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A small angle neutron scattering study of the lamellar and nematic phases of caesium
pentadecafluoro-octanoate (CsPFO)/2H2O and CsPFO/CsCl/2H2O
M. Leaver, M. Holmes
To cite this version:
M. Leaver, M. Holmes. A small angle neutron scattering study of the lamellar and nematic phases of caesium pentadecafluoro-octanoate (CsPFO)/2H2O and CsPFO/CsCl/2H2O. Journal de Physique II, EDP Sciences, 1993, 3 (1), pp.105-120. �10.1051/jp2:1993114�. �jpa-00247803�
J. Phys. II France 3 (1993) 105-120 JANUARY 1993, PAGE 105
Classification
Physics Abstracts
61.12E 61.30
A small angle neutron scattering study of the lamellar and nematic phases of caesium pentadecafluoro-octanoate
(CSPFO)/~H~O and CSPFO/Cscl/~H~O
M. S. Leaver and M. C. Holmes
Department of Physics and Astronomy, University of Central Lancashire, Preston, PRI 2TQ, Great-Britain
(Received 3 August 1992, accepted in final form 6 October 1992)
Abstract. The binary lyotropic liquid crystalline system caesium pentadecafluoro-octanoate (CSPFO)/Water (2H~O) exhibits an isotropic micellar phase, a lyotropic nematic phase and a
positionally ordered, lamellar phase whose planes contain substantial water filled defects, It has been suggested that the structural unit in all three phases is a disk shaped micelle which, as temperature is lowered, first become orientationally ordered and then positionally ordered onto
planes. Increasing the surfactant concentration apparently causes the micelles to rust increase in size and then decrease, explained by invoking an attractive inter-micellar interaction, In this small
angle neutron scattering study, we present further results on the structure of the binary and the temary systems, the latter containing added electrolyte. Our results call into question the
previously proposed model of the larnellar phase and show that the form of the temperature dependence of the aggregate dimensions depends on the structural model that has been adopted.
We show that a much more satisfactory picture of the lamellar phase is obtained if the shape of the aggregates is assumed to consist of continuous amphiphile layers pierced by irregular water filled holes increasing thq CSPFO concentration then causes the amphiphile aggregates to grow. This
structure is conflrrned by the effect of adding electrolyte (Cscl) to the lamellar phase, Here,
scattering shows explicitly that the water filled holes anneal out. The addition of electrolyte is also shown to cause changes of aggregate shape in the nematic phase. These changes in both binary and temary systems are shown to be attributable to modifications of the intra-aggregate electrostatic head-group interactions.
1. Introduction.
The structure of lyotropic lamellar (L~ phases has been well established for many years II ], Surfactant molecules aggregate in concentrated aqueous mixtures to form extended, continuous lamellae, the alkyl chains forming their interior whilst the amphiphilic head groups form a planar interface with the aqueous layer, These lamellae stack periodically with interleaving
water layers. The lamellar phase is clearly identifiable from its optical textures [2] and by its X-
ray scattering pattem which shows a series of Bragg reflections from the inter-lamellar
spacing, in the ratio : 2 : 3... Ill. Studies have shown that there are a number of lyotropic
lamellar phases which show additional structural features, The decylammonium chlo-
ride/NH4Cl/H20 system [3] can be easily ordered into a monodomain lamellar sample by cooling in a magnetic field from the isotropic phase, The scattering along the direction parallel
to the plane normal shows Bragg reflections characteristic of a lamellar phase whilst
106 JOURNAL DE PHYSIQUE II N°
perpendicular to this there are diffuse liquid like reflections from intra-lamella structure, This
was interpreted as arising from water filled defects in the lamellae [3]. Freeze fracture electron
microscopy has provided a visualization of these defects and suggests that the lamellar phase
may consist of disk micelles closely packed onto planes [4], Sodium dodecyl sulphate/H20 [5]
and sodium decyl sulphate/I-decanol/H20 [6] both show several types of structural fluctuation in different regions of their lamellar phases. The same type of diffuse reflection from intra-
lamella defects has been identified close to a phase boundary with a rectangular ribbon phase [6], For ionic surfactants, this type of defected lamellar phase seems to occur invariably when the system also shows lyotropic nematic, rectangular or other exotic phases for which it is a
precursor. A defected lamellar phase has also been reported in the nonionic binary system hexaethylene glycol-cis-13-docosenyl ether (C22EO~)/water (2H20) [7] as an intermediate
phase between a true L~ phase and a lower temperature hexagonal (Hi) phase rather than a cubic phase found in the shorter alkyl chain homologs. From the limited number of defected
lamellar phases studied so far it is not clear whether there is a single generic type of structure or whether there are several possible structures as suggested by recent theoretical work [8]. Direct evidence for the structure of the defects in a lamellar phase is difficult to obtain from scattering
alone because of Babinet's Principle. For example, layers made of micelles surrounded by a
continuous water medium give the same scattering as a continuous amphiphile medium pierced
by water filled holes.
Perhaps one of the most interesting ionic systems to exhibit an extensive lamellar phase showing intra-lamella structure is that of caesium pentadecafluoro-octanoate/2H~O (CSPFO) [9-17]. It is important and unusual in being a binary mixture without the need of added co-
surfactant or salt [9] to form either this type of lamellar phase or a nematic phase
(N~) of disk micelles. The latter phase is extensive, extending from X~ = 0.2250 to 0.6325 (X~ is the weight fraction of CSPFO) and from 285.3 K to 351.2 K. The high resolution phase
diagram, determined by 2H NMR has been previously published [9]. The N~ phase is bounded to high temperature by an isotropic micellar phase, Lj from which it is separated by a narrow
biphasic region indicating a first order phase transition. To low temperature it is bounded by a
lamellar phase previously denoted by L~ [14]. It has been proposed that the structural units in all three phases of this system are disk micelles. The transitions from isotropic micellar to nematic and from nematic to lamellar phases are simply disorder~order transitions [12, 16].
The former marks the onset of long-range orientational order of the disk micelles whilst the latter marks the onset of their long-range positional order, Thus, it is suggested that the lamellar phase in this system is a smectic A phase of disk micelles, the intra~lamella reflection
arising from the scattering from the disks in the planes. For this model it is necessary that the disk micelles show a remarkable stability in size and shape irrespective of the phase in which
they find themselves despite being essentially fluid entities with a comparatively short lifetime.
The evidence for the maintainance of disks as stable entities in the lamellar phase comes firstly from X-ray scattering [12, 14] which shows intra-lamella structure continuous with the
N~ phase. Also electrical conductivity measurements II 3, 16], sensitive to Cs+ ion diffusion, show the lamellar phase to contain water filled defects giving a continuous conduction path
and 2HNMR shows the phase to contain some highly curved edges [9]. These latter
techniques also measure parameters which are continuous across the nematic-lamellar phase
transition and therefore the structural units in both phases are inferred to be identical. Since the structural units in the ND phase are disk micelles it follows that the lamellar phase must consist of the same, the only difference being their positional order. Clearly, although a model of disk
micelles under going a disorder-order transition from N~ to a lamellar phase of disks
positionally ordered onto planes is consistent with experimental results and an attractive idea, the evidence is circumstantial and depends heavily upon the models used to interpret the
N° I A SANS STUDY OF CSPFOPH20 AND CSPFO/CSCIPH20 IQ?
results. In refering to the lamellar phase here we will use the more general notation for a lamellar phase whose lamellae are pierced by water filled defects of whatever type or structure,
Lf [7].
There are problems with the disk micelle model of the Lf phase. It has been shown from X- ray scattering studies [14] that at fixed temperature, increasing surfactant concentration causes
a decrease in micelle size as measured by the mean aggregation number, I above
X~ = 0.42. Using the model of Gelbart et al. [18] for the free energy of a micelle, this decrease
was explained only by envoking an attractive inter-micellar interaction which is contrary to
accepted ideas about the interactions in these systems. A possible candidate for this attractive interaction was suggested in reference [14] to be the attractive Coulombic interaction proposed by Sogami and Ise [19]. However, as noted in their paper [19], the attractive interaction will be
rather weak for ionic micellar systems. Further, as electrolyte is added, the mean separation
should decrease which we shall show in this paper is not the case. Also, no computational
model of disk shaped units has yet, as far as we are aware, shown the formation of stable layers
to be possible. Usually such models give columnar type phases, although this may simply be
that so far the correct form of the pair interaction potential has not been used. A further
difficulty is how the system can sustain many small micelles each with high energy, highly
curved edges, in close proximity to each other and over such a wide range of temperature and surfactant concentration.
In this paper we present the results of a study intended to explore the structure of the
Lf phase and its relationship with the N~ phase. Neutron scattering has been used rather than X-ray scattering since the latter generates pattems (from both conventional and synchrotron sources) which become more difficult to analyse as the caesium concentration increases
because of its high mass absorption coefficient. Small angle neutron scattering (SANS) avoids this difficulty and the scattering cross-section of the CSPFO binary system gives good contrast
scattering pattems in a reasonable collection period (circa 30 min). We also investigated the addition of electrolyte (Cscl) to the binary system. This modifies both intra- and inter-micellar
interactions and therefore the structure of the aggregates.
For an ionic amphiphile, the surface of the aggregate is charged and the counterions can be
thought of as occupying one of two sites, the aggregate surface or in free solution [20, 21]. A two state Boltzmann distribution may be used to describe the position of the counterions, with
those associated with the surface of a lower energy. Rapid exchange of counterions is expected
between the two « sites » [21]. The addition of extra counterions is expected to increase the number of counterions associated with the surface. It directly affects intra-aggregate forces, reducing the head group interactions, head group area and lowering the free energy per
monomer in the aggregate. Electrolyte addition stabilizes flat interfaces and promotes a growth
of larger, flatter aggregates.
The inter-aggregate potential is composed of three contributions excluded volume effects due to steric repulsion between hydrated micelles, Coulombic repulsions and long range Van der Waals attractions. It is clear that both the excluded volume terms and electrostatic
interactions will be significantly affected by the addition of an electrolyte [22]. Electrostatic
screening reduces Coulombic repulsions and it has been shown that for spherical micelles the
addition of sufficient electrolyte can screen the Coulombic interactions to the extent of
producing a negative pair potential [23, 24]. If it is assumed that the Coulombic interaction dominates, Onsager [25] showed that the addition of electrolyte increased the effective micellar dimensions by increasing the Debye screening length K ~, even if the physical size of the micelle is unaltered. He also showed that, for a colloidal suspension of long charged
cylindrical particles, the free energy term due to the orientational order increased the effective
miceilar dimensions by an amount proportional to K~~ For the CSPFOPH2° system,
108 JOURNAL DE PHYSIQUE II N°
K~ is of the order of 0.60 nm [26] and the effective mean dimensions of the micelle should increase on the addition of electrolyte. With 7.5 9b by weight of added Cscl, K has reduced to 0.38 nm and hence there 15 a decrease in the inter-aggregate Coulombic interaction [I Ii- The
effect of added electrolyte, therefore, is complicated. The modification of the intra-aggregate head-group interactions will encourage larger, flatter aggregates whilst the screening of the
Coulombic interaction between aggregates causes a change in the range of these interactions and, therefore, in the effective dimensions of the aggregates.
Rosenblatt and Zolty [11] showed, using magnetic birefringence, that for an X~ = 0.40
sample on the addition of salt (up to 7.5 9b by weight Cscl), that both transition temperatures increased. They attributed this to an increase in the micellar dimensions, assuming the micelle to be disk shaped. They also found that the temperature over which the nematic phase existed
decreased as Cscl was added.
It is not clear either from the theoretical models of micelle formation nor from the above experimental work whether the increases in transition temperatures can be attributed simply to a growth in the disk shaped micelles, their shape remaining constant or whether there are
associated structural changes.
2. Experimental.
2.I SAMPLE PREPARATION AND PHASE DtAGRAM. The synthetic procedure used to prepare
CSPFO was a modification of that of Nakayama [27] in which the pentadecafluorooctanoic
acid (Fluka chemicals purity > 95 9b) was added to distilled water and the resultant solution
titrared directly with caesium hydroxide monohydrate (Fluka chemicals purity >959b)
solution. Recrystallization from ethanol was found to give the best purity. Samples of CSPFO dissolved in CDCI~ were checked by IH NMR to verify that all traces of solvent were removed and standard X~ = 0.50 samples were made for each new batch to verify the reproducibility of the phase transitions.
The samples were made by carefully weighing out the mass of surfactant and heavy water
(2H20 Fluka chemicals purity
> 99.8 9b) into constricted tubes which were then sealed. The
samples were mixed by repeated centrifugation and storing in an oven in the isotropic phase
until they are optically uniform. A series of samples were prepared in which the mole ratio of 2H20 to CSPFO was held constant at 27.26, the same as that for an X~ =
0.50 binary sample
and increasing amounts of caesium chloride (Fluka chemicals purity
> 99.9 9b) was added up to lo 9b by weight of Csci. Care was required in mixing samples with salt greater than 8 9b by weight because of the loss of the nematic phase and their tendancy to separate in the Lj + Lf biphasic region.
2.2 NEUTRON SCATTERING. The neutron scattering experiments were carried out using the
SERC'S neutron spallation source at the Rutherford Appleton Laboratories, UK. The
instrument used was the small angle neutron scattering facility, LOQ. The samples were held in I mm path length Helma quartz glass cells with an electrical temperature control (± o-I °C). Some samples were ordered by cooling from the isotropic phase into the ND or Lf phase in an electromagnet mounted around the sample holder, generating a field of 0.5 Tesla. This gave well aligned samples with the liquid crystal director (the mean orientation of the disk normals or plane normals in the case of the Lf phase) lying along the field direction (x axis of the detector). The y axis of the detector contained information about the intra-
lamellar scattering and the z axis is defined by the neutron beam.
2.3 OPTICAL POLARISING MicRoscoPY, A Vickers M17 polarising microscope with a
Linkam TH600 hot stage and Linkam PR600 temperature control unit, with an accuracy of
N° I A SANS STUDY OF CSPFOPH20 AND CSPFO/CSCIPH20 109 360
350 ~l ~ ~D
340
~
~ 330
~
L~ + L~
320 L~
310
0.00 0.02 0.04 0.06 0.08
Welght Fraction of Cscl
Fig. I.-The phase diagram of the CSPFO/CSCIPH20 system for a constant mole ratio of
j2H~O]/[CSPFO]
=
27.26 with various amounts of Cscl. Open squares represent experimental transition temperatures for the isotropic phase and closed squares represent those for the lamellar phase obtained from the optical polarising microscope lines are best fit curves included as an aid to the eye.
o
~2 o
o
--~
' 't
/ Q
~ j
E
., j
C$
Q ,,
~
, Q
,/ Q
~, ', ~
7381
4256 °
2454 ~l.0
j '~~( 0 Do Q
4.71 Q Q
~
~~~ ~
090
052 Q
030 -2.0
~2.0 -1.5 -1,0 0.5 0.0 0.5 1.0 1.5
Q j~m-1
Fig. 2. A contour plot of the neutron scattering from a magnetically aligned W~ 0.50 CSPFO sample
at 313 K. The intense Bragg reflection from the inter-lamella spacing is located on the x-axis whilst the weak intra-lamella reflection is Visible parallel to the,t-axis. The horizontal line cutting the Bragg peak on
the left hand side is an artifact of the detector and is visible in all the scattering profiles. It has no effect on the results presented.
1lo JOURNAL DE PHYSIQUE II N°
± 0. I °C was used to determine the phase diagram, figure I, as a function of added Cscl for the sample series and confirms the 2H NMR phase diagram of Parbhu and Jolley [28]. The
N~ phase is present up to about 8 9b by weight Cscl although the extent of the phase is
decreasing, It is bounded to high temperature by an isotropic micellar, Lj phase and separated
from it by a narrow Lj + N~ biphase. To low temperature is an extensive Lf region. Above
about 8 9b by weight Cscl there is a direct Lj Lf transition via a broad biphasic region.
3. Lamellar phase of CSPFO/2H20.
3. I RESULTS. Figures 2 and 3 show the neutron scattering contour plot from a magnetically aligned X~
= 0.50 binary sample in the Lf phase at 313 K. The strong reflection on the x axis
corresponds to the inter-lamella spacing (dj ) whilst the weak, broad bands parallel to the x axis
correspond to the liquid like reflections from the intra-lamella structural dimension
a)
0
0 0.2 0.6 1-O 1.2 1.6 2.0
Qx/nm-1 b)
i
j
ho-i
1g
0.01 m
# e 0.001
o.oooi
0 0.2 0.6 1-o 1.2 1.6 2.0
Qy/nm"I
Fig. 3. Sections taken from the scattering shown in figure 2 taken along the a) x-axis and b) y-axis.
N° I A SANS STUDY OF CSPFOPH~O AND CSPFO/CSCIPH~O I I1
0 O-S I-o I-S 2.0' 2.5
Qx/nm"I
Fig. 4. An x-axis section of the scattering from a W~ = 0.75 CSPFO sample at 313 K.
(di ), figure 3. The pattem is similar to those obtained from X-ray scattering [12, 14] and the distances dj and di measured from x and y axes respectively correspond within experimental
error with those obtained previously. The series of samples in the concentration range
X~ = 0.45 to 0.75 were studied at 313 K making a direct comparison with previously published X-ray results possible. Figure 4 shows the neutron scattering profile from a powdered X~ = 0.75 sample. The reflection from intra4amellar structure is still clearly visible and, indeed, measured in disordered samples relative to the intensity of the Bragg reflection it is
increasing in intensity with increasing CSPFO concentration. The values of djj and
Table I. Experimental values ofdjj, d~ and I, the mean aggregation number jkom SANS
and SAXS [14] (shown in brackets) for the Lf phase (except for X~
=
0.45 which is in the
N~ phase) of CSPFOPH~O at 313 K.
X~ ~ba (djj ± 0.05)/nm (di ± 0.2)/nm I
0.45 0.262 4.80 6.1 150(170)
0.50 0.302 4.59 6.0 159(165)
0.55 0.346 4.38 5.5 147(lsl)
0.60 0.394 3.99 5.1 131(134)
0.65 0.446 3.72 5.0 135(127)
0.70 0.503 3.48 4.7 124
0.75 0.565 3.25 4.5 l19
