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Dielectric properties of microemulsions and of their liquid crystalline mesophases
Journal of Electrostatics, 12 (1982) 383--404 Elsevier Scientific Publishing Company, Amsterdam
383 -
-
Printed in The Netherlands
DIELECTRIC PROPERPIES OF MICROEMULSICNS AND OF THEIR LIQUID ~RYSTALLINE MESOPHASES
D~qATELLA SENATRA Liquid State Physics Laboratory, Physics Institute, University of Florence: L.E. Fermi 2; (Arcetri), 50125 Florence, Italy
ABSTRACT Isotropic w/o microe~isions
as well as the liquid crystalline
(LC) phase that
develops in the former upon water addition, were investigated as a function of increasing water concentration, mass fraction, measured:
in the interval 0.024--0.8.
the complex permettivity in the frequency range 16Hz-50~z;
We
the con-
centration dependence of both 8' and 8" at the different frequencies; the thermally stirm/lated dielectric polarization release (TSD); and the electro-optical behavior.
The different structures that develop in isotropic microemulsions as
water is added, were analyzed.
An accurate description of the tempe_rature-depen--
dent relaxation processes occurring both in the isotropic state and in the LC phase of these systems was obtained and their activation energy and relaxation time calculated.
Two different structures, of lyotropic nature, respectively of
cylindrical and lamellar type, were identified; their ooncentration range of existence and tenpe~ature dcmain were determined.
The concentration regions where the
transition occurs from an isotropic micro~aulsion to an LC mesophase and from the first to the second lyotropic LC structure
(0.3-0.48)
(0.580-0.605) were also
investigated.
IN~T43DUCTION Microemulsions are optically transparent, isotropic, stable liquid dispersicns of water, soap, alcohol and a water-insoluble liquid (oil) (refs. I-3).
Depen-
ding upcn whether water is disy~_rsed in oil (w/o) or viceversa (o/w) , two different types of microemalsion may be obtained.
0304-3886/82/0000--0000/$02.75
The soap and alcohol molecules are
© 1982 Elsevier Scientific Publishing Company
384
adsorbed in highly oriented form at the interface between the t%~ liquid phases; they constitute a stabilizing interphase whgch allows, wit~hout any input of work, the formation of a dispersed l~hase of spherical droplets with a diameter as small as 100
~.
The system ap=~ars transparent to the naked eye, since its droplet size, O
ranging between 100-1500A,
is small (I/4~), in cc~garison with the wave length of
the white light (4000,~<7000) .
Uo to this date, microemulsions have been consi-
dered thermodynamically stable; they do not separate into their original liquid phases, nor on standing, nor if spun in a laboratory centrifuge for 5 minutes at 100 G's.
A peculiar characteristic of these systems is that, the sLTple addition
of water to a fluid w/o microenmlsion, leads to the formation of a liquid crystal-. line phase (LC) with different mescrnorphic structures of lyotropic type.
As more
water is added, they invert to a perfectly fluid o/w type of dispersion.
Ibis
process is reversible. Considering the formation of the IC phase upon increasing w/o ratios, it is difficult to think of microemulsions in terms of a "very finely" dispersed twophase, hmmogeneous syste~ that forms spcntaneously and is also thermod~nqamically stable (refs. 4-@ . As a matter of fact, it has been suggested that microemulsions have nothing to do with "emulsions" hut instead are "micellar solutions"
(refs.4-5).
In this case, the system would be a single phase system cfmposed by colloidal aggregates (micellae) of surfactant(s) which can bind water or oil depending on the proportions of the different ccnstituents. tically:
Both theories need be oons±dered cri-
the difference in terminology is more than ~ s t a question of semantics.
The problem of the mechanism of formation of microemulsions is hitherto unresolved. Due to their unique properties, current interest in microemulsions arises frcm the widespread use of these systems in many different fields of everyday life. Since 1943 microesmlsions have been cn the market as:
self-polishing floor waxes,
cutting oils, detergents and dry cleaning products, polymer latices for paints, just to mention a few.
~%icroenmlsions of both types are also investigated for
tertiary oil recovery and for biophysical and biomedical ~ r p o s e s
(ref.6).
In the present paper the dencmination "microemulsion" will be adopted to identify the aforementioned system since both sides of the controversy use the same term, althcugh each ascribes a different meaning to it. The present work deals with the analysis of the dielectric and electro-optical properties of a water-in-dodecane t~_ype of microemulsion.
Tne main purpose of the
research was to have information abo/t the various structures through which the
,385 system organizes itself as a function of increasing water contents.
With this
aim, starting with a perfectly fluid, isotropic w/o microeamlsion, we investigated, upon very little water increments, the mechanism of formation of the liquid crystalline phase, and, within the latter region, the properties which both distinguish and characterize the different mesomorphic structures. We studied, by means of different experimental approaches, always the "same" microesmlsion, i.e. without modifying or changing the ccmlnosition and the proportions of its oily phase.
Within each concentration region, a conparison of the
findings was made by paralleling the results obtained during o~mpletely inde2endent both experimental runs and technical approaches, in order to verify whether the observed properties may be considered as intrinsic features of the particular system defined within the given concentration interval.
M~TERIAIB AND ~ I H O D S PreDaratic~ of the microer~ision ~he oily phase of the microenmlsion was realized with n-dodecane, n-hexanol and potassium oleate, were:
qhe proportions by weight of the different constituents
58.6% dodecane, 25.6% hexanol and 15.8% potassium oleate.
Microemmlsion
samples were produced at 293°K, by adding to the former mixture very small amounts of double distilled water from. a Super-Q-Millipore System with a 0 . ~ m Mil!ipore filter.
The water content of the sanples was expressed by the mass fraction C
(weight ratio (water/~o)) . the oily phase:
Two different procedures were followed to formulate
a) by mixing first 100 mi of dodecane, 40 ml of hexanol and 20
ml of oleic acid; then a solution of potassium hydroxide of known titre was preDared and the stoichiometric amount of KOH, needed to neutralize the oleic acid was added to the fomaer mixture.
Therefore the K~oleate was obtained "in situ"
and the initial water content, ~ae to the acid-KOH reaction, was taken into a ~ t in the calculation of the sample concentration; b) by mixing, always in the same proportions, n-dodecane and n-hexanol which were both double distilled in cur laboratory, and 99.7% pure potassium oleate in the form of a white crystalline powder produced also in our laboratory (refs.7,9) .
2.4% of water by weight was then
added to the oily mixture in order to solubilize the potassium oleate.
The latter
amount of water was acccunted for in the evaluaticn of the total water ccntent of each microen~ision sample.
386
Phase Map of the actual micro~mulsion The different structure regions that may be macroscopically observed in the actual system, as a function of increasing water contents, are synthesized in the phase~nap (Fig.l) performed in the temperature interval (253-353)~K within the concentration range 0.024-0.8 (refs.9-11) . Each point in the (C,T) diagram corresponds to an equilibrium condition of the system.
In order to detect birefrin-
gence, each sample was observed through two crossed polaroid sheets with a white light source. Four different concentration regions may be identified in the range 0.024~C_~0.8: I-the system is a stable, optically transparent isotropic w/o microemulsion with water spherical droplets; II-the system s%o~ates into twr) different phases: isotropic, the lower; birefringent.
the upper one is optically
This region was called the "Gap Region."
III-the system becomes a lyotropic liquid crystal (LLC) and is naturally birefringent. IV-the system a p ~ a r s again as optically isotropic and transparent.
In order
to envisage both the dielectric and the optical results vs. concentration, with the corresponding microemulsion structure regions, hereafter constant reference will be made to the system's phase map plotted in Fig. I. Dielectric properties The dielectric properties of the system were studied by means of +6
353
I
~
em
two different methods. +
60~-
.°r--
I *40
313
~ i~
2
~ ~\
3
~ + 0
. . . . . . . . . . . . . . . . . . . . .
°il
~
\
....
===
In the
first we measured over the fre-
NIP&IIIINT 21TIA I,,
quency range 16 Hz-50 MHz and with.
4 I hA|Spa|(|! o llllll .....
in the concentration interval
~-
0. 024&C~0.8, C
a) at T=-293"K £ 0.5~K, the frequen-
"2°~'263 !1
~2 !3
¼
!e
!6
!7 ~0
cy dependence of the relative complex permettivity at the different
Fig. I ; Micro~nulsions Phase Map
concentrations;
vs. concentration.
b) the behavior of both ~' and 6 ", at the different frequencies, as
a function of increasing water contents.
In the second, we investigated the te~-
perature dependence of the system's dielectric properties at the different con-
887 centrations, by applying a technique known as the Thermally Stimulated Depolariza-tion (TSD). Dielectric measures qhe former study was carried cut bY measuring the total impedance and-the ~ohase angle of a two terminal cell filled with the sample.
The method was specifically
devised to follow any spontanecus evolution eventually occurring in the system ~ r
test, by singly recording vs. time. at any given fixed frequency, the vari.-
ation of the complex impedance of the dielectric filled cell.
The sample holder
device was a two terminal cylindrical cell with coaxial shieldj circular, gold plane parallel electrodes with roughened surfaces and variable spacing. The electrode distance variation technique was applied to avoid electrode polarization inpedance,
The inloedance magnitude was known with an error of the order
of I%, the phase angle with an error of due order of -+ 0.5° . The calculated values of the real and the imaginary ~oart of the relative ccmplex permettivity were determined with an average uncertainty of 10% for 8' and 15% for 8 ".
The
method and the calibration procedure were described in refs. 8~9.
TSD measures In the TSD analysis the orientation induced in the sample by a polarizing field Ep at a polarizing ten~geratlLre Tp is frozen-in by immersing the sample in liquid nitrogen.
Then the field is removed, the sample connected to an electrometer,
and the depolarization current J(T) recorded as a function of linearly increasing temperab/re.
During the depolarization, the current due to a given relaxation
prooess first increases expcnentially, then reaches a maxirm/m and finally drops to zero.
If more kinds of dipolar species are present in the sample, a series of
current peaks are observed.
The resultant curve, known as the TSD spectram, offers
by means of a single measurement, a complete description of the t~r~erature dependent relaxation processes induced in the sanple by the ~
field.
Each current
peak is characterized by its maximim] peak t~mperature which is inde~Dendent of both the polarizing field strength and the polarizing te2mpe_rature, if the orientation process is of dipolar origin; on the contrary, it strongly depends on the polari-zing ~ r a t u r e
whether the polarizatien is produced by s_Dace charge build-up or
by charge transfer at the electrode-s~mple surface.
The latter behavior is usually
adopted as an experimental means of discriminating the different ccntributions to the overall TSD spectrum.
The activation energy ~ and the relaxation time ~
of
388 each isolated (see below in the text) depolarization process may be evaluated with the Bucci and Fieschi method (refs. 12, 13) , provided that the field induced polarization is of dipolar nature and follows a first order kinetic.
In this
case the thermal current J(T), interpreted as the rate of change of polarization, is analytically expressed by:
J(T) Po_._~o exp [ - _~/ K T -
tI
(b17o)-1
j
exp (- ~ /KT)dT ]
(1)
To
with J(T) = -dP/dt = -P/17 ; 17= 17o exp (~/KT) where:
(2)
Po is the initial polarization induced by ~
rate; T the absolute ~ r a t u r e ;
at Tp;
b is the heating
K the Boltzman's oonstant; ~
tively are the activation energy_ and the relaxation ~
and
170 respec-
constant of the given de--
polarization process. The values of the activation energy and of d~e relaxation time may he derived from the relation: in 17 (T) = i n ~ 0 +
~ / KT = [, in J(T')dT' t :I
- inJ(T)
(3)
The expression to the right in Eq. 3 may be evaluated by integrating the experimentally known depolarization current peak of the given process.
The total charge
released during a whole TSD spectrum is given by
Q =
J (T)dt
(4)
:O The exDerLmental conditions adopted in the TSD measures were: ature TD = 293°K, polarizing field ~
polarizing te~?er-
= 33V/cm (by applying to the sanple a +10
V, 0.5 Hz square wave), freezing tenperature Tf = 70°K and linear heating rate b = 0.1°K/s. The sample alignment at the electrode golden surface, was realized by rubbing, always in one direction, the surface of both electrodes with calibrated 2 0 ~ m Carborundum powders.
No twist was imposed to the sample.
The above treatment
was, for isotropic microe_mulsion san_~_les, equivalent to the "roughened surfaces" adopted in the dielectric study, while for LC samples it represents the "anchoring condition" at the hydromhobic electrode golden surfaoe (refs. 9, 14, 15). The current values were known with an error less than I%. values were calculated with an accuracy of 2%.
The ~
and in17o
For both isotrcpic and LC samples
independent temperature calibrations were made by measuring, in the absence of any in,pressed field ~ ,
the variation vs. time of the sample tenperature during
389
the freezing and the heating process.
Therefore the ~mpe_ rature readings were
assessed from the "inside" sample calibration values. The sample holder device used in the TSD m e a s u ~ t s
was made with circular
gold electrcdes, isolated by a very thin teflon spacer.
The cell electrode sepa-
ration, surface and volume were, respectively, d = 0.3 an; S = 2.54 c~ 2 and V = 0.763 c~ 3.
Electro-optical measures The electro-optical behavior of w/o microemulsions was investigated mostly with the aim of obtaining further expe_rimental evidence to support the TSD results on the LC samples, without having to worry about "whether the ~ r a t u r e
variation
had affected or in some way modified the structural integrity of the different LC mesophases"
(refs. 6-17).
The sar~ples were oriented at 300 _+ O.5°K, with a 33/Vcm electric field (~) realized by applying a +10V, 0.5 Hz, square wave. that adopted for Kerr effect detection.
The experimental set up was
The sanple holder device was a fused
silica parallel,~pipedon with rectangular, gold parallel electrodes: 4 c~ long, 0.3 c~ wide at 3 ~n distance.
The anchoring condition was realized by rubbing
lengthwise, in mutually parallel directions, the surface of both electrodes; no twist was imposed to the samples. (~=6328 ~) .
The light source was a 0.5 ng~ He-Ne Laser
The intensity of the transmitted beam was monitored with a linear
detector and the signal (mY), recorded as a ftmction of time. We measured:
in isotropic samples, the aneunt of induoed anisotropy; in natu--
rally birefringent sanples, the variation of the optical anisotropy due to the impressed electric ~
field.
ANALYSIS OF :4ICKOI~MUIBION PROPERTIES IN THE D I F F E ~ T
CONCENTRATION REGIONS
Isotropic and prestructural region Dielectric behavior:
the dielectric behavior of the w/o microemulsicn as a
function of increasing water content at different frequencies is characterized by twomain features:
an abrupt increase of the
8' and $" values at C = 0.3 (ref.8)
that strongly depends on the .measuring frequency and a frequency independent di-vergent behavior as the concentration approaches the value Co = 0.582 (Fig. 2a,b). The fozmer, hardly detectable at frequencies higher than 10 M~z quite well observable in the low-frequency range (10 Hz-5 ~ z )
(Fig. 2b), is on sanples with
390
z,iC"
((~)
(,.
SO_ £'---.- 0 I~%_..
70
%
4060
-aO
50
(')
/J2= 5 0 M H Z
/
30~
I
I
'lO
I I I
40 1|.
!(bt
30
10
! J I
I0.
20
~
O-
5,,,
o~,
'
m--. mm.m~-e'j
I'
! .4
o!3' o[s" ~', '
I .5
!
Fig. 2. The real and the imaginary _Dart of the relative complex permettivity of w/o microemulsicns vs. water content at a few representative frequencies.
I (pA) I(pA)
/~ . . . . . C . 0.334 - - - C . 0.31S
6.
__ / ~ / /
2~,
^
2~3
/
1 .C..101
~
~x-.~3~
2~3
C ,,0.3S3
(b)
(O) 4.
,13 "K
0
2t3
2~
Fig. 3.a) TSD Spectra of isotro-Dic w/o microe~nulsions; b) First appearance of the orientation processes linked with the system': strlctural ordering.
391 water content in the interval 0 . 3 • C ~ 0 . 4 2 . verge at Co may be expressed by: is the reduced concentration
~he manner in which 8 ' and ~ " di-
8 ~ = ~ - I / 3 and 8 " = ~
(ref. 7).
-7/5 where
@=
C-C0/Co,
~he trend as well as the frequency depen-
dence of 8 ' and ~" vs. C, were controlled during three independent exDer£~ental runs 2erformed on microemulsion saaples produced with the two different procedures explained in section 2.
The frequency dependence of the conductivity a (~), at
the different concentrations, in the interval 0.3 - 0.42, was found to display a conductivity relaxation that ca/ses a (~2) to increase at : 100 Hz. result usually characterizes the a ( ~ )
~he latter
behavior of "nematic type" liquid crystals
with positive dielectric anisotro~ny (ref. 8). T.S.D. analysis : optically isotropic and trans_oarent microemulsion samples are ~haracterized by a single peak TSD spectrum which occurs at T = 253°K.
Besides a
single band s~ectrum (Fig. 3a, curve I) observed on samples with very low water content (0.024-~C,0.13)~ within the interval 0.134 - 0.35, the depolarization process at T = 253°K is the only current .peak exhibited by the actual system (Fig. 3a, curves 2,3).
The latter persists throughout the whole concentration interval
analyzed (0.13 - 0.5) with the sane peak temperature, even by varying the polari-zing temperature, in the interval 213°K< To< 353"K, 3 to 40 min. and the polarizing field, ~ ,
frcm 1.33 to 167 V/~n, by applying both
a +10 V DC signal and a +10 V, 0.5 Hz square wave. order relaxation kinetic and is fit
the polarizing time, fp;frcm
The 253°K peak f o l l ~ s a first
quite will by the theoretical function given
in Eq. (I). Considering the above features,
the 253°K peak was interp_reted as due
to the orientation of the electrified water-oil interface of the disnersed system that represents the only eommcn feature s_hared by all samples and, therefore: called The Interface Peak.
AS water is added, a seccnd neak becomes detectable
at T = 273°K in transparent and apparently hcmogeneous samples (Fig. 3b); further additiou of water leads to the fo~nation of a characteristic current band wit~hin which a well defined current peak develops at C = 0. 371 with a maximum peak tem~Derature at T = 288°K (refs. 9-14).
The latter de~nolarization ~rooess is obser-
vable only in a restricted concentration interval (O.35
(288°K• Tp< 313°K).
Within the above
limits, however, the max. peak temperature stays constant by varying the intensity of the applied field, the polarizing ~ a t u r e
and time.
The 288°K peak is also
fit quite well by Eq. (I) and follows a first order relaxation kinetic. Since the 288"K peak is defined by both a concentration range of existence and
392 a t~n~erature domain, we interoreted this orientation process as due to the instauration of scme*kind of structure which is destroyeJ if the aforementioned limits are exceeded, and called this type of process a Structure Relaxation Process (ref.15) The peak recorded at T = 273OK inm~diately after the Interface Peak (Fig. 3b) was ascribed to the anchoring condition realized at the electrodes surface whose effect beccmes detectable as soon as same structural organization develons in the system.
The Anchoring Peak may be explained in terms of the "under field" cc~oe-
tition that arises between the wall alignment and the field alignment, i.e. between the phase anchored to the electrodes and the bulk phase of the sm~ple which results in a structured phase at the electrode surface and in an oriented structure in the bulk ohase.
The Anchoring Peak is characterized by a structure-like
relaxation process (ref. 9). It is worthy of note to observe the ~nazing correspondence between the C values at which tlqe structural depolarizaticn processes develop, and those at which both
~' and ~" exhibit t~he frequency dependent abrupt increase, plotted
in Fig. 2a.
First Gap Region With reference to the Phase Map (Fig. I) : C = 0.4 (ref. 9-11), a phase s e ~ a t i c n
as the water content approaches
occurs in the system and both an isotro-
pic microemulsion and a birefringent LC phase are observed.
In this region:
called the Ist Gap P~ion, the TSD spectra are c~haracterized by the Interfaoe Peak (253"K) and the Anchoring Peak (273°K), which occur
always with the sane
peak tenperature, as well as by a Structure Peak, whose ,max. peak intensity shifts upon water addition within the range 0.4< C < 0.5~ from 288°K to 292°K.
As soon
as ccmpletely birefringent LC sanples are formed~ the above shift stops and a structural relaxation process characteristic of the given LC mesophase develops. The latter does not change its max. peak t ~ r a t u r e
as far as the concentration
is varied inside the concentration range of existence of that given structure. The above process is grad/al and paralles the .modification of the system's struc.rural ordering;
therefore, the ~ r a t u r e
calibration performed on LC samples
was adopted to assess the temoerature readings obtained with a thermocouple placed as near as possible, but still "outside" the sample under test.
Due to the cali-
bration prooedure, the max. peak temperature of both the Interface and the anchoring peaks are shifted, respectively to 262eK and to 276°K (Fig. 4a).
The problem
393
~I[~A) 2O
20- I~A)
C =0.504 A INTERFACE
I I
t
)
(b)
1 10.
0
'° 2,;o
260
280
30
/
t '~
c1(
Fig. 4.a) Example of peak-separation performed on the TSD spectrum of a LC sarmple (T = 293°K) ; b) Typical Bucci and Fies~hi curve fit for the Interface Peak. Triangles: experimental points; dotted line, theoretical fit (Eq.1).
a.&&
2(:
c,=0~.,
,~ (a)
c2=0.~,95
/ /
¢3=0.St 9 10
0
. 8
.
.
.
16
.
2/.
m In
Fig. 5.a) Electro-optical behavior vs. time of microemulsion sanDles in the Pre-structural and in the First Cap Region. Each curve was normalized to its starting value measured before the inloosition of the field.
394
I
]
~Ibil"
(b)
r I0-
(b0)
10m0
Fig. 5.b) Inverse of the exponential coefficient "b" y = (Q exp (bt)) , as a function of concentration. Upper curve, "positi\~" values calculated on the electro-optical ~neak rise; lower curve, "negative" values calculated on the anisotrony decay
wf03o-
(b.0)
40-
curve.
10-
The I/ I b l values were plotted in .~ich a way as to adhere to the experimental behavior. ~cee text for further exn!anations.
10-
.,.
. I flea)
I(pA)
/h ,A) /
20
C--0.506 (aJ \ C=0.538{,)
20
/
•
$I|UCTURE
tl~,',
INTERFACE PEAK
PEAK
,~ ~/~
" ~ ,-, 'I
I~
C=0.663(I)
10 10
~,/ %/ STRUCTURE
j 2,;o
,,,D
2~0 ' 2~o
1o1
360 'TO'K;
240
260
280
300
T('K )
Fig. 6. Strucb~ral relaxation processes exhibited by birefringent microemulsion samples, a) TSD _spectra that characterize the first LC mesorgnase; b) the second
LC mesophase.
395 concerns only the TSD _spectra performed on samples with water
contents falling
in the neighborhood of C =0.5, at the boundary between the I st Gap and the birefringent LC region. ~he mechanism of formation of the LC phase that develops in the actual system as its water content approaches a ratio C .-0.5, was investigated by means of electro-optical anisotropy measurements in the ccncentraticn interval 9~ich extends from the isotropic microemulsion phase up to and beyGnd the Ist Gap region (0.35< C < 0.55). Despite the extremely low-level field 'Imposed, an electrically induced birefringence was observed in isotropic w/o microerallsion samples at C = 0.37.
~he
amount of induced birefringence, first raised ~aralleling the ccncentration increase, reached a maximum at C = 0.487 and then decreased gradually as the concentration C = 0.5 was approached (Fig 5a).
In the neighborhood of the latter
concentration, no effect at all could be detected.
For C ~, 0.5 the field induced
effect on naturally birefringent samples, was an anisotropy decay as shown in Fig. 5a, curve 3.
In the isotropic s6mples the field induced effect was an elec-
tro--optical peak (curve I), whose max. peak intensity shifted with time upcn increasing concentration.
In any case, the removal of the field was never follcx4ed
by a return of the system to its starting configuration. of the type of effect induced,
~gDreover, independently
it was not possible to stop the development of the
process, onoe it had been switched on by the i _mposition of the field.
In order
to have a better and more quantitative description of the above behavior, we calculated the e~qgonential coefficient b
~ y=8 exp (bt)]
of the "under field" elec-
tro-optical peak rise as well as that of the anisotrepy decay curve.
~he result
is plotted in Fig. 5b, where we reported, vs. concentration, the I/l(b)I values in such a way as to adhere to the experLmentally observed behavior, since the b-coefficient
is positive for isotropic saaples and negative for the naturall_v bire-
fringent ones.
The concentration interval investigated aLnpears divided into two
regions where the system exhibits completely different properties s the transition concentration being the value C = 0.5 (ref. 16).
The Liquid crystalline region Tne TSD analysis, as well as the electro-optical study, sh(~4 that two different mescmorphic states may be distinguished in the birefringent region, each exhibiting both a typical TSD spectrum and electro-optical behavior (Figs. 6, 9).
396 The TSD spectra of naturally birefringent LC samples are characterized by: 1)the Interface Peak, the depolarization process occurring at 262°K whose maximum peak ~ r a t u r e
does not depend on the sample water content; 2) the Structure
relaxation process associated with the given mesophase; the latter is a depolarization band at ~ 300°K for the first LC structure (Fig. 6a) and a depolarization peak at T = 272°K for the second LC structure (Fig. 6b).
qhe TSD analysis vs.
water content proved that each LC mesophase is confined in a concentration range of existence.
The tests made by orienting the samples at different polarizing
temperatures, shoved that each LC structure has its own temperature domain. larization at a Tp outside the corresponding temperature d o ~ i n rile destruction of the system's structural organization. develops at higher water contents (0.6< C<0.7)
Po-
limits/ entails
The second LC structure
than the first (0.5
in a ic~ger temperature domain (272°K and 300~K respectively). Polarization at different T[] values within the above intervals, together with the many tests performed to identify the nature of the orientation processes, demonstrate that also the struch]re relaxation processes are of dipolar origin and follow a first order relaxation kinetic. The activation energy ~
and the relaxation time ~7 of both the interface and
the structure peaks are plotted in Figs. 7a,b and 8a, b as a function of increasing concentration.
The values were calculated following the procedure described
in section 2 para.4.
In Figs. 7 and 8, depending on the number of relaxation pro-
cesses exhibited by the system, from one to three different ~ correspond at any
given C.
([) values may
These are distinguished by the _max. peak temper
ature at which the current peak developed in the TSD spectrum.
One should take
into account that in this case, to analyze the system's properties against water addition, does not _mean that the system is always the same and ~
are singly vary-
ing one of its parameters, since we are con~aring systems that differ in their state of aggregation.
In other words, "a between sanples" comparison is a "be--
tween systems analysis."
As far as the latter point is concerned however, it is
quite surprising to recognize that, despite the change of the system's water content, there are concentration intervals within which the system abides I and stun ples that differ in their water content behave as if they were the same system. On the other hand, there are concentration intervals where the system evolves and even the s~me sa~]le may display quite different characteristics as a function of time (phase transition).
In the former case the concentration may indeed be con-
397
3. ~(eV]
'~t "v'
[O1
1I
I
/
(b)
tI
'°,-''
o-2,,~o
¢-'° I
2,
•
1.
0, d
C(wlw_i
.2
.3
.4
..~
-
0.5-
,
~
.
.4
.
.
.5
.
.6 C (wlw)
Fig. 7. Activation energy of microe~ulsion relaxation processes vs. water content. a) Isotropic and Prestructural region; b) First GaD and Optically birefringent region.---~ Interface Peak ;- -O-=struct~re peaks; . ~ m Anchoring peak. - - ~ - - I st LC p h a s e ; ~ 2 nd LC Phase.
o - I sl LC peele
L~,8,
(a)
(b)
o-2-~°
,,
_l~.-=1|ucTu.m ~A= ~m,e*F~a PlAE
~o~
•
1.C
•2
0
6
~°°
C{wl~)
;I
,,/
.3
.4
.5
10 ~
.3
.4
.5
.6
C(w/w)
Fig. 8. Relaxation time diagram vs. concentration of the depolarization processes corresponding to the activation enerqies plotted in Fig. 7. ( ~ was calculated at T = 293"K) Symbols are the same as in Fig. 7.
398
sidered as one of the system's parameters; in the latter it cannot.
Due to the
propagation of the error (cfr. Eqs. 2, 3), the relaxation time data are affected by a greater dispersion than the ~ data.
However, the difference in the order
of magnitude of the ~7 values calculated is absolutely significant.
The TSD nara
meters of the Structural relaxation processes are reported in Table I.
The Inter-
face peak parameters may by found in refs. 9 and 18.
We note that the "zig-zag ~'
trend of both ~
a dispersion effect;
and ~
vs. concentration is not ~ s t
it
depends on the successive d%anges of the system's dispersed phase dimension.
The
phenomenon occurs at some specific concentrations and is evidenced by an abrupt increase of the depolarization current peak intensity. by both the interface and the structure peaks.
This behavior is followed
(cfr. for instance Figs. 3a and b
and refs. 9 - 18.) By means of the electrical anisotropy measurements on naturally birefringent samples, two different trends were observed in correspondence with the C-range of existence of the first as well as the second LC mesophase. field induced effect is a Ubirefringence decay"
For 0.5< C< 0.6; the
(Fiq. 9a); for 0.6 < C< 0.7 the
field induced effect is a "birefringence rise," often acccrnpanied by saturation effects (Fig. 9b) .
In addition, for 0 . 5 8 0 ~ C < 0.605, an intermediate behavior wa.~
recorded between t/nat of t,ype (a) and that of type (b) (Fig. 9c), (refs. 17, 18). The latter parallels the vanishing of the I st LC phase and the development of the IInd.
We recall that within the same concentration interval both
6' and 8 " ex-
hibit the frequency independent diverg~_nt behavior plotted in Fig. 2 and the TSD analysis displays extremely cc~plex relaxation spectra where both the current peaks that ~haracterize the Ist and the IInd LC mesophases are simultaneously present. With the sole
I) the angular dependence
of the transmitted light intensity at any given angle of entrance of the light electric vector; 2) whether the microemulsion lyotropic mesop_.hase exhibited any rotator7 power (in spite of the absence of chiral .molecules in cur s~]stem). For each study several tests were merformed by changing the system's anchoring condition, the sar~le holder gec~etry as well as the sample volume.
The results
may be summarized as follows: I) the dependence of the tran~nitted light intensity on the angle at which the
399 electric vector of the light wave enters the sam?les with water content in the interval 0.48 - 0.580 displays a very regular pattern which is ~ i i polar plots given in Fig. 10a.
evidenced by the
~he four-lobed array vanishes at C = 0.582 in the
interval 0.570( C < 0.6 (Fig. 10b, c), while it appears again cn samples with water content in the interval 0 . 6 < C < 0.7 (Fig. 10d).
In the latter case the lobe's
am glitude is four times smaller than that of type (a). 2) The samples belonging to the first LC meso_nhase (0.48< C < 0.6) exhibit a strong rotatory power; those of the second (0.6< C<0.7) do not.
In the concentration
interval where electro-optical recordings of type (c) (Fig. 9) we_re observed, the rotatory power decreases gradually vs. increasing water content and disappears at C ~
0.6.
Of course to talk about rotatory power in a system without any chiral molecule takes some swallowing.
The first criticism that could be made is that the effect
may be due to an interaction between the walls of the s ~ p l e holder and the sample itself.
However, in this case, since we have drastically chang..~d the gec~etry of
t/qe sam21e cell, as well as the anchoring conditions, we should have observed~ at least on one occasion, some significant difference in the "ability of the system. to rotate the plane of polarization of the light wave." error of +2 °, this has never happened.
Within the exnerJ_mental
On the contrary,, the rotatory p o ~ r
seems
not to depend on the sa~Dle holder, dimensions or shade, nor on the particular anchoring condition realized.
From a practical point of view, it is reasonable
to utilize the above results for a further characterization of the system's struc -~ tural regions, that being the akin to which t/qe measures were devoted. ~ne results indicate that in t~he microemulsion birefringent region the two lyotropic mesophases distinguished by a s_necific TSD spectrum, electr~-qptical as well as optical behavior, are separated by a Second Can Region within ~%ich they coexist.
The second gap region is identified by:(1)the behavior of ~ ' and ~ "
(Fig. 2);
the intermediate electro-optical behavior (Fig. 9c); the vanishing of
t~he rotatory p~wer previously described and the disappearance of the four-lobed array (Fig. I0c). %3%e problem is now to understand what kind of structnre may correspond to the first as well as to the second LC mesephase of the w/o microemulsion. of structures have been hypothesized (refs. I-3~ 18):
Two b_,qges
a cylindrical phase in
which each cylinder is a thin water channel covered by the hydrophilic groups of the molecules while the hydrocarbon chains fill the gap between the cylinders,
(i) divergent
400
mV
50.01 hm V
2OO 37.5. 150
1--C=.511 2~C=.536
,_c...,
'mV
1--C=.626 2-- C=.629 3--C=.638
/ / / / /~-~
2.--.
//\
,.
[
",
150-
i
100.
1 ~ C = .582 2--C=.593
20C
tOO.
50.
1
O1"0
= O 2"0 m -
~
0
b
1"o
;~0
3"0
4'0 m
Fig. 9. Typical electro-optical behavior exhibited by samples with water content falling in: a) the first LC region; b) the second LC reqion; c) the Second can region where both mesophases coexist. Due to the ~ (632B~)of the laser beam employed, the samples of the second LC mesoohase appear only slightly birefringent.
Z
0"
100"
L
270 e
~,
180"
d
Y
Fig. 10. Angular ~ene~ndence of transmitted light intensity: Polar Plots of the ratio I (min.)/I (.max.) ( ~ ) vs. the in_r~t angle ( ~ ) of the light Electric vector to the y-axis, qhe latter is re_[xDrted as seen by an observer looking tc~ward t h e incoming beam. A]C= .54e S]C= .570 C]C.-,586 D]C--. 604
401
taking up a disordered, liquid-like configuration, and a lanellar phase, built up by an ordered sequence of oil and water planar sheets (Fig. 1 I). According to what is known on liquid crystals it follows that, in the lanellar configuration the requirement of a constant interlayer thickness should not allow any helical or twisted arrangement of the strucbare,
qherefore a lamellar phase
should not exhibit any rotatory ~x)wer while a cylindrical phase (nematic) may exhibit such a behavior (ref. 19). Finally in a given system in which both structures develop, the lanellar phase usually occurs at a tempe rabare below that of the nematic dQmain.
Since the two structnres are of different symmetry, they mast
be separated by a transitiou region.
The aforementioned characteristics suit
quite well those that distinguish the microemulsion LC mesophases.
Therefore it
is reasonable to assume that the first LC mesophase consists of a cylindrical structure, the second of a lamellar (ref. 18). The divergent behavior of 8 ' and " at C = 0.582, corresponds to the transition frcrn cylinders to lamellae, due to the fact that the water-oil interface, in terms of a surface enclosing the volume of the dispersed water phase, breaks down.
As far as the rotatory ~ower is
concerned, the observed phenomenon in the actual system may be justified in terms of a "bulk effect" originated by the twisted loops of water channels within the syst~n.
This is possible even in the absence of chiral molecules.
w/omicroemulsion
(1) structur~_.~
@ o/wmicroemulsion
Fig. 11.
Struc~ral transitions in w/o microemulsions against water addition.
The thermotropic transition In the second gap region the LC system is not only lyotropic but also t~herm~. tropic, and the t~Tpera~/re dcr~/ns of the two struc~res beccme very narrow. The different mesophases are still piloted by the water content;
however, amcng
the two structures, the system tends to assume the one which is more favoured by
402
/ Z-
~
i460
/
[
C.o.oos
T .30"C
.
,.0KHz
" ~ S0MHz
10"1
2601< 0
. . . . . . 2
s 4 TIM E(hours)
II
*
8
39
10"% 40
0
I/
L /
, , ..~
50MH z
%~2 nd AP G
'
l
l
, C..._
.2
.4
o8
.1~
Fig. 12. a) Thermotrooic transition in the Second Cap_ Region Lamellar-Cylindrical transition. (See text for explanation); b) The loss tangent vs. C-calculated on the data plotted in Fig. 2 is reported in order to envisage the dielectric behavior of the system in the whole concentration interval investigated (0. 024-0.8).
',%1 11,11
(Z,)
140 110]
3'
C1= 0.,510 C2= 0.604
1,5
TIME Fig. 13. Thermotropic transition:
(hours} Cylindrical-ian~llar Transition
403
the hanlpera~ure at which the system finds itself.
This kind of thermotropic
transition characterizes both gap regions ~nd is well observable by following the variation vs. time at a fixed frequency of the sample complex 'iEQedance (Fig. 12). In Fig. 12 a sample of the 2nd gap region, with a la~ellar structure ( 1 0 ~ is kept at T = 303°K.
Z-range),
This temperature does not favour the lamellar structure.
Therefore the system undergoes a thermotropic structural transition by changing to the cylindrical configuration 160 KHz.
(K~ Z-range) .
The recording was performed at
The order of magnitude of microemulsion impedance in the different con-
centraticn regions at several .measuring frequencies are given in ref. 8.
In Fig. 13
the oon~lex J__mpedgnce behavior of a stable lyotropic s a ~ l e with cylindrical struc-ture (Zl, ~I ) is plotted, as well as that of a sample with a water content falling at the end of the second g~o region (Z2, ~ 2 ) . 293°K.
Both s ~ l e s
The measurements were carried Out at 160 KHz.
is 288°K.
qhis t ~ a t u r e
were produced at
The measuring temperature
falls inside the temperature domain at which the truly
lyotropic cylindrical phase may exist.
But due to the larger water content of
the Z 2 sample, also with cylindrical structure (K~Z-range),
the above tempera-
ture favours the transition of the latter to a l&mellar phase (I02~ Z-range). ~%is behavior which occurs in the very complex region of the system is the most interesting for biophysical purposes and applications.
TABLE I TSD analysis : Structural relaxation ~rocesses in w/omicroemulsions C (w/w)
Tma x (6K)
~(eV)
~(~)
in~ o
a(~)
~(s)
0.03 0.03 0.04 0.05
-28.00 -40.30 -63.30 -48.84
1.4 1.2 1.6 2.1
43 235 496 330
1.390 0.05 1.824 0.09 Mesophase 1.250 0.03 1.198 0.12 2.080 0.17 1.800 0.06 1.760 0.18 1.290 0.05 1.340 0.09
-50.26 -67.76
1.9 3.7
118 86
-45.30 -42.58 -78.40 -67.17 -65.00 --46.40 -47.80
1.2 4.6 1.0 2.8 7.0 2.0 3.6
67 128 51 56 108 107 182
Prestructural Re~ion 0.371 288 0.388 299 0.403 302 0.418 304 First Gap Reqion 0.446 295 0.484 292 ~Jrst Lic~id Crystalline 0.495 292 0.506 292 0.516 292 0.526 292 0.536 292 0.545 292 0.581 295
0.802 1.156 1.756 1.380
404
TABLE I continued C (w/w)
Tmax (°M)
~(eV)
O(+)
Second Liquid Crystalline Mesophase 0.596 272 1.490 0.06 0.603 272 1.340 0.07 0.610 272 1.770 0.08 0.617 272 1.500 0.07 0.623 272 1.580 0.07 6. 641 272 I. 380 0.03 0.652 272 I .509 0.07
into
O(+)
--59.30 -52.60 -71.80 -59.80 -63.70 -54.16 -59.99
2.8 2.8 3.7 2.9 2.9 1.5 2.8
~ (S)
0.7 I .6 0.2 0.6 0.3 I. 6 0.8
REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
J.H. Schulman and D.P. Riley, J. Colloid Interface Sci., 3(1948)383. W. Stoeckenius, J.H. Schulman and L.M. Prince, Kolloid Z., 169 (1960)170. L.M. Prince, J. Colloid Interface Sci., 52 (1975)182. S. Friberg, L. Mandell and M. Larsson, L. Colloid Interface Sci., 29(1969) 155. P.E. Ekwall, L. Mandell and K. Fontell, J. Colloid Interfaoe Sci., 33(1965)215 L.M. Prince, (Ed.), "Microemulsions, Theory and Practice," 1977, Academy Press Inc. D. Senatra and G. Giubilaro, J. Colloid Interface Sci., 67(1978)488. D. Senatra and G. Giubilaro, J. Colloid Interface Sci., 67(1978)457. D. Senatra, C.M.C. Gambi and A.P. Neri, J. Colloid Interface Sci.a 79(1981)443. S. Ballaro', F. Mallamace, and F. Wanderlingh, Opt. Cc[~mln., 25 (1978)144. S. Ballaro' , F. Mallamace, F. Wanderlingh, D. Senatra and G. Giubilaro; J. Phys. C, 12(1979)4729. C. Bucci and R. Fieschi, Phys. Rev. Lett., 12(1964)16. C. Bucci, R. Fieschi and G. Giudi, Phys. Rev., 148(1966) 816. D. Senatra, C.M.C.Gambi and A.P. Neri, Lett. Nuovo Cimento; 28(1980)433. D. Senatra, C.M.C. Gambi and A.P. Neri, Lett. Nuovo Cimento, 28(1980)603. D. Senatra, M. Vannini and A.P. Neri t Lett. Nuovo CJanento, 28(1980)453. D. Senatra, M. Vannini and A.P. Neri, Lett. Nuovo Cimento, 28(1980)608. D. Senatra, Ii Nuovo Cimento, 64B, (1981)151. P.G. De Gennes, "The Physics of Liquid Crystals" (M. ~&arshall and D.H. Wilkinson Eds.) , The International Series of Monograph in Physics, Oxford University Press. (Clarendon) London-New York (1975).
383 -
-
Printed in The Netherlands
DIELECTRIC PROPERPIES OF MICROEMULSICNS AND OF THEIR LIQUID ~RYSTALLINE MESOPHASES
D~qATELLA SENATRA Liquid State Physics Laboratory, Physics Institute, University of Florence: L.E. Fermi 2; (Arcetri), 50125 Florence, Italy
ABSTRACT Isotropic w/o microe~isions
as well as the liquid crystalline
(LC) phase that
develops in the former upon water addition, were investigated as a function of increasing water concentration, mass fraction, measured:
in the interval 0.024--0.8.
the complex permettivity in the frequency range 16Hz-50~z;
We
the con-
centration dependence of both 8' and 8" at the different frequencies; the thermally stirm/lated dielectric polarization release (TSD); and the electro-optical behavior.
The different structures that develop in isotropic microemulsions as
water is added, were analyzed.
An accurate description of the tempe_rature-depen--
dent relaxation processes occurring both in the isotropic state and in the LC phase of these systems was obtained and their activation energy and relaxation time calculated.
Two different structures, of lyotropic nature, respectively of
cylindrical and lamellar type, were identified; their ooncentration range of existence and tenpe~ature dcmain were determined.
The concentration regions where the
transition occurs from an isotropic micro~aulsion to an LC mesophase and from the first to the second lyotropic LC structure
(0.3-0.48)
(0.580-0.605) were also
investigated.
IN~T43DUCTION Microemulsions are optically transparent, isotropic, stable liquid dispersicns of water, soap, alcohol and a water-insoluble liquid (oil) (refs. I-3).
Depen-
ding upcn whether water is disy~_rsed in oil (w/o) or viceversa (o/w) , two different types of microemalsion may be obtained.
0304-3886/82/0000--0000/$02.75
The soap and alcohol molecules are
© 1982 Elsevier Scientific Publishing Company
384
adsorbed in highly oriented form at the interface between the t%~ liquid phases; they constitute a stabilizing interphase whgch allows, wit~hout any input of work, the formation of a dispersed l~hase of spherical droplets with a diameter as small as 100
~.
The system ap=~ars transparent to the naked eye, since its droplet size, O
ranging between 100-1500A,
is small (I/4~), in cc~garison with the wave length of
the white light (4000,~<7000) .
Uo to this date, microemulsions have been consi-
dered thermodynamically stable; they do not separate into their original liquid phases, nor on standing, nor if spun in a laboratory centrifuge for 5 minutes at 100 G's.
A peculiar characteristic of these systems is that, the sLTple addition
of water to a fluid w/o microenmlsion, leads to the formation of a liquid crystal-. line phase (LC) with different mescrnorphic structures of lyotropic type.
As more
water is added, they invert to a perfectly fluid o/w type of dispersion.
Ibis
process is reversible. Considering the formation of the IC phase upon increasing w/o ratios, it is difficult to think of microemulsions in terms of a "very finely" dispersed twophase, hmmogeneous syste~ that forms spcntaneously and is also thermod~nqamically stable (refs. 4-@ . As a matter of fact, it has been suggested that microemulsions have nothing to do with "emulsions" hut instead are "micellar solutions"
(refs.4-5).
In this case, the system would be a single phase system cfmposed by colloidal aggregates (micellae) of surfactant(s) which can bind water or oil depending on the proportions of the different ccnstituents. tically:
Both theories need be oons±dered cri-
the difference in terminology is more than ~ s t a question of semantics.
The problem of the mechanism of formation of microemulsions is hitherto unresolved. Due to their unique properties, current interest in microemulsions arises frcm the widespread use of these systems in many different fields of everyday life. Since 1943 microesmlsions have been cn the market as:
self-polishing floor waxes,
cutting oils, detergents and dry cleaning products, polymer latices for paints, just to mention a few.
~%icroenmlsions of both types are also investigated for
tertiary oil recovery and for biophysical and biomedical ~ r p o s e s
(ref.6).
In the present paper the dencmination "microemulsion" will be adopted to identify the aforementioned system since both sides of the controversy use the same term, althcugh each ascribes a different meaning to it. The present work deals with the analysis of the dielectric and electro-optical properties of a water-in-dodecane t~_ype of microemulsion.
Tne main purpose of the
research was to have information abo/t the various structures through which the
,385 system organizes itself as a function of increasing water contents.
With this
aim, starting with a perfectly fluid, isotropic w/o microeamlsion, we investigated, upon very little water increments, the mechanism of formation of the liquid crystalline phase, and, within the latter region, the properties which both distinguish and characterize the different mesomorphic structures. We studied, by means of different experimental approaches, always the "same" microesmlsion, i.e. without modifying or changing the ccmlnosition and the proportions of its oily phase.
Within each concentration region, a conparison of the
findings was made by paralleling the results obtained during o~mpletely inde2endent both experimental runs and technical approaches, in order to verify whether the observed properties may be considered as intrinsic features of the particular system defined within the given concentration interval.
M~TERIAIB AND ~ I H O D S PreDaratic~ of the microer~ision ~he oily phase of the microenmlsion was realized with n-dodecane, n-hexanol and potassium oleate, were:
qhe proportions by weight of the different constituents
58.6% dodecane, 25.6% hexanol and 15.8% potassium oleate.
Microemmlsion
samples were produced at 293°K, by adding to the former mixture very small amounts of double distilled water from. a Super-Q-Millipore System with a 0 . ~ m Mil!ipore filter.
The water content of the sanples was expressed by the mass fraction C
(weight ratio (water/~o)) . the oily phase:
Two different procedures were followed to formulate
a) by mixing first 100 mi of dodecane, 40 ml of hexanol and 20
ml of oleic acid; then a solution of potassium hydroxide of known titre was preDared and the stoichiometric amount of KOH, needed to neutralize the oleic acid was added to the fomaer mixture.
Therefore the K~oleate was obtained "in situ"
and the initial water content, ~ae to the acid-KOH reaction, was taken into a ~ t in the calculation of the sample concentration; b) by mixing, always in the same proportions, n-dodecane and n-hexanol which were both double distilled in cur laboratory, and 99.7% pure potassium oleate in the form of a white crystalline powder produced also in our laboratory (refs.7,9) .
2.4% of water by weight was then
added to the oily mixture in order to solubilize the potassium oleate.
The latter
amount of water was acccunted for in the evaluaticn of the total water ccntent of each microen~ision sample.
386
Phase Map of the actual micro~mulsion The different structure regions that may be macroscopically observed in the actual system, as a function of increasing water contents, are synthesized in the phase~nap (Fig.l) performed in the temperature interval (253-353)~K within the concentration range 0.024-0.8 (refs.9-11) . Each point in the (C,T) diagram corresponds to an equilibrium condition of the system.
In order to detect birefrin-
gence, each sample was observed through two crossed polaroid sheets with a white light source. Four different concentration regions may be identified in the range 0.024~C_~0.8: I-the system is a stable, optically transparent isotropic w/o microemulsion with water spherical droplets; II-the system s%o~ates into twr) different phases: isotropic, the lower; birefringent.
the upper one is optically
This region was called the "Gap Region."
III-the system becomes a lyotropic liquid crystal (LLC) and is naturally birefringent. IV-the system a p ~ a r s again as optically isotropic and transparent.
In order
to envisage both the dielectric and the optical results vs. concentration, with the corresponding microemulsion structure regions, hereafter constant reference will be made to the system's phase map plotted in Fig. I. Dielectric properties The dielectric properties of the system were studied by means of +6
353
I
~
em
two different methods. +
60~-
.°r--
I *40
313
~ i~
2
~ ~\
3
~ + 0
. . . . . . . . . . . . . . . . . . . . .
°il
~
\
....
===
In the
first we measured over the fre-
NIP&IIIINT 21TIA I,,
quency range 16 Hz-50 MHz and with.
4 I hA|Spa|(|! o llllll .....
in the concentration interval
~-
0. 024&C~0.8, C
a) at T=-293"K £ 0.5~K, the frequen-
"2°~'263 !1
~2 !3
¼
!e
!6
!7 ~0
cy dependence of the relative complex permettivity at the different
Fig. I ; Micro~nulsions Phase Map
concentrations;
vs. concentration.
b) the behavior of both ~' and 6 ", at the different frequencies, as
a function of increasing water contents.
In the second, we investigated the te~-
perature dependence of the system's dielectric properties at the different con-
887 centrations, by applying a technique known as the Thermally Stimulated Depolariza-tion (TSD). Dielectric measures qhe former study was carried cut bY measuring the total impedance and-the ~ohase angle of a two terminal cell filled with the sample.
The method was specifically
devised to follow any spontanecus evolution eventually occurring in the system ~ r
test, by singly recording vs. time. at any given fixed frequency, the vari.-
ation of the complex impedance of the dielectric filled cell.
The sample holder
device was a two terminal cylindrical cell with coaxial shieldj circular, gold plane parallel electrodes with roughened surfaces and variable spacing. The electrode distance variation technique was applied to avoid electrode polarization inpedance,
The inloedance magnitude was known with an error of the order
of I%, the phase angle with an error of due order of -+ 0.5° . The calculated values of the real and the imaginary ~oart of the relative ccmplex permettivity were determined with an average uncertainty of 10% for 8' and 15% for 8 ".
The
method and the calibration procedure were described in refs. 8~9.
TSD measures In the TSD analysis the orientation induced in the sample by a polarizing field Ep at a polarizing ten~geratlLre Tp is frozen-in by immersing the sample in liquid nitrogen.
Then the field is removed, the sample connected to an electrometer,
and the depolarization current J(T) recorded as a function of linearly increasing temperab/re.
During the depolarization, the current due to a given relaxation
prooess first increases expcnentially, then reaches a maxirm/m and finally drops to zero.
If more kinds of dipolar species are present in the sample, a series of
current peaks are observed.
The resultant curve, known as the TSD spectram, offers
by means of a single measurement, a complete description of the t~r~erature dependent relaxation processes induced in the sanple by the ~
field.
Each current
peak is characterized by its maximim] peak t~mperature which is inde~Dendent of both the polarizing field strength and the polarizing te2mpe_rature, if the orientation process is of dipolar origin; on the contrary, it strongly depends on the polari-zing ~ r a t u r e
whether the polarizatien is produced by s_Dace charge build-up or
by charge transfer at the electrode-s~mple surface.
The latter behavior is usually
adopted as an experimental means of discriminating the different ccntributions to the overall TSD spectrum.
The activation energy ~ and the relaxation time ~
of
388 each isolated (see below in the text) depolarization process may be evaluated with the Bucci and Fieschi method (refs. 12, 13) , provided that the field induced polarization is of dipolar nature and follows a first order kinetic.
In this
case the thermal current J(T), interpreted as the rate of change of polarization, is analytically expressed by:
J(T) Po_._~o exp [ - _~/ K T -
tI
(b17o)-1
j
exp (- ~ /KT)dT ]
(1)
To
with J(T) = -dP/dt = -P/17 ; 17= 17o exp (~/KT) where:
(2)
Po is the initial polarization induced by ~
rate; T the absolute ~ r a t u r e ;
at Tp;
b is the heating
K the Boltzman's oonstant; ~
tively are the activation energy_ and the relaxation ~
and
170 respec-
constant of the given de--
polarization process. The values of the activation energy and of d~e relaxation time may he derived from the relation: in 17 (T) = i n ~ 0 +
~ / KT = [, in J(T')dT' t :I
- inJ(T)
(3)
The expression to the right in Eq. 3 may be evaluated by integrating the experimentally known depolarization current peak of the given process.
The total charge
released during a whole TSD spectrum is given by
Q =
J (T)dt
(4)
:O The exDerLmental conditions adopted in the TSD measures were: ature TD = 293°K, polarizing field ~
polarizing te~?er-
= 33V/cm (by applying to the sanple a +10
V, 0.5 Hz square wave), freezing tenperature Tf = 70°K and linear heating rate b = 0.1°K/s. The sample alignment at the electrode golden surface, was realized by rubbing, always in one direction, the surface of both electrodes with calibrated 2 0 ~ m Carborundum powders.
No twist was imposed to the sample.
The above treatment
was, for isotropic microe_mulsion san_~_les, equivalent to the "roughened surfaces" adopted in the dielectric study, while for LC samples it represents the "anchoring condition" at the hydromhobic electrode golden surfaoe (refs. 9, 14, 15). The current values were known with an error less than I%. values were calculated with an accuracy of 2%.
The ~
and in17o
For both isotrcpic and LC samples
independent temperature calibrations were made by measuring, in the absence of any in,pressed field ~ ,
the variation vs. time of the sample tenperature during
389
the freezing and the heating process.
Therefore the ~mpe_ rature readings were
assessed from the "inside" sample calibration values. The sample holder device used in the TSD m e a s u ~ t s
was made with circular
gold electrcdes, isolated by a very thin teflon spacer.
The cell electrode sepa-
ration, surface and volume were, respectively, d = 0.3 an; S = 2.54 c~ 2 and V = 0.763 c~ 3.
Electro-optical measures The electro-optical behavior of w/o microemulsions was investigated mostly with the aim of obtaining further expe_rimental evidence to support the TSD results on the LC samples, without having to worry about "whether the ~ r a t u r e
variation
had affected or in some way modified the structural integrity of the different LC mesophases"
(refs. 6-17).
The sar~ples were oriented at 300 _+ O.5°K, with a 33/Vcm electric field (~) realized by applying a +10V, 0.5 Hz, square wave. that adopted for Kerr effect detection.
The experimental set up was
The sanple holder device was a fused
silica parallel,~pipedon with rectangular, gold parallel electrodes: 4 c~ long, 0.3 c~ wide at 3 ~n distance.
The anchoring condition was realized by rubbing
lengthwise, in mutually parallel directions, the surface of both electrodes; no twist was imposed to the samples. (~=6328 ~) .
The light source was a 0.5 ng~ He-Ne Laser
The intensity of the transmitted beam was monitored with a linear
detector and the signal (mY), recorded as a ftmction of time. We measured:
in isotropic samples, the aneunt of induoed anisotropy; in natu--
rally birefringent sanples, the variation of the optical anisotropy due to the impressed electric ~
field.
ANALYSIS OF :4ICKOI~MUIBION PROPERTIES IN THE D I F F E ~ T
CONCENTRATION REGIONS
Isotropic and prestructural region Dielectric behavior:
the dielectric behavior of the w/o microemulsicn as a
function of increasing water content at different frequencies is characterized by twomain features:
an abrupt increase of the
8' and $" values at C = 0.3 (ref.8)
that strongly depends on the .measuring frequency and a frequency independent di-vergent behavior as the concentration approaches the value Co = 0.582 (Fig. 2a,b). The fozmer, hardly detectable at frequencies higher than 10 M~z quite well observable in the low-frequency range (10 Hz-5 ~ z )
(Fig. 2b), is on sanples with
390
z,iC"
((~)
(,.
SO_ £'---.- 0 I~%_..
70
%
4060
-aO
50
(')
/J2= 5 0 M H Z
/
30~
I
I
'lO
I I I
40 1|.
!(bt
30
10
! J I
I0.
20
~
O-
5,,,
o~,
'
m--. mm.m~-e'j
I'
! .4
o!3' o[s" ~', '
I .5
!
Fig. 2. The real and the imaginary _Dart of the relative complex permettivity of w/o microemulsicns vs. water content at a few representative frequencies.
I (pA) I(pA)
/~ . . . . . C . 0.334 - - - C . 0.31S
6.
__ / ~ / /
2~,
^
2~3
/
1 .C..101
~
~x-.~3~
2~3
C ,,0.3S3
(b)
(O) 4.
,13 "K
0
2t3
2~
Fig. 3.a) TSD Spectra of isotro-Dic w/o microe~nulsions; b) First appearance of the orientation processes linked with the system': strlctural ordering.
391 water content in the interval 0 . 3 • C ~ 0 . 4 2 . verge at Co may be expressed by: is the reduced concentration
~he manner in which 8 ' and ~ " di-
8 ~ = ~ - I / 3 and 8 " = ~
(ref. 7).
-7/5 where
@=
C-C0/Co,
~he trend as well as the frequency depen-
dence of 8 ' and ~" vs. C, were controlled during three independent exDer£~ental runs 2erformed on microemulsion saaples produced with the two different procedures explained in section 2.
The frequency dependence of the conductivity a (~), at
the different concentrations, in the interval 0.3 - 0.42, was found to display a conductivity relaxation that ca/ses a (~2) to increase at : 100 Hz. result usually characterizes the a ( ~ )
~he latter
behavior of "nematic type" liquid crystals
with positive dielectric anisotro~ny (ref. 8). T.S.D. analysis : optically isotropic and trans_oarent microemulsion samples are ~haracterized by a single peak TSD spectrum which occurs at T = 253°K.
Besides a
single band s~ectrum (Fig. 3a, curve I) observed on samples with very low water content (0.024-~C,0.13)~ within the interval 0.134 - 0.35, the depolarization process at T = 253°K is the only current .peak exhibited by the actual system (Fig. 3a, curves 2,3).
The latter persists throughout the whole concentration interval
analyzed (0.13 - 0.5) with the sane peak temperature, even by varying the polari-zing temperature, in the interval 213°K< To< 353"K, 3 to 40 min. and the polarizing field, ~ ,
frcm 1.33 to 167 V/~n, by applying both
a +10 V DC signal and a +10 V, 0.5 Hz square wave. order relaxation kinetic and is fit
the polarizing time, fp;frcm
The 253°K peak f o l l ~ s a first
quite will by the theoretical function given
in Eq. (I). Considering the above features,
the 253°K peak was interp_reted as due
to the orientation of the electrified water-oil interface of the disnersed system that represents the only eommcn feature s_hared by all samples and, therefore: called The Interface Peak.
AS water is added, a seccnd neak becomes detectable
at T = 273°K in transparent and apparently hcmogeneous samples (Fig. 3b); further additiou of water leads to the fo~nation of a characteristic current band wit~hin which a well defined current peak develops at C = 0. 371 with a maximum peak tem~Derature at T = 288°K (refs. 9-14).
The latter de~nolarization ~rooess is obser-
vable only in a restricted concentration interval (O.35
(288°K• Tp< 313°K).
Within the above
limits, however, the max. peak temperature stays constant by varying the intensity of the applied field, the polarizing ~ a t u r e
and time.
The 288°K peak is also
fit quite well by Eq. (I) and follows a first order relaxation kinetic. Since the 288"K peak is defined by both a concentration range of existence and
392 a t~n~erature domain, we interoreted this orientation process as due to the instauration of scme*kind of structure which is destroyeJ if the aforementioned limits are exceeded, and called this type of process a Structure Relaxation Process (ref.15) The peak recorded at T = 273OK inm~diately after the Interface Peak (Fig. 3b) was ascribed to the anchoring condition realized at the electrodes surface whose effect beccmes detectable as soon as same structural organization develons in the system.
The Anchoring Peak may be explained in terms of the "under field" cc~oe-
tition that arises between the wall alignment and the field alignment, i.e. between the phase anchored to the electrodes and the bulk phase of the sm~ple which results in a structured phase at the electrode surface and in an oriented structure in the bulk ohase.
The Anchoring Peak is characterized by a structure-like
relaxation process (ref. 9). It is worthy of note to observe the ~nazing correspondence between the C values at which tlqe structural depolarizaticn processes develop, and those at which both
~' and ~" exhibit t~he frequency dependent abrupt increase, plotted
in Fig. 2a.
First Gap Region With reference to the Phase Map (Fig. I) : C = 0.4 (ref. 9-11), a phase s e ~ a t i c n
as the water content approaches
occurs in the system and both an isotro-
pic microemulsion and a birefringent LC phase are observed.
In this region:
called the Ist Gap P~ion, the TSD spectra are c~haracterized by the Interfaoe Peak (253"K) and the Anchoring Peak (273°K), which occur
always with the sane
peak tenperature, as well as by a Structure Peak, whose ,max. peak intensity shifts upon water addition within the range 0.4< C < 0.5~ from 288°K to 292°K.
As soon
as ccmpletely birefringent LC sanples are formed~ the above shift stops and a structural relaxation process characteristic of the given LC mesophase develops. The latter does not change its max. peak t ~ r a t u r e
as far as the concentration
is varied inside the concentration range of existence of that given structure. The above process is grad/al and paralles the .modification of the system's struc.rural ordering;
therefore, the ~ r a t u r e
calibration performed on LC samples
was adopted to assess the temoerature readings obtained with a thermocouple placed as near as possible, but still "outside" the sample under test.
Due to the cali-
bration prooedure, the max. peak temperature of both the Interface and the anchoring peaks are shifted, respectively to 262eK and to 276°K (Fig. 4a).
The problem
393
~I[~A) 2O
20- I~A)
C =0.504 A INTERFACE
I I
t
)
(b)
1 10.
0
'° 2,;o
260
280
30
/
t '~
c1(
Fig. 4.a) Example of peak-separation performed on the TSD spectrum of a LC sarmple (T = 293°K) ; b) Typical Bucci and Fies~hi curve fit for the Interface Peak. Triangles: experimental points; dotted line, theoretical fit (Eq.1).
a.&&
2(:
c,=0~.,
,~ (a)
c2=0.~,95
/ /
¢3=0.St 9 10
0
. 8
.
.
.
16
.
2/.
m In
Fig. 5.a) Electro-optical behavior vs. time of microemulsion sanDles in the Pre-structural and in the First Cap Region. Each curve was normalized to its starting value measured before the inloosition of the field.
394
I
]
~Ibil"
(b)
r I0-
(b0)
10m0
Fig. 5.b) Inverse of the exponential coefficient "b" y = (Q exp (bt)) , as a function of concentration. Upper curve, "positi\~" values calculated on the electro-optical ~neak rise; lower curve, "negative" values calculated on the anisotrony decay
wf03o-
(b.0)
40-
curve.
10-
The I/ I b l values were plotted in .~ich a way as to adhere to the experimental behavior. ~cee text for further exn!anations.
10-
.,.
. I flea)
I(pA)
/h ,A) /
20
C--0.506 (aJ \ C=0.538{,)
20
/
•
$I|UCTURE
tl~,',
INTERFACE PEAK
PEAK
,~ ~/~
" ~ ,-, 'I
I~
C=0.663(I)
10 10
~,/ %/ STRUCTURE
j 2,;o
,,,D
2~0 ' 2~o
1o1
360 'TO'K;
240
260
280
300
T('K )
Fig. 6. Strucb~ral relaxation processes exhibited by birefringent microemulsion samples, a) TSD _spectra that characterize the first LC mesorgnase; b) the second
LC mesophase.
395 concerns only the TSD _spectra performed on samples with water
contents falling
in the neighborhood of C =0.5, at the boundary between the I st Gap and the birefringent LC region. ~he mechanism of formation of the LC phase that develops in the actual system as its water content approaches a ratio C .-0.5, was investigated by means of electro-optical anisotropy measurements in the ccncentraticn interval 9~ich extends from the isotropic microemulsion phase up to and beyGnd the Ist Gap region (0.35< C < 0.55). Despite the extremely low-level field 'Imposed, an electrically induced birefringence was observed in isotropic w/o microerallsion samples at C = 0.37.
~he
amount of induced birefringence, first raised ~aralleling the ccncentration increase, reached a maximum at C = 0.487 and then decreased gradually as the concentration C = 0.5 was approached (Fig 5a).
In the neighborhood of the latter
concentration, no effect at all could be detected.
For C ~, 0.5 the field induced
effect on naturally birefringent samples, was an anisotropy decay as shown in Fig. 5a, curve 3.
In the isotropic s6mples the field induced effect was an elec-
tro--optical peak (curve I), whose max. peak intensity shifted with time upcn increasing concentration.
In any case, the removal of the field was never follcx4ed
by a return of the system to its starting configuration. of the type of effect induced,
~gDreover, independently
it was not possible to stop the development of the
process, onoe it had been switched on by the i _mposition of the field.
In order
to have a better and more quantitative description of the above behavior, we calculated the e~qgonential coefficient b
~ y=8 exp (bt)]
of the "under field" elec-
tro-optical peak rise as well as that of the anisotrepy decay curve.
~he result
is plotted in Fig. 5b, where we reported, vs. concentration, the I/l(b)I values in such a way as to adhere to the experLmentally observed behavior, since the b-coefficient
is positive for isotropic saaples and negative for the naturall_v bire-
fringent ones.
The concentration interval investigated aLnpears divided into two
regions where the system exhibits completely different properties s the transition concentration being the value C = 0.5 (ref. 16).
The Liquid crystalline region Tne TSD analysis, as well as the electro-optical study, sh(~4 that two different mescmorphic states may be distinguished in the birefringent region, each exhibiting both a typical TSD spectrum and electro-optical behavior (Figs. 6, 9).
396 The TSD spectra of naturally birefringent LC samples are characterized by: 1)the Interface Peak, the depolarization process occurring at 262°K whose maximum peak ~ r a t u r e
does not depend on the sample water content; 2) the Structure
relaxation process associated with the given mesophase; the latter is a depolarization band at ~ 300°K for the first LC structure (Fig. 6a) and a depolarization peak at T = 272°K for the second LC structure (Fig. 6b).
qhe TSD analysis vs.
water content proved that each LC mesophase is confined in a concentration range of existence.
The tests made by orienting the samples at different polarizing
temperatures, shoved that each LC structure has its own temperature domain. larization at a Tp outside the corresponding temperature d o ~ i n rile destruction of the system's structural organization. develops at higher water contents (0.6< C<0.7)
Po-
limits/ entails
The second LC structure
than the first (0.5
in a ic~ger temperature domain (272°K and 300~K respectively). Polarization at different T[] values within the above intervals, together with the many tests performed to identify the nature of the orientation processes, demonstrate that also the struch]re relaxation processes are of dipolar origin and follow a first order relaxation kinetic. The activation energy ~
and the relaxation time ~7 of both the interface and
the structure peaks are plotted in Figs. 7a,b and 8a, b as a function of increasing concentration.
The values were calculated following the procedure described
in section 2 para.4.
In Figs. 7 and 8, depending on the number of relaxation pro-
cesses exhibited by the system, from one to three different ~ correspond at any
given C.
([) values may
These are distinguished by the _max. peak temper
ature at which the current peak developed in the TSD spectrum.
One should take
into account that in this case, to analyze the system's properties against water addition, does not _mean that the system is always the same and ~
are singly vary-
ing one of its parameters, since we are con~aring systems that differ in their state of aggregation.
In other words, "a between sanples" comparison is a "be--
tween systems analysis."
As far as the latter point is concerned however, it is
quite surprising to recognize that, despite the change of the system's water content, there are concentration intervals within which the system abides I and stun ples that differ in their water content behave as if they were the same system. On the other hand, there are concentration intervals where the system evolves and even the s~me sa~]le may display quite different characteristics as a function of time (phase transition).
In the former case the concentration may indeed be con-
397
3. ~(eV]
'~t "v'
[O1
1I
I
/
(b)
tI
'°,-''
o-2,,~o
¢-'° I
2,
•
1.
0, d
C(wlw_i
.2
.3
.4
..~
-
0.5-
,
~
.
.4
.
.
.5
.
.6 C (wlw)
Fig. 7. Activation energy of microe~ulsion relaxation processes vs. water content. a) Isotropic and Prestructural region; b) First GaD and Optically birefringent region.---~ Interface Peak ;- -O-=struct~re peaks; . ~ m Anchoring peak. - - ~ - - I st LC p h a s e ; ~ 2 nd LC Phase.
o - I sl LC peele
L~,8,
(a)
(b)
o-2-~°
,,
_l~.-=1|ucTu.m ~A= ~m,e*F~a PlAE
~o~
•
1.C
•2
0
6
~°°
C{wl~)
;I
,,/
.3
.4
.5
10 ~
.3
.4
.5
.6
C(w/w)
Fig. 8. Relaxation time diagram vs. concentration of the depolarization processes corresponding to the activation enerqies plotted in Fig. 7. ( ~ was calculated at T = 293"K) Symbols are the same as in Fig. 7.
398
sidered as one of the system's parameters; in the latter it cannot.
Due to the
propagation of the error (cfr. Eqs. 2, 3), the relaxation time data are affected by a greater dispersion than the ~ data.
However, the difference in the order
of magnitude of the ~7 values calculated is absolutely significant.
The TSD nara
meters of the Structural relaxation processes are reported in Table I.
The Inter-
face peak parameters may by found in refs. 9 and 18.
We note that the "zig-zag ~'
trend of both ~
a dispersion effect;
and ~
vs. concentration is not ~ s t
it
depends on the successive d%anges of the system's dispersed phase dimension.
The
phenomenon occurs at some specific concentrations and is evidenced by an abrupt increase of the depolarization current peak intensity. by both the interface and the structure peaks.
This behavior is followed
(cfr. for instance Figs. 3a and b
and refs. 9 - 18.) By means of the electrical anisotropy measurements on naturally birefringent samples, two different trends were observed in correspondence with the C-range of existence of the first as well as the second LC mesophase. field induced effect is a Ubirefringence decay"
For 0.5< C< 0.6; the
(Fiq. 9a); for 0.6 < C< 0.7 the
field induced effect is a "birefringence rise," often acccrnpanied by saturation effects (Fig. 9b) .
In addition, for 0 . 5 8 0 ~ C < 0.605, an intermediate behavior wa.~
recorded between t/nat of t,ype (a) and that of type (b) (Fig. 9c), (refs. 17, 18). The latter parallels the vanishing of the I st LC phase and the development of the IInd.
We recall that within the same concentration interval both
6' and 8 " ex-
hibit the frequency independent diverg~_nt behavior plotted in Fig. 2 and the TSD analysis displays extremely cc~plex relaxation spectra where both the current peaks that ~haracterize the Ist and the IInd LC mesophases are simultaneously present. With the sole
I) the angular dependence
of the transmitted light intensity at any given angle of entrance of the light electric vector; 2) whether the microemulsion lyotropic mesop_.hase exhibited any rotator7 power (in spite of the absence of chiral .molecules in cur s~]stem). For each study several tests were merformed by changing the system's anchoring condition, the sar~le holder gec~etry as well as the sample volume.
The results
may be summarized as follows: I) the dependence of the tran~nitted light intensity on the angle at which the
399 electric vector of the light wave enters the sam?les with water content in the interval 0.48 - 0.580 displays a very regular pattern which is ~ i i polar plots given in Fig. 10a.
evidenced by the
~he four-lobed array vanishes at C = 0.582 in the
interval 0.570( C < 0.6 (Fig. 10b, c), while it appears again cn samples with water content in the interval 0 . 6 < C < 0.7 (Fig. 10d).
In the latter case the lobe's
am glitude is four times smaller than that of type (a). 2) The samples belonging to the first LC meso_nhase (0.48< C < 0.6) exhibit a strong rotatory power; those of the second (0.6< C<0.7) do not.
In the concentration
interval where electro-optical recordings of type (c) (Fig. 9) we_re observed, the rotatory power decreases gradually vs. increasing water content and disappears at C ~
0.6.
Of course to talk about rotatory power in a system without any chiral molecule takes some swallowing.
The first criticism that could be made is that the effect
may be due to an interaction between the walls of the s ~ p l e holder and the sample itself.
However, in this case, since we have drastically chang..~d the gec~etry of
t/qe sam21e cell, as well as the anchoring conditions, we should have observed~ at least on one occasion, some significant difference in the "ability of the system. to rotate the plane of polarization of the light wave." error of +2 °, this has never happened.
Within the exnerJ_mental
On the contrary,, the rotatory p o ~ r
seems
not to depend on the sa~Dle holder, dimensions or shade, nor on the particular anchoring condition realized.
From a practical point of view, it is reasonable
to utilize the above results for a further characterization of the system's struc -~ tural regions, that being the akin to which t/qe measures were devoted. ~ne results indicate that in t~he microemulsion birefringent region the two lyotropic mesophases distinguished by a s_necific TSD spectrum, electr~-qptical as well as optical behavior, are separated by a Second Can Region within ~%ich they coexist.
The second gap region is identified by:(1)the behavior of ~ ' and ~ "
(Fig. 2);
the intermediate electro-optical behavior (Fig. 9c); the vanishing of
t~he rotatory p~wer previously described and the disappearance of the four-lobed array (Fig. I0c). %3%e problem is now to understand what kind of structnre may correspond to the first as well as to the second LC mesephase of the w/o microemulsion. of structures have been hypothesized (refs. I-3~ 18):
Two b_,qges
a cylindrical phase in
which each cylinder is a thin water channel covered by the hydrophilic groups of the molecules while the hydrocarbon chains fill the gap between the cylinders,
(i) divergent
400
mV
50.01 hm V
2OO 37.5. 150
1--C=.511 2~C=.536
,_c...,
'mV
1--C=.626 2-- C=.629 3--C=.638
/ / / / /~-~
2.--.
//\
,.
[
",
150-
i
100.
1 ~ C = .582 2--C=.593
20C
tOO.
50.
1
O1"0
= O 2"0 m -
~
0
b
1"o
;~0
3"0
4'0 m
Fig. 9. Typical electro-optical behavior exhibited by samples with water content falling in: a) the first LC region; b) the second LC reqion; c) the Second can region where both mesophases coexist. Due to the ~ (632B~)of the laser beam employed, the samples of the second LC mesoohase appear only slightly birefringent.
Z
0"
100"
L
270 e
~,
180"
d
Y
Fig. 10. Angular ~ene~ndence of transmitted light intensity: Polar Plots of the ratio I (min.)/I (.max.) ( ~ ) vs. the in_r~t angle ( ~ ) of the light Electric vector to the y-axis, qhe latter is re_[xDrted as seen by an observer looking tc~ward t h e incoming beam. A]C= .54e S]C= .570 C]C.-,586 D]C--. 604
401
taking up a disordered, liquid-like configuration, and a lanellar phase, built up by an ordered sequence of oil and water planar sheets (Fig. 1 I). According to what is known on liquid crystals it follows that, in the lanellar configuration the requirement of a constant interlayer thickness should not allow any helical or twisted arrangement of the strucbare,
qherefore a lamellar phase
should not exhibit any rotatory ~x)wer while a cylindrical phase (nematic) may exhibit such a behavior (ref. 19). Finally in a given system in which both structures develop, the lanellar phase usually occurs at a tempe rabare below that of the nematic dQmain.
Since the two structnres are of different symmetry, they mast
be separated by a transitiou region.
The aforementioned characteristics suit
quite well those that distinguish the microemulsion LC mesophases.
Therefore it
is reasonable to assume that the first LC mesophase consists of a cylindrical structure, the second of a lamellar (ref. 18). The divergent behavior of 8 ' and " at C = 0.582, corresponds to the transition frcrn cylinders to lamellae, due to the fact that the water-oil interface, in terms of a surface enclosing the volume of the dispersed water phase, breaks down.
As far as the rotatory ~ower is
concerned, the observed phenomenon in the actual system may be justified in terms of a "bulk effect" originated by the twisted loops of water channels within the syst~n.
This is possible even in the absence of chiral molecules.
w/omicroemulsion
(1) structur~_.~
@ o/wmicroemulsion
Fig. 11.
Struc~ral transitions in w/o microemulsions against water addition.
The thermotropic transition In the second gap region the LC system is not only lyotropic but also t~herm~. tropic, and the t~Tpera~/re dcr~/ns of the two struc~res beccme very narrow. The different mesophases are still piloted by the water content;
however, amcng
the two structures, the system tends to assume the one which is more favoured by
402
/ Z-
~
i460
/
[
C.o.oos
T .30"C
.
,.0KHz
" ~ S0MHz
10"1
2601< 0
. . . . . . 2
s 4 TIM E(hours)
II
*
8
39
10"% 40
0
I/
L /
, , ..~
50MH z
%~2 nd AP G
'
l
l
, C..._
.2
.4
o8
.1~
Fig. 12. a) Thermotrooic transition in the Second Cap_ Region Lamellar-Cylindrical transition. (See text for explanation); b) The loss tangent vs. C-calculated on the data plotted in Fig. 2 is reported in order to envisage the dielectric behavior of the system in the whole concentration interval investigated (0. 024-0.8).
',%1 11,11
(Z,)
140 110]
3'
C1= 0.,510 C2= 0.604
1,5
TIME Fig. 13. Thermotropic transition:
(hours} Cylindrical-ian~llar Transition
403
the hanlpera~ure at which the system finds itself.
This kind of thermotropic
transition characterizes both gap regions ~nd is well observable by following the variation vs. time at a fixed frequency of the sample complex 'iEQedance (Fig. 12). In Fig. 12 a sample of the 2nd gap region, with a la~ellar structure ( 1 0 ~ is kept at T = 303°K.
Z-range),
This temperature does not favour the lamellar structure.
Therefore the system undergoes a thermotropic structural transition by changing to the cylindrical configuration 160 KHz.
(K~ Z-range) .
The recording was performed at
The order of magnitude of microemulsion impedance in the different con-
centraticn regions at several .measuring frequencies are given in ref. 8.
In Fig. 13
the oon~lex J__mpedgnce behavior of a stable lyotropic s a ~ l e with cylindrical struc-ture (Zl, ~I ) is plotted, as well as that of a sample with a water content falling at the end of the second g~o region (Z2, ~ 2 ) . 293°K.
Both s ~ l e s
The measurements were carried Out at 160 KHz.
is 288°K.
qhis t ~ a t u r e
were produced at
The measuring temperature
falls inside the temperature domain at which the truly
lyotropic cylindrical phase may exist.
But due to the larger water content of
the Z 2 sample, also with cylindrical structure (K~Z-range),
the above tempera-
ture favours the transition of the latter to a l&mellar phase (I02~ Z-range). ~%is behavior which occurs in the very complex region of the system is the most interesting for biophysical purposes and applications.
TABLE I TSD analysis : Structural relaxation ~rocesses in w/omicroemulsions C (w/w)
Tma x (6K)
~(eV)
~(~)
in~ o
a(~)
~(s)
0.03 0.03 0.04 0.05
-28.00 -40.30 -63.30 -48.84
1.4 1.2 1.6 2.1
43 235 496 330
1.390 0.05 1.824 0.09 Mesophase 1.250 0.03 1.198 0.12 2.080 0.17 1.800 0.06 1.760 0.18 1.290 0.05 1.340 0.09
-50.26 -67.76
1.9 3.7
118 86
-45.30 -42.58 -78.40 -67.17 -65.00 --46.40 -47.80
1.2 4.6 1.0 2.8 7.0 2.0 3.6
67 128 51 56 108 107 182
Prestructural Re~ion 0.371 288 0.388 299 0.403 302 0.418 304 First Gap Reqion 0.446 295 0.484 292 ~Jrst Lic~id Crystalline 0.495 292 0.506 292 0.516 292 0.526 292 0.536 292 0.545 292 0.581 295
0.802 1.156 1.756 1.380
404
TABLE I continued C (w/w)
Tmax (°M)
~(eV)
O(+)
Second Liquid Crystalline Mesophase 0.596 272 1.490 0.06 0.603 272 1.340 0.07 0.610 272 1.770 0.08 0.617 272 1.500 0.07 0.623 272 1.580 0.07 6. 641 272 I. 380 0.03 0.652 272 I .509 0.07
into
O(+)
--59.30 -52.60 -71.80 -59.80 -63.70 -54.16 -59.99
2.8 2.8 3.7 2.9 2.9 1.5 2.8
~ (S)
0.7 I .6 0.2 0.6 0.3 I. 6 0.8
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