. I A.
By contrast, in octane (Fig. 3), the asymptotic decay at low Q is progressively modified from a Q~' (curve I) to a Q~~ behavior (curve 3) when the concentration is increased. This
evolution is clearly highlighted in a Q~.I vs. Q plot convenient for lamellar scatterers (see expression (5)). Departures from the scattering law for fibres are also noticed with some other
solvents : figure 4 shows the variation of the scattering profile with HOA concentration in
deuterated fluorobenzene C~D~F (similar observations are made for hexafluorobenzene
C~F~). For D-nitrobenzene gels, a Q~~ behavior takes place even at very low concentrations.
Q~.I
o 3
Fig. 3. -D-HOA/octane gels : cross-section scattering curves (Q~ .I) vs. Q for lamellar scatterers
(Log-Log plot). I : c
= 2.6 fbwt (C
= 2.2 g.cm~~) ; 2 : c
= 6.7 fbwt ; 3 : c
= 25.3 fbwt. Bragg peak position 0.147 A~'
Q.1
3
2
i
~
Q
-3 ~2 ~l
Fig. 4. D-HOA/fluorobenzene (C~D~F) gels : cross-section scattering curves (Q I) vs. Q for rod-like
scatterers (vertically shifted for clarity, Log-Log plot). I : c
=
2.9 fbwt 2 c
= I1.6 fbwt. Bragg peak position : 0.143 A~' Full lines are best ajustments ; following expression (4) for ribbon-like scatterers, I : k
= I, a
= b
= lo? A,
e = 0,I following expression (5) for lamellar scatterers, 2 k
- 0,
t = iso A, e
= o.3.
For most of the dilute HOA gels, the scattering is isotropic. When the concentration is
increased in C~F~ gels, the low-angle part becomes elliptical and the Bragg peak
(Q = 0,14 h~ ~) becomes also anisotropic (see inset of Fig, lo) while the scattering remains
isotropic for benzene and cyclohexane gels.
S. Analysis.
A simplified formalism for isolated and elongated structures is used to analyze data of dilute
gels. For SANS calculations, the numerical values listed in table I are used together with M(HOA = CI~H~~O~) = 300.485, b~ =
4.8443 x 10~ cm/g, v~(HOA) = 0.98 cm~/g (esti- mated from crystallographic data of the present study and [15]).
Table I. SANS parameters of D-HOA organogels.
Solvent p~ x 10~ ~° (cm/cm~) Ab x 10~ ~° (cm/g)
C~D~ benzene 5.377 5.222
C6D12 cyclohexane 6.747 6.564
C6D~F fluorobenzene 5.097 4.947
C~DI~ octane 6.416 6.239
C6F~ hexafluorobenzene 3.864 3.739
C6D~NO~ nitrobenzene 5.568 5.420
The asymptotic Q'~ Plateau in the low-Q range of HOA/benzene gels (Fig. I), confirms that the network is formed with fibrous aggregates whose scattering can be described by the axial factor of expression (I). On the low-angle side of this plateau, no typical Q~ " behavior can be
seen (a being 2 for a Gaussian coil or 1.6 for a coil in a good solvent undergoing a self-
avoiding random walk [16, 17]) : the fibres are considered as highly rigid over distances of the order of the experimental limit of ca. 4 000 h (Q
= 0.0008 h~ ~). On the large-angle side of the
plateau, the sharp intensity decay which follows is Gaussian and due to the finite size of the cross-section. The corresponding values for the radii of gyration R~ and the molecular weight
per unit length M~, obtained from the slope of Guinier plots In (Q I vs. Q~ (Fig. 5), are listed in table II.
A variation of R~ with the surfactant concentration is noticed. For a dilute gel, the
oscillations are relatively well-defined and are typical of monodisperse cross-sections of the fibres. The Bragg peak indicates the existence of crystalline organizations within the gel
network. These can arise from the fibres themselves and more significantly from the junction
zones structurally reminiscent of the solid state [7, 10]. These microdomains are non-
uniformities of the gel network whose scattering contribution is seen in various parts of the
scattering curves. The observation of interacting aggregates of viscoelastic gels suggests that
one look for a structure factor in the whole Q-range when the concentration is increased. The shape of the excess scattering has been evaluated by dividing the scattering curve of a
concentrated gel by that of a dilute sample : it appears that it has a non-trivial profile (not discussed here) which contributes not only in the lowest-Q region but also at Q-positions corresponding to the Bragg reflections and in the vicinity of the Porod form factor oscillations.
For instance, this leads to an artefactually improved flatness of the Q~' plateau and to decreased R~ values (not discussed here). The genuine structural parameters are deduced only
from dilute samples.
At this stage, it is noticed that only rod-like fibres can grow in benzene and cyclohexane,
whatever the concentration range. On the other hand, ribbon-like aggregates (or fibres and
junction zones with very anisometric cross-sectional shapes) are developed in nitrobenzene
during the HOA aggregation process. Intermediate situations are found with octane, fluoro and hexafluorobenzene gels, which exhibit the lamellar structures only if the D-HOA concentration is increased. From table II, the mean values retained are R~ = 84, I h and
n~ = 40.5 for HOA/benzene gels and R~ = 84.5 h and
n~ = 40.5 for HOA/cyclohexane gels. With the other solvents (Tab. II), fluctuations of the low~o asymptotic behavior, R~ and M~ values indicate
that some variations and departures from the general trends above discussed, could be
correlated with geometrical variations of the cross-section.
In(Q.1)
.~
s
.... j .
~ ~
i 3
,~~~~~~~~~~_~~~
i ~y
~i
o.oooi o.oooz
Fig. 5. Guinier plots (QR~ ~ l) for rod-like scatterers. I hexafluorobenzene, c = 0.44 fbwt ; 2 : D-cydohexane, c
=
I.4 fbwt 3
: D-octane, c
= 1.8 fbwt 4 : D-fluorobenzene, c 3.0 fbwt ; 5 D-benzene, c
= 2.5 fbwt (vertically shifted by + I for clarity). For Q ~ 0.004A~', the excess-
scattering from the heterogeneities of the network is seen. Gels where only a Q~' behavior can be observed whatever the concentration range are referred by (+ ) (straight lines) by contrast to those which
can develop ribbon-like or lamellar structures (.),
